Organic solar cells achieve 21% efficiency with two-step crystallization process

Most solar cells on the market today are silicon-based, but energy engineers have recently been evaluating the performance of alternative cells based on other photovoltaic (PV) materials. These alternative options include so-called organic solar cells (OSCs), lightweight, flexible cells based on organic semiconductor materials.

The operation of OSCs relies on a structure, the so-called active layer, consisting of two different types of materials called donor and acceptor materials. Both materials absorb sunlight and generate excitons that dissociate into electrons and holes at the interface between the donor and acceptor materials. Holes are then transported within the donor material and acceptors transport electrons, facilitating the flow of electrons within the device and generating electricity.

Compared to traditional silicon-based solar cells, OSCs are more flexible, lighter, more affordable, and easier to customize for specific applications, such as changing color or transparency. Nevertheless, the efficiency of converting solar energy into electricity remains significantly lower than commercially available photovoltaics (PV).

A promising strategy to improve the efficiency of OSCs is to utilize a class of acceptor materials known as nonfullerene acceptors (NFAs). Unfortunately, the organic molecules in these materials have proven difficult to crystallize into a uniform lattice that would allow for the desired efficiency gains.

Researchers at Hong Kong Polytechnic University, Sichuan University, and other institutions recently introduced a new two-step crystallization process that can produce more homogeneous NFA. Using this approach, outlined in a paper published in Nature Energy, we were able to obtain high-quality acceptor materials that significantly improve the efficiency of OSCs.

“The active layer of OSCs, consisting of a mixture of donor (D) and acceptor (A) materials, and the nanoscale arrangement (morphology) of donor and acceptor molecules within the active layer are key to improving device efficiency, but challenges remain,” Professor Gang Li, senior author of the paper, told Tech Xplore.

“In this study, we introduced acenaphthene (AP) as a crystallization control agent to manipulate the crystallization dynamics of NFA. The resulting OSC active layer exhibits an unprecedented highly oriented stacking of NFA molecules with well-defined Bragg spots. We achieved a PCE of 21% (20.5% qualified) with a maximum FF of 83.2% (82.2%). certified).

Twenty years ago, Professor Lee published a paper in Nature Materials reporting a new approach to improving the morphology of OSCs. In this paper, he showed that self-assembly of polymer molecules can significantly improve the crystallinity of the polymer-fullerene active layer of OSCs, which can improve the efficiency of OSCs.

Based on this discovery, he set out to identify promising strategies to improve the crystallization of the active layer of OSCs. This ultimately led to his latest research, conducted in collaboration with researchers working at various research institutions across China.

“In this study, we first used a layer-by-layer method to deposit donors and acceptors to form a co-continuous phase and achieve ideal vertical phase separation in the active layer,” Professor Li explained. “The crystallization modifier acenaphthene is key to manipulating the crystallization dynamics of the NFA layer on the polymer donor film, resulting in a quasi-single-crystal level of order in the active layer.”

To evaluate the potential of the newly proposed crystallization process, the researchers monitored the formation of thin film organic acceptors using various techniques. Additionally, they relied on computational quantum mechanics-based methods (i.e., performing functional theory calculations) to simulate the interactions between the molecules and the modulators used.

“We confirmed that acenaphthene induced two-step crystallization with sustained delayed crystallization, enabling high-quality crystalline NFA and optimized OSC morphology,” Professor Li said. “This strategy has further proven to be highly effective in several state-of-the-art OSC material systems.”

The modifier-assisted deposition strategy adopted by the researchers enabled the realization of uniform NFAs, which were subsequently integrated into binary OSCs. These solar cells have been found to achieve efficiencies of up to 21%, which is remarkable for an OSC, but still not as high as the efficiency achieved by silicon solar cells.

“High efficiency and stability are important requirements for new solar cells like OSC,” Professor Li added. “By meeting these requirements, OSC technology has the potential to enable new and better solar applications, such as flexible, lightweight, portable, aesthetic and customizable translucent solar applications.

“We will continue to improve the performance of OSCs, such as efficiency and stability. We will also work on superior form factor OSCs (flexibility, aesthetics, transparency, etc.), scalable manufacturing techniques, and attempt to integrate the cells with other types of devices for real-world applications such as smart power windows.”

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