An international team of researchers has uncovered the hidden mechanism that limits the performance of organic solar cells and could help them exceed the 20 percent efficiency barrier.

The study, led by scientists from Linköping University in Sweden, the University of Potsdam in Germany, and the Paul Drude Institute in Berlin, sheds light on a long-standing challenge in the development of organic photovoltaics (OPV).

Organic photovoltaic solar cells offer an Earth-abundant, low-energy alternative to conventional photovoltaics. They also have the potential to generate electricity at a lower cost than first- and second-generation solar technologies.

According to the team, the findings could provide a roadmap for the creation of more efficient solar cells that would be able to compete with already established silicon-based technologies.

A solar mystery

Solar cell performance is determined by three factors. These include short-circuit current, open-circuit voltage, and fill factor. While all three influence how much electricity a device can generate, improving one often comes at the expense of another.

Hence, researchers have struggled with a persistent trade-off in organic solar cells for years. Attempts to increase the open-circuit voltage frequently led to lower fill factors. At the same time, improvements in fill factor often reduced voltage.

As organic solar cell efficiencies climb beyond 20 percent, this trade-off becomes increasingly difficult to overcome. To tackle the challenge, Dieter Neher, PhD, of the University of Potsdam, Feng Gao, PhD, of Linköping University, as well as Safa Shoaee, PhD, of the Paul Drude Institute for Solid State Electronics, teamed up to investigate its root cause.

Working with other experts in the field, the team examined why efficiency gains in organic solar cells begin to slow at higher performance levels. The results showed that under specific conditions, the generation of free electric charges in the active layer of the solar cell depends heavily on the electric field in the organic semiconductor material.

“This results in a previously poorly understood limitation on the fill factor, which becomes particularly relevant when voltage losses need to be minimized,” Neher, a physics professor at the University of Potsdam, pointed out.

Inside the bottleneck

When sunlight hits an organic solar cell, it creates excitons, which are bound pairs of negatively charged electrons and positively charged holes. Because these pairs cannot move freely, they must first be split into free charges that can generate electricity.

Using simulations of an entire solar cell, the team found that two factors play a critical role in this process: how long excitons survive and how much energy is released during charge transfer. Both parameters were identified as the most important determinants of the fill factor at low voltage losses.

“We were able to trace the trade-off between fill factor and open-circuit voltage back to a few physical quantities and simulate how this limitation can be significantly mitigated by increasing the exciton lifetime,” Neher concluded in a press release.

The team demonstrated that extending exciton lifetimes can significantly reduce the problem. To validate the concept, they also developed new combinations of organic materials and used them to produce solar cells. The devices delivered both high fill factors and strong overall power output.

According to the researchers, the findings provide general design principles that could guide the development of future organic photovoltaic materials and device architectures.

The study has been published in the journal Nature Photonics.

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