2-Hexylthiophene-substituted Alizarin-based (D–π–A) Organic Dyes for Dye-sensitized Solar Cell Applications: Density Functional Theory and UV–Vis Studies

Abstract

This work reports density functional theory and time-dependent density functional theory calculations of the optimized geometries, electronic structures and optical properties of molecular dyes D1, D2, D3, D4, D5, and D6 formulated through substitution of 2-hexylthiophene to alizarin using the hybrid functional B3LYP and 6-31G (d,p) basis sets. The dyes are considered as potential pigments for dye-sensitized solar cells. For all dyes, HOMO/LUMO (Highest Occupied Molecular Orbital/Lowest Unoccupied Molecular Orbital) analysis results in positive outcomes upon electron injection to semiconductors and subsequent dye regeneration by the electrolyte. It is found that charge transfer is from the thiophene and unsubstituted ring of alizarin to the substituted ring of alizarin containing C=O and OH groups. The C=O groups are observed to be very important in strengthening the dyes as they are revealed to be the anchoring group bonding to the TiO₂ semiconductor. Comparatively, dye D6 is observed to possess high absorption ability and electron injection power through a study of the light-harvesting efficiency and injection driving force (ΔG^inject). The estimated values of open-circuit voltage (V_oc) for the computed dyes are also presented. Decisively, all the considered dyes prove to be useful as potential photosensitizers in solar cells using a TiO₂ semiconductor and $I^{-}/I_3^{-}$ coupling electrolyte.

Introduction

Dye-sensitized solar cells (DSSCs) have received considerable research attention due to the importance to replace nonrenewable and limited energy sources and reduce environmental pollution.^1–5 The attraction toward DSSCs is due to their low cost and easy fabrication, unlike other solar cells.^6,7 The energy conversion efficiency of the currently used technologies such as silicon solar cells has become high and stable; however, the resulting efficiency does not transfer to reduced costs of the materials and fabrication techniques.^8,9 This limits the future application of these technologies. DSSCs are environmentally friendly and cost-effective and might have high power conversion efficiency.^9–13 However, more studies are needed to make this practical. DSSCs operate through incorporation of components such as dye sensitizers, mesoporous semiconductors (arguably a metal oxide like TiO₂), electrolytes, counter electrodes, and photoelectrodes.^14,15

Dye sensitizers play an essential role in attaining a high power conversion efficiency of DSSCs ¹⁶ and have received more research attention compared to other components as they are involved in light harvesting, electron conduction, and regeneration process. ⁹ Dyes used in DSSCs are categorized as metal-free organic and metal complexes.^17,18 The former are further classified as natural and synthetic, ¹⁹ both being conjugated molecules. This is because conjugated molecules possesses particular properties which make them precise materials for solar cells,^20–22 optoelectronic devices,^23–25 and transistors. ²⁶ Furthermore, these molecules have benefits of not being amorphous and can be synthesized easily for practical use.^27–29 Metal-free organic dyes have been reported to achieve a power conversion efficiency of 17.3%, ³⁰ while metal complexes have been reported to achieve only 13%. ³¹ Ongoing experimental and theoretical research has reported studies on metal-free organic dyes alternatively to metal-based examples due to the expense and toxicity associated with metal-based dyes. Theoretical studies have gained more interest as they analyze valuable information for use in experimental studies. ³² The information gained from theoretical studies contributes to the rationalization of the properties of known materials and helps to estimate those of unknown compounds^33–36 as the basis for any future experimental work.

Many studies have been reported on natural dyes^11,37–40 due to their easy availability, cost-effectiveness, and nontoxicity, even though their power conversion efficiency is still low.^41–44 Alizarin (1,2-dihydroxy-9,10-anthraquinone) is a previously reported natural dye and one of the first natural dyes to be duplicated synthetically. ⁴⁵ Alizarin is a prominent red dye most used previously in dyeing textile fabrics. It is naturally derived from the roots of madder plants and was first reported as a synthetic dye in 1869. ⁴⁵ Apart from other applications, alizarin is confirmed to be applicable in the DSSC technology reaching up to 3.57% conversion efficiency. ⁴⁶ Despite being one of the oldest dyes, studies on derivatives of alizarin are rarely reported, if any. This paper reports theoretical studies on the substitution of alizarin using 2-hexylthiophene as the donor group based on structures D1–D6 (Figure 1). The donor group 2-hexylthiophene was chosen for this study as it has been reported previously to be one of the best electron sources in organic dyes, ⁴⁷ albeit not in anthraquinone derivatives.

