Researchers from the Shaanxi Normal University in China have designed a perovskite solar cell based on methylammonium lead iodide (MAPbI3) through a dual passivation technique that simultaneously passivates trap defects in both the perovskite and electron transport layer (ETL) films.
'So far, most techniques for modifying perovskite solar cells focus on either the perovskite or electron transport layer,' the research group reported, noting that the ETL must have decent optical transmittance and high electron mobility to extract photo'induced carriers and contribute to the solar cell efficiency.
The dual passivation was achieved by integrating europium (Eu3+) ions in both layers in a one-step process. These ions are reportedly able to reduce defects in both layers and, as a result, to improve the photovoltaic properties of the MAPbI3. Europium and tin ions were simultaneously deposited to form films on FTO (Fluorine-doped Tin Oxide) glass, which is a transparent conductive metal oxide that is commonly used in the fabrication of transparent electrodes for solar cells.
'We observed directional diffusion of Eu3+ from the Tin(IV) oxide (SnO2) ETL to the MAPbI3 perovskite film, which leads to accumulation of large amount of Eu3+ at the perovskite/ETL interface,' the Chinese group explained. 'The champion fabricated perovskite solar cell attains a PCE as high as 20.14%, and, when exposed to the ambient atmosphere, the unsealed perovskite solar cell presents a slow degradation by only 13% after 84 days.' The cell also showed an improved fill factor, due to the reduced trap density in perovskite film, and higher open-circuit voltage, thanks to the lower reduced energy loss in MAPbI3.
The scientists emphasized that the aggregation of europium ions at the interface between the perovskite and SnO2 films is the key factor improving electron transport through the interface by reducing the charge accumulation. Furthermore, the europium ions are responsible for reducing perovskite degradation by preventing moisture permeation.