Research Article  Open Access
Theoretical Study of the πBridge Influence with Different Units of Thiophene and Thiazole in Coumarin DyeSensitized Solar Cells
Abstract
Eight coumarin derivative dyes were studied by varying the πbridge size with different thiophene and thiazole units for their potential use in dyesensitized solar cells (DSSC). Geometry optimization, the energy levels and electron density of the Highest Occupied Molecular Orbital and the Lowest Unoccupied Molecular Orbital, and ultravioletvisible absorption spectra were calculated by Density Functional Theory (DFT) and TimeDependentDFT. All molecular properties were analyzed to decide which dye was the most efficient. Furthermore, chemical reactivity parameters, such as chemical hardness, electrophilicity index, and electroaccepting power, were obtained and analyzed, whose values predicted the properties of the dyes in addition to the rest of the studied molecular properties. Our calculations allow us to qualitatively study dye molecules and choose the best for use in a DSSC. The effects of πbridges based on thiophenes, thiazoles, and combinations of the two were reviewed; dyes with three units mainly of thiazole were chosen as the best photosensitizers for DSSC.
1. Introduction
The conversion of solar light to electricity through dyesensitized solar cells (DSSC) has been widely studied since the work of O’Regan and Grätzel [1]. DSSC with desirable physical properties, such as transparency and flexibility, high conversion efficiency, and low cost, were achieved using transparent conductor oxide (TCO), TiO_{2} semiconductors, and electrolyte [2]. Initially, Ru bipyridyl complexes were studied and showed prominent results, achieving up to 12% conversion efficiency [3–5]. Metalfree organic dyes were also studied to avoid using Ru, since Ru is toxic and rare and requires purification [6]. Therefore, organic dyes are more attractive for DSSC because they are cheaper than Rubased dyes, have controllable absorption wavelengths, allow versatile design, and are easily synthesized [7]. The high conversion efficiencies achieved by metalfree organic dyes have motivated further study. In previous years, DonorπbridgeAcceptor (DπA) dyes have been outstanding DSSC dyes, such as the coumarin derivative dyes with up to 8% conversion efficiency [8, 9] and triphenylaminebased dyes with up to 10% conversion efficiency [10]; both use thiophenebased groups as the πbridge. Recently, DπA organic dyes derivatives of carbazole (ADEKA1, ADEKA2, and MK2) and triphenylamine (C257, C258) reached efficiencies of up to 12.8% using a cobalt(III/II) tris(2,20bipyridine) complex as the redox electrolyte and thiophenebased groups as the πbridge [11, 12]. Theoretical studies were carried out for a better understanding of the experimental results, mainly about the dye [13–17]. In previous works, it was common to study DπA dyes using thiophene and its derivatives as πconjugated bridges, but the use of thiazolebased groups has also achieved interesting conversion efficiency results in organic solar cells [18] and DSSC [19–21], mainly due to the redshift of the maximum absorption wavelength and the improvement of intramolecular charge transfer. Accordingly, we chose eight molecules, shown in Figure 1, with coumarin derivatives as donors and cyanoacrylic acid as acceptors/anchorages while varying the conjugated bridge. Our proposal presents the molecules NKX2587, NKX2677, and NKX2697 that were synthesized and evaluated by Hara et al. [8], which have one, two, and three thiophene units. We also studied five new proposals as sensitizers, CTH12, CTH21, CTH22, CTH122, and CTH222, which resulted in conjugated bridges of thiazole groups and the combination of thiazole and thiophene units. The number 1 represents a thiophene unit and the number 2 represents a thiazole unit; for example, CTH122 is formed from a coumarin derivative (donor), thiophenethiazolethiazole, and cyanoacrylic acid (acceptor). The sensitizers were evaluated and compared to using data reported for a TiO_{2}based DSSC to determine the effect of the πbridge. To study this effect from a theoretical point of view, we performed calculations about ultravioletvisible (UVVis) absorption spectra, electronic transitions analysis, energy levels of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), and HOMO and LUMO electrondensity distribution. An important aspect in relation to other studies is the evaluation of the chemical reactivity parameters and their influences on the performance of the dye.
2. Calculation Details
This study was performed on the basis of Density Functional Theory (DFT), using the M06 hybrid metaGGA functional [22] for the ground state geometry optimization and frequencies, the HOMO and LUMO electron density and their energy levels, and the chemical reactivity parameters; the chemical reactivity parameters were obtained by energy calculations (ionic and neutral state) such as those expressed by Parr and Pearson [23] and Gázquez et al. [24]. The M062X hybrid metaGGA functional [22] was used to obtain the UVVis absorption spectra in acetonitrile through TimeDependentDFT (TDDFT) with the nonequilibrium protocol [25, 26] and the excitation energies were processed by the SWizard program [27, 28] using the Gaussian model to establish the convolution. Both functionals were combined with two basis sets: the 631G(d) [29, 30] and the 6311G(d) [31] proposed by Pople. The effect of the solvent was considered by the integral equation formalism polarizable continuum model (IEFPCM) [32], an implicit method. The difference in energy between the dye’s LUMO level and the TiO_{2} semiconductor conduction band was reviewed, as well as some parameters of chemical reactivity, such as chemical hardness (), electrophilicity index (ω), and electroaccepting power (ω^{+}), to predict the possible behavior of the dye and thus of the DSSC. All computational calculations were carried out with Gaussian 09 Rev. D.01 [33].
