(411e) Efficient and Stable Short-Wave Infrared Colloidal Quantum Dot Photodetectors

Infrared (IR) photodetectors play a vital role in many applications including biomedical imaging, machine vision, mobile devices, and autonomous vehicles. PbS colloidal quantum dot (CQD) photodetectors are promising candidates for short-wavelength infrared (SWIR) light sensing technologies, due to their tunable absorption (850-1800 nm) and low manufacture costs enabled by solution processing. PbS CQD photodetectors typically employ a PbS CQD absorbing layer sandwiched between a hole-transport layer (HTL) and an electron-transport layer (ETL). However, the performance of CQD SWIR photodetectors is limited by the energy level mismatch between the CQD absorbing layer and the ETL which is commonly made of metal oxides. To improve their performance, we develop a new class of ETLs using n-type PbS CQDs, finding that these benefit from quantum-size effect tuning of band energies, as well as from surface ligand engineering. We demonstrate 1450 nm-operating photodetectors using PbS CQDs with tailored functionalities for each of the transport layers and the absorbing layer. By optimizing the band alignment at the absorbing layer/ETL interface, we report CQD photodetectors that combine low dark current of ~1×10^-3 mA/cm² with high external quantum efficiency (EQE) of ~66% at 1450 nm, outperforming prior reports of >1400 nm-operating CQD photodetectors that relied on metal oxides as ETLs. In addition, we find that the electric-field-induced ion migration has an adverse impact on the operating stability of CQD photodetectors. Therefore, a strategy is developed to improve the passivation of ETL CQDs using strongly-bound organic ligands. Specifically, we demonstrate that ETLs employing a strong organic ligand trans-4-(trifluoromethyl)cinnamic acid (TFCA) improve the dark current stability of CQD photodetectors by 50x compared to ETLs employing a weakly-bound inorganic ligand tetrabutylammonium iodide (TBAI). This work introduces new design principles of transport layers and provides insights into the stability mechanism of CQD photodetectors.

Reference:

Zhang, Y.; Vafaie, M.; Xu, J.; Pina, J. M.; Xia, P.; Najarian, A. M.; Atan, O.; Imran, M.; Xie, K.; Hoogland, S.; Sargent, E. H. Electron-Transport Layers Employing Strongly Bound Ligands Enhance Stability in Colloidal Quantum Dot Infrared Photodetectors. Adv. Mater. 2022, 2206884, 1–8.