Photocurrent Saturation Mechanisms in Colloidal Quantum Dot Photodetectors

Colloidal quantum dot photodiodes (QDPDs) are becoming an increasingly mature technology for infrared sensing and imaging that can offer high detectivity, short response times, and micrometer-scale pixelation. Even so, QDPDs suffer from a limited linear dynamic range (LDR), with photocurrents saturating at modest light intensities. Here, we analyze this remaining QDPD bottleneck in PbS-based QDPDs through operando transient absorption spectroscopy. As compared to isolated QDs, we find that recombination of photogenerated charge carriers accelerates in device-ready QD films and full QDPD stacks. Supported by kinetic Monte Carlo simulations, we assign this loss pathway to trion recombination mediated by hopping and doping-induced background charges, and we show that the photocurrent saturates when the rate of trion recombination and charge separation match. On the basis of this result, we argue that the widely varying literature data on the linear dynamic range of QDPDs mainly reflect different carrier extraction rates and conclude that faster carrier extraction is essential to extend the linear dynamic range.