Since the first report in 1991 [1], dye-sensitized solar cells (DSSCs) have demonstrated broad application prospects owing to their diverse raw material sources, low preparation costs, and ease of fabrication [2]. As an essential component of DSSCs, the dyes are responsible for the generation of excited electrons under irradiation. In this respect, ruthenium complex dyes [3,4], metal-free organic dyes [5,6], and zinc porphyrin dyes [7–14] have been widely employed to fabricate efficient DSSCs. As the analog of chlorophyll in photosynthesis, artificial porphyrins feature tunable optoelectronic properties upon peripheral modification [15,16], emerging as key candidates. However, porphyrin dyes exhibit an absorption valley between the Soret and Q bands, which limits the light-harvesting capability and the power conversion efficiency (PCE). Hence, intermolecular cosensitization of porphyrins with spectrally complementary organic dyes has been carried out to achieve panchromatic absorption and high PCEs [17]. For this approach, the dye adsorption faces challenging optimization process, since the soaking sequence and competitive adsorption need to be taken into consideration among other factors [18–21]. To address this issue, concerted companion (CC) dyes [22–29] has been developed, in which intramolecular co-sensitization is achieved by covalently linking a porphyrin sensitizer with an organic sensitizer. Notably, when four C18-alkoxy chains were introduced into the ortho-positions of the meso-phenyl groups in the porphyrin moiety, the organic dyes were simultaneously protected, and thus doubly concerted companion (DCC) dyes were developed for fabricating DSSCs with high PCE and long-term stability [26]. Thus, DCC dye XW96 was synthesized by introducing phenothiazine and fluorenyl indoline donors into the porphyrin and organic dye units, respectively (Fig. 1) [29], with multiple hexyl chains attached on the donors to suppress dye aggregation and charge recombination between the oxidized electrolytes and injected electrons and thus improve open-circuit voltage (V_OC) [30]. As a result, XW96 afforded an outstanding PCE of 12.50% [29].

Compared to the long chain substituents on the donors, branched ones have greater steric hindrance, which may provide better protection and thus further suppress charge recombination. In this respect, 2-ethylhexyl chain has been shown to be an effective substituent for developing high performance DSSCs [31–33]. Based on this background, we herein report DCC dye XW98, which has been synthesized by replacing the hexyl chains on the donors of XW96 with 2-ethylhexyl chains. Meanwhile, the corresponding organic dye Y4 and porphyrin dye XW97 have also been synthesized (Fig. 1). As a result, XW98 afforded lowered V_OC (773 mV), J_SC (21.70 mA/cm²), and PCE (12.13%), compared to those of XW96 (782 mV, 21.97 mA/cm², and 12.50%) [29]. Meanwhile, a lowered adsorption amount was observed for XW98, probably owing to the severe steric hindrance induced by the bulky donors, which leads to a twisted alignment of the molecule and unsatisfactory anchoring on the TiO₂ film. To release the repulsion, XW99 and XW100 have been synthesized by extending the linkage near the anchor from seven single bonds to eight and nine single bonds, respectively (Fig. 1). As expected, the DSSCs based on XW99 afford higher adsorption amount with elevated V_OC, J_SC and PCE of 784 mV, 22.08 mA/cm², and 12.54%, respectively. Notably, XW100 exhibits the highest adsorption amount among these DCC dyes. However, its PCE of 12.25% is lower than that of XW99, which may be related to the larger ratio of single anchoring according to the desorption analysis. These results reveal that judiciously designed bulky donors in combination with optimized linker length is effective for developing efficient DCC sensitizers.

The detailed synthetic routes and procedures for the dyes are outlined in Scheme S1 and described in Supporting information. The structures of all the synthesized compounds were systematically characterized by NMR and mass spectrometry analyses (refer to Supporting information for more details).

The absorption spectra of all the dyes in THF are shown in Fig. 2, with the corresponding data summarized in Table S1 (Supporting information). Obviously, porphyrin dye XW97 and organic dye Y4 exhibit complementary absorption profiles. Consequently, the corresponding DCC dyes XW98−XW100 exhibit panchromatic absorption similar to that of XW96, indicative of negligible contribution from the alkyl chains on the donors and the linker near the anchors. Upon adsorption of the dyes onto the TiO₂ film, the absorption spectra are significantly broadened (Figs. S1 and S2 in Supporting information), which markedly improves the light-harvesting capability [34]. Notably, the fluorescence spectra of DCC dyes XW98−XW100 are comparable to the addition of the corresponding spectra for the two sub-dyes, indicative of neglectable energy/electron transfer between the porphyrin and organic dye units, which is similar to our previous findings [23,26].

