- Email: [email protected]
X3− (X = I, Br)
Dyes and Pigments 120 (2015) 74e84
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Discovering the intermediate of dye regeneration in dye-sensitized solar cells: Theoretical investigations on the interaction between organic dye with different donors and X =X 3 (X ¼ I, Br) Mo Xie, Jian Wang, Fu-Quan Bai*, Li Hao, Hong-Xing Zhang* State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 20 March 2015 Accepted 24 March 2015 Available online 14 April 2015
Dye regeneration was a key process to influence lifetime and open-circuit voltage of dye-sensitized solar cells. In order to explore the mechanism of dye regeneration, the intermediate forms of this reaction were searched by first principle calculations in this study. The possible intermediat forms were obtained by analyzing the reactivity of four organic dyes with different donors and two electrolyte ions (I and Br). Then comparing their structures, interaction energy and reaction free energy, most appropriate intermediate forms were screened out. Throughout the comparison between I and Br, we found that Br/Br-3 could be a good redox couple if the redox potential matched with the energy gap of dye. In particular, steric-hindrance was likely the most influential factor in determining the intermediate forms. It is suggested that triphenylamine homologues as donor groups may both interact with electrolyte easily and accordingly keep off the electrolyte from semiconductor effectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dye regeneration Intermediate form Density functional theory Organic dye Intermolecular interaction DSSC
1. Introduction Dye-sensitized solar cell presents human being a brilliant and clean approach to solve energy and environment issue, but keeps us waiting due to its low efficiency in light to current conversion [1e4]. From the point view of DSSC operating principle, the overall efficiency of DSSC is controlled by the following processes: a) the light harvest; b) electron injection; c) electron collection on the dye-semiconductor interface; d) dye regeneration [5e7]. The light harvest, electron injection and electron collection can usually be optimized and improved within dye itself or dye-semiconductor interface, the modus efforts have been devoted to overcome the bottleneck of DSSC by covering one or more aspects of them [8e14]. Until recently, some experimental measurements [15,16] and theoretical calculations [17e20] for the regeneration of Ru-based
* Corresponding authors. Tel.: þ86 431 8849 8966; fax: þ86 431 8894 5942. E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (H.-X. Zhang). http://dx.doi.org/10.1016/j.dyepig.2015.03.026 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
dyes have been done and the research contents include the reaction mechanism [21e23], counter ion effects [24,25], optical properties [26] and deprotonation effects [27], etc. However, for those cells fabricated with donoreacceptor (DeA) organic dyes, the regeneration process is less investigated. Unlike the widely studied Ru-dyes, ordered structure would be formed during the synthesis and preparation of catenulate organic dye molecule. Thus, the structure, energy levels and conjugate nature of DeA dye may fluctuate during the whole cell operation session, namely, it is hard to picture how electrolyte interact with dyes. The most widely used electrolyte in DSSC is I =I 3 redox couple, which has been successfully pushing the overall efficiency of organic cell up to 10% [28]. Additionally, another important character of cell is open-circuit voltage (Voc), which is defined as the potential energy difference of the redox couple and semiconductor. Redox couple as reagent directly participate the dye regeneration process and, thereby, influence Voc by controlling the potential difference of the redox couple semiconductor. In current contribution, four simple DeA organic dyes (same in A, but varies in D part) were selected as an example to explore the
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from this factor, the difference and similarity of I =I 3 and Br =Br3 will be analyzed herein to understand the periodic trends from bromine to iodine. We hope to figure out a clear description of dye regeneration process at theoretical level and may provide little tips for optimizing the electrolyte and improving cell efficiency.
2. Computational details A two-step regeneration mechanism has been put forward by Clifford [22] and prove to be suitable for organic dyes by Nyhlen [31].
Fig. 1. Schematic representation of Tþ, COþ, CAþ and Qþ.
dye regeneration process within molecule level of theoretical methods. We believe the above mentioned four typical donors may cover all issues in dye regeneration process. As a reference, we extend the dye-iodine interaction to Br =Br 3 which is in the same main group with I =I 3 but less employed and investigated [29,30]. To our knowledge, the biggest reason for under-appreciated Br =Br 3 redox couple is the difference between I =I3 and Br =Br on the redox potential: 0.5 V and 1.1 V, against NHE. Few 3 sensitizers with lower energy levels of highest occupied molecular orbital have received successfully the electron from Br =Br 3 . Apart
i h dyeþ þ I / dyeþ I
(1)
h i dyeþ I þ I /dye þ I 2
(2)
In the first step, charge transfer took place between dyeþ and I and results in a formation of intermediate [dyeþI]. The intermediate [dyeþI] as the reagent of the rate determining step (reaction (2)) [22] of the regeneration reaction, played an important role in the overall regeneration process. In the present study, four commonly used donors (triphenylamine, coumarin, carbazole and quinoline) are considered in order to seek out the possible intermediate [dyeþI] configurations. The four D-A dyes employed these donors are shown in Fig. 1, marked as T, CO, CA and Q. The geometries of four dyes before and after the interaction with electrolyte ion were optimized by employing B3LYP [32] hybrid functional with Grimme's DFT-D3 [33e35] dispersion correction. B3LYP-D3 has been proved to be an applicable method for modeling noncovalent interactions [36]. Comparing with some other functions (see Table S1), we confirmed the rationality of B3LYP-D3 aiming at our system. The basis set 631 þ G(d) [37] was adopted for C, H, O, N, S atoms and Stuttgart/ Dresden ECP basis set SDDAll [38] was for I atom. Frequency calculations were performed at the same level of theory to confirm the
Fig. 2. Optimized geometry structures of Tþ, COþ, CAþ and Qþ.
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Table 1 Natural Charges (e) for main atoms.
