Nanowire-enhanced silicon solar cells

Abstract

Thin film solar cells are expected to gain market importance due to the possibility of cheap mass production. However, they suffer from rather low efficiency. Radial p-n junctions, for example along a nanowire surface, could contribute to enhance efficiency. Theoretical studies have suggested potential advantages of radial p-n junction structures over planar p-n structures. Those radial structures were fabricated from polysilicon coated single-crystalline silicon nanowires.<br />This master thesis issues the development of nanowire-enhanced solar cells based on semiconductor silicon and their characterization. Planar p-n junction devices were built according to a classical silicon thin film concept.<br />Measurements and examinations on these devices are presented as they enroll the basics for the envisaged nanowire solar cells, i.e. thin film silicon solar cells incorporating silicon nanowires as photoactive elements.<br />Integration of nanowires is performed according to a LPCVD-scheme using gold catalysts. Photovoltaic activity and diode behaviour could be shown for the nanowire solar cells.<br />The thesis is divided into three sections. The first section "Theory" provides insight into the theoretical basics of photovoltaic energy conversion and solar cell manufacture. The most important types of solar cells are presented, some semiconductor physics is reviewed and important process technological issues are discussed.<br />This gives the prerequisites to the following chapters. In the "Experimental" section the key processes for solar cell manufacture are described in detail. Silicon nanowire synthesis, silicon thin film deposition and the doping process are the basic modules for device fabrication. For each cell type, fabrication is described stepwise utilizing these modules. The "Results" section presents the findings for process control as well as device characterization. In a chronological manner, following the sequence of fabrication, results are presented and discussed. Process parameter optimization and its impact on the characteristics and performance of the solar cells is described for each cell type.