The electronic and excitonic structures of an inorganic–organic perovskite-type quantum-well crystal (C4H9NH3)2PbBr4 have been investigated by optical absorption, photoluminescence, electroabsorption, two-photon absorption, and magnetoabsorption spectroscopies. Excitons in (C4H9NH3)2PbBr4 are of the Wannier-type, and ns (n≥2) excitons form an ideal two-dimensional Wannier exciton system. The binding energy, longitudinal–transverse splitting energy, and exchange energy of 1s excitons have been determined to be 480, 70 and 31 meV, respectively. These high values originate from both a strong two-dimensional confinement and the image charge effect. These values are larger than those in (C6H13NH3)2PbI4, owing to the smaller dielectric constant of the well layer in (C4H9NH3)2PbBr4 than that in (C6H13NH3)2PbI4. The seemingly unusual electric-field dependence of excitons resonance is also reasonably understood by taking the image charge effect into account.
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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