cm5b01350_si_001.pdf (2.57 MB)
Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π‑Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells
journal contribution
posted on 2015-07-14, 00:00 authored by Ailing Tang, Chuanlang Zhan, Jiannian YaoSmall molecules with narrow bandgap
of <1.6 eV can harvest the
visible and near-infrared solar photons. In this Article, we report
a new method to achieve narrow bandgap small molecule donors by using
electron-deficient quinoidal methyl-dioxocyano-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 high-lying 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, become localizations
in different moieties, i.e., the central electron-donating and the
terminal electron-withdrawing units, respectively, which provides
necessary 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 PC71BM, showing the higher hole mobility and promising electric performance.