Improved Performance of Carbon Electrode Perovskite Solar Cells Using Urea Treatment in Two-Step Processing

Perovskite solar cell has made a steady research progress with superior photovoltaic power conversion efficiency (PCE) over 25%. The quality of perovskite film is a key factor affecting device efficiency. Lewis bases such as urea, DMSO and thiourea etc. have been applied in perovskite precursor solutions for one step processing to effectively control the film morphology. Herein, we report urea treatment in a two-step processing for obtaining high quality methylammonium lead triiodide perovskite (CH₃NH₃PbI₃) film, which is accomplished by immersing the PbI₂ film into the mixture solution of urea/methylammonium iodide (MAI). The optimized urea concentration in the MAI solution is 0.4 mg/mL, leading to larger-sized and better crystalline perovskite grains than that without urea additive. The main function of the urea additive is the formation of the adduct MAI⋅PbI₂⋅O=C(NH₂)₂, retarding the crystallization process of the perovskite film. The steady-state and time-resolved photoluminescence measurements revealed that perovskite CH₃NH₃PbI₃ grown using optimized concentration of urea has prolonged carrier lifetime and reduced carrier recombination. Finally, the carbon-based perovskite solar cells fabricated from the optimal urea concentration of 0.4 mg/mL achieved the enhanced photovoltaic performance with the highest PCE of 13.10% and an average value of 11.34%, in comparison to devices without urea treatment exhibiting an average PCE of only 8.67%.