VðrÞ ¼
Atoms
T
CO
CA
Q
N C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 O1 O2
0.025 0.058 0.068 0.128 0.031 0.129 0.070 0.060 0.069 0.129 0.028 0.128 0.067 0.067 0.062 0.116 0.039 0.117 0.060
0.022 0.138 0.235 0.333 0.062 0.338 0.081 0.133 0.041 0.052 0.338 0.335 0.231 0.140 0.161 0.067 0.124 0.119 0.054 0.382 0.258 0.212
0.011 0.056 0.033 0.128 0.039 0.031 0.062 0.029 0.079 0.081 0.130 0.038 0.097 0.144 0.343
0.048 0.051 0.342 0.336 0.230 0.152 0.342 0.089 0.131 0.110 0.117 0.041 0.116 0.258
rationality of the optimized geometry and achieve the thermodynamic properties. The basis set superposition error (BSSE) included in interaction energies was achieved by using the counterpoise method. Solvent effects were considered using the Polarizable Continuum Model (PCM [39,40]) of SCRF procedure for acetonitrile, which was also employed experimentally. The basis set superposition error (BSSE) included in interaction energies was achieved by using the counterpoise method. All calculations were preformed with the Gaussian 09 program package [41]. In order to seek the possible intermediate, we use natural population analysis (NPA) to express the atom charge, spin density to show the activity of single electron, electrostatic potential (ESP) and average local ionization energy (ALIE) to reflect the reactivity of dye cations. The NPA charge was calculated by NBO 3.1 module embedded in Gaussian 09. Electrostatic potential V(r) [42] is the potential that is created at any point r by the molecule's nuclei and electrons, and is given by
X A
ZA jRA rj
Z
rðr 0 Þdr 0 jr 0 rj
in which ZA is the charge on nucleus A, located at RA, and r(r) is the molecule's electronic density. Is V(r) positive or negative depends upon whether the positive contribution of the nuclei or the negative one of the electrons is dominant at point r. Average local ionization energy I(r) [43] is given by
P IðrÞ ¼
ri ðrÞjεi j
i
rðrÞ
Here, ri(r) is the electronic density of the ith occupied atomic or molecular orbital and εi is its energy. I(r) gives the average energy required to remove an electron at point r in which the focus is on the point in space rather than on a particular orbital. The analysis of electrostatic potential and average local ionization energy were finished by Multiwfn 3.3 [44] based on the Gaussian calculated geometries. The mapped isosurface graphs of electrostatic potential (ESP) and average local ionization energy (ALIE) on vdW surface were rendered by VMD 1.9 program [45]. The interaction energies were calculated as follows:
DE ¼ E ½Ei þ Ed where DE is interaction energy, E is total energy and Ei and Ed are energies of electrolyte and dye. The reaction free energies (DG) is the free energy change of the first step reactions dyeþþI / [dyeþI] or dyeþ þ Br / [dyeþBr].DE and DG were applied here to evaluate the reasonability of the intermediates.
3. Result and discussion From the point view of cell operation mechanism, donor loses electrons after electron excitation and injection, thereby generate a analogous hole structure which can be reduced easily. Here, we focus our attention on the interaction between four selected donors and electrolyte to highlight the donor effect in dye reduction from other effects.
Fig. 3. Spin density of Tþ, COþ, CAþ and Qþ.
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Fig. 4. ESP mapped molecular vdW surface with local maxima of ESP and minima of ALIE on vdW surface of Tþ, COþ, CAþ and Qþ. The extrema of ESP (in kcal/mol) and ALIE (in eV) in involved in donor unit are labeled; the transparent texts correspond to the values of extrema at backside of the molecule.
3.1. Searching for dye regeneration intermediates of triphenylamine dye The optimized geometry of T cation is shown in Fig. 2. NPA charges and singly occupied molecular orbital (SOMO) are
illustrated in Table 1 and Fig. 3, respectively. According to the NPA charges, N atom of TPA and the C atoms which directly connect to N atom act as a positive center. In the two benzene rings which do not connect to acceptor group, the meta-position carbon atoms are the most negative atoms and followed by ortho-position and para-
Fig. 5. Interaction energy and reaction free energy of Tþ in all obtained configurations; all values are sorted from small to large order for I.
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Fig. 6. The intermediate forms with structural parameters of [TþI¡] and [TþBr¡]. All distances are in Å.
