Volume 3 Number 2 February 2019 Pages 305–566 Sustainable Energy & Fuels Interdisciplinary research for the development of sustainable energy technologies rsc.li/sustainable-energy ISSN 2398-4902 COMMUNICATION Anders Hagfeldt, Monica Lira-Cantu et al. PbZrTiO3 ferroelectric oxide as an electron extraction material for stable halide perovskite solar cells Sustainable Energy & Fuels View Article Online COMMUNICATION View Journal | View Issue PbZrTiO3 ferroelectric oxide as an electron This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. extraction material for stable halide perovskite Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. Cite this: Sustainable Energy Fuels, 2019, 3, 382 solar cells† Received 7th September 2018 Amador Pérez-Tomas, a Haibing Xie,a Zaiwei Wang,c Hui-Seon Kim, c Ian Shirley,a Accepted 29th October 2018 Silver-Hamill Turren-Cruz, b Anna Morales-Melgares,a Benedicte Saliba,a DOI: 10.1039/c8se00451j David Tanenbaum, a Michael Saliba, b Shaik Mohammed Zakeeruddin, b Michael Gratzel,b Anders Hagfeldt *c and Monica Lira-Cantu *ac rsc.li/sustainable-energy State-of-the-art halide perovskite solar cells employ semiconductor constituent materials, as well as interface instabilities, is of oxides as electron transport materials. Defects in these oxides, such as crucial importance for commercialization.3 Semiconductor oxygen vacancies (Ovac), act as recombination centres and, in air and oxides (SO) constitute a fundamental part of highly efficient UV light, reduce the stability of the solar cell. Under the same condi- photovoltaic technologies4–11 such as PSCs.11,12 Oxides are tions, the PbZrTiO3 ferroelectric oxide employs Ovac for the creation of applied as dense thin lms or as mesoporous oxide layers, defect-dipoles responsible for photo-carrier separation and current working as hole or as electron transport materials (HTM or transport, evading device degradation. We report the application of ETM, respectively) in normal or inverted PSC congura- PbZrTiO3 as the electron extraction material in triple cation halide tions.10,11,13–15 Electron transport semiconductor oxides, like perovskite solar cells. The application of a bias voltage (poling) up to TiO2, are characterized by an oxygen vacancy (Ovac)-mediated 2 V, under UV light, is a critical step to induce charge transport in the conductivity16 caused by a deviation in stoichiometry, the ferroelectric oxide. Champion cells result in power conversion effi- presence of impurities, or both.17 In oxygen-containing atmo- ciencies of 11% after poling. Stability analysis, carried out at 1-sun AM spheres, and especially under UV light, holes generated at the 1.5 G, including UV light in air for unencapsulated devices, shows nonstoichiometric oxide surface react with the oxygen adsorbed negligible degradation for hours. Our experiments indicate the effect at Ovac increasing charge recombination and degradation of the of ferroelectricity, however alternative conducting mechanisms solar cell.17,18 Different methods have been employed to affected by the accumulation of charges or the migration of ions (or passivate or eliminate these Ovac. For example, the application the combination of them) cannot be ruled out. Our results demon- of organic interfacial modiers with anchoring groups speci- strate, for the first time, the application of a ferroelectric oxide as an cally selected to bond with oxides,3,19 the application of doped electron extraction material in efficient and stable PSCs. These find- oxides19,20 or the application of less reactive SnO2 which results ings are also a step forward in the development of next generation in less hygroscopicity, fewer Ovac at its surface, and less UV- ferroelectric oxide-based electronic and optoelectronic devices. damage.17,18 Another possibility is the application of a coating of secondary oxides, like ZrO2 or Al2O3, applied to suppress surface defects, avoid interfacial recombination, and enhance device Halide perovskite solar cells (PSCs) have emerged as a compet- stability.21–23 More recently, Seok, et al., reported a highly stable itive photovoltaic technology with power conversion efficiencies PSCs by the application of an UV-light resistant oxide, the La- (PCEs) surpassing the 22.7% mark.1 One of the main bottle- doped BaSnO3, applied as electron transport layer.24 A less- necks of the technology is their long-term stability.2 Under- explored option is the application of complex oxides with standing the different degradation mechanisms of the singular properties, such as ferroelectric, magnetic or ferroic oxides. Given their wide bandgap values and insulating prop- a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The erties, their application as electron transport materials (ETM) in Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, emerging solar cells seems unlikely.25 For example, ferroelectric E-08193 Barcelona, Spain. E-mail: monica.lira@icn2.cat oxides (FE-oxides) display spontaneous electric polarization b Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and where the application of an electric eld (poling) reorients Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland domains within individual grains. Owing to this characteristic, c Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and different sources of conduction can be observed, such as Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, polarization switching, breakdown and leakage.26,27 Conduction Switzerland through tunnel junctions28,29 as in TiO2/Al2O3 electrodes,21 or † Electronic supplementary information (ESI) available. See DOI: domain walls30,31 are other forms of current transport usually 10.1039/c8se00451j 382 | Sustainable Energy Fuels, 2019, 3, 382–389 This journal is © The Royal Society of Chemistry 2019 View Article Online Communication Sustainable Energy & Fuels observed in nanometres thick lms. Moreover, their large for the desired ABO3 perovskite oxide phase formation24 which dielectric constant implies the presence of a low exciton limits the extrinsic conductivity of the transparent conducting binding energy which translates in low electron/hole recombi- FTO substrate. We circumvent this problem by the application nation and more defect-tolerant charge carrier transport.32 of a fast (10 min) sintering method for the fabrication of PZT Other ferroelectric materials have been applied in PSCs. For thin lms on transparent FTO.11 The synthesis of the PZT, example ferroelectric polymers, such as PVDF, has been Pb(ZrxTi1x)O3 (x ¼ 0.6) (for exact composition see ESI†), is employed as an additive during the fabrication processes made by solution processing and the solution is deposited on improving the perovskite morphology and the nal device effi- the FTO by spin coating. The minimum annealing temperature ciency.33–41 It has also been applied as an insulating tunnelling at which good ferroelectric properties are retained is found at This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. layer between the halide perovskite and the electron transport 625  C. The ferroelectric perovskite crystalline structure of the Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. layer,42 increasing carrier transfer.43 Nevertheless, the low pho- PZT deposited on FTO was conrmed by X-ray diffraction (XRD) toactivity that FE oxides show under UV light makes them very and scanning electron microscopy (SEM) analysis, as shown in attractive to enhance perovskite solar cell stability. In FE-oxides, Fig. 1 respectively (see also ESI Fig. S1 and S2†). XRD theta- UV light can photoinduce the enhancement of the remnant 2theta (Fig. 1a) and cross-sectional X-TEM mapping (ESI polarization, as recently reported for PbZrTiO3 (PZT) lms.44 Fig. S1†) shows that the PZT thin lm belongs to the symmetry The formation of defect dipoles, responsible for charge sepa- point group N m with space group Pm3 m (221) cubic, where the ration and current transport, depends on the presence of defect lattice constant has been determined to be a ¼ 4.077 Å (ESI charges (Ovac). Thus, instead of acting as photoreactive sites as Fig. S1†). Hysteresis loops demonstrating the ferroelectric in SnO2 or TiO2, Ovac in PZT are removed aer poling under UV character of the PZT lm were carried out to all thin lms. light increasing its transport properties,44 suggesting a different Fig. 3j and k shows the hysteresis loops of the PZT thin lm, working mechanism which can benet the stability of solar synthesized at 625  C, before and aer irradiation with UV light cells. (Fig. 3j) and visible light (k). The analyses were made rst in the Here we report the fabrication and lifetime analysis of PSCs dark and then aer exposed to different light sources: (near- applying a thin lm of Pb[ZrxTi1x]O3 (PZT) ferroelectric oxide ultraviolet light-emitting diode (UVA-LED, 365  10 nm/3.40 as the ETM. The PZT was made as dense thin lm and applied  0.12 eV) and white diode (White-LED, 400–700 nm/1.77– in planar conguration PSCs of the type: FTO/PZT/CsMAFA/ 3.10 eV). For all the other PZT obtained at different tempera- spiro-OMeTAD/Au, where CsMAFA refers to the halide perov- tures see Fig. S2 (under ESI†). On the basis of controlled skite material with formula Cs5(MA0.17FA0.83)(95)Pb(I0.83Br0.17)3 experiments examining the effect of the PZT thin lm thickness, and PZT refers to the nominal composition Pb[Zr0.6Ti0.4]O3. For a series of different PZT thickness were fabricated (see Fig. 1c comparison purposes, reference solar cells were made applying and S4–S8 in ESI†). The screening of the thickness was SnO2 as dense ETM as in FTO/SnO2/CsMAFA/spiro-OMeTAD/ accomplished by diluting the PZT solution in acetic acid (c/co) as Au. We also explored the effect of the PZT layer on the triple described in methods (ESI Fig. S4†). Analysis revealed that PZT cation perovskite under illumination with respect to solar cell lms ranged from thick 100 nm lms (corresponding to an stability. Analysis of the solar cells were made at 1-sun AM 1.5 G, undiluted solution c/co of 1/0) to thin <10 nm lms (c/co of 1/10). including UV light, in air (no encapsulation), at 45  C and 55% Two dissimilar samples were then selected, a thick 100 nm and RH. The solar cells were poled up to 2 V in order to polarize the a thin 10 nm lm (Fig. 1a and b). The nal PZT surface PZT electrode. Poling resulted in a slow but steady improvement morphology, analysed by AFM and SEM (Fig. 1e–h), seems to be of the photovoltaic properties of the PZT-based PSC. The poorly affected by the lm thickness but also likely inuenced improvement was observed during the rst 90 minutes aer by the well-crystallized FTO substrate which has a rutile crystal which the device stabilized and then maintained