Fig. 1. Schematic diagram of a TPV device.

Fig. 2. The curves of the efficiency η varying with the voltage V for given three values .

Fig. 3. The curves of non-ideal and ideal photovoltaic cells varying with : (a) open-circuit voltages; (b) short-circuit current densities.

Fig. 4. (a) The 3D graphs of power density and (b) efficiency as a function of and V.

Fig. 5. Optimal power density and efficiency as a function of photons’ cut-off energy , where