Band-gap engineering in CdS/Cu(In,Ga)Se2 solar cells

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Abstract
Using a numerical device simulation program, the band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells is examined. The device physics of different design concepts is analysed. Normal band‐gap grading improves performance, especially due to the additional quasi‐electric field, and the analysis showed that the best results are achieved if the grading extends from the highest band‐gap value at the back up to the space charge region. The double grading concept does not yield further improvement, because the front grading—even if located in the space charge region—repels the minority carriers (electrons) away from the CdS interface, and consequently, the fill factor drops significantly. Notch structures in the base also exhibit lower performance than the uniform band‐gap base due to the lower open‐circuit voltage and poorer fill factor. Therefore, the best results are achieved by a normal grading in a Cu(In,Ga)Se2 base from the edge of the space charge region to the back contact.