Figure 1.

Structures of the dyes considered for the theoretical studies.

Results and discussion

Geometries optimization

Geometry optimization for D1, D2, D3, D4, D5, and D6 was achieved using the B3LYP/6-31G (d,p) functional. The resulting geometries of the optimized molecular dyes are presented in Figure 2. Optimization to produce the molecular dyes was performed by replacing the hydrogen atom at C3 (D2), C4 (D3), C5 (D4), C6 (D1), C7 (D6), and C8 (D5) of the alizarin precursor with 2-hexylthiophene. It should be noted here that this research work also involves substitution using other groups such as carboxylic acid, 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid and a mixture of carboxylic acid and 2-hexylthiophene and the results will be discussed in a separate paper.

Figure 2.

Optimized geometries of the considered molecular dyes.

The total energy (E_T) values of the optimized molecules D1, D2, D3, D4, D5, and D6 were found to be −1626.5878, −1626.5856, −1626.5722, −1626.5524, −1626.5519, and −1626.5690 eV, respectively. The results for E_T, the HOMO/LUMO (Highest Occupied Molecular Orbital/Lowest Unoccupied Molecular Orbital) energies, and the energy gap (E_g) in eV are presented in Table 1. A coplanar conformation was revealed in all the optimized molecular dyes, which implies efficient intramolecular electron transfer from the donor to the acceptor. This is explained by the fact that the coplanar structure improves electron transfer from the donor to the acceptor through the π-spacer and increases the dye performance. ⁴⁸

Table 1.

Electrical and optical parameters of the computed molecules in a solvent phase.

Dye	EHOMO (eV)	ELUMO (eV)	Eg (eV)	ET (eV)	μ (Debye)
D1	−6.0560	−3.2567	2.7993	−1626.5878	5.3973
D2	−5.9047	−3.2415	2.6632	−1626.5856	1.4391
D3	−5.9924	−3.2660	2.7264	−1626.5722	2.2521
D4	−6.0022	−3.2252	2.7770	−1626.5524	5.5430
D5	−5.9722	−2.9122	3.0600	−1626.5519	4.7210
D6	−5.9725	−3.0178	2.9547	−1626.5690	2.6629

HOMO: Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital.

Intramolecular charge transfer

Charge transfer from a donor to an acceptor is one of the important features of metal-free organic dyes as it is used to indicate conduction and regeneration in the cell. ⁴⁸ It is generally referred to as intramolecular charge transfer (ICT). The investigation of ICT in molecular dyes is possible through the study of HOMO and LUMO energies, which is scientifically referred to as the frontier molecular orbital (FMO) contribution.^49,50 The study of these molecular orbital energies is accurately done through the energy gap created by the two, as the energy gap directly affects the photocurrent of the dye molecule. The results of the HOMO/LUMO energy (eV), E_g (eV), E_T (eV), and dipole moment (μ) in Debye are presented in Table 1. For conjugated molecules with electron delocalization, HOMOs always show typical aromatic features.^51,52 For the dye molecules, HOMOs are concentrated at electron-rich points of the molecules with LUMOs at electron-deficient points.^48,53 Thus, HOMO represents the bonding character and LUMO represents the antibonding character of molecular dyes.^54,55 The HOMO/LUMO diagrams are presented in Figure 3, while the HOMO/LUMO energies are shown in Figure 4. The strength of the electron-donating group is directly proportional to the HOMO energy level;^56,57 a dye molecule with a strong electron-donating group gives a high HOMO energy than examples with weakly donating groups. Considering all the studied dyes, the HOMO energy levels are in the following order: D2 > D5 > D6 > D3 > D4 > D1. Thus, dye molecule D2, with E_HOMO = −5.9047 eV, expresses a stronger electron-donating ability than the other dye molecules, while D1, with E_HOMO = −6.0560 eV, has the HOMO at the lowest level. This shows that attachment of 2-hexylthiophene at the C3 position favors electron donation by the dye molecule. On the other hand, the LUMO energy level should be as low as possible to simplify electron acceptance from the HOMO, but not lower than the energy of the conduction band (E_CB) of the semiconductor. The LUMO energy levels are in the following order of D3 < D1 < D2 < D4 < D6 < D5. The dye molecule, D3, with E_LUMO = −3.2660 eV, shows a stronger electron-accepting ability than the rest of the dye molecules, even though the difference is very small in the range of 0.3538 eV. From this argument, it is unsatisfactory to come to a conclusion of the best dye for electron conduction from the HOMO to the LUMO by focusing on the orbital energy levels separately rather than by considering the average distance between the two, HOMO and LUMO orbitals. This can be achieved by investigating the energy gap (E_g) between the two orbitals. The E_g values, which show the dissimilarity of the HOMO and LUMO energy levels, are presented in Table 1. In the DSSC technology, the energy gap is used to analyze the excitation of electrons and hence evaluate the applicability of the dye in solar cell formation. In practice, E_g is inversely related to the electron excitation efficiencies. ⁵⁶ Thus, the lower the E_g values, the higher the ability of the dye to excite electrons from the HOMO to the LUMO. The energy gap is also used to indicate the spectral coverage of a dye. This is in the sense that easy excitation of electrons by the dye is enhanced by the fact that enough photons are absorbed and signifies the ability of the dye to absorb photons from the longer wavelength. Furthermore, this increases the value of J_sc and upturns the conversion efficiency (η) of the intended solar cell. As presented in Table 1, the E_g trend is as follows: D2 < D3 < D4 < D1 < D6 < D5. From the trend, dye D2 (E_g = 2.6632 eV) has the highest ability to excite electrons from the HOMO to the LUMO and is expected to allow more photon absorption than other dyes. With a small range of 0.3968 eV in the E_g values, all the considered dyes show high ability toward intramolecular charge transfer and hence potential for the DSSC technology.

