Bromide regulated film formation of CH3NH3PbI3 in low-pressure vapor-assisted deposition for efficient planar-heterojunction perovskite solar cells

Abstract

Obtaining high-quality organic–inorganic halide perovskite films that have smooth and continuous surfaces and large crystal domains and possess photoelectrical properties preferable for photovoltaic applications is of paramount importance to achieve high performance perovskite solar cells (PSCs). The introduction of other halide ions into the synthesis process of methylammonium lead triiodide, CH3NH3PbI3, to fabricate CH3NH3PbI3−xXx (X=Cl or Br) (x≈0) has been confirmed as an effective approach to optimize optoelectronic properties, thereby enhancing solar cell performance. However, the reported approaches to incorporate chlorine or bromine in perovskite films mainly take place during a liquid-phase synthesis process. Here, we report on bromide regulated CH3NH3PbI3 film formation through a low-pressure vapor-assisted deposition process. PbBr2 was used to either replace or to be mixed with PbI2 to form pre-deposited films that were then reacted with CH3NH3I vapor. Detailed structural, spectroscopic, and morphological characterizations of the perovskite films unambiguously demonstrate the formation of CH3NH3PbI3 films. However, bromine incorporation slows down the perovskite formation process through formation of an intermediate phase, CH3NH3PbBrxIy, which improves the grain domains in the as-fabricated perovskite films. Enhancements in power conversion efficiency (PCE) for the as-fabricated planar-heterojunction structured PSCs were found for samples in which bromine was incorporated. Meanwhile, transient photovoltage decay measurements revealed that carrier recombination was suppressed throughout the entire device. When mixed PbI2/PbBr2 films with a molar ratio of 1:4 were used as re-deposited films, an optimized PCE of 17.40% was obtained.