its photovol- structure with a tetragonal unit cell (P42/mnm) matching the taic properties under continuous illumination in air for hours. perovskite oxide structure. Nevertheless, careful analysis of the Our results demonstrate, for the rst time, the possibility of surface of a series of different PZT thin lms made at different applying the PZT ferroelectric oxide as ETM in air and UV-stable thicknesses showed a slight increase in surface roughness when PSCs. increasing thin lm thickness (ESI Fig. S5†). The surface morphology of the thicker lms (e.g. 100 nm) are free of micro- Results and discussion cracks, pores or holes, whereas some pinholes can be present in thinner thin lms. Growth of crystalline PZT lms In the context of emerging photovoltaics, ferroelectric oxide (FE- oxide) layers with perovskite arrangement are of particular Photovoltaic performance of the PZT-based PSCs interest since they are a class of materials with unique func- Here we fabricate complete PSCs consisting of the FTO/PZT thin tionalities, strong metal-oxide bonds, and high heat of forma- lms, followed by the CsMAFA perovskite of about 600 nm. tion resulting in very stable compounds. A highly crystalline Spiro-OMeTAD was used as the hole transport material and structure enables the ferroelectric properties of the PZT, a thin layer (80 nm) of Au as the back metal electrode. For resulting in performance dependent on grain boundaries and comparison purposes, reference solar cells were also fabricated the ferroelectric domain wall conguration.44 Nevertheless, as previously described45 applying the SnO2 as the ETM. The high temperatures, usually above 600  C, are required in general PZT-based PSC schematics are shown in Fig. 2a. In this This journal is © The Royal Society of Chemistry 2019 Sustainable Energy Fuels, 2019, 3, 382–389 | 383 View Article Online Sustainable Energy & Fuels Communication This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. Fig. 1 Synthesis of Pb[ZrxTi1x]O3 (PZT) thin film on transparent FTO. (a) X-ray diffraction analysis of the PZT thin (10 nm) and thick (100 nm) films on FTO substrates, annealed at 625  C. (b) UV vis analysis of the thick (1) and thin (2) PZT films; (c) representation of the crystal structure of the PZT, and the relation between the PZT thickness and the PZT concentration applied during thin film fabrication; (d) PZT band gap at 3.6 eV; (e–f) AFM surface analysis of the 10 nm thin PZT film; (g and h) surface SEM images (g) and grain boundaries (marked in white, (h)) of the 10 nm thin PZT film. conguration, the PZT takes the place of the electron transport lms29,46,47). For comparison purposes, we selected two PZT material (ETM). The solar cells were analysed at 1-sun AM 1.5 G lms with highly dissimilar thickness: 10 nm (c/co of 1/10) and (including UV light) under ambient conditions without encap- 100 nm (c/co of 1/1). In general, the majority of the as-prepared sulation. We collected statistical data on devices applying devices showed no photovoltaic response or exhibited poor different PZT thickness (ESI Fig. S6 and S7†) where efficiency PCEs with values around 0.2% and 4% for PSCs applying the increases as the thickness of the PZT decreases, with 100 nm and 10 nm PZT lms, respectively, as shown in Fig. 2c a maximum observed at 10 nm thickness as shown in Fig. 2c. and d (see also Table 1, and ESI Table S1†). These results show Below the 10 nm (c/co of 1/10), the photovoltaic performance an evident suppression of the electron injection from the PZT to decreases possibly due to the presence of pinholes or conduc- the FTO electrode, especially for thick PZT lms, highlighting tion via tunnelling (only observable in a-few nanometers thick the high dielectric and insulating nature of PZT. The solar cells Fig. 2 Photovoltaic performance of PSCs of the type: FTO/PZT/CsMAFA/spiro-OMeTAD/Au. (a) Schematic representation of the planar and normal configuration PSC. (b) SEM analysis of a PZT-based PSC. (c) PCE (%) observed at different PZT dilutions or concentrations (c/co). (d) Histograms of the 10 nm thin PZT-based PCSs before and after 90 min of poling. (e) and (f) I–V curves for the 10 nm thin PZT-based PSC before (e) and after (f) 90 min of poling in air. Solar cells were analysed at 1-sun AM 1.5 G, including UV light, in air (no encapsulation), at 45  C and 55% RH. 384 | Sustainable Energy Fuels, 2019, 3, 382–389 This journal is © The Royal Society of Chemistry 2019 View Article Online Communication Sustainable Energy & Fuels Table 1 Photovoltaic parameters of the champion PSC applying 10 nm thin film PZT-based PSC and the reference PSC applying SnO2 before and after poling. Values are given for the forward and reverse scans. Solar cells were analysed at 1-sun AM 1.5 G, including UV light, in air (no encapsulation), at 45  C and 55% RH Before poling Aer poling Cell Scan Jsc (mA cm2) Voc (V) FF (%) PCE (%) Scan Jsc (mA cm2) Voc (V) FF (%) PCE (%) PZT Forward 2.40 0.99 27.0 0.54 Forward 18.26 0.91 43.8 7.31 Reverse 2.28 1.01 31.8 0.64 Reverse 18.51 0.91 65.8 11.16 SnO2 Forward 22.48 1.13 0.78 19.55 Forward 9.36I 1.02 27.3 2.63 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Reverse 22.84 1.12 0.76 19.03 Reverse 2.43 0.94 27.7 0.63 Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. were then subjected to poling up to 2 V under 1 sun AM 1.5 G. P3HT:PCBM active