Side chain effect of benzodithiophene on the diketopyrrolopyrrole-based copolymer for the opto-electronic properties

This study investigates synthesis, thermal, optical, and morphological properties of two DPP-based copolymers, PBDTTT-DPP-C12 (poly{2,6-bis- 5-(2-butyloctyl)thienothiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-6-bis(5-thiophen-2-yl)-2,5-bis(2-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione}) and PBDTTT-DPP-C17 (poly{2,6-bis-5-(2–7-butyltridecyl)thienothiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-6-bis(5-thiophen-2-yl)-2,5-bis(2-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione}), with different alkyl side chains. Both polymers were synthesized using a series of reactions (alkylation, lithiation, and bromination) and Stile coupling polymerization. The obtained polymer structures were confirmed using ¹H NMR (Nuclear Magnetic Resonance). The thermal stability of both polymers, assessed via thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed high decomposition temperatures (T_d) of 377 °C and 367 °C for PBDTTT-DPP-C12 and PBDTTT-DPP-C17, respectively, which indicate sufficient thermal stability. The polymers exhibited dual-band absorption, with maximum absorption at 669 nm for PBDTTT-DPP-C12 and 700 nm for PBDTTT-DPP-C17, showing a slight red shift when transitioning from solution to film. The electrochemical analysis via cyclic voltammetry (CV) revealed similar highest occupied molecular orbital (HOMO) levels of − 5.22 eV and − 5.23 eV for both polymers, while the lowest unoccupied molecular orbital (LUMO) levels were calculated to be 1.47 eV and 1.49 eV, respectively. Grazing incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) studies revealed differences in morphology and crystallinity of the films, with PBDTTT-DPP-C12 showing a more pronounced edge-on orientation. In photovoltaic devices, PBDTTT-DPP-C17 demonstrated a higher power conversion efficiency (PCE) of 2.8%, outperforming PBDTTT-DPP-C12 with a PCE of 1.8%. This work highlights the significant role of alkyl side chain length in modulating polymer morphology, crystallinity, and miscibility with electron acceptors, thus enhancing the photovoltaic performance of DPP-based polymers for organic solar cells.

Graphical abstract

It highlights the significant role of alkyl side chain length in modulating polymer morphology, crystallinity, and the miscibility with fullerene acceptors, thus enhancing the photovoltaic performance of DPP-based polymers for organic solar cells.