Electrochemical investigations were carried out using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) to determine the redox properties of the sensitizers. As shown in Fig. S4 (Supporting information), the first oxidation potentials (E_ox) for Y4, XW97 and XW98–XW100 were found to be 0.75−0.76 V (vs. NHE) (Table S2 in Supporting information), considerably lower than that of the I^–/I₃^– redox shuttle (~0.4 V vs. NHE), providing sufficient driving forces for dye regeneration. The optical bandgaps (E₀₋₀) were determined via the intersection method from normalized absorption and fluorescence emission profiles (E₀₋₀ = 1240/λ_inter, Fig. S5 in Supporting information), and thus the lowest unoccupied molecular orbital (LUMO) level values were found to be −1.25, −1.15, −1.23, −1.21, and −1.14 V (vs. NHE) for the respective sensitizers, which are obviously higher that of the conduction band of TiO₂ (−0.5 V vs. NHE), thermodynamically favorable for electron injection into TiO₂ [35,36].

On this basis, the photovoltaic performance was evaluated under simulated sunlight (AM 1.5 G, 100 mW/cm²), using I^–/I₃^– as the electrolyte (Figs. 3a and b), and the photovoltaic parameters are summarized in Table 1. As a result, the organic dye Y4 affords slightly higher V_OC (755 mV) with respect to that of Y3 (747 mV), demonstrating the superiority of the 2-ethylhexyl chains in suppressing charge recombination. However, the bulky donor moiety of Y4 leads to reduced dye loading (Y4 = 1.53 × 10⁻⁷ mol/cm², Y3 = 1.77 × 10⁻⁷ mol/cm²), resulting in lowered J_SC (19.05 mA/cm²) compared to Y3 (19.24 mA/cm²). Because of this trade-off effect, very close PCE values were obtained for Y4 (10.28%) and Y3 (10.20%) [29]. On the other hand, the presence of branched alkyl chains on the donor of porphyrin dye XW97 enhances the V_OC to 764 mV by 20 mV, compared to reference dye XW78 [26]. Notably, the bulky donor of XW97 did not lower the adsorption amount (XW97 = 1.67 × 10⁻⁷mol/cm², XW78 = 1.65 × 10⁻⁷ mol/cm²). This observation is different from those observed for Y4 and Y3, which may result from the larger size of the porphyrin dye framework with respect to the organic dyes. Thus, the J_SC of XW97 is slightly elevated (19.24 mA/cm²) compared to XW78 (19.00 mA/cm²). As a result, XW97 exhibits a PCE of 10.48%, higher than that of 10.30% obtained for XW78. Considering the complementary absorption of Y4 and XW97, they were cosensitized, achieving a PCE of 11.32% (Table S3 in Supporting information). Compared to the intermolecular co-sensitization method, the panchromatic DCC dye XW98 affords a considerably enhanced PCE of 12.13%. However, this efficiency is still lower than that of 12.50% obtained for XW96. This observation may be related to the smaller adsorption amount of XW98 (0.85 × 10⁻⁷ mol/cm²) with respect to XW96 (0.93 × 10⁻⁷ mol/cm²), which may result from a twisted arrangement of the two sub-dyes induced by the severe steric hindrance between the bulky donors. The smaller adsorption amount of XW98 results in lowered light harvesting capability and J_SC. In addition, the loosened packing of XW98 on the TiO₂ surface also leads to aggravated charge recombination and lowered V_OC, which is consistent with the electrochemical impedance spectroscopy (EIS) measurements (vide infra). To address this issue, the linker near the anchors has been extended from seven to eight single bonds to give XW99, achieving an enhanced adsorption amount of 0.98 × 10⁻⁷ mol/cm². This observation implies that the twisted molecular alignment has been alleviated for XW99. As a result, improved V_OC (784 mV), J_SC (22.08 mA/cm²), and PCE (12.54%) have been achieved for the DSSCs based on XW99. This efficiency is slightly superior to that of XW96 (12.50%). Upon further extending the linkage to nine single bonds (XW100), the adsorption amount is further enhanced to 1.05 × 10⁻⁷ mol/cm². However, the V_OC (776 mV) and J_SC (21.72 mA/cm²) are reduced, affording a PCE of 12.25%, lower than that of XW99. Desorption studies reveal a faster desorption process for XW100, compared to XW98 and XW99 (Fig. 3c), indicative of a higher ratio of single anchoring for XW100. To further probe the anchoring behavior of the dyes, ATR-FTIR spectra of the dyes in the powder state and adsorbed on the TiO₂ films have been collected (Fig. S7 in Supporting information). All DCC dyes exhibit a characteristic stretching band of the carboxyl group at ca. 1690 cm⁻¹. Upon anchoring onto the TiO₂ films, this peak disappears for XW98 and XW99, while it is only weakened for XW100, indicative of nearly complete deprotonation and adsorption of the carboxyl groups for both XW98 and XW99. In contrast, neutral carboxyl group is still present for XW100 owing to the incomplete anchoring. These results are consistent with the desorption results. Hence, an enhanced adsorption amount is obtained together with a lowered PCE [37]. Based on these results, the DCC dyes were co-adsorbed with CDCA to further improve the photovoltages and efficiencies [38]. Upon coadsorption with 1 mmol/L CDCA, the PCEs of XW98 and XW100 could be enhanced to 12.32% and 12.35%, respectively. Notably, co-adsorption with 1 mmol/L CDCA did not obviously affect the adsorption amount of XW98, indicative of sufficient vacancies on the TiO₂ surface of the XW98-based photoanode, which is consistent with the aforementioned hypothesis of the twisted alignment of XW98 owing to the bulky donors of the two sub-dyes. By contrast, gradual addition of CDCA to XW99 affords gradually decreased adsorption amounts (0.93 × 10⁻⁷~0.83 × 10⁻⁷ mol/cm²), J_SC and PCE (12.39%~12.13%) while the V_OC remains relatively unchanged. These results indicate that XW99 possesses remarkable anti-aggregation properties because of the well matched bulky donors and the linker length.