layer even aer the application of voltages as During poling, the photovoltaic properties of the PZT-based high as 40 V.53 In the case of the halide perovskite active layer, solar cells increased gradually until stabilization. This the 10 nm PZT-based PSCs barely sustain the application of improvement was almost negligible for the thick 100 nm PZT 2 V, while the thick 100 nm PZT-based PSC requires stronger based PSC (ESI Fig. S7†) and drastic for the thin 10 nm PZT- poling showing a voltage limit at around 4–5 V. Moreover, the based PSC (Fig. 2c–f). The champion PSCs corresponds to scan rate should also be limited to low speeds, with optimum solar cells with the 10 nm PZT thin lm (Fig. 2c–f) which values below 2 mV s1. Finally, we observed that the solar cell showed a dramatic and steady increase in PCE scaling from an was unresponsive aer several scans in the forward direction initial 1.49% as-prepared, before poling (Fig. 2e), up to 11.16% (i.e. biasing the solar cell from the minimum positive voltage efficiency aer poling (Fig. 2f, Table 1). Fig. 2d depicts the value) during poling, thus poling was carried out under reverse histogram of the PCE for the PSCs using the champion cell of mode only. In summary, the poling was carried out in air at 10 nm thick PZT lm, before and aer poling, displaying under 1 sun AM 1.5 G (including UV light), through subsequent a very reproducible pattern of efficiency increase for all devices. IV-curves up to maximum of +2 V. The scan direction was the Besides the difference in thicknesses, the dissimilarity observed reverse mode (i.e. biasing the solar cell from the maximum on photovoltaic response between the solar cells with the thin positive voltage value) and the scan rate was 2 mV s1. IV curves (10 nm) and the thick (100 nm) PZT lm can be related to the were performed every 5 min until the photovoltaic parameters, surface roughness morphology and grain size of the PZT layer. in particular current density, Jsc, reached a maximum and The polarization-dependent conductivity (ferroelectric proper- stabilized (for details on poling optimization see ESI Fig. S7, S8 ties)48,49 is highly reliant on synthesis conditions, morphology, and S15†). defect concentration and the orientation and size of the oxide Fig. 3a and b shows the effect of electrical poling on the grains.50 Especially the presence of different grain size indicates photovoltaic response of the champion PZT-based PSC (red different intrinsic stress in the lms due to the difference in line) and the comparison with a reference device based on SnO2 grain boundaries, resulting in different dielectric properties.51 (blue line). The PZT-based PSC increased its photovoltaic Thus, the different thickness, surface roughness morphology response from 1.49% up to 11.16% aer poling. This response and grain size of the PZT lms could benet the photovoltaic validates the effective activation of the photovoltaic parameters response aer poling for the PSC with the 10 nm thin PZT through poling for the PZT which evolve slowly until stabiliza- which shows the largest roughness (Ra 26 nm) in comparison tion of the PCE aer 90 minutes. Poling the reference solar cell to the 100 nm thick PZT (Ra  20 nm) (ESI Fig. S2†). (made with SnO2) results in a clear degradation of the photo- voltaic response aer poling (Fig. 3b). Although this observed degradation of the SnO2-based PSC is expected (due to the Electrical poling under 1 sun AM 1.5 G severe testing environment and conditions) it permits to Poling, the process of applying an external electric eld to demonstrate the dissimilar working mechanism between both a ferroelectric material, induces the reorientation of dipole the PZT and the SnO2 ETMs. The poling effect is observed to be moments within individual grains improving the mobility of highly reproducible for a series of analysed samples as shown by charge carriers.52 Poling conditions carried out under 1 sun AM the error bars depicted in Fig. 3c–f (also in Fig. S7†). These 1.5 G, were observed to be strongly affected by the PZT thin lm results demonstrate the crucial importance of poling under thickness, the light harvester material (active layer), and the light irradiation, for the successful transformation of the method at which the electric bias is applied. In our samples, insulating PZT into an electron transport material in our solar progressive IV poling (with increasing initial voltage of 20–50 cells. mV) were observed to result in enhanced photovoltaic perfor- To analyse the evolution of the transport properties of the mance of the FE-oxide based PSC if compared to the application PZT-based PSC under UV light and poling, we carried out of a single high voltage. Thus, careful optimization of all experiments of IPCE (or EQE) against poling time. We then parameters was carried out (see ESI Fig. S2, S8–S11†). We rst compared the Jsc obtained from the IPCE and the one obtained analysed the voltage limit for poling. We have previously from the IV curves at specic times. The poling experiments observed high robust response of solar cells based on the were carried out at 0.1 h, 1.0 h, 2.5 h and 5 h following the same This journal is © The Royal Society of Chemistry 2019 Sustainable Energy Fuels, 2019, 3, 382–389 | 385 View Article Online Sustainable Energy & Fuels Communication