Organic Sensitizers with Bridged Triphenylamine Donor Unit: A DFT Study
Le-Yan Liu, Kai-Li Zhu*, Li-Qaing Xie, Rong Li and Yi Wang* Author for corresponding; e-mail address: zhukaili2008@126.com
ORCID ID: https://orcid.org/0009-0002-4888-0978
Volume: Vol.53 No.1 (January 2026)
Research Article
DOI: https://doi.org/10.12982/CMJS.2026.010
Received: 10 March 2025, Revised: 7 October 2025, Accepted: 1 December 2025, Published: 6 January 2026
Citation: Liu L.-Y., Zhu K.-L., Xie L.-Q., Li R. and Wang Y., Organic sensitizers with bridged triphenylamine donor unit: A DFT study. Chiang Mai Journal of Science, 2026; 53(1): e2026010. DOI 10.12982/CMJS.2026.010.
Graphical Abstract
Abstract
The P-series of dyes have been designed and theoretically characterized through DFT and TD-DFT to systematically explore the underlying physical origin of the bridged TPA donor unit and to investigate its influence on DSSCs. Key parameters associated with the solar cell performance were characterized and analyzed in detail. The results indicate that the methylene-bridged TPA group not only enhances the planarity of the donor group while effectively lowering λtotal and EBE, thereby elevating the Jsc, but also promotes Voc due to differences of μnormal. Furthermore, the design strategy of the bridged TPA group has been explored preliminarily. A TPA donor group bridged by an N atom or group is favorable for both Jsc and Voc. Lastly, the newly designed dye P12 combines the virtues of both C-bridged and N-bridged groups, demonstrating outstanding Jsc and Voc among all P-series dyes. This finding further validates the correctness of previous conclusions, and P12 emerges as the best alternative dye molecule in this work.
1. INTRODUCTION
Dye-sensitized solar cells (DSSCs) have received extensive attention since the significant breakthrough in 1991[1]. One of the important types of dye sensitizers in DSSCs is the dye involving triphenylamine (TPA) unit as donor group, namely TPA-type dye sensitizers. As the prominent molecule, TPA unit cannot only provide better spectral absorption by red-shifting the dye’s spectrum, but also effectively reduce molecular aggregates owing to its intrinsically helical structure. Thus, the dye sensitizers with the TPA unit as donor group have been widely studied due to the peculiarities mentioned above.
Among these studies, the use of bridged TPA unit as donor group represents an excellent strategy. Bridged TPA groups can elevate the power conversion efficiency (PCE) through increasing both the short-circuit current density (Jsc) and open-circuit photovoltage (Voc), which are considered the most critical factors to evaluate the performance of DSSCs. In 2011, JK-206, which featured a dithia-bridged triphenylamine as the donor group, was reported by Ko et al. [2] Subsequently, another series of dyes with similar C-bridged structures were published. Among the series, JK-98, containing a trimethylene-bridged TPA group, stands out as an outstanding dye, exhibiting superior cell performance with a higher PCE of 8.71% [3]. Additional series of sensitizers with trimethylene-bridged TPA unit have been reported subsequently, and have exhibited enhanced performance, especially in photocurrent density, owing to their high molar extinction coefficients and extended red-light response [4-8]. The planarization of the donor and the use of alkene linkages have proven to be extremely effective in extending the red-light response of the sensitizers, leading to a significant enhancement in photocurrent density. Moreover, other bridged TPA donor units have also been designed and investigated to explore better dye sensitizers [9-18]. In recent years, numerous theoretical studies have also been reported, particularly the work on TPA donor units by Chetti et al. [19-22]
However, a systematic study of the bridged TPA donor unit, particularly its theoretical exploration with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) has not been reported until now. Therefore, with great interest, the underlying physical origin of the bridged TPA donor has been investigated by DFT and TD-DFT calculations through comparing the reported dye sensitizer JK-98, which renamed as P0 in this paper, with the unbridged TPA donor dye R0 (Figure 1). Furthermore, to capitalize on these advantages of the bridged TPA donor and identify better dye candidates with bridged TPA unit as donor group, a new series of dyes with different atoms or groups bridging the TPA units as donor groups have been designed and theoretically studied.
2. METHODS
2.1 Theoretical Background
Generally speaking, the overall efficiency (η) of the DSSC is determined by the short-circuit current density (Jsc), the open-circuit photovoltage (Voc), the fill factor (FF) and the incident solar power on the cell (Pin) which can be expressed by the following equation [23, 24]:
$$\eta = \frac{J_{sc} V_{oc} FF}{P_{in}} \tag{1}$$
The Jsc is described as:
$$J_{sc} = \int_{\lambda} IPCE(\lambda) d\lambda \tag{2}$$$$IPCE = LHE(\lambda) \Phi_{inject.} \eta_{collect.} \tag{3}$$
where IPCE is the Incident Photon Current Efficiency, LHE(λ) is the light harvesting efficiency, Φinject. is the electron injection efficiency and ηcollect. is the charge collection efficiency.
