Realized a high performance ternary organic solar cell thanks to two isomeric non-ferrous acceptors
New record for polymer solar cell efficiency
The efficiency of organic photovoltaics exceeds a new milestone thanks to polymer solar cells that can transform 20% of the incident light into electricity. It happens in Hong Kong where a group of engineers from the polytechnic, in collaboration with colleagues from other Chinese universities, has redesigned the mixture of active layers. And the result achieved today is a milestone for organic photovoltaic technology. But to better understand innovation you need to take a few steps back.
Organic photovoltaics, the structure
Most polymer organic solar cells adopt a binary mixture of active layers in the form of mass heterojunction (BHJ – Bulk Heterojunction). This mixture is nothing more than a mix of molecules that absorb photons (donor materials) and molecules that acquire excited electrons (acceptor material). The former usually consists of a conductive polymer and the latter of fullerenes or, an alternative much more promising from the point of view of the performance, of non-fullerenic acceptors. These compounds offer several advantages, starting with the possibility of being designed to efficiently absorb light in the visible region. They are also easily tuned, which means that you can generate a system of components with complementary forbidden bands.
Over the years the binary structure has evolved into a ternary, with active layers with three components: two donors and an acceptor or vice versa. This change was largely determined by the possibility of expanding solar absorption. The organic photovoltaic “ternary”, in fact, can be specially designed with absorption bands that cover as much as possible the range of the solar spectrum, thereby increasing the short-circuit current density of the device.
A boost for the efficiency of organic photovoltaics
The search of the Chinese team fits exactly at this level. The starting element? A binary organic solar cell, based on PM6, a conjugated semiconductor polymer, and BTP-ec9, a n. non-ferrous semiconductor acceptor, the group transformed the binary cell into a ternary by adding a second acceptor to the mixture. In detail, the researchers, guided by theoretical calculations, have synthesized the o-BTP-ec9, an isomer of BTP-ec9 with distinct photoelectric properties. “Very similar chemical structures lead to an excellent miscibility of these two isomers,” the authors explain. And the results on the efficiency of organic photovoltaics were very clear.
“The o-BTP-ec9-based device improved the charge transfer state, thus significantly reducing the energy loss […] and showing an excellent power conversion efficiency of 18.7%. In addition, the new host acceptor o-BTP-ec9 has excellent miscibility, crystallinity and energy level compatibility with BTP-ec9, which allows an efficiency of 19.9% (19.5% certified) in the ternary system PM6:BTP-C9:o-BTP-ec9″.
The research was published in Nature Communications.