To further understand the photovoltage (V_OC) variation trends, electrochemical impedance spectroscopy measurements were carried out. The Nyquist plots feature two semicircles: the smaller left arcs correspond to the charge transport resistance at the counter electrode/electrolyte interface (R_tr), while the larger right arcs represent the recombination resistance (R_rec) between oxidized electrolytes and free electrons in the TiO₂ film (Fig. 3d). For the monomeric sensitizers at a fixed bias of 750 mV, the chemical capacitance (C_μ) values were found to be 0.403, 0.321, 0.630, and 0.306 (mF/cm²) for Y3, Y4, XW78, and XW97, respectively (Table S5). Concurrently, the R_rec values exhibit increasing trends of XW78 (46.82 Ω cm²) < XW97 (156.3 Ω cm²) and Y3 (112.1 Ω cm²) < Y4 (140.0 Ω cm²). As a result, the electron lifetime values (τ_rec = R_rec × C_μ) lie in the increasing order of XW78 (0.030 s) < Y3 (0.045 s) = Y4 (0.045 s) < XW97 (0.048 s). These results indicate that the 2-ethylhexyl chains can suppress electron recombination more effectively than the hexyl moieties, resulting in longer lifetimes and higher V_OC. For the DCC systems, the C_μ values were found to be 0.302, 0.274, 0.253, and 0.262 mF/cm² for XW96, XW98, XW99, and XW100, respectively, and the R_rec values lie in the increasing order of XW98 (183.4 Ω cm²) < XW100 (199.6 Ω cm²) < XW96 (200.5 Ω cm²) < XW99 (243.0 Ω cm²). As a result, the electron lifetimes follow the sequence of XW98 (0.050 s) < XW100 (0.052 s) < XW96 (0.061 s) < XW99 (0.062 s), consistent with the V_OC trend. The relatively short electron lifetime and low V_OC for XW98 may be related to the twisted structure induced by the contradiction between the large donors and short linking chain, and the good matching between the bulky donors and the optimized linker length result in the longest electron lifetime and the highest V_OC achieved for XW99 among these DCC dyes (vide supra).

To further assess the stability of the DSSCs, long-term irradiation has been performed for 500 h under simulated sunlight conditions. As a result, the PCEs of the monomeric dyes Y4 and XW97 are decreased to 82% and 83%, respectively, relative to the initial values (Fig. S7 in Supporting information). Under the same conditions, the PCEs of DCC dyes XW98, XW99, and XW100 are decreased to 89%, 91%, and 89%, respectively, obviously better than those of the corresponding monomeric dyes, indicative of excellent stability of the DCC dye-sensitized DSSCs is favorable for practical applications.

In summary, a series of DCC dyes have been synthesized by introducing 2-ethylhexyl chains to the donors and optimizing the linker lengths near the acceptors. As a result, the incorporation of 2-ethylhexyl substituents significantly enhance anti-charge recombination capabilities, resulting in improved V_OC and PCE for Y4 (755 mV and 10.28%) and XW97 (764 mV and 10.48%) with respect to the reference dyes Y3 (747 mV and 10.20%) and XW78 (744 mV and 10.30%), respectively. For the DCC dyes, the large donors and short linker for XW98 result in detrimental steric hindrance, leading to a twisted molecular alignment, lowered adsorption amount and worsened photovoltaic parameters (J_SC = 21.70 mA/cm², V_OC = 773 mV, FF = 0.7233, PCE = 12.13%), compared to those of the reference dye XW96 (J_SC = 21.97 mA/cm², V_OC = 782 mV, FF = 0.7252, PCE = 12.50%). Relative to XW98, the longer linking chain in XW99 alleviates the steric hindrance, affording improved photovoltaic performance (V_OC = 784 mV, J_SC = 22.08 mA/cm², PCE = 12.54%), which is the record efficiency for DSSCs based on the I^–/I₃^– electrolyte, to the best of our knowledge. When the linker is further extended, XW100 exhibits a higher percentage of single anchoring, resulting in decreased V_OC (776 mV), J_SC (21.72 mA/cm²) and PCE (12.25%). In brief, these results indicate that incorporation of bulky branched chains into the donors in combination with well-matched linker length provides an efficient approach for developing efficient DCC sensitizers. Inspired by the results of this work, we are now working on the design of new branched alkyl chains on the donors and linkers between the two subdye units without oxygen atoms for further developing high performance sensitizers.