This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. Fig. 3 Effect of poling in air and UV light. Evolution of the IV curves during poling for PSCs made with (a) PZT and (b) SnO2. (c–f) Evolution of the photovoltaic parameters of the solar cells with time under ambient atmosphere and UV light (no encapsulation) for the SnO2 – based PSC (blue) and the PZT-based PSC (red). IPCE (g) and normalized IPCE (h) for the champion solar cell; (i) comparison between Jsc obtained from IV curves and from the IPCE spectra at different poling times. Poling times: (1) 0.1 h, (2) 1.0 h, (3) 2.5 h, (4) 5.0 h. Solar cells were analysed at 1-sun AM 1.5 G, including UV light, in air (no encapsulation), at 45  C and 55% RH. Hysteresis loops for the PZT before and after illumination with UV light (j) and visible light (k) and conductance measurements of a PZT thin film in the dark and under 1 sun AM 1.5 G continuous illumination, including UV light, and under air atmosphere (l). poling procedure. Aer each poling period the sample was absorption of the halide perovskite CsMAFA material (Fig. 3h-1 taken out of the sun simulator and immediately analysed by to 4), an indication of charge transport throughout the PSC. We IPCE. The rst observation is that, due to the insulating prop- should also emphasize that the shape of the nal IPCE obtained erties of the PZT, very low charge transport is initially observed aer 5 h poling (Fig. 3g-4 and h-4) corresponds exactly with the (Fig. 3g-1) by increasing the poling time from 0.1 h to 5 h, the shape of the IPCE of a PSC applying the classical SnO2 oxide as conversion of photons to electrons increases from 4% to 60% ETL as shown in ESI Fig. S9†.57 (Fig. 3g-1 and 4). The transformation of the PZT with time and Finally, we compared the response observed for the current the improvement of IPCE (and solar cell efficiency) can also be density (Jsc) obtained from IV curves and IPCE aer poling explained by the shape of the IPCE spectra. Fig. 3h shows the (Fig. 3i). In both cases, the Jsc shows a steadily increase until normalized IPCE spectra shown in Fig. 3g. The initial spectra reaching similar values around 14 mA cm2 aer the poling obtained aer 0.1 h of poling (Fig. 3h-1) revealed a very weak process. We observed however, that for intermediate poling absorption of the PZT oxide at wavelengths below 340 nm, times between (1) 0.1 h, (2) 1.0 h and (3) 2.5 h, the Jsc obtained corresponding with a very poor semiconductor. At this time, no from IPCE is always lower than the Jsc obtained from the IV absorption of the halide perovskite CsMAFA (between 400– curves (Fig. 3i). The main difference between both character- 800 nm) is observed since no charge transport takes place from ization methods is that IV curves are carried out under full the PZT to the halide perovskite. As the poling under UV light illumination under the sun simulator, right aer poling. In the continues, this peak at 340 nm, red shis to 380 nm (from 3.64 case of the IPCE characterization, illumination and poling are to 3.26 eV), a response observed during the reduction of the eliminated in order to situate the sample under the IPCE set up band gap in oxides. This red shi is attributed to oxide doping for analysis. In this short period of time, the removal of UV light observed in classical oxides like TiO2 (ref. 54 and 55) but also and poling in PZT samples brings the material back to its observed in other oxides like PZT.56 It has also been attributed original state and electrostatic repulsion between polarization to the continuous and steady removal of Ovac under UV light charges and defect charges (Ovac) immediate occurs, depola- exposure or inert atmospheres for several binary oxides.6,7 The rizing the PZT lm. Since remnant polarization is dened as the increase of the poling time, from 0.1 h to 1.0 h and 2.5 h, results polarization remaining in the material aer the poling voltage in the gradual improvement of the IPCE absorption between is eliminated or reduced to zero, we attributed the increase in Jsc 400 nm and 800 nm, which is the region that corresponds to the (from IPCE) to the enhancement of the remnant polarization 386 | Sustainable Energy Fuels, 2019, 3, 382–389 This journal is © The Royal Society of Chemistry 2019 View Article Online Communication Sustainable Energy & Fuels when increasing poling time, in perfect agreement with the mechanism reported for PZT by Cao, et al.44 Thus, we infer here that the as-prepared PZT-based PSC with polycrystalline PZT thin lms (as shown in Fig. 1 and ESI Fig. S6†) have domains which are random. When the solar cell is poled under UV light, part of the defect-dipoles located at Ovac, are separated and the charges are driven to opposite interfaces of the lm. This effect is enhanced by UV light irra- diation which permits the excitation of the remaining defect- This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. dipole charges that are expelled to opposite interfaces by the Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. poling eld as reported for PZT.44 The process takes place slowly throughout the rst minutes of the analysis. With time, the PZT behaves as an electron transport material in the solar cell and delivers the best photovoltaic performance. The effect is reversible if the bias voltage and UV light is eliminated but can Fig. 4 Stability of PSCs in air and UV light (unencapsulated devices) become permanent if the poling time increases (as observed in after poling. Stability analysis of a PZT-based PSC (red) and a reference Fig. 3i), in agreement to the enhancement of the remnant PSC applying SnO2 (blue). Analyses were made at 1 sun AM 1.5 G polarization with poling as described for PZT.44 The trans- continuous illumination, including UV light, and under air atmosphere formation of the PZT from an insulator into a conductor is (55% RH, 45  C) without encapsulation. proved by the hysteresis (ferroelectric) loops analysed to the PZT before and aer light irradiation (Fig. 3j and k). Aer analysis under white-LED the hysteresis loop shows a clear ferroelectric tunnelling, conduction through domain walls and the conduc- response. Under UV-LED irradiation the PZT samples presented tion mechanisms related to defects like oxygen vacancies in signicant hysteresis suppression in the hysteresis loops as oxides58–60), there is a possibility that poling and UV light irra- observed in Fig. 3j. This effect, known as polarization diation can also be causing the accumulation of charges or the suppression58 (or ferroelectric fatigue59), is a well-known effect migration of ions (or the combination of all of them) in the in ferroelectric materials60 and it is directly related to the active, transport layer or at interfaces in our devices. conductivity of the perovskites: good ferroelectric properties can only be observed on materials with low conductivity, Stability analysis in air and UV light improving conductivity will inevitably increase the leakage We further studied the stability of PSCs under continuous 1-sun current and suppress ferroelectricity.60 These results are also AM 1.5 G light irradiation (Fig. 4). Aer poling, the PSCs were supported by conductivity measurements carried out to the PZT analysed by consecutive IV curves taken each few minutes in thin lm in the dark and aer light irradiation (Fig. 3g). The reverse mode with a scan rate of 2 mV s1. The I–V scan starting PZT samples analysed in the dark show low conductivity which voltage was established at +1.6 V and the scan nal voltage was increases once the sample is irradiated with light. The effect is 0.1 V. Degradation of halide PSCs under ambient conditions reversible once the PZT thin lm is stored in the dark again. (without encapsulation) is well-documented for devices applying Given that the defect-dipoles are reliant on the multi- electron transport oxides like TiO2 or SnO2.3,11,14 Thus, long-term crystalline nature of the PZT (density of grain boundaries and stability studies are usually carried out under encapsulation domain walls), we infer that the different response of the PSC conditions or inert atmospheres.24 In our stability tests, we want aer poling is attributed to the different PZT thickness (and to demonstrate the effective response of the PZT thin lm under surface roughness) as shown in Fig. 1 (and Fig. S6 ESI†). We also severe conditions: 1 sun continuous light irradiation (including observed that the PZT-based PSCs showed large hysteresis aer UV light), in air, at 45  C and 55% RH and without encapsula- poling and that the hysteresis index (HI) strongly depends on tion. Fig. 4 shows the results of the initial 10 h of the stability the poling time (Fig. 2 and ESI Fig. S14†). Since hysteresis is analysis carried out aer poling. Strikingly, our PZT-based PSC related to factors like applied conditioning voltage, device shows a remarkable stability under air and UV light, while the conguration or UV light, among many others61 we attribute the SnO2-based PSC degrades in minutes. Although one can argue observed hysteresis to the effect of testing conditions (ambient that the degradation of the reference device, the SnO2-based air or the presence of UV light) but especially to the large poling PSC, is most likely to be intensied due to the application of bias voltage applied. Especially, the hysteresis observed from ion voltage (poling), the fact that our PZT-based PSCs sustain and migration in halide perovskites can be strongly affected by the improve its photovoltaic properties demonstrates a different poling procedure. Identifying a specic conduction mechanism working mechanism and an improved stability in air and UV in a device comprising a ferroelectric material can be rather light. complex, especially in a dynamic system under the continuous effect of poling and UV light. Nevertheless, since several Conclusions possible conduction mechanisms can be possible (for example bulk effects including ohmic conduction, charge carrier We introduced the ferroelectric oxide Pb(ZrxTi1x)O3 (x ¼ 0.6), hopping, space charge limited conduction, interface effects, PZT, as the electron extraction layer in triple cation, This journal is © The Royal Society of Chemistry 2019 Sustainable Energy Fuels, 2019, 3, 382–389 | 387 View Article Online Sustainable Energy & Fuels Communication Cs5(MA0.17FA0.83)(95)Pb(I0.83Br0.17)3 (CsMAFA) halide perovskite Agència de Gestió d'Ajuts Universitaris i de Recerca for the solar cells. Poling the solar cells under UV light in air, triggered support to the consolidated Catalonia research group 2017 SGR- the transport properties of the PZT resulting in the enhance- 329 and the Xarxa de Referència en Materials Avançats per ment of the photovoltaic properties of the devices with time. a l'Energia (Xarmae). To the Agencia Estatal de Investigación (AEI) This improvement is directly inuenced by the thickness of the and Fondo Europeo de Desarrollo Regional (FEDER) for the PZT with the best response observed for PZT lms of 10 nm. contract ENE2015-74275-JIN to A. P. To the CERCA Programme/ The champion PZT-based solar cell resulted in 11% PCE aer Generalitat de Catalunya and to the European COST Action Sta- poling under standard 1-sun AM 1.5 G sunlight (including UV bleNextSol project MP1307. To the Fullbright fellowship program light, in air and without encapsulation). The solar cells were for the support to I. S. