Owing to rapid development in their efficiency1 and stability2, perovskite solar cells are at the forefront of emerging photovoltaic technologies. State-of-the-art cells exhibit voltage losses3,4,5,6,7,8 approaching the theoretical minimum and near-unity internal quantum efficiency9,10,11,12,13, but conversion efficiencies are limited by the fill factor (<83%, below the Shockley–Queisser limit of approximately 90%). This limitation results from non-ideal charge transport between the perovskite absorber and the cell’s electrodes5,8,13,14,15,16. Reducing the electrical series resistance of charge transport layers is therefore crucial for improving efficiency. Here we introduce a reverse-doping process to fabricate nitrogen-doped titanium oxide electron transport layers with outstanding charge transport performance. By incorporating this charge transport material into perovskite solar cells, we demonstrate 1-cm2 cells with fill factors of >86%, and an average fill factor of 85.3%. We also report a certified steady-state efficiency of 22.6% for a 1-cm2 cell (23.33% ± 0.58% from a reverse current–voltage scan).
This is a preview of subscription content, access via your institution
Subscribe to Nature+
Get immediate online access to the entire Nature family of 50+ journals
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
The data that support the findings of this study are available from the corresponding authors on reasonable request.
Green, A. M. et al. Solar cell efficiency tables (version 57). Prog. Photovolt. 29, 3–15 (2021).
Bai, S. et al. Planar perovskite solar cells with long-term stability using ionic liquid additives. Nature 571, 245–250 (2019).
Saliba, M. et al. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 354, 206–209 (2016).
Correa-Baena, J.-P. et al. Identifying and suppressing interfacial recombination to achieve high open-circuit voltage in perovskite solar cells. Energy Environ. Sci. 10, 1207–1212 (2017).
Peng, J. et al. A universal double-side passivation for high open-circuit voltage in perovskite solar cells: role of carbonyl groups in poly(methyl methacrylate). Adv. Energy Mater. 8, 1801208 (2018).
Luo, D. et al. Enhanced photovoltage for inverted planar heterojunction perovskite solar cells. Science 360, 1442–1446 (2018).
Yang, S. et al. Tailoring passivation molecular structure for extremely small open-circuit voltage loss in perovskite solar cells. J. Am. Chem. Soc. 141, 5781–5787 (2019).
Peng, J. et al. Nanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cells. Science 371, 390–395 (2021).
Kim, M. et al. Methylammonium chloride induces intermediate phase stabilization for efficient perovskite solar cells. Joule 3, 2179–2192 (2019).
Min, H. et al. Efficient, stable solar cells by using inherent bandgap of ɑ-phase formamidinium lead iodide. Science 366, 749–752 (2019).
Zhu, H. et al. Tailored amphiphilic molecular mitigators for perovskite solar cells with 23.5% efficiency. Adv. Mater. 32, 1907757 (2020).
Jiang, Q. et al. Surface passivation of perovskite film for efficient solar cells. Nat. Photon. 13, 460–466 (2019).
Jung, E. H. et al. Efficient, stable and scalable perovskite solar cells using poly(3-hexythiophene). Nature 567, 511–515 (2019).
Giordano, F. et al. Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells. Nat. Commun. 7, 10379 (2016).
Peng, J. et al. Efficient indium-doped TiOx electron transport layers for high-performance perovskite solar cells and perovskite-silicon tandems. Adv. Energy Mater. 7, 1601768 (2017).
Zhou, H. et al. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546 (2014).
Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K. & Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 293, 269–271 (2001).
Asahi, R., Morikawa, T., Irie, H. & Morikawa, T. Nitrogen-doped titanium dioxides as visible-light-sensitive photocatalyst: design, development, and prospects. Chem. Rev. 114, 9824–9852 (2014).
Shasti, M. & Mortezaali, A. The effect of nitrogen doping of TiO2 compact blocking layers on perovskite solar cell performance. Solid State Sci. 92, 68–75 (2019).
Zhang, Z. et al. Enhancement of perovskite solar cells efficiency using N-doped TiO2 nanorod arrays as electron transfer layer. Nanoscale Res. Lett. 12, 43 (2017).
NIST X-ray Photoelectron Spectroscopy Database NIST Standard Reference Database 20 (National Institute of Standards and Technology, 2000); https://doi.org/10.18434/T4T88K
Shahiduzzaman, M. D. et al. Low-temperature processed TiOx electron transport layer for efficient planar perovskite solar cells. Nanomaterials 10, 1676 (2020).
