Lifetime and recombination kinetics in a-Se thin films

Modulated photoconductivity measurements in amorphous selenium (a-Se) thin films were carried out. Especially, photocarrier lifetime as a function of applied electric field (d.c.) and temperature was determined by using the quadrature frequency-resolved spectroscopy (QFRS) method. At low temperature, two different carrier lifetime channels were observed. However, only one carrier lifetime channel was dominated at room temperature (297 K). The temperature dependence of the frequency-resolved photocurrent (FRPC) was investigated under different applied electric fields. At high temperatures, a small field independent activation energy value of 147 ± 35 meV was determined, in which hole transport is controlled by the valence band-tail states. The exponent ν in the power-law relationship (I_ph∝G^ν) between generating flux and photocurrent was obtained at different electric fields and excitation wavelengths. The value of ν increases very little with decreasing applied electric fields. However, ν shows a little stronger dependence on the excitation wavelengths. The applied electric field dependences of photocurrent at different excitation wavelengths were also directly measured. However, a little non-ohmic behaviour was observed at high applied electric fields and at low excitation wavelengths measured. We explained it in the frame of traditional models.