A simple model of graded band-gap CuInGaSe2 solar cells

Abstract

The effects of quasi-electric fields, caused by the band-gap variation of the active semiconductor, upon the illumination current density and open-circuit voltage of a solar cell will be discussed. In addition, it will be shown that open-circuit voltage improvement is possible when the band-gap is gradually increased within the space charge region from the neutral to the hetero-junction interface so that the dark saturation current density of the cell is reduced. Our estimation is that in the case of a solar cell where the band-gap increases from 1.15 eV to 1.4 eV, at the space charge region (of the order of 0.2 μm), an increase of the open-circuit voltage around 80 mV will be observed with respect to the single gap absorbing material case. A similar (increasing) band-gap variation within the bulk of the material will cause an increase of the drift-diffusion length of minority carriers by a factor of 5 with respect to a single band-gap material. Then, based on these physical concepts, two possible structures with variable band-gap CIGS layers are proposed in order to have higher efficiencies than cells without any band-gap grading. It will also be shown that very likely present record efficiency CIGS solar cells have achieved efficiencies as high as η=19.9% due to (intentional or non-intentional) band-gap grading of the CIGS layer. This band-gap grading can be optimized with the purpose of a further efficiency increase.