Evaluating the Impact of the Size of PbS Colloidal Quantum Dots on Photodetection Performance

Colloidal quantum dots (CQDs) provide size-tunable optoelectronic properties, enabling broadband photodetection from the visible to the short-wave infrared (SWIR). However, CQD photodetector performance depends on different processes such as optical absorption, charge transport, and photogain mechanisms in CQD solids. Here, we investigate the impact of PbS CQD size on the electrical conductivity and photodetection performance of CQD films and hybrid graphene/CQD photodetectors. Three CQD sizes are studied: d~3.06 nm, d~3.78 nm, and d~5.27 nm, with exciton peaks at λ_e~935 nm, λ_e~1080 nm, and λ_e~1550 nm, respectively. Electrical measurements show that the largest CQDs exhibit higher conductivity. In contrast, photodetection measurements reveal that the largest-sized CQDs (~5.27 nm) produce the lowest photoresponse, both as bare CQDs and as hybrid graphene/CQD photodetectors. Spectral and power-dependent measurements show decreasing responsivity with increasing optical power, consistent with trap-mediated photoconductive gain. These results indicate that the CQD size can have a significant effect on the optoelectronic performance of CQD devices, requiring materials, interfaces, and design optimization to maintain high photodetector performance.