Composition tuning of organic-inorganic perovskite crystals by post-treatment for high efficiency solar cells
Atsushi Kogo a, Tetsuhiko Miyadera a, Masayuki Chikamatsu a
a Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP20)
Tsukuba-shi, Japan, 2020 January 20th - 22nd
Organizers: Michio Kondo and Takurou Murakami
Oral, Atsushi Kogo, presentation 025
Publication date: 14th October 2019

Recently, solution-processable organolead halide perovskite materials have been attracting great interest as a cost-effective and high performance light-harvesting material of solar cells. Since perovskite crystals are formed by coating and drying process of precursor solutions, control of crystal growth has been important issue for high power conversion efficiency (PCE). Recently, it is reported that ratio of PbI2 and methylammonium iodide (MAI) in precursor solutions has great influence on perovskite crystal formation. Excess amount of PbI2 contained in a precursor solution promotes crystal growth of perovskite and yields the layer of high crystallinity, which gives high PCE of solar cells.[1] On the other hand, excess amount of MAI in precursor solutions yields perovskites with less trap density.[2] To obtain perovskites with high crystallinity and low trap density, in this study, we performed post-treatment with MAI solution to highly crystalline perovskite crystals formed from a PbI2-rich precursor solution. As a result, high PCE up to 20.7% was achieved.

   Perovskite layers was fabricated from a precursor solution containing 10 mol% excess PbI2. To the perovskite layers (PbI2-rich perovskite), MAI solution was spin-coated, and the substrates was annealed at 100 oC. Diffraction peak of PbI2 was observed in X-ray diffraction pattern of perovskite without the MAI treatment. When the perovskite was treated with 10 mM MAI solution, the PbI2 peak disappeared. This indicates that in the perovskite treated with >10 mM MAI solutions, PbI2 was removed and excess MAI was incorporated.

  Photovoltaic performance of perovskites with and without the MAI treatment was compared. The perovskites without MAI treatment exhibits PCE at 19.2%. Even when perovskite was treated with 10 mM MAI solution and PbI2 was removed, the PCE was same. However, by >10 mM MAI solution treatment, PCE increased up to 20.7%. This indicates that excess amount of MAI in perovskite layer is important for high PCE. Dark current analysis shows that trap density was drastically decreased in >10 mM MAI-treated perovskite. We conclude that PCE was improved due to decrease in perovskite trap density based on tuning MAI content by post-treatment.

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