The window-absorber cell structure depicted in the band diagram of Figure 5.20 is the next case we will examine. Again it is stressed that we are not engaged in any sort of an optimization procedure here; rather we are simply looking at a variety of structures with the intent of understanding more about heterojunction solar cell operation. This first window-absorber structure has the modified properties listed in the first row of Table 5.3 superimposed on the basic properties listed in Table 5.1. The configuration is seen to have a built-in potential of 0.54 eV, all of which is developed in the absorber, material 2, due to the heavy doping of the window. As noted in the figure and Table 5.3, the cell of Figure 5.20 has affinity steps that result in an “effective band gap” of 0.87 eV. As discussed earlier, window-absorber structures like this can be very useful in creating electrostatic-field built-in potentials when the absorber is available in only one doping type. They are also helpful when the front contact has very high recombination losses.