Interpretation of Slow Electroluminescence and Open Circuit Voltage Transient Response in Cs-Based Perovskite Solar Cells
The desirable small hysteresis in the current-voltage characteristics of perovskite solar cells is often understood as a result of small ionic concentration or mobility and low interface charging by depleted/accumulated ions. However, devices having very small apparent hysteresis at practical scan rates can exhibit strong ionic effects seen in the transient response to excitation events. We explore Cs-based double-cation perovskite solar cells showing vanishing hysteresis and nearly hour-long responses to light- and voltage-step excitation, which are tracked by the evolution of open-circuit voltage and injected current, together with electroluminescence emission, respectively. The observed responses, including the increase of electroluminescence with time, are explained by the modulation of the electric field within the perovskite by mobile ions under the condition of interface recombination of mobile charge carriers dominating overall recombination. This is further explored by a numerical model containing mobile ions, which requires that charge carriers recombine predominantly at the interfaces of the device in order to obtain transient responses comparable to the experiments. Further support for the predominance of interface recombination is experimentally obtained by steady-state photocarrier grating characterization, which yields diffusion lengths of photogenerated carriers above 1 mm, i.e., well above the 300 nm perovskite layer thickness. The numerical model further shows that within the case of dominating interface recombination, the shape of both the light- and voltage-step transient responses are strongly determined by the energy band offsets between the perovskite and the contact layers of the solar cell.