SCIENCE ADVANCES | RESEARCH ARTICLE MATERIALS SCIENCE Copyright © 2022 The Authors, some Enabling full-scale grain boundary mitigation rights reserved; exclusive licensee in polycrystalline perovskite solids American Association for the Advancement of Science. No claim to Lichen Zhao1,2†, Pengyi Tang3,4†, Deying Luo1, M. Ibrahim Dar5*, Felix T. Eickemeyer2, original U.S. Government Neha Arora5§, Qin Hu6,7,8*, Jingshan Luo2,9, Yuhang Liu2*, Shaik Mohammed Zakeeruddin2, Works. Distributed Anders Hagfeldt10‡, Jordi Arbiol3,11, Wei Huang12,13, Qihuang Gong1,14, Thomas P. Russell6,7, under a Creative Richard H. Friend5, Michael Grätzel2*, Rui Zhu1,14* Commons Attribution NonCommercial There exists a considerable density of interaggregate grain boundaries (GBs) and intra-aggregate GBs in polycrystal- License 4.0 (CC BY-NC). line perovskites. Mitigation of intra-aggregate GBs is equally notable to that of interaggregate GBs as intra-­ aggregate GBs can also cause detrimental effects on the photovoltaic performances of perovskite solar cells (PSCs). Here, we demonstrate full-scale GB mitigation ranging from nanoscale intra-aggregate to submicron-scale inter- aggregate GBs, by modulating the crystallization kinetics using a judiciously designed brominated arylamine trimer. The optimized GB-mitigated perovskite films exhibit reduced nonradiative recombination, and their cor- responding mesostructured PSCs show substantially enhanced device efficiency and long-term stability under Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 illumination, humidity, or heat stress. The versatility of our strategy is also verified upon applying it to different categories of PSCs. Our discovery not only specifies a rarely addressed perspective concerning fundamental studies of perovskites at nanoscale but also opens a route to obtain high-quality solution-processed polycrystalline per- ovskites for high-performance optoelectronic devices. INTRODUCTION grains of the same material that are generally oriented in different Polycrystalline halide perovskite solids have emerged as excellent crystallographic directions according to the classic material science semiconductor materials for photovoltaics (1, 2), light-emitting textbooks (10, 11). The aggregation of grains is inevitable in this diodes (3, 4), lasers (5), and photodetectors (6) because of their ease polycrystalline film (12). The grain aggregates, in turn, form larger of fabrication and outstanding optoelectronic properties (7–9). The agglomerates (domains) leading to domain boundaries, often and polycrystalline nature of the perovskites arising from rapid nucle- inadequately referred to as “GBs” in the research field of metal halide ation and crystallization from solution at relatively low temperatures perovskites. As shown in Fig. 1A, these resultant boundaries can be usually causes a considerable density of grain boundaries (GBs). In categorized as interaggregate GBs (pointed by green arrows) and intra-­ a typical polycrystalline film, a grain is a single-crystalline entity, aggregate GBs (indicated by orange arrows), respectively. and a GB is the interface between two individual homophase crystal The interaggregate GBs have been frequently and extensively ex- amined in metal halide perovskite research as they can be readily 1 State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of observed by optical microscopy, scanning electron microscopy (SEM), Physics, Frontiers Science Center for Nano-optoelectronics & Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, China. 2Laboratory of or scanning probe microscopy (13–15). By contrast, the number of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Poly- crystallographic studies on the genuine GBs at nanoscale and the technique Fédérale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland. 3Catalan structural heterogeneity inside grain aggregates by transmission elec- Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, tron microscopy (TEM) is limited to date. Previous studies mainly Bellaterra, Barcelona 08193, Catalonia, Spain. 4State Key Laboratory of Information Functional Materials, 2020 X-Lab, ShangHai Institute of Microsystem and Information concerned the interaggregate GBs as they may adversely affect the Technology, Chinese Academy of Sciences, Shanghai 200050, China. 5Cavendish charge carrier transport, facilitate ion migration, and even produce Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, some deep sub-band energy levels within the bandgap (16, 17), leading Cambridge CB3 0HE, UK. 6Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003, USA. 7Materials Sciences Division, Lawrence to major nonradiative losses that limit the photovoltaic performance Berkeley National Laboratory, Berkeley, CA 94720, USA. 8School of Microelectronics, (13, 18, 19). In addition, some imperfect structures occurring at inter- University of Science and Technology of China, Hefei 230026, China. 9Institute of Pho- aggregate GBs may influence the device stability associated with the toelectronic Thin Film Devices and Technology, Solar Energy Research Center, Nankai University, Tianjin 300350, China. 10Laboratory of Photomolecular Science, Institute presence of mobile charges or ions, oxygen, moisture, heat, and light of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, (20–24). In this regard, many approaches, such as surface passiva- Station 6, CH-1015 Lausanne, Switzerland. 11ICREA, Pg. Lluís Companys 23, Barcelona tion and functionalization (25, 26), have been widely studied to sup- 08010, Catalonia, Spain. 12Key Laboratory of Flexible Electronics & Institute of Ad- press these undesirable defects at interaggregate GBs. vanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Mate- rials, Nanjing Tech University, Nanjing 211816, China. 13Shaanxi Institute of Flexible Recently, the existence of grain aggregates and intra-aggregate GBs Electronics, Northwestern Polytechnical University, Xi’an 710072, China. 