The impact of voltage independent carriers on implied voltage measurements on silicon devices

The electrical performance a solar cell is determined from direct measurements of the current voltage relationship, while the so-called implied current-voltage measurements are often performed to estimate the performance of partially processed samples. Implied current voltage measurements are commonly obtained from quasi steady state photoconductance and quasi steady state photoluminescence measurements, where the implied voltage is inferred from the average excess carrier density. As will be shown here, this approach can be problematic due to the presence of excess carriers that do not contribute to the terminal voltage. These carriers are referred to as voltage independent carriers, a concept that is not widely known or generally accepted. This paper provides the theoretical background for the distinction of voltage dependent and voltage independent carriers. It is shown that the relative impact of voltage independent carriers on implied voltage data depends strongly on device parameters and on the illumination wavelength. Practical limits are provided for these parameters for which the voltage independent carriers can be neglected and for which an implied voltage thus accurately reflects the junction voltage.