Reduced Optical Gain Threshold by Carrier Multiplication in Semiconductor Perovskite Nanocrystals

Abstract: Carrier multiplication (CM) describes a strong charge-carrier interaction process in semiconductor colloidal nanocrystals (NCs), wherein two band-edge excitons are simultaneously created by an absorbed photon with at least twice the bandgap energy (2 Eg). While being fundamentally intriguing, it has been exclusively utilized to enhance the light-to-electricity conversion efficiencies in the photodetector and solar-cell devices. In this report, we have synthesized the core/shell perovskite FAPbI3/NdF3 NCs with a biexciton recombination lifetime of ~3.9 ns, and demonstrated that a CM efficiency of ~25.7% can be achieved under the ~355 nm laser excitation (~2.21 Eg). This CM occurrence leads to a two-fold reduction in the optical gain threshold, as compared to that obtained under the ~640 nm laser excitation (~1.23 Eg). When combined with the single-exciton and zero-threshold optical gain schemes previously developed for semiconductor colloidal NCs, the CM effect introduced here would further mitigate the optical-pumping requirement for the routine operation of continuous-wave lasing.

• The paper demonstrates that carrier multiplication in FAPbI₃/NdF₃ perovskite NCs reduces the optical gain threshold, lowering <N> from ~1.20 (640 nm) to ~0.68 (355 nm) excitation levels.
• The paper employs single-particle time-resolved PL and ensemble transient absorption spectroscopy, revealing a CM efficiency of about 25.7% under high-energy excitation.
• The paper suggests that exploiting CM can enable low-threshold, electrically pumped lasers with reduced energy consumption and improved device longevity.

Carrier Multiplication-Enabled Threshold Reduction for Optical Gain in FAPbI₃/NdF₃ Perovskite Nanocrystals

Introduction

The investigation of semiconductor colloidal nanocrystals (NCs) as gain media for lasing applications continues to advance due to their low-cost synthesis, solution processability, high fluorescence quantum yield, and narrow/definable emission spectra. Realizing optically-pumped and, ultimately, electrically-pumped continuous-wave (CW) lasing hinges on efficiently generating population inversion and overcoming Auger recombination, especially for multiexciton states. A significant bottleneck arises from the optical gain threshold—the minimum excitation required to achieve optical gain—whose reduction is vital for the practical deployment of colloidal nanocrystal lasers. This work presents a systematic study of perovskite FAPbI₃/NdF₃ core/shell NCs exhibiting carrier multiplication (CM) with high efficiencies and demonstrates that CM can substantially lower the optical gain and amplified spontaneous emission (ASE) thresholds.

Carrier Multiplication in Perovskite Nanocrystals

Carrier multiplication is a nonlinear photophysical process wherein the absorption of a single high-energy photon (with $h\nu > 2E_g$) results in the nearly simultaneous generation of multiple band-edge electron–hole pairs (excitons). While CM has previously been explored as a means to enhance quantum efficiency in optoelectronic devices such as solar cells, this work extends its application domain to lasing threshold reduction. In core/shell FAPbI₃/NdF₃ NCs with engineered type-II band alignment, biexciton Auger recombination is moderately suppressed (lifetime $\sim$3.9 ns), providing a suitable platform for isolating and studying the CM phenomenon under high-energy excitation.

Experimental measurements (single-particle time-resolved PL and ensemble transient absorption (TA)) reveal that excitation with 355 nm ($\sim 2.21E_g$) photons leads to a CM efficiency of $\sim$25.7%. The process is highly wavelength-dependent: at lower photon energies (640 nm, $\sim 1.23E_g$), CM is negligible. The efficiency compares favorably with previously reported values in other nanocrystal systems. A pronounced amplitude of the fast (biexciton) PL decay component upon high-energy excitation confirms robust multiple exciton generation, enabled by the long hot carrier relaxation times intrinsic to lead-halide perovskites.

Impact on Optical Gain and ASE Thresholds

Optical gain in colloidal NC films was characterized via nonlinear absorption and TA spectroscopy, with gain manifesting at lower average excitation densities (<N>, average photons absorbed per NC per pulse) when employing high-energy excitation. Specifically, the gain threshold is reduced from $<N> \approx 1.20$ (640 nm excitation) to $<N> \approx 0.68$ (355 nm excitation) in the presence of CM. Similarly, the ASE threshold is decreased from $<N> \approx 1.35$ to $<N> \approx 0.85$ under the same respective conditions. These results demonstrate that the CM pathway leverages the quantum confinement-enhanced Coulomb interactions in these NCs to achieve population inversion with fewer incident photons, drastically lowering the gain and lasing thresholds.

Notably, the CM-induced thresholds approach those obtained by other advanced schemes such as single-exciton gain and zero-threshold gain via trion formation, but remain compatible and potentially synergistic with these paradigms. The observed gain lifetimes (optical gain maintenance over $\sim$732 ps under 355 nm excitation) are also extended relative to traditional approaches.

Implications and Future Prospects

The exploitation of CM for lasing threshold reduction presents substantial implications for the development of low-threshold, solution-processable, perovskite-based lasers. The threshold reduction directly improves the feasibility of colloidal NC-based CW lasers and, by extension, electrically-driven laser diodes, by decreasing the intensity requirement for population inversion. The advances in CM-induced gain are particularly beneficial for designs aiming at electrically pumped devices, which suffer additional losses due to charge transport layers but could be compensated by the lower threshold enabled here.

Practical impacts include reduced energy consumption, increased device longevity (by minimizing photodamage and thermal stresses), and broadened material compatibility. Theoretically, this study invites a reassessment of gain mechanisms in quantum-confined perovskite heterostructures, highlighting hot carrier dynamics and CM as critical parameters.

Future work should pursue the electrical injection of high-energy carriers ($\sim$0) to exploit electrically-driven CM, integration with advanced cavity geometries, compositional tuning for further lifetime enhancement, and surface/interface engineering to further suppress non-radiative channels. Additionally, strategies combining CM with the zero-threshold gain frameworks may yield devices with unprecedented performance metrics.

Conclusion

This study demonstrates that carrier multiplication in FAPbI₃/NdF₃ perovskite nanocrystals enables a significant reduction in both optical gain and ASE thresholds by facilitating the generation of multiple excitons per high-energy photon. The approach is broadly compatible with other advanced gain schemes and paves the way for colloidal nanocrystal lasers with practical threshold requirements compatible with CW and electrically injected operation. These findings underscore the multifunctionality of perovskite nanocrystals and expand their application horizon beyond photovoltaics into next-generation photonic and optoelectronic devices.

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Reference:

"Reduced Optical Gain Threshold by Carrier Multiplication in Semiconductor Perovskite Nanocrystals" (2604.04628)