Improving photodetection and stability of visible-light QDs/ZnO phototransistor via Al2O3 additional layer

We have developed a visible-light phototransistor with excellent photodetection characteristics and stability via atomic layer deposition (ALD) to add a thin layer of aluminum oxide (Al2O3) to quantum dot (QD)/zinc oxide (ZnO) films. Al2O3 was used to passivate the surface of the QDs and fill the interspace of the QDs. This led to a reduction in the number of QD surface traps and prevented damage due to the migration of aggressive metal atoms during electrode deposition. Additionally, some parts of Al2O3 deposited on the surface of ZnO triggered a doping effect by interacting with ZnO. This enhanced the electrical properties of ZnO TFTs by providing an extra electron and allowed the efficient transfer of photoelectrons from the QDs to ZnO by reducing the conduction band minimum of ZnO. Therefore, upon 520 nm laser irradiation Al2O3/QD/ZnO TFTs exhibited excellent photodetection characteristics (e.g., photosensitivity: 3.8 × 107, photoresponsivity: 46.8 A W−1, and photodetectivity: 4.5 × 1014 Jones) at a gate voltage of −5 V. Excellent electrical properties, such as saturation mobility of 1.72 cm V−1 s−1 and on/off current ratio of 1.8 × 106, were also realized. Furthermore, the photo-excited charge transfer mechanism was investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and time-resolved photoluminescence. Our experimental results suggest a method for realizing enhanced optoelectronic characteristics and stability of QD/ZnO TFTs via ALD-Al2O3 deposition, which is useful for developing high-performance visible phototransistors.