Zhang, L. et al. N–TiO2-coated polyester filters for visible light—photocatalytic removal of gaseous toluene under static and dynamic flow conditions. J. Environ. Chem. Eng. 4, 357–364 (2016).
Batzill, M., Morales, E. H. & Diebold, U. Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase. Phys. Rev. Lett. 96, 026103 (2006).
Napoli, F. et al. The nitrogen photoactive centre in N-doped titanium dioxide formed via interaction of N atoms with the solid. Nature and energy level of the species. Chem. Phys. Lett. 477, 135–138 (2009).
Milošv, I., Strehblow, H.-H., Navinšek, B. & Metikoš-Huković, M. Electrochemical and thermal oxidation of TiN coatings studied by XPS. Surf. Interface Anal. 23, 529–539 (1995).
Wyckoff, R. W. G. Crystal Structures 1, 85–237 (Interscience, 1963).
Howard, C. J., Sabine, T. M. & Dickson, F. Structural and thermal parameters for rutile and anatase. Acta Cryst. B 47, 462–468 (1991).
Yang, X. et al. Dual-function electron-conductive, hole-blocking titanium nitride contacts for efficient silicon solar cells. Joule 3, 1314–1327 (2019).
Peelaers, H., Kioupakis, E. & Van de Walle, C. G. Free-carrier absorption in transparent conducting oxides: phonon and impurity scattering in SnO2. Phys. Rev. B 92, 235201 (2015).
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).
Lin, Y.-H. et al. A piperidinium salt stabilizes efficient metal-halide perovskite solar cells. Science 369, 96–102 (2020).
Hanaor, D. A. & Sorrell, C. C. Review of the anatase to rutile phase transformation. J. Mater. Sci. 46, 855–874 (2011).
Brudnik, A., Bucko, M., Radecka, M., Trenczek-Zajac, A. & Zakrzewska, K. Microstructure and optical properties of photoactive TiO2:N thin films. Vacuum 82, 936–941 (2008).
Morikawa, T., Asahi, R., Ohwaki, T., Aoki, K. & Taga, Y. Band-gap narrowing of titanium dioxide by nitrogen doping. Jpn. J. Appl. Phys. 40, L561–L563 (2001).
Ding, B. et al. Facile and scalable fabrication of highly efficient lead iodide perovskite thin-film solar cells in air using gas pump method. ACS Appl. Mater. Interfaces 8, 20067–20073 (2016).
Shi, L. et al. Accelerated lifetime testing of organic–inorganic perovskite solar cells encapsulated by polyisobutylene. ACS Appl. Mater. Interfaces 9, 25073–25081 (2017).
Stadelmann, P. A. EMS - a software package for electron diffraction analysis and HREM image simulation in materials science. Ultramicroscopy 21, 131–145 (1987).
This work was supported by the Australian Government through the Australian Renewable Energy Agency and the Australian Research Council. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. J.P. acknowledges the financial support of a Postdoc Fellowship from the Australian Centre for Advanced Photovoltaics. T.P.W. is the recipient of an Australian Research Council Future Fellowship (project number FT180100302) funded by the Australian Government. T.P.W. also acknowledges the support of the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University). D.Z. acknowledges funding from the National Natural Science Foundation of China (grant numbers 11574403 and 11974431). The work was partly conducted at the ACT node of the Australian National Fabrication Facility, and the ANU node of the Australian Microscopy and Microanalysis Facility. We thank S. Surve and S. Zhao for experimental assistance.
The Australian National University has filed a PCT patent (PCT/AU2021/051266) related to the subject matter of this manuscript.
Peer review information
Nature thanks Michael McGehee, Md. Shahiduzzaman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Bare FTO substrate. b, FTO/TiN substrate without post-annealing treatment (labeled as ‘As-deposited’). c, FTO/TiOxNy substrate annealed at 300oC (labeled as ‘300 C’). d, FTO/TiOxNy substrate annealed at 350oC (labeled as ‘350 C’). e, FTO/TiOxNy substrate annealed at 400oC (labeled as ‘400 C’). f, FTO/TiOxNy substrate annealed at 450oC (labeled as ‘450 C’). g, FTO/TiOxNy substrate annealed at 500oC (labeled as ‘500 C’). h, FTO/TiOxNy substrate annealed at 550oC (labeled as ‘550 C’). Note that the legend of ‘RMS’ represents root mean square of the surface roughness.
