Small-Bandgap Semiconducting Polymers with High Near-Infrared Photoresponse

Lowering the optical bandgap of conjugated polymers while maintaining a high efficiency for photoinduced charge transfer to suitable electron acceptors such as fullerene has remained a formidable challenge in the area of organic photovoltaics. Here we present the synthesis and application of a series of ultra-small-bandgap donor–acceptor polymers composed of diketo­pyrrolo­pyrrole as acceptor and pyrrole-based groups as strong donors. The HOMO energy levels of the polymers can be progressively increased by increasing the donor strength while the LUMO level remains similar, resulting in optical bandgaps between 1.34 and 1.13 eV. Solar cells based on these polymers blended with fullerene derivatives show a high photoresponse in the near-infrared (NIR) and good photovoltaic characteristics, with power conversion efficiencies of 2.9–5.3%. The photoresponse reaches up to 50% external quantum efficiency at 1000 nm and extends to 1200 nm. With the use of a retro-reflective foil to optimize light absorption, high photocurrents up to 23.0 mA cm^–2 are achieved under standard solar illumination conditions. These ultra-small-bandgap polymers are excellent candidates for use in multi-junction applications and NIR organic photodetectors

Multielectron Cycling of a Low-Potential Anolyte in Alkali Metal Electrolytes for Nonaqueous Redox Flow Batteries

Recent efforts have led to the design of new anolytes for nonaqueous flow batteries that exhibit reversible redox couples at low potentials. However, these molecules generally cycle through just a single electron-transfer event, which limits the overall energy density of resulting batteries on account of the undesirably high equivalent weight (i.e., ratio of anolyte/supporting electrolyte molecular weight to electrons transferred). In addition, these anolytes generally require expensive alkylammonium salts as supporting electrolytes for stable cycling, which further increases the equivalent weight of the system. The current work describes the multielectron redox cycling of a low-potential anolyte using alkali metal salts as supporting electrolytes. These studies reveal that potassium hexafluorophosphate (KPF₆) dramatically lowers the equivalent weight of the anolyte system while supporting flow cell cycling through two redox events at low potentials for 150 cycles with no detectable degradation

Efficient Tandem and Triple-Junction Polymer Solar Cells

We demonstrate tandem and triple-junction polymer solar cells with power conversion efficiencies of 8.9% and 9.6% that use a newly designed, high molecular weight, small band gap semiconducting polymer and a matching wide band gap polymer

High Quantum Efficiencies in Polymer Solar Cells at Energy Losses below 0.6 eV

Diketopyrrolopyrrole-based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open-circuit voltage below 0.6 eV

Universal Correlation between Fibril Width and Quantum Efficiency in Diketopyrrolopyrrole-Based Polymer Solar Cells

For a series of six diketopyrrolopyrrole (DPP)-based conjugated polymers, we establish a direct correlation between their external quantum efficiencies (EQE) in organic solar cells and the fibrillar microstructure in the blend. The polymers consist of electron-deficient DPP units, carrying long branched 2′-decyltetradecyl (DT) side chains for solubility, that alternate along the main chain with electron-rich aromatic segments comprising benzene, thiophene, or fused aromatic rings. The high molecular weight DT-DPP polymers were incorporated in bulk heterojunction solar cells with [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]­PCBM) as acceptor. The morphology of the DT-DPP:[70]­PCBM blends is characterized by a semicrystalline fibrillar microstructure with fibril widths between 4.5 and 30 nm as evidenced from transmission electron microscopy. A clear correlation is found between the widths of the fibrils and the EQE for photon to electron conversion. The highest EQEs (60%) and power conversion efficiencies (7.1%) are obtained for polymers with fibril widths less than 12 nm. For blends with fibrils wider than 12 nm, the EQE is low because exciton diffusion becomes limiting for charge generation. Interestingly, the correlation found here matches with previous data on related DPP-based polymers. This suggests that for this class of materials the relation between fiber width and EQE is universal. The fiber width is largely correlated with the solubility of the polymers, with less soluble DPP-based polymers giving narrower fibrils

Homocoupling Defects in Diketopyrrolopyrrole-Based Copolymers and Their Effect on Photovoltaic Performance

We study the occurrence and effect of intrachain homocoupling defects in alternating push–pull semiconducting PDPPTPT polymers based on dithienyl–diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT–TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]­PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5–5.6% for polymers containing 20% to 5% defects

Influence of the Position of the Side Chain on Crystallization and Solar Cell Performance of DPP-Based Small Molecules

Three isomeric π-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C₇₀ fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells