Novel Donor-acceptor Small Molecules: Design, Synthesis And Application In Organic Solar Cells

In this paper, we have synthesized a series of new narrow band-gap organic small molecular donor materials for organic solar cells. The main work contains two parts:In the first part, we design and synthesize a series of multifluorine substituted small molecular donor materials based on D1-A-D2-A-D1 linear frameworks. They have an IDT-based core as the central donor unit(D2), with two benzothiadiazole or difluorobenzothiadiazole as the acceptor, and two n-hexyl-substituted bithiophene as the terminal groups(D1). Moreover, it was desirable to employ an extra thiophene π bridge unit in BIT-4F-T to enhance the light-harvesting ability and π-electron delocalization and solid-state intermolecular packing. In combination with using PC71 BM as the acceptor, the BHJ-OSC devices based on BIT-4F-T exhibited PCE of 8.1%, with a high open circuit voltage(Voc) of 0.90 V, short circuit current density(Jsc) of 12.0 mA cm-2 and ultra-high FF of 0.76 for the device without using any additives. To the best of our knowledge, this is the highest PCE of solution processed BHJ solar cells based on IDT units reported to date.In the second part, we have synthesized a series of donor-accepor type small molecular donor materials based on D2-Aw-D1-As-D1-Aw-D2 linear frameworks. The two molecules consist of DPP as the central stronger acceptor unit(As), two benzothiadiazole or difluorobenzothiadiazole as the acceptor(Aw), IDT as the donor unit(D1), and two n-hexyl-substituted bithiophene as the terminal groups(D2). We mainly investigate the effect of fluorine substitution in weaker acceptor on the HOMO/LUMO energy level, charge carrier mobility and the morphologies of blend films, and their photovoltaic properties. DPPBIT and DPPBIT4 F possess similar broad and intense optical absorption covering the range from 300-900 nm and relatively low-lying HOMO energy levels. Compared with the best PCE of 2.7% for the blend films of DPPBIT and PC71 BM, the BHJ-OSC devices based on DPPBIT4 F and PC71 BM, exhibited best PCE of 5.4% and very-high FF of 0.69 upon CH2Cl2 vapor annealing for 30 s, which is among one of the best reported photovoltaic performances based on DPP-core small molecules in BHJ solar cells.