Optimization of Transparent and Reflecting Electrodes for Amorphous Silicon Solar Cells: Annual Technical Report, 1 April 1995 - 31 March 1996

The general objective is to develop methods to deposit materials that can be used to make more-efficient solar cells. The work is organized into three general tasks. Under the task to develop improved methods for depositing and using transparent conductors of fluorine-doped zinc oxide in amorphous silicon cells, we demonstrated that fluorine-doped zinc oxide is the most transparent conductor, fora given electrical sheet resistance, of any known material, and we showed how to deposit it by low-cost, high-productivity chemical vapor deposition at atmospheric pressure. Under the task to deposit and evaluate titanium oxide as a reflection-enhancing diffusion barrier between amorphous silicon and an aluminum or silver back-reflector, we found that a titanium dioxide layer could be insertedover the metal back-reflector without loss of optical or electrical characteristics, but a gain in cell stability or efficiency has not yet been demonstrated. Under the task to deposit and evaluate electrically conductive titanium oxide as a transparent conducing layer on which more efficient and more stable superstrate cells can be deposited, thin niobium-doped titanium dioxide films weredeposited on top of tin oxide films and were found to protect the tin oxide against chemical etchants. These bilayer films were also used as substrates for growing amorphous silicon solar cells. These cells had normal voltages and fill factors, but lower currents because of some optical absorption by the niobium-doped titanium dioxide.