Influence of fluorination on the optoelectronic properties of dithienyl cyclopentadithiophene-benzothiadiazole donor-acceptor alternate copolymers: experimental and computational studies

Herein, we describe the synthesis and the characterization of thermal, electrochemical, optical and molecular morphology in the solid state of two dithienyl cyclopentadithiophene-alt-benzothiadiazole polymers (CDTHPTBT and CDTHPTBTF). CDTHPTBTF was decorated with two fluorines on the benzothiadiazole unit. CDTHPTBT displayed a higher Mw (7650 Da) than CDTHPTBT (4360 Da), whereas the decomposition temperature of both copolymers is higher than 400 degrees C. In thin film, the lambda max of CDTHPTBTF (664 nm) was hypsochromic-shifted by 21 nm compared with CDTHPTBT (685 nm). However, both polymers displayed almost similar optical band gaps of about 1.47 eV. Also, the two polymers demonstrated similar HOMO levels (- 5.01 eV); but the LUMO level of CDTHPTBTF (- 3.32 eV) is slightly lower than CDTHPTBT. This is a consequence of fluorinating the benzothiadiazole-acceptor moiety. The Egelec of CDTHPTBT and CDTHPTBTF were 1.74 and 1.69 eV, respectively. XRD measurements of powdery CDTHPTBT and CDTHPTBTF confirmed that the two polymers presented similar pattern curves with narrow pi-pi stacking distances of 3.48 & Aring;. We have also performed quantum computational analysis of synthesized oligomers, CDTHPTBT and CDTHPTBTF to assess their photovoltaic efficiency and electronic structure. Frontier molecular orbital (FMOs) analysis reveal extensive HOMO charge distribution across the molecule, with LUMo localized on the benzothiadiazole ring. Notably, low orbital gaps of 3.64 eV (CDTHPTBT) and 3.85 eV (CDTHPTBTF) suggest efficient charge transfer properties. Density of states (DOS) analysis highlight the contribution of benzothiadiazole ring to LUMO and ditoluene-cyclopentadithiophene role in HOMO. Photovoltaic assessments highlight the open-circuit voltage of 1.39 eV and 1.71 eV, respectively, underscoring the potential of these oligomers for high efficiency in organic photovoltaic applications. These findings illuminate the favorable charge transfer characteristics and optoelectronic properties of our designed oligomers, positioning them as promising candidates for next-generation solar devices.