Transparent, or at least semi-transparent, solar cells would open up new possibilities for energy conversion and sustainable buildings. Now, Seunghyup Yoo of Korea Advanced Institute of Science and Technology (KAIST) and Nam-Gyu Park of Sungkyunkwan University and their teams are investigating whether perovskites might fill this niche and have developed a semi-transparent solar cell that is highly efficient and functions very effectively as a thermal mirror. [Yoo et al., Adv Energ Mater (2017); DOI: 10.1002/aenm.201502466]

Such materials could be used as construction materials, fashioned into panels that provide insulation for a building but simultaneously offer power generation when the sun shines. The new approach could circumvent, though indirectly but effectively, the well-known issue of loss of efficiency with increased transparency in a solar cell.

Conventional solar cells are fabricated from crystalline silicon, which is generally not associated with transparency nor even translucency. In contrast, however, it is possible to make solar cells with organic or dye-sensitized materials that are semi-transparent, but when compared against silicon-based cells, the power-conversion efficiencies are very low. Perovskites, hybrid organic-inorganic materials, that offer photovoltaic properties are simple and inexpensive to produce compared with crystalline silicon products and also offer the possibility of high efficiency even when they are made into translucent devices.

The team explains that what is critical to making an efficient and yet semi-transparent solar cell will be to develop a transparent electrode for the uppermost layer that is itself compatible with the photoactive material. The Korean researchers have now constructed a "top transparent electrode" (TTE) that works well with perovskite solar cells. Their TTE is based on a multilayer stack consisting of a metal film sandwiched between a layer with a high refractive index layer and an interfacial buffer layer. Simply overlaid on the upper surface of the perovskite solar cell, this TTE does not interfere with the underlying technology nor disrupt its fabrication. Moreover, unlike conventional transparent electrodes that focuses only on transmitting visible light, the Korean teams' top transparent electrode has two desirable properties in that it allows impinging visible light to pass through while simultaneously reflecting otherwise heating infrared rays.

The team reports an average power conversion efficiency as high as 13.3%, with approximately 85.5% of incident infrared radiation being reflected. Efficiencies of available crystalline silicon solar cells are much higher, up to about 25%, but they are opaque. There is a balance to be met should the semi-transparent perovskite solar cell with a TTE be further developed and scaled up to panels of practical size for "solar windows" in buildings and vehicles. Of course, work on perovskite solar cells themselves continues apace and efficiencies are on the rise.

"Our next thrust is to develop semitransparent solar cells in a form of films so that they can be seamlessly integrated with existing windows," Yoo told Materials Today. "Such work is in progress."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".