연사: 조월렴 교수 (이화여대 물리학)
제목: Fundamental aspects and materials properties in organic-inorganic lead halide perovskites for solar cell applications
초록:
Organic-inorganic halide CH3NH3Pb(I,Br)3 perovskites is one of the most promising photovoltaic materials [1]. Perovskite solar cells have many advantageous for solar cell such as a large absorption coefficient, high carrier mobility, high carrier diffusion length, and direct band gap. We explored electrical properties of CH3NH3(Pb,Sn)(I,Br)3 with the thickness of perovskite ~150 nm since it is interesting that the bandgap can be tuned by Pb/Sn and I/Br. Our best cell of perovskite has achieved ~14% conversion efficiency.
A concern of environmental hazard of Pb draws attention of Sn but the chemical stability of Sn is very low. The fabricated perovskite solar cells have a glass/FTO/blocking-TiO2/mesoporous-TiO2/perovskite/HTM(Spiro-MeOTAD)/Ag. We investigated the grain boundary properties in perovskite solar cells with different Pb/Sn and I/Br ratio by Kelvin probe force microscopy, conductive atomic force and pieozoresponse force microscopy measurements. The electrical grain boundary properties (positively charged grain boundaries) are similar to the polycrystalline CIGS and CZTSSe thin-film solar cells.
Especially, positively charged GBs grain boundaries is obtained. These positively charged grain boundaries could be enhanced electron-hole separation and suppressing recombination near grain boundaries for high efficiency in the perovskite solar cells. In addition, the lead halide perovskite single crystals are studied for elucidating their intrinsic physical properties. The grains are yet not fully controlled in terms of compositions, textures, and even believed as a trapping source of ionic migration. We report our observations on the distribution of surface electric potential and current transport on the two single crystals with iodine and bromine by Kelvin probe force microscopy and conductive atomic force microscopy, respectively. Current level of the grain-boundary free large grains is smaller than thin-films but some spots exhibit large current values at high external voltage bias.
Piezoelectric force microscopic measurement on the perovskite single crystals was also performed to decipher the potential ferroelectric properties or unequal majority/minority carrier dynamics.
[1] G. Y. Kim, S. H. Oh, B. P. Nguyen, W. Jo, B. J. Kim, D. G. Lee, H. S. Jung, “Efficient Carrier
Separation and Intriguing Switching of Bound Charges in Inorganic−Organic Lead Halide
Solar Cells”, Journal of Physical Chemistry Letters, 6 2355 (2015).
일시: 2016. 5. 12(목) 오후 4시30분
장소: 베어드홀 103호