The heterojunction solar cells is constructed from an absorber material which acts the base of the junction and an window layer which acts an emitter of the heterojunction. It is so that the highest efficiency of such hj solar cell is dominated by the absorber optoelectronic properties. One can safely say that the maximum efficeincy is limited by the single junction solar cell efficiency by Queisser which is about 33.7 percent for an aborber with energy gap of about 1,35 eV
For more information please refer to the chapter:https://www.researchgate.net/publication/323309527_Solar_cells_and_arrays_Principles_analysis_and_design