Solution-Processed Optoelectronic Devices Based on Colloidal Semiconductor Nanostructures for Photodetection

Abstract

Miniaturisation of electronic and optoelectronic devices have enabled the realization of system-on-a-chip technology in modern image sensors, where the photo sensor arrays and the corresponding signal processing circuitry are monolithically integrated in a single chip. Apart from intrinsic advantages, the drive towards miniaturisation has been further fuelled by the exotic properties exhibited by semiconductor materials at the nano scale. As the dimension of the material is gradually reduced from the bulk, interesting physical and chemical properties begin to emerge owing to the increased confinement of charge carriers in different spatial dimensions. Solution-processed optoelectronics have revolutionised the field of device physics over recent years due to the superior performance, ease of processing, substrate flexibility, cost-effective production of large-area devices and other advantages associated with the technique. In the present work, solution-processed photo detectors have been fabricated on SiO2/Si substrate facilitating the ease of integration with conventional silicon CMOS technology. The present thesis deals with the successful exploitation of most common point defects in semiconductor nanostructures to reduce the overlap of hole wave function with the envelop wave function of the ground state electron to improve photoconduction. As a result of the investigation process, successful strategies have been devised for the improvement of photoconduction by engineering the defect states. In the first study, the intrinsic copper vacancies and the capping agent thiol have been employed to trap photo holes in photo detectors based on copper indium selenide nanoparticles, thereby allowing the photoelectrons to transit the device. In the second study, the optical excitation of charge carriers into the defect-related band originating from oxygen vacancies further raises the photoconductivity of molybdenum trioxide nanobelts based photodetectors. In the third study, the absence of photoconductivity in zinc selenide based quantum dots has been attributed to the radiative recombination of photogenerated carriers at the donor-acceptor states caused by the self-compensation of point defects in the dots. In the final study, the crucial role of the energy depth of trap states in determining the carrier relaxation dynamics (temporal response) of the photodetector based on SnO2 nanowires has been discussed in detail.

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