Colloidal quantum dots (QDs) are a versatile class of materials with notable potential in optoelectronics. Offering tunable optical and electronic properties and solution-processable fabrication, QDs have been widely explored in applications such as LEDs and photodetectors (PDs), where they can be used as printable inks. QDPDs are particularly interesting in the short wave infrared (SWIR) region (1 – 2µm), which is critical for many applications, such as environmental monitoring, biomedical and adverse weather imaging, and telecommunications. Current SWIR devices rely on costly fabrication techniques, but the integration of solution-processed QDs could substantially reduce the manufacturing costs of SWIR imagers, making them appealing for the consumer market.

Yet, efficient application of QDs to SWIR devices has been restrained to the use of lead sulfide (PbS), and mercury telluride (HgTe), which face significant regulatory restrictions due to their hazardous nature. In recently years, QDPDs based on III-V compounds, particularly In(As,P), have gained attention as a cost-effective, solution-based, and regulatory-compliant alternative for SWIR PDs, driving improvements on their synthetic chemistry, surface passivation and optimization of device structures. However, thus far, the performance metrics, such as quantum efficiency and dark current, of In(As,P) QDPDs have remained subpar relative to PbS-based devices, and have not significantly benefited from ligand engineering, suggesting other factors affect device performance.

Here, we investigate the relation between the properties of the In(As,P) QDs and the PD performance. QDPDs are fabricated using an established ligand exchange chemistry, involving the replacement of oleylamine and chloride by mercaptopropanediol and butylamine [1]. Current-voltage (I-V) measurements show that the resulting QDPD stacks are rectifying, and attain an external quantum efficiency (EQE) of up to 10-20% at 1050 nm. Impedance measurements are used to obtain deeper insight in the semiconductor characteristics of the In(As,P) QD film and the voltage distribution across the stack. A striking observation is the high dark current under reverse bias, which increases with adaptations to the stack that enhance the EQE. By combining temperature-dependent I-V measurements and transient absorption spectroscopy, we propose thermal generation of charge carriers within the QD film as the main source of dark current, and we discuss the prospects of adapting the QD surface termination so as to reduce dark current and enhance detectivity of PDs based on III-V QDs.