The cell is a tandem device, where wavelength coverage is increased by using two different junctions. In this case the materials were tuned so that the junctions complement one another – the front cell absorbing 300-720nm, while the rear cell handles 720-1,000nm.

Taking the more challenging route, both cells are connected in series rather than being measured separately within the materials stack. With a series connection, current/unit area is locked together in the cells, forcing one or both to work at a non-ideal operating point.

The materials are modern complex organic compounds designed for photovoltaic use, know by the abbreviations:

• PBDB-T (electron donor for non-fullerene acceptors)
• F‐M (non-fullerene electron acceptor)
• PTB7-Th (electron donor)
• PC₇₁BM (fullerene electron acceptor for PTB7)
• O6T-4F (electron acceptor)

Unlike silicon solar cells where p and n-type semiconductors are distinct separate layers, these organic junctions are mixed types where acceptor and donor compounds are mingled in the active layer.

With light entering from the bottom, the stack-up is:

• Ag – silver electrode
• MoOx
• PBT7-Th:06F-4F:PC₇₁BM – low bandgap junction
• ZnO
• M-PEDOT – polymer conductor
• PBDB-T:F-M – high bandgap junction
• PFN-Br – conjugated polyelectrolyte electron interface
• ZnO
• ITO – indium tin oxide electrode

The team was led by Professor Chen Yongsheng from Nankai University. “This latest achievement has made organic solar cells one step closer to industrialisation,” said the University.

The materials are said to be suitable for use on flexible substrates, although glass substrates are frequently used during research and tend to yield higher efficiency – Electronics Weekly has asked for clarification.

Selection of active materials was based on what the researchers are describing as a ‘semi-empirical model’, and these predict further gains.

“Based on our semi-empirical model prediction, the highest conversion efficiency of organic solar cells can theoretically reach more than 20%,” said Chen.

A major issue with organic solar cells is lifetime under solar radiation and exposed to air and moisture. Silicon solar panel assemblies tend to be specified with a 25 year life.

“We also conducted preliminary tests on the life of the battery and found, after 166 days of experimentation, efficiency is only reduced by 4%,” said Chen. “We will continue to design new materials to further improve the energy conversion efficiency, carry out systematic experiments on life issues, and strive to take organic solar cells from the laboratory to practical applications as soon as possible.”

Nankai University worked with the National Nanoscience Center of the Chinese Academy of Sciences and South China University of Technology.

The work is published as ‘Organic and solution-processed tandem solar cells with 17.3% efficiency‘ in Science.