This is the submitted version of the following article: Pereyra C., Xie H., Lira-Cantu M.. Additive engineering for stable halide perovskite solar cells. Journal of Energy Chemistry, (2021). 60. : 599 - . 10.1016/j.jechem.2021.01.037, which has been published in final form at https://dx.doi.org/10.1016/j.jechem.2021.01.037 © https://dx.doi.org/10.1016/j.jechem.2021.01.037. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ 1 Manuscript File in Doc Template Click here to view linked References Review Additive Engineering for Stable Halide Perovskite Solar Cells Carlos Pereyraa, Haibing Xiea,* and Mónica Lira-Cantua,* a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the Barcelona Institute of Science and Technology (BIST). Building ICN2, Campus UAB E08193, Bellaterra, Barcelona, Spain. * Corresponding author. E-mail address: haibing.xie@icn2.cat (Haibing Xie), monica.lira@icn2.cat (Mónica Lira-Cantu) ABSTRACT Halide perovskite solar cells (PSCs) have already demonstrated power conversion efficiencies above 25%, which makes them one of the most attractive photovoltaic technologies. However, one of the main bottlenecks towards their commercialization is their long-term stability, which should exceed the 20-year mark. Additive engineering is an effective pathway for the enhancement of device lifetime. Additives applied as organic or inorganic compounds, improve crystal grain growth enhancing power conversion efficiency. The interaction of their functional groups with the halide perovskite (HP) absorber, as well as with the transport layers, results in defect passivation and ion immobilization improving device performance and stability [1-4]. In this review, we briefly summarize the different types of additives recently applied in PSC to enhance not only efficiency but also long-term stability. We discuss the different mechanism behind additive engineering and the role of the functional groups of these additives for defect passivation. Special emphasis is given to their effect on the stability of PSCs under environmental conditions such as humidity, atmosphere, light irradiation (UV, visible) or heat, taking into account the recently reported ISOS protocols [5]. We also discuss the relation between deep defect passivation, non-radiative recombination and device efficiency, as well as the possible relation between shallow defect passivation, ion immobilization and device operational stability. Finally, insights into the challenge and criteria for additive selection are provided for the further stability enhancement of PSCs. Keywords: Additives; additive engineering; perovskite solar cells; defect passivation; shallow defect, deep defect; stability Carlos Pereyra received his B.Sc. in 2013 from the Department of Energy, at the Polytechnic University of Chiapas (Mexico). He is currently pursuing his Ph.D. studies at the Chemistry Department of the Autonoma University of Barcelona (Spain) and at the Nanostructured Materials for Photovoltaic Energy Group at the Catalan Institute of Nanoscience and Nanotechnology, ICN2 (Spain). His current research focuses on electronic devices based on halide perovskites, especially on the fabrication of highly efficient and stable carbon-based perovskite solar cells. Haibing Xie received his Master degree in 2012 from the University of Science and Technology of China (USTC) and his Ph.D degree in 2016 from the University of Barcelona and the Catalonia Institute for Energy Research, IREC (Spain), majoring in Materials Science. He is expert on the study of Cu(In,Ga)Se2 chalcopyrite solar cells and CuZnSn(S,Se)2 kesterite solar cells. Since 2017, he has been working under a postdoctoral contract in the group of Prof. Monica Lira-Cantu at the Catalan Institute of Nanoscience and Nanotechnology (ICN2). His current work includes improving the long-term stability of perovskite solar cells and exploring emerging optoelectronic materials for photovoltaic applications. He has published more than 30 peer-reviewed high impact papers in photovoltaic field, with an h index of 16 with over 900 citations. Prof. Monica Lira-Cantu obtained her PhD at the Materials Science Institute of Barcelona, ICMAB and the Autonoma University of Barcelona (Spain) in 1997. She is Group Leader of the Nanostructured Materials for Photovoltaic Energy Group at the Catalan Institute of Nanoscience and Nanotechnology, ICN2 in Barcelona (Spain). Her research interests are the synthesis and application of nanostructured materials for next-generation thin film solar cells: Dye sensitized Hybrid, Organic, All-oxide and Perovskite Solar Cells. She has more than 120 publications, among them more than 105 published papers, 9 patents, 10 book chapters and 1 edited book. She has an h index of 46 with more than 7848 citations. 1. Introduction Halide perovskite solar cells (PSCs) have become a rapidly evolving photovoltaic technology attracting great interest due to the many advantages besides their impressive efficiency, among them are materials and optical tunability and versatility, striking optical properties or low temperature processability. In addition, their “soft” nature permits their fabrication by solution processing methods at low-cost and on flexible substrates. Metal halide perovskites (HPs) adopt the general chemical formula of ABX3 and have unit cells composed of five atoms in a cubic structure (α phase), where cation B has six-nearest neighbour anions X and cation A has twelve (Fig. 1). Under ideal conditions, the stability of the crystal structure is highly dependent on its high symmetry cubic structure; the ionic radii of A, B, and X should satisfy the requirement of a tolerance factor t close to 1 (𝑡 = (𝑅𝐴 + 𝑅𝑋)/{√2(𝑅𝐵 + 𝑅𝑋)}, where RA, RB, and RX are the ionic radii of the corresponding ions). To satisfy t = 1, the A ion must be much larger than the B ion [6-7]. Fig. 1. Schematic representation of the halide perovskite (HP) crystalline structure ABX 3. Metal halide perovskites were first synthesized and reported in the 1970s [8-10]. These materials present a chemical formula ABX3 where A+ = methylammonium (CH3NH3+ or MA+), formamidinium (HC(NH2)2+ or FA+), caesium (Cs+), rubidium (Rb+); B2+ = lead (Pb2+) or tin (Sn2+); and X− = chlorine (Cl−), bromine (Br−), or iodine (I−) [7]. The first application of HPs as the absorber in a solar cell corresponds to Prof. Tsutomu Myasaka’s group in 2009 [11]. They applied the methyl ammonium lead Iodide (CH3NH3PbI3) as the absorber resulting in devices with power conversion efficiencies (PCEs) of 3.8%. The HP was depicted as a pigment absorbed on a mesoporous TiO2 thin film surface in a device configuration based on the Dye Sensitized Solar Cell (DSSC). In the following 2 years, Prof. Nam-Gyu Park’s group was able to increase the PCE up to 6.5% [12], but the stability of such devices was rather poor due to the dissolution of the perovskite by the liquid redox electrolyte. A breakthrough was achieved in 2012 with a contribution from the research groups of Prof. Park and Prof. Michael Gratzel [13] and, in parallel, to the groups of Prof. Henry Snaith and Prof. Myasaka [14] who, independently, reported the use of a solid-state hole transporter material, the 2,2’,7,7’-tetrakis (N,N-di-p-methoxyphenylamine) -9,9’-spirobifluorene (spiro-OMeTAD) in replacement of the liquid electrolyte. As a result, an increase of the PCE above 9% was reported. After these pioneering studies, the efficiency of these PSCs has observed a rapid improvement that triggered an explosive research effort worldwide to improve PSC manufacturing with demonstrated rapid advances in optimizing device architectures and perovskite properties. Fig. 2 depicts the evolution reported on the PSC’s efficiency in the last 11 years. Currently, the certified record PCE for a single-junction PSC has reached 25.2% [15]. Among the most relevant works we can mention Heo et al. who reported the application of several polymeric hole selective contacts in PSCs applying the CH3NH3Pbl3 as the absorber, leading to a PCE of 12% [16]. Burschka et al. modified the deposition method of the perovskite layer using a two-step technique, increasing the PCE up to 14,14%; [17] Liu et al. reported also a modification of the deposition method via physical vapour deposition, which resulted in a PCE of 15.4% [18]. With the aim of increasing uniformity and perovskite thin film density, solvent and composition engineering was carried out by Jeon et al. who reached 17.91% efficiency [19-20]. Yang et al. obtained a certified 20.11% PSC via intermolecular exchange in a FAPbI3–based PSCs [21] and Saliba et al. increased the PCE to 21.1% and 21.6% by using a triple and a quadruple cation perovskite system, respectively [22-23]. Other examples worth mentioning are those carried out by Yang et al. who reported a certified PCE of 22.1% using iodide management in a double cation perovskite system [24]. Jung et al. obtained a 22.67% efficiency using an alternative hole- transport material, the Poly (3-hexylthiophene) (P3HT), which showed excellent optoelectronic properties [25]. Jiang et al. reported the use of an organic halide salt, the phenethylammonium iodide (PEAI), in mixed perovskite films for surface defect passivation with certified PCE of 23.32% [26]. The details of the device architectures for the last three examples with 23.7%, 24.2% and 25.2% efficiency (Fig. 2) are still not available [15]. Fig. 2. Evolution of the power conversion efficiency of PSCs with time of selected lab-scale perovskite-based solar cells [3-26]. Liquid electrolytes were first employed following the DSSC technology (black), followed by solid state devices (red). The use of additives has recently allowed the highest efficiencies reported for the technology (blue). Several issues associated with PSCs need to be addressed in order for the technology to become competitive. However, the critical hurdle of PSCs is their operational stability. The ionic-electronic conductivity and the “soft material” nature of halide perovskites indicates a strong component for ion movement and ion migration. This is more pronounced under operational conditions (e.g., light irradiation, moisture, heat) and translates into device ion vacancy formation (charge trap formation), hysteresis, phase separation and device instability. Incredible gains in device stability have been achieved in recent years and in a relatively short period of time due to the employment of different strategies, among them is the interface/bulk engineering that primarily employs the use of additives. The inclusion of small amount of chemical additives in the perovskite precursor solution provides advantages in terms of attractive optical and electrical properties which influence the performance of the resulting device. Fig. 3 summarizes the efficiency and performance loss (in stability) of PSCs selected from the literature (for the years 2018-2020) with (w) and without (w/o) the application of additives within the perovskite absorber layer. The graphs are divided by the type of additive employed. In all cases, we observe a clear efficiency increase when additives are employed (Fig. 3a). This effect is attributed to the enhanced thin film properties and defect passivation effect of the additive on the HP thin film. In the case of performance loss (Fig. 3b), this is, the difference in percentage between the initial PCE and the PCE observed after stability analysis, we can also observe an improvement of the stability (or a reduced performance loss) after every stability analyses. The latter demonstrates the beneficial effect of additive engineering on the performance and lifetime of PSCs. Fig. 3. Efficiency (a) and Performance loss after stability analysis (b) of PSCs with different types of additives (graphs based on data depicted in Table 1). Given the current rate of progress and continued improvements in PSCs, great efforts have been made through designing new device structures [14][27], interface engineering [28-30] and defect passivation [31], exploring new perovskite materials [20] [23] [32-34]and controlling the perovskite crystal growth [19][35-36]. The growth of the perovskite crystal plays an important role in optimizing the performance of PSC device. We based this review on recent advances in additive engineering reported in the literature since 2018, with the aim to enhance PSCs stability. We include topics that range from physical material degradation to non-destructive phenomena that, nevertheless hamper efficient device operation. We will discuss the working mechanisms and important roles of additives in PSCs focusing on issues such as (see also Fig.5): a) Perovskite thin film processing, including the growth of the perovskite crystal, which is related to perovskite morphology, and plays an important role in optimizing device performance; b) Device performance: in order to achieve high device performance in PSCs, great efforts have been made to design new device structures, manipulate their surface by additive engineering and to explore new perovskite materials; c) Device lifetime: one of the most important criteria for PSCs is the