Investigation on the role of Lewis bases in the ripening process of perovskite films for highly efficient perovskite solar cells

Introduction of Lewis acid-base adducts is an efficient way for achieving high quality perovskite films. In this work, we investigate the influence of Lewis bases on the annealing process of perovskite films, and three Lewis bases with different melting points and boiling points including DMSO, urea and thiourea are chosen. The interaction between the Lewis base and PbI2 as well as the residual time of Lewis bases in perovskite films while annealing has been investigated. In the meantime, directly spin-coating urea or thiourea on perovskite films is also performed to study the effect of Lewis bases on the perovskite crystallinity and morphology during annealing. It is found that the existence of Lewis bases in the annealing process could significantly promote the perovskite grain growth. DMSO can be removed within a few minutes whereas the residual time of thiourea is longer than that of DMSO and urea, consistent with their boiling points. As a result, the grain sizes of the thiourea-based perovskite film increase continuously whereas the DMSO-based sample shows an obvious ripening effect only in the first few minutes. To take advantage of the ripening effect, synergistic systems of DMSO and a small amount of urea or thiourea are further adopted for fabricating perovskite solar cells (PSCs). The device based on DMSO/urea presents the best PCE of 20.06%, higher than 18.8% for the device based on DMSO and 8.35% for the device based on urea.