In single-junction solar cells, organic-inorganic halide perovskites that are solution processed have proven to be excellent absorbers. Halide based perovskites also offer bandgap tunability which make them good candidates for wide bandgap top-cell in a dual junction solar cell. Organic-inorganic halide perovskites are however prone to degradation due to low formation energies and tolerance factors. We aim to develop a stable perovskite phase with bandgap close to 1.75 eV to develop a high-efficiency tandem device with CIGS thin film solar cells. Recent CsPbI2Br films processed in our laboratory with bandgap of 1.9 eV have demonstrated close to 12% efficiency devices with t80 life of > 600 hours at 1-sun and 65 oC and Voc conditions in air.
To improve lifetime of CsPbI2Br devices and tailor bandgap, we study the effect of mixed A and B cations via addition of Rb and Cd to partially replace Cs and Pb respectively. Thin films of (Rb,Cs)(Pb,Cd)I2Br being processed via spin coating and doctor blade process, with thickness < 500 nm, and effect of composition on bandgap and structure is determined via UV-Vis-NIR spectroscopy and X-ray diffraction. Results of a design of experiments with mixed A and B cations, and annealing temperature will be presented. Preliminary DFT results indicate addition of Cd lowers the tendency of halide segregation, thereby making the films more stable. Also, a stabilization of perovskite phase is observed with mixed Rb and Cs cations due to entropic gains and the small internal energy input required for the formation of their solid solution. Superstrate p-i-n devices for (Rb,Cs)PbI2Br and CsPbI2Br with stabilized power conversion efficiency >12% have been demonstrated and stability results will be presented. Development of (Rb,Cs)(Pb,Cd)I2Br is currently under progress. (Rb,Cs)PbI2Br and CsPbI2Br devices do not show any measurable halide segregation under light soaking tests, however, a slow and steady decrease in Voc is observed. The devices are fabricated with F:SnO2/Zn(O,S) front contact and carbon-based back contact. As previously presented, (Rb,Cs)PbI2Br and CsPbI2Br devices show a positive temperature coefficient measured between 25 oC to 65 oC. The mixed cation and mixed anion perovskites show good stability, but have lower power conversion efficiency as compared to FA/MA mixed organic cation formulations. Further improvement is needed to lower Voc deficit for these wide bandgap devices.