Halogen-free donor polymers based on dicyanobenzotriazole for additive-free organic solar cells

Two halogen-free donor polymers PCN1 (zigzag-shape backbone) and PCN2 (linear-shape backbone) were constructed with dicyanobenzotriazole as the acceptor (A) block. Their absorption, energy levels, charge transport properties, and applications as donors in organic solar cells (OSCs) were thoroughly investigated. PCN2 with strong absorption at 400-700 nm, maximum extinction coefficient of up to 8.67 x 10(4) M-1 cm(-1), low highest occupied molecular orbital (-5.50 eV), and the high hole mobility (1.42 x 10(-3) cm2 V-1 s(-1)) is an excellent donor polymer. The OSCs based on PCN2:Y6 reached a high PCE of 15.20% without the addition of any additives and any post-treatment, with an open-circuit voltage (Voc) of 0.862 V, which is by far the highest PCE among OSCs with donor polymers based on benzotriazole derivatives, and it is also one of the highest Voc reported for binary OSCs matched to Y6 in halogen-free donor polymers. The measurements of atomic force microscopy, transmission electron microscopy, and grazing-incidence wide-angle X-ray scattering show that PCN2 has a more orderly arrangement and stronger pi-pi stacking, and the PCN2:Y6 device exhibits better charge transport, higher and more balanced carrier mobility, less exciton recombination loss, suitable phase separation size, which lead to its higher photovoltaic performance.