The spectral dependence of Voc is studied in a-Si:H based p-i-n junctions by using chopped monochromatic light. A dispersion ΔVoc = Voc(blue) - Voc(red) is observed which depends systematically on a low level doping of the central layer. While ΔVoc is largest and positive in phosphorous doped cells it is negative in boron doped cells. Voc is found to be enhanced by an additional white bias light.
The dispersion data are discussed by considering the contributions to Voc arising from drift in an inhomogeneous field and from carrier diffusion.