Researchers have shown how arrangements of molecules in organic solar cells can improve light absorption, leading to better and cheaper solar panels.

The research, published today in Nature Chemistry, was carried out by scientists from the University of Cambridge and Imperial College London.

Organic solar cells use small organic molecules or organic polymers to absorb and transform sunlight into electricity. The molecules can be produced synthetically at high throughput, and the resulting cells are lightweight, flexible and inexpensive to make.  This makes them potentially cheaper, sustainable and more flexible than traditional cells made of silicon.

Our models show a new design strategy that could result in higher efficiency photon to electric energy conversion. Jeroen Royakkers University of Cambridge

When light hits an organic solar cell, it forces the molecules to transfer electrons, which generates an electrical current.  The efficiency of the process depends on the arrangement of the molecules and how well they interact.

One problem is that organic solar cells are still much less efficient than silicon solar cells. Being able to understand and improve the molecular arrangements in the cells is the key to making them more efficient and commercially competitive. However, the molecular configurations are difficult to characterize because they are buried deep within the system, and their structure has remained a mystery.

Now, co-first authors Jeroen Royakkers from the University of Cambridge and Hanbo Yang of the Department of Physics at Imperial, respectively, have come up with a new way to construct model interfaces, which will allow scientists to study the molecular structures in detail to determine which structures are more efficient.

A combined theory and experiment approach

Royakkers and Yang designed a synthetic strategy to design and control model interfaces. They then used these interfaces to study and model the efficiency of molecular transfer at different locations.

Co-author Dr Jarvist Moore Frost, from the Department of Chemistry at Imperial, said: “A key approach was to simulate the molecular dynamics of these materials, and then use these as ‘snapshots’ that we put back into the quantum mechanical simulations. At room temperature, these materials are very flexible and constantly moving around.

“We can then directly simulate the laser measurements, but we have the information from our calculations about where the quantum-mechanical wavefunction of the electron is moving from and to.”

Royakkers added: “The aim of this investigation was to study the processes that control the initial charge separation process, rather than to achieve high power conversion efficiency in these devices. But our models do show a new design strategy that could result in higher efficiency photon to electric energy conversion.”

A new design

Co-author Dr Flurin Eisner, Lecturer in Green Energy at Queen Mary University of London, said: “Our research delves into the inner workings of light-harvesting molecules by analysing the colours of light they emit. We observed distinct colour shifts between molecules that were arranged in different configurations.

“This told us that the arrangement of molecules matters a lot for how efficiently they separate electrical charges, which is crucial for solar cell performance. Excitingly, our experiments closely matched theoretical predictions, solidifying our understanding of these materials. This paves the way for the development of next-generation organic solar cells with enhanced efficiencies.”

“We have shown in this research that certain arrangements of these molecules make this process better, meaning that we can now design new materials which will improve the efficiency of solar panels,” said Professor Hugo Bronstein, who holds joint roles within this department and the Department of Physics, and who led the research with Professor Jenny Nelson and Dr Jarvist Moore Frost at Imperial College London.

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‘Synthesis of model heterojunction interfaces reveals molecular-configuration-dependent photoinduced charge transfer’, by J. Royakkers, H. Yang et al. is published in Nature Chemistry.

Based on a press release by the University of Cambridge.