14Collabora- has been confirmed by electron backscatter diffraction and TEM tive Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China. (27–31). Li et al. (29) and Cai et al. (30) start to correlate the perform­ *Corresponding author. Email: iamzhurui@pku.edu.cn (R.Z.); qinhu20@ustc.edu.cn (Q.H.); yuhang.liu@epfl.ch (Y.L.); id338@cam.ac.uk (M.I.D.); michael.graetzel@ ances of perovskite solar cells (PSCs) with the density and structure epfl.ch (M.G.) of intra-aggregate GBs in polycrystalline perovskite films. These intra-­ †These authors contributed equally to this work. aggregate heterogeneities have been validated to evoke detrimental ‡Present address: Department of Chemistry-Ångström Laboratory, Uppsala Uni- effects on the performances of PSCs including power conversion effi- versity, Box 523, SE-751 20 Uppsala, Sweden. §Present address: Department of Chemistry, University College London, 20 Gordon ciency (PCE), open-circuit voltage (VOC), current density–voltage Street, London WC1H 0AJ, UK. (J–V) hysteresis, and stability of PSCs, implying that the mitigation Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 1 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 Fig. 1. Illustration of solid polycrystalline films and perovskite film morphologies. (A) Schematic diagram of a solid polycrystalline film with GBs, illustrating interag- gregate GBs and intra-aggregate GBs. (B) Chemical structure of TTABr. (C and D) Top-view SEM images of the control (C) and GBMP (D) films. (E and F) Cross-sectional SEM images of the control (E) and GBMP (F) films. Scale bars, 400 nm (C to F). of intra-aggregate GBs is of equal significance to that of interaggre- enhancements in the device efficiency from 21.1 to 23.2% and long- gate GBs. Full-scale mitigation from intra- to interaggregate GBs is term operational stability under illumination, humidity, or heat stress. thus urgently warranted to maximize the device performances toward The versatility of our strategy is also verified by applying it to differ- theoretical limit. However, the deficient understanding of the fun- ent categories of PSCs. This discovery specifies a previously un- damental origin of GBs renders it hard to develop effective full-scale addressed perspective concerning the full-scale mitigation of GBs in GB mitigation strategies. perovskites from nanoscale to submicroscale, opening a route to Here, we first systematically perform the crystallographic study obtain high-­quality solution-processed polycrystalline perovskite on the grain aggregates and GBs, and we show full-scale GB mitigation films for high-­performance optoelectronic devices. from genuine GBs formed between small nanocrystals to interaggre- gate GBs in polycrystalline perovskite solids. We control precisely the crystal growth during the film formation process through the RESULTS introduction of the brominated arylamine trimer N4,N4′-bis(4- Structure characterizations of perovskite films bromophenyl)-N4-(4′-((4-bromophenyl)(phenyl)amino)-[1,1′-biphenyl]- Mixed-cation halide perovskites have shown their superiority in photo- 4-yl)-N4′-phenyl-[1,1′-biphenyl]-4,4′-diamine (TTABr; Fig. 1B). A voltaic performance, delivering record-level certified PCEs over a brief unique feature of TTABr is its solubility in both polar and nonpolar period in the perovskite research community (32–35). Here, we focus solvents used for dissolving in the perovskite precursors and as anti- on the mitigation of GBs for mixed-cation lead mixed-halide solvent, respectively. This enables it to act as a crystallization aid and perovskite films with the composition of FA0.851MA0.149PbI2.552Br0.448 to access GB sites, allowing it to mitigate defects. This results in the with 4.48 mole percent (mol %) excess PbI2, where MA and FA de- suppression of nonradiative carrier recombination, increasing sub- note methylammonium and formamidinium, respectively, via the stantially the VOC of PSCs. By virtue of its more favorable redox intervention of the brominated arylamine trimer, TTABr. TTABr potential, TTABr also enhances intergrain hole transfer, improving was fully characterized by mass spectrometry (MS) and nuclear charge carrier collection and the fill factor (FF) of the devices. We magnetic resonance (NMR) spectroscopy. The differential scan- analyze the detailed crystallographic information and link the underly- ning calorimetry (DSC) and thermogravimetry (TG) analyses (fig. ing GB mitigation to the variation in microscopic structures, crystal S1) further reveal its good thermal stability. The GB-mitigated size distributions, defect concentrations, charge carrier lifetimes, and perovskite is abbreviated as “GBMP” below. photovoltaic performances. We demonstrate regular mesostructured We first investigated the morphological variation of the control perovskite photovoltaics based on GB-mitigated films with large and GBMP films using SEM. Top-view SEM images in Fig. 1 (C and D) Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 2 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE show that the films with and without TTABr exhibit remarkably differ- ent surface morphologies. The control film is dominated by tightly packed angular blocks with sizes of ~200 to 400 nm. As mentioned above, these blocks observed with SEM are not single-crystalline grains but rather grain aggregates. We verified this by cross-sectional TEM analysis of the lamellae obtained from the devices with and without TTABr (fig. S2). The resulting interaggregate boundaries formed during crystal growth in the control film exhibit broken-line features, whereas, as shown in Fig. 1D, the angular nature of the GBMP film is substantially impaired as the grain aggregate outlines become rounded. The corresponding atomic force microscopy (AFM) and conductive AFM (c-AFM) images (fig. S3) of perovskite surfaces in- dicate larger current flow in the GBMP film than in the control and the GBs in GBMP film showing darker contrast. From the cross-sectional SEM images in Fig. 1 (E and F), we note that there are noticeable interaggregate boundaries across the control film, resembling the sur- face morphology. However, the GBMP film shows few obvious visible interaggregate boundaries across the entire cross section. We con- Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 sider these differences to be related to the intervention of TTABr. To access the crystalline phase evolution of the grain aggregates in the absence and presence of TTABr, we carried out the cross-sectional TEM measurements of the devices prepared with and without GBMP films. After