Computer simulation of transient experiments for determining the transport parameters in amorphous silicon solar cells

Computer simulation of a time-of-flight experiment is used to predict and interpret the terminal response following pulsed excitation in amorphous silicon solar cells. The device under consideration is a 0.62 mu m thick a-Si:H cell under short-circuit conditions. A 6 ps pulse of blue light having an absorption coefficient alpha =10⁶ cm^-1 and a flux density A=5*10¹⁸ cm^-2 s^-1 shines through the P⁺ layer. Numerical solutions and the associated physical interpretation illuminate the transport physics and make it possible to assess the accuracy of the method. The interaction between free carriers and gap states is discussed on the basis of numerical results on the internal variable profiles. The influence on the transient photocurrent of band mobilities, electric field, transient trapping, and transient emission is investigated. Certain analytical approximations are derived based on an interpretation of the computer solutions. It is shown that increasing the fail state density from 10²⁰ to 10²¹ cm^-3 eV^-1 changes the transport from nondispersive to dispersive and reduces the drift mobility by a factor of five.