Auger generation effect on the thermodynamic efficiency of Cu(In,Ga)Se ₂ thin film solar cells

The effect of Auger generation on the thermodynamic efficiency of a reverse graded band gap Cu(In,Ga)Se₂ thin film solar cell is studied using a detailed balance approach. For the graded profile (1.5-1 eV), a wider range of photons can be considered as high energy photons to produce hot electrons which can pump an additional electron from the valence band to the conduction band. The effects of the carrier multiplication probability, P, and grading strength of the front band gap on the variation of efficiency are studied. Auger mechanism is more effective at lower band gaps due to higher multiplication probability. A thermodynamic efficiency of about 41% is obtained, which is higher than the maximum efficiency of 31% for a graded band gap cell without Auger generation. Moreover, we study the effect of Auger generation on the current-voltage characteristics of the cell. The carrier multiplication increases the short-circuit current due to the increased photogenerated electrons and almost does not change the open-circuit voltage. However, the band gap grading enhances the open-circuit voltage by reducing the recombination rate. Also, a known result is the red shift in the optimum band gap towards a lower band gap (1.15 eV) with respect to the Shockley-Queisser limit.