Morphological, redox, spectroscopic and spectroelectrochemical properties of alternating copolymers containing benzothiadiazole-type and benzodithiophene units and their potential application in solar energy conversion

Three alternating donor-acceptor-type copolymers were synthesized which consisted of a common donor (fluorine derivative of benzodithiophene) and three different acceptors, namely benzothiadiazole (polymer P1 ), naphthobisthiadiazole (polymer P2 ) and dithienobenzothiadiazole (polymer P3 ). Their quasi -monolayers deposited on HOPG graphite showed molecular mass-dependent morphology – “network meshes”-type for P1 and P2 and densely packed oriented fibers-type in the case of P3 whose molecular mass was significantly higher. Their optical band gaps ranged from 1.59 to 1.95 eV and correlated with the electron-withdrawing strength of the acceptor unit, following the sequence P3 > P1 > P2 . Consistent with increasing electron withdrawing strength of the acceptor, their electron affinity |EA| changed in the range 2.56 eV – 2.97 eV in the following sequence P2 > P1 > P3. The ionization potentials (IPs) of all three polymers were only slightly affected by the nature of the acceptor unit, varying within a narrow range of 5.38–5.53 eV . The investigated polymers exhibited interesting spectroelectrochemical behavior in the oxidation mode, involving bleaching of the absorption bands in the visible part of the spectrum with concomitant growth of oxidation absorption bands in the NIR part of the spectrum. For this reason, they could be considered as promising NIR electrochromic materials. Considering the good overlap between their absorption spectra and the solar spectrum, these polymers were evaluated in preliminary studies as donor components of all-organic photovoltaic cells using L8-BO as the acceptor. The highest power conversion efficiency (PCE), approaching 10%, was achieved for the device employing P3 as the donor.