Different roles of trap states in semiconductors for solar energy conversion

The solar cell industry has grown dramatically in the last decade. To further boost solar energy utilization, two popular strategies are a) to increase distributed generation and energy harvesting on buildings and personal devices; and b) store solar energy in various forms, including hydrogen fuel from water splitting. Organic photovoltaics (OPVs) and photoelectrochemical cells (PECs) hold great promise to address the above challenges. Polymer semiconductors and corrosion-resistant oxides are the key materials in OPVs and PECs, respectively. In these two types of materials, electronic trap states induced by morphology, impurities and defects play an important role in determining the electrical properties. Thus, characterizations of these defect states and understanding their effects are critically important for achieving high solar energy conversion efficiency in OPVs and PECs. This dissertation will first focus on a study of the molecular doping effect on two typical polymer:fullerene OPV systems. Organic semiconductors have a high density of trap states due to their disordered nature. Previous studies found that doping improved the performance of organic solar cells. The proposed explanation pointed to trap passivation, albeit lacking a detailed understanding of this effect. Through probing the relationship between photovoltage and photogenerated charge carrier density, we determine in this work that the dopant-induced carriers fill up the trap states, leading to a moderate enhancement of the open circuit voltage. However, this improvement is limited due to the low doping efficiency and the complications of morphology change at higher doping levels. Therefore, we conclude that doping is not a promising way to improve organic solar cells. On the other hand, "trap" states in amorphous titanium oxide (a-TiO2) have attracted research interests due to a different reason—the in-gap states, possibly induced by oxygen vacancies, have been hypothesized to be responsible for the surprisingly high hole conductivity in a-TiO2, an n-type wide bandgap semiconductor. The high hole conductivity, combined with good photostability, has enabled a new application of a-TiO2 as a conductive protection layer in PECs, though the hole transport characteristics and mechanism in a-TiO2 still need to be better understood. This work focuses on silicon/a-TiO2/iridium anodes, where the a-TiO2 thin films are grown by atomic layer deposition (ALD). The bias-dependent electrochemical impedance spectra of the anodes strongly indicate the coexistence of electron conduction and trap-mediated hole conduction in the a-TiO2. A three-rail transmission line model is built to understand how the electron and hole resistances as well as the chemical capacitances are reflected in the impedance features. The model successfully explains the impedance spectra of a-TiO2 with different thicknesses, as-prepared or annealed, and the influence of the metal interfacing the a-TiO2. Our findings support the design principle of using np+-Si over n-Si for a-TiO2-protected photoanodes, as the equilibrium hole injection from the p+-Si layer into the a-TiO2 enhances a-TiO2's hole conductivity, therefore allowing low photovoltage loss and high oxygen evolution reaction efficiency.

Keywords: Materials Science and Engineering.
Advisor: Salleo Alberto
Lindenberg Aaron Michael 
McIntyre P. (Paul)