Strong Photocurrent Amplification in Perovskite Solar Cells with a Porous TiO2 Blocking Layer under Reverse Bias

We investigate two different types of TiO2 blocking layer (BL) deposition techniques commonly used in solid-state methylammonium lead triiodide perovskite (MaPbI(3))-based solar cells. Although these BLs lead to similar photovoltaic device performance, their structure and blocking capability is actually very different. In one case, the "blocking" layer is porous, allowing an intimate contact of the perovskite with the fluorine-doped tin-dioxide (FTO)-covered glass substrate serving as transparent electron collector. This interface between the perovskite and the FTO shows rectifying behavior. Reverse biasing of such a solar cell allows the determination of the valence-band position of the MaPbI(3) and the theoretical maximum attainable photovoltage. We show that under reverse bias strong photocurrent amplification is observed, permitting the cell to work as a high-gain photodetector at low voltage. Without BL, the solar-cell performance decreased, but the photocurrent amplification increased. At 1 V reverse bias, the photocurrent amplification is above a factor of 10 for AM 1.5 solar light and over 100 for lower light intensities.