What Limits the Open-Circuit Voltage of Bromide Perovskite-Based Solar Cells?

High band gap Pb bromide perovskite (APbBr ₃ )-based solar cells, where A is a mixture of formamidinium, methylammonium, and Cs, show significantly higher, relative, V _OC losses than their iodide analogs. Using photoluminescence-, quantum efficiency-, and surface photovoltage-spectroscopy measurements, we show the absence of any significant electronically active tail states within the bulk of the (FA _0.85 MA _0.1 Cs _0.05 )PbBr ₃ absorber. All methods confirm that E _G = 2.28 eV for this halide perovskite, HaP. Contact potential difference measurements for this HaP, on different substrates, reveal a Z-shape dependence between the substrate work functions and that of the HaP, deposited on it, indicating that the HaP is relatively low doped and that its Fermi level is affected by the substrate onto which it is deposited. We confirm results from electron beam-induced current (EBIC) and other measurements that most voltage loss of cells, made with these HaP films, is at the HaP/selective-contact interface, specifically the TiO ₂ /HaP one, and provide a complete account of these cells' V _OC losses. Capacitance measurements indicate that 350 mV V _OC could be gained by eliminating (fast) interfacial states, emphasizing the importance of interface passivation. Still, even passivating the TiO ₂ /HaP interface cannot eliminate the band misalignment with Br-based HaPs.