Electron-Transport Layers Employing Strongly Bound Ligands Enhance Stability in Colloidal Quantum Dot Infrared
Yangning Zhang1, Maral Vafaie1, Jian Xu1
1Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario, M5S 3G4, Canada.
Abstract:
Solution-processed photodetectors based on colloidal quantum dots (CQDs) are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low dark current of ≈1 × 10-3 mA cm-2 with a high external quantum efficiency of ≈66% at 1 V are reported, outperforming prior reports of CQD photodetectors operating at >1400 nm that rely on metal oxides as ETLs. It is shown that stable CQD photodetectors rely on well-passivated CQDs: for ETL CQDs, a strongly bound organic ligand trans-4-(trifluoromethyl)cinnamic acid (TFCA) provides improved passivation compared to the weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). TFCA suppresses bias-induced ion migration inside the ETL and improves the operating stability of photodetectors by 50× compared to TBAI.
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