Structure-property correlations are often hampered by insufficient structural insight in the crystal packing of polymer semiconductors widely used in electronic devices such as organic solar cells. Herein, both the semi-crystalline morphology and the crystalline structures of two high performance polymer semiconductors showing more than 9% efficencies, namely PDTBT-TT and PBT4T, are established by a combination of oriented crystallization and transmission electron microscopy. PDTBT-TT and PBT4T form layered structures with alternation of -stacked backbones and layers of disordered alkyl side chains. -stacking is such that benzothiadiazol and the co-monomer (quaterthiophene or thienothiophene-bithiophene) segregate to form distinct stacks. This segregated stacking is preferentially obtained in thin films aligned by high temperature rubbing at T=200°C-230°C. However, the two polymers show different stabilities of this polymorph versus temperature. The segregated stacking of PDTBT-TT is stable up to near the melting temperature whereas for PBT4T, it transforms to a layered structure with significant intra-stack disorder at T≥250°C. The intensity of the 0-0 component of the vibronic progression in the absorption spectrum is enhanced for the polymorph with long-range segregated -stacks. The structural models determined for the two polymers suggest that both the position of alkyl side chains and the preferential -stacking interactions between co-monomers determine the polymorphism and corresponding thermal stability.

The rise of halogenated organic semiconducting materials has led to a significant increase of organic photovoltaic power conversion efficiencies in recent years. However, the impact of halogen atoms on optoelectronic, structural and photovoltaic properties is not yet fully understood. In particular, because of different physico-chemical properties, the design of polymers using chlorine atoms instead of fluorine atoms still needs to be rationalized. In this paper, we investigate a series of 4 halogenated D-A electron donor copolymers, by varying not only the number of halogen atoms, but also their nature. The in-depth experimental and theoretical study of these new polymers, using the non-halogenated polymer as a reference, allowed us to rationalize the impact of these chemical modifications. In particular, we found that the structural properties and blending morphologies are mainly influenced by the nature of the halogen in this series of polymers. We have also demonstrated that a reasonable reduction in the number of fluorine atoms along the polymer backbone can be a good strategy to improve thin film processing conditions while keeping efficiencies at an acceptable level.

Several bottlenecks need to be passed to reach market readiness for organic photovoltaic modules. To avoid scarce and costly materials such as indium is an important technological issue. To be able to process the active layer from solutions prepared from non-halogenated solvents and harmless additives is also a crucial step. Herein, a fluorinated polymer is used as an electron-donor material and PC 71 BM as an electron-accepting material in a bulk-heterojunction configuration, to demonstrate that indium-tin-oxide (ITO)-free modules with an active area greater than 60 cm 2 and processed from non-halogenated solvents and harmless additives can reach a high power conversion efficiency of above 6%.

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<p>Nowadays, climate change is a reality because energy demand is mostly satisfied by fossil fuels which are limited resources and also responsible for greenhouse gas emissions. Actions have to be undertaken to overcome this issue. Among the solutions proposed to this is the development and use of new energy sources called renewable energies. By renewable energy, we understand energies coming from the sun, wind, geothermal, water, or biomass. Of these, solar energy is one of the most abundant, clean, effective, and easily deployed. One of the efficient ways to exploit solar energy is photovoltaics.</p><p>Two decades of research have allowed organic photovoltaics to appear today as an alternative to their conventional and inorganic counterparts. However, several issues have to be addressed in order to ease their production on an industrial level. Bulk heterojunction (BHJ) solar cells based on the blend of two types of conjugated molecules acting as an electron donor (hole transport) and an electron acceptor (electron transport) are the most efficient organic solar cells. Further, using non-fullerene acceptors (or NFA) in these BHJ solar cells have recently gained a broad interest due to their great potential to realize high conversion efficiencies (more than 18%) with a long lifetime over the conventional polymer/fullerene blend solar cells.</p><p>Here we provide an overview of the recent progress of different existing and growing photovoltaic technologies. We also provide prospects for the future development of organic photovoltaic devices.</p>