systematically analyzing the high-resolution TEM (HRTEM) images and the corresponding temperature-colored fast Fourier trans- form (FFT) patterns as displayed in Fig. 2 and figs. S2 and S4, we Fig. 2. Direct observations of perovskite grains, grain aggregates, and intra-­ confirm that both the control and GBMP films contain two different aggregate GBs. (A) (1 to 3) Three representative HRTEM images and the corresponding phases, including a tetragonal phase with space group 140 (I4/MCM) temperature-colored FFT patterns from the control film. Right: The magnified HRTEM and a cubic phase with space group 221 (PM3M) (36, 37), which is image of the red squared area selected in (A3) and the corresponding FFT pattern. similar to the single-cation perovskites reported previously in the It indicates that the crystal structure corresponds to a cubic perovskite phase with literature (38). Moreover, the HRTEM images and the correspond­ space group 221 (PM3M) visualized from [100] direction. (B) (1 to 3) Three repre- ing temperature-colored FFT patterns in Fig. 2A (1 to 3) and figs. S2, sentative HRTEM images and the corresponding temperature-colored FFT patterns S4A, and S5A demonstrate the polycrystalline nature for these from the GBMP film. Right: The magnified HRTEM image of the red squared area selected in (B3) and the corresponding FFT pattern. It indicates that the crystal observed regions of fixed size in the control film, whereas for the structure corresponds to a cubic perovskite phase with space group 221 (PM3M) GBMP film (Fig. 2B, 1 to 3, and figs. S4B and S5B), the single-­ visualized from [110] direction. (C) Left: Representative HRTEM image of the region crystalline nature is realized for the observed regions of equiva- with intra-aggregate GBs of the GBMP film. Right top: The magnified HRTEM of the lent size. blue squared area and the corresponding FFT pattern. It indicates that the crystal It is noteworthy that the whole GBMP film containing TTABr is structure corresponds to a cubic perovskite phase with space group 221 (PM3M) not a single crystal as some regions without a crystallographic tex- visualized from [111] direction. Right bottom: The magnified HRTEM of the red ture are observed lying between different perovskite grains (Fig. 2C), squared area and the corresponding FFT pattern. It indicates that the crystal structure whereas for the control film, we will obtain the overlapped imaging corresponds to a cubic perovskite phase with space group 221 (PM3M) visualized of all the stacked perovskite grains in the observation region from from [011] direction. the vertical direction (i.e., parallel to the electron beam direction) in the HRTEM image. This hinders the atomic visualization of detailed structure for GBs in the control lamellae based on the two-dimensional and of different samples in various batches (fig. S5). All the HRTEM (2D) TEM images. The amorphous structures localized at GBs are and FFT results show that the GBMP film indeed has larger perovskite highlighted in white dotted lines in Fig. 2C and shown in fig. S5C. The grain sizes, delivering lower GB densities. By combining SEM and prevalent single-crystalline perovskite nature observed in the HRTEM TEM results, we obtained a consistent result of mitigation in the den- images obtained from the GBMP film confirms its larger grain sizes sity of intra-aggregate GBs. From HRTEM and FFT measurements, in comparison to the control film since the control film shows the we also observe that there are isolated lead halide species [triclinic polycrystalline nature in the commensurate field of view. In addition, phase with space group 186 (P63MC)] with the sizes ranging from by applying Scherrer equation to the most intense reflection in x-ray several nanometers to tens of nanometers distributed across the whole diffraction (XRD) pattern, i.e., (110) peak, we roughly estimated an cross section of the control film (fig. S7). This is not unexpected average grain size of 72 nm for the GBMP sample, which is substantially since the films were processed from a nonstoichiometric perovskite larger than the grain size of 42 nm estimated for the control sample precursor solution with excess lead halide. Our TEM results provide (fig. S6). The increased grain sizes are indicative of the reductions in direct evidence that the lead halide species are embedded in the the density of GBs and unwanted defects in the polycrystalline films. perovskite grain aggregates distributed in the whole film (fig. S8). To corroborate the reproducibility and reliability of TEM obser- We further conducted annular dark-field scanning TEM (ADF- vations, we also probed crystallographic information via HRTEM STEM) and electron energy-loss spectroscopy (EELS) measurements images and FFT patterns in different regions of an identical sample (4, 25) to probe the spatial distribution of TTABr in the GBMP film. Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 3 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE As shown in the cross-sectional low-magnification STEM image of (42, 43). 2D GIXD profiles acquired from the control sample reveal the GBMP film (Fig. 3A) and the control film (fig. S9A), there are the presence of discrete Bragg spots, which are suppressed by the pres- some dark “gaps” with low Z contrast, indicating fewer perovskite ence of TTABr in the GBMP sample (fig. S13, A and D). From this species or lighter species. In the case of the control film, we do ob- observation, we surmise that TTABr molecules interact with the inter- serve some interaggregate domain boundaries present outside the mediate adduct and interrupt the alignment of corner-sharing [PbI6]4− packed angular grain aggregates via SEM, suggesting that the dark octahedra in certain directions over an extended range. The scattering gaps observed in the STEM for the control film arise from a lack of features from the intermediate phases diminish with time, whereas the perovskite material at the interaggregate (domain) boundary sites. features corresponding to the 3D perovskite phase intensify, indi- By contrast, the cross-sectional SEM image of the GBMP film exhibits cating the transformation of the adducts into the 3D perovskite phase. fewer interaggregate boundaries, indicating that the dark