Bifacial CIGS solar cells consist of very thin layers deposited on top of a transparent electrical contact. The CIGS polycrystalline layer can absorb light from both front and rear sides of the solar cell. Image: Empa.
Bifacial CIGS solar cells consist of very thin layers deposited on top of a transparent electrical contact. The CIGS polycrystalline layer can absorb light from both front and rear sides of the solar cell. Image: Empa.

The idea is as straightforward as it is simple: if a solar cell can collect both direct sunlight and reflected light, via its rear side, this should increase the yield of energy the cell produces. Potential applications would include building-integrated photovoltaics, agrivoltaics – the simultaneous use of areas of land for both photovoltaic power generation and agriculture – and vertically or high-tilt installed solar modules on high-altitude grounds.

This is the idea behind the bifacial solar cell. According to the International Technology Roadmap of Photovoltaics, bifacial solar cells could capture a 70% share of the overall photovoltaics market by 2030.

Although bifacial solar cells based on silicon wafers are already on the market, thin-film solar cells made from copper, indium, gallium and selenium (Cu(In,Ga)Se2; CIGS) have so far lagged behind. This is, at least in part, due to the rather low efficiency of bifacial CIGS thin-film solar cells, which is caused by a critical bottleneck problem. For any bifacial solar cell to be able to collect reflected sunlight at its rear side, an optically transparent electrical contact is required. This is achieved by using a transparent conductive oxide (TCO) to replace the opaque back contact found in conventional – i.e. mono-facial – solar cells.

The problem is that high-efficiency CIGS solar cells are generally produced by a high-temperature deposition process, i.e. above 550°C. At these temperatures, however, a chemical reaction occurs between the gallium in the CIGS layer and the oxygen in the transparent back contact. The resulting gallium oxide interface layer blocks the flow of sunlight-generated current and thus reduces the energy conversion efficiency of the cell.

The highest values achieved so far in a single CIGS solar cell are 9.0% for the front side and 7.1% for the rear side. “It’s really difficult to have a good energy-conversion efficiency for solar cells with both front and rear transparent conducting contacts,” says Ayodhya Tiwari, who leads the Thin Film and Photovoltaics lab at the Swiss Federal Laboratories for Materials Science and Technology (EMPA).

So, Shih-Chi Yang, a PhD student in Tiwari's lab, developed a new low-temperature deposition process that should produce much less of the detrimental gallium oxide – ideally none at all. Yang used a tiny amount of silver as a secret ingredient of sorts, to lower the melting point of the CIGS alloy. This allowed him to obtain absorber layers with good electronic properties at a deposition temperature of just 350°C. Sure enough, when Yang and his colleagues analyzed the multilayer structure with high-resolution transmission electron microscopy, they could not detect any gallium oxide at the interface at all.

The resulting CIGS solar cell had a drastically improved energy-conversion efficiency, with measured values of 19.8% for front illumination and 10.9% for rear illumination, as independently certified by the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany. The team reports this advance in a paper in Nature Energy.

What's more, the team succeeded in fabricating, for the very first time, a bifacial CIGS solar cell on a flexible polymer substrate, which – due to its light weight and flexibility – widens the spectrum of potential applications. The team also managed to combine two photovoltaic technologies – CIGS and perovskite solar cells – to produce a bifacial ‘tandem’ cell.

According to Tiwari, bifacial CIGS technology has the potential to yield energy-conversion efficiencies beyond 33%, thus opening up further opportunities for thin-film solar cells in the future. At the moment, he is trying to establish a collaborative effort with key labs and companies across Europe to expedite the technological development of bifacial CIGS solar cells and their industrial manufacturability on a larger scale.

This story is adapted from material from EMPA, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.