Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure–Property Correlation of Thiophene Oligomers

Semiconducting conjugated oligomers having same end group (<i>N</i>-ethylrhodanine) but different central core (thiophene: <b>OT–T</b>, bithiophene: <b>OT–BT</b>, thienothiophene: <b>OT–TT</b>) 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 <b>OT–TT</b> has significantly higher photoconductivity than <b>OT–T</b> and <b>OT–BT</b> 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 <b>OT–T</b> and <b>OT–BT</b>. On the other hand, 5-fold enhancement in the device efficiency was achieved in <b>OT–TT</b> (PCE: 3.52%, <i>V</i>_OC: 0.80 V, <i>J</i>_SC: 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