A team of researchers in Germany has fabricated a non-fullerene acceptor-based organic photovoltaic cell featuring a bilayer solution-processed hole transporting layer. It recorded a power conversion efficiency of 17.1% and maintained 93% of its initial efficiency after 1,800 hours under continuous solar cell operation at 60 C.

A team of scientists from Germany's Friedrich-Alexander-Universität has presented an organic photovoltaic cell based a hole transport layer (HTL) based on PEDOT:PSS, a polymer known for its low cost and easy preparation properties.

In the research paper, “A polymer bilayer hole transporting layer architecture for high-efficiency and stable organic solar cells,” published in the journal Joule, the researchers said that despite significant progress on laboratory-scale organic solar cells, there is still a lack of interface materials that can be solution-processed on top of the active layer, are compatible with novel non-fullerene acceptors (NFAs) and also provide sufficient long-term stability.

The scientists explained that the proposed HTL provides “exceptional stability” for cells based on NFA materials and inverted architecture. The bilayer HTL is composed of doped poly[bis(4-phenyl)(2,5,6-trimethylphenyl)amine (PTAA) nanoparticles and the polymer PEDOT:PSS.

The researchers said doped PTAA nanoparticles (D-PTAAnp) serve as a buffer layer, provide proper alignment of the hole transport levels and enable efficient hole extraction, while PEDOT:PSS forms a dense layer, protecting the active layer and “furthermore enabling Ohmic contact to the Ag electrode”.

Upon testing, the researchers found that the cell can achieve a power conversion efficiency of 17.1%, which they say is among the highest reported efficiencies for inverted OPV cells using PEDOT:PSS as HTL.  

After 1,600 hours under metal-halide lamp illumination at room temperature, the device retained 95% of its initial performance. After 1,800 hours operating under metal-halide lamp illumination at 60 C it retained 93% of its performance, leading the team to conclude the device's “excellent operational stability”. 

To demonstrate the universality of the interlayer, the researchers then selected four organic semiconductor composites, including PM7:Y6, PTQ11:Y6, PM6:DT-Y6, and PM6:BTP-eC9:L8-BO. The PM6:BTP-eC9:L8-BO device based on the bilayer HTL achieved a PCE of 17.1%.

“To the best of our knowledge, this is among the highest efficiencies reported thus far for inverted OPV cells with solution-processed HTL,” the scientists said. “Overall, the performance and stability values achieved while including a solution-processed top PEDOT:PSS layer present a key enabling tool toward industrially scalable PM6:Y6 modules.

In December, another team of scientists at Germany’s Friedrich-Alexander Universität set an efficiency record of 14.46% for an organic PV module.