Our research explores the phenomenon of “photon recycling” or “photorecycling” in perovskite materials. This process, where emitted photons are reabsorbed and re-emitted within a material, is a mechanism for achieving high luminescent efficiency and enabling novel device architectures. Our work demonstrates this concept across scales, from engineered perovskite waveguides down to the interaction between perovskite matrices and nanocrystal sensitizers.
Radiation Trapping in Perovskite Waveguides
In a foundational study, efficient photon recycling in cesium lead halide perovskite crystalline waveguides was directly observed and quantified [1]. This work provided evidence of the optical properties of perovskite materials that make them ideal for light-management applications. Using temperature-dependent time-resolved photoluminescence (TRPL) measurements on single-crystal waveguides, a significant elongation of the PL lifetime at low temperatures was discovered. This effect is not due to a change in the intrinsic radiative lifetime but is a signature of radiation trapping: photons emitted from the crystal’s bulk are waveguided via total internal reflection and reabsorbed by the perovskite itself, creating new electron-hole pairs. The characteristic signature of this effect is a divergence between the measured PL lifetime and the actual radiative lifetime at cryogenic temperatures, where non-radiative channels are frozen out. Kinetic modeling confirmed that the long-lived PL decay components were due to this photon recycling process. This demonstrated that perovskite crystals can act as their own perfect “cavities,” effectively increasing the path length of light and enhancing the probability of emission from the crystal edges. This insight is critical for designing high-efficiency perovskite-based lasers and light-emitting devices where light outcoupling is essential.
Photorecycling for Energy Transfer in Hybrid Materials
A recent application of the photorecycling concept involves using a perovskite matrix not just as an active emitter, but as a “photon bath” for sensitizing other nanomaterials. This was demonstrated in hybrid light-emitting devices incorporating quantum shells (QSs) [2]. Light-emitting electrochemical cells (LECs) was fabricated using a blend of large-core CdS/CdSe/CdS quantum shells and a CsPbBr₃ perovskite matrix. The device showed a strong electroluminescence (EL) enhancement, with a radiant exitance nearly triple that of a perovskite-only device and a 2.3-fold increase in external quantum efficiency (EQE). The enhancement is attributed to a sensitization process driven by photon recycling within the perovskite. The perovskite layer is electrically pumped and emits green light. A portion of this emitted light is not directly escaped but is reabsorbed by the perovskite matrix. Some of these “recycled” photons are subsequently absorbed by the red-emitting QSs dispersed within the film, populating their excitonic states and causing them to luminesce very efficiently. This work moves beyond simple Förster resonance energy transfer (FRET) by leveraging the macroscopic photon recycling capability of the perovskite. It demonstrates a novel strategy for creating hybrid devices where a high-performance host material can efficiently pump a guest emitter through a photonic energy transfer channel, leading to enhanced device performance.
(a) Linear absorption spectrum of an individual microwire. Inset: Left part, schematics of linear absorption measurements using white light; right part, variable length excitation–collection measurements using the same pair of objectives, with the bottom one translated along the microwire. (b) PL spectra as a function of separation s. Spectra are normalized at the maximum emission intensity. Excitation power 20 μW. Inset: PL emission intensity at the 530 nm peak as a function of separation distance s.
At the Macro/Meso Scale (Waveguides), photon recycling manifests as radiation trapping, where the perovskite’s high luminescence efficiency and large absorption coefficient create an ideal environment for light to be recycled, leading to elongated apparent PL lifetimes and directed light emission from the crystal edges [1]. At the Nanoscale (Hybrid Films), the same fundamental property—efficient emission and reabsorption—is harnessed for sensitization. The perovskite acts as an internal light source, and its photon recycling capability increases the probability that energy will be transferred to a secondary emitter (the quantum shells) within a blended film, boosting overall electroluminescence [2].
The investigation of photon recycling reveals perovskites as exceptional optical materials. This process is a powerful tool that can be engineered. It is crucial for improving light emission efficiency in pure perovskite devices and can be exploited in hybrid systems to create highly efficient, sensitized light-emitting structures. This understanding continues to guide our development of next-generation optoelectronic devices.
[1] Dursun, I. et al. Efficient Photon Recycling and Radiation Trapping in Cesium Lead Halide Perovskite Waveguides. ACS Energy Lett. 2018, *3* (7), 1492–1498. DOI: 10.1021/acsenergylett.8b00758
[2] Marder, A. A. et al. CdS/CdSe/CdS Spherical Quantum Wells with Near-Unity Biexciton Quantum Yield for Light-Emitting-Device Applications. ACS Materials Lett. 2023, *5*, 1411–1419. DOI: 10.1021/acsmaterialslett.3c00110