Researchers image real-time structural changes in perovskite solar cells

Researchers from Korea and the USA have used an imaging technique to observe structural changes at the atomic level suggesting strategies to reduce perovskite solar cell degradation.

Perovskite solar cells (PSCs) tend to degrade quickly. When they are exposed to sunlight, freely moving ion vacancies form in the structure and migrate towards the electrodes. In dark conditions, the effect is reversed, and the ions are once again redistributed in the perovskite structure. Repeated cycles of this ion transport during the operation of the solar cell permanently degrade the cell and result in short lifetimes. However, degradation at the atomic level due to ion migration has not been directly observed.

Researchers from Korea and the USA were recently successful in observing changes in the perovskite structure using an in-situ transmission electron microscopy (TEM) imaging technique. "This work offers vital insights on understanding ion migration and addressing instability challenges of perovskite optoelectronics", explains Assistant Professor Min-cheol Kim of the Department of Mechanical Engineering at Pusan University, who led the study.

While electron microscopy has the scope to image structures at the atomic scale, the instability of the perovskite structure towards an external electrical bias (voltage) makes it challenging to monitor it for structural changes. The researchers circumvented the problem by imaging perovskites under a stable voltage. They first prepared a thin layer (or lamella) of a nano perovskite solar cell. The lamella was placed between the electrodes of an electronic chip and a stable voltage was applied across the sample. This setup enabled the researchers to stimulate the photovoltage experienced by the solar cell under working conditions. A constant voltage of 1 V was applied across the lamella and the structure was monitored by TEM.

The researchers observed a gradual disappearance of the lattice fringes of perovskite crystals, which indicated the "amorphization" of the structure. As is the case with perovskite crystals, amorphization was observed to be reversible under dark conditions. However, using TEM, the researchers noticed that recrystallization was faster when the sample was subjected to mild heating at 50°C. "This work not only demonstrates the ability to achieve real-time imaging of perovskite amorphization but also provides an effective method to recover the degraded performance of PSCs related to ion instability," comments Prof. Kim

Elucidating the changes in perovskite structure could enable researchers detect points of failure and build more resilient solar cells. In addition, observing the rate of recrystallization can help determine the effectiveness of strategies used to restore the performance of solar cells.