Recently, the low-cost organolead halide perovskites have emerged as the most promising absorber materials for the development of next generation high efficiency and cost-effective photovoltaic devices. Owing to its impressive properties such as high absorption coefficient over a broad region of visible light spectrum and extremely long carrier diffusion lengths, a device power conversion efficiency (PCE) as high as 25 % could well be within reach in the future. This is comparable to the best commercial single-crystalline silicon solar cells which are substantially more expensive than the perovskite materials. Although there have been a number of reports on the development of high efficiency perovskite-based solar cells demonstrate significant potential in achieving high device efficiency, the basic understanding of the materials, device properties, working mechanisms as well as the manufacturing processes are still at the early stages of development.
In this work, we report on the fabrication and systematic investigations of high efficiency planar CH3NH3PbI3-based solar cells (FTO/TiO2 compact layer/CH3NH3PbI3/spiro-MeOTAD/metal electrode). A two-step spin coating technique was used to fabricate the devices. Through careful optimization of the fabrication and film formation processes we have achieved a high PCE of 15.4% measured under the calibrated ABET Technologies SUN 2000 solar simulator equipped with AM 1.5 filter at 100mW/cm2, which is a record efficiency, at the time of the composition of this abstract, for all-solution processed CH3NH3PbI3-based devices with a planar structure. Detailed investigations, including I-V characteristics, external quantum efficiencies, carrier lifetimes, impedance spectroscopy and low-frequency noise measurements, were performed on the devices to examine the underlying mechanisms responsible for the observed improvements in the PCEs of the devices. In particular, systematic studies on the impact of the optimized fabrication process on the density of the localized states and their effects on the performance of the devices were performed.
From the experimental results, it is observed that performance of perovskite solar cells is strongly affected by concentration of the material defects which could be highly sensitive not only to the processing parameters but also the post-deposition treatments of the films. The results of our investigations point to a direction for future improvements of perovskite-based solar cells.