ability to maintain stable power output under standard working conditions; d) Others: a section where we include additive’s molecular configuration and size (e.g. chain length). Fig. 4. Schematic diagram of three common perovskite solar cell architectures: mesoporous n-i-p (a), planar n–i–p (b), and planar p–i–n (c). For the latter, we divided this work in four sections starting with an overview of the different working mechanisms observed when additives (organic and inorganic) are employed in PSCs (Section 2). We include the most recent examples of organic additives depending on their functional group, chain length and molecular configuration and their effect in 2D perovskites. In Section 3 we elaborate on the effect of additives on perovskite grain size and their effect on grain boundaries and surface passivation. Section 4 encompass different examples where additives are employed to enhance PSC stability, here we include a brief discussion about the role of defects, and we finalize this work giving a general outlook of the future of additives in PSC future evolution. Fig. 5. The impact of additives in PSCs: Effect on device fabrication and reproducibility, device efficiency and device stability. Table 1 includes the published research works on additive engineering for PSCs reported between 2019 and the middle part of 2020. The table is organized by the type of additive and includes only those published works which report on any type of stability analysis. Stability test results and conditions are included in the corresponding columns. Table 1. PSCs employing additives to enhance device performance (PCE and Stability). PCE Stability Performance loss Condition Perovskite Additive name Molecular formula Testing Ref. composition Control w/additive time/Temperature/RH%/Light Control w/additive with UV or not/Encapsulation (E) or not (NE) POLYMER [37] MAPbI3 Polymer poly(4-vinylpyridine) (C7H7N)n 18,05 % 20,23 % 40 % 15 % 2160h/25ºC/40%/Dark/NE 200h/85ºC/40-60%/Light (w/o [38] MAPbI3 Ethyl 2-cyanoacrylate C6H7NO2 19,43 % 21,03 % 50 % 10 % UV)NE 1000h/25ºC/40-60%/Light (w/o [39] MAPbI3 Ethyl 2-cyanoacrylate C6H7NO2 18,39 % 20,71 % 20 % 11,7 % UV)/NE FA0.87Cs0.13P H−(O−CH2−CH2)n 336h/25ºC/88%/Light (w/o [40] b(I0.87Br0.13)3 Poly(ethylene oxide) −OH 15,29 % 16,46 % 50 % 20 % UV)/NE [41] MAPbI3 Ethyl cellulose 17,11 % 19,41 % 30 % 20 % 720h/25ºC/45%/Dark/NE (FAI)0.81(PbI2 [42] )0.85(MABr)0.1 Poly(methyl methacrylate) 5 (C5O2H8)n 21,6 % (PbBr2)0.15 (FAPbI3)1−x( Poly[(thiophene)-alt-(6,7-difluoro- [43] PTQ10 18,91 % 21,21 % 40 % 14 % 1480h/25°C/40%/Dark/NE MAPbBr3)x 2-(2-hexyldecyloxy)-quinoxaline)] Cs0.05(MA0.17 (CF3CF2CH2NH3+, [44] FA0.83)0.95]0.95 Hydro-phobic polyfluorinated 20,69 % 22,86 % 50 % 20 % 3000h/25°C/65%/Dark/NE 5F-PA+) Pb(Br0.17I0.83) cations [45] MAPbI 3 Polymer Doping (C66H85F2N3S6Si2)n 17,43 % 19,19 % 20 % 10 % 336h/25°C/45%/Dark/NE (J71) 400h/25°C/45%/Light (w/o [46] MAPbI3 Trimethylolpropane triacrylate C15H20O6 19,06 % 20,22 % 25 % UV)/NE H−(O−CH2−CH2)n 300h/25°C/70%/Light (w/o [47] MAPbI3 Polymer (PEG)-scaffold 14 % 16 % 30 % UV)/NE −OH [48] (FAPbI3)0.85( MAPbBr3)0.15 Polyaniline (C6H7N)x 16,96 % 19,09 % 20 % 14 % 1600h/25°C/50%/Dark/NE Cs0.07Rb0.03F [49] A0.765MA0.135 Poly(methyl methacrylate) (C5O2H8)n 18,50 % 20,86 % PbI2.55Br0.45 Group of polarized ferroelectric (polyinylidene fluoride- [50] MAPbI3 (C2H2F2)n(C2HF3)m 17,21 % 21,38 % 20 % 10 % 500h/25ºC/40%/Dark/NE trifluoroethylene polymer P(VDF- TrFE)) (Cs Poly[bis(4- [C6H4N(C6H2(CH3) phenyl)(2,4,6trimethylphenyl)amine 18,64 % 20,05 % 5% 1440h/25°C/40%/Dark/NE [51] 0.2FA0.8)P 3)C6H4]n b(I0.95Br0.05)3 SOLVENT (FAPbI Alkyl ammonium [52] 3)0.92( bromide/chloroform CH3NH3Br/CHCl3 21,3 % 22,3 % 30 % 15 % 100h/25ºC/40%/Light (w/o UV)/E MAPbBr3)0.08 [53] MAPbI3 Ammonium benzenesulfonate C6H9NO3S 18,69 % 20,62 % 20 % 15 % 1440h/25ºC/30%/Dark/NE [54] MAPbI3 Dimethylsulphoxide (CH3)2SO 16,19 % 18,19 % [55] MAPbI3 Methanol CH3OH 16,53 % 19,51 % 50 % 15 % 720h/25ºC/Dark/NE [56] MAPbI3 Tetraethyl orthosilicate SiC8H20O4 15,96 % 18,38 % 13 % 7% 672h/25ºC/30%/Dark/NE Cs0.05(MA0.17 Dimethyl [57] FA0.83)0.95Pb(I sulfoxide/Chlorobenzene (CH3)2SO/C6H5Cl 16,6 % 18,4 % 0.83Br0.17)3 [58] MAPbI3 2-Propanol C3H8O 16,02 % 19,70 % 50 % 15 % 960h/25ºC/40%/Dark/NE [59] MAPbI3 Water H2O 7,40 % 18 % 10 % 960h/25ºC/40%/Dark/NE [60] MAPbI3 Water H2O 9,78 % 20,1 % 10 % 720h/25ºC/40%/Dark/NE [61] MAPbI3 2-Methylanisole CH3C6H4OCH3 18,3 % 20 % 40 % 10 % 720h/25ºC/50%/Dark/NE [62] a-CsPbI2Br 2-Propanol C3H8O 12,65 % 16,07 % 30 % 5% 1000h/25ºC/30%/Dark/NE 35h/25ºC/60%/Light (w/o [63] MAPbI3 Acetonitrile C2H3N 15,04 % 17,81 % UV)/NE 1600h/25ºC/40%/Light (w/o [64] MAPbI3 Benzoquinone C6H4(=O)2 10,73 % 15,66 % UV)/NE [65] FAPbI3 Benzylamine C7H9N 14,20 % 19,20 % 90 % 50 % 2880h/25ºC/50%/Dark/NE [66] MAPbI3 Dimethyl sulfide C2H6S 16,46 % 17,56 % 50 % 14 % 1440h/25ºC/35%/Dark/NE [67] MAPbI3 4-tert butylpyridine C9H13N 16,20 % 17,30 % [68] MAPbI3 Methyl acetate C3H6O2 16,3 % 20 % 360h/25ºC/60%/Dark/NE Cs0.05(MA0.17 [69] FA0.83)0.95 Pb Pyridine C5H5N 16,94 % 19,03 % (I0∙83Br0.17)3 [70] MAPbI3 4-tert-butylpyridine C9H13N 14,49 % 17,41 % 23,4 % 11 % 720h/25ºC/45%/Dark/NE [71] MAPbI3 Alkylamine R–NH2 18,30 % 21,50 % 50 % 10 % 1000h/25ºC/45%/Dark/NE [72] FAI:MACl:M 2,2,2-trifluoroethanol ABr C2H3F3O 19,17 % 20,92 % 24 % 10 % 720h/25°C/40%/Dark/NE [73] MAPbI3 Methyl acetate CH3COOCH3 16,3 % 20 % 360h/60%/Dark/NE [74] CsPbI3 Ethylenediamine NH2CH2CH2NH2 7,66 % 11,86 % 720h/25°C/45%/Dark/NE 1020h/85°C/45%/Light (w/o [75] CsFAMA Alkylamine ligands R–NH2 20.5 % 22,3 % 39 % 10 % UV)/E FULLERENE [6,6]-(4-pyridinyl)- Pyridine-functionalized fullerene C -ethyl acid ethyl 17,61 % [76] MAPbI3 61 19,82 % 30 % 17 % 720h/25ºC/30%/Dark/NE derivative ester (PyCEE) 200h/25ºC/40%/Light (w/o [77] MAPbI3 Tetratetracontane (TTC) CH3(CH2)42CH3 17,34 % 20,05 % 40 % 23 % UV)/NE ([6,6]-phenyl-C61-butyric-acid [78] MAPbI3 methyl ester) C72H14O2 6,90% 12.78 % Lithium-ion 1000h/30ºC/40%/Light (w/o [79] MAPbI3 containing[60]fullerenetri-fluoro Li+@C60]TFSI- 12,30 % 16,70 % methanesulfonylimide salt UV)/E CH [80] 3NH3Pb0.7 Fullerene C60 12,5 % 13,7 % 60 % 20 % 168h/25ºC/50%/Dark/NE 5Sn0.25I3 Carboxylic-acid/fullerene [81] MAPbI3 derivative R–COOH/C60 11,75 % 13,97 % CH [6,6]-phenyl-C61-butyric styryl [82] 3NH3PbIx dendron ester C72H14O2 12,23 % 17,21 % 50 % 15,4 % 720h/25ºC/40%/Dark/NE Cl3-x Phenyl-C61-butyric acid methyl [83] MAPbI3 ester C72H14O2 14,9 % Phenyl-C61-butyric acid methyl [84] MAPbI3 ester C72H14O2 14,4 % Phenyl-C61-butyric acid methyl [85] MAPbI3 ester C72H14O2 11,4 % 16 % FA0.85MA0.15 Fullerene derivative/bis phenyl (α bis PCBM)/ [86] Pb(I0.85Br0.15) C61 butyric acid methyl ester 18,8 % 20,8 % 50 % 4% 600h/25ºC/Light (w/o UV)/NE C72H14O2 3,5- (CF3)2C6H3CHO/C6 bis(perfluorooctyl)benzaldehyde 15,67 % 18,11 % 50 % 17 % 720h/25ºC/60%/Dark/NE [87] MAPbI3 and fullerene 0 ITIC-Th (3,9-bis(2-methylene-(3- MA0.17FA0.83 (1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(5- [88] Pb(I0.83Br0.17) 18,07 % 19,20 % 5% 960/25°C/45%/Dark/NE 3 hexylthienyl) dithieno[2,3-d:2′,3′d′]-s indaceno[1,2-b:5,6-b′] dithiophene) Phenyl-C61-butyric acid methyl [89] MAPbI3−xClx C 13,5 % 15,7 % 10 % 2h/80°C/45%/Dark/NE ester 72H14O2 hydrophilic [6,6]-phenyl-C61- butyric acid-(3,4,5-tris(2-(2-(2- 1440h/25°C/50%/Light (w/o [90] CH3NH3PbI3 18,62 % 19,74 % 50 % 2% UV)/NE methoxyethoxy)ethoxy)ethoxy)ph enyl)methanol ester Photo-crosslinked [6,6]phenylC61- [91] MAPbI3 butyric oxetane dendron ester C-PCBOD 14,7 % 20,4 % 80 % 28 % 960h/25ºC/25%/Dark/NE NANOPARTICLE Cs0.05(MA0.17 [92] FA0.83)0.95Pb(I Carbon NPs C 17,7 % 18,3 % 0.83Br0.17)3 (Cs0.05FA0.80 500h/25ºC/60%/Light (w/o [93] MA0.15)Pb(I0.8 Europium-doped titania Eu-TiO2 19,22 % 21,40 % 40 % 25 % UV)/NE 5Br0.15)3 [94] MAPbI3 Cesium, Lead and Bromine NPs CsPbBr3 18,51 % 20,46 % 26 % 10 % 500h/25ºC/30-40%/ Dark/NE [95] MAPbI3 Zinc oxide NPs ZnO 13,40 % 18,34 % Barium hydroxide hybridized [96] MAPbI3 B:ZnO 17,63 % 20,53 % 25 % 20 % 2400h/25ºC/Dark/NE boron-doped ZnO [97] 3NH3PbI3- CH Metal–Organic Framework 10,10 % 12 % xClx Nanocrystals [98] MAPbI3 Carbon nanodots C 15,11 % 19,50 % 7% 500h/25ºC/40%/Dark/NE Cs0.05(MA0.15 [99] FA0.85)0.95Pb(I Cobalt(II) oxide NPs CoO 18,75 % 20,72 % 79,39 % 17,84 % 720h/25°C/50%/Dark/NE 0.85Br0.15)3 FA0.85MA0.15 [100] Pb(I0.85Br0.15) Cesium, Lead and Bromine NCs CsPbBr3 17,95 % 20,56 % 1440h/25°C/45%/Dark/NE METAL OR METAL ORGANIC SALTS [101] CsxFA1-xPbI3 GaAA3 Gallium(III) 14% 17 % 30 % 10 % 800h/25ºC/50%/Dark/NE FA 2400h/25ºC/45%/Light (w/o 0.83Cs0.17P C4H12IN 15,3 % 19,5 % 15 % 20 % UV)/E [102] n-butylammonium iodide b(IyBr1−y)3 (FA,MA,Cs)P (HOCH2CH2)3N · [103] b(I,Br)3 Triethanolamine hydrochloride HCl 19,97 % 21,01 % 20 % 10 % 1500h/25ºC/35-60%/Dark/NE FA [104] 0.85MA0.15 Cesium Cs 19,4 % 21,7 % 40 % 140h/25ºC/Light (w UV)/NE PbI2.55Br0.45 MA0.03FA0.97 [105] Pb(I0.97Br0.03) Cesium chloride CsCl 21,2 % 22,1 % 20 % 10 % 500h/25ºC/65%/Dark/NE 3 [106] 3NH3PbI3CH 100min/25ºC/40%/Light (w xClx Cesium bromide CsBr 13,1 % 16,3 % UV)/NE (CsI)0.17(FAI) [107] 0.83(PbI2)10.5x(PbBr2)0.5x Lead (II) thiocyanate Pb(SCN)2 14,35 % 18,60 % FAI, FABr, [108] MABr, CsI, PbI2 Potassium triiodide I3K 20 % 21,2 % 20 % 10 % 1680h/25ºC/Light (w/o UV)/E Csx(MA0.17F A [109] 0.83)(100- Insoluble CsCl CsCl 18,03 % 19,12 % x)Pb(I0.83Br0.1 7)3 (CsI)0.04(FAI) [110] 0.82(PbI2)0.86( Potassium-intercalated rubrene K2Rubrene 17,82 % 18,14 % MAPbBr3)0.14 [111] MAPbI3 Calcium iodide CaI2 16,7 % 19,25 7% 300h/25ºC/40%/Light (w UV)/NE [112] MAPbI3 n-Butylammonium iodide C16H36IN 17,75 % 19,56 % 30 % 5% 100h/95ºC/Dark/NE [113] MAPbI3 Zinc Chloride ZnCl 16,4 % 18,2 % 56 % 7% 720h/25ºC/30%/Dark/NE [114] MAPbI3 Aluminium acetylacetonate Al(C5H7O2)3 17,1 % 19,1 % Csx(MA0.17F Cesium as additive into triple [115] A0.83)(100x)Pb( Cs 17,42 % 20,96 % 60 % 10 % 250h/25ºC/40%/Dark/NE perovskite I0.83Br0.17)3 [116] MAPbI3 Potassium K 19,21 % 20,56 % 1000h/25ºC/10%/NE FA [117] 0.8Cs0.2Pb( Lead (II) thiocyanate Pb(SCN)2 14,20 % 17,18 % I1-xBrx)3 [118] MAPbIxCl3–x Indium (III) chloride InCl3 12,61 % 17,55 % 350h/25ºC/40%/Dark/NE [119] MAPbI3 Hydrogen iodide HI 14,30 % 17,60 % 1,1,1-trifluoro-ethyl ammonium [120] MAPbI3 iodide CF3CH2I 15,60 % 18 % (FAPbI [121] 3)x(M Theophylline C7H8N4O2 19,34 % 21,32 % 80 % 10 % 500h/40ºC/40%/Light (w/o UV)/E APbBr3)1-x [122] MAPbI3 Lead (II) thiocyanate Pb(SCN)2 16,41 % 18,15 % (FAPbI3)0.85 [123] (MAPbBr3)0.1 Lead (II) iodide PbI2 17,7 % 19 % 5 (FAPbI3)0.85 [124] (MAPbBr3)0.1 Lead (II) iodide PbI2 18,12 % 5 Cs0.06FA0.79M [125] A0.15Pb(I0.85B Potassium Iodide KI 17,3 % 21,5 % 30 % 20 % 350h/25ºC/45%/Light (w UV)/NE r0.15)3 (FAPbI 3)0.85( KI 20,55 % [126] Potassium Iodide MAPbBr3)0.15 (FAPbI 3)0.85( KI 19,10 % 19,26 % [127] Potassium Iodide MAPbBr3)0.15 BAx(FA0.83Cs (iso-butylammonium iodide) and [128] 0.17)1−xPb(I0.6B iBAI and FAI 21,7 % 13 % 912h/25ºC/20-75%/Dark/NE formammi-dinium iodide r0.4)3 [129] 0.85MA0.1C FA s0.05PbI2.7Br0.3 Potassium Iodide KI 17,99 % 18,20 % (Cs0.05FA0.54 1000h/25ºC/45%/Light (w/o [130] MA0.41)Pb(I0.9 Sodium Fluoride NaF 19,68 % 21,92 % 20 % 5% UV)/NE 8Br0.02)3 (FA,MA,Cs)P 500h/25ºC/45%/Light (w/o [131] b(I,Br)3(Cl) Europium(III) oxide Eu2O3 20,9 % 9% UV)/NE (CsFAMA)Pb [132] Rubidium iodide RbI 21,8 % 5% 500h/85ºC/Light (w/o UV)/NE (IBr)3 [133] MAPbI3 Magnesium iodide MgI2 17,8 % 10 % 600h/25ºC/40%/Dark/NE [134] MAPbI3 Nickel(II) chloride NiCl2 17,25 % 20,6 % Butylphosphonic acid 4ammonium [135] MAPbI3 chloride C4H13ClNO3P 8,8 % 16,55 % 60 % 20 % 360h/25°C/55%/Dark/NE [136] MAPbI3 Monoammonium zinc porphyrin ZnP 17,8 % 19,4 % 10 % 1000h/85°C/45%/NE MA [137] n−1PbnI3n+ Zinc phthalocyanine C32H16N8Zn 19 % 20,3 % 90 % 10 % 1000h/85°C/45%/Dark/NE 1 [138] MAPbI3−xClx HPbI3 HPbI3 15,36 % 18,59 % 39,6 % 28,4 % 1008h/25°C/30%/Dark/NE organic ammoniums (FAPbI (nbutylammonium (BA), C4H12IN, C8H20IN [139] 3)x(M 13,13 % 16,58 % 50 % 10 % 4000h/25ºC/30%/Dark/NE octylammonium (OCA), and and C18H38IN APbBr3)1-x oleylammonium (OLA)) ACIDS γ-Aminobutyric acid and Hydriodic NH2(CH2)3COOH [140] MAPbI3 acid and HI 16,34 % 17,35 % 32 % 9% 312h/25ºC/75%/Dark/NE [141] MAPbI3 Terephthalic acid C8H6O4 15,53 % 18,51 % 80 % 49 % 35h/25ºC/30%/Dark/NE (FAI)0.81(PbI2 Lewis base BrPh-ThR and Lewis 3600h/25ºC/20%/Light (w/o [142] )0.85(MABr)0.1 acid bis-PCBM (N-(45(PbBr2)0.15 19,30 % 21,70 % 68 % 8% bromophenyl)thiourea) UV)/NE FA 0.85Cs0.15P H3PO4 15,20 % 17,70 % [143] Phosphoric acid bI3 FA [144] 0.95Cs0.05P Ethylene diamine tetraacetic acid C10H16N2O8 18,93 % 21,60 % 26 % 8% 2880h/25ºC/35%/Dark/NE bI3 [145] MAPbI3 Trimesic acid C9H6O6 13,57 % 15,81 % [146] MAPbI3 Terephthalic acid C8H6O4 15,53 % 18,51 % 82 % 49 % 840h/25°C/30%/Dark/NE HOCO-C6H4- [147] MAPbI3 Aminobutyric acid NH3+I− 13,63 % 16,80 % ORGANIC COMPOUNDS FA 1−xMAxPbI CH3NH3Cl 13,24 % 20,65 % 20 % 10% 500h/85ºC/Light (w/o UV)/NE [148] Methylammonium chloride 3 [149] MA/FA/Cs Methylammonium chloride CH3NH3Cl 21,65 % 17 % 4% 1440h/25ºC/30%/Dark/NE (FA0.83MA0.17 1-butyl-3-methylimidazolium 100h/60-65ºC/40-50%/Light (w C8H15BF4N2 18,5 % 19,8 % 40 % 14% [150] )0.95Cs0.05Pb(I UV)/NE tetrafluoroborate 0.9Br0.1)3 FA 1−xMAxPbI C8H12IN 20,95 % 23,10 % 10 % 5% 500h/85ºC/Dark/NE [26] Phenethylammonium iodide 3 (FAPbI 3)0.85( C8H12IN 18,85 % 20,08 % 50 % 10 % 800h/50ºC/60%/Light (w/o UV)/E [151] Phenethylammonium iodide MAPbBr3)0.15 1440h/25ºC/55%/Light (w/o [152] MAPbI3 Phenethylammonium iodide C8H9NH3 15, 3 % UV)/NE [153] MAPbI3 Propylene carbonate C4H6O3 17,88 % 20,06 % 35 % 5% 1008h/25ºC/Dark/NE [154] MAPbI3 Urea CH4N2O 17,34 % 18,55 % 43 % 35 % 648h/25ºC/Dark/NE 2-(6-bromo-1,3-dioxo- FA 1Hbenzo[de]isoquinolin- [155] 0.83MA0.17 144h/25ºC/60%/Light (w/o 2(3H)yl)ethan-1-ammonium 2-NAM 18,6 % 20,0 % 51 % 35 % UV)/NE PbI2.51Br0.49 iodide (2- NAM) CH3NH2 · Methylammonium acetate and 1000h/25ºC/25%/Light (w/o [156] MAPbI3 CH3COOH and 19,19 % 10 % thio-semicarbazide UV)/NE CH5N3S (FAPbI HN=CHNH2 · HCl Formamidinium chloride and [157] 3)0.87( · and C10H17N · 19,43 % 21,2 % 33 % 12 % 700h/25ºC/40%/Dark/NE 1adamantylamine hydrochloride MAPbBr3)0.13 HCl Methylammonium chloride and CH3NH3Cl and [158] FAI·PbI3 N-methyl-2-pyrrolidone C5H9NO 15,68 % 19,38 % 10 % 5520h/25ºC/20%/Dark/NE [159] MAPbI3 Trimethylammonium chloride (CH3)3N · HCl 18,67 % 20,36 % 20 % 10 % 600h/25ºC/70%/Dark/E Cs0.05(FA0.83 1174h/45-50ºC/45-55%/Light [160] MA0.17)0.95]0.9 t-Butylammonium iodide C4H12IN 18,67 % 20,62 % 37 % 20 % (w/o UV)/NE Pb(I0.83Br0.17) [161] MAPbI 3 Pyrrole C4H5N 18,58 % 20,07 % 26 % 10 % 768h/25ºC/30%/Dark/NE FA [162] 0.88Cs0.12P 2,2′-Bipyridine C10H8N2 17,58 % 19,02 % 43 % 23 % 192h/85ºC/Dark/NE bI3 Methylammonium chloride and CH3NH3Cl and 120h/25ºC/15%/Light (w/o [163] CH3NH3PbI3 methylammonium acetate CH3NH2 · 15,6 % 19,64 % 20 % 10 % UV)/NE Cs0.05FA0.81M CHHN=CHNHCOOH [164] A0.14PbI2.55Br Formamidine acetate 2· 20,77 % 21,81 % 50 % 5% 300h/25ºC/60%/Dark/NE CH3COOH C6H7N3S 15,5 % 18,2 % 12 % 8% 720h/25ºC/55%/Dark/NE [165] CH3NH3PbI3 2-Pyridylthiourea n-octyltrimethylammonium [166] MAPbI3 bromide C11H26BrN 15,80 % 18,32 % 26 % 8% 720h/70ºC/40%/Dark/NE HN=CHNH2 · [167] MAPbI3 Formamidine acetate salt 12,13 % 18,90 % 10% 1920h/25ºC/40%/Dark/NE CH3COOH [168] MAPbI3 Methylammonium chloride CH3NH3Cl 17,15 % 19,71 % Hydroxyl and carbonyl functional [169] MAPbI3 -OH and C=O 15,67 % 18,24 % 84 % 27 % 48h/25ºC/80%/Dark/NE groups (FAPbI 3)0.875( C3H7N6+ • I- • H2O 15,86 % 17,32 % 31 % 15 % 96h/25ºC/65%/Dark/NE [170] Melaminium iodide CsPbBr3)0.125 Cs0.05(MA0.17 240h/25ºC/40%/Light (w/o [171] FA0.83)0.95Pb(I Potassium thiocyanate KSCN 17,83 % 19,23 % 40 % 10 % UV)/NE 0.83Br0.17)3 [172] MAPbI3 3,4-dihydroxybenzhydrazide C7H8N2O3 14,47 % 17,58 % 30 % 15 % 840h/25ºC/Dark/NE [173] MAPbI3 Phenothiazine C12H9NS 14,38 % 15,50 % 168h/25ºC/50%/Dark/NE [174] MAPbI3 Potassium thiocyanate KSCN 19,30 % 19,62 % [175] MAPbI3 Methylammonium thiocyanate C2H6N2S 2,04 % 18,22 % 60 % 10 % 1000h/25ºC/25%/Dark/NE 1200h/40ºC/25%/Light (w/o [176] FAPbI3 Methylammonium chloride CH3NH3Cl 23,48 % 10 % UV)/NE 65h/25ºC/60%/Light (w/o [177] MAPbI3 Rhodanine C3H3NOS2 12,1 % 20,3 % 70 % 15 % UV)/NE [178] MAPbI3 Methylamine CH3NH2 16,92 % 18,82 % 3120h/25ºC/35%/Dark/NE [179] MAPbI3 Copper(I) bromide CuBr 13,7 % 15,2 % [180] CsFAMA n-Octyl-ammonium sulfate C16H40N2O4S 19,16 % 21,1 % 15,8 % 3,2 % 1200h/65ºC/45%/Light (w UV)/E [181] 0.17FA0.83P Cs 1-butyl-1-methylpiperidinium 1200h/85ºC/40%/Light (w/o b(I0.90Br0.10)3 tetrafluoroborate ([BMP]+[BF4]-) 17,6 % 20,1 % 50 % 5% UV)/NE [182] MAPbI3- xClx Methylammonium chloride CH3NH3Cl 8,80 % 15,41 % [183] FAPbI3 Phenylethylammonium iodide C8H12IN 17,17 % 50 % 10 % 384h/25ºC/40%/Dark/NE [184] CsPbI3 Methylammonium iodide CH3NH2 • HI 6,80 % 12,10 % 40 % 7% 720h/25ºC/60%/Dark/NE FA0.98Cs0.02P C5H9NO 20,19 % [185] N-methyl-2-pyrrolidone bI3 CH4N2S and [186] MAPbI3 Thiourea and ethyl acetate (EA) 13,22 % 17,57 % 80 % 15 % 1200h/25°C/50%/Dark/NE C4H8O2 Sodium dodecyl benzene CH3(CH2)11C6H4S [187] MAPbI3 16,26 % 20,15 % 20 % 10 % 1008h/25°C/Dark/NE sulfonate O3Na Rb0.05{Cs0.05[ (FAPbI [188] 3)0.85( Thiourea CH4N2S 17,68 % 20,92 % 40 % 5% 100h/65°C/30%/Dark/NE MAPbBr3)0.15 ]0.95}0.95 1440h/25°C/20%/Light (w/o [189] MAPbI3 Thiourea CH4N2S 18,08 % 19,57 % 63 % 2% UV)/NE [190] MAPbI3 Methylammonium chloride CH3NH3Cl 15,5 % 18,2 % 30 % 40h/85°C/65%/NE (ThMA) [191] 2(MA Methylammonium chloride CH3NH3Cl 1,74 % 15,42 % 50 % 10 % 1000h/25°C/30%/Dark/NE )n−1PbnI3n+1 [192] MAPbI3 Ammonium chloride NH4Cl 15,6 % 250h/25°C/30%/Dark/E (PEA) [193] 2(MA)4 NH4SCN + NH4Cl NH4SCN + NH4Cl 1,1 % 14,1 % 50 % 10 % 1080h/25°C/30%/Dark/E Pb5I16 FA0.85MA0.15 (CH3)3N(Cl)CH2C [194] Pb(I0.85Br0.15) Choline chloride 20 % 21 % 720h/25°C/45%/Dark/E 3 H2OH [195] FAPbI3 BA2PbBr4 BA2PbBr4 19,02 % 20,54 % 80 % 20 % 1440h/25°C/40%/Dark/NE (FA0,65MA0.20 [196] Cs0.15)Pb(I0.8 PE2APbI4 PE2APbI4 17,1 % 1200h/25°C/70%/Dark/NE Br0.2)3 1,1,1-trifluoro-ethyl ammonium [197] MAPbI3 iodide CF3CH2I 15,6 % 18 % 79 % 8% 2880h/25°C/45%/Dark/NE [198] MAPbI3 Imidazolium iodide C3H5IN2 18,78 % 19,25 % [199] MAPbI3 Imidazolium iodide C3H5IN2 18,22 % 20,13 % 1000h/25°C/45%/Light (w/o [200] MAPbI3 Aliphatic fluorinated amphiphilic CF3CH2NH3I 15.6 % 18 % Methyl-3-(1H,1H,2H,2H- (FAPbI 3)0.85( 12,48 % 15,38 % 1248h/25°C/60%/Dark/NE [201] nonafluorohexyl)-imidazolium MAPbBr3)0.15 iodide Cs0.05(MA0.15 Methyl-3-(1H,1H,2H,2H- [202] FA0.85)0.95Pb(I nonafluorohexyl)-imidazolium 16,07 % 16,32 % 3600h/25°C/60%/Dark/NE 0.85Br0.15)3 iodide 1-methyl-3-(1H,1H,2H,2Hnonafluorohexyl)- imidazolium Iodide with -CH2-CH=CH2 460h/25°C/45%/Light (w/o [203] MAPbI3 17,92 % 19,14 % 65 % 45 % UV)/NE 1-ethylamine hydrobromide-3methylimidazolium [204] MAPbI3 10,08 % 13,01 % 48 % 6% 840h/25°C/20%/Dark/NE hexafluorophosphate Salt 1-(4-ethenylbenzyl)- 3(3,3,4,4,5,5,6,6,7,7,8,8,8tridecafluorooctylimida- 700h/60°C/45%/Light (w/o [205] MAPbI3 zolium iodide 19,21 % 19,51 % 51 % 17 % UV)/E Lead salt of pyridine-2carboxylate [206] MAPbI3 PbPyA2 18,86 % 19,96 % 76,4 % 20 % 480h/90°C/60%/Dark/NE [207] MAPbI3 Methylammonium formate C2H7NO2 19,1 % 19,5 % OTHERS [208] Iodide ions 20,3 % 21,6 % CH3NH3PbI3 3-(5-Mercapto-1H-tetrazol-1- (FAI) 1000h/55-60ºC/20%/Light 0.9Cs0.1( C7H8IN5S 18,7 % 20,5 % 12 % 2% (w/o [209] yl)benzenaminium PbI2)1.05 UV)/NE Iodide (SN) [210] MAPbI3 Eu-porphyrin complex (Eu-pyP) Eu-pyP 18,96 % 18,2 % 50 % 20 % 1000h/25ºC/45%/Dark/E 1-alkyl-4-amino-1,2,4-triazolium” (termed as RATZ; R represents [211] MAPbI3 16,13 % 20,03 % 30 % 20 % 3200h/25ºC/40%/Dark/NE alkyl chain, and ATZ represents 4amino-1,2,4- triazolium [CH(NH2)2]X[ 1000h/60ºC/45%/Light (w/o [212] CH3NH3]1- Chlorinated graphene oxide Cl-GO 20 % 21,08 % 50 % 10 % UV)/E XPb1+YI3 (FAPbI [213] 3)0.85( Nitrogen-doped reduced graphene N-rGO 20,37 % 10 % 1000h/25ºC/70%/Dark/NE MAPbBr3)0.15 oxide [214] MAPbI3 Alkali metal cation (Na+) Na+ 15,56 % 18,61 % Cs [215] 0.15FA0.85)P Zn:CuGaO2 Zn:CuGaO2 20 % 20,67 % 10% 5% 60h/25ºC/40-70%/Dark/NE b(I0.9Br0.1)3 Diiodoperfluoroalkyl additives of 1,4-diiodooctafluorobutane [216] MAPbI3 (C4F8I2) along with C4F8I2/ C4H8I2 17,46 % 20,12 % nonfluuorinated 1,4 diodobutane C4H8I2 [217] MAPbI3 Graphitic carbon nitride g-C3N4 16,8 % 20,2 % 54 % 10 % 500h/25ºC/40%/Light (w/o UV)/E (1,3,4-thiadiazolidine-2-thione, [218] MAPbI3 TDZT and 1,3,4-thiadiazolidine2,5 15,61 % 19,04 % 60 % 9% 1200h/25ºC/65%/Dark/NE dithione TDZDT) Lithium bis(trifluoromethanesulfonyl)imid [219] MAPbI3 11,02 % 18,17 % 50 % 10 % 960h/25ºC/30%/Dark/NE e (LiTFSI) and 4-tertbutylpyridine (tBP) 6,6,12,12-tetrakis(4-hexylphenyl) indacenobis(dithieno[3,2-b;2′,3′d]thiophene) [220] MAPbI3 end-capped by 3- (1,1- 19,60 % 21,70 % 80 % 30 % 1440h/25ºC/40%/Dark/NE dicyanomethylene)-5/6fluoro-1-indanone terminal groups [221] MAPbI3 Quantum Dots CsPbBrCl2 QDs 20,60 % 21,50 % 60 % 20 % 500h/25ºC/50%/Light (w/o UV)/E 370h/25ºC/40%/Light (w/o [222] CsPbI2Br Europium-Doped CsPbI0.96Eu0.06Br 10,21 % 13,71 % 7% UV)/NE [223] MAPbI3 1,3,7-trimethylxanthine C8H12N4O2 17,59 % 20,25 % 40 % 14 % 1300h/85ºC/Light (w/o UV)/NE [224] 3NH3PbI3CH xClx 1,8-diiodooctane ICH2(CH2)6CH2I 7,90 % 10,30 % (phenyl-C61-butyric acid methyl ester) (PCBM), (3,9-bis(2methylene- (3-(1,1dicyanomethylene)- indanone))5,5,11,11-tetrakis(4- hexylphenyl)dithieno[2,3-d:2′,3′-d′]- [225] MAPbI3 sindaceno[1,2-b:5,6- 17,52 % 19,28 % 50 % 13 % 960h/25ºC/50%/Dark/NE b′]dithiophene) (ITIC), 7,7′-[4,4Bis(2-ethylhexyl)- 4H-silolo[3,2b:4,5-b′]dithiophene- 2,6diyl]bis[6- fluoro-4-(5′-hexyl-[2,2′bithiophen]-5-yl)benzo[c] [1,2,5]thiadiazole]) (pDTS(FBTTh2)2 Cs0.05FA0.81M [226] A0.14PbI2.55Br D-4-tert-butylphenylalanine C13H19NO2 19,10 % 21,40 % 0.45 [227] MAPbI3 1H,1H-perfluorooctylamine C8H4F15N 16,48 % 21,31 % 50 % 5% 2500h/25ºC/75%/Dark/NE 700h/25ºC/45%/Light (w/o [228] MAPbI3 Methimazole C4H6N2S 17,48 % 19,57 % 50 % 20 % UV)/NE Nonfullerene small molecule [229] MAPbI3 (IT4F) 15,59 % 18,35 % 59,8 % 20,2 % 792h/15ºC/10%/Dark/NE Cs0.05FA0.81M [230] A0.14PbBr0.45I 3-alkylthiophene C10H16S 18,08 % 19,89 % 50 % 20 % 700h/25°C/40%/Dark/NE 2.55 [231] CsPbI2Br Carbide–metal oxide (C–MOX) (C–MOX) 14 % 10 % 1000h/45ºC/45%/Dark/E [232] MAPbI3 2-Pyridylthiourea C6H7N3S 15,5 % 18,2 % 60 % 8% 720h/25°C/55%/Dark/NE [233] MAPbI3 1,3,4-thiadiazolidine-2,5-dithione C2H2N2S3 15,61 % 19,04 % 25,4 % 9,8 % 1200h/25°C/65%/Dark/NE [234] MAPbI3 N,N-dibutylaminophenyl 18,19 % 20,09 % 14 % 720h/25°C/30%/Dark/NE [235] MAPbI3 DRSBDT-2OT and mixed HTM 17,13 % 21,31 % 43,7 % 15,9 % 768h/25°C/35%/Dark/NE 6,6,12,12-tetrakis(4 FAI/MAI/M hexylphenyl)indacenobis(dithieno [3,2- [236] b;2′,3′-d]thiophene) end- capped by 3- 17,9 % 19,8 % 1440h/25°C/45%/Dark/NE ACl/MABr (1,1-dicyanomethylene)-5/6-fluoro-1- indanone [237] MAPbI3 Guanidine thiocyanate C2H6N4S 15,24 % 16,92 % Pentafluorophenylethylammoniu 1000h/25°C/40%/Light (w/o [238] A2PbI4 FEA m 20 % 22,1 % 57 % 10 % UV)/NE [239] 0.15FA0.85P Cs b(I0.9Br0.1)3 Lead(II) chloride PbCl2 20,03 % 21,30 % 19 % 5% 8760h/25°C/40%/Dark/NE 2. Working mechanism of additives in PSCs Different inorganic additives can be applied in PSC to passivate defects and enhance device performance. Some examples include the use of potassium Iodide (KI), redox shuttles like the pair Eu+3/Eu+2, the use of graphene and doped graphene or among others. In general, additives form strong ion bonds with uncoordinated Pb2+ and hydrogen bonds are created between the additive and MA, FA and halide ions to enhance the Pb-I bonding and MA-I bonding. In this way, MA, iodine vacancies and other deep defects can be reduced and passivated. At the same time non-radiative recombination and ion migration can be suppressed. The final result is the enhancement of the PCE and the increase of the stability of the final PSCs. 2.1. Inorganic additives PSCs with inorganic additives have attracted great interest recently due of their prominent photoelectric properties and thermal stability for stable, highly efficient PSCs. Stability improvements have been reported through the incorporation of partial Cs ion in the FA-site leading to the contraction of the cubo-octahedral volume of the perovskite. The partial replacement of the Cs cation by the FA cation results in suppressed charge recombination and was attributed to the reduced trap density being close to the maximum of the valence band, resulting in an increase of the shunt resistance and a decrease in the saturation current [240]. Furthermore, it has been found that the segregation of cations and halides during the growth of mixed halide lead perovskite films is influenced by the formation of two competing phases: an MA-I-rich solvated phase and FA-Br-rich hexagonal phase. The roles of addition of Cs+ and Rb+, individually and collectively, it is of vital importance on the crystallization of perovskite thin films. The use of these components enables the uniform distribution of halides, preventing the formation of isolated halide- and cation-rich phases. However, the optimal addition of Cs+ and Rb+ into the perovskite precursor results in well-defined mixed-halide mixed cation perovskite crystals that correlate with excellent device performance. This improved performance is attributed to the synergistic effect of Cs + and Rb+ in suppressing inherent halide segregation by promoting the unique formation of the desired 3C phase (also referred to as α) during antisolvent dripping [241]. Son et al. (Fig. 6a) demonstrated that adding a small amount of potassium iodide (KI) in HPs is a universal approach toward hysteresis-free PSCs.