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Photodetection in Disordered Materials


This dissertation offers two different mechanisms for photodetection in disordered materials. Photodetectors made of amorphous materials enable low cost optical imaging and communications over non-semiconductor platforms. The key challenges are to improve efficiency, sensitivity, and frequency response. Using the localized surface plasmon resonance (LSPR) effect and an efficient carrier multiplication process, cycling excitation process (CEP), the plasmonically enhanced amorphous silicon photodetector (PEASP) with a thin (60 nm) absorption layer achieves a high external quantum efficiency with a record fast impulse response of 170 ps (FWHM). This approach offers the possibility of making detectors out of amorphous material for high frame rate imaging and optical communications in spite of the material’s low carrier mobility.

Spin-coating thin film is another approach in obtaining disordered materials. Organometallic halide perovskites attract strong interests for their high photoresponsivity and solar cell efficiency. However, there was no systematic study of their power and frequency dependent photoresponsivity. Two different power-dependent photoresponse types in methylammonium lead iodide perovskite (MAPbI3) photodetectors were identified. In the first type, photoresponse remains constant from 5 Hz to 800 MHz. In the second type, absorption of a single photon can generate a persistent photoconductivity of 30 pA under an applied electric field of 2. 5 ×10^4 V/cm. Additional absorbed photons, up to 8, linearly increase the persistent photoconductivity, which saturates with absorption of more than 10 photons. This is different than single photon avalanche detectors (SPADs) because the single photon response is persistent as long as the device is under bias, providing unique opportunities for novel electronic and photonic devices such as analog memories for neuromorphic computing. An avalanche-like process for iodine ions was proposed, with the estimation that absorption of a single 0.38 aJ photon triggers motion of 108-9 ions, resulting in accumulations of ions and charged vacancies at the MAPbI3/electrode interfaces to cause band bending and change of material electric properties. This is the first observation that single-digit photon absorption can alter the macroscopic electric and optoelectronic properties of a perovskite thin film. DFT toy model calculations were also conducted to further support the proposed ionic impact ionization.

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