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Improved reverse bias stability in p–i–n perovskite solar cells with optimized hole transport materials and less reactive electrodes

Abstract

As perovskite photovoltaics stride towards commercialization, reverse bias degradation in shaded cells that must current match illuminated cells is a serious challenge. Previous research has emphasized the role of iodide and silver oxidation, and the role of hole tunnelling from the electron-transport layer into the perovskite to enable the flow of current under reverse bias in causing degradation. Here we show that device architecture engineering has a significant impact on the reverse bias behaviour of perovskite solar cells. By implementing both a ~35-nm-thick conjugated polymer hole transport layer and a more electrochemically stable back electrode, we demonstrate average breakdown voltages exceeding −15 V, comparable to those of silicon cells. Our strategy for increasing the breakdown voltage reduces the number of bypass diodes needed to protect a solar module that is partially shaded, which has been proven to be an effective strategy for silicon solar panels.

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Fig. 1: Reverse bias stability study in an archetypal p–i–n-structured perovskite solar cell.
Fig. 2: The effect of different HTLs on Vrb.
Fig. 3: The effect of electrochemically stable Au electrode on Vrb.
Fig. 4: Schematic of perovskite solar cell degradation mechanisms under reverse bias.

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Data availability

All data are available in the main texts and its Supplementary Information. The raw data supporting Figs. 13 are publicly available via figshare at https://doi.org/10.6084/m9.figshare.24069768 (ref. 83). Individual JV parameters behind the datasets in Supplementary Figs. 3, 9c, 10b, 13, 14b, 15, 16b, 17b, 3133, 42 and 44 and Supplementary Table 1 have also been uploaded as Supplementary Data files. Source data are provided with this paper.

References

  1. Best Research-Cell Efficiency Chart (NREL, 2024); https://www.nrel.gov/pv/cell-efficiency.html

  2. Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643–647 (2012).

    Article  Google Scholar 

  3. Jeon, N. J. et al. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 13, 897–903 (2014).

    Article  Google Scholar 

  4. Yang, W. S. et al. Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells. Science 356, 1376–1379 (2017).

    Article  Google Scholar 

  5. Lu, H. et al. Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells. Science 370, eabb8985 (2020).

    Article  Google Scholar 

  6. Jeon, N. J. et al. Compositional engineering of perovskite materials for high-performance solar cells. Nature 517, 476–480 (2015).

    Article  Google Scholar 

  7. Yang, W. S. et al. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 348, 1234–1237 (2015).

    Article  Google Scholar 

  8. Li, X. et al. A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells. Science 353, 58–62 (2016).

    Article  Google Scholar 

  9. Zhou, H. et al. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546 (2014).

    Article  Google Scholar 

  10. Jeong, J. et al. Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells. Nature 592, 381–385 (2021).

    Article  Google Scholar 

  11. Li, N. et al. Liquid medium annealing for fabricating durable perovskite solar cells with improved reproducibility. Science 373, 561–567 (2021).

    Article  Google Scholar 

  12. Khenkin, M. V. et al. Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures. Nat. Energy 5, 35–49 (2020).

    Article  Google Scholar 

  13. Li, C. et al. Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells. Science 379, 690–694 (2023).

    Article  Google Scholar 

  14. Azmi, R. et al. Damp heat–stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions. Science 376, 73–77 (2022).

    Article  Google Scholar 

  15. PACT—Perovskite PV Accelerator for Commercializing Technologies. Sandia National Laboratories https://pvpact.sandia.gov/ (2024).

  16. Lan, D. & Green, M. A. Combatting temperature and reverse-bias challenges facing perovskite solar cells. Joule 6, 1782–1797 (2022).

    Article  Google Scholar 

  17. Wang, C. et al. Perovskite solar cells in the shadow: understanding the mechanism of reverse-bias behavior toward suppressed reverse-bias breakdown and reverse-bias induced degradation. Adv. Energy Mater. 13, 2203596 (2023).

    Article  Google Scholar 

  18. Boyd, C. C., Cheacharoen, R., Leijtens, T. & McGehee, M. D. Understanding degradation mechanisms and improving stability of perovskite photovoltaics. Chem. Rev. 119, 3418–3451 (2019).

    Article  Google Scholar 

  19. Razera, R. A. Z. et al. Instability of p–i–n perovskite solar cells under reverse bias. J. Mater. Chem. A 8, 242–250 (2020).

    Article  Google Scholar 

  20. Bowring, A. R., Bertoluzzi, L., O’Regan, B. C. & McGehee, M. D. Reverse bias behavior of halide perovskite solar cells. Adv. Energy Mater. 8, 1702365 (2018).

