Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.136
Publication date: 21st February 2018
The highest efficiency Perovskite Solar Cells (PSC) often contain two [1], three [2] or even four [3] cations. As well a higher efficiency, mixed cation devices show higher reproducibility and stability [2]. However, the reasons behind the improvements are still being investigated and are not clearly understood. Here we present the results of a systematic study looking at nine different sized cations, from ammonium (ionic radius 146 pm) to guanidinium (ionic radius 278 pm).
5% of each cation was substituted into methyl ammonium lead iodide (MAPI) and the degree of substitution was confirmed by NMR. We chose 5% substitution as all the materials showed the same crystal structure, the same band gap, and no 2D perovskite phases were formed. This allowed us to make a direct comparison between our results and understand the large changes in material and device properties introduced by some of the cations. Three of the partially substituted MAPI powders were studied by muon spectroscopy (μ-SR). We recently showed that μ-SR is able to detect both cation dynamics and iodide diffusion in perovskite materials [4] and we have extended the technique to multi-cation systems. We also prepared inverted cells containing the nine different perovskite materials and compared them using a range of characterisation techniques, including impedance spectroscopy. The results are striking, confirming that all of the cations have an effect at just 5% substitution, while some of the cations substantially change the fundamental properties of the perovskite itself.
[1] Nature Energy, 2017, 2, 972–979
[2] Energy Environ. Sci., 2016, 9, 1989-1997
[3] Energy Environ. Sci., 2017,10, 2509-2515
[4] https://arxiv.org/abs/1801.03845
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