Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Abstract

•Perovskite surface treatment generates shallow iodine interstitial defects (Ii)•Ii aggravates the instability of FAPbI3 perovskites•Universal strategy to avoid Ii generation and iodine vacancy passivation•Over 90% of initial performance retained after near 1,000 h of illumination Shallow defects, by definition, do not constitute non-radiative recombination centers and are, therefore, mostly benign and of less importance in inorganic covalent semiconductors, such as silicon or gallium arsenide. Perhaps, for this reason, shallow defects have received comparatively less attention from the metal halide perovskite community. In this work, we observed through systematic experimental and computational studies that shallow iodine interstitial defects (Ii) can be unintentionally generated during commonly used surface treatments for state-of-the-art perovskite solar cells (PSCs). Although shallow and, thus, not particularly detrimental to initial device performance, Ii aggravates the phase instability of FAPbI3 perovskites to accelerate its degradation. A universal strategy is proposed to resolve this issue to significantly elongate the operational lifetime of PSCs. Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects (Ii) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of Ii and the more effective passivation of iodine vacancies (VI) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing. Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects (Ii) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of Ii and the more effective passivation of iodine vacancies (VI) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing. The record power conversion efficiencies (PCEs) of single-junction metal halide perovskite solar cells (PSCs) have catapulted to over 25% in just over a decade.1Kim H.S. Lee C.R. Im J.H. Lee K.B. Moehl T. Marchioro A. Moon S.J. Humphry-Baker R. Yum J.H. Moser J.E. et al.Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.Sci. Rep. 2012; 2: 591Crossref PubMed Scopus (5617) Google Scholar, 2Yang W.S. Noh J.H. Jeon N.J. Kim Y.C. Ryu S. Seo J. Seok S.I.l. Solar cells. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange.Science. 2015; 348: 1234-1237Crossref PubMed Scopus (4733) Google Scholar, 3Yang W.S. Park B.W. Jung E.H. Jeon N.J. Kim Y.C. Lee D.U. Shin S.S. Seo J. Kim E.K. Noh J.H. Seok S.Il Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.Science. 2017; 356: 1376-1379Crossref PubMed Scopus (3843) Google Scholar, 4Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photonics. 2019; 13: 460-466Crossref Scopus (1790) Google Scholar, 5Min H. Kim M. Lee S.U. Kim H. Kim G. Choi K. Lee J.H. Seok S.I.l. Efficient, stable solar cells by using inherent bandgap of α-phase formamidinium lead iodide.Science. 2019; 366: 749-753Crossref PubMed Scopus (405) Google Scholar With attainable PCEs now rivaling that of mature photovoltaic technologies based on conventional inorganic semiconductors, the attention of the community has turned to address the notorious instability issues of PSCs. Shallow defects, by definition, do not constitute non-radiative recombination centers because of their low transition energies and are, therefore, considered mostly benign and of less importance in inorganic covalent semiconductors, such as silicon or gallium arsenide. In contrast, due to the characteristic ionic nature of metal halide perovskites, intrinsic point defects are charged,6Yin W.-J. Shi T. Yan Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber.Appl. Phys. Lett. 2014; 104: 1-4Crossref Scopus (1605) Google Scholar, 7Liu N. Yam C. First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.Phys. Chem. Chem. Phys. 2018; 20: 6800-6804Crossref PubMed Google Scholar, 8Walsh A. Scanlon D.O. Chen S. Gong X.G. Wei S.H. Self-regulation mechanism for charged point defects in hybrid halide perovskites.Angew. Chem. Int. Ed. Engl. 2015; 54: 1791-1794Crossref PubMed Scopus (371) Google Scholar and this has significant implications on its defect physics. Particularly, the migration of charged defects in response to a potential gradient is known to seriously degrade the long-term operational stability of PSCs.9Azpiroz J.M. Mosconi E. Bisquert J. De Angelis F. Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation.Energy Environ. Sci. 2015; 8: 2118-2127Crossref Google Scholar, 10Domanski K. Roose B. Matsui T. Saliba M. Turren-Cruz S.H. Correa-Baena J.P. Carmona C.R. Richardson G. Foster J.M. De Angelis F. et al.Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells.Energy Environ. Sci. 2017; 10: 604-613Crossref Google Scholar, 11Lee J.-W. Kim S.-G. Yang J.-M. Yang Y. Park N.-G. Verification and mitigation of ion migration in perovskite solar cells.APL Mater. 2019; 7: 1-12Crossref Scopus (93) Google Scholar, 12Aristidou N. Eames C. Sanchez-Molina I. Bu X. Kosco J. Islam M.S. Haque S.A. Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells.Nat. Commun. 2017; 8: 15218Crossref PubMed Scopus (518) Google Scholar Notably, several shallow defects are theoretically predicted to have low formation and migration activation energies.6Yin W.-J. Shi T. Yan Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber.Appl. Phys. Lett. 2014; 104: 1-4Crossref Scopus (1605) Google Scholar,7Liu N. Yam C. First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.Phys. Chem. Chem. Phys. 2018; 20: 6800-6804Crossref PubMed Google Scholar,9Azpiroz J.M. Mosconi E. Bisquert J. De Angelis F. Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation.Energy Environ. Sci. 2015; 8: 2118-2127Crossref Google Scholar,12Aristidou N. Eames C. Sanchez-Molina I. Bu X. Kosco J. Islam M.S. Haque S.A. Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells.Nat. Commun. 2017; 8: 15218Crossref PubMed Scopus (518) Google Scholar,13Meloni S. Moehl T. Tress W. Franckevic M. Saliba M. Lee Y.H. Gao P. Nazeeruddin M.K. Zakeeruddin S.M. Rothlisberger U. Gratzel M. Ionic polarization-induced current–voltage hysteresis in CH3NH3PbX3 perovskite solar cells.Nat. Commun. 2016; 7: 10334Crossref PubMed Scopus (450) Google Scholar This implies the possible generation of shallow defects during film fabrication and post-treatment processes. Although the initial device photovoltaic performance is mostly unaffected by the generated defects due to their shallow nature, the long-term operational stability of the PSCs can potentially be impacted. Therefore, systematic investigations on the effects of shallow defects are required. In particular, fewer studies have been done on the formamidinium lead triiodide (FAPbI3) perovskite as compared with the prototypical methylammonium lead triiodide (MAPbI3),14Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y. 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, 15Aristidou N. Sanchez-Molina I. Chotchuangchutchaval T. Brown M. Martinez L. Rath T. Haque S.A. The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactiveLayers.Angew. Chem. Int. Ed. Engl. 2015; 54: 8208-8212Crossref PubMed Scopus (559) Google Scholar, 16Conings B. Drijkoningen J. Gauquelin N. Babayigit A. D’Haen J. D’Olieslaeger L. Ethirajan A. Verbeeck J. Manca J. Mosconi E. et al.Intrinsic thermal instability of methylammonium lead trihalide perovskite.Adv. Energy Mater. 2015; 5: 1500477Crossref Scopus (1214) Google Scholar, 17Yuan Y. Wang Q. Shao Y. Lu H. Li T. Gruverman A. Huang J. Electric-field-driven reversible conversion Between methylammonium lead triiodide perovskites and lead iodide at elevated temperatures.Adv. Energy Mater. 2016; 6: 1-7Crossref Scopus (201) Google Scholar whereas the vast majority of high-performance PSCs are based on a FAPbI3 dominant composition.3Yang W.S. Park B.W. Jung E.H. Jeon N.J. Kim Y.C. Lee D.U. Shin S.S. Seo J. Kim E.K. Noh J.H. Seok S.Il Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.Science. 2017; 356: 1376-1379Crossref PubMed Scopus (3843) Google Scholar,4Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photonics. 2019; 13: 460-466Crossref Scopus (1790) Google Scholar This is significant because of the different degradation mechanisms between the two—FAPbI3, for instance, phase degrades from the photoactive cubic α-FAPbI3 perovskite phase to the hexagonal non-perovskite δ-FAPbI3 phase, whereas the MAPbI3 perovskite phase degrades into PbI2.14Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y. 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,16Conings B. Drijkoningen J. Gauquelin N. Babayigit A. D’Haen J. D’Olieslaeger L. Ethirajan A. Verbeeck J. Manca J. Mosconi E. et al.Intrinsic thermal instability of methylammonium lead trihalide perovskite.Adv. Energy Mater. 2015; 5: 1500477Crossref Scopus (1214) Google Scholar,18Lee J.-W. Kim D.-H. Kim H.-S. Seo S.