    Article  Google Scholar 

  21. Wolf, E. J., Gould, I. E., Bliss, L. B., Berry, J. J. & McGehee, M. D. Designing modules to prevent reverse bias degradation in perovskite solar cells when partial shading occurs. Sol. RRL 6, 2100239 (2022).

    Article  Google Scholar 

  22. Bogachuk, D. et al. Perovskite photovoltaic devices with carbon-based electrodes withstanding reverse-bias voltages up to –9 V and surpassing IEC 61215:2016 international standard. Sol. RRL 6, 2100527 (2022).

    Article  Google Scholar 

  23. Ni, Z. et al. Evolution of defects during the degradation of metal halide perovskite solar cells under reverse bias and illumination. Nat. Energy 7, 65–73 (2021).

    Article  Google Scholar 

  24. Najafi, L. et al. Reverse-bias and temperature behaviors of perovskite solar cells at extended voltage range. ACS Appl. Energy Mater. 5, 1378–1384 (2022).

    Article  Google Scholar 

  25. Li, W. et al. Sparkling hot spots in perovskite solar cells under reverse bias. ChemPhysMater 1, 71–76 (2022).

    Article  Google Scholar 

  26. Qian, J. et al. Destructive reverse bias pinning in perovskite/silicon tandem solar modules caused by perovskite hysteresis under dynamic shading. Sustain. Energy Fuels 4, 4067–4075 (2020).

    Article  Google Scholar 

  27. Xu, Z. et al. Reverse-bias resilience of monolithic perovskite/silicon tandem solar cells. Joule 7, 1992–2002 (2023).

    Article  Google Scholar 

  28. Wang, C. et al. Abnormal dynamic reverse bias behavior and variable reverse breakdown voltage of ETL-free perovskite solar cells. Sol. RRL 7, 2300456 (2023).

    Article  Google Scholar 

  29. Vieira, R., De Araújo, F., Dhimish, M. & Guerra, M. A comprehensive review on bypass diode application on photovoltaic modules. Energies 13, 2472 (2020).

    Article  Google Scholar 

  30. Green, M. A., Gauja, E. & Withayachamnankul, W. Silicon solar cells with integral bypass diodes. Sol. Cells 3, 233–244 (1981).

    Article  Google Scholar 

  31. Klasen, N., Lux, F., Weber, J., Roessler, T. & Kraft, A. A comprehensive study of module layouts for silicon solar cells under partial shading. IEEE J. Photovolt. 12, 546–556 (2022).

    Article  Google Scholar 

  32. Ma, Y. et al. Suppressing ion migration across perovskite grain boundaries by polymer additives. Adv. Funct. Mater. 31, 2006802 (2021).

    Article  Google Scholar 

  33. Jeangros, Q. et al. In situ TEM analysis of organic–inorganic metal-halide perovskite solar cells under electrical bias. Nano Lett. 16, 7013–7018 (2016).

    Article  Google Scholar 

  34. Kim, D. et al. Light- and bias-induced structural variations in metal halide perovskites. Nat. Commun. 10, 444 (2019).

    Article  Google Scholar 

  35. Bertoluzzi, L. et al. Incorporating electrochemical halide oxidation into drift-diffusion models to explain performance losses in perovskite solar cells under prolonged reverse bias. Adv. Energy Mater. 11, 2002614 (2021).

    Article  Google Scholar 

  36. Breitenstein, O. et al. Understanding junction breakdown in multicrystalline solar cells. J. Appl. Phys. 109, 071101 (2011).

    Article  Google Scholar 

  37. Xu, Z. et al. Halogen redox shuttle explains voltage-induced halide redistribution in mixed-halide perovskite devices. ACS Energy Lett. 8, 513–520 (2023).

    Article  Google Scholar 

  38. Xu, J. et al. Triple-halide wide–band gap perovskites with suppressed phase segregation for efficient tandems. Science 367, 1097–1104 (2020).

    Article  Google Scholar 

  39. Eperon, G. E., Hörantner, M. T. & Snaith, H. J. Metal halide perovskite tandem and multiple-junction photovoltaics. Nat. Rev. Chem. 1, 0095 (2017).

    Article  Google Scholar 

  40. Fu, F. et al. Monolithic perovskite-silicon tandem solar cells: from the lab to fab? Adv. Mater. 34, 2106540 (2022).

    Article  Google Scholar 

  41. Al-Ashouri, A. et al. Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction. Science 370, 1300–1309 (2020).

    Article  Google Scholar 

  42. Liu, J. et al. Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgFx. Science 377, 302–306 (2022).

    Article  Google Scholar 

  43. Aydin, E. et al. Enhanced optoelectronic coupling for perovskite/silicon tandem solar cells. Nature 623, 732–738 (2023).