zones ob- We further analyzed the transformation of adducts into the served by STEM of the GBMP films originate from TTABr embed- perovskite phase from the time-resolved integral GIXD profiles shown ded in the boundary regions. This is consistent with the HRTEM in fig. S14 by estimating the integrated intensity of diffraction peaks evidence for the presence of amorphous regions in the GBMP film. through Gaussian fitting. The peak intensity profiles reveal relatively From the STEM-EELS chemical composition mapping of the slower kinetics for the transformation of adduct into the 3D perovskite control film in fig. S9, weak signals of I, Br, and Pb are detected in phase due to the presence of the TTABr, resulting in larger perovskite the dark zones between the grains. In contrast, the sequential EELS grains, thus lowering the GB density for GBMP films. This is further chemical composition mapping of GBMP film (Fig. 3A) shows that supported by color evolution of the control and GBMP films upon the dark gaps with bubble-like structures are not only richer in C annealing, shown in fig. S15A. We propose a scheme demonstrat- Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 and N elements compared to the control film but also present less I, ing how TTABr influences the crystallization and growth of various Br, and Pb contents. Therefore, we conclude that TTABr, mainly phases during the formation process (Fig. 3, F and G, and fig. S15B). consisting of C and N, is located at GB sites in the upper part away from the bottom surface of the GBMP film, in keeping with the GB mitigation–induced variety in optoelectronic finding that the dark gaps are wider than those in the control film. properties of perovskites We posit that TTABr possibly inserts its bromine substituents into To understand the influence of the GB mitigation on the optoelec- the halide vacancies of metal halide perovskites at GBs. This inter- tronic properties of perovskite films, we quantified the hole mobility, action may be explained by a Lewis acid–Lewis base type interaction trap densities by space charge–limited current (SCLC) measurement between undercoordinated Pb and lone pair–containing nitrogen in fig. S16 (A to C), charge carrier lifetimes by time-resolved photo- as established by systematic shifts in the core-level binding energies luminescence (TRPL), and energetic disorder using photothermal corresponding to Pb, N, and Br atoms revealed by x-ray photoelectron deflection spectroscopy (PDS). We obtained hole mobilities of 10.51 spectroscopy (fig. S10) (39–41). The N → Pb interaction is further and 6.24 cm2 V−1 s−1 for the GBMP and control device, respectively, supported by vibrational spectroscopy (fig. S11). confirming the enhancement of the hole mobility caused by the unique To explore whether the intervention of TTABr influences the GB mitigator TTABr. In addition, the trap density in the GBMP film grain orientations, we applied depth-dependent grazing incidence is calculated to be 2.25 × 1015 cm−3, less than half of that for the XRD (GIXD) on the control and GBMP films through varying the control (6.87 × 1015 cm−3). The charge carrier lifetime increased x-ray incidence angle. The 2D pattern in fig. S12A and the integral over 10-fold from 340 ns for the control to 4.2 s for the GBMP film GIXD profile in fig. S12B show that TTABr is of amorphous nature, (fig. S16D), demonstrating the marked suppression of nonradiative in good agreement with the TEM results. The arc shape in 2D GIXD carrier recombination by TTABr. By measuring the sub-bandgap patterns both at a shallow incidence angle of 0.2° (fig. S12, C and G) absorption using PDS, we estimated the energetic disorder of perovskite and a larger incidence angle of 0.6° (shown in Fig. 3, B and C) indi- films (fig. S16, E and F). The lower Urbach energy (EU = 14.2 meV) cates the random orientation of the grains from the surface and for GBMP films as compared to the control films (EU = 15.1 meV) within the bulk, manifesting the inherent polycrystallinity of the proves less energetic disorder of the GBMP films. perovskite films (42). For the GIXD integral line files in Fig. 3 (D and E) The GBMP film in a complete solar cell architecture, i.e., with at various grazing incident angles from 0.2° to 0.6°, neither an obvi- electron transport layer and hole transport layer, showed more than ous peak shift nor a new diffraction peak is obtained in the GBMP a doubled higher-average PL quantum yield (PLQY) (fig. S16G), demon- film in comparison with the control. We therefore conclude that the strating that its superior optical quality is maintained even when the large TTABr molecule could not incorporate into the 3D perovskite perovskite film is embedded between electron and hole transport layers. structure, which is also consistent with the presence of TTABr at From the PLQY, the quasi-Fermi level splitting EF q−1 can be de- GBs observed in STEM-EELS measurements. termined, which is the internal VOC of the absorber layer and rep- resents the maximum VOC that a device can achieve. From the PLQY The influence of TTABr intervention increase, an average EF q−1 increase of ~24 mV is determined (fig. S16, on the crystallization process H and I), which matches the trend of the device VOC values as dis- To explore the time course of the crystallization process, we per- cussed further below. In summary, improved charge carrier dynamics, formed in situ GIXD while annealing the control and GBMP samples. minimization of defects, low energetic disorder, and increased lu- A series of 2D GIXD images acquired at different stages of anneal- minescence quantum yield suggest that the GB mitigation has desired ing (100°C) are shown in fig. S13, as well as in movies S1 and S2. In effects on the optoelectronic properties of the perovskite films. both cases, the scattering features at q ∼ 0.46 Å−1, q ∼ 0.50 Å−1, and q ∼ 0.64 Å−1 that can be indexed to FA(MA)X-PbX2-DMSO(DMF) Device architecture and photovoltaic performances intermediate adducts (X = Br− and I−) and the diffraction feature at We fabricated the regular mesostructured PSCs to evaluate the rela- q ~ 1.0 Å−1 corresponding to the 3D perovskite phase are observed tionship between GB mitigation and device performance. In a fully Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 