The Voc can be determined by the following equation [25, 26]:
$$V_{oc} = \frac{E_{CB}}{q} + \frac{kT}{q} \ln(\frac{n_c}{N_{CB}}) - \frac{E_{redox}}{q} \tag{4}$$
where ECB is the conduction band edge of the semiconductor substrate, Eredox is the electrolyte Fermi level, q is the unit charge, nc is the number of electron in the conduction band, NCB is the accessible density of conduction band (CB) states and ΔCB is the shift of ECB and can be expressed as [27-29]:
$$\Delta CB = -\frac{q \mu_{normal} \gamma}{\varepsilon_0 \varepsilon} \tag{5}$$
where q is the electron charge, μnormal is the dipole moment of individual dye molecules perpendicular to the surface of the semiconductor substrate.
2.2 Computational Details
All the density functional theory [30] (DFT) calculations were performed with the Gaussian 09 package [31] and ORCA 5.0 program [32-34]. Full ground state geometry optimization of all dyes in vacuum as well as the cationic and anionic molecules of all isolated dyes were carried out using the B3LYP [35] hybrid functional combined with 6-31G(d) for C, H, O, N, S atoms. Frequency calculations were performed on the fully optimized geometries at the same level to ensure that they correspond to the lowest point on the potential energy surface. A variety of different XC functionals, including PBE0[36], M06-2X [37], BHandHLYP [38], CAM-B3LYP [39], MPW1K [40], combined with different basis set were involved in TD-DFT calculations to evaluate the appropriate level on the optimized structure.
Different functionals vary in their ability to describe molecular electronic structures. By testing these combinations, we aim to identify the method yielding λmax values closest to experimental data, ensuring the reliability and comparability of subsequent calculations for all dye molecules. The results show that the TD-CAM-B3LYP-CPCM//6-31+G(d) level is the best fit among these different XC functionals listed above (Table 1 and Figure S1). The oxidation potentials of all isolated molecules of the ground and excited states were carried out at the B3LYP-CPCM/6-311G(d,p) level through single-point calculations based on their neutral molecules optimized under B3LYP/6-31G(d) level. Furthermore, the electron density differences maps (EDDMs) were obtained using Multiwfn 3.3[41, 42] to conduct an in-depth study on the electron transfer and separation of the excited state. The charge transfer amount (qCT) and the distance between the barycenters of the density increment and depletion regions upon electronic excitation (DCT)[43] were calculated through Multiwfn 3.3 based on the optimized structure under B3LYP/6-31G (d) level. Additionally, the natural population analysis (NPA) was performed to assess the number of photo-injected electrons in the conduction band at the B3LYP-CPCM/6-311G(d,p) level.
Furthermore, the electron density differences maps (EDDMs) were obtained using Multiwfn 3.3[41, 42] to conduct an in-depth study on the electron transfer and separation of the excited state. The charge transfer amount (qCT) and the distance between the barycenters of the density increment and depletion regions upon electronic excitation (DCT)[43] were calculated through Multiwfn 3.3 based on the optimized structure under B3LYP/6-31G (d) level. Additionally, the natural population analysis (NPA) was performed to assess the number of photo-injected electrons in the conduction band at the B3LYP-CPCM/6-311G(d,p) level.
3. RESULTS AND DISCUSSION
3.1 Molecular Design and Optimization
To systematically explore the theoretical preponderance of the bridged TPA donor unit, 11 new dyes of P series (P1-P11) were designed by introducing different atoms or groups as the bridging unit. (Figure 2).
3.2 Comparative Study of P0 And R0
To explore the underlying advantage of the bridged TPA donor unit, comparative study of P0 involving bridged TPA unit as donor unit and original R0 has been conducted to disclose the intramolecular charge transfer (ICT) property, represent one of the critical property of dye sensitizers. The side view of these two dyes shows that, the original TPA unit in R0 exhibits a typical helical structure, while the bridged TPA unit displays planar characteristics with the three benzene rings being nearly coplanar. As shown in Table S1, the frontier molecular orbitals (FMO) of these two contrastive dyes shows that, the HOMO electronic density of both dyes are localized mainly on the donor group, while the LUMO ones are localized mainly on the anchor moiety. This reveals better ICT from donor to acceptor moieties. EDDMs and other ICT parameters in Table 2 also demonstrate enhanced ICT characteristics. Comparatively, the qCT of the P0 is larger than R0 while the DCT is not much difference, which should be attributed to the bridged planar donor group. Overall, both P0 and R0 display better ICT characteristic.
