Cracking perylene diimide backbone for fullerene-free polymer solar cells

• Three naphthalimide dimers with different bridge were synthesized.
• These compounds were used to fabricate non-fullerene polymer solar cells.
• Effect of bridge units on optical and electronic properties was investigated.

Through cracking perylene diimide by inserting a single bond, small π-conjugation unit thiophene and large π-conjugation unit indaceno[1,2-b:5,6-b′]dithiophene, three molecules were synthesized and used as electron acceptors for solution-processed polymer solar cells. These three compounds showed excellent thermal stability with decomposition temperatures over 370 °C. As the size of inserted conjugation units increased, the extinction coefficient increased from 2.9 × 104 to 6.5 × 104 M−1 cm−1, the absorption wavelength maximum in thin film red shifted from 360 to 488 nm, the optical band gap decreased from 2.98 to 2.25 eV, the highest occupied molecular orbital energy level was upshifted from −6.18 to −5.67 eV, the lowest unoccupied molecular orbital energy level was downshifted from −3.59 to −3.68 eV, power conversion efficiencies of solar cells based on blends of poly(3-hexylthiophene) donor and these acceptors increased from 0.28% to 2.36%.

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