Reverse Bias Behavior of Halide Perovskite Solar Cells

DOI: 10.1002/aenm.201702365 solar cells have a breakdown voltage (VBD) at which current starts to flow in reverse bias. When current flows in reverse bias, the shaded cell dissipates power rather than producing it, and this can cause local heating, which damages the cell.[8] Silicon cells generally breakdown in reverse bias by avalanche breakdown; the carriers gain enough kinetic energy from the applied electric field to generate additional carriers through impact ionization. VBDs for silicon cells are typically >15 V. If the pn junction is highly doped, the depletion width can narrow enough to allow tunneling in reverse bias. With either mechanism, breakdown current can get localized by uneven doping, crystalline defects, trace processing contaminants, etch sites, or edge effects causing damaging hot spots. CIGS and CdTe exhibit VBDs < 10 V and a decrease in VBD under illumination. This has been attributed to tunneling through defects at the buffer layer/CIGS interface.[9] Partial shading has been shown to cause local current flow and the damage is exacerbated by the light dependence of the VBD, which causes even more of the current to selectively flow through the illuminated region. This can cause localized shunting those results primarily in a permanent decrease in fill factor.[10] Stability in reverse bias has not been explored for perovskite solar cells, but there have been studies of MAPbI3 memristors,[11–14] which require biasing in both forward and reverse directions, and photodetectors, which function in reverse bias.[15] Some memristors operate via the formation of metallic filaments through the perovskite[11,12]and others seem to function based on mobile defects in the perovskite.[12,13] Photodiodes of the structure fluorine-doped tin oxide (FTO)/porous TiO2/MAPbI3/ Spiro-OMeTAD/Au show current multiplication in reverse bias, which has been attributed to mobile ion accumulation.[15] Mobile ions have also been used to explain hysteresis in current–voltage measurements.[16,17] In most cells, preconditioning at lower voltages makes cells worse, which is why scans from JSC to VOC tend to give lower efficiencies. It has also been demonstrated that mobile ions can cause band bending that can turn a symmetric device with nonselective contacts into a diode that functions as a solar cell.[18] Mobile ions likely also play an important role in the behavior of perovskite solar cells in reverse bias. In this paper, we first present a phenomenological study of reverse bias breakdown in halide perovskite solar cells. We characterize cells that have been held at constant current in the dark as they would be in a series connected module if only one cell were completely shaded. We also provide constant voltage measurements. We show how the reverse breakdown The future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells under reverse bias conditions. Reverse bias stability is important because shaded cells in a module are put into reverse bias by the illuminated cells. In this paper, a phenomenological study is presented of the reverse bias behavior of halide perovskite solar cells and it is shown that reverse bias can lead to a partially recoverable loss in efficiency, primarily caused by a decrease in VOC. A general mechanism is proposed, supported by drift–diffusion simulations, to explain how these cells breakdown via tunneling caused by accumulated ionic defects and suggests that the reversible loss in efficiency may be due to an electrochemical reaction of these defects. Finally, the implications of these phenomena are discussed and how they can possibly be addressed is also discussed.