Polymorphous Silicon: Transport Properties and Solar Cell Applications
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501 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
SAMPLES AND EXPERIMENTS The samples studied were deposited in a radio frequency powered (13.56 MHz) PECVD system [6] at a substrate temperature Ts of 150 'C. For the polymorphous films, the silane was highly diluted into hydrogen (3% of SiH 4 in 97 % of H2 ) and the RF power Prf was of the order of 110 mW/cm 2 . The total pressure P in the chamber was varied between 80 and 293 Pa. The transport properties of these samples were compared to those of an a-Si:H sample prepared under standard conditions (T,=150 'C, Pif =5 mWcm-2 , P= 5 Pa). For both types of materials different types of samples were prepared: simple films deposited onto 7059 Coming glass, Schottky diodes and p-i-n diodes, the i-layer being made of either pm-Si:H or a-Si:H. Two parallel ohmic aluminum electrodes 2 mm apart were thermally evaporated on the films deposited onto glass in order to perform the following experiments: conductivity and steady-state photoconductivity (SSPC) at different light intensities, modulated photocurrent (MPC) and steadystate photocarrier grating (SSPG). From these experiments we determined respectively the value of the dark conductivity and of the activation energy, the mobility-lifetime (latte) product of the
majority carriers, here assumed to be the electrons, the shape of the density of states above the Fermi level, hereafter referred to as the MPC-DOS, and the mobility-lifetime (4th)product for the minority carriers (i.e. the holes). On the Schottky diodes we performed capacitance measurements as a function of frequency and temperature to determine the density of states at the Fermi level. We also performed time of flight measurements to determine the drift mobilities of the carriers. Finally, the I-V characteristics of the solar cells were measured at different light intensities to obtain the effective axt of carriers in the intrinsic layer of the pm-Si:H cells by means of the variable irradiance method (VIM) [7]. The samples were studied in their as-deposited, annealed and light-soaked states. The light soaking was achieved under white light with a flux of the order of 400 mW/cm2 at a temperature of 80 'C during 70 hours for the films and 50 'C during 24 hours for the pin diodes. For the films the kinetics of the light soaking process was monitored by the evolution of the dark and photo conductivity measured at 20'C. RESULTS For all the studied samples the room temperature dark conductivities and activation energies range between 3x1012 and 6x1011 S.cmI and 0.7 and 0.9 eV, respectively. In Fig. 1 we present the evolution with pressure of the mobility-lifetime product for electrons and holes measured 17 under red light (?,=670 nm) with a flux of 10 cm- s . These products were measured before and after light-soaking. The comparison with the results obtained on a standard a-Si:H sample shows that in the as-deposited state the ltt products for holes and electrons are of the same order of magnitude or slightly lower than those measured on the st
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