position ones contrasted to the positive atoms (C1 and C7). This rule does not suit the other benzene ring because the acceptor connected carbon atom (C16) is positive. The spin density demonstrates that the single electron distributes evenly on TPA. Therefore,
the N atom of TPA and surrounding C atoms are the most possible atoms to attract electron in TPA. From the perspective of electrostatic attraction (Figure S1), the negatively charged electrolyte ion is probably around this area. It is far from comprehensive to confirm the location of electrolyte ion depend on the analysis of NPA and spin density only. Thus, more advanced ESP and ALIE will be analysed here to predict the intermolecular dye regeneration interaction. ESP on molecular surface has been employed to predict reactivity successfully [46]. The most positive site of ESP possesses the strongest ability to attract nucleophiles and is the most possible site to combine the negatively charged electrolyte ion in this study. The local maxima of ESP are labeled in molecular surface in Fig. 4. The maxima of ESP almost locate in the space among three benzene rings and the most positive one is around the N atom. Fig. 4 also shows the ALIE on vdW surface. ALIE at point r is interpreted as the average energy needed to remove an electron at the point r. Thus, the smaller the ALIE is, the more active the electron is. The minima of ALIE distribute on all three benzene rings and almost locate above the CeC bonds, but the differences of these values are small. From the point of electrostatic attraction and electron activity, the electrolyte ion may attack the area around the extreme point of ESP and ALIE on molecular surface. Based on above conjectures, several [TþI] and [TþBr] intermediate configurations were designed and the structures were fully optimized. A pair of configurations where electrolyte ions lie on either side of N atom were marked as T1 and T1′ The configurations where electrolyte ions lie on either sides of the two benzene rings which is not connect to acceptor were marked as T2 and T2′, T3 and T3′. The last one that electrolyte ion lies between two benzene rings was marked as T4.The stable structures of [TþI] and [TþBr] are illustrated in Figure S2, whose rationality were confirmed by frequency calculations. In order to select the most possible [TþI] and [TþBr] configurations from them, the reaction free energies (DG) and the interaction energies (DE)were compared in Fig. 5 followed the value order of I. In general, the DE between Tþ and I are stronger than Br except for T4. The lowest DE is consistently in T1′ of both I and Br. The highest DE is in T4 for I and T3 for Br. However, there are differences between interaction energies and reaction free energies in the arrangement of values. The lowest and highest DG for Br are still in T1′ and T4, respectively. As for I, lowest and highest DG are in T1 and T4. It is worth noting that the DG of Br are lower than that of I in T3 and T3′. Depend on above energy analysis, The T4 configuration can be firstly eliminated to be the most possible intermediate form for both I and Br. It is easy to confirm the T1′ is the most possible intermediate form for Br, because both interaction energy and reaction free energy of T1′ the lowest. For another electrolyte ion I, the lowest DE appears in T1′ and next in T1, the results is on the contrary for DG. Although the lowest DE and DG for I belong to different interadiate configurations (T1′ and T1), the energy values do not appear to be much different from the second lowest ones (T1 and T1′). We thus speculate both of T1 and T1′ all could be the intermediate forms for I. In addition, Fig. 6 shows the structural parameters of intermediates. In T1, the IeN distance is 3.87 Å, which is similar to the van der Waals radii [47] of bonding atoms. T1′ configuration for I is structural similar to the {L0þ,I} which is found by Nyhlen [31], but the distances between I and some near atoms in T1′ are smaller. Moreover, the distances between Br and some near atoms in T1′ are even smaller, this result may be explained by the size of ionic radius. In this configuration, electrolyte ion may interact with reactive region of adjacent benzene rings.
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Fig. 7. Interaction energy and reaction free energy of COþ in all obtained configurations; all values are sorted from small to large order for I.
3.2. Searching for dye regeneration intermediates of coumarin dye Similar analyses were employed on coumarin dye cations (CO). The NPA charges of COþ show that the positive atoms are primarily the N atom and the benzene ring which contains C6, C7 and C8. The corresponding spin density shows that the single electron is scarcely distributed on the two hexatomic rings contained N atom but distributed evenly on benzopyran ring. As can be seen from Fig. 4, the positive regions of ESP are around N atom and O atom of coumarin and the maxima of ESP almost located in these regions. Two additional maxima of ESP are near the acceptor part. Furthermore, the minima of ALIE all distribute on benzopyran ring. Overall, all criteria point toward the similar result: the benzopyran ring is the reactive region of coumarin. Electrolyte ions were sited close the reactive region from different directions, the obtained stable structures and the naming of them are listed in Figure S3. The results confirmed what we speculated. Seen from Figure S2, all electrolyte ions are in the reactive region and around N atom. It is noticeable that I and Br influence the geometry of CO dye in CO3 significantly, but this phenomenon is not obvious in CO1, CO2 and CO2′. The interaction energies (DE) together with reaction free energies (DG) of these stable structures are illustrated in Fig. 7. The numeral values of I series DE are similar to that of Br series in any configurations. As for DG, the numeral values of Br series are smallar than that of I series except for CO1 configuration. The CO3 configuration can still be eliminated to be the possible intermediate form for both I and Br. Owing to the relatively high DG of CO1, expecially for Br, this configuration is unlikely the most stable intermediat form despite it poccesses the lowest DE. Comparing CO2 with CO2′, the DE and DG of CO2′ are lower. Thus, CO2′ may be the most possible intermediate form and its structural parameters were showed in Fig. 8. In intermediate configuration, I and Br situate below C7 and the distances between Br and CO cation are still smaller. This location provides enough space for electrolyte ion to attack the N atom and surrounding C atoms. 3.3. Searching for dye regeneration intermediates of carbazole dye For simply structural carbazole dye cation CA: i) positive charges are concentrated on N atom and C1, C7, C9 and C12; ii) the spin density is relatively even in hole molecule, but a little more in the pyrrole unit; iii) large positive values of ESP mainly distribute
in two regions: around the N atom and bond NeC13, beside C8 and C9. The maxima of ESP mostly appear near N, C13, C11 and C8 atoms; iv) the two benzene rings all have the distribution of ALIE minima. In the one which do not directly connect to acceptor parts, values of minima are smaller. The stable structures of CA throughout optimization and frequency calculations are much more than that of T and CO. From Figure S4, it can be seen that all situations of electrolyte ions are beside molecule plan. It is supposed that good planarity of CA cation results in the decreasing of steric hindrance so that the attacking of electrolyte ions is multifaceted. The interaction energies (DE) and reaction free energies (DG) of these stable structures are illustrated in Fig. 9. Different from the previous energy results of T and CO, the DE and DG for all combined Br configurations are higher than that for I. It is easy to distinguish that CA5′ is the intermediate form for both Br and I because both of two energy values indicate CA5′ is the lowest energetic configuration. Fig. 10 shows the structural parameters of CA5′ for Br and I. There is big space superiority of electrolyte ions to attack dye cation in CA5′. Electrolyte ions may fully interact with the benzene ring which not directly connects to the acceptor part. In addition, a similar conclusion can be also obtained: Br is closer to CAþ than I. 3.4. Searching for dye regeneration intermediates of quinoline dye As for quinoline dye cation (Q): i) the positively charged atoms are dispersed in benzene ring and hexatomic ring, there isn't internal connection among them possibly; ii) the single electron is active around the benzene ring and N atom, inactive in three methyls connected to the hexatomic ring; iii) large positive values of ESP mainly distribute in three areas: around N atom, the methyl connected to N and the hexatomic ring. The maxima of ESP almost appear around N atom and inside the hexatomic ring, they scarcely locate in benzene ring; iv) The minima of ALIE all situate beside the benzene ring. It is a little bit complicated in this system because the four different criteria point to different situations. In order to seeking the intermediate form widely, electrolyte ions were sited on each capable location, and five possible intermediate configurations were obtained. (Figure S5). The analysis of interaction energies (DE) and reaction free energies (DG) were also done to screen out the most possible intermediate configurations. As can be seen Fig. 11, the DE values of Br
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3.5. Assessment the electrolyte effects and deprotonation effects for intermediates
Fig. 8. The intermediate forms with structural parameters of [COþI¡] and [COþBr-]. All distances are in Å.
