Date of Award
Physics and Astronomy
Mohammad Matin, Ph.D.
Sean Shaheen, Ph.D.
Organic Photovoltaic (OPV) devices, Time-of-Flight (TOF) measurement, Marcus theory
In this dissertation, we discussed the shape, energy level and density of defects in Organic Photovoltaic (OPV) devices by both theoretical and experimental methods. The energy band structure and intrinsic junctions of general standard and inverted OPV devices are investigated in detail. Transportation of separated holes and electrons in both structures are well analyzed. In chapter II, we developed a Monte-Carlo model to simulate charge transport process in organic materials. Marcus theory is applied to calculate the carrier transfer rate to build up hopping process in ideal lattices. Results of this model showed several properties of carriers regarding mobility, reorganization energy and Poole-Frenkel behavior. Time-of-Flight (TOF) measurement is simulated by our Monte-Carlo model. Four typical phases of TOF are fitted in the simulation, corresponding well to the experiment data. In chapter III, we introduced Impedance Spectroscopy as a useful technique to examine overall electronic component and internal structure. Geometric capacitance and depletion regions are found to describe the equivalent circuit of OPV. Another section of this chapter showed that impedance data can be also used by Mott-Schottky analysis to calculate the doping density and built-in voltage of Schottky junction, which is widely considered existing in standard OPV devices. Degradation and lifetime studies are the major topics of chapter IV. We compared three devices within different thickness of active layers under different illumination intensity. Open-circuit voltage, short-circuit current and fill factor are deeply analyzed to explain the decrease of conversion efficiency during degradation process. In addition, we utilize a simple method to estimate the lifetime of free carriers. In chapter V, impedance spectroscopy is used to investigate the electronic structure of OPV devices, with the goal of understanding the role of defect states. Capacitance-voltage measurements in the temperature range of 77 - 300 K are carried out to determine the activation energy of trap states. Preliminary results show at least one type of defect state residing at 166 meV in standard blends of P3HT:PCBM. Furthermore we compared three different molecular weight of P3HT in both standard and inverted devices. The result shows that higher molecular weight provides deeper defects energy level, and the density of defects generated during fabrication is different for standard and inverted structures. In chapter VI, JV curve behavior, capacitor-voltage analysis and efficiency comparisons before annealing and after annealing are performed to conclude that a p-n junction is formed at the ZnO/P3HT interface in inverted devices. We also discussed the free carrier transportation in both structure and explain why annealing leads to different conversion efficiency changes. Finally the equivalent circuit is confirmed by agreement with the geometric capacitance, as well as the structure and transportation analysis discussed in rest of the dissertation.
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Jiang, Xin, "Defects and Dark Current in Organic Photovoltaics and Impacts on Device Physics" (2013). Electronic Theses and Dissertations. 318.
Received from ProQuest