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

Abstract

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 °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 °C. The pre-heated substrate led to higher photocurrent and voltage than the non-pre-heated one. When T_sub increased to 60 °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.