Development of Efficient Wide-bandgap Perovskite Solar Cells with Composition and Interface Engineering

Metal halide perovskites are considered the most promising solar energy technology because of their distinct properties, such as defect tolerance, low cost, easy fabrication due to solution-processibility, band tunability, etc. Due to these properties, the efficiency of perovskite solar cells reaches more than 25% and approaches the limit of singlejunction within last few years. To increase the efficiency further in a more cost-effective way, double junction tandem solar cells with an efficient ‘top’ wide-bandgap cell is desired. But wide-bandgap perovskites still face some critical issues, such as poor morphology, smaller grain size, the formation of excessive lead halides, light-induced halide segregation, high surface recombination, etc. Higher bromide concentration in wide-bandgap perovskites causes charge-carrier traps/defects that can increase recombination rates reducing overall performance, stability and causing significant current density-voltage hysteresis. To develop a low-cost, stable, and highly efficient wide-bandgap perovskite, we have made efforts to understand these critical issues and solve them. Firstly, we designed a new composition of 1.78 eV quadruple cations (FA0.79MA0.16Cs0.05)0.95Rb0.05Pb(I0.6Br0.4)3 wide-bandgap perovskite, where rubidium (Rb) is used to increase the grain size of the perovskite. We also developed a post-growth strategy known as secondary growth (SG) using guanidinium iodide to convert the excess lead halides to perovskites. Our CPD analysis using AFM indicated the type of defects in grain boundaries, and the nanoscale spatial mapping visualized and quantified the charge carrier dynamics after the passivation of this wide-bandgap structure by Rb incorporation and performing SG treatment. The synergy of these two strategies increases the power conversion efficiency of the devices from 14.17% to 17.71%. In our second project, we developed a facile crystallization technique utilizing poly (methyl methacrylate) (PMMA) to control the growth of our novel wide-bandgap perovskite. PMMA was applied on the surface of the perovskite film to slow down the crystallization and to lock the position of the perovskite components, which resulted in an excess lead halide free uniform perovskite film. Perovskite films using this strategy delivered superior electrical and optical properties with an efficiency of 18.18%. Finally, we developed a rapid and cost-effective way to process our wide-bandgap perovskite using the microwave. After optimizing the microwave power and time, we achieved perovskite film with similar properties to the thermal annealing. Our microwave-processed film achieved a little higher efficiency than the thermal annealed devices with significantly reduced process time from ~1 hour to 90 sec. Advanced characterization techniques conducted in these projects introduce an effective way to analyze grain boundary defects. Moreover, our developed novel widebandgap structure, defect passivation and crystallization strategies, and the low-cost fabrication method can provide a facile approach to realize efficient and more stable wide-bandgap devices to fabricate tandem structures