Origin of the modulated photocurrent in the presence of bias light in quantum efficiency measurements

It is demonstrated mathematically that in quantum efficiency measurement in the presence of a constant bias illumination, one of three possible phenomena occurs: (1) monochromatic-light-produced field redistribution has negligible impact and almost all photocarriers in the quantum efficiency measurement are from the probe monochromatic light; (2) the monochromatic light causes appreciable improvement in the electric field in some part of the device, causing a channeling of bias-light-produced photocarriers into the modulated current response. This creates the possibility of apparent quantum efficiencies greater than unity; and (3) the monochromatic light has a significant detrimental influence on the electric field in some part of the device. This may result in negative values of quantum efficiency, although the lock-in measurement must be suitably modified to detect the effect. Using our detailed computer-analysis model, two conditions are identified in which quantum efficiencies greater than unity can be achieved in a hydrogenated amorphous silicon Schottky barrier photovoltaic device.