Photocharge Transport and Recombination Measurements in Amorphous Silicon Films and Solar Cells by Photoconductive Frequency Mixing: Annual Subcontract Report, 20 April 1999 - 19 April 2000

This report describes research focused on improving the individual component cells from which the multijunction devices are fabricated. The Mid-Bandgap and Metastability subteam and the Low-Bandgap subteam have the responsibility to develop appropriate materials for the respective layer of the triple-junction solar cell. To this end, it is necessary to characterize the materials that are preparedfor the appropriate layer to optimize the devices and to develop an understanding of the conditions responsible for light-induced degradation so as to develop means to mitigate the degradation. Using the photomixing technique, UCLA was able to determine the mobility and lifetime separately of a number of semiconductor materials. We have established that different kinetics of degradation occurfor mobility and lifetime. We have found that the drift mobility is electric-field dependent, and we developed a model for the charge transport through long-range potential fluctuations that enable a determination of the range and the depth of these fluctuations for material in the annealed and light-soaked states. UCLA has continued to provide transport parameters for the Mid-Gap,Metastability, and Low-Band teams. The materials studied were prepared by various deposition techniques. In phase II of this program, we investigated in detail the charge-transport properties by photomixing of a-Si:H, ?c-Si:H and a-SiGe:H alloy films prepared by hot-wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) techniques, particularly under theconditions of high deposition rate and the transition from amorphous to microcrystalline state. Photomixing experiments were initiated to compare intrinsic film properties and device performance, and to study the impact of the changed contact geometry on the results of our photomixing measurements. We also attempted to employ the photomixing technique to measure the drift mobility of thetransparent conducting oxide. Following our previous measurements of the transport parameters under hydrostatic pressure, we initiated the hydrostatic pressure dependence of small-angle X-ray scattering measurements to find the origin of the inelastic effect. Time-resolved photo- and thermoelectric effects (TTE) were used to simultaneously determine the thermal diffusivity, carrier lifetime,carrier mobility, and trap-level density in crystalline and amorphous Si (a-Si:H) and Si/Ge (a-Si/Ge:H) samples.