Degradation of CIGS solar cells

Large scale commercial introduction of CIGS photovoltaics (PV) requires modules with low costs, high efficiencies and long and predictable lifetimes. Unfortunately,knowledge about the lifetime of CIGS PV is limited, which is reflected in the results of field studies: degradation rates varying from 0% to about 4% per year have been observed. Since warrantees are given out that the modules will still yield 80% of their initial power after 20 years of field exposure, degradation rates are often too high as well as too unpredictable. In order to decreasing these degradation rates, knowledge about the degradation behaviour is very important. Knowledge about this degradation would also enable lower production costs: Since both field and accelerated laboratory tests have already shown that elevated humidity and temperature have a negative impact on CIGS PV, modules are laminated with barrier materials to keep the moisture out. For rigid modules, glass is an excellent barrier choice, but for flexible modules, expensive organic-inorganic multilayer coatings are often used. An intrinsically more stable CIGS solar cell would therefore also limit the barrier costs and facilitate the large scale market introduction of flexible CIGS PV. Therefore, I have focused in this work on the identification of de degradation mechanisms in thin film CIGS solar cells. Since these cells consist of at least five individual layers, that all influence each other and the degradation behaviour, various layers have also been studied individually. The knowledge obtained on these individual layers was then used to interpret the degradation behaviour of complete CIGS solar cells. Within this thesis, I propose the degradation mechanisms for the molybdenum back contact and the zinc oxide front contact as well as for complete Cu(In,Ga)Se2 solar cells. The study of these mechanisms has been executed by the exposure of samplesdeposited at different conditions to various accelerated lifetime testing. This was combined with extensive analysis of the samples before, during and after lifetime tests. These tests should give an indication about the long term field exposure of CIGS modules, but it should be mentioned that the validity of the comparison of laboratory testing on solar cells and field exposure on modules it still under discussion. The following tests have been used in this study. 1. Standard ‘damp heat’ – exposure of samples to 85oC and 85% relative humidity (RH). A thousand hours exposure to these standard conditions should be comparable to 20 years of field exposure in Miami (acceleration factor 175x). It should be noted that e.g. literature reported that the acceleration factors of these tests range from 10x to 700x, so this extrapolation should be treated with care. 2. ‘Hybrid degradation’ test (https://www.youtube.com/watch?v=Zmy5tb-2NK8) – a combination of damp heat exposure and AM 1.5 illumination. This allows simultaneous sample degradation with elevated temperature and humidity as well as illumination as loads, while the samples can also be insitu monitored, in order to learn more about the degradation behaviour. This setup has been designed and built within this study and attracted interest from external parties. Therefore, a consortium of three Dutch SMEs (Eternal Sun, Hielkema Testequipment and ReRa Solutions) is currently working on the commercialisation of this setup. 3. ‘Atmospheric species’ exposure – the cells and layers have been exposed to individual species occurring in air, like oxygen, nitrogen, carbon dioxide and water, and combination of these species. This helps the identification of the species that are responsible for the degradation of CIGS solar cells. This can lead to the identification of the degradation mechanisms and can allow optimisation of cost-effective and effective barrier materials. This setup has been designed and built as part of this study. Barrier materials are not used in any of these tests in order to even further accelerate the degradation rate. These tests, combined with extensive analysis of samples with among others IV, EQE, IV(T), EL, PL, lock-in thermography, 4PP, Hall, UV-VIS-NIR, Raman, XRD, SEM-EDX, HIM and SIMS, have led to the following conclusion on the degradation of CIGS solar cells and its constituents.