Perovskite solar cells (PSCs) are commonly fabricated from precursors containing excess lead iodide (PbI2), and unreacted PbI2 crystals remaining in the film were shown by Tumen-Ulzii et al. (Adv. Mater. 2020) to photodecompose under continuous illumination into metallic lead (Pb0) and iodine, accelerating device degradation. That work explicitly left three coupled mechanistic questions open: (i) the physical origin of the strongly enhanced, reversible current-voltage (J-V) hysteresis and burn-in observed specifically in PbI2-rich devices, and its connection to ion migration; (ii) the spatial distribution and formation pathway of Pb0 within complete device stacks, which surface-sensitive X-ray photoelectron spectroscopy (XPS) could not detect on exposed film surfaces, leading the authors to hypothesize formation near the buried SnO2/perovskite interface; and (iii) the reason for divergent, composition-dependent effects of excess precursor constituents across perovskite chemistries. Later work (2019-2026) shows substantial but partial progress: nanoscale microscopy has localized metallic-lead conversion to defective phase impurities in films of the same composition class, and light-induced reversible defects arising from PbI2 residue have been characterized, but no work establishes the microscopic mechanism linking PbI2 domains to enhanced hysteresis and mobile-ion dynamics, nor provides depth-resolved quantification of Pb0 in operating complete devices. The problem remains open.

Background

Halide perovskite solar cells (PSCs) are thin-film photovoltaic devices based on ABX3 semiconductors (A = organic or inorganic cation, B = Pb, X = halide) that reach power conversion efficiencies (PCEs) above 25% but whose operational stability under continuous illumination limits commercialization. The highest-performing 'triple-cation' compositions, often denoted CsFAMA, are mixed-cation mixed-halide perovskites of approximate formula Cs0.05(FA1−xMAx)0.95Pb(I1−yBry)3\mathrm{Cs}_{0.05}(\mathrm{FA}_{1-x}\mathrm{MA}_{x})_{0.95}\mathrm{Pb}(\mathrm{I}_{1-y}\mathrm{Br}_{y})_{3}, combining cesium (Cs), formamidinium (FA) and methylammonium (MA) cations. A widespread empirical practice is to use a precursor solution containing excess lead iodide (PbI2\mathrm{PbI}_{2}) relative to the stoichiometric ratio: small amounts of residual PbI2\mathrm{PbI}_{2} were reported to passivate defects and improve efficiency, e.g. by reducing the halide-vacancy concentration (Park et al. 2018), while other studies found excess PbI2\mathrm{PbI}_{2} harmful, making it a 'double-edged sword' whose effect depends on amount, location, and processing (Jacobsson et al. 2016).

Key physical concepts are: (1) J-V hysteresis — the dependence of the measured current density-voltage curve on voltage scan direction and rate, generally attributed to drift of mobile ionic species (iodide vacancies/interstitials) that screen the internal electric field and alter interfacial charge extraction; (2) burn-in — the rapid early-stage loss of PCE during the first hours of illumination, which in some devices is partially reversible after dark rest, consistent with mobile-ion redistribution rather than permanent chemical damage; (3) photolysis of PbI2\mathrm{PbI}_{2} — the light-driven decomposition PbI2→Pb0(s)+I2(g)\mathrm{PbI}_{2} \rightarrow \mathrm{Pb}^{0}(\mathrm{s}) + \mathrm{I}_{2}(\mathrm{g}), producing metallic lead that acts as a non-radiative recombination center; and (4) device geometry — planar PSCs stack a transparent electrode of indium tin oxide (ITO), an electron-transport layer of tin oxide (SnO2\mathrm{SnO}_{2}), the perovskite absorber, a hole-transport layer of the organic semiconductor Spiro-OMeTAD (which extracts the photo-generated holes), and a metal electrode (Au), so chemical analysis of 'buried' interfaces requires destructive depth-profiling or cross-sectional methods.

Tumen-Ulzii et al. (Adv. Mater. 2020) compared CsFAMA devices made from precursors with 1.12, 1.15, and 1.18 m PbI2\mathrm{PbI}_{2}: only the highest concentration produced unreacted PbI2\mathrm{PbI}_{2} crystals (detected by scanning electron microscopy and by the X-ray diffraction peak at 2θ=12.5∘2\theta = 12.5^{\circ}), and those devices retained only 47% of initial PCE after 520 h of continuous 1-sun illumination (standard simulated sunlight, 100 mW cm−2\mathrm{mW\,cm^{-2}}), versus 92-99% for the others. They traced the loss to photolysis of the unreacted PbI2\mathrm{PbI}_{2} — the PbI2\mathrm{PbI}_{2} diffraction peak disappeared, a cubic Pb0 peak appeared near 2θ≈31∘2\theta \approx 31^{\circ}, and Pb enrichment was confirmed by energy-dispersive spectroscopy — and to Pb0 acting as a non-radiative recombination center (reduced steady-state photoluminescence). The same detrimental trend was reproduced in MAPbI3\mathrm{MAPbI}_{3} cells (methylammonium lead triiodide, the archetypal single-cation perovskite) with excess PbI2\mathrm{PbI}_{2}. Within this settled picture, the authors acknowledged that the microscopic physics underneath three observations was not understood, and these form the present problem.

