Additive regulated crystallization and film formation of CH3NH3PbI3−xBrx for highly efficient planar-heterojunction solar cells

Abstract

Hybrid CH₃NH₃PbI_3−xBr_x have recently been found to be excellent photovoltaic materials showing high power conversion efficiency, small hysteresis and low voltage loss in solar devices. In the synthesis, the reaction between lead bromide (iodide) and methylamine iodide (bromide) usually leads to a CH₃NH₃PbI_3−xBr_x film with a rough surface and low coverage, which is thus unable to generate high efficiency in planar heterojunction solar devices. The high efficiency is exclusively achieved in devices with mesoporous TiO₂ nanoparticle network supported hybrid CH₃NH₃PbI_3−xBr_x. To improve the efficiency in planar heterojunction device architectures, herein we attempt to fabricate a uniform thin film composed of CH₃NH₃PbI_3−xBr_x by using NH₄Cl as a low-cost additive. The results show that the incorporation of NH₄Cl in the reaction system can always improve the film formability regardless of the molar ratio between I and Br in the final compound. We also identified that the device based on CH₃NH₃PbI_2.4Br_0.6 can lead to an efficiency of 12.1%, which is the highest value for the planar heterojunction solar cell based on CH₃NH₃PbI_3−xBr_x ever reported with a normal device configuration. This work provides a facile method for the fabrication of high quality CH₃NH₃PbI_3−xBr_x films for highly efficient solar energy conversion.