and I are similar for all configurations. The DG values of I are lower than DG values of Br in Q2′ and Q4, and the situation is on contrary in other configurations. The highest value for both DE and DG are obtained in Q4, so it is the impossible configuration. Q1 configuration have the lowest DE for Br and I, and third lowest DG. The lowest DG for Br and I appear in Q2 and Q2′,respectively. The corresponding DE of Q2 and Q2′ are secondary. This observation well confirmed that Q2 and Q2′ are the possible intermediate forms for Br and I, and their structural parameters are presented in Fig. 12. Br locates on the opposite position comparing with I, and Br is still closer dye cation than I.
In all discovered configurations, electrolyte ions situated near N atom of donors except for CA. It is probably because a): N atom and the surrounding atoms act as a positive center of donor; b) planar CA cation gains the stronger space superiority, other areas of donor can attract electrolyte ions easily beside N neighborhood. As a consequence, the number of stable configurations is the most in CA. Under ideal condition, electrolyte only reduce oxidized dye and do not recombinate the charge from semiconductor. Planar donor group can interact with electrolyte easily but hardly keep off electrolyte from semiconductor. However, TPA may relatively balance this two aspects. It will be very useful to examine the deprotonation effects on the intermediates to estimate the interact behavior between deprotonated dye and electrolyte ions. The geometry optimization, interaction energy and reaction free energy of the intermediates with the proton removal of the cyanoacrylate acid were calculated. Some structural results are illustrated in Fig. 13 and the energetic results are summarized in Supporting Information. In the deprotonation situation, the analogous intermediate configurations with previous mentioned strategy were obtained. Some of them are not stable and few intermediate configurations exhibited slight different from the corresponding structures without deprotonation. For easy identification and comparison, we labeled the deprotonated configurations as added ‘d’ following the used names to distinguish the deprotonated ones. In deprotonated T system, I-dT1, I-dT1′ and I-dT2 are more stable configurations in I-based intermediates. I-dT1′ is the dominate one among three configurations in both interaction energy and reaction free energy. Stable Br-dT3 and Br-dT4 are not obtained in the calculation. The interaction energy and reaction free energy of other Br-based intermediate configurations do not correlate very well. Br-dT1 and Br-dT1′ have their respective advantages. Fig. 13 shows the structural fluctuation before and after deprotonation of the selected intermediate forms. The configurations of selected intermediate do not change much. Besides the distortion of the cyanoacrylate acid and two outward benzenes of TPA, the structural fluctuation also reflected in the similar movement of I and Br from their original positions to the one benzene ring of TPA. All corresponding deprotonated intermediate configurations were obtained in CO system. The numeral orders of interaction energy and reaction free energy for I and Br are correlated well. We can easily identify that dCO2′ is the most stable configuration for both I and Br in deprotonation situation. It seems like that the selecting of the most appropriate intermediate form is not effected by deprotonation. From the perspective of structural changes before and after deprotonation, whole configuration maintains well, the acceptor part warps slight and both I and Br shift to where is more close to N atom of donor. In deprotonated Q system, different initial structures (Q2 and Q3) were optimized to the similar final structure. dQ2 and dQ3 in deprotonation situation can be regarded as the same configurations of Q2 and Q3 approximately. dQ2 possesses the lowest interaction energy and reaction free energy for both I and Br, leading to the highest stability of itself. By looking at the structural fluctuation in Fig. 13, we can summarize: 1) besides the distortion of cyanoacrylate acid, the deformation of Q dye under depronotation is slight; 2) I and Br are all more close to N atom of donor against two different initial states. Similar to CO system, the energy values of deprotonated Br-based intermediate are lower than deprotonated I-based intermediate.
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Fig. 9. Interaction energy and reaction free energy of CAþ in all obtained configurations; all values are sorted from small to large order for I.
