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. [ABSTRACT FROM AUTHOR]