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ELECTRON MOBILITY IN AMORPHOUS SILICON It has been shown that CH and the associated increases in microstructure can be related to poor electron mobility 4. The trap limited hole transport as determined by time-of-flight measurement also seems to adversely affected by CH and the associated microstructure; however, the steady-state hole mobility does not appear to be a strong function of the deposition conditions and CH '. The degradation rate 6-9 and the high temperature solar cell saturated defected state 10appear to be a functions of CH and microstructure. For the low microvoid, high Ts material (T. = 350' C), t has a value of - 13 cm 2/V sec. The low values for p, and the relationships between p, and CH are not easily explained in terms of classical crystalline semiconductor physics. To address these issues a theory which describes electronic transport in amorphous silicon as a hopping process was developed ". According to this theory electrons move by tunneling to an adjacent-localized-conduction band site which is centered on silicon atoms. The hopping process is mediated by atomic vibrations which act as a perturbation on the potential barrier between these sites; therefore, the tunneling and electron jump frequencies will on average be related. The tunneling probability is temperature dependant because the barrier width is a function of the average bond length. Also, the vibrational amplitude is a function of temperature. These vibrations are those in which two adjacent atoms move out of phase, and therefore, are analogous to the crystalline optical modes (at k=O) which have frequencies of M-I0' 4/s. Experimental investigations of the pressure dependence of resistivity 12 suggest that low hydrogen content (-7%) device quality amorphous silicon has a pressure dependent mobility. The pressure dependence can be understood in terms of the hopping conduction mechanisms described in reference ". The mobility is expected to increase with increasing compressive stress. Compressive stress is a function of deposition conditions including hydrogen dilution, substrate temperature, and the resulting hydrogen content. However the stress is related to these factors in a complicated manner. For example, the stress in th films appears to reach a maximum compressive stress at a particular hydrogen content for each substrate temperature 13 THE DISPERSIVE TRANSPORT: MOBILITY, LIFETIME and STABILITY The relationship between solar cell performance and the mobility in a dispersive material such as amoprhous silicon is not as straight forward as one might guess. The thermodynamic equilibrium model 2 and dispersive carrier transport description 14 are used to elucidate one of the linkages among carrier transport, solar cell configuration and stability. Since in these materials the mobility is both involved with the speed at which carriers move to contacts (the collection process) and the speed at which carriers move to recombinaiton centers (a loss mechanism) the two factors tend to cancel out. The capture rate, R, of electrons by a