It is well known that the surface trap states and electronic disorders in the solution-processed CH NH PbI perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surface-trap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V ) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed V as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5 % in the polymer-modified devices (PCE=15 %) with V more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50 % humidity).

We have measured the temperature dependence of the reflection and photoluminescence spectra of a lead bromide hybrid material (CH 3 NH 3 PbBr 3 ) using high-quality macroscopic single crystals. Single crystals provide clear and sampleindependent reflection spectra that permit discussion of the properties of the photoexcited carriers. We have found that excitons still exist in CH 3 NH 3 PbBr 3 , even at room temperature. We have also determined the temperature dependence of the exciton resonance energy and the line width.In recent years, lead halide organicinorganic hybrid perovskite materials have attracted considerable attention for photovoltaic applications. 16 The reported power conversion efficiencies (PCE) of the hybrid perovskite-type solar cells with lead iodide materials (CH 3 NH 3 PbI 3¹x Cl x ) are more than 20%. Meanwhile, such materials have been studied from the viewpoint that they form ideal low-dimensional systems and have stable excitons. 812 Even in three-dimensional hybrid materials, the exciton binding energies were reported to be 3750 meV. 13This suggests that stable excitons might exist even at room temperature, and seems to contradict the fact that these materials function as excellent photovoltaic materials, for which dissociation of photoexcited electronhole pairs is necessary to extract the photocurrent.While most studies of the excitonic properties have been performed at low temperatures, solar cells work at room temperature. Therefore, in order to resolve the contradiction stated above, it is important to study the temperature dependence of photoexcited carrier properties in these materials. Recent studies of CH 3 NH 3 PbI 3 indicate that the static dielectric constant shows a rapid increase with increasing temperature, suggesting that this effectively decreases the exciton binding energy. 14,15 Consequently, these results support the view that the excitons dissociate at room temperature in CH 3 NH 3 PbI 3 . In order to clarify the intrinsic properties of these materials, however, such investigations should be performed with high-quality single crystals, which are less affected by defects or impurities. Nevertheless, there are few studies on hybrid perovskite single crystals. 1618In this work, we used high-quality macroscopic single crystals of lead bromide hybrid material (CH 3 NH 3 PbBr 3 ), which has similar properties to CH 3 NH 3 PbI 3 but has a larger band gap (ca. 2.35 eV at room temperature).13 Although CH 3 NH 3 PbBr 3 does not absorb near infrared light, a PCE of more than 10% was reported. 19 The reason why we chose CH 3 NH 3 PbBr 3 is that macroscopic single crystals are more easily synthesized than those of CH 3 NH 3 PbI 3 . Using the single crystals, we have measured the temperature dependence of the reflection and photoluminescence (PL) spectra to discuss the properties of the photoexcited carriers. Our results indicate that excitons still exist in CH 3 NH 3 PbBr 3 , even at room temperature. In addition, we have determined the temperature dependence of the exciton reso...

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation-halide perovskite (FAPbI)(MAPbBr). The mixed perovskite was crystallized on a low-temperature prepared brookite TiO mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm device and 15.2% for a 1.0 × 1.0 cm device.

We have measured photoluminescence spectra in CH3NH3PbBr3 at low temperatures and found a nonlinear emission at the lower energy side of the exciton resonance. Considering that this signal shows a rapid decay, we have assigned it as originating from the exciton–exciton inelastic scattering process called P-emission. The energy difference between this P-emission and the free exciton resonance is 20–30 meV, which is consistent with recent studies of the exciton binding energy.

The nature of metal oxide scaffold played a pivotal role for the growth of high quality perovskites and subsequently facilitates efficient photovoltaics devices. We demonstrate an effective way to fabricate a low-temperature TiO2 brookite scaffold layer with a uniform and pinhole-free layer for enhancing photovoltaic properties of perovskite solar cells. Various concentrations of TiCl4 were used to modify brookite TiO2 for efficient charge generation and fast charge extraction. We observed that the brookite layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the substrates, whereas TiCl4 treatment further improves the contact of the TiO2/perovskite interface which facilitates charge extraction and drastically influenced charge recombination. The surface treated brookite scaffolds perovskite devices showed an improved performance with an average power conversion efficiency more than 17%. The time resolved photoluminescence showed that the treated samples have obvious effect on the charge carrier dynamics. The striking observation of this study was very low appearance of hysteresis and high reproducibility in the treated samples, which opens up the possibilities for the fabrication of high efficient devices at relatively low temperatures with negligible hysteresis via facile surface modifications.