Efforts to increase the relative permittivities of organic semiconducting materials and their effect on organic solar cell performance are evaluated.

Gradually increasing dielectric constants (up to 6.3) are obtained for a series of PCPDTTPD polymers.

The influence of cross-conjugation on the optical and electrochemical properties of donor−acceptor copolymers is investigated. Isoindigo, substituted at the 6,6′ or the 5,5′ positions, and thieno[3,2-b]thiophene and thieno [2,3-b]thiophene are taken as conjugated and cross-conjugated electron-deficient and electron-rich building blocks from which four isomeric donor−acceptor polymers were synthesized. Introducing cross-conjugation in isoindigo has only a small effect on the electrochemical band gap and on the onset of the absorption, which remains in the near-infrared. The optical absorption spectra, however, differ strongly because crossconjugation strongly reduces the absorption coefficient. DFT calculations confirm that the transition to the lowest excited singlet state has a small oscillator strength in cross-conjugated isoindigo model compounds. Cross-conjugation in thienothiophene exerts a different effect. It causes a moderate but distinct blue-shift of the optical absorption and a deeper HOMO energy level.

Direct arylation affords organic semiconductors enabling high solar cell efficiency upon careful removal of the ‘miss-coupled’ side products.

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