The situation of CA system is most complicated. Firstly, IdCA6′ and Br-dCA1′ were not obtained by the optimization. Secondly, interaction energy and reaction free energy for I and Br are not correlate and the numeral trends are unordered except for dCA5 and dCA5′. Thirdly, the deprotonated configurations of dCA2′ and dCA3′, dCA5 and dCA5′ are almost identical. We can easily found that dCA5 or dCA5’ is the most possible intermediate form in deprotonation situation. As can be seen in Fig. 13, the location of electrolyte ions is quite different from the original position from both sides of CA dye. The electrolyte ions in dCA5 located beside the H atom and parallel with molecular plane of CA dye. 4. Conclusion In this study, we have performed theoretical prediction on the intermediate configurations for four organic dyes with different typical donors in DSSC. Four dye cations have been analyzed from a wide variety of aspects, including geometry, charge distribution, spin density, electrostatic potential and average local ionization energy. Based on these judgements, we designed and optimized possible intermediate forms then verified their
stability by frequency calculations. In addition, interaction energies and reaction free energies of these intermediate structures were calculated and analyzed in detail. The most appropriate intermediate forms of four dye cations have been flitered throughout the comparison of the two kinds of energy values of all possible configurations. In compared processes, Br is sometimes superior to I as a result of the lower value of interaction energy and reaction free energy. In the appropriate intermediate forms, Br is always closer to dye cations than I. To our best knowledge, Br/Br 3 was not commonly used owing to the high redox potential. Few dye possess such large energy gap to match the redox potential (Eredox) and reduction potential of the conduction band of semiconductor (ECB) simultaneously although high redox potential generate large open-circuit voltage (Voc). If the newly designed dye would accommodate the high redox potential, Br =Br 3 is a good redox couple. The obtained intermediate forms show that electrolyte ions interact with dye cations by attacking N atoms and the groups with strong space superiority. This is a reason why planar CA dye cation can provide more reactive sites to attract electrolyte ions than other dyes. However, to avoid the recombination of electrons from semiconductor with intermediate state, electrolyte should stay away
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Fig. 12. The intermediate forms with structural parameters of [QþI] and [QþBr]. All distances are in Å.
Fig. 10. The intermediate forms with structural parameters of [CAþI¡] and [CAþBr¡]. All distances are in Å.
from semiconductor. Planar dyes with strong space superiority could not fix the electrolyte ions around donor groups only, so it seems to balance the dye regeneration and recombination hardly. We suggest that TPA donor group can not only interact with
electrolyte but also keep electrolyte away from semiconductor. Moreover, the deprotonation effects upon the interact behavior between dyes and electrolyte ions were probed. The results indicate that values of interaction energy and reaction free energy reduce significantly and the intermediate configurations shift slightly. The investigation of the reactivity of dye cation is an effective pathway to predict the dye regeneration mechanism, further studies are required to confirm the transition states and reaction pathways.
Fig. 11. Interaction energy and reaction free energy of Qþ in all obtained configurations; all values are sorted from small to large order for I.
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Fig. 13. The structural fluctuation of the intermediate before (transparent) and after (solid) deprotonation.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21173096) and the State Key Development Program for Basic Research of China (Grant No.2013CB834801) and the Jilin Provincial Natural Science Foundation (Grant No. 201215031) and The Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20110061110018) and Research Fund for the Doctoral Program of Higher Education (Grant No.20130061120025) Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.03.026. References [1] O’regan B, Grfitzeli M. A low-cost, high-efficiency solar cell based on dyesensitized. Nature 1991;353:24. [2] Kalyanasundaram K, Gr€ atzel M. Applications of functionalized transition metal complexes in photonic and optoelectronic devices. Coord Chem Rev 1998;177(1):347e414. €tzel M. Molecular photovoltaics. Acc Chem Res 2000;33(5): [3] Hagfeldt A, Gra 269e77. €tzel M. Photoelectrochemical cells. Nature 2001;414(6861):338e44. [4] Gra €tzel M. Dye-sensitized solar cells. J Photochem Photobiol C Photochem Rev [5] Gra 2003;4(2):145e53. [6] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nanowire dye-sensitized solar cells. Nat Mater 2005;4(6):455e9. [7] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev 2010;110(11):6595e663. [8] Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. A high-light-harvesting-efficiency coumarin dye for stable dye-sensitized solar cells. Adv Mater 2007;19(8):1138e41. [9] Chen CY, Wu SJ, Wu CG, Chen JG, Ho KC. A ruthenium complex with superhigh light-harvesting capacity for dye-sensitized solar cells. Angew Chem 2006;118(35):5954e7. [10] Mozer AJ, Griffith MJ, Tsekouras G, Wagner P, Wallace GG, Mori S, et al. ZnZn porphyrin dimer-sensitized solar cells: toward 3-D light harvesting. J Am Chem Soc 2009;131(43):15621e3. [11] Koops SE, O'Regan BC, Barnes PR, Durrant JR. Parameters influencing the efficiency of electron injection in dye-sensitized solar cells. J Am Chem Soc 2009;131(13):4808e18. [12] Lü X, Mou X, Wu J, Zhang D, Zhang L, Huang F, et al. Improved-performance dye-sensitized solar cells using Nb-doped TiO2 electrodes: efficient electron injection and transfer. Adv Funct Mater 2010;20(3):509e15. [13] Martinson AB, Elam JW, Liu J, Pellin MJ, Marks TJ, Hupp JT. Radial electron collection in dye-sensitized solar cells. Nano Lett 2008;8(9):2862e6.
