Improved Crystallization and Reduced Defect Density by Water Additive for High Performance Single-step Inverted Perovskite Solar Cells
Tze Chien Sum a, Ankur Solanki a, Lim Swee Sien b
a Physics and Applied Physics, Nanyang Technological University, Nanyang link, School of Physical and Mathmetical Sciences, Singapore, 637371
b Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, Singapore , 639798
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Oral, Ankur Solanki, presentation 064
Publication date: 7th November 2016

Low power conversion efficiencies and  material instability caused by moisture effects under ambient conditions are the pertinent issues for perovskite solar cells.  Herein, an optimal small amount of water additive into methyl ammonium lead iodide (CH3NH3PbI3) solution helps to improve perovskite crystallization and stability of perovskite films. Addition of optimal fraction of water into N, N -dimethylformamide (DMF) results in controlled perovskite growth due to the higher vapour pressure and lower boiling point of water compared to DMF. The optimal water concentration of 1% by volume was found to yield preferential crystallization in the (110) plane, together with improved grain size,smaller amount of pin holes and defects compared to non-water added film. The device performance improved from 10.1% to 12.3% in 1% water added perovskite films, but decreases upon further inclusion of water content. Pump fluence dependent photoluminescence shows the reduced trap density from 4.8´1017 cm-3 to 3.2´1017 cm-3 on inclusion of 1% water to the perovskite film. The presence of the optimal amount of water enhances the charge carrier lifetimes and diffusion lengths in blend film confirmed by the transient measurements and hence improved device performance. We conclude that these detects in perovskite are dominated by surface trap states and inclusion of an optimal amount of water as an additive help to passivate these traps. The findings in this work correlates the device performance, thin film morphology with transient dynamics and provide a route to control the growth of crystal perovskites and new insights for improving the stability of organic–inorganic halide perovskites.  

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