Tuning the photovoltaic parameters of thiophene-linked donor–acceptor liquid crystalline copolymers for organic photovoltaics

Abstract

Three new donor–acceptor copolymers containing the electron acceptor di-2-thienyl-2′,1′,3′-benzothiadiazole (DTBT) linked to the electron donors 9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene (Ftp) with terphenyl mesogen side chain viathiophene units are synthesized to study polymer structure–properties relationships in organic photovoltaic devices. All the random copolymers (PFtpTDTBT-a, b, c) were prepared via Stille polycondensations based on variable contents of DTBT and Ftp. The results of absorption spectra, fluorescence spectra and electrochemistry shows that increasing the ratio of fluorine units and decreasing the electron withdrawing groups of DTBT not only control the bandgaps but also the liquid crystallinity of the copolymers. The structural variation of mesomorphism behaviors induced by terphenyl pendants is characterized by POM, DSC, XRD and polarized absorption. When the polymers were blended with fullerene derivative as an electron acceptor, the photovoltaic performance reveals the good correlation between the Ftp and DTBT ratio in the main chain of the copolymers with desirable HOMO levels and the open circuit voltage (V_oc). Therefore, the deepest HOMO energy level of polymer PFtpTDTBT-a leads to the highest V_oc of 0.86 V, while the polymer PFtpTDTBT-c possesses a superior absorption coefficient resulting in the largest short current density (J_sc = 4.58 mA cm⁻²). Additionally, the packing alignments of the three polymers guided by the orientation of mesogenic pendants pose a significantly effect on the morphology with different microphase separation after annealing treatment. Nevertheless, the solar cell based on PFtpTDTBT-b shows the best performance among the thiophene incorporated polymers, which demonstrated the importance of designing materials with a balance in the properties of HOMO/LUMO energy levels, the absorption coefficient and morphological variations.