Exciton ‘surfing’ could boost the efficiency of organic solar cells

Physics

Surfing excitons: Cambridge’s Alexander Sneyd with the transient-absorption microscopy setup. (Courtesy: Alexander Sneyd)

Organic solar cells (OSCs) are fascinating devices where layers of organic molecules or polymers carry out light absorption and subsequent transport of energy – the tasks that make a solar cell work. Until now, the efficiency of OSCs has been thought to be constrained by the speed at which energy carriers called excitons move between localized sites in the organic material layer of the device. Now, an international team of scientists led by Akshay Rao at the UK’s University of Cambridge have shown that this is not the case. What is more, they have discovered a new quantum mechanical transport mechanism called transient delocalization, which allows OSCs to reach much higher efficiencies.

When light is absorbed by a solar cell, it creates electron-hole pairs called excitons and the motion of these excitons plays a crucial role in the operation of the device. An example of an organic material layer where light absorption and transport of excitons takes place, is in a film of well-ordered poly(3-hexylthiophene) nanofibres. To study exciton transport, the team shone laser pulses at such a nanofibre film and observed its response.

Exciton wave functions were thought to be localized due to strong couplings with lattice vibrations (phonons) and electron-hole interactions. This means the excitons would move slowly from one localized site to the next. However, the team observed that the excitons were diffusing at speeds 1000 times greater than what had been shown for similar samples in previous research. These speeds correspond to a ground-breaking diffusion length of about 300 nm for such crystalline films. This means energy can be transported much faster and more efficiently than previously thought.

Surfing vibrational waves

To further understand the phenomenon, the lead author of a paper describing the research Alexander Sneyd explains the next task for the researchers: “We are very interested in testing whether this new mechanism occurs in other organic materials to see whether it is a general phenomenon, as appears to be the case for transient charge delocalization. We also want to extend the set of design principles which permit transient delocalization, while also exploring its behaviour in different temperature and pressure regimes”.

Rao and colleagues propose that this unusually fast transport of the excitons is due to a phenomenon called transient delocalization, where the excitons “surf” large distances rapidly in the nanofibre films by accessing a wave of delocalized states via energy exchange with vibrational modes in the material. In other words, quantum mechanical effects allow the excitons to temporarily become delocalized and therefore travel much further over the span of a second than if only the nearest neighbor site could be accessed by the exciton at any time.

Having demonstrated this new highly efficient transport mechanism, the team has established that current models for energy transport in OSCs need to be reconsidered. “This new transport mechanism opens up new avenues to explore highly efficient energy transport in OSC materials, both from a fundamental point of view, and in the context of devices, where better transport will allow one to explore entirely new kinds of device architectures which don’t rely on the traditional nanoscale bulk heterojunction architecture,” says Sneyd.

The research is described in Science Advances.

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