Photovoltaics
Module with five-junction solar cell achieves efficiency of 32 per cent

Originally developed as an energy source for space travel, multi-junction solar cells are currently used for concentrated photovoltaics in sunny regions of the Earth. Using high-quality lens systems, the technology currently achieves a module efficiency of around 29 per cent. This high efficiency is due to special multi-junction solar cells made of various semiconductor materials. Within the QuintUMM project, the researchers have developed a new five-junction solar cell, the so-called quintuple solar cell. Modules with these new solar cells achieve an efficiency of up to 32 per cent and thus generate over 10 per cent more energy than systems with triple-junction solar cells.

Concentrated photovoltaics currently uses triple-junction solar cells

Manufacturers of high-concentration photovoltaic systems currently usually use triple-junction solar cells, which consist of three solar cells stacked on top of each other. Each of these partial solar cells converts a certain wavelength range of sunlight into electrical energy. The efficiency of these cells is up to 44 per cent. This is due to the various semiconductor layers, which are usually composed of gallium indium phosphide, indium gallium arsenide and germanium. Triple-junction solar cells use sunlight much more efficiently than silicon solar cells. These cannot use the high-intensity light particles efficiently. Therefore they only achieve a comparatively low voltage. A considerable part of the energy is lost in the form of heat. In contrast, multi-junction solar cells achieve a higher voltage and consequently a higher efficiency. Such solar cells are relatively expensive. They are therefore unsuitable for conventional, full-surface solar modules.

Efficient production of semiconductor stacks with new materials

Engineers use a special method, Metal Organic Vapour Phase Epitaxy (MOVPE), to produce the semiconductor stacks of the multi-junction solar cells. In this process, molecules of the various semiconductor materials are successively deposited from the gas phase on a heated substrate, the germanium wafer. The technical term for this process is epitaxial layer growth. This takes place in a production reactor, which simultaneously deposits various semiconductors on several germanium wafers. Within the research project, the scientists developed this method for the new structure of the five partial solar cells, which are stacked on top of each other. Furthermore, the tertiary and quaternary semiconductor materials (InGaAs, AlInGaAs, InGaP and AlInGaP) have a special material composition.

The individual sub-cells of the multi-junction solar cell are arranged vertically on the germanium wafer and connected internally by tunnel diodes and other functional semiconductor layers. The tunnel diodes serve to interconnect the individual sub-cells in series. This adds up the voltage of the individual cells. In the end, the desired higher output is achieved. The researchers optimised the material composition of the tunnel diodes in further investigations. They are now suitable for use in concentrated photovoltaic systems at 1000 times the concentration of incident solar radiation. Furthermore, the uppermost tunnel diode of the semiconductor stack is particularly transparent. This allows sufficient light to pass through to the four sub-cells below.