Efficient semi-transparent planar perovskite solar cells using a 'molecular glue'

Semitransparent perovskite solar cells are promising for their use in building integrated photovoltaic applications. One critical requirement for this application is the ability to thin down the perovsldte absorber layer without sacrificing much of its optimum device efficiency. However, the production of high quality and defect-free very thin films of perovskites in an all low temperature solution processed p-i-n type device structure has been elusive due to the sensitivity of the materials crystallization dynamics and the variability of surface specific interactions. Here, we show that a unique surface passivation method based on thiourea vapor treatment can effectively produce high quality CH3NH3PbI3 thin-films up to 110 nm of thickness on top of a poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) surface. The thiourea vapor is believed to act as a 'molecular glue' by introducing favorable surface interactions in the PEDOT: PSS film with the soft, polarizable inorganic species (i.e. Pb and I) present in the thin-film precursors, and helps to achieve continuous, pinhole-free perovskite films. Using these solution-processed thin and high quality CH3NH3PbI3 films, semitransparent p-i-n planar perovskite-PCBM solar cells are fabricated with an evaporated transparent Ag top electrode, and the solar cells show a high average visible transparency of 34% at a power conversion efficiency level of 8.2%, which are among the highest reported performance values for semitransparent perovskite solar cells. This demonstration provides an excellent example of how surface passivation can be an effective approach to grow high quality all solution-processed semitransparent perovskite films, and will lead to more efficient thin-film optoelectronic devices in the future.