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

The high open-circuit potential (<i>V</i><sub>oc</sub>) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The <i>V</i><sub>oc</sub> analysis is essential to understand their working mechanisms. A large number of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> PSCs were fabricated on single large-area substrates and their <i>V</i><sub>oc</sub> dependencies on illumination intensity, <i>I</i><sub>0</sub>, were measured showing three distinctive regions. Similar results obtained in Al<sub>2</sub>O<sub>3</sub> based PSCs relate the effect to the compact TiO<sub>2</sub> rather than the mesoporous oxide. We propose that two working mechanisms control the <i>V</i><sub>oc</sub> in PSCs. The rise of <i>V</i><sub>oc</sub> at low <i>I</i><sub>0</sub> is determined by the employed semiconductor n-type contact (TiO<sub>2</sub> or MgO coated TiO<sub>2</sub>). In contrast, at <i>I</i><sub>0</sub> close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated <i>E</i><sub>Fn</sub> (as in the dye sensitized solar cells) to an <i>E</i><sub>Fn</sub>, directly determined by the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> absorber is suggested.