Improving the efficiency of converting sunlight into electricity is a fundamental task if humanity is to overcome its dependence on fossil fuels. When light is shone onto a semiconductor, for example in a photovoltaic cell, one photon usually generates one excited state. However, a remarkable photophysical mechanism known as “singlet fission” can occur in some systems, whereby one photon generates two excited states. For the first time, the intermediate coupled states have been observed under ambient conditions – namely at room temperature and in air.
In a well-known system, namely pentacene in p-terphenyl, researchers from the LCN at both Imperial College and UCL in a collaboration with researchers at Oxford University and Kobe University, have found a correlation between the orientation of the pentacene molecules in the sample and the lifetimes of their excited states. This information is essential for the design of more efficient photovoltaic devices.
Pentacene, a well-known organic semiconductor, has a herringbone structure. This means that neighbouring molecules can either be parallel to each other or at an angle. Until now, this difference in geometry had not been directly associated with the lifetime of their excited state. However, while investigating the “singlet fission” mechanism, the researchers led by Professor Sandrine Heutz (Imperial College) and Professor Chris Kay (UCL), have shown a link between the geometry of the pentacene molecules and the lifetime of their excited states.
Singlet fission involves two neighbouring molecules interacting with each other. The relative arrangement of these molecules dictates how long-lived the excited states are. It was found that the parallel geometry more readily dissociates into free charges. Conversely, non-parallel pairs form a trap. This becomes a loss mechanism in any optoelectronic device, such as solar cells, although it highlights strategies for maximising communication between spin states, which could be useful for information technologies.
Figure: Schematic of the triplet and quintet kinetics: (i) formation of 5(TT)||, (ii) formation of ISC (T) and of the 5(TT)/\, (iii) dissociation of the quintets into triplets, (iv) equilibrium state between (TT)/∖⇌T+T(TT)/∖⇌T+T, forming a trap-state, whilst (TT)||→T+T forms long-lived triplets
Singlet fission has typically, with a few recent exceptions, been studied using optical spectroscopy techniques but the limitations of this method meant the precise nature of the excited states could not be identified. Daphné Lubert-Perquel, lead author of the paper, comments: "In this work we used magnetic resonance - akin to MRI scanners used in hospitals – to directly observe these excitations. Our observations have implications for the broader research community that typically uses optical methods to study singlet fission optically, as we are able to provide details of the type of states as well as their lifetimes.”
Dr Enrico Salvadori, co-author, adds: “Our work is unique as we have successfully performed these intricate measurements at room temperature for the first time, making the study relevant for the development of real devices.”
Understanding which geometries lead to efficient charge generation and which cause traps is crucial to provide new material design axioms for devices. Building on this work, the team will study ways to control the orientation and generate a system with only the desired molecular geometry. This will lead to optimised layers in the optoelectronic devices and thus enhanced conversion efficiency in solar cells.
Link to article in Nature Communications