Bifacial thin film solar cells based on copper indium gallium diselenide or CIGS can collect solar energy from both their front and their rear side – and thus potentially yield more solar electricity than their conventional counterparts. So far, however, their fabrication has led to only modest energy conversion efficiencies. An Empa team has now developed a new low-temperature production process resulting in record efficiencies of 19.8% for front and 10.9% for rear illumination.
Moreover, they also produced the first bifacial perovskite-CIGS tandem solar cell, as recently published in Nature Energy (“Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process”), opening up the possibility of even higher energy yields in future.

The idea is as straightforward as it is simple: If I can collect both direct sunlight as well as its reflection via the rear end of my solar cell, this should increase the yield of energy the cell produces. Potential applications are, for instance, 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. Enter the bifacial solar cell. According to the International Technology Roadmap of Photovoltaics, bifacial solar cells could capture a market share of 70% of the overall photovoltaics market by 2030.
Although bifacial solar cells based on silicon wafers are already on the market, thin film solar cells have so far lagged behind. This is, at least in part, due to the rather low efficiency of bifacial CIGS thin film solar cells caused by a critical bottleneck problem: For any bifacial solar cell to be able to collect reflected sunlight at the rear side, an optically transparent electrical contact is a prerequisite. This is achieved by using a transparent conductive oxide (TCO) that replaces the opaque back contact in conventional – i.e. mono-facial – solar cells made of molybdenum.