Advantageous light management in Cu(In,Ga)Se2 superstrate solar cells

• CIGS deposition onto ZnO:Al window layers leads to an in-situ annealing of the ZnO window layer, reducing its parasitic light absorption as well as the required ZnO layer thickness.
• A thin layer of the high work-function material MoO3−x is shown to allow the use of low work-function and highly reflective metals as the back contact material in CIGS superstrate devices.
• It is shown, that light scattering is more efficient within CIGS devices fabricated in the superstrate configuration as in the substrate configuration.
• The layer thickness of all active layers can be strongly reduced in the superstrate configuration compared to the substrate configuration while potentially increasing the power conversion efficiency.

The superstrate configuration of Cu(In,Ga)Se2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p-type CIGS and the n-type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO3−x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production.

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