Complementary absorbing star-shaped small molecules for the preparation of ternary cascade energy structures in organic photovoltaic cells

Two anthracene-based star-shaped conjugated small molecules, 5',5a″-(9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(5-hexyl-2, 2'-bithiophene), HBantHBT, and 5',5a″-(9,10-bis(phenylethynyl)anthracene- 2,6-diyl)bis(5-hexyl-2,2'-bithiophene), BantHBT, are used as electron-cascade donor materials by incorporating them into organic photovoltaic cells prepared using a poly((5,5-E-alpha-((2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2, 6-[(1,5-didecyloxy)naphthalene])) (PBTADN):[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) blend. The small molecules penetrate the PBTADN:PC₇₁BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short-circuit current (J_SC) and fill factor (FF) are obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN:BantHBT:PC₇₁BM blend solar cells is characterized by a J_SC of 11.0 mA cm^-2, an open circuit voltage (V_OC) of 0.91 V, a FF of 56.4%, and a power conversion efficiency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW^-2). The effects of the small molecules on the ternary blend are investigated by comparison with the traditional poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) system.