Figure 3.

HOMO and LUMO diagrams of the computed dyes.

Figure 4.

HOMO and LUMO energies of the computed dyes D1–D6.

As observed in Figure 3, the HOMOs in all the considered molecules are located more in the conjugated unsubstituted ring of the alizarin and thiophene side, thereby showing stronger electron-donating behavior. The LUMOs are located more on the two substituted rings of the alizarin molecules with the carbonyl and hydroxy groups, which indicates that the anchoring group for the dyes is C=O. This could lead to a strong electronic coupling with the semiconductor (TiO₂) surface and improve the electron injection efficiency. The higher electron injection power will subsequently enrich the J_sc value of the dyes. As shown in Figure 4, the LUMO energy levels of the studied dyes are higher than those of the TiO₂ conduction band edge, which is −4.0 eV. ⁵⁸ Thus, injection of electrons from the excited dyes to the semiconductor, TiO₂, is certain. In addition, the corresponding HOMO energies are lower than that of the electrolyte, $I^{-}/I_3^{-}$ , which is −4.8 eV ⁵⁹ ; hence, easy restoring of the excited electron from the HOMO by the electrolyte is possible. For this reason, electron injection from the LUMO of the dyes to the conduction band of the semiconductor and successive regeneration of the HOMO of the dyes by the electrolyte are strongly guaranteed. Therefore, we can argue that all the considered dye molecules are applicable in the DSSC technology.

The dipole moments of the dyes are presented in Table 1. The dipole moment indicates the symmetry in the distribution of the electronic charge of the dye molecule. It should be noted that a larger dipole moment is attributed to the asymmetric behavior in the distribution of the electronic charge. This behavior makes a molecule more reactive and sensitive to the external electric field.^48,53 From the results in Table 1, the dipole moment of dye D4 is greater than those of other dyes; hence, it is highly sensitive to the external electric field.

Photovoltaic properties

The parameters to explain the photovoltaic properties of the computed dyes are presented in Table 2. This includes the oxidation potential energy of the dye in the ground state (E^dye), the electronic injection free energy (ΔG^inject), the light-harvesting efficiency (LHE), the oxidation potential energy of the dye in the excited state (E^dye*), and the open-circuit voltage (V_oc). The other more important property, short-circuit current (Jsc), is not discussed in this paper as it can only be determined experimentally, instead it is analyzed through LHE and ΔG^inject. The ability of the dye to absorb photons from the sun and inject photoexcited electrons from the LUMO to the conduction band is explained through these vital parameters.

Table 2.

Photovoltaic properties of the computed molecules in a solvent phase.