Problem Statement

Establish the microscopic mechanisms by which unreacted PbI2 governs the operation and degradation of PbI2-containing perovskite solar cells, as left open by Tumen-Ulzii et al. (Adv. Mater. 2020). The single common objective is a predictive, mechanism-level account of PbI2-mediated device degradation, with three interlinked deliverables, each anchored in an observation of the source work: (1) Identify and quantitatively test the physical origin of the markedly enhanced, reversible J-V hysteresis and initial burn-in of devices containing unreacted PbI2 — in particular, determine whether PbI2 domains alter one or several of the mobile-ion population, ion immobilization or migration kinetics, and interfacial energetics, quantify the contribution of each factor that is altered, and connect these changes to the reversible character of the early-stage loss. (2) Determine the spatial distribution and formation pathway of metallic lead (Pb0) inside complete device stacks, given that surface-sensitive XPS did not detect Pb0 on exposed film surfaces and the buried SnO2/perovskite interface was only hypothesized as the formation site: provide direct spatially resolved chemical identification and quantification of Pb0 production under realistic continuous-illumination operating conditions, taking care to distinguish genuine Pb0 from XPS-analysis artifacts. (3) Explain, mechanistically, why the effects of excess precursor constituents diverge across perovskite chemistries — for instance why excess organic cations have been reported to improve some MAPbI3 devices whereas they reduced PCE in the CsFAMA system — so that precursor stoichiometry can be chosen predictively rather than empirically. Answers that only suppress the symptoms (e.g. additive engineering that removes excess PbI2 or passivates interfaces) without identifying the underlying mechanisms do not resolve this problem.

