Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

Abstract

Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near infrared solar photons. In this paper, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyldioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A−π−D−π−A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (0T−5T) π-bridge. The affording small molecules, namely nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about −3.7 eV) and the highest occupied molecular orbital (HOMO) energy level (around −5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, becomes localizations in different moieties, i.e. the central electron-donating and the terminal electron-withdrawing units, respectively, which provides neccesary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from "outward" to "inward", the crystalline nature, fibril length and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the "inward" alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15−20 nm when blended with PC 71 BM, showing the higher hole mobility and promising electric performance.

Single Component Organic Solar Cells Based on Oligothiophene‐Fullerene Conjugate

“…Detailed synthesis procedures are provided in Scheme and supporting information. Briefly, compounds 1 and 2 were prepared according to the reported procedure and then coupled together to form the p‐type block (benzodithiophene‐rhodanine, BDTRh) via the Knoevenagel condensation . BDTRh–[6,6]‐phenyl‐C 61 butyric acid methyl ester (PCBM) was synthesized in 42% yield by connecting BDTRh with two PCBM wings via an esterification reaction .…”

Improving optoelectronic and charge transport properties of D–π–D type diketopyrrolopyrrole-pyrene derivatives as multifunctional materials for organic solar cell applications

“…23,24 They can efficiently tune the FMOs energy levels through an intramolecular charge transfer, which leads to a narrow band gap, intense light absorption, and high PCEs. [25][26][27] Additionally, the intermolecular stacking, lm morphology, and charge transport properties can be determined by the type and length of side chains of D-A molecular structures. 28,29 It has been reported that a…”

Effect of the π-conjugation length on the properties and photovoltaic performance of A–π–D–π–A type oligothiophenes with a 4,8-bis(thienyl)benzo[1,2-b:4,5-b′]dithiophene core

“…In this respect, oligothiophenes, including monotiophene [16,20], bithiophene [16], terthiophene [8,14–15 17–19 21–23 26], quaterthiophene and quinquethiophene [27], and cyclopentadithiophene [28] have been utilized as the π-bridge in constructing conjugated molecules with a BDT core. Among these 3,3''-dihexyl-2,2':5':2''-terthiophene (3T) is the most widely used π-bridge.…”

“…In order to minimize the synthesis efforts, structurally symmetric oligothiophene units are mostly used for constructing A–π–D–π–A-type molecules with a BDT core. Interestingly, although various terthiophene-based derivatives with a BDT core have been reported, there is only one paper that reported the synthesis and characterization of BDT derivatives based on oligothiophene π-bridges with more than three thiophene units [27], where symmetric quater- and quinquethiophenes were used as the π-conjugation bridge. Surprisingly, the quaterthiophene-bridged compound showed the worst photovoltaic performance when blending with a fullerene derivative as the photoactive layer.…”