Energy Level Engineering of Donor Polymers via Inductive and Resonance Effects for Polymer Solar Cells: Effects of Cyano and Alkoxy Substituents

Fine tuning the energy levels of donor polymers is a critically important step toward achieving high power conversion efficiencies in polymer solar cells (PSCs). We systematically controlled the energy levels of donor polymers by introducing cyano (CN) and alkoxy (OR) groups into the 4,4′-didodecyl-2,2′-bithiophene (BT) unit in a step-by-step fashion, thereby varying the inductive and resonance effects. The three monomer units (BT, BTC, and BTCox) were polymerized with benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene (BDT) as a counter unit to afford three polymers (PBDT-BT, PBDT-BTC, and PBDT-BTCox). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels decreased significantly upon the introduction of CN groups, and these levels increased slightly upon attachment of the OR groups, in good agreement with the measured open-circuit voltages of the three polymer devices. The strong inductive and resonance effects present in PBDT-BTCox narrowed the polymer band gap to 1.74 eV to afford a power conversion efficiency of 5.06%, the highest value achieved among the three polymers

Medium-Bandgap Conjugated Polymers Containing Fused Dithienobenzochalcogenadiazoles: Chalcogen Atom Effects on Organic Photovoltaics

We designed, synthesized, and characterized a series of three medium-bandgap conjugated polymers (PBDT­fDTBO, PBDT­fDTBT, and PBDT­fDTBS) consisting of fused dithienobenzo­chalcogenadiazole (fDTBX)-based weak electron-deficient and planar building blocks, which possess bandgaps of ∼2.01 eV. The fDTBX-based medium-bandgap polymers exhibit deep-lying HOMO levels (∼5.51 eV), which is beneficial for use in multijunction polymer solar cell applications. The resulting polymers with chalcogen atomic substitutions revealed that the difference in the electron negativity and atomic size of heavy atoms highly affects an intrinsic property, morphological feature, and photovoltaic property in polymer solar cells. The polymer solar cells based on sulfur-substituted medium-bandgap polymer showed power conversion efficiencies above 6% when blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester in a typical bulk-heterojunction single cell. These results suggest that the fDTBX-based medium-bandgap polymer is a promising alternative material for P3HT in tandem polymer solar cells for achieving high efficiency