The current±voltage (J(V)) data measured in light and dark from a-Si-based solar cells have been analyzed to yield six parameters that completely specify the illuminated J(V) curve from reverse bias to beyond open-circuit voltage (V oc ). A simple photocurrent collection model is used, which assumes drift collection in a uniform ®eld. The method has been applied to J(V) data from over 20 single-junction a-Si or a-SiGe devices from ®ve laboratories measured under standard simulated sunlight. Very good agreement is obtained between measured and calculated J(V) performance with only one adjustable parameter, the ratio of collection length to thickness L c /D. Some of these devices have also been analyzed after extended light soaking or under ®ltered illumination. The eect of the voltage-dependent photocurrent collection on the ®ll factor and V oc is considered in detail. Results under 1 sun illumination for both a-Si and a-SiGe devices are consistent with hole-limited collection. Photocurrent collection in very thin devices (D $ 0X1 m mm), or thicker devices under blue light, may be strongly in¯uenced by interface recombination or back diusion. The ¯atband voltage (V fb ) is dependent on the intensity and spectrum of illumination, and hence is not a fundamental device property and is not equivalent to the built-in potential. The V oc is limited by V fb , not junction recombination current J o , in typical devices. The illuminated solar cell performance is nearly independent of the forward diode current for low values of L c /D, as occurs after light soaking or with a-SiGe. The model is also useful to investigate the intensity dependence of the ®ll factor and to predict the in¯uence of L c /D and V fb on solar cell performance.