A simple theoretical model for pin/nip-type μc-Si:H solar cells is presented. It is based on a superposition of a drift-dominated collection model and of the classical drift-diffusion transport model of the pn-diode. The model is the basis for a solar cell equivalent circuit, identical to the one introduced by Merten et al. [1], for amorphous cells. The equivalent circuit serves as framework for the diagnosis of faulty solar cells, by selected experimental tools, such as: J(V) curves, Quantum efficiency (QE) curves, Raman spectroscopy, Fourier-Transform Photo Spectroscopy (FTPS), Variable Intensity Measurements (VIM) [1]. The main parameter that characterizes solar cell "quality" is the fill factor (FF). For best cells FF is over 75%. FF can be reduced by (1) collection problems (characterized by a drop in the collection voltage Vcoll); (2) shunts (characterized by low shunt resistance Rshunt); (3) excessive series resistance. Thanks to VIM analysis, it is possible to discriminate experimentally between these 3 types of deficiencies. It is also possible to measure Vcoll very easily and link it to fill factor reduction AFF. Selected examples of solar cell series and case studies of defective and degraded cells are given. © 2006 IEEE.