Researchers at the National Renewable Energy Laboratory (NREL) and University of North Carolina at Chapel Hill have reported a systematic study on the degradation mechanisms of p–i–n structure perovskite solar cells (PSCs) under reverse bias. Reverse bias is a phenomenon that can occur when, for example, an individual cell is shaded and other cells in the module try to push a higher current through it, increasing the temperature and potential damage to the cells. These conditions make solar cells unstable and deteriorate their performance over time.
The team's new strategy could improve the stability of PSCs under reverse bias conditions and facilitate the future deployment of perovskite-based photovoltaics (PVs) in real-world settings.
Perovskite cells are known to have a significantly thinner photoactive layer than other existing PVs. As a result, the electric field induced by reverse bias conditions can be far larger in this type of cell. The scientists also explained that the ion migration in perovskites is another factor that makes them much less stable under reverse bias. In many past studies, PSCs were reported to break down or degrade after a few seconds to a few minutes under reverse bias of a few volts. Solving this issue is vital for future adoption of PSCs, otherwise future perovskite modules will need many bypass diodes to protect them, pushing up their fabrication costs.
When reviewing previous literature, the team observed that some PSCs were significantly more stable under reverse bias conditions. This motivated them to investigate the mechanisms underpinning this greater stability, in the hope of devising an effective method to stabilize perovskite cells. In their study, they chose p-i-n structured PSCs as they have shown good operational stability in previous studies.
The scientists used an optimized perovskite composition to minimize other possible degradation pathways. they applied various reverse bias values to PSCs, while changing the device stacks. They then closely examined what happened in the devices and tried to determine whether the phenomena they observed were correlated to the device's performance.
They were thus able to identify the degradation mechanism inside the devices - carefully distinguishing the degradation behavior with breakdown and gradual degradation. They explained that the former happens under high reverse bias in a short period, while the latter happens under low reverse bias in a longer duration.
The experiments carried out by the team unveiled a series of electrochemical reactions that were associated with the degradation of PSCs under reverse bias conditions. These reactions entailed the generation of iodine, which led to the corrosion of the Cu electrode in the cells. This process, in turn, triggered the breakdown and degradation of the solar cells.
Inspired by their findings, the scientists used a device stacking of lithium fluoride/tin oxide/indium tin oxide, which also makes devices stable under light and heat, to inhibit the iodine formation under reverse bias the electrode corrosion. By doing so, they found that the lifetime of modified PSCs could last up to 1,000 hours under reverse bias of -1.6 V.
The researchers plan to conduct additional studies, which would be aimed at delineating the upper limit of the reverse bias stability for PSCs.
