Balanced Carrier Injection and Charge Separation of CuInS₂ Quantum Dots for Bifunctional Light-Emitting and Photodetection Devices

The ligand exchange of 6-mercaptohexanol on the surface CuInS₂ quantum dots not only improves their solution processability in alcoholic solvents such as methanol, ethanol, and N,N-dimethylformamide but also modulates their electrical band gap and thus the charge injection and extraction at the charge transport interfaces. Bifunctional light-emitting and photodetection devices based on these alcohol-soluble CuInS₂ quantum dots are realized adopting an inverted structure with ZnO as the electron transport layer and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4′-(N-(4-butylphenyl)diphenylaminel)] and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate as the hole transport layers. The optimized device with selected active layer thickness exhibits red emission at 647 nm with a maximum luminance of 1600 cd/m² under forward bias and works as a photodetector at zero bias with a maximum responsibility of 0.53 mA/W and detectivity of 2.5 × 10¹⁰ jones. Furthermore, with interface engineering of the polyethylenimine ethoxylated (PEIE) layer at the electron transport side, more balanced charge injection is achieved, leading to reducing electroluminescence roll-off effect. The insulating PEIE layer also blocks the current leakage, giving rise to reduced dark current and improved detectivity of 3.5 × 10¹⁰ jones. The effective bidirectional charge transfer achieved under simplified device design using the alcohol-soluble quantum dots brings a new candidate for multifunctional devices.