Nature-Inspired Conjugated Molecules for Future Organic Solar Cell Materials
Authors: Su Haw-Lih , Bronstein Hugo , Marks Tobin , Bazzi Hassan , Al-Hashimi Mohammed 
Published: July 2019

In 2009, the average energy consumption was about 16.1 TW. 81 % of it was supplied by non-renewable fossil fuels with emission of 29 × 1012tons of the carbon dioxide (CO2) into the atmosphere.1 Predicting the rise of energy consumption, the world is facing an urgent need for environmentally friendly and renewable energy technologies. As the direct exploitation of the ultimate energy source of the earth surface, solar power is a critical objective for our future. With recent $1 Billion investment in the polycrystalline silicon solar cell production, Qatar is clearly building a global leadership position in the alternative energymarketplace.2Current solar materials is primarily based on crystalline silicon, which is expensive on the energy- and water-intensive production processes due to the nature of the inorganic deposition.3 The newly developed large-scale and low-cost materials/technologies are thus needed for the next-generation solar energy production. The earth-abundant, non-toxic organic polymeric materials (“plastics”) have recently attracted much attention because of their cost-effectivity, flexibility, light weight and potential use in large- area flexible devices. In addition, organic active layers offer versatile design space for the polymer architectures, providing potential for layer designs and tunability to suit specific energy supply criteria.3Indigo is the most produced natural dye with a highly planer bis-lactam structure.4 Such kind of planer bis-lactam structures, such as perylene diimide (PDI), 2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), and their structure derivatives, have attracted considerable interests as acceptor materials among various optelectronic devices in past decades.3In addition to the structure benefit, its isomer, isoindigo has better conjugate prosperity since its lactam ring conducting with an extended π system throughout the bis-oxindole framework, which results in the strong electron-withdrawing nature alongside their high degree of coplanarity.3,4 Isoindigo was first introduced into organic semiconductors in 20105 and has been widely studied in the following years. More than 100 isoindigo-based molecules had been developed by 2014 and up to ∼7% organic photovoltaic (OPV) efficiencies and 3.62 cm2V-1s-1 hole mobilities in organic field-effect transistor (OFET) have been reached.6,7Among the various modifications of isoindigo-based molecules, the low bandgap donor-acceptor copolymers containing thienoisoindigos (TIIs) and thiazolisoindigos are particularly of interest to us. Replacing the outer phenyl rings of isoindigo with thiophene and thiazole rings, these molecules could further enhance planarity (via S-O interactions) along the backbone,5 resulting in better packing and higher mobilities for both holes and electrons with very low bandgaps through internal charge transfer interactions.Another indigo derivative, 7,14-diphenyldiindolo[3,2,1-de:3′,2′,1′-ij][1,5]naphthyridine-6,13-dione, containing the core of another synthetic dye cibalackrot, is also of interest to us. The cibalackrot was first synthesized by the condensation reaction of nature abundant indigo andarylacetyl chloride in 1914 but its potential usages in semiconductorswas not noticed until 2014.8,9 Our research team is one of the pioneers in this area. Recently, we published a polymer exhibiting OFET devices with holes and electrons exhibiting mobilities of 0.23 and 0.48 cm2V-1s-1, respectively. The OPV device efficiencies reached 2.35% with the light absorbance up to 950 nm, suggesting the potential of this novel monomer unit for implementation in near-IR OPV devices.4 Othercibalackrot containing copolymers are currently being explored.Reference:1. International Energy Agency, Key World Energy Statistics, IEA, Paris, 2011.2. https://www.jccp.or.jp/international/conference/docs/14assessment-of-solar-and-wind-energy-potential-in.pdf3. Guo, X.; Facchetti, A.; Marks, T. J. Chem. Rev. 2014, 114, 8943-9021.4. Fallon, K. J.; Wijeyasinghe, N.; Yaacobi-Gross, N.; Ashraf, R. S.; Freeman, D. M. E.; Palgrave, R. G.; Al-Hashimi, M.; Marks, T. J.; McCulloch, I.; Anthopoulos, T. D.; Bronstein, H. Macromolecules 2015, 48, 5148-5154.5. Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12, 660.6. Wang, E.; Mammo, W.; Andersson, M. R. Adv. Mater. 2014, 26, 1801-1826.7. Dutta, G. K.; Han, A.-R.; Lee, J.; Kim, Y.; Oh, J. H.; Yang, C. Adv. Funct. Mater. 2013, 23, 5317-5325.8. He, B.; Pun, A. B.; Zherebetskyy, D.; Liu, Y.; Liu, F.; Klivansky, L. M.; McGough, A. M.; Zhang, B. A.; Lo, K.; Russell, T. P.; Wang, L.; Liu, Y. J. Am. Chem. Soc. 2014, 136, 15093 − 15101.9. http://www.nano2014.org/thesis/view/4220