Spring 2022 BSAC Research Review Presentation
Defects in conventional semiconductors substantially lower the photoluminescence (PL) quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device efficiency. Two-dimensional transition-metal dichalcogenides (TMDCs), such as monolayer MoS2, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. Moreover, most optoelectronic devices operate at high photocarrier densities where all semiconductors suffer from enhanced nonradiative recombination. In this talk, I will discuss how PL QY of as-processed MoS2, WSe2 and WS2 monolayers can reach near-unity at all photocarrier densities, when they are made intrinsic through electrostatic or chemical doping and their band structure is favorably altered by strain. Surprisingly, neutral exciton recombination is entirely radiative at all exciton concentrations even in the presence of a high native defect density. Finally, I will discuss light emitting devices without efficiency roll-off and how the photophysics evolves if indirect excitons are also present. These findings enable TMDC semiconductors for optoelectronic device applications as the stringent requirement of low defect density is eased and facilitate light-emitting devices that retain high efficiency at all brightness levels.