Transparent photovoltaics (TPV) is a disruptive approach in which the solar cells can selectively transmit the visible light to human eyes harvesting UV and/or NIR photons.[1] TPV is attractive as it widens the deployment of PV into new sectors, like building integrated photovoltaics (BIPV), greenhouses, car windows and sunglasses, thus providing an immense potential to generate solar electricity beyond the conventional rooftops and solar power plants. One possible approach to TPV is based on wavelength-selective absorbers where the chromophore requires an absorption far from the photopic response of the human eye.

DSSCs offer a sustainable solution for transparent and colorless photovoltaic cells, thanks to the exceptional tunability of both dyes and electrolytes. However limited classes of dyes possess energetic levels that can ensure an efficient injection while having a bandgap sufficiently narrow to selectively absorb the NIR region.[2] Among these classes, polymethine dyes (cyanines and squaraines) are promising for their high molar extinction coefficient and easily tunable properties through modification of central core or lateral units. Cyanines in particular have already been investigated for dye sensitized solar cell (DSSC) devices with promising results in terms of transparency and performance.[3]

In recent years, our research group has developed a series of polymethine-based NIR dyes for wavelength-selective DSSCs, as well as different colorless redox shuttle. Most of these molecules were synthesized using microwave-assisted procedures aligned with green chemistry principles, enabling efficient and cost-effective production. This work presents the design, synthesis, and device integration of NIR dyes that achieve high transparency coupled with acceptable performances. Additionally, we report on the development of compatible colorless redox mediators and transparent electrodes. A comprehensive discussion is provided on the interplay of materials and the current technological challenges in realizing fully transparent and colorless NIR-DSSCs for BIPV applications.

These new molecules have been deeply characterized in terms of their optical, photophysical and electrochemical properties, showing interesting structure/property relationships. Finally, photovoltaic performances have been evaluated in lab-scale DSSCs and optimized by different anode modifications and electrolyte formulations.