Photocharge Transport and Recombination Measurements in Amorphous Silicon Films and Solar Cells by Photoconductive Frequency Mixing: Final Subcontract Report, 20 April 1998-30 June 2002

The tasks carried out under this subcontract focused on characterizing the charge transport, opto-electronic, and structural properties of a number of amorphous and microcrystalline semiconductors prepared by several techniques. The dominant approach to accomplish the tasks of the present phase of the program is the photoconductive frequency mixing technique. This technique enabled us todetermine separately the drift mobility and the photomixing lifetime of the photogenerated carriers. The technique is based on the idea of heterodyne detection for photoconductors. When two similarly polarized monochromatic optical beams of slightly different frequencies are incident on a photoconductor, the photocurrent produced, when a dc bias is applied, will contain components resulting fromthe square of the sum of the incident electric fields. Consequently, a photocurrent composed of a dc and a microwave current due to the beat frequency of the incident fields will be produced; these two currents allow a separate determination of the drift mobility and the photomixing lifetime. In the present work, we improved the instrumentation of the photomixing measurements by applying biaspulses of arbitrary width and frequency. The longitudinal modes of a He-Ne laser were used to generate a beat frequency of 252 MHz; all the measurements were performed at this frequency for the data indicated in the accompanying figures and tables. Results from this technique, as well as FTIR, XRD, SAXS, and optical spectroscopy, are presented in the full report.