[129] In 2018, Mojtaba Abdi-Jalebi et al. introduced the same KI additive to triple-cation PSCs. The excess halides filled these vacancies, thereby passivating the non-radiative recombination pathways and leading to high luminescence efficiencies. They discovered that the device power conversion efficiency (PCE) increases from 17.3% (x = 0) to 21.5% (x = 0.1) upon passivation, with the elimination of hysteresis in the latter case consistent with the inhibition of ion migration. This is also consistent with a rapid increase in stabilized efficiency to a value of 21.3%, compared to a slower increase to just 17.2% for the control. (Fig. 6b) [125]. Fig. 6. (a) Eliminating hysteresis by KI additives. KI eliminates the hysteresis by preventing the formation of Frenkel defect. Reproduced with permission. Copyright 2018, American Chemical Society [129]. (b) schematic of a crosssection of a film showing halide-vacancy management in cases of excess halides, in which the surplus halide is immobilized through complexing with potassium into benign compounds at the grain boundaries and surfaces. J–V curves in forward (open symbols) and reverse (closed symbols) mode of the best-performing solar cells with (Cs,MA,FA)Pb(I0.85Br0.15)3 and (Cs,MA,FA) Pb(I0.4Br0.6)3 absorbers without (x = 0) and with (x = 0.1) passivation. Reproduced with permission. Copyright 2018, Springer Nature [125]. The fabrication of PSC through solution processing methods results in the generation of Pb0 and iodine I0 defects. This indicates, for example, that Pb is unsaturated and thus, iodine deficiencies may also be present in the perovskite lattice of the reference sample. These defects serve as not only recombination centers to deteriorate device efficiency but also degradation initiators to hamper device lifetimes. Moreover, it has been shown that the formed I2 could further accelerate the decomposition of the HPs. The application of additives could effectively suppress the formation of metallic Pb via either direct electron donation to unsaturated Pb or immobilization of iodine. To avoid the formation and accumulation of these defects, the incorporation of europium ion pair Eu3+-Eu2+ has been employed to act as "redox shuttle" that selectively oxidized Pb0 and reduces I0 defects simultaneously in a cyclic transition. As a result, the final PSCs showed exceptional stability improvement and high PCE (Fig. 7a) [131]. The application of wide-bandgap lead oxysalt layers reduces defect density on the perovskite surfaces by passivating under coordinated surface lead centers, which are defect-nucleating sites. The group Huang used lead oxysalt to increase the carrier recombination lifetime and to boost the efficiency of the solar cells. Its use also improved the water resistance of perovskite films by forming strong chemical bonds. Therefore, the top layer of lead sulfate on the perovskite surface efficiently suppresses ion migration, possibly due to the tightly bonded lead sulfate layer which immobilizes surface defects. [180] The devices stabilized by the lead oxysalt exhibited an efficiency of 21.1% and maintain 96.8% of their initial efficiency after operation at maximum power point under simulated air mass (AM) 1.5 G irradiation for 1200 hours at 65°C (Fig. 2b). Fig. 7. (a) Proposed mechanism diagram of cyclically elimination of Pb0 and I0 defects and regeneration of Eu3+Eu2+ metal ion pair. Long-term stability and original performance evolution of PSCs. Reproduced with permission. Copyright 2019, Science [131]. (b) schematic illustration of protection of perovskites through in situ formation of a lead sulfate top layer on the perovskite surface. Time-resolved PL of the perovskite films with and without lead sulfate layers. Stability test of encapsulated solar cell devices based on control (blue) and sulfate-treated (red) CsFAMA perovskite active layers. Reproduced with permission. Copyright 2019, Science [180]. Due to the weak bonding nature in HPs, the soft crystal lattice is easily decomposed, usually starting at the surface, and leads to great difficulties in forming stable heterostructures on perovskite surfaces. The formation of strong chemical bonds on the surface of soft perovskite films can greatly prevent perovskite loss, thus reducing damage to organic charge transport semiconductors. Wang et al. used a perovskite film with a surface-rich Pb and a chlorinated graphene oxide (Cl-GO) layer, where strong Pb–Cl and Pb–O bonds are formed to join the two layers. A certified stabilized PCE of 18.7% was achieved based on Cl-GO modified devices for longer than 1000 hours (Fig. 8a) [212]. In addition, the control of the reaction rate and formation of extremely dense, textured, smooth and large grains of the perovskite layer, is crucial to ensure highly efficient PSCs. Mali et al. (Fig. 8b) developed a new simple dual-retarded reaction processing (DRP) method to synthesize a high-quality mixed-cation perovskite thin film via intermediate phase and incorporation of nitrogen-doped reduced graphene oxide (N-rGO). This DRP process allows the fabrication of PSCs with a maximum conversion efficiency higher than 20.3% [213]. Fig. 8. (a) Schematic representation of the pristine perovskite layer conversion through the treatment with Pb(SCN)2, and the Pb surface–rich perovskite layer. IPCE spectrum and integrated Jsc of the PSC applying the perovskite/ClGO interface. Operational stability of the control cell and the cell with GO and Cl-GO. Reproduced with permission. Copyright 2019, Science [212]. (b) cross sectional FESEM image and schematic representation of a complete perovskite solar cell fabricated by DRP method, where a textured and compact perovskite with N-rGO capping layer fully covers the mesoporous mp-TiO2 layer infiltrated with perovskite. J–V curves for the best cell using the DRP method recorded in reverse and forward scanning directions. Steady-state photocurrent stability of SRP-based device under constant illumination and respective efficiency of champion cell recorded at the maximum power point. Reproduced with permission. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [213]. 2.2. Organic additives The application of organic additives has a long tradition in the development of highly efficient and highly stable PSCs. Organic additives can be selected depending on their properties which are directly related to their functional group(s), molecular structure or chain length. In this section, we present the most recent works reported on the development of stable PSC applying organic additives, including a section on additives for 2D perovskites. 2.2.1. Functional groups The introduction of additives (organic molecules or ligands) in the HP absorber, was initially proposed with the aim of increasing device efficiency. Different molecules have been employed to passivate defects reducing the non-radiative recombination loss and increase device efficiency. A recent review by Choy, et al., enlists the use of different molecules in PSCs and categorizes them depending on the type of ligand. Most organic additives introduce functional groups such as carboxylic or ammonium. Being the monoammonium or diammonium groups the most utilized [242-247]. The type of bond between the additive and the PSC material dictates not only the optoelectronic properties of materials but also the intrinsic and extrinsic stability of devices. For example, additives with functional groups S=O or P=O or C=O can control the halide perovskite thin film crystallization and passivate uncoordinated Pb2+ defects by the creation Metal-O bonds. Hydroxyl (-OH) and carboxyl (-COOH) groups can act as Lewis bases passivating uncoordinated Pb. Especially, under illumination, the hydrogen bonding energy between organic cations and Pb2I framework can be weakened and the organic cations become easier to migrate, which, without an organic additive, would accelerate the decomposition of the halide perovskite [248]. Recently, the inhibition of ion migration, due to the type and binding strength of the additive applied, was linked to the long-term stability of PSCs. For example, Bark, et al., introduced anchoring alkyl amine ligands (AAL) with different chain lengths in the perovskite layer. Defect passivation and ion immobilization was effective due to the filling of the amine group into the A-site vacancies of the perovskite. In this case, the amount of efficiency (and Voc) increase, as well as the stability improvement, is related to the length of the organic molecule. The activation energy, Ea, of the pristine halide perovskite was 243 meV, in comparison with the 422 meV observed with the AAL additive, a clear indication of ion immobilization [249]. As a consequence, the PSC showed no loss in efficiency after 1000 h under continuous irradiation of 1 sun. Li et al., introduced rubrene in the perovskite layer. The supramolecular cation- interaction (a kind of noncovalent force) between the aromatic rubrene and the organic cations of the halide perovskite ensured ion immobilization. The energy of the cation–π interaction have been found to be as strong as 1.5 eV (1500 meV), which is enough to immobilize the organic cations in HP [248]. The chemical bonding modulation of the HP was also studied by tuning the bond strength of additives. The introduction of small amounts of alkali halide (NaX) at the FAI-terminated surfaces, was reported to stabilize the local structure via increase bonding with Pb, and via hydrogen bonds with FA ions. This was observed for Fluoride which forms a stronger bond in comparison with other halides such as I, Br or Cl [130]. The PSC retain 90 % of the initial efficiency after 1000 h under continuous light irradiation. The chemical shift trend varies considerably depending on the functional group participating in the hydrogen bonding [244][250]. Quantum dots have also been employed to enhance PSC efficiency and stability. For example, PbS [251], PbSe [252], SnS [253] or CdSe [254]. In quantum dot sensitized solar cells with tuneable band gap, several organic molecules with different end-groups (–CH3, –NH2, –COOH) showed that the nature of the end-group did not significantly affect the uptake of CdS quantum dots on TiO2, nor the final optical properties of the device. The power conversion efficiencies in devices made with phosphonic acids were up to 3 times higher compared to those without any anchoring agent, possibly due to the PA monolayers acting as recombination barriers or passivating defects at the TiO2 surface [255].The latter indicates that binding of functional groups in the HP is a complicated process that involves not only the chemical environment and the materials themselves, but also, the type of the molecule including chain type and length, as well as bonding strength. All of these characteristics of the functional group affect the final properties of the HP thin films and the interfaces with the transport layers of the device. In the case of long-term stability, the activation energy for ion migration is highly dependent on chemical bonding and crystal structure of the materials [256]. Therefore, structural engineering of perovskite crystals can be used to modulate the strength and geometry of the chemical bonds and thus enhance the activation energies to modulate and control ion migration. However, the mechanisms behind binding modes in perovskite-additive interactions and the correlation between defect passivation, non-radiative recombination and device stability remain poorly understood. Li et al. reported a one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient PSCs with enhanced stability. The introduction of molecularly engineered alkylphosphonic acid ω-ammonium cations as crosslinking agents facilitates the growth of perovskite crystals in the mesoporous network and creates smooth coating layers, enhancing the material’s photovoltaic performance from 8.8% to 16.7% as well as its resistance to moisture (Fig. 9a) [135]. Interestingly, multifunctional molecules have also been used as templates to direct nucleation or crystal growth. Dongqin Bi et al. demonstrated a concept of multifunctional molecular modulation of stable PSCs. The designed bifunctional molecular modulator SN links the mercapto-tetrazolium (S) and phenylammonium (N) moieties, which passivate the surface defects. As a result, molecular modulation simultaneously enhances the perovskite grain size and crystallinity and reduces the level of defects acting as centers for no-radiative charge carrier recombination (Fig. 9b) [209]. Fig. 9. (a) From left to right, HAADF and EDS mapping (in STEM mode) of the Ti, Pb and P components of the 4ABPA-anchored perovskite grains with the mp-TiO2 scaffold. Cross-sectional SEM images of pristine and 4- ABPAanchored (CH3NH3PbI3–ABPA) perovskite films deposited on TiO2/FTO substrates. Reproduced with permission. Copyright 2015, NATURE CHEMISTRY [135]. (b) plane-view (top) SEM images of the pristine (control) and modulator-containing (N, S, and SN) perovskite films deposited on the mesoporous-TiO2/compact- TiO2/FTO. Structure of N, S, and SN modulators employed in the study, with the corresponding tautomeric forms of SN. Evolution of the PCE over time measured by maximum power point tracking of unsealed PSCs of aperture area of 0.16 cm2 under light soaking with full solar intensity at temperature between 55 and 60 °C. Cell was kept under an Ar atmosphere. Reproduced with permission. Copyright 2018, NATURE COMMUNICATIONS [209]. Zhang et al. (Fig. 10a) employed two amino acids, 4-aminobenzoic acid (GABA) and 4-aminobutyric acid (PABA), with similar molecular size and closed chain length to study the molecular rigidity of the organic coupling agents affecting the quality of perovskite films. The cells with PABA as coupling agent improved the quality of perovskite films by reducing the defects and enhancing the crystalline, thus leaded to a significant improvement of the photocurrent and efficiency of the solar cells. The PCE of PABA-treated cell was increased from 13.6% up to 16.8%, mainly due to the enhancement of the short-circuit current density (Jsc) from 19.12 to 22.00 mA. [147] It has been shown that polymers with functional groups can interact with the perovskite to passivate defects. Jun Peng et al. reported double-side passivated perovskite cells by inserting ultrathin poly(methyl methacrylate) PMMA films at both the perovskite/ETL and perovskite/HTL interfaces, finding strong evidence that the excellent passivation provided by PMMA films is associated with the Lewis-base nature of the oxygen atoms in the carbonyl (C=O) groups of PMMA, where donor electrons can reduce the charge state of Pb2+ defect ions at the perovskite/ETL and perovskite/HTL interfaces, effectively reducing nonradiative recombination. The passivation properties of PMMA allowed open circuit voltages up to 1.22 V, achieving a high PCE of 20.8% with negligible hysteresis for champion passivated cells (Fig. 10b) [49]. Fig. 10. (a) Schematic representation of HOCO-C6H4-NH3+I− (PABA) and HOCO-C3H6-NH3+I− (GABA) and the proposed mechanism of amino acid MAPbI3 surface (a) on PABA-treated TiO2, and (b) on GABA-treated TiO2. J– V curves of the best PSCs based on the MAPbI3 films on bare-TiO2, GABA-treated TiO2, and PABA-treated TiO2. Reproduced with permission. Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [147]. (b) SEM cross-sectional image of the perovskite cell. J–V curves of the double-side passivated cell with a structure of FTO/cIn-TiOx/m-TiO2/PMMA:PCBM/Perovskite/PMMA/Spiro-OMeTAD/Au, and its corresponding PL image. Reproduced with permission. Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [49]. 2.2.2. Chain length and molecular configuration Organic additives with different alkyl chain lengths and end-groups would play a key role in the development of high-quality halide perovskite films, as they participate in perovskite formation via dissociated carbon–halogen bonds to generate extra halogen ions. An excessively long alkyl chain length additive could result in steric hindrance for chelation with Pb2+ and an enhanced non-polar nature of the additives [257]. As an example, Zheng et al. (Fig. 11a) demonstrated that the use of alkylamine ligands (AALs) added to the precursor solution suppresses nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films, particularly long-alkyl-chain AALs. The resulting AAL surface modified films exhibit a prominent (100) orientation and lower trap-state density, as well as enhanced carrier mobility and diffusion lengths, demonstrating p–i–n structured devices with >23.0% efficiency (22.3% certified) and good operational stability. [75] Another example is given by Wang et al., who analyzed the chemical environment of functional groups that can be activated for defect passivation. He employed theophylline, caffeine and theobromine, demonstrating PSCs with high efficiency when applying the (FAPbI3)x(MAPbBr3)1-x perovskite. In these molecules, the conjugated structure as well as the dipoles induced by the hetero atoms, tends to increase the intermolecular interaction. The xanthine core also helps maintain the coplanarity of the carbonyl group and the N–H. Unlike other small molecules with flexible alkyl chains, this rigidity allows to define the configuration and distance between the carbonyl group and N–H when they are interacting with the defects (Fig. 11b) [121]. Fig. 11. (a) Illustration of the influence of the short-chain AALs a) and long-chain AALs b) on the crystallization of the perovskite films. Illustration of long-chain AALs assembled on perovskite film surface c) and blocking the holes at the perovskite and C60 interfaces d). Reproduced with permission. Copyright 2020, NATURE ENERGY [75]. (b) top view of the various types of surface defects. Theoretical models of perovskite with molecular surface passivation of PbI antisite with theophylline, caffeine, and theobromine. J-V curves of PSCs with or without small-molecules treatment under reverse scan direction. Reproduced with permission. Copyright 2019, Science [121]. 2.2.3. Application of stable 2D perovskites It has been reported recently that the instability of lead halide perovskites, which are promising materials for high efficiency and low-cost solar cells, can be improved by reducing the dimensions of the perovskites from the three-dimensional (3D) bulk to that of two-dimensional (2D) films. To overcome the limited stability of 3D perovskites, the development of two-dimensional (2D) halide perovskites has been proposed due to their own superior stability, but at the expense of lower efficiency, mainly due to their contrasting optoelectronic properties (i.e., energy high exciton binding, less absorption). Reported results demonstrate that AX-terminated 2D APbX3 are thermodynamically more stable than the PbX2-terminated 2D structures. Although the formation energies of 2D CsPbBr3 and PbI2-terminated MAPbI3 are higher than their 3D counterpart, the stability of MAI-terminated 2D MAPbI3 becomes comparable to, or even better, than their 3D bulk. The stability of 2D AX-terminated perovskites stems from the asymmetric surface properties of these materials: fewer surface dangling bonds and higher AX layer ionicity, as well as significant surface relaxation. These factors lead to a low surface energy for the AX finished surface [258]. The 2D perovskite materials are prepared by the insertion of larger organic cations which isolate the anionic metal layers disrupting the 3D perovskite structure, for example, C6H5(CH2)2NH3+(PEA+) and CH3(CH2)3NH3+(BA+) into the 3D MAPbI3 lattice [259]. Cho and co-workers reported a perovskite solar cell employing a thin 2D perovskite layer on top of a 3D perovskite film with improved stability. They applied the phenethylammonium iodide (PEAI) additive to obtain a 2D perovskite in the form of PEA2PbI4. By intercalating a wide bandgap 2D perovskite layer, the charge carrier recombination at the interface was reduced, and the efficiency was enhanced. The devices retained 85% of the initial PCE stressed under one sun illumination for 800 hours at 50 oC (Fig. 12a)[151]. Another study by Wang et al. reported the formation of 2D perovskite platelets, interspersed between highly oriented 3D perovskite grains. Non-radiative recombination was effectively reduced due to interfacial grain boundary passivation when incorporating n-butylammonium cations into a mixed-cation lead mixed-halide FA0.83Cs0.17Pb(IyBr1−y)3 3D perovskite. The interface between these heterostructures appears to be electronically clean, and consequently inhibits unwanted trap-assisted recombination, leading to reduced current–voltage hysteresis, enhanced efficiency and significantly improved operational stability in the ensuing solar cells. The best performing device shows an efficiency of 19.5% which maintains 80% stability of its "post-burn" efficiency after 1,000 h in air, and close to 4,000 h when encapsulated (Fig. 12b) [102]. Fig. 12. (a) Energy band diagram of the L-CFM/P (layered perovskite with Cs0.1FA0.74MA0.13PbI2.48Br0.39 and quasiPEA2PbI4, a 2D perovskite) device and description of how the 2D perovskite capping layer improved the PCE. J–V curves and hysteresis of PSCs at a scan rate of 25 mV/s. Photo-stability test from maximum power point tracking under 800 h continuous illumination of full intensity in an inert gas (blank ones) and ambient (filled ones) environment with encapsulated glass. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry [151]. (b) BA-enhanced 3D perovskite crystalline growth. 2D–3D perovskite heterostructure. Device performances of the BA/FA/Cs PSCs. Reproduced with permission. Copyright 2017, NATURE ENERGY [102]. Liu et al. introduced a two-dimensional (2D) A2PbI4 perovskite layer using pentafluorophenylethylammonium (FEA) as a fluoroarene cation inserted between the 3D light-harvesting perovskite film and the hole-transporting material (HTM) (Fig. 13a). The introduction of (2D) perovskite layer effectively enhances the hole extraction and suppress non-radiative recombination, resulting in a PCE of over 22%. In addition, PSC devices retained 90% of their efficiency during photovoltaic operation for 1000 hours in humid air under simulated sunlight [238]. Cho et al. demonstrated a 2D–3D mixed passivation treatment by using a mixture of bulky organic ammonium iodide (iso-butylammonium iodide, iBAI) and formammidinium iodide (FAI) (Fig. 13b). The applied iBAI/FAI can efficiently passivate the trap states on surface and the interface of perovskite/CTL, achieving a stabilized power conversion efficiency of 21.7% and sustaining over 87% of the original performance after 912 hours storage in ambient environment under 75 ± 20% relative humidity [128]. Fig. 13. (a) Ambient atmosphere aging results of a 3D PSC and a 3D/2D PSC, with the relative humidity shown in the inset. I-V curves of a 3D PSC and a 3D/2D PSC, with inset showing MPP tracking. Plot of contact resistance against voltage. Reproduced with permission. Copyright 2019, American Association for the Advancement of Science [238]. (b) schematic of the mixed passivation (MP) treatment method with FAI and iso-butylammonium iodide (iBAI). Champion device J–V curves under reverse (MP50 – green, reference – black) and forward (MP50 – bright green, reference gray). Inset: stabilized PCEs. Stability test; PCEs monitored in 75% RH condition over a period of 38 d. Reproduced with permission. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [128]. 3. The role of additives in PSCs fabrication The development of PSC has focused on two important issues regarding practical applications. One is photoelectric conversion efficiency and the other is stability. The PCE of PSCs has been increased to over 25% [7] in just a few years through material design and additive engineering based on the desired optoelectronic properties. Therefore, the great challenge lies in the long-term stability of the PSCs. A high-quality perovskite film is crucial for the fabrication of stable of PSCs. The perovskite film is very sensitive to the external environment conditions leading to the degradation of the PSCs, which is reflected in the considerable PCEs decrease. In general, the stability of the perovskite film depends on its chemical components, crystal structure, and the quality of the perovskite film. 3.1. Grain size and crystal orientation The PSCs performance is highly sensitive to fabrication conditions, which have critical impact on the morphology of perovskite films. Poor morphology of the perovskite thin film has been cited as being highly detrimental to the performance of the device because it not only causes problems such as electrical short circuits, but also affects charge dissociation, charge transport and recombination [260-261]. Due to the sensitive dependence of growth kinetics on interfacial energy, solution concentration, precursor composition, solvent choice and deposition temperature, improving perovskite morphology and coverage through controlling crystallization during film deposition and annealing is an attractive route to device optimization. Optimal perovskite film morphology can be achieved by finding effective ways to manipulate its nucleation and growth [28][260-261]. At present, one of the main challenges in perovskite thin film fabrication is the control of grain growth and crystal orientation during bulk perovskite growth applying various additives. Wu et al. proposed an additive engineering strategy that modulates the film morphology and crystalline quality by using an ionic-liquid additive of methylammonium acetate (MAAc) and a molecular additive of thio-semicarbazide (TSC) (Fig. 14a). Their combination allowed fabrication of uniform perovskite films with large crystals, low density of defects, and ultrasmooth surface over inch-sized planar substrates. The introduction of MAAc (10–15% molar ratio) is capable of generating highly uniform perovskite films with full coverage morphology through a simple one-step spin-coating process and a small amount (3–5% molar ratio) of addition of TSC to efficiently increase the perovskite crystal size. The high crystalline quality of perovskite is found to simultaneously improves the PCE and durability of PSCs [156]. Fei et al. reported that introducing thiourea into perovskite precursor with two-step ethyl acetate (EA) interfacial processing can grow compact micro-sized and monolithically grained perovskite films (Fig. 14b). The crystal grain size increases substantially from 500 nm to over 2 μm, confirmed that thiourea in the precursor has exerted a crucial impact on the nucleation and subsequent crystal growth processes. With the EA post processing, grain boundaries blocked by the intermediate phase were opened resulting in a subtle decrease in grain size. The resulting PSCs with sequential processing demonstrated an impressive power conversion efficiency of 18.46% and an average steady state PCE of 18.05%. [186] Yang et al. reported the use ethylene diamine tetra acetic acid (EDTA) provides excellent modification of electron transporting layers (ETLs) in PSCs owing to its strong chelation function. The EDTA- complexed SnO2 (E- SnO2) ETLs were prepared by complexing EDTA with SnO2 nanoparticles for planartype PSCs. The electron mobility of E-SnO2 increases by about three times compared to that of SnO2, leading to negligible current density–voltage (J–V) hysteresis due to improved electron extraction from the perovskite absorber. Furthermore, it was found that the SnO2 surface becomes more hydrophilic with EDTA treatment, which decreases Gibbs free energy for heterogeneous nucleation, resulting in higher quality perovskite films with larger grains and lower trap density (Fig. 14c) [144]. Bae et al. demonstrated that adding dimethyl sulfoxide (DMSO) and methoxyammonium salt (MeO) simultaneously (dual additives) to a precursor solution is a promising strategy to control PSCs morphology. MeO additive helps the precursors form a stable intermediated PbI2DMSO adduct during film formation by retarding the kinetics of conversion of the adduct to the perovskite. MeO is eliminated during annealing while the optical band gap and crystal structure of PSC are not affected by the addition of the additive. When a small amount of MeO was added, all devices parameters were enhanced, especially for MeO10 devices (10% addition) with a champion 19.71 % PCE achieved (VOC = 1.13 V, JSC = 22.89 mA cm−2, FF = 0.76) (Fig. 14d) [168]. Fig. 14. (a) Additive engineering by combining an ionic additive of MAAc and a molecular additive of TSC in stoichiometric PbI2 MAI precursor solution with DMF as solvent. Schematic diagram of the additive assisted one step deposition of perovskite thin films. Top view SEM images of MAPbI3 films fabricated with additives of MAAc. Reproduced with permission. Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [156]. (b) schematically fabrication process of perovskite films with thiourea containing precursor and ethyl acetate post processing. Schematic reaction process from the precursor to monolithic perovskite grains. Copyright 2017, WILEYVCH Verlag GmbH & Co. KGaA, Weinheim [186]. (c) top-view SEM images of perovskite films coated on SnO2, EDTA and E-SnO2 substrates. The grain size distribution of perovskite deposited on various substrates. Reproduced with permission. Copyright 2018, NATURE COMMUNICATIONS [144]. (d) SEM images of perovskite films prepared using MeO0 and MeO10. Schematic representation for the formation of MAPbI3 perovskite films using different amounts of MeO additives during annealing. Reproduced with permission. Copyright 2019, American Chemical Society [168]. In another report, Han et al. applied methylammonium thiocyanate (MASCN) as an additive to the precursor and a rapid vacuum-based drying approach, high quality MAPbI3 films were achieved (Fig. 15a). Result indicated that the Thiocyanate (SCN-) anion, rather than the MA+ cation, plays the critical role in increasing grain size at room temperature. At a sufficiently high centration, SCN- likely reduces the interaction between Pb2+ ions and DMSO through Pb2+ ion coordination (i.e., SCN out-competes DMSO for coordination sites), enabling more facile DMSO removal under mild vacuum conditions [175]. The fabrication of efficient, stable, and reproducible PSCs is possible through the application of triple A-cation perovskites based on methylammonium (MA), formamidinium (FA), and caesium (Cs) (Fig. 15b) [105]. However, fast nucleation and growth rate of the perovskite film due to the presence of Cs atoms result in a thin film with small grain size, which can increase the carrier recombination and decrease its diffusion length. To address this issue, MACl, was added into the perovskite solution before deposition of planar triple A-cation (Cs/MA/FA) PSC using a structure based on FTO/TiO2/SnO2/perovskite/spiroOMeTAD/Au. The effect of the additive was an increase in grain size from 200 to over 1000 nm (Fig. 15c), where the MACl has no effect on the absorption or photoluminescence (PL) spectra of the perovskite films [149]. As we mentioned earlier, searching for high crystalline quality of perovskite thin film is essential for efficient PSCs. Recently, Wu et al. reported the use of an anti-solvent wash treatment using a versatile electron-rich heterocycle (phenothiazine, PTZ) as an additive in sec-butyl alcohol enhances MAPbI3 perovskite crystals growth. The strong interaction between PTZ and Pb2+ ions not only promote the size of the MAPbI3 crystals from 0.58 mm to 0.73 mm (Fig. 15d), but also is beneficial for the formation of a preferential orientation perovskite film, thereby delivering improved electronic properties. As can be seen from X-ray diffraction (XRD) patterns, both films possess the dominant [110] lattice reflection, which is the main preferential orientation for the MAPbI3 films. They defined as the ratio of the intensity of [110] and [222] peaks, namely, = I[110]/I[222], where the calculations suggest that the values of the PSC films upon incorporation of PTZ increased to 2.93 from 2.42 in reference samples. This consequence indicates that the [110] crystals grow larger while other randomly oriented neighboring grains are suppressed. The oriented growth of the grains in the PSC films also promotes to both the decrement of defect density and enhancement of charge transport of the device. Thus, the enhancement of absorption of PSC layer may result from the more preferential orientation crystallinity and larger crystal size by introducing a certain content of PTZ [173]. Fig. 15. (a) Schematic flow diagram of the room-temperature process to deposit MAPbI3 films and top view SEM images of perovskite thin films with MASCN addition. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry [175]. (b) current–voltage scans for the best performing Cs5M device showing PCEs exceeding 21%. The inset shows the power output under maximum power point tracking for 60 s. Aging for 250 h of a high performance Cs5M and Cs0M devices in a nitrogen atmosphere held at room temperature under constant illumination and maximum power point tracking. Cross-sectional scanning electron microscopy (SEM) images of low magnification Cs5M devices. Reproduced with permission. Copyright 2016, The Royal Society of Chemistry [115]. (c) schematics of the triple A-cation perovskite films without and with MACl additive, and after passivation with iodine. Crystal structure of perovskite films with MACl additive before annealing, after annealing, and after passivation of ionic vacancies. Top-view SEM images of triple A-cation perovskite films without and with MACl additive in the starting solution. Reproduced with permission. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [149]. (d) SEM images of MAPbI3 and optimized PTZ-modified MAPbI3 films. XRD of MAPbI3, optimized PTZ-modified MAPbI3 films and simulated powder tetragonal MAPbI3, and absorption spectra of MAPbI3 and optimized PTZ-modified MAPbI3 films. Reproduced with permission. Copyright, 2020, Elsevier B.V [173]. 3.2. Grain boundaries and surface passivation Recently, many studies have focused additive engineering to passivate grain boundaries (GBs) and grain surfaces to further increase the power conversion efficiency and open circuit voltage (VOC) of PSCs, because GBs and surfaces are the main sources of recombination sites. An additive used is the PTAA {poly [bis (4-phenyl) (2,4,6trimethylphenyl) amine]} that improved the interfacial trap-states passivation, the transfer and collection of the carrier significantly due to the formation of the mixed interlayer of perovskite [51]. As mentioned above, MACl was used to obtain highquality perovskite film, but to further improve the performance of the triple A- cation PSC device, the researchers passivated the surface of the perovskite film with iodine dissolved in Isopropyl alcohol to mitigate recombination sites and vacancies of iodide on the perovskite layer surface (Fig. 16c). The final device performance demonstrates that the additive plays an important role in obtaining a highly efficient PSC with a potential loss of only ≈370 mV [149]. Jiang et al. [26] reported the use of an organic halide salt, phenethylammonium iodide (PEAI), for post-treatment of mixed perovskites FA1−xMAxPbI3 (FA: HC(NH2)2; MA: CH3NH3) to suppress the surface defects of perovskite polycrystalline films for efficient solar cells, with a device structure of glass/ITO/SnO2/perovskite/spiro-OMeTAD/Au. They showed that the additive molecules were not incorporated into the perovskite crystal lattice and likely resided at the GBs and surfaces. Therefore, this reduces the height difference between the grain surface and the GBs and provides an efficient and simple method for passivation of the perovskite surface through which the defects are significantly reduced, and the recombination is suppressed. As a result, they showed a certified efficiency of 23.32% by adopting a flat structure processed in a low temperature (Fig. 16a). Fu et al. [206] prepared high-performance PSCs incorporated a pyridine-2carboxylic lead salt (PbPyA2). The pyridine and carboxyl groups on PbPyA2 can not only control crystallization, but also passivate GBs, which result in the high-quality perovskite film with larger grains and fewer defects. During the film preparation, the pyridine and carboxyl groups of PyA− have strong interaction with Pb2+ and MA+ ions, thereby control perovskite crystallization to suppress the morphological defect formation. After the perovskite deposition, the PbPyA2 molecules mainly distribute around GBs and not only interact with the GBs by the organic acid ions, but also the excess Pb2+ ions around GBs. This can cap the volatile composition to decrease the deep defects, thereby benefits for suppressing ions migration, depressing composition volatilization and insolating external violation, leading to lower hysteresis and increased VOC in PSCs (Fig. 16b). Zuo et al. demonstrated the use of polymers with coordination groups to improve the performance of PSCs. They proposed that the binding interaction between perovskite and polymers is a key parameter that affects passivation. The mechanism regarding the performance of the device is attributed to the passivation of the trap states in the GBs throughout the perovskite films with different polymers. The use of these polymer modifiers results in a remarkable change in the electronic properties of the films, as measured by both carrier dynamics and overall device performance. The devices grown with the polymer poly(4-vinylpyridine) (PVP) showed significantly enhanced power conversion efficiency from 16.9 ± 0.7% to 18.8 ± 0.8% (champion efficiency, 20.2%) from a reverse scan and stabilized champion efficiency from 17.5 to 19.1% compared to controls without any passivation (Fig.16c) [37]. Yang et al. showed the influence of molecular structure of molecules on their passivation effect to hybrid perovskites. They systematically designed the structures of molecular functional groups, including carboxyl, amine, isopropyl, phenethyl, and tertbutylphenethyl groups. Results demonstrated that the carboxyl and amine groups would heal charged defects through electrostatic interactions, and defects related to neutral iodine can be reduced by aromatic structures. The defect passivation in the interfaces and surfaces reduces the VOC deficit of the p−i−n structured device to 0.34 V and boosts the efficiency to 21.4% (Fig. 16d) [226]. Fig. 16. (a) Device structure. Passivation mechanism of the PEAI layer for the perovskite film. The thermal stability (85 °C) of the devices with and without PEAI layers. The typical J–V curve of the device with PEAI (20 mM) and without PEAI treatment under one-sun (100 mW cm−2) conditions. Reproduced with permission. Copyright 2019, NATURE PHOTONICS [26]. (b) the schematic illustration of PbPyA2 molecules located at the GBs after film formation and strongly interacted with perovskite component to passivate GBs. The J–V curses of champion devices from the bare and doped films. Reproducibility analysis of the device’s efficiency. Reproduced with permission. