Improving semitransparent solar cells

Swiss researchers have developed semitransparent organic solar cells that apparently achieve better efficiency and transparency than existing ones.

Semitransparent organic solar cells (OSCs) have potential for providing low-cost, large-area energy conversion devices for various applications such as windows, glass roofs, facades and greenhouses. However, it is challenging to achieve semitransparent OSCs with high power conversion efficiency (PCE) and high transparency at the same time.

Usually, the active materials of OSCs consist of a binary blend of a visibly absorbing donor polymer and a fullerene acceptor. The average visible transmittance (AVT) of the cell can be increased by decreasing the binary film thickness; however, this is at the expense of the PCE because less sunlight is absorbed by a thinner layer.

Researchers from the Swiss Federal Institute for Materials Science and Technology, led by Mohammed Makha, bypassed this trade-off between transparency and efficiency of OSCs by using a ternary mixture. In addition, they used a flexible and transparent top electrode that was applied via a lamination step. The lamination process was simple and compatible with roll-to-roll systems for OSC production from solution.

The team added to a visibly absorbing binary polymer-fullerene blend a dye as a third minority component. The dye absorbs light exclusively in the near-infrared (NIR) wavelength region and therefore does not reduce the visible transparency of the OSC. Due to the additional current generated via NIR absorption, the polymer content could be reduced without compromising the cell performance.

Writing in the journal Science and Technology of Advanced Materials, the researchers revealed that they achieved semitransparent OSCs with a uniform AVT of 51% and a PCE of 3%. The team believes that their ternary blend performed so well because of a specific intermixed phase between the NIR dye and the fullerene; therefore, the system could successfully work with other polymers.