Quantum mechanical phenomena control in organic solar cells
Qingzhen Bian had to do the experiment many times, but the result was the same. Credit: Magnus Johansson

Doctoral student Qingzhen Bian obtained remarkable results when he experimented on optimizing a solar cell material consisting of two light absorbing polymers and an acceptor material. Olle Inganäs, professor emeritus at the Department of Biomolecular and Organic Electronics, asked him to remit to exclude measurement errors. Time and time again, in experiments performed both at LiU and at colleagues in Lund, the same thing happened: in the signature from the optical absorption and the formation of a photocurrent in the photovoltaic material, a small periodic waveform occurred for a few hundred fifty seconds. Why?

The explanation has been published in Nature Communications.

The coherence occurs

Little background: When light in the form of photons is absorbed in a semiconducting polymer, a so-called excited state is created. It consists of holes in the ground state that the electrons are still attracted to – the electrons do not release and the charge transport, the photocurrent, fails. When the electron-donating polymer is mixed with a polymer that accepts electrons, the electrons can be released. The electron then only needs to take a small leap to get loose and the energy loss is minimized. The holes the electrons leave behind accelerate the charge transport and the solar cell begins to supply electricity.

So far, the researchers agree: but then the strange waveform appeared in the experiments.

Olle InganäsOlle Inganäs, professor emeritusTHOR BALKHED– The only conceivable explanation is that there is a coherence between the excited system and the separated charges. We asked quantum chemists to look at this and what we see in experiments after experiments is well in line with their calculations, says Olle Inganäs.

At the quantum mechanical scale, atoms vibrate, especially when heated. It is these vibrations that somehow interact with each other and with the excited system of electrons – the wave phases follow one another and coherence occurs.
– Coherence contributes to creating the charges that give the photo stream, and it is done at room temperature, but we do not yet know why or how, says Olle Inganäs.

molecular Design

The same quantum mechanical coherence exists in the biological world.
– The phenomenon exists and is significant, but more than that we do not know yet. In biophysics, a heated debate rages whether photosynthetic systems have learned to use coherence or not. I consider it unlikely that nature has not used the phenomenon to its advantage during years of development, says Olle Inganäs.

– If we understood better how charge carriers are formed, what it is that controls the process, we should be able to utilize it to increase the efficiency of the organic solar cells. The vibrations depend on the structure of the molecule and if we can design molecules that help increase the photo stream we can also use the phenomenon to our advantage, he says.

The study was published in Linköping University