Two-dimensional (2D) organic-inorganic hybrid perovskite multiple quantum wells that consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies of order of 0.3 eV due to the dielectric confinement between the inorganic and organic layers. We have investigated the exciton characteristics of 2D butylammonium lead iodide, (CHNH)PbI using photoluminescence and UV-vis absorption in the temperature range of 10 K to 300 K, and electroabsorption spectroscopy. The evolution of an additional absorption/emission at low temperature indicates that this compound undergoes a phase transition at ≈250 K. We found that the electroabsorption spectrum of each structural phase contains contributions from both quantum confined exciton Stark effect and Franz-Keldysh oscillation of the continuum band, from which we could determine more accurately the 1s exciton, continuum band edge, and the exciton binding energy.

The Rashba splitting in hybrid organic–inorganic lead–halide perovskites (HOIP) is particularly promising and yet controversial, due to questions surrounding the presence or absence of inversion symmetry. Here we utilize two-photon absorption spectroscopy to study inversion symmetry breaking in different phases of these materials. This is an all-optical technique to observe and quantify the Rashba effect as it probes the bulk of the materials. In particular, we measure two-photon excitation spectra of the photoluminescence in 2D, 3D, and anionic mixed HOIP crystals, and show that an additional band above, but close to the optical gap is the signature of new two-photon transition channels that originate from the Rashba splitting. The inversion symmetry breaking is believed to arise from ionic impurities that induce local electric fields. The observation of the Rashba splitting in the bulk of HOIP has significant implications for the understanding of their spintronic and optoelectronic device properties.

While research on derivatives of both bulk and low-dimensional metal halide perovskite (MHP) semiconductors has grown exponentially over the past decade, the understanding and intentional applications of electronic doping have lagged behind. In this Focus Review, we take a critical look at these challenges by considering the different potential doping routes, the advantages and pitfalls of each route, and the unique properties of MHP systems that may contribute to the inherent difficulties of realizing successful electronic doping. We specifically consider low-dimensional MHP derivatives as a case study, given that the mechanistic understanding of how defect chemistry affects electronic doping has been studied less extensively in these systems than in their three-dimensional counterparts, but we also consider lessons learned from the prototypical bulk methylammonium lead iodide perovskite semiconductor to inform our discussion. We discuss the potential roles that the partially ionic nature of the chemical bonds and the soft, polarizable nature of the lattice may play in the realization of doping in MHPs, with an emphasis on defect chemistry, redox side reactions, and polaronic stabilization. Informed by relevant case studies, we illustrate lessons taken from the literature and our own experience in an effort to provide a foundation for successful electronic doping of MHPs. We conclude that the successful realization of doped MHPs will likely hinge upon careful consideration and application of doping strategies and mechanisms that have been established in both the inorganic and organic semiconductor fields over the past several decades.

Excited-state interactions between organic and inorganic components in hybrid metal halide semiconductors open up the possibility of moving charge and energy in deliberate ways, including energy funneling, triplet energy harvesting,...