The direct conversion of sunlight into electricity using energy harvesting devices is key for the provision of renewable energy in the future. Efficient use of existing large surface areas in residential buildings could be made by the development of a transparent photovoltaic technology that can be integrated onto window panes. Transparent and flexible solar cells could also be integrated into greenhouses, foldable curtains or used as chargers for portable electronics, such as screens of laptops and mobile phones. In addition, such transparent devices are interesting for powering "invisible" electronic circuits that are integrated, for example, in automobile windscreens.
The use of inorganic semiconductors for such applications is limited because of the band like absorption spectra of these materials. In contrast, organic molecules show discrete absorption bands in narrow spectral regions. Transparent organic energy harvesting devices require organic molecules where the visible light transmittance is maximized while ultraviolet (UV) and near-infrared (NIR) radiation is absorbed. Research has mainly presented semi-transparent and coloured organic solar cells so far, that might find applications, for example, as an architectural and design motif.
A unique feature of cyanines is that dyes can be synthesized that are fully transparent in the visible wavelength region. The synthesis of cyanine dyes is well-established and up-scalable. There are already several NIR cyanine dyes commercially available. An important merit of cyanines is that they are cheap compared to other established semiconducting materials. In recent work, we could demonstrate cyanine-based organic solar cells with over 2% power conversion efficiency and a very high level of transparency.