4 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 Fig. 3. The STEM-EELS chemical composition mapping and GIXD for perovskite films. (A) Left: Low-magnification ADF image showing the cross-sectional structure of a GBMP-based lamella. Scale bar, 500 nm. Right: EELS chemical composition mapping obtained from the blue rectangle area on the left ADF micrograph including individual elemental mapping for C (red), N (yellow), Pb (orange), I (blue), and Br (green) and composite distribution mapping. (B and C) 2D GIXD images with the x-ray incidence angle of 0.6° for the control (B) and GBMP (C) films. (D and E) Integral GIXD profiles at various incidence angles for films corresponding to (D) and (E), respectively. (F and G) The intensity of the characteristic peaks of films without (F) and with (G) TTABr annealed at 100°C as a function of time (frames), where peaks 1 to 3 are indexed to intermediate adducts, and peak 4 is referred to the typical perovskite phase. a.u., arbitrary units. Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 5 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE assembled device shown in Fig. 4A, TTABr an organic hole conduc- The GB mitigation in the GBMP film results in substantial improve- tor could facilitate the hole transfer process owing to its favorable ments of the photovoltaic performance. Comparing to the control energy levels as confirmed by ultraviolet photoelectron spectros- devices, key metrics, including VOC and FF, have been improved copy (UPS), cyclic voltammetry (CV), and steady-state PL spectra, simultaneously without compromising on the short-circuit current as shown in Fig. 4B and fig. S16 (J to L). Figure 4C shows the statis- density (JSC). On the basis of the optimized TTABr concentration tics of photovoltaic metrics for the control and GBMP-based PSCs. (table S1) along with careful device engineering, we achieved the Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 Fig. 4. Enhanced photovoltaic performances. (A) Cross-sectional SEM image of a typical GBMP-based device. Scale bar, 500 nm. (B) Energy level alignment diagram of TiO2, perovskite, TTABr, and spiro-OMeTAD. (C) VOC, JSC, FF, and PCE metrics and statistics of the devices fabricated with the control and GBMP films from the same batch. (D) J-V curves for the champion control device and GBMP-based device measured in forward and reverse scans under simulated AM 1.5G illumination of 100 mW cm−2. (E) The stabilized photocurrents and SPOs of the two abovementioned devices at the MPP. (F) Thermal stability results of the nonencapsulated typical control and GBMP-based devices after aging for more than 1100 hours at 85°C in a nitrogen atmosphere. (G) MPP tracking of the nonencapsulated control and GBMP-based devices at room temperature under continuous 1-sun-equivalent white light-emitting diode arrays (with no ultraviolet filters) in a nitrogen atmosphere. Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 6 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE champion device with a GBMP film, delivering an efficiency of genuine (intra-aggregate) GBs to submicron-scale interaggregate 23.16% (reverse scan) with negligible hysteresis (a PCE of 22.90% GBs. The mitigation of GBs by modulating the crystallization in the forward scan) and a stabilized power output (SPO) of 23.05% kinetics using the judiciously designed mitigator TTABr led to the at the maximum power point (MPP) under 1-sun AM 1.5G irradi- formation of perovskite films exhibiting low trap-state density and ance (Fig. 4, D and E). By contrast, the control device shows a lower energetic disorder, reduced nonradiative recombination, and improved SPO of 20.88%. The incident photon-to-electron conversion effi- PLQY, which are beneficial for realizing highly efficient and stable ciency (IPCE) spectra, as shown in fig. S17, yield an integrated JSC PSCs. TTABr also accelerates intergrain hole transfer and charge of ~23.4 mA cm−2 in accordance with the J–V data, showing the ab- carrier collection and thus amplifies the VOC, FF, and PCE of the PSCs, sence of any serious spectral mismatch between simulated and the which can be extended to various perovskite compositions and actual sunlight. device architectures. The in-depth structural characterization not only Further analysis (figs. S18 to S22) establishes that the highly benefi- clarifies the existing terminology in the perovskite field but also puts cial effect of TTABr on the photovoltaic metrics, in particular, the forth a new perspective to modulate GBs. Our results indicate that VOC and FF, extends over a wide range of perovskite compositions full-scale GB mitigation is particularly notable and promising to and device architectures, demonstrating the versatility of our strategy. boost the quality of optoelectronic devices and are likely to find The fact that the PCE improvement arises predominantly from an applications in the engineering of a whole range of polycrystalline increase in the VOC and FF is in keeping with the reduction of defect materials. concentration, prolonged charge carrier lifetime, and enhanced in- tergrain hole transfer as a result of GB mitigation in GBMP films. Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 The trends observed for the VOC values and the Fermi level splitting MATERIALS AND METHODS determined from the absolute PL efficiency are in agreement (fig. S16I), Materials implying that the VOC increase arises mainly from the reduction in Methylammonium bromide (MABr), formamidinium iodide (FAI), trap-assisted nonradiative radiation. The small difference for the con- methylammonium chloride (MACl), and titanium oxide paste (TiO2 trol film is caused by energy level misalignment (44), which is re- paste; 30 NR-D) were purchased from GreatCell Solar via Advanced duced for the GBMP film (Fig. 4B). Election Technology Co. Ltd. (China). Lead diiodide (PbI2; 99.99%) We probed the thermal stability of nonencapsulated control and and lead dibromide (PbBr2; 99%) were purchased from Tokyo Chemical GBMP-based devices at 85°C in a nitrogen atmosphere and present Industry Co. Ltd. 