Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Abstract

their great success in photovoltaics (PVs), with a phenomenally rapid rise of the power conversion efficiency (PCE) from 3.8% [1] to over 23% [2,3] in the past few years. Such high PCE of perovskite solar cells has been ascribed to the ultralong carrier lifetimes, [4][5][6][7][8] long carrier diffusion lengths, [9][10][11][12][13][14] and the extraordinarily defect tolerance. [15][16][17] Following the success of perovskites in photovoltaics, research on light-emitting diodes (LEDs), [18,19] amplified spontaneous emission (ASE) or lasers, [20][21][22][23][24] and photodetectors [25][26][27][28][29] have also gained substantial interests. Meanwhile, the rapid progress of MHPs on various applications spurred a flurry of photophysical studies in order to understand the origin of the high performance of these devices, of which the nature of the photoexcitation species has been mostly debated. [5,22,30,31] Since the photoexcitation states near the bandgap affect the key processes such as charge transport and light emission of optoelectronic devices, there is no doubt that the investigation of electronic excitations near the optical band edge is crucial for MHPs. Such knowledge is essential not only for understanding the correlation between the fundamental photophysics and device performances, but also offering a guideline for their further applications with improved performances. Generally, there are two kinds of photoexcitations near the band edge for direct bandgap semiconductors: free carriers and excitons. Exciton binding energy that reflects the Coulomb interaction strength between photoexcited electrons and holes determines the balance of the populations between the two species. Typically, inorganic semiconductors are free-carrier materials with the exciton binding energies only in few meV at room temperature and their excited states being populated principally by free carriers. While organic semiconductors are excitonic materials with the exciton binding energies in hundreds of meV and thus excitons prevailing in the excited states. Whereas, MHPs that combine some merits of organic and inorganic semiconductors seem to stand for an exotic class of semiconductors between these two limiting cases, with the experimentally determined exciton binding energy varying in a wide range of 2 to more than 50 meV for the prototype perovskite MAPbI 3 . [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] Such large variations of the exciton binding energies reported for MHPs give rise to a strongly debated question, that is, whether free carriers or excitons are generated upon photoexcitation? In other words, what is the nature of the photoexcitation species Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid unders...

Advances in inorganic and hybrid perovskites for miniaturized lasers

“…For example, the E b values of MAPbCl 3 and MAPbBr 3 have been reported as ranging from ~50-106 meV [51, 52] and 21-100 meV [51,53,54], respectively, higher than that of MAPbI 3 . Similar trends have been shown in FAPbX 3 (~8.1-110 meV) and CsPbX 3 (~20-72 meV) [41,47]. The perovskite E b values from several to hundreds of meV are favorable for highperformance and versatile optoelectronic devices.…”

“…At room temperature, when the E b is smaller than the thermal fluctuation energy (K b T ~ 26 meV), the generated exciton is likely to dissociate into free carriers (electron and hole), which is highly desired for photovoltaic device. On the contrary, the large E b signifies the existence and stability of excitons, which is more favorable for light-emitting devices due to the efficiently radiative recombination achieved through the excitons at relatively lower carrier densities [41].…”

“…MAPbI 3 has attracted extensive attention as a common and important light absorption layer in perovskite solar cells. The initial E b values of MAPbI 3 have been reported in a wide range of 2-50 meV [41,49,50]. Furthermore, the E b of the analogues of MA-based perovskites has been studied and shown to increase with substitution of halide elements from I, Br to Cl.…”

Recent Progress in Engineering Metal Halide Perovskites for Efficient Visible‐Light‐Driven Photocatalysis

“…Furthermore, the composition, structure, and size of MHPs can also be tuned by postsynthetic transformations, such as ion exchange, heteroatom doping, and surface/interface modification and heterojunction engineering, which pave the way for further tuning of their physicochemical properties. For more details about the structural, synthetic, size/shape control, and composition tuning of MHPs, please refer to previously published reviews …”

Solution‐Processed High‐Quality Cesium Lead Bromine Perovskite Photodetectors with High Detectivity for Application in Visible Light Communication

“…The absorbance spectra of the perovskite films are shown in Figure 2b. The absorbance edge behaves a clear redshift when the CsPb 2 Br 5 film is gradually transformed to CsPbBr 3 ‐CsPb 2 Br 5 by increasing the CsBr‐treatment cycle, which indicates that perovskite grains grow bigger and the exciton binding energy becomes stronger 29. Consistent with the previous XRD results, the pristine film with the pure CsPb 2 Br 5 phase and the 5‐cycle film with the almost pure CsPbBr 3 phase show merely single absorbance edge at 350 and 525 nm, respectively.…”