Mitigation of Vacuum and Illumination-Induced Degradation in Perovskite Solar Cells by Structure Engineering

•Vacuum substantially decreases operational lifetime of perovskite solar cells•Vacuum accelerates absorber degradation and ion migration in perovskite solar cells•Mitigation of vacuum-induced degradation pathways is realized•Perovskite solar cells show a projected T80 lifetime of 4,750 h Organic-inorganic hybrid perovskite (OHP) solar cells have attracted wide attention for space applications since 2017. However, OHP slowly decomposes by outgassing under vacuum as encountered in space, raising concern for its long-term stability. Here, we show that OHP solar cells (ITO/SnO2/perovskite/Spiro-MeOTAD/Au) degrade ten times faster upon reducing the operating pressure from 900 to 50 mbar. Vacuum accelerates illumination-induced OHP decomposition, accompanied by outgassing and defect formation, which further accelerates ion migration (Li+, Au, I−, and Br−) across the device. We propose an OHP solar cell structure, i.e., ITO/PTAA/OHP/PCBM/ZnO/AZO/(Ni/Al grid), that can effectively minimize outgassing of OHP and avoid accumulation of the mobile ions at charge transporting layers and related interfaces, resulting in a 588-fold operational stability improvement. This work opens new avenues for engineering OHP solar cells against the hard vacuum in the space environment. High specific power, high stowed packing efficiency, low processing cost, and high tolerance against environmental threats (high energy and charged particle radiation) make perovskite solar cell (PSC) a promising candidate for power generation in space. However, vacuum, as encountered in space, causes perovskite outgassing, raising concern for its long-term stability. In this work, we find that PSCs (ITO/SnO2/perovskite/Spiro-MeOTAD/Au) degrade ten times faster upon reducing the pressure from 9 × 104 to 5 × 103 Pa during operation, due to acceleration of the perovskite transformation and ion migration. Gas permeability of the layers atop perovskite and mobile ion-induced chemical reactions at charge transporting layers and related interfaces are two critical factors. We develop a PSC structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates vacuum and illumination-induced degradation pathways, enabling PSCs to realize a low PCE loss rate of 0.007%/h over 1,037 h at the maximum power point under 100 mW cm−2 illumination at 5 × 103 Pa. High specific power, high stowed packing efficiency, low processing cost, and high tolerance against environmental threats (high energy and charged particle radiation) make perovskite solar cell (PSC) a promising candidate for power generation in space. However, vacuum, as encountered in space, causes perovskite outgassing, raising concern for its long-term stability. In this work, we find that PSCs (ITO/SnO2/perovskite/Spiro-MeOTAD/Au) degrade ten times faster upon reducing the pressure from 9 × 104 to 5 × 103 Pa during operation, due to acceleration of the perovskite transformation and ion migration. Gas permeability of the layers atop perovskite and mobile ion-induced chemical reactions at charge transporting layers and related interfaces are two critical factors. We develop a PSC structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates vacuum and illumination-induced degradation pathways, enabling PSCs to realize a low PCE loss rate of 0.007%/h over 1,037 h at the maximum power point under 100 mW cm−2 illumination at 5 × 103 Pa. Satellites, spacecraft, and the international space station operating in the inner solar system rely on the use of photovoltaic (PV) solar cells to derive electricity from sunlight.1National Aeronautics and Space AdministrationInternational Space Station solar arrays (NASA, 2020).https://www.nasa.gov/mission_pages/station/structure/elements/solar_arrays.htmlDate: 2017Google Scholar Among the various available PV technologies, crystalline-silicon (c-Si), gallium arsenide (GaAs)-based single- and multi-junction solar cells are the only PV technologies deployed for space applications due to their excellent solar-to-electrical energy conversion efficiency2NRELBest research-cell efficiency chart.https://www.nrel.gov/pv/cell-efficiency.htmlDate: 2020Google Scholar and long-term operational stability in the harsh space environment.3Bailey S. Raffaelle R. Space solar cells and arrays.in: Luque A. Hegedus S. Handbook of Photovoltaic Science and Engineering. 