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)- the results in Fig. 4F. This test follows the International Summit on 9,9′-­spirobifluorene (spiro-OMeTAD) was purchased from Merck. Organic Photovoltaic Stability-Dark storage-Intrinsic stability at ele- TTABr {1H NMR (400 MHz, CDCl3) : 7.47 parts per million (ppm) vated temperature (ISOS-D-2I) protocol proposed in a recent con- (dd, J = 8.6 Hz, 4.6 Hz, 8H), 7.38 to 7.32 ppm (m, 6H), 7.30 to 7.26 ppm sensus statement to address the intrinsic stability of PSCs in inert (m, 4H), 7.15 ppm (d, J = 8.4 Hz, 4H), 7.11 ppm (d, J = 8.4 Hz, 8H), atmosphere (45). GBMP-based devices show superior thermal sta- 7.08 to 7.02 ppm (m, 4H), and 6.99 ppm (d, J = 8.8 Hz, 4H); 13C bility as the devices retained more than 80% of their initial PCEs NMR (101 MHz, CDCl3) : 147.39, 147.03, 146.58, 135.25, 132.45, after annealing at 85°C for >1150 hours, while the control devices 132.38, 129.59, 127.70, 127.66, 125.66, 125.49, 124.74, 124.63, 124.46, 123.59, degraded severely over the same time period, retaining <20% of 115.38, 115.18, and 77.36 ppm; high-resolution MS (atmospheric their initial PCEs. We also tracked the MPP of the nonencapsulated pressure chemical ionization/quadrupole orthogonal acceleration– control and GBMP-­based devices under continuous full-sun-intensity time-of-flight) mass/charge ratio: [M + H]+ calcd for C54H39Br3N3+ illumination, and results are presented in Fig. 4G. This test follows 969.0575; found 969.064} was obtained from Chilworth Technical the ISOS-L-1I (with electrical bias) protocol. The spectrum of the Centre, Merck Chemicals Ltd. (Southampton England). It is soluble light-emitting diode array is shown in fig. S23A. GBMP-based not only in the antisolvent [chlorobenzene (CB)] during solution devices show excellent operational stability, maintaining more than processing but also in the solvent of perovskite precursor [the mixture 90% of their initial SPO after 1000 hours, whereas the control shows of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)]. a faster degradation, with their PCE decreasing to less than 80% of Bis(trifluoromethylsulfonyl)imide lithium salt (LiTFSI) and 4-tert-­ their initial values over the same time period. From the larger water butylpyridine (tBP; 96%), titanium diisopropoxide bis(acetylacetonate) droplet contact angle of 38.2° measured for the GBMP sample as (75% in 2-propanol), cesium iodide (CsI; 99.95%), and bis(acetylac- compared to 12.1° for the control film, we anticipate better water re- etonate) were purchased from Sigma-Aldrich. Poly(triaryl)amine sistance of GBMP-based PSCs due to the hydrophobicity of TTABr. (PTAA; molecular weight ≤ 6000 g mol−1) was purchased from We confirm experimentally that under exposure of devices to a rela- Xi’an Polymer Light Technology Corp (China). Some solvents in- tive humidity of ~50 ± 10% at ~24° ± 4°C, the GBMP-based PSCs cluding DMF (99.8%), DMSO (99.7%), 1-methyl-­2-pyrrolidinone retained ~90% of their initial PCE values after 15 days, whereas the (NMP; 99%), CB (99.8%), dichloromethane, and acetonitrile (ACN; control device lost more than 50% of its initial efficiency within 6 days, 99.8%) were purchased from Acros Organics. SnO2 colloidal solu- as shown in fig. S23B [ISOS-Dark storage-Stability in the ambient tion (15% in H2O colloidal dispersion) was purchased from Alfa condition (ISOS-D-1)]. Thus, by combining a series of tests, we Aesar. All the reagents were used as received. provide clues rendering a clear picture of the device performance enhancement through GB mitigation. Preparation of compact and mesoporous TiO2 layers and SnO2 layer Fluorine-doped tin oxide (FTO) glass substrates (NSG 10, Nippon DISCUSSION Sheet Glass, Japan) were cleaned by ultrasonication in a special cleaning In summary, our study opens up new perspectives on mitigating liquid (Hellmanex II, 2 volume percent in deionized water), ace- GBs in polycrystalline perovskite films ranging from the nanoscale tone, and ethanol for 15 min, respectively, and then dried with dry Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 7 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE air flow. A compact TiO2 (c-TiO2) layer of ~30 nm was subsequently equipped with a Schott K113 Tempax sunlight filter (Praezisions Glas deposited on the clean FTO glass substrates via spray pyrolysis at and Optik GmbH) was used as the simulated AM 1.5G standard 450°C from a precursor solution of 0.6 ml of titanium diisopropoxide light source for photovoltaic measurements. The exact light inten- bis(acetylacetonate) and 0.4 ml of bis(acetylacetonate) diluted in sity was determined by a reference-calibrated Si diode equipped 9 ml of anhydrous ethanol and oxygen as a carrier gas. Then, a with an infrared cutoff filter (KG-3, Schott) before each measure- mesoporous TiO2 (m-TiO2) layer was deposited onto the c-TiO2 ment. During the measurement, a black-colored metal mask was layer by spin-coating a diluted TiO2 paste [30 NR-D (w:w = 1:6 in used to define the photoactive area of 0.16 or 0.09 cm2 to ensure the ethanol)], which was later sintered at 450°C for 30 min in dry air accuracy of the JSC obtained from J–V curves. The IPCE measure- flow. The substrates composed of FTO glass/c-TiO2/m-TiO2 were ments were carried out using a 300-W xenon light source (ILC transferred into a glovebox containing dry air (25°C, humidity Technology). The sweeping conditions are reverse scan (scan rate of of <2%) after cooling to 150°C, aiming to complete perovskite film 10 mV s−1 and no delay time) and forward scan (scan rate of 10 mV deposition. s−1 and no delay time). All the above measurements were performed For SnO2-based PSCs, indium tin oxide (ITO) transparent con- in air at room temperature with nonencapsulated PSCs. The long- ductive substrates were precleaned for the electron-extraction layer term SPO tracking of nonencapsulated PSCs in a nitrogen atmo- deposition. The SnO2 layer was deposited with the process reported sphere at the MPP under 1-sun illumination at room temperature in our previous work (46). was measured by a homemade electronic board with an eight-­ channel MPP capability. The light source that consisted of an array Perovskite film fabrication of white light-emitting diodes was powered by a constant current, Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 For the perovskite films with the composition of FA0.851MA0.149PbI2.552 and no ultraviolet filters were used. The intensity of the light source Br0.448(4.48 mol % excess PbI2), the perovskite precursor solution was calibrated by a reference-calibrated Si diode equipped with a was prepared by mixing FAI (1.14 M), PbI2 (1.20 M), MABr (0.20 M), KG-3 filter, equivalent to 1-sun sunlight of 100 mW cm−2. The and PbBr2 (0.20 M) in a mixed anhydrous solvent of DMF/DMSO setup was calibrated periodically using a Keithley 2602B source-­ (v:v = 4:1) or NMP/DMSO/DMF (v:v:v = 1:4:15). The precursor measuring unit. solution was spin-coated onto the FTO glass/c-TiO2/m-TiO2 or the ITO/SnO2 substrates via a two-step program at 2000 rpm with a Mass spectroscopy ramping rate of 200 rpm s−1 for 10 s and 6000 rpm with a ramping MS spectrum of TTABr was collected on a HITACHI-80 mass rate of 2000 rpm s−1 for 30 s, respectively. During the second step, spectrometer. 