Electrical Transport in Thin Film Systems for Energy Harvesting.

Abstract

Many energy conversion technologies rely on the properties of thin films. In many cases, the fundamental physics underlying the structure-property-performance interrelationship are not completely understood. So it is impossible to fully exploit the true capabilities of these systems. Therefore, investigating and understanding such interrelationships in different systems is of both scientific and technological importance. In this dissertation, both conjugated polymer systems for photovoltaic application and strained silicon system for thermoelectric application are investigated in order to develop a clearer understanding of the effect of film thickness and microstructural features on electrical transport. Morphological features like domain size, phase purity are investigated in the polymers to understand the effects on charge mobility, recombination and further on device performance. With regard to silicon, the effects of lattice strain on electrical conductivity and thermopower are studied. The out-of-plane hole mobility was investigated in regioregular P3HT thin films. It was shown that the hole mobilities monotonically increased an order of magnitude when film thickness increased from 80 nm to 700 nm. Based on X-ray diffraction, spectroscopic ellipsometry and simulations, this thickness-dependent mobility is associated with substrate induced anisotropies of the P3HT film structure. The role of microstructural features on the performance characteristics of the archetypal P3HT:PCBM (1:1) bulk heterojunction solar cell was investigated. It is demonstrated that small domain sizes and correspondingly large interfacial areas accommodated a high initial carrier density. However in these materials, non-geminate recombination of carriers could be significant, leading to low open circuit voltages and low fill factors. The purity of the domains also influenced the charge carrier mobilities and non-germinate recombination. One important finding from this study is that high short circuit currents were readily achieved with smaller domain sizes than 10 nm, which is believed to the best domain size. With regard to the thermoelectric characterizations in the strained silicon thin film possessing nanomesh topology, the electrical conductivity was found to increase several folds and the power factor doubled. This enhancement is attributed to the splitting of silicon conduction band under the biaxial tensile strain, which affects the effective mass, inter-valley scattering and energy distribution of transporting electrons.