Quantum-dot based photodiodes (QDPDs) are important candidates for low-cost light detectors in the short-wave infrared (SWIR) wavelength range. PbS-based QDPDs in particular are well-established and have been used in commercial SWIR imagers. In recent years, significant progress has been made in terms of reducing dark current, reducing response time, miniaturization and using more sustainable heavy-metal free materials [1] . However, the photocurrent of these devices tends to saturate for input powers above ~100 mW/cm2 [2]. This behaviour limits the use of QDPDs in integrated photonic devices, e.g., for interconnects or LIDAR, where the optical power density is high due to the confinement of light in waveguides with small cross sections [2]. The mechanism leading to detector saturation remains unknown to date, with indications of Auger-type processes playing a role despite the low overall excitation density powers of 100 mW/cm2 generated in these devices.

Here, we use in-operando transient absorption spectroscopy to study the charge decay PbS QDPDs designed for ns response times [3]. We combine measurements on the QDs in solution, on the individual (doped) device layers and for the first time in-situ measurements on the full device. Comparing the transient absorption decay of QDs in solution with the ligand-exchanged film confirmed that the ligand exchange passivates the surface and reduces the trap density. However, n/p doping (by the ligand exchange) induces another fast decay channel with a decay rate that scales with the square of the excitation density. This can be attributed to a trion Auger decay of the photo-generated electron-hole pair with an extrinsic background charge due to doping. The intrinsic disorder in the energy landscape brings photo-induced and extrensic charges together, resulting in a fast decay [4]. Inside a working device, this recombination mechanism outpaces the charge extraction already at a density of 0.02 excitations per QD. We measured the extraction efficiency as a function of pump power in these diodes and found a saturation at the same excitation density.

Our results add to the fundamental understanding of QD-based photo-detection devices and allow for better engineering of these devices. In particular, a better understanding of saturation in relation to doping levels could lead to an increased dynamic range.