100 l of CB that contains various concentrations of TTABr ranging from 0.5 to 2 mg ml−1 was dripped on the spinning substrate at 15 s NMR spectroscopy 1 before the end of this program. The film was then annealed at 100°C H NMR and 13C NMR spectra of TTABr were recorded on Bruker for 45 min. AvanceIII-400 MHz NMR spectrometer. Chemical shifts are re- For the perovskite films with the composition of FA0.851MA0.149PbI3 ported in parts per million (). 1H NMR spectrum was referenced to with 4.48 mol % excess PbI2, the perovskite precursor solution was tetramethylsilane (0 ppm) or solvent residual peak (7.26 ppm for 1H prepared by mixing FAI (1.14 M), PbI2 (1.40 M), MAI (0.20 M), and NMR) as an internal standard. MACl (0.45 M) in a mixed anhydrous solvent of DMF/DMSO (v:v = 4:1). The film deposition process is the same as above. For the Photothermal deflection spectroscopy perovskite films with the composition of MACl-stabilized FAPbI3, For PDS measurements, a monochromatic pump light beam is shined the perovskite precursor solution and films were prepared com- on the sample [film on quartz substrate immersed in an inert liquid pletely according to the process in our previous work (47), however, FC-72 Fluorinert (3M Company)], which, on absorption, produces without any posttreatment. a thermal gradient near the sample surface via nonradiative relaxation and induces heating. A fixed-wavelength continuous-wave laser Hole transporting layer and gold electrode deposition probe beam is passed through this refractive index gradient, produc- To obtain the desirable solution of hole transporting material, ing a deflection proportional to the absorbed light at that particular spiro-OMeTAD was dissolved into CB (70 mM) and doped with wavelength, which is detected by a photodiode and lock-in amplifier LiTFSI (520 mg ml−1 in ACN) and tBP by a molar ratio of 0.5 and combination. Scanning through different wavelengths produces a 3.3, respectively. Then, the solution was deposited on the perovskite complete absorption spectrum. film in a dry air glovebox with humidity of <2%, by spin-coating at 4000 rpm with a ramping rate of 2000 rpm s−1 for 20 s. Last, device Photoluminescence quantum yield fabrication was finished by thermally evaporating 80 nm of gold PLQY was measured using an integrating sphere (Gigahertz-Optik), layer as an electrode contact, wherein the overlap area between the an Andor Kymera 193i spectrograph, and a 660-nm continuous-wave FTO and metal electrodes is 0.25 or 0.103 cm2. The devices were laser (OBIS, Coherent) set at 1-sun-equivalent photon flux (790-m stored in a dry air glovebox for the oxidation of spiro-OMeTAD beam full width at half maximum, 330 W); the PL signal was col- before all the characterizations. lected at normal incidence using an optical fiber. We used the tech- nique described by de Mello et al. (48). For the calibration of the Solar cell characterization PLQY measurements, we used a halogen lamp (BN-LH250-V01, The J–V curves and SPOs of the PSCs were recorded with a digital Gigahertz-Optik), and to proof the correctness of the calibration, source meter (2400 Series SourceMeter, Keithley Instruments, we measured the PLQY of a dye solution of indocyanine green in USA). To reduce the mismatch between the simulated and actual DMSO in the same setup and measured PLQY values within 15% solar spectrum to less than 2%, a 450-W xenon lamp (Oriel, USA) deviation from the literature values. Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 8 of 11 SCIENCE ADVANCES | RESEARCH ARTICLE Steady-state PL and TRPL Wide-angle GIXD measurements The steady-state PL spectra were performed on an Edinburgh Instru- GIXD measurements for the perovskite films deposited on FTO ments photoluminescence spectrometer (Edinburg, UK) additionally glass/c-TiO2/m-TiO2 substrates and TTABr film deposited on Si equipped with an integrating sphere. The TRPL spectra were measured substrate were conducted on beamline 7.3.3 at Advanced Light via time-correlated single-photon counting using a LifeSpec II Source, Lawrence Berkeley National Laboratory. The wavelength of (Edinburgh Instruments) fluorescence spectrometer with a pico- x-ray was 1.240 Å, and the scattering intensity was detected by a second pulsed diode laser (EPL-510, Edinburgh Instruments) at PILATUS 2M detector. For the in situ thermal heating GIXD exper- 510-nm wavelength and 85-ps pulse width. The laser fluence used iment, the fresh samples removed from the spin coater were heated was 5 nJ cm−2. at 100°C. All the data were collected in a tiled mode with an expo- sure time of 1.0 s. For the control sample, there was no delay for Cyclic voltammetry frames 1 to 10, while the frame interval was set to 30 s for frames 11 to CV curves were recorded on an electrochemical workstation (Autolab 50. For the GBMP samples, there was no delay for frames 1 to 20, PGSTAT302N, Metrohm, Switzerland). and then the frame interval was changed to 30 s for frames 21 to 60. Data analysis was performed using IGOR Pro software with the DSC and TG Nika package. The DSC and TG data of TTABr were obtained from a commercial equipment (Q600, TA Instruments, USA) with a heating rate of 5°C Quantifying the effect of TTABr on defect density and hole min−1 under nitrogen atmosphere. mobility in the perovskite Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 Current density–versus–bias voltage curves of the hole-only devices Scanning