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Space Environmental Effects, International Space Station (ISS) Researcher’s Guide, NASA. NASA ISS Program Science Office, 2017Google Scholar Overall, these results indicate that PSCs have a promising potential for space applications. On the other hand, OHP is a structurally soft material compared with c-Si and GaAs and decomposes by outgassing under vacuum as encountered in space. The release of organic gas components, such as CH3NH2, HI, CH3I, NH3, and I2 from the CH3NH3PbI3 (MAPbI3) powder,27Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y.B. Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability.J. Mater. Chem. A. 2018; 6: 9604-9612Crossref Google Scholar and HN=CHNH2, HI, HCN, and NH3 from CH(NH2)2PbI3 (FAPbI3) powder28Juarez-Perez E.J. Ono L.K. Qi Y.B. Thermal degradation of formamidinium based lead halide perovskites into sym-triazine and hydrogen cyanide observed by coupled thermogravimetry-mass spectrometry analysis.J. Mater. Chem. A. 2019; 7: 16912-16919Crossref Google Scholar were observed under both dark and light conditions at 10−4 Pa. Illumination accelerates the outgassing rates. These organic gases can contaminate line-of-sight surfaces and affect optical properties of vehicle and payload surfaces and spacecraft performance.16Finckenor M.M. Groh K.K. Space Environmental Effects, International Space Station (ISS) Researcher’s Guide, NASA. NASA ISS Program Science Office, 2017Google Scholar Encapsulation of the device can slow down the outgassing22Miyazawa Y. Ikegami M. Chen H.W. Ohshima T. Imaizumi M. Hirose K. Miyasaka T. Tolerance of perovskite solar cell to high-energy particle irradiations in space environment.iScience. 2018; 2: 148-155Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar but do not completely eliminate it.16Finckenor M.M. Groh K.K. Space Environmental Effects, International Space Station (ISS) Researcher’s Guide, NASA. NASA ISS Program Science Office, 2017Google Scholar Encouragingly, decomposition and reformation of OHP are partially reversible at temperature relevant to solar cell application (Equations 1 and 2).27Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y.B. Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability.J. Mater. Chem. A. 2018; 6: 9604-9612Crossref Google Scholar,28Juarez-Perez E.J. Ono L.K. Qi Y.B. Thermal degradation of formamidinium based lead halide perovskites into sym-triazine and hydrogen cyanide observed by coupled thermogravimetry-mass spectrometry analysis.J. Mater. Chem. A. 2019; 7: 16912-16919Crossref Google ScholarMAPbI3(s) ↔ PbI2(s) + Pb0(s) + I2(g) + CH3NH2(g) + HI(g)(Equation 1) FAPbI3(s) ↔ PbI2(s) + Pb0(s) + I2(g) + HN = CHNH2(g) + HI(g) + HCN(g) + NH3(g)(Equation 2) When the OHP film is sandwiched in a solar cell, the reverse reaction is kinetically favorable and degradation of OHP is retarded.27Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y.B. Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability.J. Mater. Chem. A. 2018; 6: 9604-9612Crossref Google Scholar Vacuum stability of the OHP also depends on the preparation methods. The MAPbI3 film prepared with the dimethyl formamide (DMF)-based precursor solution, when stored in dark and vacuum, degrades to PbI2, which further dissociates into metallic lead (Pb0) and I2− ions. While the film prepared from DMF and dimethyl sulfoxide (DMSO) precursor shows negligible degradation in the same conditions.29Gunasekaran R.K. Chinnadurai D. Selvaraj A.R. Rajendiran R. Senthil K. Prabakar K. Revealing the self-degradation mechanisms in methylammonium lead iodide perovskites in dark and vacuum.Chem. Phys. Chem. 2018; 19: 1507-1513Crossref Scopus (42) Google Scholar In addition, degradation rate of the triple-cation perovskite film is much lower than MAPbI3 under the same vacuum and illumination condition because of its higher thermal stability.30Yang J. Hong Q. Yuan Z. Xu R. Guo X. Xiong S. Liu X. Braun S. Li Y. Tang J. et al.Unraveling photostability of mixed cation perovskite films in extreme environment.Adv. Opt. 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Cells. 2018; 182: 121-127Crossref Scopus (109) Google Scholar Here in this work, we give a clear argument based on statistical results that PSCs with the widely investigated planar structure, i.e., indium tin oxide (ITO)/SnO2/OHP/Spiro-MeOTAD/Au show a 10-fold faster degradation rate upon reducing the pressure from 9 × 104 to 5 × 103 Pa during operation at the maximum power point (MPP) condition under 1-sun illumination. By applying several independent experimental characterization techniques, we show that low pressure accelerates OHP decomposition, phase-segregation, and transition starting from the OHP grain boundary, accompanied by outgassing and defect formation. These defects further accelerate ion migration (Li+, Au, I−, and Br−) across the device. In addition, we propose a straightforward explanation on vacuum and illumination-induced solar cell degradation that relies on gas permeability of the layers atop the OHP layer and mobile ion-induced chemical reactions at charge transporting layer and related interfaces. Furthermore, we develop two other device structures, with one structure only delaying the gas permeability and the other structure minimizing both of the two detrimental effects. Device degradation rates slow down by a 9-fold and a 588-fold factor, respectively. In particular, the non-encapsulated devices with a structure of ITO/poly(triaryl amine) (PTAA)/OHP/phenyl-C61-butyric acid methyl ester (PCBM)/ZnO/aluminum-doped zinc oxide (AZO)/(Ni/Al grid) show a PCE loss rate of 0.007%/h at the MPP condition over 1,037 h at 5 × 103 Pa, equaling to a projected average T80 lifetime (the time when PCE decays to 80% of the initial value) of 4,750 h. We studied the effect of low pressure on operational stability of PSCs with the widely investigated planar structure (ITO/SnO2/OHP/Spiro-MeOTAD/Au, designated as device structure A) (Figure 1A; Note S1).33Jiang Q. Chu Z. Wang P. Yang X. Liu H. Wang Y. Yin Z. Wu J. Zhang X. You J. Planar-structure perovskite solar cells with efficiency beyond 21%.Adv. Mater. 2017; 29: 1703852Crossref Scopus (847) Google Scholar The triple-cation perovskite with composition of (Cs0.1FA0.9PbI3)0.97(MAPbBr3)0.03 was prepared using a DMF and DMSO mixed precursor solution, which showed high vacuum stability regarding to its composition and processing method.29Gunasekaran R.K. Chinnadurai D. Selvaraj A.R. Rajendiran R. Senthil K. Prabakar K. Revealing the self-degradation mechanisms in methylammonium lead iodide perovskites in dark and vacuum.Chem. Phys. Chem. 2018; 19: 1507-1513Crossref Scopus (42) Google Scholar,30Yang J. Hong Q. Yuan Z. Xu R. Guo X. Xiong S. Liu X. Braun S. Li Y. Tang J. et al.Unraveling photostability of mixed cation perovskite films in extreme environment.Adv. Opt. Mater. 2018; 6: 1800262Crossref Scopus (43) Google Scholar A small amount of KI was added in the OHP to reduce the J-V hysteresis.34Son D.Y. Kim S.G. Seo J.Y. Lee S.H. Shin H. Lee D. Park N.G. Universal approach toward hysteresis-free perovskite solar cell via defect engineering.J. Am. Chem. Soc. 2018; 140: 1358-1364Crossref PubMed Scopus (571) Google Scholar Band gap of the perovskite is 1.58 eV (Figure S1). The PCEs of these devices are between 17% and 18% with very small J-V hysteresis, and the steady-state efficiencies are over 17% (Figures 1B and S2; Tables S1 and S2). Short-circuit current density (Jsc) of the best PSC is 22.5 mA/cm2 from J-V measurement, agrees well with the value (22.4 mA/cm2) obtained from integration of the external quantum efficiency (EQE) spectrum (Figure S3). Three batches of PSCs were measured at the solar cell MPP tracking conditions under 100 mW/cm2 illumination intensity from a white LED lamp at three pressure conditions, i.e., 9 × 104, 2.2 × 104, and 5 × 103 Pa. All the devices are non-encapsulated in order to maximize effects of the low pressure (Note S2). At 9 × 104 Pa, PSCs degrade to 80% of the initial efficiencies (T80 lifetime) in an average of 109 ± 5 h (Figures 1C and 1G), close to the value obtained at the atmospheric condition (1 × 105 Pa).35Yang D. Yang R. Wang K. Wu C. Zhu X. Feng J. Ren X. Fang G. Priya S. Liu S.F. High efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO2.Nat. Commun. 2018; 9: 3239Crossref PubMed Scopus (848) Google Scholar Efficiency loss is due to decrease in fill factor (FF) (0.133%/h), open-circuit voltage (Voc) (0.058%/h) and Jsc (0.019%/h) (Figures 1D–1F and S4; Table S3, loss rates are shown in the relative values). Among all the possible PCE loss mechanisms (recombination losses, optical losses, series resistive losses, etc.), recombination losses, either at interface or bulk, plays a great role as it has strong influence on the Voc and FF.36Sherkar T.S. Momblona C. Gil-Escrig L. Ávila J. Sessolo M. Bolink H.J. Koster L.J.A. Recombination in perovskite solar cells: significance of grain boundaries, interface traps, and defect ions.ACS Energy Lett. 2017; 2: 1214-1222Crossref PubMed Scopus (649) Google Scholar,37Jiang Y. Yu B.B. Liu J. Li Z.H. Sun J.K. Zhong X.H. Hu J.S. Song W.G. Wan L.J. Boosting the open circuit voltage and fill factor of QDSSCs using hierarchically assembled [email protected]2S nanowire array counter electrodes.Nano Lett. 2015; 15: 3088-3095Crossref PubMed Scopus (82) Google Scholar When the operating pressure is decreased to 2.2 × 104 Pa, the average T80 lifetime drops to 