-W. Cho S.M. Park N.-G. Formamidinium and cesium hybridization for photo- and moisture-stable perovskite solar cell.Adv. Energy Mater. 2015; 5: 1-9Crossref PubMed Scopus (978) Google Scholar Point defects disrupt the original lattice arrangement, and thus might affect the thermodynamic energetics of the perovskite,19Kapur S.S. Prasad M. Crocker J.C. Sinno T. Role of configurational entropy in the thermodynamics of clusters of point defects in crystalline solids.Phys. Rev. B. 2005; 72: 1-12Crossref Scopus (45) Google Scholar,20Bulyarskii S.V. Oleinikov V.P. Thermodynamics of defect formation and defect interaction in compound semiconductors.Phys. Stat. Sol. (b). 1988; 146: 439-447Crossref Scopus (18) Google Scholar especially relevant for FAPbI3 because of its metastable nature. Therefore, it is of critical importance to understand the effects of the intrinsic point defects to mitigate the instability issues of FAPbI3-based PSCs. In this study, we observed that shallow iodine interstitial defects (Ii) can be generated unintentionally during commonly used post-fabrication treatments. We show that Ii can lower the cubic-to-hexagonal phase transformation activation energy barrier of FAPbI3-based perovskites to accelerate its degradation. We demonstrate that concurrently avoiding the generation of Ii and the more effective passivation of iodine vacancy defects (VI) improves the thermodynamic phase stability and operational stability of the perovskite films and devices. Consequently, our most stable PSC devices demonstrated a more than 5-fold increased average lifetime over the control devices under continuous illumination operational stability testing. The champion device retained 92.1 % of its initial performance after nearly 1,000 h of testing. Iodine-related shallow defects are theoretically predicted to have relatively low (bulk) formation energies.6Yin W.-J. Shi T. Yan Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber.Appl. Phys. Lett. 2014; 104: 1-4Crossref Scopus (1605) Google Scholar, 7Liu N. Yam C. First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.Phys. Chem. Chem. Phys. 2018; 20: 6800-6804Crossref PubMed Google Scholar, 8Walsh A. Scanlon D.O. Chen S. Gong X.G. Wei S.H. Self-regulation mechanism for charged point defects in hybrid halide perovskites.Angew. Chem. Int. Ed. Engl. 2015; 54: 1791-1794Crossref PubMed Scopus (371) Google Scholar Recently, post-fabrication surface treatment strategies using organic iodides have been reported by many groups. The iodide anion (I−) has been reported to passivate VI defects.21Abdi-Jalebi M. Andaji-Garmaroudi Z. Cacovich S. Stavrakas C. Philippe B. Richter J.M. Alsari M. Booker E.P. Hutter E.M. Pearson A.J. et al.Maximizing and stabilizing luminescence from halide perovskites with potassium passivation.Nature. 2018; 555: 497-501Crossref PubMed Scopus (820) Google Scholar, 22Li N. Tao S. Chen Y. Niu X. Onwudinanti C.K. Hu C. Qiu Z. Xu Z. Zheng G. Wang L. et al.Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells.Nat. Energy. 2019; 4: 408-415Crossref Scopus (371) Google Scholar, 23Wu W.Q. Rudd P.N. Ni Z. Brackle C.H. Van Wei H. Wang Q. Ecker B.R. Gao Y. Huang J. Reducing surface halide deficiency for efficient and stable iodide- based perovskite solar cells.J. Am. Chem. Soc. 2020; 142: 3989-3996Crossref PubMed Scopus (90) Google Scholar However, we speculated that excessive I− coated on the perovskite surface possibly generates shallow iodine-related defects, so we tried to investigate this by mimicking the surface environments while excluding the organic cation. All control samples for this study are based on a FAPbI3 composition with 5 mol % of added MAPbBr3 fabricated by a one-step antisolvent quenching method. The as-fabricated perovskite films were then post-treated with different concentrations of pure iodine (I2) dissolved in isopropanol3Yang W.S. Park B.W. Jung E.H. Jeon N.J. Kim Y.C. Lee D.U. Shin S.S. Seo J. Kim E.K. Noh J.H. Seok S.Il Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.Science. 