Impact of Hydroxyl Groups Boosting Heterogeneous Nucleation on Perovskite Grains and Photovoltaic Performances

Surface energy is a key factor in controlling the kinetics of nucleation and growth of perovskite, which are crucial for the formation of high quality films and the photovoltaic efficiency of solar cells. It has been reported that substrate wettability and perovskite grain size are to be compromised with necessity, as promoted heterogeneous nucleation that occurs on a hydrophilic surface reduces the grain size for a two-step deposition method. Herein, the increase in grain size on hydrophilic surfaces in the presence of hydroxyl groups and the direct correlation between the perovskite grain formation and photovoltaic performance are investigated. The surface energy of the hole transport layer in planar p-i-n type perovskite solar cells is modulated by the introduction of polymer surfactant additive, poly(ethylene glycol) tridecyl ether (PTE). Perovskite films deposited on a hydrophilic surface by a two-step method contain small grain size, leading to a reduction in photovoltaic performance. In contrast, surface hydroxyl groups were found to induce the preferential (110) orientation and large grain size in the perovskite films deposited by means of a one-step method. Nucleation and growth mechanisms are proposed to explain those different behaviors of the dependence of grain size on surface energy. The enlarged perovskite grains on hydrophilic surfaces lead to an efficiency improvement owing to an increase in the short-circuit current and fill factor. Our study highlights that the grain size increase and high crystallinity can be achieved even with accelerated heterogeneous nucleation on a hydrophilic substrate surface.