Field-effect Photodetectors Based On Polymer-quantum Dots Composites

The emergency and development of semiconductor materials and devices have promoted the development of productive forces, and have changed people’s life dramastically. However, the semiconductor devices still strongly rely on semiconductors such as silicon and germanium. To satisfy the requirements of miniaturization, integration and low power consumption for semiconductor devices, as well as to adapt with the application in different situations, researchers are exploring new materials, structure and preparation routes to discover novel devices. In the 20 th century, the organic semiconductors and nanotechnology were proposed to open up new ways for the development of semiconductor devices.Photodetector is a kind of semiconductor device that photo-signal can be converted to electricity, and it can be used in photo-switch, light triggered amplification and detection circuits. Also, photodetector is an important part of image sensors which decide the quality of images. Traditional photodetectors based on silicon is expensive, complex production, flexible device incompatible and the operating wavelength is limited. The various kinds of nano-semiconductor materials provide multiple choices for developing new-type photodetectors, and the novel physical and chemical properties of nano-materials supply more possibilities, especially for the quantum dots with remarkable quantum confinement effect and controllable band gap. In addition, organic semiconductor owns the superiority of flexible, solution processing and low cost. Therefore, this work is aiming at the combination of quantum dots and organic semiconductor(especially polymer semiconductors), tremendously utilizing the advantage of each material, and finally exploring new routes to new-type photodetectors with high performance.Field effect transistor(FET) based photodetectors is a novel device with great potentials. In our work, nano-material and organic semiconductors are combined and applied in photodetectors, for its low cost processing methods, such as spin-coating and thermal evaporation of metal materials. Moreover, the influence of different device configurations, dielectric materials, and processing conditions on the device performance has been investigated. The main results are listed as following:(1) By using top gate configuration, we fabricated our photodetector Au(Source/Drain) /P3HT/PMMA/Al(Gate),in which poly(3-hexylthiophene)(P3HT) was used as the active layer and poly(methyl methacrylate)(PMMA) as dielectric layer. To avoid “solution corrosion”, acetone was chosen as the solvent of PMMA. Active layers with different thickness of 38 nm、150 nm and223 nm were prepared for comparison. Our experimental data showed that the device with 223 nm P3 HT layer owns best electrical and photo responsive property, and the effective hole mobility was about 0.01 cm2·V-1·s-1; and its response to the 350 ~ 650 nm light with a photoresponsivity of 15 ~ 26 mA/W and the largest photo-to-dark current ratio reached 2400. The device made of 150 nm is chosen to be further optimized the dielectric layer. We changed the solvent of PMMA from acetone to butyl acetate, the effective hole mobility was enhanced to 0.08 cm2·V-1·s-1. In this way, this device got more sensitive to incident light as compared to the one with acetone as solvent, and the largest photo-to-dark current ratio reached 75 with a photoresponsivity of 0.3 A/W.(2) By using bottom-gate/top-contact FET configuration and cross-linked poly(4-vinylphenol)(cr-PVP) as dielectric layer, we had successfully fabricated photodetector ITO(Gate)/cr-PVP/P3HT/Au(Source/Drain). To improve the performance of P3 HTphotodetector, PMMA was blended into P3 HT as the active layer in different weight ratio(K = MPMMA:M(P3HT+PMMA)) of 20%、50% and 60%, respectively. Our experimental data showed that the K=20% PMMA device owns the best electrical property with a largest “on/off” current ratio and mobility of 329 and 1.58×10-3 cm2·V-1·s-1. In addition, the K=50% device was most sensitive to illumination with a largest photo-to-dark current ratio of 44 and a photoresponsivity of 166.45 mA/W. By measuring its UV-vis absorption spectra and atomic force microscopy, we found the crystalline and conjugation length of P3 HT was improved after suitable doping of PMMA.(3) To expand the responsive wavelength of our photodetector into the infrared region, we chose the narrow bandgap lead sulfide quantum dots(PbS QDs) and mixed them into the P3 HT as the active layer to prepare photodetector ITO(Gate)/cr-PVP/P3HT:PbS QDs/Au(Source/Drain). By optimizing the weight ratio of P3 HT to PbS QDs(K = MP3HT:MPb S = 1:1, 1:3, 1:5 and 1:7), we found that the electrical performance of the photodetector keeps declining with increasing the content of PbS QDs,. However, the device with K = 1:5 showed best photo-response performance under 980 nm laser. The largest photo-to-dark current ratio reached 869 and best photoresponsivity reached 135 mA/W with a specific detectivity of 7.28×1010 Jones. The device performance did not show any obvious degeneration after 50 days deposition.