2017; 356: 1376-1379Crossref PubMed Scopus (3843) Google Scholar (hereafter, concentrations given in terms of an equivalent concentration of atomic iodine, see Supplemental Experimental Procedures for clarification) at 5,000 rpm, followed by annealing at 100°C for 5 min. The concentrations and deposition conditions were set to be comparable with those commonly reported for organic iodide salts (Table S1). The control films were treated with blank (pure) isopropanol and underwent the same annealing. PSC devices of architecture indium tin oxide (ITO)/SnO2/perovskite/spiro-MeOTAD/Au were fabricated without or with surface treatment. We observed that the device photovoltaic parameters, including the open-circuit voltage (VOC), measured in reverse current density-voltage (J-V) scan, changed marginally with surface treatment (Figure S1), while the scan direction-dependent J-V hysteresis (Figures 1A , S1, and S2) of the devices became increasingly more pronounced. This hints at the formation of shallow iodine-related defects, given that defect migration is responsible for the hysteresis behavior in perovskites.10Domanski K. Roose B. Matsui T. Saliba M. Turren-Cruz S.H. Correa-Baena J.P. Carmona C.R. Richardson G. Foster J.M. De Angelis F. et al.Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells.Energy Environ. Sci. 2017; 10: 604-613Crossref Google Scholar,11Lee J.-W. Kim S.-G. Yang J.-M. Yang Y. Park N.-G. Verification and mitigation of ion migration in perovskite solar cells.APL Mater. 2019; 7: 1-12Crossref Scopus (93) Google Scholar,24Tress W. Marinova N. Moehl T. Zakeeruddin S.M. Nazeeruddin M.K. Grätzel M. Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field.Energy Environ. Sci. 2015; 8: 995-1004Crossref Google Scholar Particularly, on the longer timescales of the stabilized power output (SPO) measurements (insets of Figure 1A), where defects have sufficient time to migrate and screen the applied bias,10Domanski K. Roose B. Matsui T. Saliba M. Turren-Cruz S.H. Correa-Baena J.P. Carmona C.R. Richardson G. Foster J.M. De Angelis F. et al.Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells.Energy Environ. Sci. 2017; 10: 604-613Crossref Google Scholar,25Eames C. Frost J.M. Barnes P.R.F. O’Regan B.C. Walsh A. Islam M.S. Ionic transport in hybrid lead iodide perovskite solar cells.Nat. Commun. 2015; 6: 7497Crossref PubMed Scopus (1418) Google Scholar the SPO is seen to stabilize slower and decays to a lower value with increased surface treatment concentration. First-principles density functional theory (DFT) calculations were performed to identify the probable defect species. The surface formation energies (Figure 1B) of iodine-related neutral point defects were computed for FAPbI3 grown in stoichiometric conditions for a FAI terminated surface (most thermodynamically stable, see Figure S3 and Supplemental Experimental Procedures for details) and compared with the bulk formation energies reproduced from a previous report.7Liu N. Yam C. First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.Phys. Chem. Chem. Phys. 2018; 20: 6800-6804Crossref PubMed Google Scholar Figure 1B shows that the bulk formation energies of VI, Ii, FA-I antisite defects (FAI), and I-FA antisite defects (IFA) are approximately comparable at ~1 eV. The trend, however, changes dramatically at the surface. The calculated formation energies increased from their bulk values for VI (1.16 to 2.71 eV), FAI(1.27 to 4.56 eV), and IFA (1.37 to 2.78 eV) but decreased for Ii (1.13 to 0.97 eV). The Fermi-level (EF) dependence of the formation energies at the surface was further computed for Iiq, where q denotes the defect charge state (Figure S4). Negatively charged Ii− has the lowest formation energy across the band gap among the different Iiq charged states. Near the valence band maximum, the formation energies for Ii0 and Ii− are comparable and relatively low at ~1 eV. The position of the (q/q') transitions (transition levels) in relation to the band gap determines the susceptibility of the defect to trap or release charge carriers, i.e., the depth of the (shallow or deep) trap state. The (0/−1) transition occurs 0.03 eV above the valence band edge, whereas the (+1/0) and (+1/−1) transitions occur within the valence band, indicating that Ii is a shallow hole trap with low formation energy. The theoretical results suggest that Ii, predominantly in its Ii− charged state (because of its relatively lower formation energy) is the likely generated species. Additionally, Ii− has a low migration activation energy,9Azpiroz J.M. Mosconi E. Bisquert J. De Angelis F. Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation.Energy Environ. Sci. 2015; 8: 2118-2127Crossref Google Scholar,13Meloni S. Moehl T. Tress W. Franckevic M. Saliba M. Lee Y.H. Gao P. Nazeeruddin M.K. Zakeeruddin S.M. Rothlisberger U. Gratzel M. Ionic polarization-induced current–voltage hysteresis in CH3NH3PbX3 perovskite solar cells.Nat. Commun. 2016; 7: 10334Crossref PubMed Scopus (450) Google Scholar,25Eames C. Frost J.M. Barnes P.R.F. O’Regan B.C. Walsh A. Islam M.S. Ionic transport in hybrid lead iodide perovskite solar cells.Nat. Commun. 2015; 6: 7497Crossref PubMed Scopus (1418) Google Scholar and is, thus, a plausible candidate to induce the J-V hysteresis. We further note that additional concurrent phenomena reported for MAPbI3 might perhaps also explain the benign nature of Ii−, including kinetic deactivation due to a fast hole trapping and/or de-trapping process.26Angelis F. De Petrozza A. Clues from defect photochemistry.Nat. Mater. 2018; 17: 377-384Crossref Scopus (29) Google Scholar We further probed the perovskite films by using positron annihilation spectroscopy (PAS) to investigate the existence of the defects. A basic schematic of the measurement mechanism is shown in Figure S5 (with details in the Experimental Procedures and Supplemental Experimental Procedures). Positively charged positrons (antiparticle to the electron) are implanted from the perovskite surface and annihilate with electrons from a free lattice site or after trapping at negatively charged or neutral (but not positive) vacancies and/or interstitial defects to emit two gamma photons.27Barthe M.-F. Labrim H. Gentils A. Desgardin P. Corbel C. Esnouf S. Piron J.P. Positron annihilation characteristics in UO2: for lattice and vacancy defects induced by electron irradiation.Phys. Status Solidi (c). 2007; 4: 3627-3632Crossref Scopus (19) Google Scholar, 28Wiktor J. Jomard G. Torrent M. Bertolus M. First-principles calculations of momentum distributions of annihilating electron–positron pairs in defects in UO2.J. Phys. Condens. Matter. 2017; 29035503Crossref PubMed Scopus (8) Google Scholar, 29Xue J. Wang R. Yang Y. The surface of halide perovskites from nano to bulk.Nat. Rev. Mater. 2020; https://doi.org/10.1038/s41578-020-0221-1Crossref Scopus (59) Google Scholar The shape (S) parameter is then extracted from the doppler broadening of the gamma spectrum at each implantation depth. The S parameter increases with the increasing density of defect sites.27Barthe M.-F. Labrim H. Gentils A. Desgardin P. Corbel C. Esnouf S. Piron J.P. Positron annihilation characteristics in UO2: for lattice and vacancy defects induced by electron irradiation.Phys. Status Solidi (c). 2007; 4: 3627-3632Crossref Scopus (19) Google Scholar,28Wiktor J. Jomard G. Torrent M. Bertolus M. First-principles calculations of momentum distributions of annihilating electron–positron pairs in defects in UO2.J. Phys. Condens. Matter. 2017; 29035503Crossref PubMed Scopus (8) Google Scholar Here, we varied the implantation depth of the positron by changing its kinetic energy (Ek) to investigate the depth-dependent defect density. The results show that the S parameter of the films near the top surface (mean depth < 10 nm, shaded in yellow in Figure 1C) is higher with increasing surface treatment concentration, indicating a higher density of negatively charged or neutral defects at the surface region. Notably, an inverse uptick (Figure 1C) right at the top surface (mean depth < 0.2 nm) is observed for the treated films but not the control film. The selective enhancement in defect density and, thus, S parameter at the top surface (Ek < 0.8 keV, mean depth < 0.24 nm) compared with the bulk region (0.8 keV < Ek < 6 keV, 6.73 nm < mean depth < 170 nm) is more obviously seen in Figure 1D. Together with the theoretical and device results, the selectivity of PAS to detect only negatively charged or neutral defects supports the generation of Ii−, i.e., I−occupying an interstitial site. I3−occupying an interstitial site is equivalent to the Ii+point defect,26Angelis F. De Petrozza A. Clues from defect photochemistry.Nat. Mater. 