Simulation of Quantum Efficiency Spectroscopy for Amorphous Silicon P-I-N Junctions

Quantum efficiency(QE) spectroscopy of amorphous silicon and alloy solar cells has been used for many years now to determine the mobility-lifetime products for minority carriers. Similarly, matching of I(V) curves, assuming a linear model for collection as a function of applied voltage, has been used to quantify the effects of degradation on cell performance by estimating changes in the collection length [or range] of holes. In this paper, we do a numerical simulation of these techniques, using the AMPS I-D model developed by Fonash and his coworkers. The simulation shows that neither the lifetime nor the electric field in the devices is constant as a function of position. Nor is the electric field a linear function of applied voltage, particularly when the voltage exceeds about half the built-in voltage. The uniformity of the lifetime depends on the applied bias and on the defect densities in the material. This variation in electric field and lifetime and nonlinearity with applied voltage makes questionable some of the conclusions drawn from fitting device I(V) curves, particularly under forward bias. However, when one uses only a limited range of forward bias, or, preferably, make measurements in cells with thicker i layers under reverse bias, one c.an make reasonable estimates of the hole mobility-lifetime(μτ) product or the collection lengthl The simulations also show that indeed, it is the hole μτ product which is the limiting parameter.