Solution-processed inorganic solar cells are promising low-cost alternatives to first-generation devices but there is a problem in that most of these cells are made from toxic elements, such as lead or cadmium, or scarce elements like tellurium or indium. Other established thin-film technologies need to be fabricated using high-temperature processes such as selenization or sintering, or rely on vacuum deposition techniques.

Researchers led by Gerasimos Konstantatos of the Institut de Ciències Fotòniques (ICFO) at the Barcelona Institute of Science and Technology and the Institució Catalana de Recerca i Estudis Avançats (ICREA) say that they may have overcome these problems with their AgBiS2-based solar cell.

Promising certified PCEs

“Our new photovoltaic material can be processed at low temperatures and ambient pressures,” explains Konstantatos. “It is a very strong panchromatic absorber up to wavelengths of 1000 nm and is made up of non-toxic, earth-abundant elements. It also has promising certified power conversion efficiencies (PCEs) of more than 6% using only 35 nm thick light absorbing layers.”

The new material heralds a move away from traditionally employed nanomaterials based on lead and cadmium, he adds. “It may lead to a new era of ‘green photovoltaics’ that could be installed everywhere – in buildings, vehicles, clothing and consumer electronics, for example, without any regulation.”

Thicker films need to perform just as well

The researchers made their AgBiS2-based solar cells by first building an electron transport/hole blocking substrate made from a 50 nm thin layer of ZnO. Then by spincasting, they added a 35 nm layer of their AgBiS2 nanocrystal material to this substrate and completed the device by spincasting an ultrathin 5-10 nm electron blocking hole transport polymer layer on top.

“At the moment, the most efficient cells based on this material are only 35 nm thick, which makes them very promising for semi-transparent photovoltaic applications,” explains Konstantatos. However, for other applications, we need to be able to make thicker films that perform just as well. We are thus now working on improving charge (electron and hole) collection in such thicker films.

A "tour-de-force" of chemistry and engineering

In fact, our next milestone is to actually double the PCE in this material and then work on making module-level prototype devices,” he tells

Ted Sargent of the University of Toronto, who was not involved in this study, says that Konstantatos and colleagues’ new work is a “tour-de-force” of innovative materials chemistry and next-generation device engineering. “It addresses a topic of essential importance to the world’s sustainability challenge: realizing solar cells that are ultimately efficient, cost-effective, liquid-processed, and low in their energy cost. The team’s breakthrough defines an important step forward towards next-generation solar energy harvesters.”

The research is detailed in Nature Photonics doi:10.1038/nphoton.2016.108.