Effects of thiourea on the perovskite crystallization for fully printable solar cells

Fully printable HTL free perovskite solar cells (PSCs) are the most attractive candidates for large scale production, due to their low cost and high stability. In this kind of PSCs, perovskite precursor solutions are dropped on the three layer mesoscopic scaffolds, so it is important to improve the crystallinity as well as the penetrability into the mesoscopic scaffolds. Thiourea that has two NH2 groups and one S-donor can form hydrogen bond with the perovskite crystals or lewis acid-base adducts to retard the growth of perovskite crystals. In this work, it is found that thiourea facilitate the penetrability of perovskite precursor solution into the mesoscopic scaffolds and the crystallinity of perovskite crystals. It is suggested that such effects are associated with the cross-liker function and with the retarding function of thiourea. With adding thiourea into the perovskite precursor solution, electron transfer from perovskite crystals to FTO and the recombination resistance are enhanced, so that the performance of thiourea based device was improved from 10.20% (the pristine device) to 13.03%. Along with this, the thiourea can reduce iodine molecule to I- ion, so that perovskite films with thiourea have a higher stability than the pristine film.