Excess iodine as the interface recombination center limiting the open-circuit voltage of CuI-based perovskite planar solar cell

Copper iodide (CuI) thin film grown by a direct vacuum thermal evaporation method at low temperature is utilized as inorganic hole transport layer in planar CH₃NH₃PbI_3−xCl_x-based perovskite solar cell. The device performance is mainly controlled by the deposition rate of CuI layer for optimized thickness. The reason was investigated in detail by transient photovoltage/photocurrent spectroscopy (TPV/TPC), X-ray photoelectron spectroscopy (XPS), and the energy-dispersive X-ray spectrum (EDS). The results indicate that larger deposition rate leads to a non-stoichiometric CuI layer with excess iodine. More importantly, the redundant iodine is illustrated as the recombination center that results in interface recombination. Under optimized CuI growth condition, a power conversion efficiency of 6.4% has been achieved with a short-circuit current density (J_SC) of 16.4 mA/cm², an open-circuit voltage (V_OC) of 730 mV, and a fill factor (FF) of 54%. Our results open a possibility for further improvement of perovskite solar cells by controlling the concentration and distribution of iodine in the perovskite films.