Fundamental Challenges in Lead-Free Halide Double Perovskite Optoelectronic Applications
Fuxiang Ji a b, Feng Wang a, Gerrit Boschloo b, Feng Gao a
a Department of Physics, Chemistry and Biology Linkoping University 58183, Linkoping, Sweden
b Department of Chemistry, Angstrom Laboratory, Uppsala University, Sweden
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
London, United Kingdom, 2023 June 12th - 14th
Organizers: Tracey Clarke, James Durrant and Trystan Watson
Poster, Fuxiang Ji, 261
Publication date: 30th March 2023

Lead-free halide double perovskites with the formula of A2B+B3+X6 are received significantly growing interest due to their potential to address the toxicity and instability issue of lead-based perovskites APbX3. They show high stability, a wide range of possible combinations, rich substitutional chemistry, and attractive optoelectronic properties, making them successfully applied in various optoelectronic devices, such as solar cells, photodetectors, light-emitting diodes, etc. However, their device performance still lags far behind that of the state-of-the-art lead-based perovskite ones. Thus, we first summarize the main fundamental challenges that limit high-efficiency optoelectronic from both the materials development and device fabrication, including large bandgap, indirect bandgap nature, parity-forbidden transitions, low electronic dimensionality, weak or no photoluminescence, short carrier diffusion length, difficulties to fabricate and dope/alloy HDP films, contact issues, high defect density in thin films, etc.1 Equally important, we highlight current possible solutions to these challenges, such as metal doping/alloying, B-site metal ions order to disorder, B-site metal ions arrangement, post-treatment, thermal evaporation, appropriate charge transport layers, etc.1 Following this thinking and taking the large bandgap issue as an example, we successfully modify the bandgap and/or optical absorption properties of several double perovskites through metal doping, B-site metal ions order to disorder, and thermal heating. Specifically, we tune the bandgap of Cs2AgInCl6 over a range from 2.8 eV to 1.6 eV by Fe3+ alloying.2 For Cs2AgBiBr6, its optical absorption edge is broadened from 610 to around 860 nm by incorporating Cu in the lattice, which is attributed to the sub-bandgap formation rather than bandgap narrowing.3 Besides metal doping/alloying, we also develop a crystallization control approach to narrow the bandgap of Cs2AgBiBr6 by simply increasing the crystal growth temperature from 60 oC to 150 oC. As a result, its bandgap reduced from 1.98 eV to 1.72 eV, which is the lowest reported bandgap for Cs2AgBiBr6 at ambient conditions. The underlying reason is hypothesized to be related to the increased level of Ag–Bi disorder.4 We also observe a remarkable and fully reversible thermochromism in the double perovskite Cs2NaFeCl6, indicating its bandgap can be simply adjusted by temperature. The bandgap of Cs2NaFeCl6 crystal is reduced from 2.5 to 1.87 eV when the temperature increases from 6.8 to 423 K, significantly exceeding the values reported in lead-based perovskites and many conventional group-IV and III-V semiconductors. The first-principles calculations reveal that this dramatic thermochromism is attributed to a strong electron-phonon coupling rather than thermal expansion or Na/Fe order-disorder transition. Until now, halide double perovskites are still in their infancy and require enormous efforts to realize their full potential.

I thank all co-authors listed in the references. This work was financially supported by the Swedish Research Council (VR starting grant: 2018-04809), Carl Tryggers Stiftelse, Olle Engkvist Byggmästare Stiftelse, STINT grant (CH2018-7655), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices (Soochow University), and the Åforsk ( 21-32), Knut and Alice Wallenberg Foundation (Dnr. KAW 2019.0082), the Swedish Energy Agency (2018-004357), the Grant Agency of the Czech Republic (Grant GA19-05259S), VR Starting Grant (2019-05279), Carl Tryggers Stiftelse, and Olle Engkvist Byggmästare Stiftelse. F.J. was supported by the China Scholarship Council (CSC). Theoretical analysis of calculated properties was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of the Increase Competitiveness Program of NUST “MISIS” (Grant No. K2-2019-001) implemented by a governmental decree dated 16 March 2013, No. 211. The support from Swedish Research Council (VR) (Project No. 2019-05551) is acknowledged by S.I.S. and J.K. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC Centre for High Performance Computing (PDC-HPC) and the National Supercomputer Center (NSC) partially funded by the Swedish Research Council through grant agreement no. 2016-07213.

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