TLTL · Transport layer engineering towards lower threshold for perovskite lasers

Transport layer engineering towards lower threshold for perovskite lasers

Backgrounds: Electrically driven surface-emitting lasers, with an output of a coherent light source, are a fast-growing technology in various applications such as information transmission, medical treatment, and 3D sensing. However, the current successful industrial fabrication of surface-emitting lasers is only based on a few inorganic materials, heavily limited by the expensive and complex high vacuum manufacturing processes. Recently, metal halide perovskites emerge as a promising candidate for future lasers owing to various advantages such as low-cost solution processability, bandgap-tunable luminescence with high color purity and high photoluminescence quantum yields (PLQY), outstanding optical gain coefficient, and excellent optoelectronic properties. And the recent advances in metal halide perovskites have inspired a growing research interest in pursuing electrically pumped perovskite lasers, a “holy grail” in the field of optoelectronics yet to be realized in the family of solution-processed semiconductors. Motivation 1: Towards demonstrating electrically pumped lasers, it is necessary to sandwich the perovskite thin film into transport layers for the integration of electrical devices. However, it remains unclear how transport layers will influence perovskite lasing actions. Especially, the appearance of perovskite/transport layer interfaces, which are absent in optically pumped perovskite lasers, is usually considered a detrimental factor to light emission in perovskite light-emitting diodes due to the transport layer induced photocarrier quenching. Moving from light-emitting diodes towards lasers, there were reports realizing high injection current densities (> 1 kA/cm2) by carefully integrating the perovskite films into electrical device structures; yet lasing actions have not been realized. This might be caused by the reduced optical gain properties or even higher lasing threshold of perovskites when the perovskite films are sandwiched by transport layers in electrical devices. Hence, it is very important to reveal the effects of transport layers on the perovskite lasing threshold and develop excellent transport layers that will maintain the optical gain properties or even reduce the lasing threshold of perovskites when integrating them with perovskite films. Motivation 2: For successful stimulated emission and eventual lasing, a low threshold carrier density for population inversion in the active medium is demanded, which is a key parameter associated with many basic properties in the materials and device, such as carrier distributions near the band edges and absorption losses, etc., but less studied in detail on the perovskites in literature. The threshold carrier density of a material can be estimated from conventional optical pumping experiments. The estimates obtained by those measurements cannot however be simply applied to metal halide perovskites. The slow cooling of hot carriers excited to the high-energy bands delays the carrier accumulation for stimulated emission, consequently resulting in an over estimated threshold carrier density.

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