Dye	Eoo (eV)	Edye(eV)	Edye*(eV)	ΔGinject (eV)	LHE	Voc (eV)
D1	2.4259	6.0560	3.6301	−0.3699	0.5255	0.7433
D2	2.2702	5.9047	3.6345	−0.3655	0.2824	0.7585
D3	2.1220	5.9924	3.8704	−0.1296	0.0742	0.7340
D4	2.3014	6.0022	3.7008	−0.2992	0.0503	0.7748
D5	2.4335	5.9722	3.5387	−0.4613	0.1766	1.0878
D6	2.5427	5.9725	3.4298	−0.5702	0.4743	0.9822

LHE: light-harvesting efficiency.

Considering the results from Table 2, all the computed dyes have negative values of ΔG^inject: −0.3699, −0.3655, −0.1296, −0.2992, −0.4613, and −0.5702 eV for D1, D2, D3, D4, D5, and D6, respectively. This result shows the positioning of the LUMO in the sense that the excited state of the dye is located above the conduction band edge of the semiconductor. This assures injection of excited electrons into the semiconductor. ⁴⁸ From the results, dye D6 has the largest value of ΔG^inject and dye D3 has the smallest value. The LHEs of the computed dyes are also given in Table 2. Dye D1 possesses the highest LHE value of 0.5255, while dye D4 has the lowest (0.0503). From the results of LHE and ΔG^inject, it is worth mentioning that dye D1 would possess the highest value of J_sc as the LHE and ΔG^inject are directly proportional to J_sc.^56,57 The fact that these two dyes have higher values of J_sc is concluded under the argument that the difference in ΔG^inject between D6 and D1 is small (0.2003 eV), while D1 has a very high LHE compared to D6 with a difference of 0.4752. Focusing on the E^dye* values, the trend is D6 < D5 < D1 < D2 < D4 < D3, which indicates that the most convenient oxidizing dye in this case is D6 and the worst oxidizing is D3.

Another parameter which influences the power conversion efficiency (η) of the dye is the open-circuit voltage (V_oc). In molecular dyes, a higher value of V_oc demonstrates higher electron injection power. From Table 2, the values of V_oc for all the computed dyes show insignificant differences in the range of 0.3538 eV. The highest values are observed in D5 and D6 with values of 1.0878 and 0.9822 eV, respectively, and the trend is in the following order D5 > D6 > D4 > D2 > D1 > D3. Thus, dye D6 is observed to perform better with respect to the photovoltaic properties based on the better values of LHE, ΔG^inject, and V_oc and hence has better efficiency than the other dyes.

Absorption properties

To analyze the absorption behavior of the dyes, the oscillator strength (f), the vertical excited singlet state and the transition energy level are considered in the gas phase and in acetonitrile solvent (Table 3). In the same media, significant variation is observed between the molecular dyes for the vertical excited energy. In contrast, under the same excitation level, the dyes show insignificant differences between the solvent and gas media. Representatively, the first vertical excited energy decreases in the following order D3, D2, D4, D1, D5, and D6 with values of 2.1222, 2.2705, 2.3017, 2.4262, 2.4338, and 2.5431 eV. From the results, the observed difference is trivial; hence, the dyes possess almost the same value for the vertical excitation energy.

Table 3.

Absorption properties of the considered dye derivatives in the gas phase and acetonitrile.

