Efficient mesoscopic perovskite solar cells from emulsion-based bottom-up self-assembled TiO2 microspheres

Mesoscopic perovskite solar cells (PSCs) usually show superior photovoltaic performance to their planar counterparts because of the mesoporous electron transport layers (ETLs) benefiting for perovskite crystal growth and electron transport. However, the preparation of high-quality mesoporous ETLs needs selecting sizable semiconductor nanocrystals commonly within 20–50 nm, smaller or larger nanocrystals will lead to either too narrow pores to effectively infiltrate perovskite materials or too small surface area to efficiently extract charges from perovskite materials. Using sizable semiconductor microspheres can solve the issue, whereas it is also not easy to synthesis sizable semiconductor microspheres. Here, a novel emulsion-based bottom-up self-assembly strategy is used to prepare small size (about 150 nm) TiO 2 microspheres from ultra-small (about 3.6 nm) TiO 2 nanocrystals. The as-prepared mesoscopic PSCs can yield a champion PCE as high as 19.27% with a much smaller current–voltage hysteresis compared with those of the planar one. Specially, the emulsion-based bottom-up self-assembly strategy is a general way for preparing microspheres from wide kinds of semiconductor nanocrystals, so it will greatly expand the material selection range for preparing efficient mesoscopic PSCs and even inverted mesoscopic devices.