NIR sensitive organic dyes for tandem solar cells and transparent photodiodes

Due to advantages such as mechanical flexibility, light weight and the prospect to use low-cost roll-to-roll manufacturing processes, organic semiconductors have been widely investi-gated in many application areas as alternatives for their inorganic counterpart. In organic sem-iconductors, the rather weak Van der Waals interactions holding together the molecular build-ing blocks result in narrow absorption bands which endow organic electronics with important advantages for the development of smart functionalities. Transparent organic electronics (TOEs), for example, incorporate devices through which visible light is transmitted. Among other semiconducting devices, it is actually possible to construct sensors and photovoltaic de-vices that solely use ultraviolet (UV) and near infrared (NIR) light to produce electrical energy or signal. TOEs have been proposed for easy integration with other electronic devices. Among the different molecular materials, cyanine dyes stand out by sharp, intense absorption bands exhibiting the highest molar extinction coefficients. The absorption peak can be easily shifted into the NIR wavelength region by increasing the length of the conjugated polymethine chain. For example, NIR light absorbing heptamethine cyanine dyes (Cy7) are promising candidates as transparent and colorless photoactive film materials. In this thesis work, highly efficient TOE devices such as transparent solar cells and transparent photodetectors using NIR absorbing cyanine dyes as photosensitive materials have been successfully fabricated. To optimize these multilayer devices, various cyanine dyes were in-vestigated, device architecture and interfaces were engineered. Optical simulations of the stacked thin film structures allowed understanding and tuning device performance. Moreover, organic solar cells which are transparent in the visible range have been integrated into tandem and triple junction solar cells. Low bandgap materials that absorb NIR light were combined with cyanine cells which absorb visible light, thereby more sunlight could be harvested and power conversion efficiency was dramatically enhanced in such tandem solar cells. The photo-stability investigation of cyanine solar cells showed that cyanine dyes were photostable when illuminated in the absence of oxygen and water vapor. We found that the initial degradation of cyanine dye devices during operation was due to the photo-polymerization of the widely used electron acceptor material fullerene C60 and photo-chromism of the hole extraction interfacial layer molybdenum oxide (MoO3).