    Article  Google Scholar 

  44. Chin, X. Y. et al. Interface passivation for 31.25%-efficient perovskite/silicon tandem solar cells. Science 381, 59–63 (2023).

    Article  Google Scholar 

  45. Duan, L. et al. Stability challenges for the commercialization of perovskite–silicon tandem solar cells. Nat. Rev. Mater. 8, 261–281 (2023).

    Article  Google Scholar 

  46. Mariotti, S. et al. Interface engineering for high-performance, triple-halide perovskite–silicon tandem solar cells. Science 381, 63–69 (2023).

    Article  Google Scholar 

  47. Jiang, Q. et al. Surface reaction for efficient and stable inverted perovskite solar cells. Nature 611, 278–283 (2022).

    Article  Google Scholar 

  48. Li, L. et al. Flexible all-perovskite tandem solar cells approaching 25% efficiency with molecule-bridged hole-selective contact. Nat. Energy 7, 708–717 (2022).

    Article  Google Scholar 

  49. Shi, Y. et al. (3-Aminopropyl)trimethoxysilane surface passivation improves perovskite solar cell performance by reducing surface recombination velocity. ACS Energy Lett. 7, 4081–4088 (2022).

    Article  Google Scholar 

  50. Taddei, M. et al. Ethylenediamine addition improves performance and suppresses phase instabilities in mixed-halide perovskites. ACS Energy Lett. 7, 4265–4273 (2022).

    Article  Google Scholar 

  51. Bertoluzzi, L. et al. Mobile ion concentration measurement and open-access band diagram simulation platform for halide perovskite solar cells. Joule 4, 109–127 (2020).

    Article  Google Scholar 

  52. Jariwala, S. et al. Reducing surface recombination velocity of methylammonium-free mixed-cation mixed-halide perovskites via surface passivation. Chem. Mater. 33, 5035–5044 (2021).

    Article  Google Scholar 

  53. Pothoof, J., Westbrook, R. J. E., Giridharagopal, R., Breshears, M. D. & Ginger, D. S. Surface passivation suppresses local ion motion in halide perovskites. J. Phys. Chem. Lett. 14, 6092–6098 (2023).

    Article  Google Scholar 

  54. Akrami, F., Jiang, F., Giridharagopal, R. & Ginger, D. S. Kinetic suppression of photoinduced halide migration in wide bandgap perovskites via surface passivation. J. Phys. Chem. Lett. 14, 9310–9315 (2023).

    Article  Google Scholar 

  55. Guo, H. et al. Immobilizing surface halide in perovskite solar cells via calix[4]pyrrole. Adv. Mater. 35, 2301871 (2023).

    Article  Google Scholar 

  56. Boehm, A. M., Liu, T., Park, S. M., Abtahi, A. & Graham, K. R. Influence of surface ligands on energetics at FASnI3/C60 interfaces and their impact on photovoltaic performance. ACS Appl. Mater. Interfaces 12, 5209–5218 (2020).

    Article  Google Scholar 

  57. Al Kurdi, K. et al. A naphthalene diimide side-chain polymer as an electron-extraction layer for stable perovskite solar cells. Mater. Chem. Front. 5, 450–457 (2021).

    Article  Google Scholar 

  58. Birkhold, S. T. et al. Interplay of mobile ions and injected carriers creates recombination centers in metal halide perovskites under bias. ACS Energy Lett. 3, 1279–1286 (2018).

    Article  Google Scholar 

  59. Phung, N. et al. Enhanced self-assembled monolayer surface coverage by ALD NiO in p-i-n perovskite solar cells. ACS Appl. Mater. Interfaces 14, 2166–2176 (2022).

    Article  Google Scholar 

  60. Paniagua, S. A. et al. Phosphonic acids for interfacial engineering of transparent conductive oxides. Chem. Rev. 116, 7117–7158 (2016).

    Article  Google Scholar 

  61. Jiang, Y., Liu, T. & Zhou, Y. Recent advances of synthesis, properties, film fabrication methods, modifications of poly(3,4-ethylenedioxythiophene), and applications in solution-processed photovoltaics. Adv. Funct. Mater. 30, 2006213 (2020).

    Article  Google Scholar 

  62. Tsai, H. et al. Addressing the voltage induced instability problem of perovskite semiconductor detectors. ACS Energy Lett. 7, 3871–3879 (2022).