electron microscopy with the architecture of ITO-coated glass/NiOx/perovskite/PTAA/ The surface and cross-sectional morphologies of the control and Ag were based on the control and GBMP films, where these two GBMP films coated on FTO glass/c-TiO2/m-TiO2 substrates were films were processed from the approach identifying with the device acquired using a high-resolution scanning electron microscope fabrication. Single-carrier hole-only devices were measured in the (Gemini-SEM 300). An electron beam accelerated to 5.0 kV was dark under bias from 0.0025 to 3.500 V and 3.500 to 0.0025 V using used with either the In-Lens or SE2 detector. an electrochemical workstation (Autolab PGSTAT302N, Metrohm, Switzerland). Transmission electron microscopy All the lamellae for HRTEM, ADF, STEM, and EELS investigations SUPPLEMENTARY MATERIALS were prepared from complete solar cell devices by focused ion beam Supplementary material for this article is available at https://science.org/doi/10.1126/ (FIB) cutting. Before the sample preparation with the FIB cutting, a sciadv.abo3733 Pt/carbon layer was deposited on the Au electrode to protect the sample from milling damage. The lamellae were carefully thinned REFERENCES AND NOTES down with FIB by gradually decreasing the ion acceleration voltages, 1. J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. 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Pucci, Dielectric function and degradation process acknowledge the GIXD measurements at beamline 7.3.3 of Advanced Light Source (LBNL), of poly(triarylamine) (PTAA). Org. Electron. 15, 2959–2963 (2014). which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. P.T. and 54. X. Yang, Y. Ni, Y. Zhang, Y. Wang, W. Yang, D. Luo, Y. Tu, Q. Gong, H. Yu, R. Zhu, J.A. acknowledge the funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish Multiple-defect management for efficient perovskite photovoltaics. ACS Energy Lett. 6, MINECO project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from 2404–2412 (2021). Spanish MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Programme/ 55. M. A. Green, A. Ho-Baillie, H. J. Snaith, The emergence of perovskite solar cells. Nat. Generalitat de Catalunya. Author contributions: R.Z., L.Z., M.G., and Q.H. conceived the idea. Photon. 8, 506–514 (2014). L.Z., D.L., and R.Z. designed all the experiments. L.Z. fabricated and characterized the solar cells 56. R. T. Ross, Some thermodynamics of photochemical systems. J. Chem. Phys. 46, and prepared the samples for characterizations. P.T. and L.Z. conducted the TEM, STEM, and 4590–4593 (1967). EELS. P.T., J.A., and L.Z. analyzed the TEM, STEM, and EELS. Q.H. measured and analyzed the GIXD. Y.L. synthesized extra TTABr and measured MS and NMR. N.A., M.I.D., and R.H.F. recorded Acknowledgments: The FIB sample preparation for (S)TEM experiments was conducted in the and analyzed the PDS data. L.Z. measured and analyzed SCLC. L.Z. and D.L. measured and Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon-Universidad de analyzed DSC and UPS. Q.G. and W.H. contributed to the analysis of characterizations such as Zaragoza. We acknowledge the LMA-INA for offering access to instruments and expertise. We AFM, c-AFM, and PL. A.H. provided the platform for long-term MPP tracking and contributed acknowledge Z. Wang from École Polytechnique Fédérale de Lausanne (EPFL) for measuring to the measurement. F.T.E. carried out the TRPL and PLQY measurements and analysis. S.M.Z., SEM for SCLC, A. Agarwalla from EPFL for long-term MPP tracking, W. Yang from Peking A.H., T.P.R., J.A., W.H., and Q.G. gave suggestions and comments. R.Z., M.G., and Q.H. directed University for analyzing the SEM and PLQY, and L. Ge from NT-MDT SI Beijing office for and supervised the project. L.Z., D.L., Y.L., and M.I.D. wrote the manuscript. L.Z., D.L., P.T., J.A., measuring AFM and c-AFM. Funding: This work was financially supported by Beijing Natural W.H., Q.G., T.P.R., A.H., F.T.E., M.I.D., Y.L., Q.H., M.G., and R.Z. revised the paper. All the authors Science Foundation (JQ21005), the National Key R&D Program of China (2021YFB3800100 and discussed the results and commented on the paper. Competing interests: The authors 2021YFB3800101), the National Natural Science Foundation of China (91733301 and declare that they have no competing interests. Data and materials availability: All data Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 62104221), the China Postdoctoral Science Foundation (2020M670036), and the R&D Fruit needed to evaluate the conclusions in the paper are present in the paper and/or the Fund (20210001). M.G. and S.M.Z. thank the King Abdulaziz City for Science and Technology Supplementary Materials. (KACST) for financial support. M.I.D. acknowledges funding from the Royal Society. N.A. and R.H.F. thank the EPSRC project SUNRISE (EP/P032591/1). Q.H. and T.P.R. were supported by the Submitted 8 February 2022 U.S. Office of Naval Research under contract N00014-17-1-2241. Q.H. thanks the support from Accepted 18 July 2022 USTC Research funds of the Double First-Class Initiative (YD2100002007). We thank the Published 2 September 2022 support for sample preparation at Molecular Foundry of Lawrence Berkeley National 10.1126/sciadv.abo3733 Zhao et al., Sci. Adv. 8, eabo3733 (2022) 2 September 2022 11 of 11 Enabling full-scale grain boundary mitigation in polycrystalline perovskite solids Lichen Zhao, Pengyi Tang, Deying Luo, M. Ibrahim Dar, Felix T. Eickemeyer, Neha Arora, Qin Hu, Jingshan Luo, Yuhang Liu, Shaik Mohammed Zakeeruddin, Anders Hagfeldt, Jordi Arbiol, Wei Huang, Qihuang Gong, Thomas P. Russell, Richard H. Friend, Michael Grtzel, and Rui Zhu Sci. Adv., 8 (35), eabo3733. DOI: 10.1126/sciadv.abo3733 Downloaded from https://www.science.org at Universitat Autnoma de Barcelona on February 01, 2023 View the article online https://www.science.org/doi/10.1126/sciadv.abo3733 Permissions https://www.science.org/help/reprints-and-permissions Use of this article is subject to the Terms of service Science Advances (ISSN ) is published by the American Association for the Advancement of Science. 1200 New York Avenue NW, Washington, DC 20005. The title Science Advances is a registered trademark of AAAS. Copyright © 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).