A One-Step Strategy for End-Functionalized Donor–Acceptor Conjugated Polymers

A modular and robust method for preparing end-functionalized donor–acceptor (D–A) narrow bandgap conjugated polymers is reported that avoids multistep reactions and postpolymerization modification. The strategy is well-controlled and affords functional materials with predictable molecular weight and high end-group fidelity. To exemplify this synthetic strategy, narrow bandgap conjugated polymers based on PDPP2FT were prepared that contain perylene diimide (PDI) units at the chain-ends. Monte Carlo simulations confirm the high degree of chain-end functionalization while photoluminescence studies reveal the unique photophysical properties of the end-functional polymers with efficient charge transfer occurring between the main polymer chain and PDI end-groups that results exclusively from their covalent linkage

A One-Step Strategy for End-Functionalized Donor–Acceptor Conjugated Polymers

A modular and robust method for preparing end-functionalized donor–acceptor (D–A) narrow bandgap conjugated polymers is reported that avoids multistep reactions and postpolymerization modification. The strategy is well-controlled and affords functional materials with predictable molecular weight and high end-group fidelity. To exemplify this synthetic strategy, narrow bandgap conjugated polymers based on PDPP2FT were prepared that contain perylene diimide (PDI) units at the chain-ends. Monte Carlo simulations confirm the high degree of chain-end functionalization while photoluminescence studies reveal the unique photophysical properties of the end-functional polymers with efficient charge transfer occurring between the main polymer chain and PDI end-groups that results exclusively from their covalent linkage

Energy Transfer Directly to Bilayer Interfaces to Improve Exciton Collection in Organic Photovoltaics

Ternary blends and energy cascades are gaining popularity as ways to engineer absorption as well as exciton and charge collection in organic solar cells. Here, we use kinetic Monte Carlo simulations to investigate energy cascade designs for improving exciton collection in bilayer solar cells via a Förster energy transfer mechanism. We determine that an interfacial monolayer (C) between the donor and acceptor with a D → A → C energy cascade will lead to good exciton collection, allowing for >90% collection, even for energy donor layers up to 75 nm thick. We further examine how roughening the interface, increasing the exciton diffusion length, and using other energy cascade designs affect the enhancement from the energy transfer. We also propose using the inherent charge transfer states at the interfaces as energy acceptors and estimate that the Förster radius could be as large as 3.4 nm, leading to nearly 70% improvement in exciton collection, without the need for a third material

A One-Step Strategy for End-Functionalized Donor–Acceptor Conjugated Polymers

A modular and robust method for preparing end-functionalized donor–acceptor (D–A) narrow bandgap conjugated polymers is reported that avoids multistep reactions and postpolymerization modification. The strategy is well-controlled and affords functional materials with predictable molecular weight and high end-group fidelity. To exemplify this synthetic strategy, narrow bandgap conjugated polymers based on PDPP2FT were prepared that contain perylene diimide (PDI) units at the chain-ends. Monte Carlo simulations confirm the high degree of chain-end functionalization while photoluminescence studies reveal the unique photophysical properties of the end-functional polymers with efficient charge transfer occurring between the main polymer chain and PDI end-groups that results exclusively from their covalent linkage

Effect of Bridging Atom Identity on the Morphological Behavior of Solution-Processed Small Molecule Bulk Heterojunction Photovoltaics

We examined the effects of changing the central bridging atom identity from carbon (d-CDT­(PTTh₂)₂) to silicon (d-DTS­(PTTh₂)₂) in the cyclopentadithiophene unit in a small molecule donor material. The substitution left the optical and electrical properties largely unchanged but significantly modified the melting/crystallization behavior and the formation of crystalline domains in thin film blends with PC₇₁BM. Solar cells made with the d-CDT­(PTTh₂)₂:PC₇₁BM had efficiencies less than 1%, while thermally annealed solar cells made with d-DTS­(PTTh₂)₂:PC₇₁BM achieved efficiencies up to 3.4%. Morphological analyses of the active layer film morphology were done with polarized optical microscopy, grazing incidence wide-angle X-ray scattering, and transmission electron microscopy and showed that large (micrometer scale) crystals formed in the d-CDT­(PTTh₂)₂ based films while smaller (25 to 50 nm) crystals formed in the d-DTS­(PTTh₂)₂, largely explaining the difference in device performance. Thermally activated photocurrent was observed in devices suggest that the additional current at elevated temperatures results from thermally activated charge generation. Charge transfer excitons were also investigated using external quantum efficiency measurements. Sharper band tails for the small molecule donors suggest less disorder than in P3HT:PCBM and other polymer systems