References

  1. Tumen-Ulzii, Ganbaatar; Qin, Chuanjiang; Klotz, Dino; Leyden, Matthew R.; Wang, Pangpang; Auffray, Morgan; Fujihara, Takashi; Matsushima, Toshinori; Lee, Jin-Wook; Lee, Sung-Joon; Yang, Yang; Adachi, Chihaya. 'Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells.' Advanced Materials (2020). DOI: 10.1002/adma.201905035, https://doi.org/10.1002/adma.201905035
  2. Holovský, Jakub; Peter Amalathas, Amalraj; Landová, Lucie; Dzurňák, Branislav; Conrad, Brianna; Ledinský, Martin; Hájková, Zdeňka; Pop-Georgievski, Ognen; Svoboda, Jan; Yang, Terry Chien-Jen; Jeangros, Quentin. 'Lead Halide Residue as a Source of Light-Induced Reversible Defects in Hybrid Perovskite Layers and Solar Cells.' ACS Energy Letters (2019). DOI: 10.1021/acsenergylett.9b02080, https://doi.org/10.1021/acsenergylett.9b02080
  3. Macpherson, Stuart; Doherty, Tiarnan A. S.; Winchester, Andrew J.; Kosar, Sofiia; Johnstone, Duncan N.; Chiang, Yu-Hsien; Galkowski, Krzysztof; Anaya, Miguel; Frohna, Kyle; Iqbal, Affan N.; Nagane, Satyawan; Roose, Bart; Andaji-Garmaroudi, Zahra; Orr, Kieran W. P.; Parker, Julia E.; Midgley, Paul A.; Dani, Keshav M.; Stranks, Samuel D. 'Local nanoscale phase impurities are degradation sites in halide perovskites.' Nature (2022). DOI: 10.1038/s41586-022-04872-1, https://doi.org/10.1038/s41586-022-04872-1 (preprint arXiv:2107.09549, posted under the title 'Local Nanoscale Defective Phase Impurities Are the Sites of Degradation in Halide Perovskite Devices', https://arxiv.org/abs/2107.09549)
  4. Haque, Farjana; Hoang, Nhu Thi To; Ji, Jeoungmin; Mativenga, Mallory. 'Effect of Precursor Composition on Ion Migration in Hybrid Perovskite CH3NH3PbI3.' IEEE Electron Device Letters (2019). DOI: 10.1109/LED.2019.2939363, https://doi.org/10.1109/LED.2019.2939363
  5. Jacobsson, T. Jesper; Correa-Baena, Juan-Pablo; Halvani Anaraki, Elham; Philippe, Bertrand; Stranks, Samuel D.; Bouduban, Marine E. F.; Tress, Wolfgang; Schenk, Kurt; Teuscher, Joël; Moser, Jacques-E.; Rensmo, Håkan; Hagfeldt, Anders. 'Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells.' Journal of the American Chemical Society (2016). DOI: 10.1021/jacs.6b06320, https://doi.org/10.1021/jacs.6b06320
  6. Park, Byung-wook; Kedem, Nir; Kulbak, Michael; Lee, Do Yoon; Yang, Woon Seok; Jeon, Nam Joong; Seo, Jangwon; Kim, Geonhwa; Kim, Ki Jeong; Shin, Tae Joo; Hodes, Gary; Cahen, David; Seok, Sang Il. 'Understanding how excess lead iodide precursor improves halide perovskite solar cell performance.' Nature Communications (2018). DOI: 10.1038/s41467-018-05583-w, https://doi.org/10.1038/s41467-018-05583-w
  7. Hu, Zhelu; An, Qingzhi; Xiang, Hengyang; Aigouy, Lionel; Sun, Baoquan; Vaynzof, Yana; Chen, Zhuoying. 'Enhancing the Efficiency and Stability of Triple-Cation Perovskite Solar Cells by Eliminating Excess PbI2 from the Perovskite/Hole Transport Layer Interface.' ACS Applied Materials & Interfaces (2020). DOI: 10.1021/acsami.0c17258, https://doi.org/10.1021/acsami.0c17258
  8. Chen, Li; Chen, Jingde; Wang, Chenyue; Ren, Hao; Luo, Yu-Xin; Shen, Kong-Chao; Li, Yanqing; Song, Fei; Gao, Xingyu; Tang, Jian-Xin. 'High-Light-Tolerance PbI2 Boosting the Stability and Efficiency of Perovskite Solar Cells.' ACS Applied Materials & Interfaces (2021). DOI: 10.1021/acsami.1c02929, https://doi.org/10.1021/acsami.1c02929
  9. Ray, Aniruddha; Martín-García, Beatriz; Prato, Mirko; Moliterni, Anna; Bordignon, Simone; Spirito, Davide; Marras, Sergio; Goldoni, Luca; Boopathi, Karunakara Moorthy; Moro, Fabrizio; Casati, Nicola Pietro Maria; Giacobbe, Carlotta; Saidaminov, Makhsud I.; Giannini, Cinzia; Chierotti, Michele R.; Krahne, Roman; Manna, Liberato; Abdelhady, Ahmed L. 'Mixed Organic Cations Promote Ambient Light-Induced Formation of Metallic Lead in Lead Halide Perovskite Crystals.' ACS Applied Materials & Interfaces (2023). DOI: 10.1021/acsami.3c03366, https://doi.org/10.1021/acsami.3c03366
  10. McGettrick, James D.; Hooper, Katherine; Pockett, Adam; Baker, Jenny; Troughton, Joel; Carnie, Matthew; Watson, Trystan. 'Sources of Pb(0) artefacts during XPS analysis of lead halide perovskites.' Materials Letters (2019). DOI: 10.1016/j.matlet.2019.04.081, https://doi.org/10.1016/j.matlet.2019.04.081
  11. Gao, You; Ren, Fumeng; Sun, Derun; Li, Sibo; Zheng, Guanhaojie; Wang, Jianan; Raza, Hasan; Chen, Rui; Wang, Haixin; Liu, Sanwan; Yu, Peng; Meng, Xin; He, Jizhou; Zhou, Jing; Hu, Xiaodong; Zhang, Zhengping; Qiu, Longbin; Chen, Wei; Liu, Zonghao. 'Elimination of unstable residual lead iodide near the buried interface for the stability improvement of perovskite solar cells.' Energy & Environmental Science (2023). DOI: 10.1039/d3ee00293d, https://doi.org/10.1039/d3ee00293d
  12. Kosar, Sofiia; Pininti, Anil R.; Hnapovskyi, Vladyslav; Jurado, José P.; Mannar, Subhashri; Mardegan, Lorenzo; Subbiah, Anand S.; Laquai, Frédéric; De Wolf, Stefaan. 'High-Throughput Imaging of Degradation-Inducing Microscopic Impurities in Perovskite Solar Cells.' arXiv preprint (2026). arXiv:2609.35510, https://arxiv.org/abs/2609.35510