TECHNOLOGYPHOTOVOLTAICS
the cells are monolithically connected in series and metal contacts are only on the front and back sides of the device (see Figure 2). The sun- simulator is limited to characterising the current- voltage curve of the triple-junction, and can also extract values for the short-circuit current, the open-circuit voltage, the fill factor and the efficiency of the entire device.
Figure 2: Schematic of a GaInP/GaInAs/ germanium lattice matched triple-junction solar cell. The
bandgap energies and emitting wavelengths of each subcell are indicated
The foundations of our EL technique are theoretical developments detailed in two landmark papers. Uwe Rau from Forschungszentrum Jülich wrote one of these; the lead author for the other was Rau’s colleague Thomas Kirchartz. Co-authors on this paper were Rau; Anke Helbig and Jürgen H. Werner from the Institute for Physical Electronics, Stuttgart; and Martin Hermle and Andreas Bett (an author of this feature) who both work at the Fraunhofer Institute for Solar Energy Systems (see Further Reading for paper references).
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One cornerstone of this theoretical work is the so- called reciprocity relation. By measuring the spectrally resolved electroluminescence signal and the external quantum efficiency for each sub-cell, it is possible to access its dark current-voltage and illuminated current-voltage characteristics.
We have adopted this approach to study a
Ga0.50In0.50P/Ga0.99In0.01As/germanium lattice- matched triple-junction cell, extracting information that can be used to predict the power performance of this photovoltaic device under realistic operation conditions.
The spectral reciprocity relation connects the intensity of the EL signal with the energy of an
Figure 3: (a) Spectrally resolved EL spectrum at a constant injection current-density of 37.5 mA/cm2 and (b) external quantum efficiency (EQE) of a
2x2 cm2 lattice-matched Ga0.50In0.50P/ Ga0.99In0.01As/germanium triple-junction solar cell. Each EL peak is related to one of the sub-
cells. The three EL peaks emerge at the same energy as the declining slope of the corresponding EQE because both are related to the band gap energy of the sub-cell
emitted photon, the internal voltage of the sub-cell and the external quantum efficiency of the sub-cell. This efficiency depends on the black body photon flux, which is described by Planck’s formula.
The first step is to measure the EL at a range of injection current densities when no light is incident on the cells. Once this is complete, the external quantum efficiency for each of the sub-cells is measured (see Figure 3). By applying the spectral reciprocity relation, the voltage of each sub-cell can then be calculated as a function of the dark- current density – this provides the dark current- voltage characteristics for each sub-cell.
Terrestrial concentrator system of III-V multi-junction GaInP/GaInAs/ germanium solar cells in Spain. Image courtesy of Concentrix Solar, Division of the Soitec Group
Once the ‘dark’ current-voltage characteristics of the respective sub-cells are known, the illuminated current-voltage characteristics of individual sub- cells and of the whole multi-junction device can be easily calculated for any desired spectral condition using superposition principles.
Our studies have shown that there is excellent agreement between the resulting current-voltage
www.solar-pv-management.com Issue III 2011
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