A theory is proposed to quantify the collection efficiency of a thin-film metal-insulator-semiconductor (MIS) solar cell with back-surface field. The model incorporates two loss mechanisms: first, the image force effect producing an electron potential maximum located at xm near the surface within which all photogenerated electrons as well as holes are collected and thus do not contribute to the photocurrent, and second, the collection of the majority carriers, photogenerated in the depletion region, which diffuse against the electric field to xm and thus reduce the photocurrent. In addition to being affected by doping density, majority carrier mobility, minority carrier diffusion length, and effective surface collection velocity, the collection efficiency is shown to be thickness dependent when the semiconductor is fully depleted. Calculations are included for the specific case of electrodeposited n-type CdTe MIS solar cells. Methods to improve the collection efficiency include: designing a high-low doping profile; reducing the surface collection velocity; and optimizing the thickness and doping for the semiconductor.