Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition

摘要

Organometal halide perovskites (CH_3NH_3PbI_(3-x)Cl_x and CH_3NH_3PbI_3) have recently attracted tremendous attention as promising materials for solar energy conversion. The pioneer devices demonstrated a power conversion efficiency (PCE) of 4%, as reported by Miyasaka and co-workers in 2009. The PCEs soon evolved to exceed 12% in a few years both in meso-superstructure-type and planar-type cells. The planar-type device architecture is particularly interesting due to the simple cell configuration and possible low-temperature fabrication on flexible substrates. However, unlike meso-superstructure-type devices, in which perovskite can be scaffolded by mesoporous matrices, incomplete and non-uniform coverage of perovskite films was usually observed in planar-type perovskite solar cells and has been regarded as the major factor resulting in decreased device performance. Many efforts have been made to control the morphology of perovskite thin films including optimization of the annealing time and temperature, selection of the under-layer material and thickness, and the use of alternative deposition methods such as two-step deposition and vacuum sublimation. Among these methods, the vacuum thermal co-evaporation of CH_3NH_3I and PbCl_2 (or PbI_2) and the resulting perovskite thin films exhibited the most homogeneous morphology and the highest thin-film coverage, leading to a high performance of 12-15% PCE. Despite the promising results, however, to date, only limited reports have utilized this vacuum sublimation technique to fabricate perovskite layers. The main reason could be due to the small molecular weight of CH_3NH_3I, which results in a random diffusion of molecules inside the vacuum chamber and causes difficulty with the monitoring and control of the CH_3NH_3I deposition rate using quartz microbalance sensors.