Abstract:
Semiconducting conjugated oligomers having same end group (Nethylrhodanine)
but different central core (thiophene: OT−T, bithiophene: OT−BT,
thienothiophene: OT−TT) connected through thiophene pi-linker (alkylated
terthiophene) were synthesized for solution processable bulk-heterojunction solar
cells. The effect of the incorporation of an extra thiophene to the central thiophene unit
either through C−C bond linkage to form bithiophene or by fusing two thiophenes
together to form thienothiophene on the optoelectronic properties and photovoltaic
performances of the oligomers were studied in detail. Flash photolysis time-resolved
microwave conductivity (FP−TRMC) technique shows OT−TT has significantly
higher photoconductivity than OT−T and OT−BT implying that the former can
outperform the latter two derivatives by a wide margin under identical conditions in a
bulk-heterojunction solar cell device. However, the initial photovoltaic devices
fabricated from all three oligomers (with PC71BM as the acceptor) gave power
conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC
observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in
the PCE of OT−T and OT−BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT−TT
(PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm−2, FF: 0.50), which was in correlation with the TRMC results. The structure−
property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were
deduced through UV−vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence
quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.