[14] Dang X, Yi H, Ham M-H, Qi J, Yun DS, Ladewski R, et al. Virus-templated selfassembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nat Nanotechnol 2011;6(6):377e84. [15] Feldt SM, Lohse PW, Kessler F, Nazeeruddin MK, Gratzel M, Boschloo G, et al. Regeneration and recombination kinetics in cobalt polypyridine based dyesensitized solar cells, explained using Marcus theory. Phys Chem Chem Phys 2013;15(19):7087e97. [16] Farnum BH, Jou JJ, Meyer GJ. Visible light generation of I-I bonds by Rutris(diimine) excited states. Proc Natl Acad Sci U S A 2012;109(39):15628e33. [17] Privalov T, Boschloo G, Hagfeldt A, Svensson PH, Kloo L. A study of the interactions between I-/I-3 redox mediators and organometallic sensitizing dyes in solar cells. J Phys Chem C 2009;113(2):783e90. [18] Hu C-H, Asaduzzaman AM, Schreckenbach G. Computational studies of the interaction between ruthenium dyes and X and X 2 , X¼Br, I, at. Implications for dye-sensitized solar cells. J Phys Chem C 2010;114(35):15165e73. [19] Asaduzzaman AM, Chappellaz GA, Schreckenbach G. Relationship between dyeeiodine binding and cell voltage in dye-sensitized solar cells: a quantummechanical look. J Comput Chem 2012;33(31):2492e7. [20] Asaduzzaman AM, Schreckenbach G. Interactions of the N3 dye with the iodide redox shuttle: quantum chemical mechanistic studies of the dye regeneration in the dye-sensitized solar cell. Phys Chem Chem Phys 2011;13(33):15148e57. [21] Schiffmann F, VandeVondele J, Hutter J, Urakawa A, Wirz R, Baiker A. An atomistic picture of the regeneration process in dye sensitized solar cells. Proc Natl Acad Sci U S A 2010;107(11):4830e3. [22] Clifford JN, Palomares E, Nazeeruddin MK, Gratzel M, Durrant JR. Dye dependent regeneration dynamics in dye sensitized nanocrystalline solar cells: evidence for the formation of a ruthenium bipyridyl cation/iodide intermediate. J Phys Chem C 2007;111(17):6561e7. [23] Mosconi E, Yum J-H, Kessler F, Gomez Garcia CJ, Zuccaccia C, Cinti A, et al. Cobalt electrolyte/dye interactions in dye-sensitized solar cells: a combined computational and experimental study. J Am Chem Soc 2012;134(47): 19438e53. [24] Asaduzzaman AM, Schreckenbach G. Computational studies on the interactions among redox couples, additives and TiO2: implications for dyesensitized solar cells. Phys Chem Chem Phys 2010;12(43):14609e18. [25] Kislenko SA, Amirov RH, Samoylov IS. Effect of cations on the TiO2/acetonitrile interface structure: a molecular dynamics study. J Phys Chem C 2013;117(20): 10589e96. [26] Lobello MG, Fantacci S, De Angelis F. Computational spectroscopy characterization of the species involved in dye oxidation and regeneration processes in dye-sensitized solar cells. J Phys Chem C 2011;115(38): 18863e72. [27] Xie M, Chen J, Bai F-Q, Wei W, Zhang H-X. Theoretical studies on the interaction of ruthenium sensitizers and redox couple in different deprotonation situations. J Phys Chem A 2014;118(12):2244e52. [28] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized solar cells with an organic photosensitizer featuring orderly conjugated ethylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22(5): 1915e25. [29] Wang Z-S, Sayama K, Sugihara H. Efficient eosin Y dye-sensitized solar cell containing Br-/Br3-electrolyte. J Phys Chem B 2005;109(47):22449e55. [30] Teng C, Yang X, Li S, Cheng M, Hagfeldt A, Wu LZ, et al. Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br/Br3 electrolytes. Chem A Eur J 2010;16(44):13127e38. ́ ́
84
M. Xie et al. / Dyes and Pigments 120 (2015) 74e84
[31] Nyhlen J, Boschloo G, Hagfeldt A, Kloo L, Privalov T. Regeneration of oxidized organic photo-sensitizers in Gratzel solar cells: quantum-chemical portrait of a general mechanism. Chem Phys Chem 2010;11(9):1858e62. [32] Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phy 1993;98(7):5648e52. [33] Grimme S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006;27(15): 1787e99. [34] Grimme S. Accurate description of van der Waals complexes by density functional theory including empirical corrections. J Comput Chem 2004;25(12):1463e73. [35] Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 2010;132(15):154104. [36] Torres E, DiLabio GA. A (nearly) universally applicable method for modeling noncovalent interactions using B3LYP. J Phys Chem Lett 2012;3(13): 1738e44. [37] Hariharan PC, Pople JA. The influence of polarization functions on molecular orbital hydrogenation energies. Theor Chim Acta 1973;28(3): 213e22. [38] Igel-Mann G, Stoll H, Preuss H. Pseudopotentials for main group elements (IIIa through VIIa). Mol Phys 1988;65(6):1321e8.
[39] Barone V, Cossi M. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J Phys Chem A 1998;102(11):1995e2001. [40] Cossi M, Barone V, Cammi R, Tomasi J. Ab initio study of solvated molecules: a new implementation of the polarizable continuum model. Chem Phys Lett 1996;255(4):327e35. [41] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian, Inc., Wallingford CT, gaussian 09, Revision D.01. Wallingford CT: Gaussian, Inc; 2009. [42] Murray JS, Sen K. Molecular electrostatic potentials: concepts and applications, vol. 3. Elsevier; 1996. [43] Sjoberg P, Murray JS, Brinck T, Politzer P. Average local ionization energies on the molecular surfaces of aromatic systems as guides to chemical reactivity. Can J Chem 1990;68(8):1440e3. [44] Lu T, Chen F. Multiwfn: a multifunctional wavefunction analyzer. J Comput Chem 2012;33(5):580e92. [45] Humphrey W, Dalke A, Schulten K. VMD: visual molecular dynamics. J Mol Graph 1996;14(1):33e8. [46] Manzetti S, Lu T. The geometry and electronic structure of aristolochic acid: possible implications for a frozen resonance. J Phys Org Chem 2013;26: 473e83. [47] Bondi A. van der Waals volumes and radii. J Phys Chem 1964;68(3):441e51.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Discovering the intermediate of dye regeneration in dye-sensitized solar cells: Theoretical investigations on the interaction between organic dye with different donors and X =X 3 (X ¼ I, Br) Mo Xie, Jian Wang, Fu-Quan Bai*, Li Hao, Hong-Xing Zhang* State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 20 March 2015 Accepted 24 March 2015 Available online 14 April 2015
