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The fundamental efficiency limitation in a single solar cell results from the trade off between a low bandgap, which maximizes light absorption and hence the output current, and a high band gap, which maximizes output voltage. In tandem cells having two or more series-connected cells with different bandgaps the top cell converts the high energy photons (UV and visible photons) and the bottom cell, made of a material with smaller bandgap, converts transparency losses of the the top cell.

Recent years have shown GaInP/GaAs tandem solar cells with AM0 (sunlight incidence in space=1.35 kW/m²) efficiencies in excess of 25%  Efficiencies above 30% will be available by substituting the GaAs cell (band gap of 1.42 eV) with a cell that more efficiently absorbs lower energy photons and is crystalographically lattice-matched to commonly used Ge or GaAs substrates. But most common semiconductors having bandgaps in the range of interest, such as ternary In_X Ga_1-XAs alloys, are lattice-mismatched to GaAs. (Lattice mismatch for a 1.2 eV In_0.2 Ga_0.8 As is about 1.4%). In order to avoid defect generation and minority carrier performance degradation, only very thin layers (a few hundred Angstroms thick) of these materials can be grown on GaAs. The thin layers are not thick enough for the fabrication of efficient conventional cells. As a result the efficiency and radiation hardness of the existing tandem devices are mainly limited by the photocurrent output and the radiation induced degradation of the GaAs bottom cell. Highest efficiencies are achieved by reducing the thickness (and performance) of the top GaInP cell to below one (1) micron, to favor higher photon flux in the bottom GaAs.

Multi-Quantum Well Design

What is needed is a bottom cell of a tandem solar cell that is capable of producing increased electrical current from the bottom or GaAs cell by absorbing photons having insufficient energy to be absorbed in the top or GaInP cell. The bottom cell should have characteristics such that reducing the thickness of the GaInP cell is not required and it should be lattice-matched to a GaAs or Ge substrate so as to avoid crystalline defects in the cell. 

At CAM, the Photovoltaics and Nanostructures Laboritories are developing a patented materials system (US 6147296,  US 6372980 B1) that utilizes an intrinsic layer of quantum wells (below) that balances lattice stresses without dislocations.  A tandem solar cell including the GaAs cell with MQW provides a near-ideal spectral matching between top and bottom cells, while maintaining the entire structure lattice-matched to commonly used GaAs or Ge substrates. Radiation-induced degradation of a tandem cell is no longer controlled by degradation of GaAs but follows essentially that of the extremely radiation-tolerant InGaP cell. Radiation-induced degradation of the MQW cell, assuming a reduction of minority carrier lifetimes in the conventional p/n part of the cell, is greatly reduced, enhancing end-of-life efficiencies.