Thin Films and Photovoltaics  
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Tandem Cells

Tandem Cells

The maximum achievable efficiency of solar cells is theoretically described by the Shockley Queisser limit. This model takes as assumption that all photons with higher energy than the bandgap will be absorbed and converted to an electron hole pair, and all recombination processes are neglected. For solar cells with a bandgap of 1.1 eV, this limit is about 30%.

However, the photon energy exceeding the bandgap will be lost as the excited electron hole pair will instantly thermalize with the lattice and drop to the conduction or valence band edge for electrons and holes, respectively. To minimize these losses, several solar cells with varying bandgaps can be stacked on top of each other. Thus, the photons with higher energy will be absorbed in a wide gap absorber, while those with lower energy will be transmitted to a lower cell which has a smaller bandgap. This results in a better utilization of the photon energy.

With a triple junction device using monocrystalline III-V semiconductors, efficiencies exceeding 40% could be achieved with this approach under concentrated light. However, those epitactically grown cells are expensive in production. Thus, we investigate the possiblity to implement this approach in thin film solar cells.

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Schematics of a Monolithically Integrated Tandem Cell
The challenges in this work are on the one hand to grow the top cell on an optically transparent substrate (e.g. ITO, ZnO:Al) to ensure the transmission of the lower energy photons to the bottom cells without significant losses. On the other hand, when using a series connection of the individual subcells, the current density generated in the subcells should be identically to prohibit electrical losses (in a series connection, the lowest current determines the overall current in the circuit). Furhtermore, to minimize losses due to absorption or reflection, a monolithic integration of the device is desired, i.e., it should be possible to grow the top cell onto the bottom cell or the other way round. To achieve that, in many processes the heat stability of the finished subcell at the time of growing the second cell is a necessary condition.
In our group, we investigate the possibility of using CIGS solar cells in tandem devices. The investigated or planned-to-investigate structures include CGS/CIS, dye sensitized cell (DSC)/CIGS, a-Si/CIGS, as well as (DSC?)/CdTe(CI(G)S tandem cells.
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