A thermophotovoltaic cell and diode photodetector having improved open circuit voltage and high internal efficiency includes a semiconductor body having regions of n-type conductivity and p-type conductivity adjacent each other to form a p-n junction therebetween. The p-type region is of a material having a band gap which will absorb black-body radiation and the n-type region is of a material having a wider band gap than that of the p-type region. This forms a heterojunction between the two regions. The region of n-type region has a doping level which is an order of magnitude less than the doping level in the p-type region. This structure forms a cell having a space charge region in the n-type region.