Dye regeneration was a key process to influence lifetime and open-circuit voltage of dye-sensitized solar cells. In order to explore the mechanism of dye regeneration, the intermediate forms of this reaction were searched by first principle calculations in this study. The possible intermediat forms were obtained by analyzing the reactivity of four organic dyes with different donors and two electrolyte ions (I and Br). Then comparing their structures, interaction energy and reaction free energy, most appropriate intermediate forms were screened out. Throughout the comparison between I and Br, we found that Br/Br-3 could be a good redox couple if the redox potential matched with the energy gap of dye. In particular, steric-hindrance was likely the most influential factor in determining the intermediate forms. It is suggested that triphenylamine homologues as donor groups may both interact with electrolyte easily and accordingly keep off the electrolyte from semiconductor effectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dye regeneration Intermediate form Density functional theory Organic dye Intermolecular interaction DSSC
1. Introduction Dye-sensitized solar cell presents human being a brilliant and clean approach to solve energy and environment issue, but keeps us waiting due to its low efficiency in light to current conversion [1e4]. From the point view of DSSC operating principle, the overall efficiency of DSSC is controlled by the following processes: a) the light harvest; b) electron injection; c) electron collection on the dye-semiconductor interface; d) dye regeneration [5e7]. The light harvest, electron injection and electron collection can usually be optimized and improved within dye itself or dye-semiconductor interface, the modus efforts have been devoted to overcome the bottleneck of DSSC by covering one or more aspects of them [8e14]. Until recently, some experimental measurements [15,16] and theoretical calculations [17e20] for the regeneration of Ru-based
* Corresponding authors. Tel.: þ86 431 8849 8966; fax: þ86 431 8894 5942. E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (H.-X. Zhang). http://dx.doi.org/10.1016/j.dyepig.2015.03.026 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
dyes have been done and the research contents include the reaction mechanism [21e23], counter ion effects [24,25], optical properties [26] and deprotonation effects [27], etc. However, for those cells fabricated with donoreacceptor (DeA) organic dyes, the regeneration process is less investigated. Unlike the widely studied Ru-dyes, ordered structure would be formed during the synthesis and preparation of catenulate organic dye molecule. Thus, the structure, energy levels and conjugate nature of DeA dye may fluctuate during the whole cell operation session, namely, it is hard to picture how electrolyte interact with dyes. The most widely used electrolyte in DSSC is I =I 3 redox couple, which has been successfully pushing the overall efficiency of organic cell up to 10% [28]. Additionally, another important character of cell is open-circuit voltage (Voc), which is defined as the potential energy difference of the redox couple and semiconductor. Redox couple as reagent directly participate the dye regeneration process and, thereby, influence Voc by controlling the potential difference of the redox couple semiconductor. In current contribution, four simple DeA organic dyes (same in A, but varies in D part) were selected as an example to explore the
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from this factor, the difference and similarity of I =I 3 and Br =Br3 will be analyzed herein to understand the periodic trends from bromine to iodine. We hope to figure out a clear description of dye regeneration process at theoretical level and may provide little tips for optimizing the electrolyte and improving cell efficiency.
2. Computational details A two-step regeneration mechanism has been put forward by Clifford [22] and prove to be suitable for organic dyes by Nyhlen [31].
Fig. 1. Schematic representation of Tþ, COþ, CAþ and Qþ.
dye regeneration process within molecule level of theoretical methods. We believe the above mentioned four typical donors may cover all issues in dye regeneration process. As a reference, we extend the dye-iodine interaction to Br =Br 3 which is in the same main group with I =I 3 but less employed and investigated [29,30]. To our knowledge, the biggest reason for under-appreciated Br =Br 3 redox couple is the difference between I =I3 and Br =Br on the redox potential: 0.5 V and 1.1 V, against NHE. Few 3 sensitizers with lower energy levels of highest occupied molecular orbital have received successfully the electron from Br =Br 3 . Apart
i h dyeþ þ I / dyeþ I
(1)
h i dyeþ I þ I /dye þ I 2
(2)
In the first step, charge transfer took place between dyeþ and I and results in a formation of intermediate [dyeþI]. The intermediate [dyeþI] as the reagent of the rate determining step (reaction (2)) [22] of the regeneration reaction, played an important role in the overall regeneration process. In the present study, four commonly used donors (triphenylamine, coumarin, carbazole and quinoline) are considered in order to seek out the possible intermediate [dyeþI] configurations. The four D-A dyes employed these donors are shown in Fig. 1, marked as T, CO, CA and Q. The geometries of four dyes before and after the interaction with electrolyte ion were optimized by employing B3LYP [32] hybrid functional with Grimme's DFT-D3 [33e35] dispersion correction. B3LYP-D3 has been proved to be an applicable method for modeling noncovalent interactions [36]. Comparing with some other functions (see Table S1), we confirmed the rationality of B3LYP-D3 aiming at our system. The basis set 631 þ G(d) [37] was adopted for C, H, O, N, S atoms and Stuttgart/ Dresden ECP basis set SDDAll [38] was for I atom. Frequency calculations were performed at the same level of theory to confirm the
Fig. 2. Optimized geometry structures of Tþ, COþ, CAþ and Qþ.
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Table 1 Natural Charges (e) for main atoms.
