We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer.

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).

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We have experimentally compared the excitonic properties of hybrid multiple quantum wells, (C 6 H 5 -C 2 H 4 NH 3 ) 2 PbBr 4 and (C 4 H 9 NH 3 ) 2 PbBr 4 , using photoluminescence, reflection, and photoluminescence excitation measurements. We focused on the contribution of the image charge effect (ICE) to the excitonic energy structure in these materials which have different dielectric constants of the barrier layers. We have found that the binding energies of the 2s and 3s excitons are considerably enhanced by ICE, while the contribution of ICE to the 1s excitons is smaller because of the small Bohr radius, which is comparable to the well width.

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...