Tailoring properties of the photoactive layer through blending polymers with different functional groups

- Polymer absorption properties are tuned by modifications to functional side chains.
- Amide-functionalized fluorothieno[3,4-b]thiophene leads to blue shift in absorbance.
- Ternary blends of differently functionalized polymers broaden absorption.
- A more favorable morphology was achieved from random copolymer design.
- A device with a certified efficiency of 10.1% was achieved in one polymer blend.

Opportunities for fine-tuning conjugated polymer properties are needed to continue enhancing photovoltaic performance. Herein an amide- or ester-functionalized 3-fluorothieno[3,4-b]thiophene (FTT(N) or FTT(E)) were used in alternating copolymers with benzo(1,2-b:4,5-b')dithiophene (BDT). The amide-functionalized polymer had a blue-shifted absorption spectrum and was blended with the ester-functionalized polymer in photovoltaic devices in an effort to increase light absorption of the photoactive layer. A 50:50 blend of the two polymers resulted in unfavorable morphology, leading to decreased power conversion efficiency (PCE), however, the blended mixture had a slightly enhanced stability relative to devices containing a single PBDT-FTT(E) polymer. Reducing the loading of the amide-functionalized polymer, PBDT-FTT(N), to 3 wt.% led to a certified PCE of 10.1%. To allow a higher loading of the FTT(N) monomer in the photoactive layer, a random copolymer containing both FTT(N) and FTT(E) was also prepared, which exhibited broader absorption and reached a PCE of 7.9%.

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