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Block Copolymers for High Efficient Solar Cells with novel Structures

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Tuning the active-layer morphology of solar cells for improved performance

EU-funded scientists sought to fabricate organic photovoltaics (OPVs) with enhanced solar-cell efficiency, improved morphological stability and prolonged lifetime.

Climate Change and Environment

Low-production costs, green processes for material synthesis and material flexibility make OPVs preferable with respect to their inorganic counterparts. However, their efficiency is affected by poor morphology and limited lifetimes — a major bottleneck to their commercialisation. In the EU-funded project 'Block copolymers for high efficient solar cells with novel structures' (CHESS), scientists incorporated block copolymers in the blend that forms the OPV active layer. They applied process techniques to fabricate bulk heterojunction OPVs with enhanced stability for eventual industrial use. Enhancing efficiency and increasing lifetime of OPVs should make them more competitive in comparison to inorganic photovoltaic cells. Block copolymers are able to form well-controlled nanostructures and act as compatibilisers in the respective homopolymer blends. Scientists used these properties to form stable nano-morphologies with optimum domain size —specifications desirable for OPV applications. An integrated study was performed, starting from copolymer design and synthesis until their incorporation in photovoltaic devices and their performance evaluation. In particular, rod-coil block copolymers that comprise an electron-donor semiconducting block and a wisely chosen coil block (e.g. a low glass-transition temperature polymer) were synthesised and blended with well-established donor/acceptor organic materials. The team characterised the blends with a thermoanalytical technique, complemented by small- and wide-angle X-ray scattering experiments to derive their complete phase diagrams. Project members extensively studied the ternary blend morphology in the novel solar cell active layers. They concluded that the copolymer presence drives an optimised bulk heterojunction network formation that stimulates photon absorption, efficient exciton dissociation and improved charge transport. Subsequently, a maximum power conversion efficiency as high as 4.5 % was achieved. Concurrently, device stability over time was prolonged. Large-scale integration of devices should facilitate the commercialisation of project members' products, benefitting the EU at community level and enhancing its competitiveness.

Keywords

Organic photovoltaics, solar cell, lifetime, block copolymer, active layer, stability, compatibiliser, homopolymer blend, nano-morphology, X-ray scattering

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