3. Results and Discussion
The maximum absorption wavelengths (), HOMO and LUMO energy levels and electron density, and the chemical reactivity are presented to choose the best dye molecule to be employed in a DSSC. M062X was the best functional to reproduce the UVVis absorption spectra according to the experimental results of NKXtype coumarins reported in the literature [8, 34]. The selection of the functional is due to the molecule’s size, as demonstrated in the theoretical study of chargetransfer excitations in organic dyes by Dev et al. [35]; M06 and M062X are two methods from the same family of functionals with different HartreeFock exchanges. All calculations were carried out using the 631G(d) and 6311G(d) basis sets, with very similar results. Only the HOMO and LUMO energy levels and chemical reactivity are presented with both basis sets; the rest of the results are presented only with the 6311G(d) basis set.
3.1. UltravioletVisible Spectra
The absorption maximum wavelength of the eight molecules is around 500 nm, as shown in Figure 2. When the molecular systems contain a thiazoleπbridge in their structure, these have a redshift of compared to those with only thiophene groups; the molecules with combined groups (thiazole and thiophene) in the πbridge present intermediate between the above; for example, is 509, 538, and 540 nm for NKX2697, CTH122, and CTH222, respectively. The same behavior is observed for NKX2677 (501 nm) and CTH22 (512 nm); the absorption band of the latter is comparable to NKX2697 (three thiophenes in the πbridge).
It is preferable that a photosensitizer’s is closer to red and hence has a lower energy gap between the HOMO and LUMO [36], which helps the DSSC to have better charge injection and therefore better efficiency. In this case, the difference among the of all dyes is small, but the CTH222 is slightly better, followed by CTH122 and NKX2697; the remaining molecular structures have the following trend: CTH22 > CTH21 (502 nm) > CTH12 (501 nm) > NKX2677 > NKX2587 (479 nm). Further, the is more redshifted when the units in the πbridge are augmented (thiophenes, thiazoles, or both); this effect already has been reported [37].
Table 1 shows the absorption bands, oscillator strength , and electron transitions for each band of the proposed sensitizers; further the experimental maximum absorption wavelengths available in the literature are shown. In the NKX molecules, if the πbridge is augmented with a thiophene unit (from NKX2587 to NKX2677), the contribution corresponding to the of the HOMO to LUMO decreases by 10%, but f increases by 0.43; from NKX2677 to NKX2697, f increases slightly by 0.02. In CTH molecules, when the πbridge is augmented with a thiazole unit, the HOMOLUMO transition remains around 80%. Furthermore, by using thiazole groups, f decreases by 0.29 from NKX2677 to CTH22 and by 0.67 from NKX2697 to CTH222. In the case of CTH12, CTH21, and CTH122, f decreases, with values of 1.5737, 1.3246, and 1.1430; this affirms the behavior of f with the inclusion of thiazole groups. A second band is observed for all molecular systems in the UVVis spectrum with a transition. The higher values of f for these absorption bands, in order from highest to lowest, correspond to CTH222, CTH122, and NKX2697; the rest of the absorption bands keep a slight variation. The above data suggest that NKX2697 has higher light harvesting in the HOMO to LUMO transition, followed by NKX2677 and CTH21.
 
= experimental maximum absorption wavelength was measured in acetonitrile as solvent. Lower contributions to 2% are not shown. 
3.2. Molecular Orbitals and Energy Levels
The HOMO, HOMO1, LUMO, and LUMO + 1 energy levels are shown in Figure 3 with the M06 functional and the 631G(d) and 6311G(d) basis sets. Similar results were achieved between both basis sets, presenting a negative displacement of the energy levels around 0.2 eV from 631G(d) to 6311G(d); however, the tendency of the energy levels among the dyes is maintained, and hence any method can be used adequately. The eight molecules are good photosensitizer candidates because the LUMO level is above the conduction band (CB) of the TiO_{2} semiconductor (−4.0 eV) [38] and the HOMO level is below the redox potential of the electrolyte (−4.8 eV) [38], which ensures electron injection from the dye to TiO_{2} and the regeneration of the dye, obtaining electrons from the electrolyte. In this sense, with both basis sets a more efficient electron transfer using CTH222 is expected considering the LUMO level, since it is closer to the TiO_{2} conduction band, followed by CTH122, CTH22, and NKX2697. From the perspective of LUMO + 1, a more efficient electron injection is also expected for CTH222. On the other hand, the best regeneration is obtained when the HOMO level is closer to the redox potential of the electrolyte; thus, the NKX2697 molecule has the best regeneration. In general, the dyes with thiophene units in the πbridge have HOMO levels closer to the redox potential than the dyes with thiazole units. With the M06/6311G(d) level of calculation, the HOMOLUMO energy gaps have the following trend: CTH122 < CTH222 < NKX2697 < NKX2677 < CTH12 < CTH22 < CTH21 < NKX2587. According to the above and considering that charge injection is the most important parameter in this study, the best dye molecule should be CTH222 or CTH122.
