Pavel Dutta

Document Type

Thesis - University Access Only

Award Date

2011

Degree Name

Doctor of Philosophy (PhD)

Department / School

Electrical Engineering and Computer Science

Abstract

Disordered organic and inorganic semiconductor based solar cells have gained significant attention due to their potential for low-cost and large-area processibility. However, in spite of considerable efforts the efficiency and stability of these solar cells is still low. One of the primary reasons for this may be the lack of a fundamental understanding of charge transport in these materials. Moreover, the role of microstructure or morphology in determining nanoscale and overall bulk charge transport and electrical conductivity is still not understood. A detailed fundamental understanding of these issues is important to engineer cost-effective, stable and efficient photovoltaic devices. The overarching goal of the project was to understand charge transport processes in inorganic and organic disordered semiconductors at the nanoscale and their correlation with device performance using spatially resolved scanning probe and device measurements. In addition, the role of microstructure and morphology in influencing nanoscale charge transport was examined. Two technologically important disordered material classes used in photovoltaic applications were selected for study: inorganic amorphous (a-Si, a-Si:H)/nanocrystalline silicon (nc-Si:H) thin films and organic polymer/fullerene blend films and bulk heterojunction (BHJ) solar cells. The primary current route in nc-Si:H was grain interiors while grain boundaries were highly resistive. Nanoscale electrical conductivity and overall electronic quality of nc-Si:H was significantly improved by modification of the microstructure of the films using low power (0.1 W/cm2) RF hydrogen plasma treatments for short durations (10 s) at room temperature. Prolonged light soaking resulted in the modification of the surface microstructure (surface roughness and grain size) and a decrease in the nanoscale and bulk electrical conductivity of a-Si, a-Si:H and nc-Si:H. Grain boundaries played a crucial role in reducing the nanoscale electrical conduction during light induced degradation. Of the three films nc-Si:H had the lowest photodegradation. Morphology of the polymer/fullerene blends also played a critical role in determining nanoscale charge transport through the donor and acceptor phases and final device efficiency. The use of high boiling point and strongly polar spin casting solvents during film processing led to finer phase separation which facilitated efficient nanoscale charge transport, prevented recombination and improved the efficiency of BHJ solar cells. Techniques to improve the nanoscale charge transport, overall electrical conductivity and final device efficiency by modifying the material microstructure/morphology were demonstrated for both inorganic and organic disordered materials. This study will aid in engineering the electronic and structural properties of these materials not only for solar cell applications, but also for other opto-electronic devices.

Library of Congress Subject Headings

Amorphous semiconductors

Silicon solar cells

Charge transfer

Nanoelectronics

Format

application/pdf

Number of Pages

263

Publisher

South Dakota State University

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