Asymmetric triphenylamine–phenothiazine based small molecules with varying terminal acceptors for solution processed bulk-heterojunction organic solar cells

Abstract

Three compounds consisting of the electron-donating triphenylamine-phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PCBM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79-7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.

Aggregation‐induced Emission Properties of Triphenylamine Chalcone Compounds

“…The potential of the lowest unoccupied molecular orbital of compound and compound 2 was obtained according to the formula E LUMO =E HOMO +E opt (eV). The energy values (E opt ) of the optical band gap of compound 1 and compound 2 were 2.24V and 2.35V, respectively, which may be caused by the different degree of molecular conjugation [44] .…”

Structure-induced optoelectronic properties of phenothiazine-based materials

“…For instance, D-A chromophores can be built by covalently attaching electron-withdrawing (EW) or donating groups at C-3 and C-7 positions. 16,28 The N-10 active site can be decorated with alkyl, oligo, aryl, or aralkyl groups to induce solutionprocessable properties and/or to modify the degree of crystallinity and/or to influence the packing motif. 22 Similarly, aromatic S-heterocycles can be easily oxidized/reduced to achieve sulphur in different oxidation states (sulphide, sulfoxide, or sulfone) and to affect the redox and spectroscopic properties.…”

“…Furthermore, Weng et al also prepared a series of small molecules based on A-D-A and A-D-D-A type configuration with the PTZ as a core and studied the correlation between different end groups Table 1. The effect of molecular configuration, optoelectronic properties, carrier mobilities, morphology, and PV properties on the molecular design of oligomers based on PTZ moiety was explored in detail by Revoju et al 16,28 In particular, the authors studied the tuning of the end groups in the molecular structures and its effect on the electron-accepting ability and polarity. In one study, they developed two symmetrical A-p-D-p-A configured oligomers 26 and 27 with a BDT as a central donor core, PTZ as a p-conjugated bridge, and malononitrile or 1,3-indandione as terminal acceptor units.…”

“…16 Devices based on 27 showed higher PCE than those employing 26 owing to the high hole mobility, better nanoscale morphology, and more controlled interpenetrating network. In another study, the same authors synthesized three asymmetrical small molecules (28)(29)(30) with an identical triphenylamine-phenothiazine conjugate backbone but end-capped with different acceptors, namely 3-ethylrhodanine (Rh), malononitrile (MN) and 1,3-indandione (IND). 28 In both the studies, oligomers with a stronger electronwithdrawing group (IND in both cases) exhibited broader absorption bandwidth, smaller bandgap, and deeper LUMO levels due to the increased effective conjugation length and a stronger ICT.…”

“…In another study, the same authors synthesized three asymmetrical small molecules (28)(29)(30) with an identical triphenylamine-phenothiazine conjugate backbone but end-capped with different acceptors, namely 3-ethylrhodanine (Rh), malononitrile (MN) and 1,3-indandione (IND). 28 In both the studies, oligomers with a stronger electronwithdrawing group (IND in both cases) exhibited broader absorption bandwidth, smaller bandgap, and deeper LUMO levels due to the increased effective conjugation length and a stronger ICT. The optimized solar devices based on 28, 29, and 30 showed overall PCEs of 4.79%, 6.18%, and 7.25%, respectively ( Fig.…”