Carbon-Based Organic Solar Cells Likely to be a Better Alternative to Traditional Silicon-Based Cells

With the carbon-based organic solar cell or OSC, solar-paneled roofs on top of commercial, residential and industrial buildings might get a makeover. OSCs may be a good alternative to conventional silicon-based cells due to their thinness and flexibility.

However, not all OSCs are the same. Even with a low offset of 0.1 eV, those focused on non-fullerene-acceptors, or NFAs, have been reported to produce electricity efficiently. NFA-based OSCs have a considerably higher power conversion efficiency than traditional fullerene-based types.

We then asked ourselves how this was achieved, and what materials we would need to develop in order to obtain the low offset.

Yasunari Tamai, Kyoto University

Yasunari Tamai’s team made the discovery.

The preferred semiconductors being used in OSCs, also known as organic photovoltaics, or OPVs, have conventionally been a combination of so-called p-type polymers and n-type fullerene derivatives. To drive photovoltaic conversion, a difference in energy levels, or offset, of more than 0.3 eV is usually required. These traditional polymers have a power conversion efficacy of up to 10%–11%.

On the other hand, a large offset also drags down the open-circuit voltage. Efficient power conversion requires a trade-off between electric current and voltage in the form of a low offset.

Yasunari Tamai, Kyoto University

Thinking outside the box, or, in this case, reversing the thinking can sometimes lead to the best solutions. Here, the solution is to lose the fullerene.

NFA-based OSCs have recently been discovered to efficiently generate free carriers even with a 0.1 eV offset, outperforming regular fullerene-based OSCs by an impressive 10% or more.

To monitor free carrier generation over time, the researchers used transient absorption spectroscopy. Relaxed charges transfer freely down the energy cascade generated in the solar cells, similar to how a slalom skier glides down the hill from gate to gate.

We hope that our research will help move the world closer to this practical application of organic solar cell technology to harness the virtually non-depletable energy source from our sun.

Yasunari Tamai, Kyoto University

Journal Reference:

Natsuda, S.-I., et al. (2022) Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells. Energy & Environmental Science. doi.org/10.1039/D1EE03565G.