Synthesis And Properties Of Asymmetrical Organic Small Molecular Photovoltaic Materials

How to effectively and conveniently convert solar energy into electricity has been a hot issue in the photovoltaic technologies. Organic photovoltaic materials have attracted much attention compared to traditional inorganic semiconductors, due to their outstanding merits of synthetic variability, light-weight, low-cost and flexibility in large-scale fabrication.Organic small molecules (OSMs) have demonstrated promising advantages over their polymer counterparts in terms of well-defined molecular structure, easier purification and better batch-to-batch reproducibility. Up to now, OSMs have shown an encouraging power conversion efficiency (PCE) of up to9.02%, making them be competitive candidates for the next generation of organic solr cells. However, further increasing the PCEs exceeding10%is still needed for their commercialization.Nowadays, most investigations of OSMs are focusing on symmetrical D-A-D or A-D-A systerm, while asymmetrical materials are seldom explored. Therefore, a series of small molecular donors with asymmetrical structure were designed and successfully synthesized to deeply study the relationship between molecular structure and properties.We selected triphenylamine (TPA) as electron-donating (D) unit, diketopyrrolopyrrole (DPP) and benzothiadiazole (BT) as two different electron-accepting (A) units, adopting C-C, C=C, cyano vinyl and C=C as π-linkage respectively to construct DPP series and BT series with D-A backbone. To further improve the PCEs of OSMs by modification the molecular structure, an acceptor unit p-cyanophenyl (CP) was end-capped to DPP series and hence afforded D-A-A typed DPPCN series.The effects that the structural variation on their optical, electrochemical and photovoltaic properties were carefully investigated through density functional theory (DFT) calculations, UV-vis absorption spectroscopy, cyclic voltammetry and experiments on solution-processed bulk heterojunction (BHJ) photovoltaic devices.(1) The lower HOMO of DPP series and BT series could effectively increase the open-circuit voltage (Voc).(2) The end-capped CP unit in DPPCN series could obviously reduce the band gaps and improve the light harvesting abilities so as to improve their short circuit current density (Jsc).(3) Both cyano vinyl and C=C as π-bridge could lower the HOMO. However, the coplanar and rigid nature of C=C was devoted to a higher degree of electron packing and promoted intramolecular charge transfer (ICT). Among the twelve donor materials, TPATDPPCN showed a remarkable PCE of5.94%with a Voc of0.93V, a Jsc of14.86mA·cm-2, and a fill factor (FF) of43%. To date, the attractive PCE of TPATDPPCN is the highest efficiency for solution-processed OSMs with asymmetrical structure as well as OSMs containing TPA and DPP.The attractive results were of great value to the development of asymmetrical OSMs.