(a)
(b)
The electron densities of the HOMO and LUMO molecular orbitals are shown in Figure 4 at the M06/6311G(d) level of theory. In this representation, the charge separation is displayed: the HOMO density is concentrated mostly in the electrondonating region (coumarin) and the πbridge (thiophene, thiazole, or both), whereas the LUMO density is focused in the πbridge and the cyanoacrylic acid acceptor, which is attached to the TiO_{2} surface. This suggests that the charge separation occurs in the πbridge and is more evident when one increases the number of thiophene or thiazole units. Furthermore, the molecules with two units in the πbridge do not present differences but the molecules with three thiazole units show good charge separation in the bridge. This good charge separation may promote a more efficient charge transfer from HOMO to LUMO, and hence from LUMO to the TiO_{2} conduction band [39].
3.3. Chemical Reactivity Study
Using the M06 functional with 631G(d) and 6311G(d) basis sets, the molecular energies for the ionic and neutral species were obtained taking into account the gas phase geometry. After that, the chemical reactivity parameters electron affinity (A), ionization potential (I), chemical hardness (), electrophilicity index (ω), and electroaccepting power (ω^{+}) were calculated (in eV); the values are shown in Table 2. Both levels of calculation have good concordance; between the 631G(d) and 6311G(d) basis sets, an average variation in eV is presented as follows: 0.2 eV for A, I, and ω^{+} and 0.3 eV for ω; the η parameter does not have a significant variation. For better information analysis, the values of the chemical reactivity parameters with M06/6311G(d) are represented in Figure 5; electron affinity and ionization potential are not discussed since these are used for obtaining the rest of the parameters and therefore can be redundant. Considering chemical hardness as the resistance to intramolecular charge transfer [23, 40], a lower chemical hardness is desired. In Figure 5, decreases while the πbridge increases with thiophene or thiazole units, but in dyes with thiophene groups, the is lower than in dyes with thiazole groups.

(a)
(b)
The dyes NKX2697 and CTH122 have the same values (2.7 eV), 0.05 eV lower than of CTH222. NKX2697 and CTH122 are proposed as the best dyes for charge transfer and hence a better short circuit current density (), which is consistent with the charge separation shown in Figure 4 and a previous study by our group [41]. According to Parr et al. [42], ω is the measure of the stabilization energy of the molecular system; with the contribution of Gázquez et al. [24], the highest ω^{+} represents the highest electronaccepting ability. With the above in mind, higher ω and ω^{+} are desired. CTH222 has the highest ω among the coumarin dyes (4.13 eV); this value is 0.12 eV higher than CTH122 and 0.34 and 0.52 eV greater than CTH22 and NKX2697, respectively.
Besides, the ω^{+} presents the same tendency than ω, CTH222 being the highest with a difference of 0.07 eV in CTH122, 0.29 eV in CTH22, and 0.36 eV in NKX2697. Therefore, CTH222 has the best stabilization energy and the best electronwithdrawing ability, suggesting that it has the best charge transfer of HOMO to LUMO and therefore we expect convenient values. Further, CTH122 presents values very close to CTH222. As discussed, the previous analysis was performed considering the M06/6311G(d) level of calculation, but a very similar process can be applied with the 631G(d) basis set and reaches the same conclusions. On the other hand, it is more convenient to place the thiophene unit at the beginning of the πbridge; for example, CTH12 has lower , higher ω, higher ω^{+}, and higher f than CTH21.
4. Conclusions
The methodology employed in this study with the M06 functional and the 631G(d) and 6311G(d) basis sets showed good results. The chemical reactivity parameters were in agreement with the energy levels and HOMO/LUMO orbital densities. It can be a good and fast methodology to study a group of candidate molecules as photosensitizers, allowing one to work with a big group of molecules and leave a smaller group for more intensive study. Molecules with better photophysical properties were obtained by increasing the thiophene and/or thiazole units in the πbridge. The CTH122 molecule can be considered the best dye when considering all studies, which results in a DSSC with a higher efficiency; however, CTH222 showed very similar properties to CTH122. Regarding the molecules with two units, the best was CTH22 (using thiazole units instead of thiophene units), but CTH12 had similar properties. Hence, using a combination of thiophenes and thiazoles in the πbridge may be a good idea.
Disclosure
Daniel GlossmanMitnik is a researcher of CIMAV and CONACYT. Jesús BaldenebroLópez is a professor and researcher at the UAS and CONACYT.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT) and Centro de Investigación en Materiales Avanzados, S.C. (CIMAV) and the Universidad Autónoma de Sinaloa (UAS). Rody SotoRojo gratefully acknowledges a fellowship from CONACYT.
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Copyright © 2016 Rody SotoRojo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.