The Effect of Light Soaking on Electron Mobilities in Hydrogenated Amorphous Silicon

  • PDF / 359,910 Bytes
  • 6 Pages / 420.48 x 639 pts Page_size
  • 39 Downloads / 226 Views

DOWNLOAD

REPORT


THE EFFECT OF LIGHT SOAKING ON ELECTRON MOBILITIES IN HYDROGENATED AMORPHOUS SILICON HOMER ANTONIADIS,* QI WANG,* E. A. SCHIFF* and S. GUHAt *Dept. of Physics, Syracuse University, Syracuse, NY 13244-1130 t United Solar Systems Corporation, 1100 W. Maple Road, Troy, MI 48084 Abstract Electron drift mobilities ue were measured in undoped hydrogenated amorphous silicon specimens for different light soaking states. Standard transient photocurrent and time-of-flight techniques were employed; the light soaking state was monitored using the electron deep-trapping mobility-lifetime product lIre,t. Two different measurement series are reported. The first series measured a specimen from the reactor at Syracuse at roomtemperature. No significant effects of light soaking on Ite were found. The second series measured a solar-optimized specimen from Energy Conversion Devices, Inc. below 200'K. In these measurements we found a decrease in lAe of about a factor 3.

Introduction Since the discovery of the Staebler-Wronski effect in a-Si [1] a very large number of investigations have confirmed that light soaking diminishes both electron and hole mobilitylifetime products. These products are primarily determined by deep levels. Most workers have assumed that the underlying drift-mobilities determined by the bandtails are unaffected by the light soaking, and early work by Street [2] supported this conclusion for electrons. In addition Johnson, et al. [3] found that defective and electronic quality amorphous silicon had comparable electron drift mobilities. On the other hand, traveling-wave measurements of Takada and Fritzsche [4] on lightly phosphorus-doped a-Si:H, showed significant effects of light soaking on electron drift mobilities at room temperature. In this paper we report new experiments on the effects of light soaking on the electron drift-mobility in a-Si:H. We have performed transient photocurrent and time-of-flight measurements for two specimens at different light soaked states. For an undoped solaroptimized a-Si:H specimen prepared at Energy Conversion Devices, Inc. we found that the electron drift mobility at 160°K and 130°K declined as much as a factor of 3 due to light soaking. At room temperature the electron deep-trapping mobility-lifetime product dropped from 6.6 x 10-8 cm 2/V to 1.8 x 10-8 cm 2 /V with light soaking. However, measurements on an undoped specimen deposited at Syracuse showed no significant change in ;ie at room temperature in agreement with the earlier report by Street [2]. In this specimen the electron deep-trapping Ar-product declined from 3.5 x 10-7 cm 2 /V to 6 x 10-" cm 2 /V with light soaking. We discuss standard bandtail and transport edge models for these effects at the conclusion of the paper.

Specimens and Intrumentation The specimens used for the transient experimens are listed in Table I. The pin diode (denoted ECD) was prepared at Energy Conversion Devices, Inc.. This specimen had 20 nm p+ top and n+ bottom layers and was deposited onto a Cr bottom electrode. Semitransparent Pd top el

Data Loading...