Abstract
Organo-lead halide perovskite solar cells (PSC) are one of the most promising photovoltaic devices because of their extraordinary power conversion efficiencies. It is known that the performances of perovskite solar cells depend heavily on the uniformity, surface coverage, and thickness of the perovskite active layer. Typically, solution processing of perovskite films in PSCs is carried out using one-step or two-step method. In the one-step method, the perovskite films are directly deposited onto mesoporous metal oxide using a mixture of PbX2 (X = Cl, Br, I) and CH3NH3X in a polar solvent. However, it is difficult to control the morphology of the resulting perovskite layer in the one-step method, leading to a wide fluctuation in photovoltaic performances. In the two-step method, PbI2 is first deposited onto the mesoporous TiO2 scaffold layer. The CH3NH3I (MAI) solution is then introduced to the PbI2 layer to react and form the CH3NH3PbI3 layer. It is much easier to control the morphology of the perovskite layer with the two-step method than with the one-step method. But the main challenge for the two-step method is the volume expansion issue during the formation of the perovskite, leading to incomplete conversion of PbI2, which is detrimental to the photovoltaic performance. In this work, we choose CH3NH3PbI3-xBrx-based perovskites as the active layer and further modify the traditional two-step method in order to address the volume expansion issue of the PbI2 precursor film. Here we develop a solvent soaking method to produce porous PbX2 layers. We use binary mixture organic solvent, diethyl ether and toluene (ether/toluene) to produce porous PbX2, enabling easier MAI solution infiltration into the entire pore space accessing rapidly the interior of the mesoporous PbX2 layer and thus more complete conversion of PbX2 to perovskite. From XRD, UV-Vis, PL, and TR-PL analyses, it is proved that the perovskite films derived from the ether/toluene treatment have better crystallinity, fewer defect states, and longer lifetime than the perovskite films derived from the traditional two-step method. As a result, the average power conversion efficiency was enhanced from 10.03% for the control device up to 12.32% for the ether/toluene treated device under AM 1.5G solar illumination of 100 mW cm-2. From the IPCE data, we found that the highest efficiency was boosted from 65% to 75% in the visible light region as a result of the high quality perovskite layer derived from the ether/toluene treated PbX2 films. Furthermore, by optimizing the processing condition, we found that the increase in the PbX2 annealing temperature from 70℃ up to 100℃ created the optimal fraction of voids in the PbX2 films, leading to larger grains and more dense perovskite capping layer, which can reduce the trap states in the perovskite films and enhance the light harvesting efficiency. The power conversion efficiency of the champion cell is up to 13.16%.