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. stable in air and UV light for several hours demonstrating the Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. stability of the CsMAFA perovskite. We also demonstrated References a different working mechanism of the PZT in comparison with a classical oxides like SnO2 or TiO2, establishing for the rst 1 W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, time the possibility to apply (and transform) an insulating D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh and ferroelectric oxide to work as electron extraction layer in PSCs. S. I. Seok, Science, 2017, 356, 1376–1379. Our experiments indicate the effect of ferroelectricity, however 2 G. Grancini, C. Roldán-Carmona, I. Zimmermann, alternative conducting mechanisms affected by the accumula- E. Mosconi, X. Lee, D. Martineau, S. Narbey, F. Oswald, tion of charges or the migration of ions (or the combination of F. De Angelis, M. Graetzel and M. K. Nazeeruddin, Nat. them) cannot be ruled out. We expect our ndings to have Commun., 2017, 8, 15684. a major impact in a new generation of ETMs for PSCs that can 3 M. Lira-Cantú, Nat. Energy, 2017, 2, 17115. also affect the long-term stability of emerging photovoltaics, but 4 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, also to open up new possible functionalities of ferroelectric Chem. Rev., 2010, 110, 6595–6663. oxides in a wide range of novel electronic and optoelectronic 5 J. Xia, N. Masaki, M. Lira-Cantu, Y. Kim, K. Jiang and applications. S. Yanagida, J. Am. Chem. Soc., 2008, 130, 1258–1263. 6 M. Lira-Cantu and F. C. Krebs, Sol. Energy Mater. Sol. Cells, Author contributions 2006, 90, 2076–2086. 7 M. Lira-Cantu, K. Norrman, J. W. Andreasen and F. C. Krebs, M. L.-C. wrote the manuscript with feedback from the co- Chem. Mater., 2006, 18, 5684–5690. authors. M. L.-C. and A. H. supervised the study. M. L.-C., 8 G. Teran-Escobar, J. Pampel, J. M. Caicedo and M. Lira- H. X. and A. P conceived and designed the experiments. H. X., Cantu, Energy Environ. Sci., 2013, 6, 3088–3098. Z. W., H.-S. K. and S.-H. T.-C. fabricated and characterized the 9 J. Zhang, B. Xu, L. Yang, A. Mingorance, C. Ruan, Y. Hua, devices. H. X. carried out the conductivity measurements. L. Wang, N. Vlachopoulos, M. Lira-Cantú, G. Boschloo, A. P. and I. S. fabricated and characterized the PZT substrates A. Hagfeldt, L. Sun and E. M. J. Johansson, Adv. Energy and developed and optimized the poling method. A. M. performed Mater., 2017, 7, 1602736. the SEM and AFM analysis. B. S. carried out the UV-vis and catal- 10 A. Perez-Tomas, A. Mingorance, Y. Reyna and M. Lira-Cantu, ysis analyses. D. T., M. S., S. M. Z and M. G. coordinated the work. in The Future of Semiconductor Oxides in Next Generation Solar Cells, ed. M.Lira-Cantu, Elsevier Singapore, 2017, Conflicts of interest p. 566. 11 M. Lira-Cantu, The future of semiconductor oxides in next There are no conicts to declare. generation solar cells, Elsevier Singapore, 2017. 12 M. Saliba, T. Matsui, J. Y. Seo, K. Domanski, J. P. Correa- Acknowledgements Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt and M. Grätzel, Energy Environ. Sci., Authors would like to thank Prof. Gustau Catalán for the hyster- 2016, 9, 1989–1997. esis loops made at his laboratory. To Dr Belen Ballesteros for the 13 A. Mingorance, H. Xie, H.-S. Kim, Z. Wang, M. Balsells, X-TEM images and analysis. M. G. and S. M. Z. thank the Euro- A. Morales-Melgares, N. Domingo, N. Kazuteru, W. Tress, pean Union's Horizon 2020 programme, through a FET Open J. Fraxedas, N. Vlachopoulos, A. Hagfeldt and M. Lira- research and innovation action under grant agreement No. Cantu, Adv. Mater. Interfaces, 2018, 0, 1800367. 687008 and the King Abdulaziz City for Science and Technology 14 Y. Reyna, A. Perez-Tomas, A. Mingorance and M. Lira-Cantu, (KACST) for the nancial support. H.-S. K. is grateful for the in Molecular Devices for Solar Energy Conversion and Storage, postdoctoral fellowship grant (NRF-2016R1A6A3A03012393). M. S. ed. H.Tian, G.Boschloo and A.Hagfeldt, Springer-Verlag acknowledges support from the co-funded Marie Skłodowska Berlin, Berlin, 2018, pp. 477–531. Curie fellowship, H2020 grant agreement no. 665667. To the 15 A. Hagfeldt and M. Lira-Cantu, Appl. Surf. Sci., 2018, In Press. Spanish MINECO through the Severo Ochoa Centers of Excellence 16 M. K. Nowotny, T. Bak and J. Nowotny, J. Phys. Chem. B, 2006, Program under Grant SEV-2013-0295 and for the postdoctoral 110, 16270–16282. contract for H. X.; for the grant ENE2016-79282-C5-2-R and the 17 A. Raccanelli and A. Maddalena, J. Am. Ceram. Soc., 1976, 59, OrgEnergy Excelence Network CTQ2016-81911-REDT. To the 425–430. 