    Article  Google Scholar 

  63. Li, R. et al. Layered perovskites enhanced perovskite photodiodes. J. Phys. Chem. Lett. 12, 1726–1733 (2021).

    Article  Google Scholar 

  64. Henzel, J. et al. Impact of the current on reverse bias degradation of perovskite solar cells. ACS Appl. Energy Mater. 6, 11429–11432 (2023).

    Article  Google Scholar 

  65. Bard, A. J. Standard Potentials in Aqueous Solution (Routledge, 2017).

    Book  Google Scholar 

  66. Xu, Z. et al. Origins of photoluminescence instabilities at halide perovskite/organic hole transport layer interfaces. J. Am. Chem. Soc. 145, 11846–11858 (2023).

    Article  Google Scholar 

  67. Park, S.-J. et al. Enhancement of light extraction efficiency of OLEDs using Si3N4-based optical scattering layer. Opt. Express 22, 12392 (2014).

    Article  Google Scholar 

  68. Sohn, S., Kim, S., Shim, J. W., Jung, S. K. & Jung, S. Printed organic light-emitting diodes on fabric with roll-to-roll sputtered ITO anode and poly(vinyl alcohol) planarization layer. ACS Appl. Mater. Interfaces 13, 28521–28528 (2021).

    Article  Google Scholar 

  69. Donie, Y. J. et al. Planarized and compact light scattering layers based on disordered titania nanopillars for light extraction in organic light emitting diodes. Adv. Opt. Mater. 9, 2001610 (2021).

    Article  Google Scholar 

  70. Dykstra, E. et al. OLEDs on planarized light outcoupling-enhancing structures in plastic. Org. Electron. 111, 106648 (2022).

    Article  Google Scholar 

  71. Guo, F. et al. The fabrication of color-tunable organic light-emitting diode displays via solution processing. Light. Sci. Appl. 6, e17094 (2017).

    Article  Google Scholar 

  72. Song, J., Lee, H., Jeong, E. G., Choi, K. C. & Yoo, S. Organic light-emitting diodes: pushing toward the limits and beyond. Adv. Mater. 32, 1907539 (2020).

    Article  Google Scholar 

  73. Liang, J. et al. Origins and influences of metallic lead in perovskite solar cells. Joule 6, 816–833 (2022).

    Article  Google Scholar 

  74. Hu, J., Kerner, R. A., Pelczer, I., Rand, B. P. & Schwartz, J. Organoammonium-ion-based perovskites can degrade to Pb0 via amine–Pb(II) coordination. ACS Energy Lett. 6, 2262–2267 (2021).

    Article  Google Scholar 

  75. Lin, W.-C. et al. In situ XPS investigation of the X-ray-triggered decomposition of perovskites in ultrahigh vacuum condition. npj Mater. Degrad. 5, 13 (2021).

    Article  Google Scholar 

  76. Bitton, S. & Tessler, N. Perovskite ionics—elucidating degradation mechanisms in perovskite solar cells via device modelling and iodine chemistry. Energy Environ. Sci. 16, 2621–2628 (2023).

    Article  Google Scholar 

  77. Galatopoulos, F., Papadas, I. T., Armatas, G. S. & Choulis, S. A. Long thermal stability of inverted perovskite photovoltaics incorporating fullerene‐based diffusion blocking layer. Adv. Mater. Interfaces 5, 1800280 (2018).

    Article  Google Scholar 

  78. Li, M. et al. Interface modification by ionic liquid: a promising candidate for indoor light harvesting and stability improvement of planar perovskite solar cells. Adv. Energy Mater. 8, 1801509 (2018).

    Article  Google Scholar 

  79. Arlinghaus, H. F. in Surface and Thin Film Analysis (eds Friedbacher, G. & Bubert, H.) 115–139 (Wiley, 2011).

  80. Gao, Y., Marie, Y., Saldi, F. & Migeon, H. N. Secondary ion mass spectrometry—SIMS IX. In Proc. of the 9th International Conference on Secondary Ion Mass Spectrometry (eds Benninghoven, A. et al.) 382–385 (Wiley, 1993).

  81. Marseilhan, D., Barnes, J. P., Fillot, F., Hartmann, J. M. & Holliger, P. Quantification of SiGe layer composition using MCs+ and MCs2+ secondary ions in ToF-SIMS and magnetic SIMS. Appl. Surf. Sci. 255, 1412–1414 (2008).

    Article  Google Scholar 

  82. Holliger, P., Laugier, F. & Dupuy, J. C. SIMS depth profiling of ultrashallow P, Ge and As implants in Si using MCs2+ ions. Surf. Interface Anal. 34, 472–476 (2002).

