With the continued rise in the uptake of solar cells, consumers are now looking at less obtrusive ways to incorporate these in buildings and vehicles. Transparent or semi-transparent cells provide greater flexibility and visual appeal than standard, opaque silicon solar cells, however their relatively high-cost and poor efficiencies have meant that their adoption has been slow. To help remedy this, researchers working at the Hong Kong Polytechnic University (PolyU) have created semi-transparent, efficient, low-cost perovskite solar cells with graphene electrodes.

First generation silicon solar cells have been the mainstay of photovoltaic (PV) energy conversion for many years now due to their high stability and efficient energy conversion, but their opacity and expense mean that alternatives are now being actively sought for modern building and vehicle applications. Thin film PVs (second generation solar cells) are lightweight and flexible, but are expensive because they are created from rare materials using complex structures requiring high-temperature production processes.

Now, utilizing such materials as thin-film perovskite, the third generation of solar cell is currently being developed for commercial use in the not-too-distant future with the promise of greater power conversion efficiencies, simpler fabrication processes, and lower cost.

In this vein, the PolyU researchers have developed their own version of the third generation solar cell using semitransparent perovskite with graphene used as the electrodes. Being exceptionally thin but with high conductivity and low cost, graphene makes an ideal choice for semitransparent solar cells as it allows light to be absorbed from both sides. As such, the researchers envisage these devices potentially able to be used in windows, louvers, and building roof surfaces, thereby increasing the available surface area for collecting solar energy.

With a claimed power conversion efficiency of around 12 percent, the PolyU solar cells outperform standard transparent and semi-transparent versions hands-down. The potential to be produced at less than HK $0.50 (US $0.06)/Watt also means a greater than 50 percent saving on the cost of conventional silicon solar cells.

While graphene has been around for more than a decade now and is highly-efficient as a conductor in its own right, the PolyU researchers decided to further enhance the conductivity of graphene to meet their specific requirements. To do this, the graphene was coated with a patina of PEDOT:PSS conductive polymer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) – the same ingredient recently used by KAIST scientists in the production of weavable LED fibers – that also acted as an adhesion layer to the perovskite during the process of lamination.

To promote power conversion efficiency, the researchers found that by multilayering graphene through chemical vapor deposition to create transparent electrodes, the sheet resistance of the electrodes was additionally reduced while the exceptional transparency of the electrodes was retained. Finally, the performance of the device was further improved by enhancing the degree of contact between the top graphene electrodes and the hole transport layer on the perovskite film.

According to the researchers, the exceptional flexibility of graphene and the simplified preparation of the cells means that the PolyU device could be eminently suitable for mass production via direct printing or using a roll-to-roll process. In this way, semitransparent solar cells may well provide a greater uptake of PV panels across markets not currently serviced by traditional, opaque devices.

The results of this research were published in the journal Advanced Materials.