Towards improved plastic solar cells

Neutron spectroscopy provides a range of tools to study the structural dynamics of semiconducting organic materials for novel solar cells with improved power conversion efficiency. 

Solar cells – devices that convert sunlight into electricity – are a key component in the provision of future green energy. Typical solar panels rely on two layers of the semiconductor silicon, processed so that one layer has an excess of electrons carrying negative charges while the other is electron deficient carrying positive charges. This charge difference generates an electric field between the layers. When a light ray hits the panel, the electrons are released leaving behind positively charged ‘holes’. Separated by the electric field, the two types of charge carriers then travel in opposite directions around a circuit to create electricity.

There is also a huge worldwide interest in fabricating solar cells made from plastic materials. They would be cheaper to manufacture, being fabricated using low-cost ink-jet processing, or as a coating from solution in a continuous process. The plastic device would be lightweight and flexible, and could be scaled up to any desired size. 

The most favoured design relies on a combination of an electron donor, i.e. an electron-rich semiconducting polymer, and an electron acceptor, i.e. an electron-deficient semiconducting small molecule. When blended together, these two components form an electronically active layer which upon illumination is subjected to the generation and subsequent migration of charge carriers. Indeed, light falling on this active layer creates pairs of electrons and holes in either semiconducting components. Unlike in silicon, these pairs do not spontaneously separate; they are bound. Thus, to generate an electrical current and prevent the electrons and holes recombining (efficiency loss), the pairs must be fully separated and the resulting charge carriers transported through the material to the appropriate electrodes. The first successful plastic solar cells used a derivative of fullerene (a sphere-like carbon molecule) as an acceptor, together with a semiconducting polymer such as P3HT – poly(3-hexylthiophene-2,5-diyl) as a donor. They have achieved solar conversion efficiencies of up to 11 per cent.

Recently, interest has turned to blends using non-fullerene electron acceptors (NFAs). Unlike fullerenes, they resemble the donor polymer in design, and as such absorb visible light. However, they are small molecules having an almost planar core geometry made of carbon rings linked to carbon chains (alkyls). New families of NFAs have made it possible to aim reaching an efficiency milestone as high as 20%. In this context, the molecular-by-design route plays a key role towards optimising the microstructural properties of the donor–acceptor blend forming the plastic solar cell active layer. Maximising the charge separation and transport will improve the photo-current generation. This requires the blend components, donor and acceptor(s), to match certain morphological characteristics of the blend structure, i.e. balancing fine blend of the components in the amorphous domains to achieve a better charge separation while maintaining the crystallinity aspects of the donor and acceptor domains to boost the mobility is fundamental to reach the potential of those new materials. The challenge is thus to find the right composition and mixing of components that generate the ideal stable microstructure for efficient power conversion. Introducing a second NFA to form an acceptor-donor-acceptor organic solar cell ternary blend offers a further flexibility in reaching this goal.

Probing structural dynamics with neutrons

To explore this possibility further, we studied systems based on P3HT polymer as the donor, blended with two structurally-related NFA molecules known as O-IDTBR and O-IDFBR. Probing the energies (gained or lost through the called inelastic process) of neutrons exchanged with a sample as they pass through provides a measure of what the atoms of the different molecules are doing over the probed time – since energy and time are intimately related. We used a range of techniques that cover molecular motions, either single and/or collective behaviours of molecules and atoms, taking place at the nanometre scale and over timescales ranging from picosecond to nanoseconds, as well as probing vibrational degrees of freedom observed within the femtosecond range.

Neutrons are very sensitive to hydrogen atoms and the signal stemming from their interaction is very strong - strong enough to potentially hide signals from the other atomic species a material is made from. Therefore, neutrons can also efficiently probe the behaviour of materials containing hydrogens. Therefore, by substituting the hydrogen atoms in either the NFA molecules or the P3HT polymer with the heavier isotope deuterium (via a technique called deuteration) - which gives a weaker signal than hydrogen – it is possible to highlight selectively the dynamics of the blend components. Using these techniques, we probed the dynamics of both binary and ternary blends of the NFA molecules O-IDTBR and O-IDFBR with the donor polymer P3HT over a range of temperatures relevant to operating conditions of plastic solar cells. In this way, we could follow the dynamical behaviour of the structure and gain insights into the morphology of the blends. 

Our studies validated further the use of neutrons to study such systems. The results highlight a contrasted blending behaviour between the two NFAs, O- IDTBR and O-IDFBR, but – unlike mixtures containing fullerene acceptors – blending them with P3HT did not alter their dynamics. This is probably because the chemical structures of these NFAs and the polymer are closely similar. We plan to explore further the multifaceted behaviour of these interesting solar cell materials.