Operation Mechanism of Perovskite Quantum Dot Solar Cells Probed by Impedance Spectroscopy

We fabricated perovskite quantum dot solar cells (PQDSCs) and varied the thickness of the QD layer by controlling the number of deposition cycles; the cells were systematically investigated with impedance spectroscopy. Despite the evident structural differences with respect to standard perovskite solar cells (PSCs), similar impedance spectra were obtained for PQDSCs, pointing to similar working principles in terms of the active layer. We distinguish two different regimes: At low illumination, recombination is ruled by multiple trapping with trap distributions and/or shunting. However, at higher light intensities, Shockley-Read-Hall recombination is observed. In addition, the low-frequency capacitance, C-LF, of PQDSCs increases several orders of magnitude when the illumination is varied from dark to 1-sun conditions. This feature has not been observed in other kinds of photovoltaic devices and is characteristic of PSCs. Although there is no consensus about the exact mechanism responsible for C-LF, the suggested models point to an ion migration origin. Its observation in thin-film and PQDSCs devices implies a similar effect in both.