Dye	Gas phase					Acetonitrile
	λmax (nm)	λmax (eV)	f	Transition		λmax (nm)	λmax (eV)	F	Transition
D1	476.56	2.6020	0.0000	HOMO → LUMO	95.20%	511.09	2.4262	0.3238	HOMO → LUMO	97.42%
	445.16	2.7855	0.1588	HOMO-1 → LUMO	95.82%	448.36	2.7656	0.1728	HOMO-1 → LUMO	97.15%
	436.28	2.8422	0.0006	HOMO-2 → LUMO	62.93%	424.04	2.9243	0.0001	HOMO-4 → LUMO	93.75%
D2	676.05	1.8342	0.0756	HOMO → LUMO	98.11%	546.13	2.2705	0.1441	HOMO → LUMO	97.89%
	476.56	2.6020	0.1295	HOMO-1 → LUMO	94.69%	430.51	2.8803	0.1790	HOMO-1 → LUMO	94.82%
	467.72	2.6512	0.0000	HOMO-3 → LUMO	95.08%	421.12	2.9445	0.0001	HOMO-3 → LUMO	94.15%
D3	573.15	2.1635	0.1015	HOMO → LUMO	91.30%	584.29	2.1222	0.0335	HOMO → LUMO	92.4%
	442.58	2.8018	0.0313	HOMO-1 → LUMO	38.15%	445.96	2.7805	0.0974	HOMO-1 → LUMO	64.43%
	412.52	3.0059	0.0447	HOMO-1 → LUMO	50.97%	423.79	2.9260	0.0228	HOMO-2 → LUMO	72.31%
D4	513.12	2.4166	0.0846	HOMO → LUMO	88.09%	538.73	2.3017	0.0954	HOMO → LUMO	91.71%
	439.52	2.8213	0.0165	HOMO-3 → LUMO	78.89%	424.78	2.9192	0.0356	HOMO-3 → LUMO	76.55%
	399.94	3.1005	0.0072	HOMO-4 → LUMO	35.58%	408.13	3.0382	0.1105	HOMO-1 → LUMO	88.78%
D5	502.78	2.4663	0.0630	HOMO → LUMO	87.05%	509.49	2.4338	0.0844	HOMO → LUMO	89.60%
	419.85	2.9534	0.0237	HOMO-3 → LUMO	51.60%	412.59	3.0054	0.0854	HOMO-3 → LUMO	34.20%
	397.58	3.1189	0.0024	HOMO-4 → LUMO	60.28%	403.02	3.0768	0.0596	HOMO-1 → LUMO	62.92%
D6	457.81	2.7085	0.1311	HOMO → LUMO	58.01%	487.60	2.5431	0.2793	HOMO → LUMO	96.23%
	453.82	2.7324	0.0930	HOMO-2 → LUMO	56.54%	438.17	2.8300	0.0004	HOMO-2 → LUMO	88.16%
	408.49	3.0356	0.0000	HOMO-4 → LUMO	92.40%	408.34	3.0367	0.1269	HOMO-1 → LUMO	95.09%

HOMO: Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital.

Another parameter presented in Table 3 is the oscillator strength. The relationship between oscillator strength and wavelength is shown in Figures 5 and 6. The oscillator strength of the dyes trends randomly for the reflected excitations. Mostly, the values of the oscillator strengths are seen to be higher in the solvent phase, as represented in Figure 5, than in the gas phase, as shown in Figure 6. From the results in Figures 5 and 6, D1 exhibited the highest value of oscillator strength, specifically in the solvent phase for the first excitation even though the dye revealed poor results in the gas phase for first and third excitations. This phenomenon highlights the poor ability of dye D1 to harvest solar radiation in the gas phase, but is better in the solvent phase. Moreover, the other parameter presented in Table 3 and Figures 5 and 6 is the maximum wavelength (λ_max). As observed, the λ_max values for D1, D2, D3, D4, D5, and D6 are 476.56, 676.05, 573.15, 513.12, 502.78, and 457.81 nm in the gas phase and 511.09, 546.13, 584.29, 538.73, 509.49, and 487.60 nm in the solvent phase. In both these media, the λ_max values are higher in the solvent phase than in the gas phase. The variation between dyes in the same media is not pronounced; hence, arguably, the dyes have similar coverage in their absorption spectra.

Figure 5.

Theoretical UV–Vis spectra for the computed molecules in the solvent phase.

Figure 6.

Theoretical UV–Vis spectra for the computed molecules in the gas phase.

Methodology

Theoretical details

To analyze the applicability of the computed dyes in DSSCs, different parameters were calculated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. One of the crucial parameters in DSSCs is the power conversion efficiency (η), which is obtained using the formula below ⁶⁰

$$\eta =FF\frac{V_{\mathrm{oc}}{J}_{\mathrm{sc}}}{P_{\mathrm{inc}}}$$

(1)

where FF is the fill factor, J_sc is the short-circuit current density, V_oc is the open-circuit voltage, and P_inc is the incident solar power on the cell. J_sc and FF values are experimental parameters, and J_sc is calculated by the following relation ⁴⁸

$$J_{\mathrm{sc}}={\int }^{\mathrm{}}LHE(\lambda ){\Phi }_{\mathrm{inject}}{\eta }_{\mathrm{collect}}\mathrm{d}\lambda$$

