Work performed:
Part 1:
We focus our first study on the benchmark methylammonium lead bromide (MAPbBr3) perovskite film because of its low amplified spontaneous emission (ASE) threshold and intensive previous investigations on the optically pumped lasing actions. Here, the ASE is realized on the condition that the population of excited states exceeds the population of ground states (so-called population inversion condition). Since this is the same required condition for the laser emission, the ASE threshold provides a comparable indication for the laser threshold.
In the first work, different transport layers are introduced on top of perovskite thin films and the resulting ASE thresholds are characterized to study the effects of transport layers on perovskite lasing actions. With introducing an additional hole transport layer PTAA, we surprisingly find that the ASE threshold is reduced by 22.9% and the ASE intensity is improved by 18.8%. These changes cannot be explained by the possible defect passivation effects induced by the additional PTAA layer as PTAA reduces the spontaneous emission of MAPbBr3. Further transient photoluminescence and transient absorption measurements suggest that the threshold in this case is determined by the hot carrier cooling process. Specifically, the relatively slow hot hole cooling process within the bare MAPbBr3 film inevitably enlarges the ASE threshold due to increased Auger loss. By extracting the holes with PTAA, the cooling process is facilitated, which largely reduces the Auger loss and thus reduces the ASE threshold. In addition, we also attempt to directly avoid excess hot hole generation in perovskites by switching the pumping wavelength from 400 nm to 500 nm, and the resulting ASE threshold is further reduced from 25.7 to 7.2 mJ/cm2, providing another direct evidence to support our argument of the hot hole effects on the ASE threshold of perovskites. Our first work provides practical design strategies towards the realization of electrically pumped perovskite lasers.
Part 2:
Our second work aims at finding a rational way to estimate the required threshold carrier density that is useful for the design of electrically pumped perovskite laser diodes. We investigate the excitation photon energy dependence of amplified spontaneous emissions (ASE) in state-of-the-art bromide- and iodine-based perovskites. By reducing the excitation photon energy, we observe a large reduction of the threshold carrier density of ASEs and the pronounced bandgap renormalization (BGR) effect. We attribute our results to the carrier cooling process that is affected by coupling of the phonon scattering, giving rise to changes in time delay for carrier to accumulate at the band edge.
This mechanism is verified in two-photon pumping process, where carrier cooling to the band edge is faster than one photon excitation with the equivalent excitation energy, due to different momentum requirement in connection to the band structure. Based on the above results, we evaluate the threshold carrier density of metal halide perovskites to the band edge excitation, which is considered as a rational analog to direct carrier injection through the electrical pumping, by extrapolating the experimental data to the excitation energy close to the bandgap. The extrapolated threshold values for the resonance excitation, which corresponds to the eventual threshold carrier densities required for the electrical injection laser diodes are one order of magnitude lower than the previous reported estimate based on optically pumped ASE experiments with only one specific excitation energy (wavelength) in both MAPbBr3 and FAPbI3 films.
Main achievements:
1. With hole extraction via a PTAA layer on top, the hot hole-induced Auger recombination can be suppressed, resulting in amplified spontaneous emission (ASE) threshold reduction in perovskite thin film. This finding provides an inspiring indication that the transport layer can reduce the lasing threshold of perovskite, eliminating the community’s common concern that transport layers increase loss.
2. The hot carrier-induced Auger loss is proven detrimental to perovskite lasing actions. The loss can be reduced by hole extraction or low-energy excitation, which facilitates the hot carrier cooling process and decreases hot carrier generation, respectively. This discovery provides useful guidelines for reducing the lasing threshold of perovskites, which will spur new material designs for perovskite lasers.
3. The effect of hot carrier cooling on optically pumped amplified spontaneous emission (ASE) thresholds is observed as a universal phenomenon in both bromide- and iodine-based perovskite films. The ASE threshold carrier density decreases with reducing the excitation energy, due to coupling of multiple phonon emissions. This provides evidence that the hot carrier cooling process leads to overestimation of the ASE threshold, and the estimates obtained by only one excitation wavelength are significantly higher than needed.
4. The two-photon up-conversion ASE is also studied by introducing the excitation photon energies between 0.5 Eg and 1.0 Eg. The cooling process becomes faster compared to the one-photon excitation with the same energy. This observation is attributed to the reduced momentum for cooling in the unique band structures with Rashba splitting in metal halide perovskites, and subsequently benefits lower threshold carrier densities. This, on one hand, opens up great opportunities for upcoversion lasers based on perovskites, on the other hand rationalizes the phenomenon that upconversion ASE has been widely observed in perovskites.
