Organic semiconductor bulk heterojunction diodes with low dark current for photovoltaic, photodetection, and scintillator-free ionizing radiation detection applications
Khan, Talha Mansur
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Solid-state organic semiconductor-based photovoltaics (OPV) are an emerging technology being developed to generate clean and sustainable electricity in light weight, flexible and shatter-proof form factors. In this dissertation, firstly, a method to enhance the environmental stability of OPV is developed. Previous work has shown that the physical adsorption of the polymer polyethylenimine on the surface of conductors results in a significant reduction of the conductor work function. In this work, it is demonstrated that the reduction in work function of the conductors using polyethylenimine is independent of the order of deposition and can also be achieved by depositing the conductors on top of polyethylenimine. Consequently, novel OPV architectures are developed in which the commonly used but particularly air unstable calcium electrode is replaced by polyethylenimine in conjunction with various air stable metals as top electron-collecting electrodes. The performance of the novel calcium-free OPV is found to be comparable to reference OPV containing calcium electrodes. Secondly, it is experimentally demonstrated that the energy of the charge-transfer complex formed by molecular interactions between the donor and acceptor components of the photoactive layer in OPV is the energy gap relevant for the thermal generation of carriers that leads to the reverse saturation current. Organic photodetectors that define the state-of-the-art in terms of dark current density (JDark on the order of 6 pA/cm2) and specific detectivity (D* > 10^14 Jones at visible wavelengths) are demonstrated by employing a donor-acceptor pair with a large charge-transfer complex energy, in conjunction with devices with large shunt resistance. The approach to reproducibly fabricate high shunt resistance devices is detailed, which includes the optimization of the photoactive layer morphology, the photoactive layer thickness, and the work function of the charge collecting-electrodes. A proof of concept for scintillator-free organic semiconductor detectors of ionizing radiation enabled by this approach is also demonstrated.