The atomic ratio of O to Ti and N to Ti for TiOxNy thin films annealed at different temperatures.
a, TiOxNy without annealing (‘As-deposited’). b, TiOxNy annealed at 300oC. c, TiOxNy annealed at 400oC. d, TiOxNy annealed at 500oC.
a and b, TiOxNy annealed at 350oC. c and d, TiOxNy annealed at 450oC. e and f, TiOxNy annealed at 550oC. Note that the non-continuous diffraction ring visible in Fig. 4f diffraction pattern is caused by the low crystal density within the specimen.
Tauc plots for the as-deposited TiN film and TiOxNy films annealed at different temperatures. Note that all films (~50 nm) were deposited on quartz substrates.
a, The cross-sectional SEM image of the TiOxNy-based cell with a structure of glass/FTO/TiOxNy (~40 nm)/meso-TiO2 (~50 nm)/ultrathin PMMA:PCBM (<3nm)/Perovskite (~500 nm)/ultrathin PMMA (<3 nm)/P3HT:CuPc (~65 nm)/Gold. b, The cross-sectional SEM image of the TiOx-based cell with a structure of glass/FTO/TiOx (~50 nm)/meso-TiO2 (~50 nm)/ultrathin PMMA:PCBM (<3 nm)/Perovskite (~500 nm)/ultrathin PMMA (<3 nm)/P3HT:CuPc (~65 nm)/Gold. Note that the meso-TiO2 and perovskite represent mesoporous TiO2 and Cs0.05FA0.9MA0.05PbI2.74Br0.26, respectively.
a, VOC and JSC distribution for the TiOx-based cells (12 cells) and the TiOxNy-based cells (14 cells). b, The J-V curve of the perovskite cells based on the TiOxNy ETLs, which were annealed at 550oC. c, The J-V parameters distribution of the TiOxNy (annealed at 550oC) based cells (15 cells).
Extended Data Fig. 8 Effects of electron transport layers with different carrier densities on the performance of perovskite solar cells.
a, Simulated J-V curves where only ETL doping is varied using dopant densities taken from Hall effect measurements. b, Experimental J-V curves for the reference TiOx and TiOxNy PSCs. Reduction in FF and increase in Ohmic series resistance is predominantly due to electron depletion in the ETL layer. c, Energy level diagram of electron quasi-Fermi levels and conduction band, illustrating the resistive voltage loss in the ETL for the lowly-doped cases. Note that Energy levels and electron concentrations are calculated at 22.5 mA/cm2 for ETLs with doping densities equivalent to TiOx (5x1014 cm−3) and TiOxNy (3x1017 cm−3). d, Electron concentration in the ETL for doping levels from 5x1014 – 1018 cm−3. Note that the dash-dot horizontal lines mark the dopant defect concentration, indicating the magnitude of electron depletion. With the exception of extremely high doping at 1018 cm−3, in no case does the free electron concentration reach even the same order of magnitude as the fixed dopant concentration. e-f, Contour plots of perovskite solar cell fill factor (e) and open-circuit voltage (f) across a range of ETL doping levels and electron affinity. Note that the stars mark the conditions simulated in Fig. 9a. The vertical dashed lines mark the doping levels of the TiOx and optimized TiOxNy films fabricated in this work. Electron depletion from the ETL suggests that high doping is in general necessary to achieve fill factors on the order of 85% or above.
a, Light-soaking stability tests. b, Damp-heat stability tests. Note that SPO represents steady-state power output measured by maximum power point voltage (VMPP) tracking under continuous 1 sun illumination intensity. The device structure of the encapsulated cells is glass/FTO/TiOxNy (or TiOx)/m-TiO2/PMMA:PCBM/Perovskite/PMMA/P3HT:CuPc/MoOx (~10 nm)/IZO (~40 nm)/Au, where the perovskite is Cs0.05FA0.9MA0.05PbI2.74Br0.26. Details of encapsulation are provided in the experimental section.
About this article
Cite this article
Peng, J., Kremer, F., Walter, D. et al. Centimetre-scale perovskite solar cells with fill factors of more than 86 per cent. Nature 601, 573–578 (2022). https://doi.org/10.1038/s41586-021-04216-5