Table 3 presents the key theoretical parameters of P0 and R0. Among these parameters, light harvesting efficiency (LHE), exciton binding energy (EBE), and intramolecular recombination energy (λₜₒₜₐₗ) are critical factors affecting the Jsc: the larger the LHE and the smaller the EBE and λₜₒₜₐₗ, the better the Jsc performance. Additionally, the difference in charge transfer amount between the ground and excited states (∆q) and the vertical dipole moment (μₙₒᵣₘₐₗ) are key parameters regulating the Voc, with larger values of both parameters being more favorable for Voc enhancement. As presented in Tab. 3, the energy gap (ΔE) and λmax of P0 and R0 are not significantly different. Additionally, P0 and R0 display almost the same LHE (97.39% vs. 97.64%) but significantly different exciton binding energy (EBE: 1.85 vs. 2.07 eV) and intramolecular recombination energy (λtotal: 0.36 vs. 0.50 eV). Therefore, it can be concluded that the bridged TPA unit effectively reduces the EBE and λtotal, thereby enhancing the Jsc of the dye. Regarding the parameters affecting Voc, the ∆q(0.103 vs. 0.106 e) is basically the same, while the μnormal of P0 (12.35 D) is much larger than that of R0 (11.73 D). Due to the structural similarity between P0 and R0, the donor-acceptor distance and adsorption sites of these two dyes are identical, resulting in equal charge recombination. As a consequence, the difference in Voc is determined by μnormal.
3.3 Theoretical Study of the Designed P-series Dyes
In order to explore design principles and identify more advantageous dyes of the bridged TPA donor moieties, 11 new dyes (P1-P11) were designed and theoretically characterized by introducing different atoms or groups as bridging unit.
First, the suitability of these dyes was preliminarily evaluated based on the calculated ground and excited state oxidation potentials, as listed in Table S2. As the table shows, Edye values of several designed dyes, including P3-P5, are below -4.8 eV, indicating that the regeneration of these dyes cannot be guaranteed. Thus, in this aspect, P3-P5 are considered unqualified. Regarding the planarity shown in Figure 3, P0 is the most planar molecule among P-series dyes, and P3 being a close second, which was designed with an N atom as the bridged atom. Moreover, P1 (O-bridging), P5 (N-bridging) and P9 (B-bridgeing) also exhibit better planarity. Therefore, in terms of the planarization degree of the bridged donor group, none of these 11 dyes can match the planarization degree of P0 (CH2-bridging).
Table S3 and Table 4 list the figures of FMO and EDDM of P-series dyes as well as other ICT parameters, qCT and DCT. As may be seen from figures of FMO, all P-series dyes exhibit favorable ICT characteristics except P9 and P10, whose FMOs are not ideal enough. As shown in Table 4, all P-series dyes demonstrate equivalent or slightly inferior ICT features.
As shown in Figure 4 and Table 5, the introduction of an N atom as the bridged unit can effectually red-shift the absorption spectra and narrow down the optical band gap of dye molecule by involving N atom as the bridged unit. This is supposed to be the p-π conjugation between N atoms and benzene rings, which enhances the degree of delocalization of electronic cloud density, thereby red-shifting the absorption spectra.
Additionally, the molar extinction coefficients of P3, P4 and P5 are significantly lower than those of other P-series dyes. Moreover, the B atoms as well as the carbonyl group neither significantly red-shifts the absorption, nor reduces the molar extinction coefficients in the UV-Vis spectrum.
As shown in Table 5, the LHE values of the N-bridging dyes P3-P5 are significantly reduced, while those of P6 (carbonyl-bridging), P9 (B-bridging), and P10 (Si-bridging) are higher. The LHE values of P7 and P8 is 97.63% and 97.84%, which represent successive enhancement comparing to P0 likely due to the introduction of alkoxy groups. In other words, incorporating alkoxy groups into the bridged TPA units can slightly increase the LHE. The driving force values of all P-series dyes are sufficiently large to guarantee the complete electron injection. The driving force values of P3-P5 are in the first level, while P9 is the lowest one, which means that introducing N atom as the bridged group can effectively elevate the driving force, but B atom can reduces it drastically. As seen in this table, the values of the reorganization energy (λtotal) of all dyes are increased to varying degrees, especially dyes bridged with O, N, B and Si atoms. The increases of λtotal for dyes P2, P6-8 as well as P11 are relatively small. The EBE values of N-bridging TPA donor dyes are effectually lowered, while that of O-bridging dyes as well as those with alkoxy side chain on the TPA donor group are increased in some degree in contrast. By reason of the foregoing, the Jsc of designed 11 dyes are corresponding or lower than that of P0.
