Abstract
Due to an impending energy shortage, renewable and clean energy should be considered as a next generation energy resource. CIGS is the best thin film solar cell technology among the second generation solar cells owing to its various outstanding characteristics, such as highest absorption coefficient, ease of integration with Si-based technology, and the highest efficiency in thin film solar cell form. As a result, it has already attracted tremendous attention in academic and industrial fields. To date, the highest efficiency of ~21.7 % has been demonstrated by ZSW in Germany. But there is still little research toward CIGS flexible devices for applications on specific building shapes or curve designs on mobile devices. In stacked structures used in solar cells AZO has been used as transparent conducting oxide (TCO) before, but its intrinsic resistivity is high so industrial applications prefer to use ITO as alternative. This inspired the development of sandwich structures. Initially, metal layers are embedded between TCOs to enhance conductivity. For example, Cu film over 16 nm can reach 1.6×10-3 Ω-cm or even below 5.0×10-4 Ω-cm when over 18 nm. But the maximum transmittance is just around 65 % (at wavelength of 600 nm) which isn’t good for solar cell applications. In the case of Ag as a metal layer, it has lower resistivity but there’s aggregation when the thickness is too thin that makes the surface of the film discontinuous and leads to higher resistance and worse current. This thesis focuses on a low temperature TCO process using Ag NWs embedded to maintain their optical transparency and electrical conductivity. In our research, flexibility of AZO is worse than ITO and that the resistance increases with the number of bending cycles. We can see from in SEM images that AZO has a wider crack seam and line-shaped structure after bending. It’s expected that after Ag NWs are embedded, they can form metal networks between films effectively. There might be interface or surface roughness issues in the beginning of bending but when it reaches certain bending cycles, nanowires can act as effective interface connections to ensure its conductive characteristics are maintained. Finally, we fix the parameters of the Ag NWs with using a figure of merit (FOM) that can be applied on devices. SEM images show the great qualities of Ag NWs clad with AZO continuously. We observe better roughness in AZO/Ag NWs/AZO than for AZO/Ag NWs from AFM measurements. IV plot shows CIGS with sandwich structure TCO has similar performance with one with ITO due to its lower series resistance. Cell with single layer AZO shows lower efficiency than single layer ITO. We have already developed a simple and fast production method for flexible CIGS solar cells improving efficiency from 4.4 % to 6 % CIGS solar cells. It’s believed such an approach can further improve future performance and developments for energy and mobile applications based on chalcopyrite photovoltaics.