    Article  Google Scholar 

  83. Jiang, F. Raw data for manuscript. figshare https://doi.org/10.6084/m9.figshare.24069768 (2024).

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Acknowledgements

This work was primarily supported by the Office of Naval Research (award number N00014-20-1-2587): F.J., Y.S., D.P.M., J.A.S., M.G.C., H.C., S.B., S.R.M, H.J.S. and D.S.G. In addition, F.J. and D.S.G. acknowledge the institutional support from the B. Seymour Rabinovitch Endowment and the state of Washington. We acknowledge the use of facilities and instruments at the Photonics Research Center (PRC) at the Department of Chemistry, University of Washington, and at the Research Training Testbed (RTT), part of the Washington Clean Energy Testbeds system. Part of this work was carried out at the Molecular Analysis Facility (MAF), a National Nanotechnology Coordinated Infrastructure site at the University of Washington, which is supported in part by the National Science Foundation (NNCI-1542101, NNCI-2025489), the Molecular Engineering & Sciences Institute and the Clean Energy Institute. We also acknowledge S. L. Young from MAF for conducting the XPS measurements. I.E.G, D.P.M. and M.D.M acknowledge support by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under Solar Energy Technologies Office (SETO) agreement number DE-EE0009513. T.R.R. and J.D.M. acknowledge support from the Washington Research Foundation, the University of Washington Clean Energy Institute’s Washington Clean Energy Testbeds and the Department of Energy’s SETO through the Perovskite Photovoltaic Accelerator for Commercializing Technologies programme. ToF-SIMS analysis was carried out with support provided by the National Science Foundation CBET-1626418. This work conducted in part using resources of the Shared Equipment Authority at Rice University. F.J. especially acknowledges J. Guo (University of Washington) for Labview programming, R. Giridharagopal (University of Washington), S. E. Chen (University of Washington), R. Kerner (National Renewable Energy Laboratory) and S. A. Johnson (University of Colorado, Boulder) for valuable scientific discussions.

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Authors

Contributions

F.J. and D.S.G. conceived the project, designed the experiments and discussed the results together. F.J. performed the majority of the experiments and analysed the data. Y.S. performed the AFM, XRD and profilometer measurements and contributed largely to the data processing/analysis. M.Y.Y. conducted the SEM measurements. T.R.R. and J.D.M. synthesized and provided NiOx. D.P.M., S.B. and S.R.M. provided the NDI-1 electron-transporting material. D.M., I.E.G., M.D.M. and A.D.M. contributed to the electric field screening calculation and discussions. T.T., F.M. and A.D.M contributed to the ToF-SIMS measurements and discussions. J.A.S., M.G.C. and H.J.S. helped with the standardization of JV characterizations and definition of breakdown voltage. H.C. helped with the data analysis. All authors contributed to the interpretation of the data and the presentation of this manuscript. All authors approved the submission. F.J. wrote the first manuscript. F.J., M.D.M and D.S.G. revised the manuscript with input from all the authors.

Corresponding author

Correspondence to David S. Ginger.

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M.D.M. is an advisor to Swift Solar. H.J.S. is a co-founder, CSO and a director of Oxford PV. The other authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–44, Notes 1–20 and Tables 1–4.

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Supplementary Data 1

Source data for Supplementary Fig. 3.

Supplementary Data 2

Source data for Supplementary Fig. 9c.

Supplementary Data 3

Source data for Supplementary Fig. 10b.

Supplementary Data 4

Source data for Supplementary Fig. 13.

Supplementary Data 5

Source data for Supplementary Fig. 14b.

Supplementary Data 6

Source data for Supplementary Fig. 15.

Supplementary Data 7

Source data for Supplementary Fig. 16b.

Supplementary Data 8

Source data for Supplementary Fig. 17b.

Supplementary Data 9

Source data for Supplementary Fig. 31.

Supplementary Data 10

Source data for Supplementary Fig. 32.

Supplementary Data 11

Source data for Supplementary Fig. 33.

Supplementary Data 12

Source data for Supplementary Fig. 42.

Supplementary Data 13

Source data for Supplementary Fig. 44.

Supplementary Data 14

Source data for Supplementary Table 1.

Source data

Source Data Fig. 1

Source data for Fig. 1b–e.

Source Data Fig. 2

Source data for Fig. 2b.

Source Data Fig. 3

Source data for Fig. 3b–f.

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Jiang, F., Shi, Y., Rana, T.R. et al. Improved reverse bias stability in p–i–n perovskite solar cells with optimized hole transport materials and less reactive electrodes. Nat Energy (2024). https://doi.org/10.1038/s41560-024-01600-z

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