Dependence of Photoresponsivity and On/Off Ratio on Quantum Dot Density in Quantum Dot Sensitized MoS₂ Photodetector

• Main Authors: Yung-Yu Lai, Yen-Wei Yeh, An-Jye Tzou, Yi-Yuan Chen, YewChung Sermon Wu, Yuh-Jen Cheng, Hao-Chung Kuo
• Format: Article
• Language: English
• Published: MDPI AG 2020-09-01
• Series: Nanomaterials
• Subjects: molybdenum disulfide; quantum dots; photodetectors; non-radiative energy transfer
• Online Access: https://www.mdpi.com/2079-4991/10/9/1828

Non-radiative energy transfer (NRET) from quantum dots (QDs) to monolayer MoS₂ has been shown to greatly enhance the photoresponsivity of the MoS₂ photodetector, lifting the limitations imposed by monolayer absorption thickness. Studies were often performed on a photodetector with a channel length of only a few μm and an active area of a few μm². Here, we demonstrate a QD sensitized monolayer MoS₂ photodetector with a large channel length of 40 μm and an active area of 0.13 mm². The QD sensitizing coating greatly enhances photoresponsivity by 14-fold at 1.3 μW illumination power, as compared with a plain monolayer MoS₂ photodetector without QD coating. The photoresponsivity enhancement increases as QD coating density increases. However, QD coating also causes dark current to increase due to charge doping from QD on MoS₂. At low QD density, the increase of photocurrent is much larger than the increase of dark current, resulting in a significant enhancement of the signal on/off ratio. As QD density increases, the increase of photocurrent becomes slower than the increase of dark current. As a result, photoresponsivity increases, but the on/off ratio decreases. This inverse dependence on QD density is an important factor to consider in the QD sensitized photodetector design.