The Bright Side and Dark Side of Hybrid Organic–Inorganic Perovskites

Abstract

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher energy bright and lower energy dark electrons (holes). The spatial separation of the dark electrons and the dark holes and the k-space separation of the bright and the dark charge carriers reduce electron hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets.

Photophysics of Localized Deep Defect States in Hybrid Organic–Inorganic Perovskites

“…This is consistent with the fast diffusion of iodine atoms observed in MAPbI 3 and with the light induced halide interstitial mediated phase segregation in MAPb­(I 1– x Br x ) 3 alloys . It is also in general agreement with theoretical calculations that place the (0/−) charge transition state of I interstitial acceptor in the lower half of the band gap. , The identification of the localized deep defect as a halide interstitial suggests its close relationship with the BANDs which are two different configurations of iodine vacancy, as V I donor and a relaxed V MA + MA I acceptor configuration. , This connection has very important implications for understanding the photophysical properties and stability of HOIPs under illumination and will be discussed in a forthcoming work.…”

“…More specifically, we demonstrated that bistable amphoteric native defects (BANDs) play an indispensable role in the formation of p−i−n junctions 6 and can account for the remarkable photophysical properties of HOIPs. 7 As has been shown previously 7 the BANDs represented by the anion dangling bond donor and halide dangling bond acceptor defects do not act as nonradiative recombination centers but rather reduce the recombination by partially separating photoexcited electrons from photoexcited holes.…”

“…26,27 The identification of the localized deep defect as a halide interstitial suggests its close relationship with the BANDs which are two different configurations of iodine vacancy, as V I donor and a relaxed V MA + MA I acceptor configuration. 6,7 This connection has very important implications for understanding the photophysical properties and stability of HOIPs under illumination and will be discussed in a forthcoming work.…”

Facile Preparation of Large-Area, Ultrathin, Flexible Semi-Transparent Perovskite Solar Cells via Spin-Coating

“…Of course, one could expect such a shift in the case of very thin samples (as it can be related to quantum confinement and built-in strain), but here it was clearly observed in the whole investigated thickness range. Another possible explanation is related to the hypothesis that emission in HOIPs such as MAPbI 3 is governed by bistable amphoteric native defects (BANDs), i.e., defects whose nature (donor/acceptor) and formation energy are controlled by the location of Fermi energy. , Assuming that due to the increased surface-to-volume ratio in thinner films, the concentration of BANDs is higher, one can expect such a behavior of the PL peak. A further interesting feature in PL spectra is related to the thickness range between 50 and 225 nm.…”

Facile Preparation of Large-Area, Ultrathin, Flexible Semi-Transparent Perovskite Solar Cells via Spin-Coating

“…defects whose nature (donor/acceptor) and formation energy are controlled by the location of Fermi energy. 42,43 Assuming that due to increased surface to volume ratio in thinner films the concentration of BANDs is higher one can expect such a behavior of PL peak. A further interesting feature in PL spectra is related to the thickness range between 50 nm and 225 nm.…”