TECHNOLOGYPHOTOVOLTAICS
C
Figure 1: (a) GaInP
onversion efficiency is the key characteristic for solar cells. Increase this and photovoltaic installations become more attractive because fewer cells are needed to generate a given power. This makes the installation more affordable and also reduces the amount of space required for the photovoltaic system.
The most efficient solar cells utilize a very high proportion of the sun’s radiation, which spans a spectral range extending from the ultraviolet to mid-infrared. To capture this radiation and convert it to electrical power effectively photovoltaics employ a collection of sub-cells designed to operate in different spectral ranges. In these devices several p-n junctions made of semiconductors with different bandgap energies are stacked on top of each other.
One way to optimise the overall efficiency of these multi-junction devices is to first characterise each sub-cell, and then optimise its contribution to the overall performance. Extracting this information is far from easy, but our partnership between the Fraunhofer Institute for Solar Energy Systems and EADS Astrium has pioneered a technique that can do just this, based on electroluminescence (EL) measurements. Today, no other technique can yield the information that is garnered by this approach.
We have used this EL technique to investigate multi-junction solar cells made of the semiconductors GaInP, GaInAs and germanium, which together can yield record efficiencies of more than 42 percent under concentrated sunlight. Our technique is not restricted to this class of device and can also be applied to multi- junction cells based on organic materials, silicon and chalcopyrites.
The monolithically stacked, triple-junction photovoltaic cells that we are studying power satellites and generate electricity on earth, where they are deployed in systems that use mirrors or lenses to focus light by a factor of several hundred. For both these applications, characterizing individual sub-cells not only aids device development – it also enables qualification testing during processing (see Figure 1)
Lateral and vertical characterization Our EL technique involves forward biasing of the solar cell so that it operates as a light-emitting device. Each sub-cell has a different bandgap, so emits a different
top cell EL image of an 8 x4 cm2 triple junction space solar cell showing lateral inhomogeneities and (b) GaInAs middle cell EL image of a
mechanically damaged cell. None of this information is accessible by (c) a standard optical inspection
27 range of wavelengths (see Figure 2).
EL-emission can be detected in different ways. By recording the EL intensity distribution with a CCD sensor it is possible to expose lateral inhomogenities, a particularly important consideration in large space solar cells. Inserting output filters before the detector can select single sub-cell performance, allowing crystal defects that act as shunting paths to be detected, even if they only affect one particular sub-cell. It is also possible to use this approach to provide qualitative quality control, because this technique can expose material issues, such as cracks (see Figure 1b). In comparison, standard optical inspection reveals none of the lateral and vertical information accessible via EL-characterization (see Figure 1c). Even more detailed characterization of the cells can be realized by recording the EL-images of all of the sub-cells at a range of injection current densities.
Extracting cell I-V curves
The EL technique that we are pioneering also yields additional, incredible valuable information – the current- voltage curves of individual sub-cells, information that can drive improvements in multi-junction device performance and enable the introduction of dedicated in-line inspection procedures.
Current-voltage curves for individual sub-cells cannot be accessed with a sun-simulator because
www.solar-pv-management.com Issue III 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48