Synthesis and photovoltaic performance of DPP-based small molecules with tunable energy levels by altering the molecular terminals

Four novel small molecules, Ph(DPP-Th)₂, Ph(DPP-Py)₂, Ph(DPP-ThCN)₂ and Ph(DPP-ThR)₂ were synthesized and characterized, in which diketopyrrolopyrrole-phenyl-diketopyrrolopyrrole was used as the core, donor units of 2-octylthiophene (Th), pyrene (Py), acceptor units octyl 2-cyano-3-(thiophen-2-yl)-acrylate (ThCN) and 3-octyl-5-(thiophen-2-ylmethylene)rodanine (ThR) were used as the terminals, respectively. By changing the terminals of small molecules, their corresponding thermal, absorptive, electro-chemical and photovoltaic properties were rationally adjusted for applications as donor materials in organic solar cells (OSCs). All of small molecules exhibit broad absorption spectra from the visible to near-infrared region, and Ph(DPP-ThR)₂ showed the minimum optical band gap of 1.48 eV. The small molecules with acceptor terminal units had much deeper HOMO energy levels than those with donor terminal units, which is important to increase the open-circuit voltage (V_oc). A maximum power conversion efficiency of 3.22%, short-circuit current density of 10.55 mA cm^-2, V_oc of 0.85 V, and fill factor of 39% were obtained in the Ph(DPP-Py)₂/PC₇₁BM based cells under AM 1.5 G irradiation (100 mW cm^-1). These results demonstrate valuable guidelines for the rational design of solution-processed small molecules for OSCs.