A novel, NIR absorbing organic small molecular donor material denoted as ICT3 with an A-D-D-D-A architecture having dithieno[3,2-b:2',3'-d]pyrrole (DTP) and butylrhodanine as donor and acceptor moieties, respectively, is synthesized and its thermal, photophysical, electrochemical and photovoltaic properties are explored. ICT3 has excellent stability over a broad range of temperatures with a decomposition temperature (T corresponds to 5% weight loss) of 372 °C, soluble in most common organic solvents (solubility up to 30 mg mL) and suitable for solution processing during device fabrication. ICT3 has broad (520-820 nm) and intense visible region absorption (molar excitation coefficient is 1.69 × 10 mol cm) and has suitable HOMO and LUMO energy levels with the [6,6]-phenyl-C-butyric acid methyl ester (PCBM) acceptor for efficient exciton dissociation and charge transfer. Bulk heterojunction solar cells (BHJSCs) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ICT3:PCBM/poly(9,9-bis(3'-(N,N-dimethylamino)propyl)fluorene-2,7-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl) (PFN)/aluminium (Al) structure are fabricated and the BHJSCs with the active layer as cast from chloroform solution displayed a power conversion efficiency (PCE) of 3.04% (J = 8.22 mA cm, V = 0.86 V and FF = 0.43). Annealing the active layer significantly improved the PCE of these BHJSCs. While thermal annealing of the active layer improved the PCE of the BHJSCs to 4.94%, thermal followed by solvent vapour annealing enhanced the PCE to 6.53%. X-ray diffraction and atomic force microscopy analyses are carried out on the active layer and these results revealed that annealing treatment improves the crystallinity and nanoscale morphology of the active layer, enriches the device exciton generation and dissociation efficiency, charge transport and collection efficiency and reduces carrier recombination. The observed higher PCE (6.53%) of the BHJSCs having ICT3 with a DTP donor moiety broadens the scope to develop new, efficient DTP based small molecular donor materials for BHJSCs.
Two new small molecule donors, namely ICT4 and ICT6 with D-A-D-A-D architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10 mol cm) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C-butyric acid methyl ester (PCBM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PCBM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PCBM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.
A series of seven unsymmetrical diketopyrrolopyrrole (DPP) derivatives with donor-pi-acceptor-pi-donor (D-π-A-π-D) architecture have been designed, synthesized and well characterised. Effect of the electron donating capacity and extent of electronic conjugation of the end-capping units on the thermal, photo-physical and electrochemical properties of the synthesized materials was thoroughly investigated using various experimental techniques and theoretical calculations. Organic field-effect transistors (OFETS) were fabricated using these materials to obtain their hole/electron transporting characteristics. All these materials show moderate to good hole transporting ability and the OFET fabricated using DPPderivative with benzofuran and pyrene end groups exhibited the hole mobility of 6.7 x 10 -4 cm 2 /Vs with VT of -9 V. The observed photo-physical, electrochemical, thermal, and charge carrier properties of the synthesized DPP-derivatives indicated their applicability in various areas like organic photovoltaics, disposable electronics and biomedical devices.Seven unsymmetrical DPP-derivatives with D-π-A-π-D architecture have been synthesized and the effect of the electron donating ability and extent of electronic conjugation of the end capping units on their thermal, photo-physical, electrochemical and charge carrier transporting properties is explored.
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