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Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D–A1–D–A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs. © 2020 Wiley-VCH GmbH

Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D–A1–D–A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs. © 2020 Wiley-VCH GmbH

In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7 × 1015 cm-3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells.

Polymer solar cells (PSCs) have recently attracted great attention because of their potential advantages, including flexibility, light weight, and low fabrication cost.[1 4] Photon absorption by conjugated polymers often creates bound electron/hole pairs (i.e., excitons). Charge collection, therefore, requires dissociation of the excitons, a process which is known to be favorable at the interface between conjugated polymers as donors and C60 derivatives as acceptors. Since [6,6]-phenyl-C61-butyric acid ester (PCBM) is still one of the best electron acceptors, most recent efforts have focused on the development of low bandgap donor polymers.[5 22] Examples of low bandgap polymers include conjugated polymers having electron-donating (D) and electron-accepting (A) moieties in the main chain.[21,22] The bandgap of the D- -A polymers can be effectively reduced through intramolecular charge transfer while their physical properties can be readily tuned by tailoring structures of the D and A moieties and/or the linking bridge. An ideal donor polymer should have a broad absorption spectrum for an efficient solar photon harvesting, a low HOMO level for a high opencircuit voltage (VOC), a good compatibility with PCBM for the formation of a bi-continuous network (i.e., bulk heterojunction, BHJ) with a large interface area, and a high hole mobility for an efficient transportation of the photo-generated charge carriers.[ 4] When designing donor polymers, therefore, a delicate balance between the crystallinity and the solubility needs to be taken into consideration.[5] Highly crystalline conjugated polymers with closely packed polymer chains in the solid state often show high hole mobility, but low solubility in organic solvents. Although the high hole mobility is an asset to PSCs, highly crystalline polymers with poor solution-processability rarely form the desired bi-continuous morphology with PCBM, and hence usually exhibit poor photovoltaic performance., Published in Advanced Materials, v24 p538-542, 2012. Prepared in cooperation with the Department of Macromolecular Science and Engineering, Case School of Engineering, Case Western Reserve University, Cleveland, OH and the Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan, South Korea.