Effect of TiO2 morphology on efficiency of CH3NH3PbI3 Perovskite solar cells

Abstract

A TiO2 compact (cp) layer or blocking layer (bl) plays a crucial role in a hybrid organic-inorganic lead halide perovskite solar cell (PSC) because it prevents the carrier recombination at the interface of fluorine-doped tin oxide (FTO) and perovskite layers. In this work, fabrication, characterizations of cp-TiO2 and their effects on the PSC were studied. The cp-TiO2 layers in this work were fabricated by radio frequency (RF)-magnetron sputtering method and spin coating method with various conditions. The cp-TiO2 layer was incorporated into the PSC device structure consisting of Au/spiro-OMeTAD/MAPbI3/mp-TiO2/cp-TiO2/FTO. For the sputtering method, cp-TiO2 could selectively block the transport of holes from TiO2 to the FTO layer, but it might not be appropriate to transfer electrons from the perovskite layer to the FTO. For a spin coating method, TiO2 layer cannot completely block the transport of holes from TiO2 to the FTO layer, but it can be used as an electron transporting layer for PSCs. Energy gaps of TiO2 by sputtering and spin coating were found to be in the range of 3.65 – 3.77 eV corresponding to the anatase phase. However, morphologies of TiO2 fabricated by sputtering and spin coating methods are not the same. For the sputtering method, morphology of TiO2 layer is similar to FTO. For spin coating, there are very fine round grains of TiO2 on FTO surface as observed in the AFM phase images. This directly affects the performance of the PSCs. It is found that the best condition for spin coating of compact and mesoporous TiO2 layers is 5,000 rpm for 30 s with the corresponding maximum power conversion efficiency of about 6.6%.