Joe Briscoe¹,Tian Du¹,Sinclair Ratnasingham¹,Thomas Macdonald¹,Felix Kasasih²,Ruoxi Yang³,Meng Li⁴,Zhongyao Jiang⁵,Lokeshwari Mohan¹,Adriana Augurio¹,Huda Ahli⁵,Shengda Xu⁵,Weidong Xu⁵,Zhenhuang Su⁶,Xingyu Gao⁶,Richard Whiteley¹,Chieh-Ting Lin⁵,Ganghong Min⁵,Saif Haque⁵,Kristin Persson³,Caterina Ducati²,James Durrant⁵,Martyn McLachlan⁵

Queen Mary University of London¹,University of Cambridge²,Lawrence Berkeley National Laboratory³,Henan University⁴,Imperial College London⁵,Shanghai Institute of Applied Sciences⁶

Abstract

Queen Mary University of London¹,University of Cambridge²,Lawrence Berkeley National Laboratory³,Henan University⁴,Imperial College London⁵,Shanghai Institute of Applied Sciences⁶

We have previously demonstrated a new approach to solvent-based post-treatment of perovskite films that achieves excellent reproducibility and uniformity by exposing a perovskite film to vapourised solvents delivered via an aerosol-assisted chemical vapour deposition system [1]. Using this process we achieved enhancement of both photovoltaic power conversion efficiency (PCE) and stability in a range of perovskite compositions and device architectures, which we have linked to improvements in both the nanoscale and macroscale uniformity of the material, including a reduction of defects and associated trap states. Here I will discuss our more recent developments of this process to include organo-halide salts in the solvent aerosol, such as MACl, leading to substantial enhancement of the post-crystallisation grain growth and recrystallisation process [2]. This leads to films that are comprised of ultra-large grains (~1-5 μm) where local traps are almost completely eliminated, as confirmed at the nanoscale via photoconductive atomic force microscopy. The large grains have also allowed us to visualise local variations in photoluminescence (PL) emission at the grain boundaries using hyperspectral PL mapping. Finally, the substantial reduction in trap states and increase of film homogeneity leads to photodetectors that can operate at ~1.5 orders of magnitude lower light levels than those made with conventional spin-coated films. Expanding beyond solvent post-treatment, we have also applied this technique to pure formamidinium lead iodide (FAPbI3) [3]. Using aerosol-assisted crystallization (AAC) of the FAPbI3 films enhanced by Lewis base additives in the solvent aerosol we are able to crystallise pure black-phase α-FAPbI3 in only 2.5 minutes at 100°C, compared to 20 minutes at 150°C for conventional thermal annealing. Not only does this open up wider processing options for the material, but we demonstrate improvement in PCE and, importantly, phase stability of pure α-FAPbI3 compared to thermally annealed control samples. Using X-ray diffraction, X-ray scattering and density functional theory simulation, we identify that relaxation of residual tensile strains due to the lower annealing temperature and post-crystallization crystal growth during AAC are key factors that facilitate the formation of phase-stable α-FAPbI3. References [1] T. Du, S.R. Ratnasingham, F.U. Kosasih, T.J. Macdonald, L. Mohan, A. Augurio, H. Ahli, C.T. Lin, S. Xu, W. Xu, R. Binions, C. Ducati, J.R. Durrant, J. Briscoe,* M.A. McLachlan,*Adv. Energy Mater. 2101420 (2021). [2] T. Du, F. Richheimer, K. Frohna, N. Gasparini, L. Mohan, G. Min, W. Xu, T. J. Macdonald, H. Yuan, S. R. Ratnasingham, S. Haque, F. A. Castro, J. R. Durrant, S. D. Stranks, S. Wood, M. A. McLachlan and J. Briscoe, Nano Lett. 22, 979-988 (2022). [3] T. Du, T.J. Macdonald, R.X. Yang, M. Li, Z. Jiang, L. Mohan, W. Xu, Z. Su, X. Gao, R. Whiteley, C.T. Lin, G. Min, S.A. Haque, J.R. Durrant, K.A. Persson, M.A. McLachlan and J. Briscoe, Adv. Mater. 34, 2107850 (2022).