Modification of a Buried Interface with Bulky Organic Cations for Highly Stable Flexible Perovskite Solar Cells

Flexible perovskite solar cells triggered a vast interest within the scientific community, thanks to their broad commercialization prospects. However, the stability of these devices still poses one of the major concerns on the way to rapid industrial deployment. Here, we demonstrate an effective strategy to improve the technical aspects of this technology, improving the reliability and efficiency values of these devices. We apply large organic ammonium molecules for modifying a buried interface between a hole-transporting layer (HTL) and perovskite-absorbing material. With the 4-fluorophenethylammonium iodide (FPEAI), we achieve 18.66% efficiency for the large-area (1 cm2) flexible solar cell, a significant improvement over the pristine device without modification. The applied passivation strategy results in a better hole extraction and reduced nonradiative recombination loss at the buried interface. Moreover, we demonstrate the formation of low-dimensional perovskite phases in the vicinity of the hole-transporting material upon the incorporation of large ammonium cations. This results in a significantly enhanced thermal and light-soaking stability of fabricated devices. We obtained no loss in 1000 h of aging at 85 degrees C, no loss in 1000 h of light soaking at open circuit, and less than 10% drop in 1000 h of operation at maximum power point for the optimized passivation treatment with the FPEAI. We also demonstrate a method for monitoring the structural stability of perovskite thin films upon prolonged illumination, ensued by the amount of molecular iodine being released from the layer.