15.76% efficiency perovskite solar cells prepared under high relative humidity: importance of PbI2 morphology in two-step deposition of CH3NH3PbI3

We report here an efficient method for preparing high efficiency CH3NH3PbI3 perovskite solar cells under high relative humidity, where the morphology of PbI2 was found to be of crucial importance. CH3NH3PbI3 was formed on a mesoporous TiO2 layer by a two-step spin coating method. During the first-step spin-coating procedure to form a PbI2 layer, an FTO glass substrate was pre-heated at temperatures ranging from room temperature (without pre-heating) to 60 degrees C. An average power conversion efficiency (PCE) of 11.16% was achieved without pre-heating, which was improved to 15.31% as the temperature of the substrate (T-sub) was raised to 50 degrees C. The pre-heated substrate led to higher photocurrent and voltage than the non-pre-heated one. When Tsub increased to 60 degrees C, the PCE declined to 10.49% due to the large portion of unreacted PbI2. Compared to the non-pre-heated substrate, unreacted PbI2 was present on the pre-heated substrates after the second-step spin-coating of CH3NH3I as confirmed by X-ray diffraction and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profile analyses. The improved crystallinity of PbI2 induced by substrate pre-heating was responsible for incomplete conversion of PbI2 to CH3NH3PbI3. Nevertheless, the increase in photocurrent and voltage by pre-heating was attributed to better pore filling and surface coverage of the perovskite layer, as observed by focused ion beam assisted scanning electron microscopy (FIB-SEM) images, which was associated with the morphology of the PbI2 layer. According to a study on the effect of CH3NH3PbI3 thickness controlled by the concentration of PbI2, the substrate temperature was found to play a predominant role in determining the photovoltaic performance rather than thickness. A best PCE of 15.76% was achieved along with a photocurrent density of 21.27 mA cm(-2), a voltage of 1.033 V and a fill factor of 0.718 from the perovskite solar cell prepared under 50% relative humidity.