Comparison of experiment and theory of the photoconductivity of a-Si:H up to a generation rate 10(28)cm(-3)s(-1)

We have studied the dependence of the photoconductivity sigma(p) on photocarrier generation rate G in intrinsic a-Si:H at 300K between G=10(12)c(m-)3s(-1) and 10(28)cm(-3)s(-1). Below a certain value G(0), we find sigma(0)=AG gamma with gamma=0.9+/-0.05 and the values of A vary considerably with defect concentration N-d which signifies monomolecular recombination through defects. Above G(0) the recombination is bimolecular, gamma=0.5+/-0.02 and A=(6+/-3)x10(-15)Omega(-1)cm(1/2)s(1/2) is indpendent of N-d. The transition value G(0) is about 3x10(20)cm(-3)s(-1) for high quality annealed a-Si:H and increases with N-d. A simulation of sigma(p)(G) assuming conduction in and recombination from extended states fits our experiments within a capture coefficient C-t=(6+/-2)x10(-9)cm(3)s(-1) of carriers to their opposite tail states. Our C-t is close to the value (5+/-2)x10(-9)cm(3)s(-1) obtained from optical measurements but higher than (0.5+/-0.1)x10(-9)cm(3)s(-1) determined from photoelectric studies. Below T=150K our model calculations overestimate sigma(p) because the tunneling transitions, becoming important for recombination and conduction, are neglected.