Based on a GaSb thermophotovoltaic (TPV) cell with a box-shaped Zn diffusion profile, a theoretical model of the generation and drift of the photogenerated minority carriers in the cell was established, and the internal quantum efficiency (IQE) of the cell was predicted with the classical semiconductor theory. The calculated results agreed well with the measurements. It is determined that reducing the front surface recombination velocity (S-n) could improve the IQE of the emitter region, whereas increasing the lifetime of holes (tau(h)) could improve the IQE of the base region. The Zn diffusion duration has a large influence on the IQE at short wavelengths below 1200 nm. Compared with the IQE for the 5 h diffusion time, the IQE of the 2 h counterpart was improved dramatically. Front surface etching could increase the IQE at short wavelengths below 700 nm while there was a decrease in the IQE for wavelengths above 700 nm. Thus, front surface etching is not necessary for a cell with a box-shaped Zn diffusion profile because the IQE for the near infrared wavelengths are the most important. The calculated results were elucidated by analyzing the distribution of the minority carriers generated in the cell and their recombination processes. (C) 2014 Elsevier B.V. All rights reserved.