(2)

where ${\Phi }_{\mathrm{inject}}$ is the electron injection efficiency, ${\eta }_{\mathrm{collect}}$ is the charge collection efficiency, and LHE(λ) refers to the LHE corresponding to a particular wavelength (λ). Being experimental parameters, in this work, J_sc and FF are homogeneously investigated in view of the other key factors affecting them. In addition, LHE, which is used to calculate the experimental parameter J_sc, can be obtained theoretically using the following formula ⁶¹

$$LHE=\mathrm{1}-\mathrm{1}{0}^{-f}$$

(3)

where f is the oscillator strength of the dye associated with the maximum wavelength (λ_max). It may be noted that LHE and f, which are presented in equation (3), are directly related. The increase in oscillator strength influences the increase in LHE and vice versa. The other parameter in the calculation of J_sc, as shown in equation (2), is ${\Phi }_{\mathrm{inject}}$ . The ${\Phi }_{\mathrm{inject}}$ value is also related to the injection driving force ΔG^inject of the dye, which is calculated using equation (4). ⁶² It is used to denote the ability of the dye in injecting electrons to the semiconductor for power generation

$$\Delta G^{\mathrm{inject}}=E^{\mathrm{dye}*}+E_{\mathrm{CB}}$$

(4)

where E^dye* denotes the oxidation potential energy of the dye in the excited state and E_CB denotes the reduction potential of the conduction band of the semiconductor. In this study, we considered TiO₂ as a semiconductor, following on from previous works^7,62,63 where this semiconductor gave the best performance. Moreover, the E^dye* value from equation (4) is calculated using equation (5)^7,63,64

$$E^{\mathrm{dye}*}=E^{\mathrm{dye}}-E_{00}$$

(5)

where E^dye is the ground-state oxidation potential of the dye, which in this work is approximated to be −E_HOMO, and E₀₀ is the electronic vertical transition energy corresponding to λ_max.

The analysis of electron conduction from dye to semiconductor is related to V_oc and the energy of the LUMO (E_LUMO) as in the following relation ⁶⁴

$$V_{\mathrm{oc}}=E_{\mathrm{LUMO}}-E_{\mathrm{CB}}$$

(6)

It should be noted that in this work the reduction potential (E_CB) for the considered TiO₂ semiconductor is −4.0 eV. ⁵⁸

Computational details

Presented in this work are molecular dyes obtained by substitution of 2-hexylthiophene on the alizarin molecule (see Figure 1). All calculations were performed with the Gaussian 09W package ⁶⁵ using DFT and TD-DFT methods. The methods used Becke’s three-parameter functional and the Lee–Yang–Parr functional (B3LYP)^66–68 coupled with the 6-31G (d,p) basis set. ⁶⁹ The UV–Vis spectra of the computed dyes were obtained by performing single-point TD-DFT calculations through the use of optimized ground-state geometries. The TD-DFT method was also used to calculate the electronic absorption spectra based on allowed excitations and oscillator strength together with excited-state properties, which allowed derivation of the HOMO and LUMO energy values. The polarizable continuum model (PCM) ⁷⁰ was used in the calculation performed in the solvent phase; the solvent used is acetonitrile.

Conclusion

This paper has presented a discussion on the sensitizer donor effects of 2-hexylthiophene-substituted alizarin photosensitizers for use in DSCCs using DFT and TD-DFT. Different parameters were considered, including geometric, photovoltaic, absorptional, and charge transfer ability aiming at finding potential sensitizers for use in solar cells. Generally, all the considered dyes showed better performance with insignificant variation, and especially the results of dye D6 were found to be slightly better than those of the other dyes. For all dyes, electron transfer was revealed to originate from the thiophene group to the alizarin moiety, which suggests the anchoring group to be C=O. The analysis of the driving force energy of the considered dyes indicates that electron injection of dye D6 took place more certainly due to the relatively larger driving force energy. Substitution of 2-hexylthiophene at position C4 (dye D3) led to slightly less performance comparatively in most of the other properties. According to the results, it is worth mentioning here that, for all the six dyes studied, the electron injection from the dyes to the conduction band of the semiconductor and successive regeneration of the dyes by the electrolyte are energetically assured, thereby virtually guaranteeing the applicability of these dyes in the DSSC technology.