Cu(In, Ga)Se2 has the best potential to reach more than 15% module efficiency in the near future. Mini-modules ranging in area from 20 to 90 cm2 that use the process sequence anticipated for larger area commercial modules have already reached efficiencies around 14-15%. Recently, Siemens Solar Industries fabricated a 1 ft x 4 ft power module (-44 watts) with an independently verified efficiency of 12.1%. Using a totally different approach to the deposition of the absorber layer, the ZSW fabricated a 30 cm x 30 cm module with a verified efficiency of 12.7%. Based on the same co-evaporation process, Wurth Solar GmbH, Stuttgart, reported an efficiency of 12.5% for a module of aperture area 5932 cm2 [95]. More results of the different processes are summarised in Table 2. Note that, because of the promising results from the laboratory scale and the first approaches of up-scaling, several companies other than those mentioned in Table 2 now plan commercial production.
A further challenge is to develop CIGS cells on flexible substrates and hence to extend their area of applications. There are ongoing efforts to produce cells on various kinds of substrates like stainless steel, polymide and at the same time to retain the performance achieved with devices on soda-lime glass [96]. For space applications it is important to reduce the weight by depositing the cells on lightweight foil substrates. Highest small area efficiencies on polymide films formed by spin coating on a glass substrate reach 12.8% [97]. Roll to roll coating on metal foils [98] and polymer films [99, 100] has already reached the stage of pilot production.
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Table 2 Comparison of efficiencies ij and areas A of laboratory cells, mini-modules, and commercial-size modules achieved with Cu(In, Ga)Se2 thin films based on the co-evaporation and the selenisation process. NREL denotes the National Renewable Energy Laboratories (USA), ZSW is the Center for Solar Energy and Hydrogen Research (Germany), EPV is Energy Photovoltaics (USA), ASC is the Angstrom Solar Centre (Sweden)
a Flexible cells. |
3.1.5 Stability
The long-term stability is a critical issue of any solar cell technology because the module lifetime contributes as much to the ratio between produced energy and invested cost as does the initial efficiency. Cu(In, Ga)Se2 modules fabricated by Shell Solar Industries more than 10 years ago show until today very good stability during outdoor operation [101, 102]. However, intense accelerated lifetime testing is made for the now commercially available Cu(In, Ga)Se2 modules. Careful sealing and encapsulation appears mandatory, especially because of the sensitivity of Cu(In, Ga)Se2 to humidity. For non-encapsulated modules, corrosion of the molybdenum contact and the degradation of zinc oxide were found to be the dominating degradation mechanisms [103] during the so- called damp heat test (1000 hours in hot (85°C) and humid (85% humidity) atmosphere). Investigations of non-encapsulated cells [104-106] unveiled further a humidity induced degradation of the Cu(In, Ga)Se2 absorber material. Despite of the sensibility of Cu(In, Ga)Se2 with respect to humidity, well encapsulated modules pass the damp heat test [107].
Recent work of Guillemoles et al. [108, 109] investigates the chemical and electronic stability of Cu(In, Ga)Se2 based solar cells and possible fundamental instabilities of the material system, namely, interface reactions, defect metastability, and constituent element (Cu) mobility. Guillemoles et al. conclude that all reasonably anticipated detrimental interface reactions at the Mo/ Cu(In, Ga)Se2, the Cu(In, Ga)Se2 /CdS, or the CdS/ZnO interface are either thermodynamically or kinetically limited. Furthermore, Cu mobility does not contradict long-term stability [108,109].
