Close-Space Sublimation as a Scalable Method for Perovskite Solar Cells.

Vacuum techniques for perovskite photovoltaics (PV) are promising for their scalability but are rarely studied with techniques readily adaptable for industry. In this work, we study the use of close-space sublimation (CSS) for making perovskite solar cells, a technique that has seen widespread use in industry, including in PV, and benefits from high material-transfer and low working pressures. A pressed pellet of formamidinium iodide (FAI) can be used multiple times as an organic source, without needing replacement. Using CSS at a rough vacuum (10 mbar), efficient cesium formamidinium lead iodide perovskite based solar cells are obtained reaching a maximum photoconversion efficiency (PCE) of 18.7%. They maintain their performance for >650 h when thermally stressed at 85 °C in a nitrogen environment. To explain the initial rise in PCE upon heating, we used drift-diffusion simulations and identified a reduction in bulk trap density as the primary factor.