The diffusion of the heavy alkali element rubidium (Rb) in Cu(In,Ga)Se2 (CIGS) layers was investigated over a temperature range from 148 °C to 311 °C by outdiffusion from a rubidium fluoride layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along grain boundaries could be distinguished from diffusion into the grain interior. Rb was found to diffuse from the CIGS surface along grain boundaries but also within the grain bulk. Based on these data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation DV (Rb) = 3.8·10−8 exp(−0.44 eV/kBT) cm2 s−1 and the fast diffusion along the grain boundaries by DGB (Rb) = 5.7·10−9 exp(−0.29 eV/kBT) cm2 s−1. Further, the effect of Na on Rb diffusion was investigated by comparing Rb diffusion into a Na-containing CIGS layer in contrast to Rb diffusion into an alkali-free CIGS layer. This comparison revealed some aspects of the ion exchange mechanism. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. Rb was found to enhance the open-circuit voltage, the fill factor, and charge carrier density in a similar manner as observed for potassium and sodium.

Thin-film solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) have demonstrated both high efficiencies and a high costreduction potential in industrial production. This way, future CIGS module production lines can be profitable even for scales below the GW range. Among the different technologies, only the coevaporation method has demonstrated efficiencies above 20%, approaching the record values of polycrystalline Si cells. The main focus of this contribution is on the new results of the ZSW cell line with efficiencies above 20%, as well as on the mini-module line on glass substrates. Mini modules (10 cm × 10 cm) with efficiencies in the range of 17% give a proof of concept for industrial-sized modules. ZSW is also developing flexible cells and modules, transferring the processes from the glass-based technology. We achieved 18.6% cell efficiency on metal substrates and a 15.4% efficient mini module could be demonstrated with adapted methods of module patterning. In order to develop industrially relevant processes for foils, we are running a roll-to-roll deposition plant. Additionally, we have improved CIGS cell efficiencies with alternative buffers to certified 19.0% for solution-grown Zn(O,S), to 16.4% for sputtered Zn(O,S), and 17.1% for evaporated In 2 S 3 . Our cells deposited by vacuumfree methods exhibit an efficiency of 8.5% with a nanoparticlebased process.

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An effective encapsulation solution for flexible CIGS is urgently needed to ensure a competitive market entry of the technology. In this work, we demonstrate the feasibility to effectively encapsulate module-level (10×10 cm 2 ) CIGS/glass solar cells by employing a thin Al2O3 barrier layer grown by atomic layer deposition (ALD). As determined by a direct methodology, 10 nm ALD-Al2O3 is proved to be sufficient in preventing electrical degradation of the Al:ZnO (AZO) window layer upon exposure to damp heat test (DHT) and equally effective to encapsulate 10×10 cm 2 CIGS/glass mini-modules by efficient blockage of moisture ingress. CIGS mini-modules encapsulated by ALD-Al2O3 barrier layer retain an average of 80% and 72% of initial efficiency after 1000 and 2000 h of DHT, respectively. Whereas unencapsulated modules drop to an average of 67% (1000 h DHT) and 22% (2000 h DHT) of initial efficiency. Thanks to the presence of ALD-Al2O3 barrier layer, less electrical degradation occurred in AZO window layer and P3 interconnection; also less shunting paths appeared -both led to a lower FF drop in encapsulated CIGS mini-modules. However, an issue of Na migration out of the CIGS layer is observed, which negatively impacts the module stability during DHT.

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