Changes from Bulk to Surface Recombination Mechanisms between Pristine and Cycled Perovskite Solar Cells

Several aspects on the photophysical characterization of lead halide hybrid organic inorganic perovskite solar cells remain unsolved. It has been observed that ionic transport and polarization of the interfaces cause very slow changes that interfere with transient measurements with effects that cannot be separated from recombination kinetics. Here we establish a protocol of initial measurement of the solar cell that provides information on recombination characteristics prior to applying any voltage cycling. The photovoltaic device is measured by several methods (photovoltage versus light intensity, open-circuit voltage decay, and impedance spectroscopy) while minimizing the exposure to external voltage stimulus to avoid ionic migration to the contacts. Results are independently confirmed by the analysis of samples with interdigitated electrodes. We show that the high-efficiency perovskite solar cells behave very closely to a bulk recombination ideal photovoltaic model. However, when voltage is scanned to determine current density voltage curves and impedance spectroscopy at fixed illumination intensity, the cell undergoes significant changes, which we attribute to a dominance of recombination at contacts that have been modified by ionic polarization. Our method provides an effective approach for determining quantitatively the rather significant changes that occur to perovskite solar cells during standard measurements, such as current voltage curves.