Excited State Properties of Hybrid Perovskites

Abstract

Metal halide perovskites have come to the attention of the scientific community for the progress achieved in solar light conversion. Energy sustainability is one of the priorities of our society, and materials advancements resulting in low-cost but efficient solar cells and large-area lighting devices represent a major goal for applied research. From a basic point of view, perovskites are an exotic class of hybrid materials combining some merits of organic and inorganic semiconductors: large optical absorption, large mobilities, and tunable band gap together with the possibility to be processed in solution. When a novel class of promising semiconductors comes into the limelight, lively discussions ensue on the photophysics of band-edge excitations, because just the states close to the band edge are entailed in energy/charge transport and light emission. This was the case several decades ago for III-V semiconductors, it has been up to 10 years ago for organics, and it is currently the case for perovskites. Our aim in this Account is to rationalize the body of experimental evidence on perovskite photophysics in a coherent theoretical framework, borrowing from the knowledge acquired over the years in materials optoelectronics. A crucial question is whether photon absorption leads to a population of unbound, conductive free charges or instead excitons, neutral and insulating bound states created by Coulomb interaction just below the energy of the band gap. We first focus on the experimental estimates of the exciton binding energy (Eb): at room temperature, Eb is comparable to the thermal energy kBT in MAPbI3 and increases up to values 2-3kBT in wide band gap MAPbBr3 and MAPbCl3. Statistical considerations predict that these values, even though comparable to or larger than thermal energy, let free carriers prevail over bound excitons for all levels of excitation densities relevant for devices. The analysis of photophysics evidence confirms that all hybrid halide perovskites behave as free-charge semiconductors. Thanks to such property, in combination with band gap energies covering the entire solar spectrum, perovskites represent a promising materials platform for highly efficient, single and multijunction solar cells. Concerning the use of perovskites as color-tunable materials in light emitting devices, free-charges are not the preferred species, as they recombine radiatively through a bimolecular process that is inefficient at the charge-injection levels typical of LED operation. Strategies to overcome this limit, and thus extend the use of perovskite materials beyond solar energy conversion, could be borrowed from inorganic semiconductor optoelectronics and include the fabrication of nanostructures with reduced dimensionality to alter the electronic density of states, as well as engineering composite materials.

Intriguing Optoelectronic Properties of Metal Halide Perovskites

“…182 Comprehensive modeling of the convoluted band edge absorption in CH 3 NH 3 PbX 3 has been carried out in a number of studies. 58,59,184,189,225,233 Excitonic and Coulomb-enhanced absorption are accounted for in Elliot's theory of Wannier excitons according to 234…”

Continuous‐Wave Lasing in Cesium Lead Bromide Perovskite Nanowires

“…The Rabi splitting of Ω = 200 meV determined here for CsPbBr 3 NWs is very similar to values from 2D layered organic-inorganic hybrid perovskite [31][32][33] or 3D CsPbBr 3 nanoplates [25] in DBR microcavities, suggesting that strong light-matter interaction is intrinsic to this class of materials. In perovskites, while an above gap excitation creates predominantly charge carriers at room temperature, [40,41] there is a strong excitonic resonance in absorption and fluorescence emission, with an exciton binding energy of ≈40 meV for CsPbBr 3 perovskite. The vacuum Rabi splitting measured for CsPbBr 3 NWs represents the highest value for inorganic semiconductor lasers where previous studies reported Ω up to ≈25 meV in the near-IR to visible region in, e.g., GaAs and CdTe, and up to ≈150 meV in the UV region for large bandgap materials, such as ZnO and GaN.…”

Color-Pure Violet-Light-Emitting Diodes Based on Layered Lead Halide Perovskite Nanoplates

“…38 The quantum confinement and high excitons binding energies are believed to increase the LED efficiency. 39 However, to date, such 2D perovskites have not been successfully integrated as the light emitting material into LEDs that can be operated at room-temperature. Green LEDs incorporating 2D PEA lead iodide [(PEA)2PbI4] were previously reported to function exclusively at liquid nitrogen temperatures, 40 thus precluding their practical applications.…”