Key Parameters and Thresholds Values for Obtaining High Performance Perovskite Solar Cells Indoors from Full Br Compositional and Bandgap Engineering

Perovskite solar cells (PSCs) have different theoretical optimal bandgaps (Eg) for outdoor and indoor light harvesting due to the different spectral distributions of the sun and indoor lamps. This work focuses on understanding how both indoor and outdoor photovoltaic (PV) performance of Cs0.05(MA0.17FA0.83)0.95Pb(I1-xBrx)3 PSCs depend on Br- content (x) spanning the whole 0-100% range, not only efficiency but also stability. Eg increases linearly with x: Eg/eV = 0.75x+1.48. Cells with x = 0.17 delivered the highest efficiency under indoor illumination, which did not correspond to the optimal theoretical bandgap. Via in depth analysis of crystal structure, morphology, and optoelectronic properties, we propose five key parameters and associated threshold values to be surpassed that enable one to achieve indoor efficiencies greater than 25% (1000 lx). First, films should possess average grain sizes greater than 300 nm (i.e., grain sizes > 70% of film thickness) and intergrain spacing ≪ 10 nm. Additionally, On/Off dark current and shunt/series resistance ratios should be higher than 102. Lastly the ratio between current density under indoor illumination and recombination currents in the dark should be >10. The aging rate of cells measured indoors (a fall of 65%) was higher than under 1 sun (41% fall), indicating that device performance is more sensitive to defects arising upon aging when measured under low intensity indoor light. Our investigation provides key parameters that can become a useful tool for researchers aiming to develop improved PSCs for indoor applications.