Schematic of the green-solvent-processible doctor-blade printed self-organization method.
Schematic of the green-solvent-processible doctor-blade printed self-organization method.

Organic solar cells (OSCs) promise cheap, lightweight, flexible, easy-to-fabricate, and eco-friendly power sources. But while competitive power conversion efficiencies of over 18% have been achieved, improvements in long-term stability, processability with green solvents, and scalability are still needed for the commercialization of OSCs. Now, however, researchers report a green-solvent-processable, open-air printable self-assembly strategy for the fabrication of stable, efficient OSCs [Tang et al., Materials Today (2022),].

“This work aims to close the lab-to-fab gap of OSCs towards cheap, stable, efficient, scalable, and ecofriendly requirements,” explains Gang Li of The Hong Kong Polytechnic University, who led the effort with colleagues from Chongqing Institute of Green and Intelligent Technology, Chongqing University, University of Science and Technology of China, Hefei, King Abdullah University of Science and Technology (KAUST), and Ulsan National Institute of Science and Technology (UNIST).

The team combined a simple self-assembly strategy with doctor-blade printing, a type of roll-to-roll compatible printing in which a blade is used to coat ink into a uniform solar film. The green ‘solar ink’ is prepared by dissolving bulk-heterojunction (BHJ) active layer and cathode interlayer (CIL) materials in a hydrocarbon solvent. The ink is suitable for doctor-blade printing in air onto an indium tin oxide (ITO) surface where self-organization takes place. The self-assembly of the CIL and BHJ materials simultaneously simplifies both the device structure and manufacturing process.

The BHJ(DPO) device achieves a power conversion efficiency of just 13-15%, comparable with similar devices produced by traditional step-by-step methods, and has the potential for scale-up. The devices are also more stable in air, more thermally stable and more light stable.

The researchers believe the self-assembly strategy reduces the density of trap-assisted recombination centers that impact conversion efficiency. The self-assembled BHJ(DPO) film also demonstrates the highest absorption efficiency, which the researchers believe is a result of the DPO determining the aggregation rate and ensuring optimal morphology of the self-assembled layers. Improved morphology in these layers leads to better hole and electron mobilities, which boost charge transport.

According to the researchers, the new green-solvent processable, open-air printed self-assembled OSCs are among the best performing devices of their type. The devices are highly stable, maintaining 80% of their PCE after 5700 hours on the shelf and demonstrating an operational lifetime of over 190 hours, as well as when exposed to light and high temperatures.

“Our approach effectively balances the efficiency, stability, cost, green processibility, and scalability of OSCs, paving the way to closing the lab-to-fab gap toward commercialization,” says Li.