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Morphological and functional characterizations of SnO2 electron extraction layer on transparent conductive oxides in lead-halide perovskite solar cells

Optimization of carrier extraction and/or transport layers is an important factor for the development of perovskite semiconductor devices. In particular, tin dioxide, SnO2, is being frequently used as an electron transport layer (ETL) in perovskite solar cells. However, a systematic study on preparation and characterization of the SnO2-ETL is still lacking, and thus, morphological and electronic-functional roles are not fully understood. In this paper, we systematically investigate the SnO2-ETL prepared on fluorine-doped tin oxide (FTO) substrates by a spin-coating technique. Using microscopic observations, we morphologically study how the SnO2 film covers the FTO surface with large unevenness. Optical characterizations are employed for investigating an electronic band alignment of the perovskite/SnO2 interface varied with the SnO2 concentration in a solution. Furthermore, we systematically evaluate photovoltaic properties of FTO-based solar cell devices. A major finding from these investigations is the fact that while the SnO2-ETL prepared at the adequate condition exhibits an ideal band alignment, the excessive SnO2 deposition causes a poor electron extraction and device performance degradation. Furthermore, we show that the spin-coated SnO2-ETL can cover the FTO surface as an ultrathin wrapping layer. These results highlight the importance of the SnO2-ETL and pave the way for optoelectronic device applications of perovskite materials.