Improved photovoltaic performance of D–A–D-type small molecules with isoindigo and pyrene units by inserting different π-conjugated bridge

Two novel D-π–A–π-D-type small molecules (SMs) of IID(Ac–Py)₂and IID(Th–Py)₂were synthesized and characterized, in which isoindigo (IID), acetylene (Ac) or thiophene (Th), and pyrene (Py) were used as the acceptor (A) core, extended π-bridges, and donor (D) terminal units, respectively. Significantly improved photophysical and photovoltaic performances were observed for both SMs containing extended π-bridge in comparison with those for their parent D–A–D type molecule of IID(Py)₂. Compared to IID(Ac–Py)₂, IID(Th–Py)₂shows a better photovoltaic performance due to broader absorption, deeper HOMO levels (−5.44 eV) and better morphology of the devices. A maximum power conversion efficiency (PCE) of 1.88% with a short-circuit current density (J_sc) of 6.90 mA cm⁻²was obtained in the IID(Th–Py)₂/PC₇₁BM-based solar cells, which is 2.35 times the value of IID(Ac–Py)₂-based cells. Particularly, the IID(Th–Py)₂/PC₆₁BM-based device exhibited a V_ocvalue up to 1.01 V, which is 0.27 V and 0.12 V higher than IID(Py)₂/and IID(Ac–Py)₂/PC₆₁BM-based devices, respectively. To our best knowledge, the V_ocof 1.01 V is among the highest values in literature for solution processed SMs OSCs without any post processing.