2018; 17: 377-384Crossref Scopus (29) Google Scholar,30Meggiolaro D. Motti S.G. Mosconi E. Barker A.J. Ball J. Andrea Riccardo Perini C.A.R. Deschler F. Petrozza A. Angelis F. D.e. Iodine chemistry determines the defect tolerance of lead-halide perovskites.Energy Environ. Sci. 2018; 11: 702-713Crossref Google Scholar given that Ii+ consists of one central I+ bounded by two I− (coordinating with Pb) on both sides,30Meggiolaro D. Motti S.G. Mosconi E. Barker A.J. Ball J. Andrea Riccardo Perini C.A.R. Deschler F. Petrozza A. Angelis F. D.e. Iodine chemistry determines the defect tolerance of lead-halide perovskites.Energy Environ. Sci. 2018; 11: 702-713Crossref Google Scholar i.e., the trimer structure of I3−. The dripping solution consists of I3− in dynamic equilibrium:3Yang W.S. Park B.W. Jung E.H. Jeon N.J. Kim Y.C. Lee D.U. Shin S.S. Seo J. Kim E.K. Noh J.H. Seok S.Il Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.Science. 2017; 356: 1376-1379Crossref PubMed Scopus (3843) Google ScholarI2+I−↔I3− We, thus, speculate that the I− can possibly be generated from (1) the evaporation of I2 upon post-treatment annealing shifts the dynamic equilibrium toward the reactants to form I− according to Le Chatelier`s principle, and/or (2) the thermal decomposition of I3− into I− from the annealing.31Awtrey A.D. Connick R.E. The absorption spectra of I2, I3-, I-, I03-, S406- and S203=. heat of the reaction I3- = I2 + I-.J. Am. Chem. Soc. 1951; 73: 1842-1843Crossref Scopus (329) Google Scholar Further PAS measurements on samples surface-treated without annealing (Figure S6) support this, in that I− must have been generated from some back reaction, and that I3− (Ii+) cannot be detected by positrons. We can further exclude any post-treatment induced damage (discussed below). Regardless of the source of I−, the results suggest that surface treatment with organic iodide salts can unintentionally generate shallow Ii− defects. Although the generated Ii seem to not act as non-radiative electronic traps, we speculated that the intrinsic stability of the perovskite might be affected.,32Wang S. Jiang Y. Juarez-Perez E.J. Ono L.K. Qi Y. Accelerated degradation of methylammonium lead iodide perovskites induced by exposure to iodine vapour.Nat. Energy. 2016; 2: 16195Crossref Scopus (321) Google Scholar, 33Fu F. Pisoni S. Jeangros Q. Sastre-Pellicer J. Kawecki M. Paracchino A. Moser T. Werner J. Andres C. Duchêne L. et al.I2 vapor-induced degradation of formamidinium lead iodide based perovskite solar cells under heat–light soaking conditions.Energy Environ. Sci. 2019; 12: 3074-3088Crossref Google Scholar Specifically, it was shown that excess I2 added to MAPbI3 accelerated its degradation into PbI2 because of an intrinsic degradation pathway whereby the excess I− in the perovskite undergoes an autocatalytic chain reaction with self-generated I2 vapor.32Wang S. Jiang Y. Juarez-Perez E.J. Ono L.K. Qi Y. Accelerated degradation of methylammonium lead iodide perovskites induced by exposure to iodine vapour.Nat. Energy. 2016; 2: 16195Crossref Scopus (321) Google Scholar Similar degradation by I2 vapor was also observed for FAPbI3. I2 vapor inevitably and readily self-forms from within the perovskite,32Wang S. Jiang Y. Juarez-Perez E.J. Ono L.K. Qi Y. Accelerated degradation of methylammonium lead iodide perovskites induced by exposure to iodine vapour.Nat. Energy. 2016; 2: 16195Crossref Scopus (321) Google Scholar, 33Fu F. Pisoni S. Jeangros Q. Sastre-Pellicer J. Kawecki M. Paracchino A. Moser T. Werner J. Andres C. Duchêne L. et al.I2 vapor-induced degradation of formamidinium lead iodide based perovskite solar cells under heat–light soaking conditions.Energy Environ. Sci. 2019; 12: 3074-3088Crossref Google Scholar during PSC operation due to the effect of illumination,14Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y. 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,33Fu F. Pisoni S. Jeangros Q. Sastre-Pellicer J. Kawecki M. Paracchino A. Moser T. Werner J. Andres C. Duchêne L. et al.I2 vapor-induced degradation of formamidinium lead iodide based perovskite solar cells under heat–light soaking conditions.Energy Environ. Sci. 2019; 12: 3074-3088Crossref Google Scholar oxygen,15Aristidou N. Sanchez-Molina I. Chotchuangchutchaval T. Brown M. Martinez L. Rath T. Haque S.A. The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactiveLayers.Angew. Chem. Int. Ed. Engl. 2015; 54: 8208-8212Crossref PubMed Scopus (559) Google Scholar heat,14Juarez-Perez E.J. Ono L.K. Maeda M. Jiang Y. Hawash Z. Qi Y. 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,16Conings B. Drijkoningen J. Gauquelin N. Babayigit A. D’Haen J. D’Olieslaeger L. Ethirajan A. Verbeeck J. Manca J. Mosconi E. et al.Intrinsic thermal instability of met