Scientists at Empa, Swiss Laboratories for Material Science & Technology, said they have developed a low-temperature technique using silver to produce a bifacial perovskite-CIGS (copper indium gallium selenide) tandem solar cell, wherein the front side of the cell achieved record efficiency of 19.8% and the rear side displayed 10.9% efficiency.

In the work published in Nature Energy, the experts at EMPA captured both the direct sunlight and its reflection on the two sides of the tandem solar cell.

To overcome the issue of achieving limited efficiency, the scientists used a conductive oxide (TCO) to establish an optically transparent electrical contact which enabled the rear side of the bifacial CIGS tandem cell to capture the sun’s reflection.

The TCO became a substitute for largely used opaque back contact which is mono-facial, where solar cells are made of molybdenum which is a transition metal.

The use of silver raises the current flow

However, the scientists found that the flow of the current produced on the rear side of the cells is largely blocked due to the formation of a gallium oxide (GaOx) interface, lowering the energy conversion efficiency of the cell.

A chemical reaction occurs between the gallium of the CIGS layer and the oxygen of the transparent rear contact, when the production of highly efficient CIGS solar cells is subjected to an increased temperature deposition process, for instance, under 550 degrees Celsius.

According to the Head of EMPA’s Thin Film and Photovoltaics lab, Ayodhya N. Tiwari, the highest efficiencies achieved in a single cell are 9% 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,” he said.

Source: Empa

To significantly void the presence of GaOx interface, the team then used little silver as a secret element to reduce the melting point of the CIGS alloy.

They focussed on obtaining absorber layers with enhanced electronic properties at a deposition temperature of 353 degrees Celsius. On keenly observing the multilayer structure of the cell with high-resolution transmission electron microscopy, gallium oxide was almost nil.

The low-temperature deposition method using a tiny amount of silver boosted the energy conversion efficiencies on the front side to 19.8% and 10.9% on the rear side of the bifacial-CIGS tandem solar cell.

The efficiencies achieved by the EMPA scientists align with the record power conversion efficiency of 47.6% achieved by researchers from the Fraunhofer ISE’s Center for High-Efficiency Solar Cells. The team at Fraunhofer optimized each layer of the complex solar cell with multiple junctions using an anti-reflection coat at a concentration of 665 suns (heat concentration level equalling 665 suns).

EMPA scientists claimed to have achieved an efficiency of 22.2% for CIGS flexible solar cells. This is an improvement of 0.8% from the previous value of 21.4%, which was announced by the same lab.