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.

Aging-induced light soaking and hysteretic behavior of methylammonium lead trihalide perovskite solar cells can be seen dependent on the morphology of copper iodide (CuI) utilized as a hole-transport layer.

Crystallographic orientation has a significant impact on the optoelectronic properties of films of quasi‐2D perovskite quantum wells. Here, oxygen‐bearing Lewis bases are employed as additives to explore their ability to modulate spatial uniformity of crystallization and orientation of crystal phases. Different Lewis bases added into the precursor solutions incorporating the large organic ammonium cation, phenethylammonium (PEA+), lead to different crystallization kinetics, which are attributed to the varying stability of intermediate complexes. The microscopic photoluminescence heterogeneity and 2D X‐ray diffraction patterns of the thin films reveal that inclusion of Lewis bases can lead to spatially more uniform crystallization and random orientation, resulting in an enhancement in light‐emitting diode performance. In contrast, quasi‐2D phases formed without Lewis bases show poorer uniformity and preferentially vertical orientation. Comparing the Lewis base properties such as Mayer order unsaturation and polarizability suggests that the ability to weakly coordinate with lead and strongly interact with the large organic ammonium is a key factor in controlling the phase composition favorably toward highly luminescent light‐emitting diodes. This work may be of help to provide insight of what kinds of Lewis bases can be helpful to realize the desired phase composition for high performance of optoelectronic applications.

The open-circuit voltage (V oc) of perovskite photovoltaic diodes depends largely on the selection of charge transport layers (CTLs) and surface passivation, which makes it important to understand the physical processes occurring at the interface between the perovskite and a CTL. We provide a direct correlation between V oc and the interfacial characteristics of perovskites tuned through stoichiometry engineering of precursor solutions and surface modification of the underlying poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer. Poor quality interfacial perovskite crystals were observed on top of the PEDOT:PSS layer, resulting in strong interfacial recombination and a low V oc. In contrast, the growth of the interfacial perovskite crystals was significantly improved by the synergic effects of varying the precursor solution composition and covering the surface with a pH-neutral conjugated polyelectrolyte, poly­[2,6-(4,4-bis­(potassium butanylsulfonate)-4H-cyclopenta­[2,1-b;3,4-b′]­dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-K), which possesses potassium ions as counter ions. The influence of the energy-level alignment at the interface on V oc was also discussed. Our findings highlight that improved perovskite crystallization at the interface can facilitate bulk growth of perovskite grains in the vertical direction and effectively suppress nonradiative surface charge recombination, thus enhancing the short-circuit current and V oc.

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