The need for solution-processed organic semiconductors

80% of global energy production in 2011 was derived from unsustainable fossil fuels. At the same time, global retail consumer electronics sales, estimated at $680 billion annually, are increasing and account for roughly 15% of our household energy use. It is production of consumer electronics, rather than daily use, that dominates its energy cost (depending on the appliance). For example, production accounts for up to 80% of the energy required to make and run a computer over its lifetime. In order to reduce this energy burden, we need to develop consumer electronics which have a much lower energy production cost.

Organic semiconductors could solve this problem: solution-processed organic photovoltaics, for example, use roughly 10 times less energy to produce than any other photovoltaic technology. In addition, they rely only on earth-abundant, elements such as C, N and O and can potentially be synthesised from waste biological feedstock.

Solution processing: how to create order

Until recently, it was thought that solution (i.e. low-energy, sustainable) processing of organic semiconductors would never produce the efficient devices needed to compete with standard semiconductors. The major hurdle with solution-processing was thought to be disorder. Work in the last 4 years has changed this paradigm. Solution-processed highly ordered single crystal transistors now demonstrate field-effect mobilities above 40cm2/Vs, competitive with polycrystalline silicon. Only 10 years ago such ultra-high mobilities at room temperature in thin organic films would have been considered unachievable by many experts in the field.

The physics of highly ordered organic semiconductors

These new ultra-high mobility thin films pose a considerable challenge to our understanding of charge and energy transport. They operate in what is known as the 'intermediate coupling' regime where energy and charge transfer can be described as being somewhere between the band-like transfer that occurs in highly ordered inorganic semiconductors and the purely hopping transfer that occurs in very disordered systems. The previous unavailability of such materials (particularly materials with high optical quality) coupled with very recent advances in spectroscopic measurement techniques (such as highly sensitive transient absorption, capable of measuring optically-induced changes in transmission of 10⁻⁶ over decades in time (5fs-1ms)) and new theoretical developments opens up the exciting possibility of tackling questions such as:

  • how to describe energy and charge transport in the intermediate coupling regime,

  • the roles of radiative and non-radiative excitation deactivation processes in this regime, and

  • how both of these depend on crystal and molecular structure

for the first time. In order to create efficient and sustainable consumer electronic devices (solar cells, lasers, LEDs) based on this breakthrough, we need this basic physical understanding of the materials.

Relevant Papers

[1] Triplet Dynamics in Pentacene Crystals: Applications to Fission-Sensitized Photovoltaics A.D. Poletayev, J. Clark, M.W.B. Wilson, A. Rao, Y. Makino, S. Hotta, R.H. Friend, Adv. Mater., 26, 919 (2014)

[2] Temperature-Independent Singlet Exciton Fission in Tetracene M.W.B Wilson, A. Rao, K. Johnson, S. Gélinas, R. di Pietro, J. Clark, R.H. Friend, J. Am. Chem. Soc., 135, 16680 (2013)

[3] Ultrafast Dynamics of Exciton Fission in Polycrystalline Pentacene, M.W.B. Wilson, A. Rao, J. Clark, R.S.S. Kumar, D. Brida, G. Cerullo, and R.H. Friend, J. Am. Chem. Soc., 133, 11830-11833 (2011)