30 h (Figures 1C and 1G). FF losses remain the main contributor to the overall PCE losses that increase considerably to 0.558%/h. In addition, the Jsc losses weigh eight times higher than the one measured at 9 × 104 Pa (Table S3), suggesting the existence of the optical losses, substantial recombination losses (e.g., reduction in carrier diffusion length) or series resistive losses. The T80 lifetime decreases dramatically to 8.5 h when the operating pressure is at 5 × 103 Pa (Figures 1C and 1G). All the PV parameters decrease at faster rates. Losses in FF and Jsc are the dominant factors (Table S3). In addition, the T80 lifetime exponentially decreases as a function of operational pressure (Figure 1G), meaning that vacuum accelerates the failure of the PSCs with the widely investigated planar structure. The result also implies that vacuum can be considered as an external stress parameter for development of new accelerated stability testing protocols specifically for PSCs. Solar cell spatial inhomogeneities were characterized by using electroluminescence (EL) on a freshly prepared PSC (designated as sample I) and three aged PSCs that were kept at MPP conditions at 9 × 104 Pa for 450 h (sample II), 2.2 × 104 Pa for 140 h (sample III), and 5 × 103 Pa for 140 h (sample IV), respectively. EL intensity stabilized in 60 s when a constant current of 2 mA was applied (Figure 1H). In order to compare the EL spatial inhomogeneity, we adjusted the exposure time for each sample to achieve a similar total brightness (Figure S5; Note S3). Samples I and II show similar and high spatial homogeneity, reflected by the narrow gray value distributions (Figures 1H and S6). On the other hand, samples III and IV show less spatial homogeneity, with existence of dark region (low gray value region) and bright region (saturated gray value region) both in large fractions. It is clear that vacuum leads to device degradation spatially at different rates on the macroscopic scale. The fast and inhomogeneous device degradation is due to the synergistic effects of illumination and low pressure (Figures S7A and S7B; Note S4). Effect of illumination i.e., in terms of photothermal effect, strain from photostriction or thermal expansion, photoexcited charge carriers, and electric field have been well documented.38Chen B. Song J. Dai X. Liu Y. Rudd P.N. Hong X. Huang J. Synergistic effect of elevated device temperature and excess charge carriers on the rapid light-induced degradation of perovskite solar cells.Adv. Mater. 2019; 31: e1902413Crossref PubMed Scopus (64) Google Scholar But the role of vacuum on illumination-induced device degradation is not clear, which is necessary to lay the foundation for advancement of PSC against the high vacuum space environment. The PSC contains a stack of several layers. A transformation in any layer may result in device failure. For example, Yang et al. showed that the triple-cation perovskite bare absorber forms pinholes under vacuum and illumination.30Yang J. Hong Q. Yuan Z. Xu R. Guo X. Xiong S. Liu X. Braun S. Li Y. Tang J. et al.Unraveling photostability of mixed cation perovskite films in extreme environment.Adv. Opt. Mater. 2018; 6: 1800262Crossref Scopus (43) Google Scholar Jena et al. suggested that large voids appear in the Spiro-MeOTAD layer by releasing gaseous components when PSCs are heated at a temperature above 60°C.39Jena A.K. Numata Y. Ikegami M. Miyasaka T. Role of spiro-OMeTAD in performance deterioration of perovskite solar cells at high temperature and reuse of the perovskite films to avoid Pb-waste.J. Mater. Chem. A. 2018; 6: 2219-2230Crossref Google Scholar To investigate effects of vacuum during device operation from a morphology point of view, we carried out scanning electron microscopy (SEM) measurements on samples I, II, III, and IV. Morphology changes on the aged samples originate from the OHP layer, suggesting that the OHP layer is at least one of the bottlenecks at such a condition (Figures 2A–2D ). In addition, higher vacuum accelerates the decomposition of OHP film, in the same trend of device degradation. Specifically, after MPP 450 h at 9 × 104 pa sample II shows similar stack structure as sample I (Figures 2A and 2B). In sharp contrast, regions with a different contrast form between the Spiro-MeOTAD layer and the OHP layer for sample III where the OHP starts to decompose (Figures 2C, S8A, and S8B).40Sanehira E.M. Tremolet de Villers B.J. Schulz P. Reese M.O. Ferrere S. Zhu K. Lin L.Y. Berry J.J. Luther J.M. Influence of electrode interfaces on the stability of perovskite solar cells: reduced degradation using MoOx/Al for hole collection.ACS E