In this work, we studied the photovoltaic performance of two novel metal-free organic dyes (MR-3 and MR-4) with the molecular motif D–π–π–A carrying the same donor (trimethoxy benzene) and terthiophene (π) units but with different anchoring moieties. MR-3 and MR-4 displayed PCE of 4.54% and 2.38%, respectively. Furthermore, MR-3 co-sensitized with NCSU-10 exhibited improved efficiency of 9.09% than NCSU alone (8.74%). One of the merits in the studied structure motif D–π–π–A was modulating the molecular geometry of the sensitizers and eliminating the steric effect between the donor and π-spacer (terthiophene) to furnish a coplanar molecular structure and to maximize electronic conjugation. In addition, coupling-steric effect precludes effective conjugation and reduces light harvesting as it increases the band gap. To eliminate the steric effect of the aryl moiety at ortho position of 2,4,6-trimethoxyphenyl donor, another conjugated spacer, alkene (CHCH), was inserted between the donor and the π-spacer (terthiophene), which maximized the electron conjugation throughout the coplanar system; the optimized molecular geometry of the dyes was calculated using DFT. Another molecular geometry feature that was studied was the influence of different anchoring groups on coplanarity of the dye structure and consequently the photovoltaic performance. In the case of MR-3, where the anchoring group is based on the cyanoacrylic group, the optimized geometry was found coplanar. However, when rhodanine moiety was used as an anchoring group, the calculated optimized geometry of the dye (MR-4) was not coplanar, which resulted in weak electronic coupling and inferior electronic injection into TiO₂ conduction band edge. The inter-relationship between the optimized geometry of the dyes and their photovoltaic performance is discussed.

Herein, we report the photovoltaic performance studies of four D–π–A configured carbazole based co-sensitizers, P_1–4, in DSSCs sensitized with Ru-II complexes, i.e.NCSU-10/N3. The organic co-sensitizers (P_1–4) comprise carbazole as a donor scaffold, a phenylene ring as a π-spacer and electron withdrawing functional groups, viz. cyanoacetic acid (P₁), rhodanine-3-acetic acid (P_1–2), barbituric acid (P₃), and thiobarbituric acid (P₄) as acceptor/anchoring units. From the results, it is evident that the device fabricated using co-sensitizer P₁ carrying cyanoacetic acid, at the concentration of 0.2 mM NCSU-10, exhibited an enhanced photon conversion efficiency (PCE) of 9.19% with a J_SC of 21.20 mA cm⁻², V_OC of 0.658 V and FF of 65.85%, while P₃ containing barbituric acid displayed a PCE of 8.75% with a J_SC of 22.23 mA cm⁻², V_OC of 0.671 V and FF of 58.64%, whereas NCSU-10 (0.2 mM) alone displayed a PCE of 8.28% with a J_SC of 20.38 mA cm⁻², V_OC of 0.665 V and FF of 61.09%, but the dyes P₂ carrying rhodanine-3-acetic acid and P₄ containing thiobarbituric acid showed considerably lower performance. The co-sensitized devices of N3 with P_1–4 displayed inferior photovoltaic performance compared to N3 itself, probably due to inefficient suppression of back current. The observed results have thrown new light upon the selection of appropriate and matchable co-sensitizers for improving photovoltaic performance of Ru-II based sensitizers.

Three metal-free organic dyes, coded FS10, FS11, and FS12, were designed and synthesized by employing indolo[2,3-b]quinoxaline (IQ) as the main building block and three different edged donors, viz., triphenylamine, carbazole, and phenothiazine, respectively. In order to comprehend the feasibility of the novel dyes as probable sensitizers/co-sensitizers, they were well-characterized and their photochemical, electrochemical, and theoretical studies were processed. The photovoltaic properties were evaluated under 1.5 AM standard illumination conditions. FS10 acquired the best DSSC performance (η = 5.27%) with supreme V_OC (0.676 V) and FF (70.1%). The contribution from the incorporated IQ was examined theoretically by using DFT and time-dependent DFT, and the results indicated that all the novel dyes displayed clear charge separation at the frontier occupied molecular orbitals. The investigation of the co-sensitization effect was carried out by employing the HD-2 ruthenium sensitizer. The corresponding photovoltaic performance revealed that FS10 gave the most positive co-sensitization effect by improving the efficiency to 8.32% (J_SC = 18.68 mA cm⁻², V_OC = 0.672 V, FF = 66.2%), while HD-2 alone exhibited the efficiency of 7.47% (J_SC = 19.00 mA cm⁻², V_OC = 0.650 V, FF = 60.5%). The increased V_OC and FF resulted from the optimized anchoring mode on the TiO₂ surface with the combination of FS10 and HD-2 and more uniform coverage, leading to the hindrance of dye aggregation and charge recombination. Additionally, despite the reduced J_SC with co-sensitizers, the corresponding IPCE curves displayed more complex increasing as well as decreasing behavior as a function of wavelengths, thus reflecting that sensitizers and co-sensitizers not only compete but also compensate the light harvesting abilities.