388 | Sustainable Energy Fuels, 2019, 3, 382–389 This journal is © The Royal Society of Chemistry 2019 View Article Online Communication Sustainable Energy & Fuels 18 J. W. Erickson and S. Semancik, Surf. Sci., 1987, 187, L658– 39 S. Zhang, Y. Lu, B. Lin, Y. Zhu, K. Zhang, N.-Y. Yuan, L668. J.-N. Ding and B. Fang, Sol. Energy Mater. Sol. Cells, 2017, 19 Y. Hou, X. Du, S. Scheiner, D. P. McMeekin, Z. Wang, N. Li, 170, 178–186. M. S. Killian, H. Chen, M. Richter, I. Levchuk, N. Schrenker, 40 A.-N. Cho and N.-G. Park, ChemSusChem, 2017, 10, 3687– E. Spiecker, T. Stubhan, N. A. Luechinger, A. Hirsch, 3704. P. Schmuki, H.-P. Steinrück, R. H. Fink, M. Halik, 41 C. Sun, Y. Guo, B. Fang, J. Yang, B. Qin, H. Duan, Y. Chen, H. J. Snaith and C. J. Brabec, Science, 2017, 358, 1192–1197. H. Li and H. Liu, J. Phys. Chem. C, 2016, 120, 12980–12988. 20 W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, 42 Q. Wang, Q. Dong, T. Li, A. Gruverman and J. Huang, Adv. E. Bi, I. Ashraful, M. Grätzel and L. Han, Science, 2015, 350, Mater., 2016, 28, 6734–6739. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. 944–948. 43 K. Yang, Z.-Y. Deng and H.-J. Feng, Appl. Phys. Lett., 2017, Open Access Article. Published on 05 December 2018. Downloaded on 6/6/2019 1:56:40 PM. 21 Y. H. Lee, J. Luo, M.-K. Son, P. Gao, K. T. Cho, J. Seo, 111, 143902. S. M. Zakeeruddin, M. Grätzel and M. K. Nazeeruddin, Adv. 44 D. Cao, C. Wang, F. Zheng, L. Fang, W. Dong and M. Shen, Mater., 2016, 28, 3966–3972. J. Mater. Chem., 2012, 22, 12592–12598. 22 E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz and 45 M. Saliba, T. Matsui, K. Domanski, J. Y. Seo, J. R. Durrant, Chem. Commun., 2002, 1464–1465. A. Ummadisingu, S. M. Zakeeruddin, J. P. Correa-Baena, 23 A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, W. R. Tress, A. Abate, A. Hagfeldt and M. Grätzel, Science, J. Chen, Y. Yang, M. Grätzel and H. Han, Science, 2014, 2016, 354, 206–209. 345, 295–298. 46 V. Garcia and M. Bibes, Nat. Commun., 2014, 5, 4289. 24 S. S. Shin, E. J. Yeom, W. S. Yang, S. Hur, M. G. Kim, J. Im, 47 E. Y. Tsymbal and H. Kohlstedt, Science, 2006, 313, 181–183. J. Seo, J. H. Noh and S. I. Seok, Science, 2017, 356, 167–171. 48 S. E. Bernacki, MRS Proceedings, 2011, 243. 25 A. Pérez-Tomás, M. Lira-Cantú and G. Catalan, Adv. Mater., 49 D.-F. Pan, G.-F. Bi, G.-Y. Chen, H. Zhang, J.-M. Liu, 2016, 28, 9644–9647. G.-H. Wang and J.-G. Wan, Sci. Rep., 2016, 6, 22948. 26 L. Pintilie, in Ferroelectrics - Physical Effects, ed. M.Lallart, 50 M. V. Kamenshchikov, A. V. Solnyshkin, A. A. Bogomolov InTech, Rijeka, 2011, p. ch. 05. and I. P. Pronin, Phys. Solid State, 2011, 53, 2080. 27 I. Stolichnov and A. Tagantsev, Ferroelectrics, 1999, 225, 147– 51 L. Lian and N. R. Sottos, J. Appl. Phys., 2000, 87, 3941–3949. 154. 52 S. Y. Yang, J. Seidel, S. J. Byrnes, P. Shafer, C. H. Yang, 28 Z. Wen, C. Li, D. Wu, A. Li and N. Ming, Nat. Mater., 2013, 12, M. D. Rossell, P. Yu, Y. H. Chu, J. F. Scott, J. W. Ager, 617–621. L. W. Martin and R. Ramesh, Nat. Nanotechnol., 2010, 5, 29 E. Y. Tsymbal and A. Gruverman, Nat. Mater., 2013, 12, 602–604. 143–147. 30 T. Sluka, A. K. Tagantsev, P. Bednyakov and N. Setter, Nat. 53 A. Pérez-Tomás, A. Lima, Q. Bilion, I. Shirley, G. Catalan and Commun., 2013, 4, 1808. M. Lira-Cantú, Adv. Funct. Mater., 2018, 28, 1707099. 31 J. Guyonnet, I. Gaponenko, S. Gariglio and P. Paruch, Adv. 54 N. Serpone, J. Phys. Chem. B, 2006, 110, 24287–24293. Mater., 2011, 23, 5377–5382. 55 M. M. Khan, S. A. Ansari, D. Pradhan, M. O. Ansari, 32 S. K. Wallace, K. Svane, W. P. Huhn, T. Zhu, D. B. Mitzi, D. H. Han, J. Lee and M. H. Cho, J. Mater. Chem. A, 2014, V. Blum and A. Walsh, Sustainable Energy Fuels, 2017, 2, 637–644. 1339–1350. 56 S. Samanta, M. Muralidhar, V. Sankaranarayanan, 33 Y. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, K. Sethupathi, M. S. Ramachandra Rao and M. Murakami, A. Gruverman, Y. Yang and J. Huang, Nat. Mater., 2011, 10, 296. J. Mater. Sci., 2017, 52, 13012–13022. 34 B. Yang, Y. Yuan, P. Sharma, S. Poddar, R. Korlacki, 57 M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa- S. Ducharme, A. Gruverman, R. Saraf and J. Huang, Adv. Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, Mater., 2012, 24, 1455–1460. A. Abate, A. Hagfeldtd and M. Gratzel, Energy Environ. Sci., 35 S. Y. Yang, J. Seidel, S. J. Byrnes, P. Shafer, C. H. Yang, 2016, 9, 1989–1997. M. D. Rossell, P. Yu, Y. H. Chu, J. F. Scott, J. W. Ager Iii, 58 W. L. Warren, D. Dimos, B. A. Tuttle, G. E. Pike, L. W. Martin and R. Ramesh, Nat. Nanotechnol., 2010, 5, 143. R. W. Schwartz, P. J. Clews and D. C. McIntyre, J. Appl. 36 J. Huang, J. Yu, Z. Guan and Y. Jiang, Appl. Phys. Lett., 2010, Phys., 1995, 77, 6695–6702. 97, 143301. 59 Y. A. Genenko, J. Glaum, M. J. Hoffmann and K. Albe, 37 K. S. Nalwa, J. A. Carr, R. C. Mahadevapuram, H. K. Kodali, J. Mater. Sci. Eng. B, 2015, 192, 52–82. S. Bose, Y. Chen, J. W. Petrich, B. Ganapathysubramanian 60 Z. Fan, K. Sun and J. Wang, J. Mater. Chem. A, 2015, 3, 18809– and S. Chaudhary, Energy Environ. Sci., 2012, 5, 7042–7049. 18828. 38 K.-S. Shin, T. Y. Kim, G. C. Yoon, M. K. Gupta, S. K. Kim, 61 B. Chen, M. Yang, S. Priya and K. Zhu, J. Phys. Chem. Lett., W. Seung, H. Kim, S. Kim, S. Kim and S.-W. Kim, Adv. 2016, 7, 905–917. Mater., 2014, 26, 5619–5625. This journal is © The Royal Society of Chemistry 2019 Sustainable Energy Fuels, 2019, 3, 382–389 | 389