As mentioned above, there are two key characteristic parameters to measure Voc, ∆q and μnormal. As shown in Tab. 5, the N-bridging dyes (P3, P4 and P5) emerge as the top-performing group due to their larger ∆q and μnormal among the P-series dyes, especially dyes P3 with the largest ∆q (0.584e) and the second-highest μnormal (20.79D) while P5 with the second-largest ∆q (0.433e) and the highest μnormal (23.72D). This indicates that N-bridging TPA donors can not only significantly enhance the ∆q, but also markedly elevate the μnormal, thereby promote the Voc efficiently. Other P-series dyes show little difference in these parameters compared to P0. It is worth noting that P10 (Si-bridging) and P11 (Se-bridging) also display relatively larger μnormal values (13.35D and 13.45D, respectively). However, P6, which incorporates a C=O group as the bridged unit, exhibits the smallest μnormal (3.2D) and the penultimate ∆q (0.088e), attributable to the strong electron-withdrawing nature of the carbonyl group. In a word, dyes P3-P5, P10 as well as P11 should possess superior Voc performance, especially P3 and P5.
In addition, an interesting conclusion can be drawn regarding the alkyl chain bonded to the bridging atoms that the alkyl groups (-C6H13) can maintain or enhance the key parameters of Jsc and Voc at a comparatively excellent level. The alkyl chain also helps to keep charge recombination at a lower level. This result enriches our previous research on the effects of side chains [44].
According to the corresponding results of the above study, methylene or substituted methylene groups perform best in maintaining the planarity of the bridged TPA group. The N-bridging TPA group can significantly enhance the parameter of Voc. Therefore, a new P-series dye, P12, has been designed and theoretically characterized, It involves a N-bridging group as the bridging group, with the purpose of improving Voc as much as possible while trying to preserving the planarity of the donor group.
Just as expected, the bridging TPA group of P12 molecule displays better planarity (Figure 3). Additionally, the FMOs (Table S3) and EDDM (Table 4) of P12 also demonstrate enhanced ICT characteristic. As shown in Figure 4 and Table 5, the λmax of P12 is 506nm, which is 31nm red-shifted compared to P0. Moreover, the LHE and the λtotal show a little change, while the EBE exhibits a noticeable change comparing with P0, which contribute to a slight enhancement of the Jsc eventually.
As listed in Tab. 5, the ∆q and μnormal of P12 are significantly increased, which effectively enlarge the Voc. In other words, introducing a N atom into the bridged TPA group can increase ∆q and μnormal appreciably, thereby favorably impacting Voc. This finding is consistent with the previous conclusion, which further validating the design strategy of bridged TPA group. It also provide valuable theoretical guidance for the design and experimental research of dyes.
4. CONCLUSIONS
In this work, the underlying physical origin of the bridged TPA donor was theoretically explored by DFT and TD-DFT calculations through a comparative study of R0 and P0. Furthermore, P-series dye sensitizers were designed and theoretically evaluated to systematically investigate the influence of the bridged TPA donor group. The following conclusions were reached through detailed theoretical analysis.
Firstly, the methylene-bridged TPA group not only enhances the planarity of the donor group and effectively reduces λtotal and EBE, thereby further elevating the Jsc, but also promote the Voc due to the difference of μnormal. Thus incorporating -CH2- as the bridged group is favourable for both Jsc and Voc. In addition, methylene-bridged TPA group exhibits superior planarity compared to other atoms or groups investigated in this work, revealing the underlying physical origin of the bridged TPA donor group.
Secondly, the design strategy of the bridged TPA group was preliminarily investigated. N-bridged TPA donor group exhibits the most significant red-shift effect and effectively enhances the ΔG while reducing the λtotal and EBE, thereby improving Jsc. Additionally, it also efficiently increases the ∆q and μnormal, which is beneficial for enlarging Voc.
Thirdly, based on the above design strategy, the newly designed dye P12 combines the virtues of both C-bridging group and N-bridging group, demonstrating outstanding Jsc and Voc among all P-series dyes. This further validates the validity of the previous conclusions, and P12 emerges as the best alternative dye molecule in this work.
Finally, the validated design strategies and empirical results provide crucial insights and valuable guidance for future research endeavors. Future work will focus on the identification and evaluation of novel bridging groups, and the strategic integration of advanced materials to further advance the field of DSSCs. We anticipate that this work will significantly contribute to the theoretical foundation for the rational design and synthesis of dye sensitizers, thereby enhancing the overall performance of DSSCs.
AUTHOR CONTRIBUTIONS
Le-Yan Liu: Computation, Writing - Original draft preparation.
Kai-Li Zhu: Conceptualization, Methodology, Project administration.
Li-Qiang Xie: Formal analysis, Writing - Reviewing and Editing.
Rong Li: Data curation.
Yi Wang: Data analysis, Visualization.
CONFLICT OF INTEREST STATEMENT
All authors have contributed significantly to the research and have read and approved the manuscript. We have no conflicts of interest to declare.
FUNDING
This research was financially supported by Young PhD Support Program of Gansu Provincial Department of Education under project [Grant Number 2023QB-101 and 2023QB-100].
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