Open Circuit Potential Build-Up in Perovskite Solar Cells from Dark Conditions to 1 Sun

The high open-circuit potential (V_oc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The V_oc analysis is essential to understand their working mechanisms. A large number of CH₃NH₃PbI_3–xCl_x PSCs were fabricated on single large-area substrates and their V_oc dependencies on illumination intensity, I₀, were measured showing three distinctive regions. Similar results obtained in Al₂O₃ based PSCs relate the effect to the compact TiO₂ rather than the mesoporous oxide. We propose that two working mechanisms control the V_oc in PSCs. The rise of V_oc at low I₀ is determined by the employed semiconductor n-type contact (TiO₂ or MgO coated TiO₂). In contrast, at I₀ close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated E_Fn (as in the dye sensitized solar cells) to an E_Fn, directly determined by the CH₃NH₃PbI_3–xCl_x absorber is suggested.