STRUCTURE WITH INTERFACE RECOMBINATION AND A BUILT-IN POTENTIAL
In this section numerical analysis is utilized to explore the structure of Section 188.8.131.52 but with various built-in electrostatic potentials added. This is done by varying the back-contact barrier height （4>BR） while keeping the front contact barrier height and all contact recombination speeds
FIGURE 5.38 Band diagrams in TE resulting from the systematic variation of the HJ back-contact barrier height （done by varying the back contact workfunction）. Contact recombination speeds were not changed from those of Table 5.4. The front-contact barrier height was kept constant. The value ^BR = 0.63 eV is the baseline cell situation （no built-in potential）.
FIGURE 5.39 Log J-V behavior under light as a function of ^BR. The interface recombination strength used for the simulations is ^ = 2 X 10——14.
at the values seen in Table 5.4. Figure 5.38 shows this systematic variation of the back barrier height （measured at TE from the Fermi level to the acceptor LUMo）. Looking at the diagrams, one would expect values of 4>br > 0.63 eV to be disadvantageous to cell performance and 4>BR < 0.63 eV to be advantageous. Figure 5.39 shows the J-V simulation results for the different <^>BR values. Surprisingly, changing the back-contact barrier heights （and therefore the built-in potential） over the whole range given in Figure 5.38 has no significant effect on Voc. It does, however, significantly affect the fill factor. This result has been selected to once again stress an interesting difference between heterojunctions with absorbers dire-cly producing free electron-hole pairs and those with absorbers that produce excitons: in a heterojunction cell with an exciton-producing p-type absorber and HJ-caused exciton dissociation, the free electrons are generated in the n-type material and the free holes are generated in the p-type material. In a heterojunction cell with a free electron-hole pair-producing p-type absorber, the carriers are generated everywhere in the absorber and electrons must be collected to the n-type material. The excitonic cell free carrier generation pattern and collection requirements are very unique. In the cells of Section 184.108.40.206, Voc is controlled by the interface recombination, so the band bending only influences the efficiency of the （majority） holes getting out of the
p-material absorber and of the （majority） electrons getting out of the n-material acceptor material; i.e., the effect of the built-in electrostatic potential is to change what we have seen appearing phenomenologically in cell characteristics as a series resistance and fill factor problem.