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [206]. (c) schematic diagram for incorporating the polymer into the perovskite films. SEM images of homogeneous PbI 2 film without slow growth, mesoporous PbI2 film with slow growth, and perovskite film processed from slowly grown PbI 2 with PVP polymer. Reproduced with permission. Copyright 2017, American Association for the Advancement of Science [37]. (d) schematic illustration of the origin of D4TBP passivation effect on different defect sites. Reproduced with permission. Grain size distribution of the perovskite films. Insets show the adsorption of monodentate and bidentate ligands affected by the steric effects. Schematic of the PL measurement of the grain boundary passivated perovskite films. Copyright 2019, American Chemical Society [226]. Shao et al. (Fig. 17a) showed that the trap states on the surface and grain boundaries of the perovskite materials are the origin of photocurrent hysteresis and reported that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. This conclusion is due to the higher response speed of the device after passivation. The control devices without PCBM show a slow rising of photocurrent to maximum value during a long duration (75 s) on turning on the illumination, corresponding to the trap filling process, while the photocurrent rises almost instantly to the maximum in the optimized devices [83]. However, Zheng et al. demonstrated the use of quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite, compared with the devices with PCBM passivation. These quaternary ammonium halides (QAHs) with a structure of NR+4X−, where R is an alkyl or aryl group and X is halide, can efficiently passivate negative and positive charged defects in HPs with quaternary ammonium and halide ions. The QAH passivation remarkably reduces the trap density and prolongs the carrier lifetime, which universally enhances the VOC of the HP planar heterojunction devices with different bandgaps and consequently increases the PCE by 10%–35% [194]. Fig. 17. (a) Device structure with PCBM layer. Photocurrents for devices without a PCBM layer (orange), with PCBM layers thermally annealed for 15 min (green) and 45 min (blue), respectively. Schematic of the surface recombination reduction by passivating the trap states. Reproduced with permission. Copyright 2014, NATURE COMMUNICATIONS [83]. (b) the device structure of perovskite planar heterojunction solar cells. Schematic illustration of quaternary ammonium halides (QAHs) assembled on the defect sites. The statistics of VOC distribution for devices with PCBM (blue), L-α-phosphatidylcholine (green), choline iodide (violet) and choline chloride (red). The solid lines represent the Gauss distribution fitting for the statistic of VOC. Reproduced with permission. Copyright 2017, NATURE ENERGY [194]. 4. Impact of additive on the stability of PSCs One of the most important criteria for a PSC today is the ability to maintain a stable power output under standard working conditions. Therefore, the real challenge lies on the long-term stability of the PSCs. Non-encapsulated PSCs have demonstrated hundreds of hours of stability when stored in the dark and measured every several hours or days. To be marketable within the industry, PSCs must be able to operate continuously for about 25 years under real outdoor conditions. However, PSCs are prone to decomposition when exposed to moisture, oxygen, heat, light, etc. [262]. Recently published ISOS protocols for stability assessment of PSCs suggest that a PSC can only be called “stable” if it can stand more than 1000 h under continuous light irradiation under 1 Sun (ISOS-1-L) without losing more than 10 % of their initial efficiency [263-265]. Some reported stability studies made to PSCs showing no more than 10 % performance loss after 1000 h stability analysis under continuous 1 sun AM 1.5G illumination are summarized in Fig. 18. The graph indicates that long operational stability can be achieved (i) in any of the three PSCs configurations (Fig. 18a): normal (green area), inverted (yellow area) and C-based (purple area); (ii) applying 3D, 3D/2D [266] and 2D [267] halide perovskites (HPs) structures containing Cs, Rb, methylammonium (MA) and/or formamidinium (FA) cations (Fig. 18b), as in MAPbI3 [268-270], double cation MAFA [271] or CsFA [272][209], triple cation RbCsFA [273] or CsMAFA [274-275] and quadruple cation RbCsMAFA (this work) (iii) by replacing the unstable Spiro-OMeTAD with novel organic hole transporting materials (HTMs) like EH44 [275], BDPSO [270] and PDCBT [271] (Fig. 18c); (iv) if metal oxides are applied as transporting layers in the form of doped [269][271], complex oxides [268], as oxide multilayers [266][276] or as metal oxide layers functionalized with organic molecules [275], polymers [271][273] or C60 derivatives [269][271][277] (Fig. 18d); or (v) by the application of additives for the controlled crystal growth of the perovskite absorbers [266][209][276]. Fig. 18. Operational stability of halide perovskite solar cells under continuous illumination of 1 sun. (a) PSC with normal (green area), inverted (yellow area) and C-based (purple area) configurations. Materials applied in PSCs (b) HP cations (c), HTMs other than metal oxides (d) and ETMs other than metal oxides (e) additives. Results from ICN2 are represented by red stars. [278] Although a great variety of materials and device configurations are displayed in Fig. 18a, the recipe to achieve high efficiencies maintaining long operational stability (≥ 1000 h) with minimal performance loss (≤ 10%), seems to reside in the engineering and passivation of defects that can be found at the transport layers, interfaces or within the HP absorbers [279-282]. The reduction of defect density mitigates recombination and prolongs the lifetime of charges, leading to efficient and stable PSCs. Less defect concentration is found in perovskite absorbers if synthesized as single crystals. Their superior properties in comparison to polycrystalline thin films are due to the presence of 2 – 4 orders of magnitude lower trap densities [283]. Nevertheless, the use of single crystals is incompatible with solution processing and large-scale device fabrication. Thus, the use of additives, usually incorporated in the perovskite solution before thin film deposition, has become one of the most popular methods to enhance PSC performance and lifetime. For example, the addition of the ionic-liquid methylammonium acetate (MAAc)/thio-semicarbazide (TSC) additive in MAPbI3 (MAPI)[284] or the application of 3-(5-mercapto-1H-tetrazol-1-yl)benzenaminium iodide (SN)[209] results in thin films with large grain size and low density of defects and thus, highly efficient and stable PSCs. 4.1. Decoupling the effect of defects governing efficiency and stability Halide perovskite are ionic-electronic conductors and “soft” nanomaterials [285-286]. They are characterized by their weak ionic bonds and van der Waals interactions which are highly sensitive to the stress factors usually found under long-term operational solar cell conditions (light, heat, humidity, electric field, etc.). Their ionic nature translates into high ionic motion, hysteresis and device degradation. Polycrystalline thin films fabricated by solution processing methods, result in high defect concentration which are undesirable in high quality solar cell devices and should be engineered in order to eliminate or reduce their presence in the film. We can find many types of point defects in halide perovskites. Deep level traps are under coordinated halides ions, under coordinated Pb2+ ions, lead clusters, among others, while shallow level traps can be I or MA vacancies in the bulk of the materials. Deep defects have strong impact in nonradiative recombination affecting Voc and power conversion efficiency of the solar cell. Shallow defects are no related to non-radiative recombination but affect the ionic constituents of the halide perovskite characterized by their low activation energies for ionic motion (0.58 eV for I-, 0.84 eV for MA+, and 2.31 eV for Pb2+) [287]. Thus, shallow defects can intensify the degradation of the long-term stability of PSCs under various operational conditions. Although the science behind point defects and defect passivation is ample and complex, we can relate the efficiency of the PSC to deep defects and thus, increasing the efficiency of the PSC should include passivation of defects through chemical bonding (coordination or ionic). To enhance the long-term stability of solar cell under operational conditions, we should avoid ion migration and phase segregation and thus, one should look for the passivation, annihilation or attenuation of shallow point defects. The equilibrium between high power conversion efficiency and long-term operational stability is not an easy task and very few publications can be found in the literature achieving this goal (See Table 1). Especially rare are the reports under high temperature (60-85ºC) conditions [274]. In general, we have observed different scenarios followed by the PV community: Most published work search for additive engineering for deep-defect passivation which results in enhanced power conversion efficiency of the PSC. However, without ion immobilization the devices are not always highly stable [94][159][173]. Very recent results have shown that the engineering of shallow defects through additive manipulation can greatly enhanced the long-term stability of the PSCs [288]. In these cases, the efficiency of the PSC applying additives is good (> 20%) but not superior than the control device (without additive). We believe the latter has limited the publication of such results in the literature. However, a careful optimization of the additive concentration that can passivate shallow defects, could permit to obtain excellent PSC’s efficiencies safeguarding, at the same time, the long-term stability. The application of halide perovskites with passivated/compensated shallow defects to inhibit ion migration in the original halide perovskite material is a method very recently proposed [248][288-290]. Very few literature examples report on simultaneous ion immobilization and defect passivation/compensation showing superior PCE and also excellent long-term stability [289]. Some examples include PSC with additives such as ionic liquids [181][291-293]. Therefore, the understanding of the influence of the additives on the environmental, light and thermal stability of PSCs is of utmost importance in order to develop reproducible techniques for highly efficient and stable PSCs. In this section, we will describe the influence of additives on PSCs stability due to environmental aspects (presence of oxygen and moisture), UV light irradiation or heat. 4.2. Influence of additive on environmental stability of PSCs PSCs offer simple processing and extremely high photoconversion efficiency. Unfortunately, during the process of assembling and testing, such devices are impeded by the poor stability of the components. A major source of instability is the exposure to oxygen and moisture. The process of photo-oxidation, where an excited electron in the conduction band can be oxidized by molecular oxygen, is almost inevitable. Other investigations deal with the formation of reactive super-oxide species, attributed to charge transfer between O2 molecules and photo-excited perovskite. While oxidation and photo-oxidation are commonly seen in a wide range of semiconductors, the current class of hybrid perovskites faces another challenge: they are extremely unstable to water or even moisture in the air. First, due to the high sensitivity of CH3NH3PbI3 to water, halide perovskite tends to hydrolyze in the presence of moisture, which leads to the degradation, as shown in the following: 𝐶𝐻3𝑁𝐻3𝑃𝑏𝐼3 (𝑠) ↔ 𝑃𝑏𝐼2 (𝑠) + 𝐶𝐻3𝑁𝐻3𝐼 (𝑎𝑞) (1) 𝐶𝐻3𝑁𝐻3𝐼 (𝑎𝑞) ↔ 𝐶𝐻3𝑁𝐻2 (𝑎𝑞) + 𝐻𝐼 (𝑎𝑞) (1𝑎) 4𝐻𝐼 (𝑎𝑞) + 𝑂2 (𝑔) ↔ 2𝐼2 (𝑠) + 2𝐻2𝑂 (𝑙) (1𝑏) 2𝐻𝐼 (𝑎𝑞) ↔ 𝐻2 (𝑔) + 𝐼2 (𝑠) (1𝑐) Furthermore, equilibration of reaction (1a) leads to the coexistence of CH3NH3I, CH3NH2 and HI in the films. There are two paths for HI to degrade in the next step. One path is a redox reaction in the presence of oxygen (1b); The other method is a photochemical reaction, in which HI can be broken down into H2 and I2 under UV radiation (1c). HI consumption, according to reactions (1b) and (1c), drives the entire degradation process [294]. Zhao et al. reported perovskite solar cell architecture based on an insulating polymer Polyethylene glycol (PEG)-scaffold structure. PEG plays an important role by forming tangled long-chain molecules, which act as a three-dimensional skeleton to support perovskite crystals, enabling the coverage on the substrate more uniformly. The improved stability and self-healing effect of the PSC devices can be ascribed to the excellent hygroscopicity of the PEG molecules and their strong interaction with the perovskite. The omnipresent PEG molecules can absorb water efficiently to form a compact moisture barrier around perovskite grains with little water penetrating into the film. The instant decomposition-regeneration mechanism explains the fast self-healing process in the PEG scaffold perovskite film (Fig. 19a) [47]. Bi et al. used an aliphatic fluorinated amphiphilic additive (CF3CH2NH3I) in MAPbI3 perovskite, to enhance the stability and performance of the PSCs. More importantly, moisture resistance is enhanced without formation of 2D perovskite (sacrificing performance) due to the hydrophobic -CF3 terminal group over the surface of the 3D perovskite. A significant enhancement from average PCE of 15.6% to 18.0% was achieved by introducing 3 mol% aliphatic fluorinated amphiphilic (FEAI) additive into the perovskite precursor solutions to improve the morphology of perovskite thin films (Fig. 19b) [200]. Fig. 19. (a) Schematic diagram to show mechanisms for the self-healing properties in PPSCs: (1) Water absorb on perovskite; (2) Perovskite hydrolysis into PbI2 and MAI-H2O by water; (3). Restrained MAI by PEG react with nearby PbI2 to form perovskite again after water evaporates. PEG has a strong interaction with MAI, preventing it from evaporating, subsequently MAI and PbI2 react in situ to form perovskite after the film was removed away from the vapour source. Schematic diagram showing the fabrication process of perovskite film with polymer scaffold using one-step spin coating method. J–V curves of PSCs before and after water spray, revealing a complete recover of the cells in one minute when it puts back to ambient air. Reproduced with permission. Copyright 2016, NATURE COMMUNICATIONS [47]. (b) schematic model of the FEAI intercalating on the crystal surface of a 3D perovskite MAPbI3. Current–voltage (J – V) curves for one optimized device with respect to forward and reverse scan direction at a scan rate of 5 mV/s under simulated AM1.5G solar illumination. Reproduced with permission. Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [200]. Quan and co-workers reported reduced-dimensionality (quasi-2D) perovskite films that exhibit improved stability while retaining the high performance of conventional threedimensional perovskites. The intermediates between 3D and 2D are layered perovskites systematically synthesized by introducing a large organic cation, phenylethylammonium (PEA, C8H9NH3). The dimensionality of metal halide perovskite compounds can be continuously adjusted by mixing stoichiometric amounts of lead iodide (PbI2), methylammonium iodide (MAI, CH3NH3I) and phenylethylammonium iodide (PEAI), to produce compounds with different values of (n) in the series PEA2(CH3NH3)n-1 PbnI3n+1. In this notation, the limit n = ∞ corresponds to the 3D cubic perovskite, CH3NH3PbI3, while the other values describe 2D perovskite structures (n = 1) or quasi-2D (n > 1). PEA inserted perovskite (2D) showed large stability in water or humidity because the energy required to remove PEA from perovskite is more than MA due to its high formation energy (Fig. 20a) [152]. Gao et al. showed a novel method for improving the stability by introducing thiourea. Thiourea passivates perovskite by the formation of the Pb─S bond on the outermost layer of perovskite and lath-shaped grains among perovskites, which contributes to the improvement of oxygen, light, thermal stability, and little hysteresis. The unencapsulated devices retain 98% and 93% of original efficiencies after two months under air conditions for active areas of 0.1 and 1.0 cm2, respectively (Fig. 20b) [189]. Fig. 20. (a) Unit cell structure of (C8H9NH3)2(CH3NH3)n 1PbnI3n+1 perovskites with different n values, showing the evolution of dimensionality from 2D (n = 1) to 3D (n = ∞). DFT simulation of the formation energy of perovskite with different n values in different atmospheres. Device performance as a function of n value, which shows that increased performance was achieved with increased n value; however, in the meantime, stability was decreased. Reproduced with permission. Copyright 2016, American Chemical Society [152]. (b) TG spectra, DSC spectra, schematic diagram of oxygen content measurement, photographs of the perovskite films with and without a thiourea additive. SEM morphologies of the perovskite film without thiourea and the perovskite film with thiourea. Reproduced with permission. Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [189]. 4.3. Influence of additive on ultraviolet (UV)-light stability of PSCs In PSCs, the most widely used photoanodes are composed of compact or mesoporous TiO2. However, instability in TiO2-based PSCs is attributed to light-induced desorption of oxygen molecules adsorbed on the surface. A possible proposed mechanism to explain the degradation process in the film under exposure to light is as follows: (1) TiO2 contain many oxygen vacancies (Ti3+) “particularly at the surface” and it acts as deep electron-donating sites which have placed at approximately 1 eV below the CB edge. These deep electron-donating sites adsorb oxygen molecules by donating electrons to oxygen molecules and formed a resultant complex (O2- -Ti4+). (2) Upon UV light exposure, photo-carriers are generated (photoelectrons in CB and holes in VB). These photo-generated holes further react with the oxygen radicles which have placed below the CB sites (oxygen vacancy sites). Molecular oxygen desorbed by leaving unoccupied states and free electrons per sites. These free electrons further recombined by hole conducting material and hence leads to instability in TiO2 based PSC devices under the explosion of ultraviolet (UV)-light [295]. Cesium bromide (CsBr), as an interface modifier between the electron collection layer and the CH3NH3PbI3-xClx absorber layer, has been shown to effectively improve the stability of planar heterojunction devices under UV light soaking. The CsBr modification (coating) reduces the areal density of pinholes the FTO and results in a negative shift in the work of function of c-TiO2 from 4.07 to 3.90 eV. Most significantly, the CsBr modification dramatically inhibits UV-induced degradation of perovskite films and devices. The presence of CsBr retards the PL decay process with the fast decay component lifetime fast, increasing from 5.0 to 20.8 ns and the slow component, slow, from 118.0 to 177.6 ns (Fig. 21a) [106]. Recently, Chen et al. reported that a synthesized luminescent EuTiO2 thin films via chemical-bath deposition at low-temperature (70 °C), not only function as efficient charge transport layers but also effectively convert damaging UV light to extra visible luminescence. The controllable doping of Eu3+ also leads to a more optimized energy band alignment and therefore facilitates the electron extraction of EuTiO2- based PSCs. This results in an increased photocurrent and open-circuit voltage, yielding an enhanced PCE of 21.40% with relative to pristine TiO2 device (19.22%). PSCs based on Eu-TiO2 exhibited remarkably improved long-term UV stability, retaining 75% of the peak PCE after 500 h under continuous UV light illumination (5 mW cm-2) (Fig. 21b) [93]. Fig. 21. (a) UPS spectra of c-TiO2 with (w) and without (w/o) CsBr modification. Energy level diagram of the device with the structure of FTO/c-TiO2(CsBr)/ perovskite/spiro-OMeTAD/Au. Time-resolved photoluminescence spectra of perovskite thin films on glass collected at 400 kHz. Reproduced with permission. Copyright 2016, The Royal Society of Chemistry [106]. (b) schematic illustrations of the device configuration of n-i-p planar PSCs employing Eu- TiO2. Energy band alignment diagram of different layers of PSCs. The J-V curves of the champion device based on 5% Eu-TiO2 film under forward and reverse scans, IPCE measurement and the integrated Jsc of PSCs based on TiO2 and 5% Eu-TiO2 films. Reproduced with permission. Copyright, 2019, Elsevier B.V [93]. 4.4. Influence of additive on thermal stability of PSCs As already mentioned, PSCs have attracted enormous attention in the last few years due to their rapid improvement and high certified efficiencies over 20%. However, PSC thermal stability is still an obstacle to their commercialization. At low temperatures, the devices present very stable values (average loss <5%), but as the temperature increases significant decreases in the open circuit potential and short-circuit current are observed. Temperature is particularly critical as the normal operating environment for photovoltaic devices is under direct sunlight, which means the cell temperature can be up to 45 °C or even to 85 °C in desert. To date, temperature dependent studies on PSCs have been very limited compared with the numerous studies on improving their power output at room temperature. In attempts to overcome thermal instability, different additives have been applied to perovskites. Li et al. introduced an additive of ethyl 2cyanoacrylate (E2CA) to MAPbI3 PSCs. E2CA was found and chemically anchored to GB in perovskite films due to its coordination effect with PbI2, effectively passivating the defects at GBs. As a result, E2CA PSCs exhibited improved stability, retaining ~90% of maximum efficiency after aging over 1000 h. Furthermore, even at high temperature (85 °C) in moisture air, unencapsulated MAPbI3-E2CA devices still showed good thermal stability, retaining over 90% of the post burn-in efficiency after aging 200 h (Fig. 22a) [38]. On the other hand, Li et al. reported a chemical bond modulation of state-of-the-art triple cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 through the addition of a small amount of alkaline halide, sodium fluoride (NaF). NaF is present on the perovskite film and forms hydrogen bonds with organic cations within the perovskite crystals, retarding the diffusion of these cations and their dissociation. Fluoride ions also form the ionic bonds with Pb2+, immobilizing both organic cations and halide anions. In this way, the fluoride ions effectively block the materials degradation pathway at the corresponding interfaces by passivating the surfaces and grain boundaries through chemical bonding. The resulted CsFAMA-F-based devices retain 90% of their initial PCE after annealing at 85°C for 1,000h, while CsFAMA PSCs retain only 50% of their initial PCE (Fig. 22b) [130]. Fig. 22. (a) Schematic representation of spontaneous grain polymerization in MAPbI3-E2CA films. E2CA chemically anchors to GBs with C=O and C≡N groups and spontaneously polymerizes to a polymer at GBs when exposed in moisture air. Proposed working mechanism of E2CA in perovskite films. Moisture stability of PSCs stored in air (relative humidity: 40–60%). Thermal stability of PSCs stored at 85 °C in air (relative humidity: 40– 60%). Reproduced with permission. Copyright, 2019, Elsevier B.V [38]. (b) optimized structures of a Na–I unit and a Na–F unit adjacent to the FAPbI3 surface. Stability performance of PSCs under various conditions. Reproduced with permission. Copyright 2019, NATURE ENERGY [130]. 5. Conclusion and perspectives Due to the soft chemical nature of halide perovskites, their fabrication as polycrystalline thin films, when fabricated by solution processing methods, results in the unavoidable formation of defects (interstitials, vacancies). Defects are responsible for charge recombination and ion migration that reduce PSC efficiency and stability, respectively. The application of additives, organic or inorganic, are employed to eliminate or reduce the presence of defects. Additives, through their functional groups, can form robust chemical bonds with the halide perovskite, act as a redox shuttle or occupy the interstitial sites, suppressing defects and immobilization ions, leading to enhanced efficiency and stability. One of the main roles of additives in PSCs is to promote large grain size formation and crystal orientation and to passivate grain boundaries and interfaces. Although numerous studies on additives in PSCs are reported, the understanding of the type of bonding between perovskite and additives, especially those organic additives with multi-functional groups, is still limited. Currently, the progress of additives is mainly based on the trial-and-error method and effective design rules are still needed. The challenge lies in the difficulties to know the exact defect types present in the halide perovskite film which will require the compilation of databases of additives. Another important issue is the screen criteria of additives for PSCs. Additives should improve the initial efficiency and environmental stability of PSCs, but also improve the thermal stability of PSCs under 65-85 (some additives may evaporate or react with transporting layers under elevated temperature and lose their effect). The ultimate aim of the additive should be to enable operational stability of PSCs under continuous light irradiation of one sun at 65-85 under maximum power point (MPP) tracking without reducing the device efficiency. Should the design rules and the database of additives were constructed and the strict screen criteria of additives was applied, the stability of PSCs will be further improved and the commercialization of PSCs will be one step approaching. Acknowledgments We thank the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295 for the postdoctoral contract to H.X. To the SolarEra.Net Cofund 2 (EU) and the AEI (Spain) for the project PrOperPhotoMiLe (Ref 12 and PCI2020-112185). To the Spanish Ministry of Science and Innovation for the grant GraPErOs (ENE2016-79282-C5-2-R), Self-Power (PID2019-104272RB-C54) and the OrgEnergy Excellence Network (CTQ2016-81911-REDT). To the Agència de Gestiód'Ajuts Universitaris i de Recerca (AGAUR) for the support to the consolidated Catalonia research group 217 SGR 329 and the Xarxa d’R+D+I Energy for Society (XRE4S). Part of this work is under the Materials Science Ph.D. programme for C.P. of the Universitat Autonoma de Barcelona (UAB, Spain). We thank CONACYT for the scholarship to C.P. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme / Generalitat de Catalunya. References [1] C. Ran, J. Xu, W. Gao, C. Huang, and S. Dou, “Defects in metal triiodide perovskite materials towards high-performance solar cells: Origin, impact, characterization, and engineering,” Chemical Society Reviews, vol. 47, no. 12. Royal Society of Chemistry, pp. 4581–4610, 21-Jun-2018. [2] A. Rajagopal, K. Yao, and A. K. Y. 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