VðrÞ ¼
Atoms
T
CO
CA
Q
N C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 O1 O2
0.025 0.058 0.068 0.128 0.031 0.129 0.070 0.060 0.069 0.129 0.028 0.128 0.067 0.067 0.062 0.116 0.039 0.117 0.060
0.022 0.138 0.235 0.333 0.062 0.338 0.081 0.133 0.041 0.052 0.338 0.335 0.231 0.140 0.161 0.067 0.124 0.119 0.054 0.382 0.258 0.212
0.011 0.056 0.033 0.128 0.039 0.031 0.062 0.029 0.079 0.081 0.130 0.038 0.097 0.144 0.343
0.048 0.051 0.342 0.336 0.230 0.152 0.342 0.089 0.131 0.110 0.117 0.041 0.116 0.258
rationality of the optimized geometry and achieve the thermodynamic properties. The basis set superposition error (BSSE) included in interaction energies was achieved by using the counterpoise method. Solvent effects were considered using the Polarizable Continuum Model (PCM [39,40]) of SCRF procedure for acetonitrile, which was also employed experimentally. The basis set superposition error (BSSE) included in interaction energies was achieved by using the counterpoise method. All calculations were preformed with the Gaussian 09 program package [41]. In order to seek the possible intermediate, we use natural population analysis (NPA) to express the atom charge, spin density to show the activity of single electron, electrostatic potential (ESP) and average local ionization energy (ALIE) to reflect the reactivity of dye cations. The NPA charge was calculated by NBO 3.1 module embedded in Gaussian 09. Electrostatic potential V(r) [42] is the potential that is created at any point r by the molecule's nuclei and electrons, and is given by
X A
ZA jRA rj
Z
rðr 0 Þdr 0 jr 0 rj
in which ZA is the charge on nucleus A, located at RA, and r(r) is the molecule's electronic density. Is V(r) positive or negative depends upon whether the positive contribution of the nuclei or the negative one of the electrons is dominant at point r. Average local ionization energy I(r) [43] is given by
P IðrÞ ¼
ri ðrÞjεi j
i
rðrÞ
Here, ri(r) is the electronic density of the ith occupied atomic or molecular orbital and εi is its energy. I(r) gives the average energy required to remove an electron at point r in which the focus is on the point in space rather than on a particular orbital. The analysis of electrostatic potential and average local ionization energy were finished by Multiwfn 3.3 [44] based on the Gaussian calculated geometries. The mapped isosurface graphs of electrostatic potential (ESP) and average local ionization energy (ALIE) on vdW surface were rendered by VMD 1.9 program [45]. The interaction energies were calculated as follows:
DE ¼ E ½Ei þ Ed where DE is interaction energy, E is total energy and Ei and Ed are energies of electrolyte and dye. The reaction free energies (DG) is the free energy change of the first step reactions dyeþþI / [dyeþI] or dyeþ þ Br / [dyeþBr].DE and DG were applied here to evaluate the reasonability of the intermediates.
3. Result and discussion From the point view of cell operation mechanism, donor loses electrons after electron excitation and injection, thereby generate a analogous hole structure which can be reduced easily. Here, we focus our attention on the interaction between four selected donors and electrolyte to highlight the donor effect in dye reduction from other effects.
Fig. 3. Spin density of Tþ, COþ, CAþ and Qþ.
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Fig. 4. ESP mapped molecular vdW surface with local maxima of ESP and minima of ALIE on vdW surface of Tþ, COþ, CAþ and Qþ. The extrema of ESP (in kcal/mol) and ALIE (in eV) in involved in donor unit are labeled; the transparent texts correspond to the values of extrema at backside of the molecule.
3.1. Searching for dye regeneration intermediates of triphenylamine dye The optimized geometry of T cation is shown in Fig. 2. NPA charges and singly occupied molecular orbital (SOMO) are
illustrated in Table 1 and Fig. 3, respectively. According to the NPA charges, N atom of TPA and the C atoms which directly connect to N atom act as a positive center. In the two benzene rings which do not connect to acceptor group, the meta-position carbon atoms are the most negative atoms and followed by ortho-position and para-
Fig. 5. Interaction energy and reaction free energy of Tþ in all obtained configurations; all values are sorted from small to large order for I.
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Fig. 6. The intermediate forms with structural parameters of [TþI¡] and [TþBr¡]. All distances are in Å.
position ones contrasted to the positive atoms (C1 and C7). This rule does not suit the other benzene ring because the acceptor connected carbon atom (C16) is positive. The spin density demonstrates that the single electron distributes evenly on TPA. Therefore,
the N atom of TPA and surrounding C atoms are the most possible atoms to attract electron in TPA. From the perspective of electrostatic attraction (Figure S1), the negatively charged electrolyte ion is probably around this area. It is far from comprehensive to confirm the location of electrolyte ion depend on the analysis of NPA and spin density only. Thus, more advanced ESP and ALIE will be analysed here to predict the intermolecular dye regeneration interaction. ESP on molecular surface has been employed to predict reactivity successfully [46]. The most positive site of ESP possesses the strongest ability to attract nucleophiles and is the most possible site to combine the negatively charged electrolyte ion in this study. The local maxima of ESP are labeled in molecular surface in Fig. 4. The maxima of ESP almost locate in the space among three benzene rings and the most positive one is around the N atom. Fig. 4 also shows the ALIE on vdW surface. ALIE at point r is interpreted as the average energy needed to remove an electron at the point r. Thus, the smaller the ALIE is, the more active the electron is. The minima of ALIE distribute on all three benzene rings and almost locate above the CeC bonds, but the differences of these values are small. From the point of electrostatic attraction and electron activity, the electrolyte ion may attack the area around the extreme point of ESP and ALIE on molecular surface. Based on above conjectures, several [TþI] and [TþBr] intermediate configurations were designed and the structures were fully optimized. A pair of configurations where electrolyte ions lie on either side of N atom were marked as T1 and T1′ The configurations where electrolyte ions lie on either sides of the two benzene rings which is not connect to acceptor were marked as T2 and T2′, T3 and T3′. The last one that electrolyte ion lies between two benzene rings was marked as T4.The stable structures of [TþI] and [TþBr] are illustrated in Figure S2, whose rationality were confirmed by frequency calculations. In order to select the most possible [TþI] and [TþBr] configurations from them, the reaction free energies (DG) and the interaction energies (DE)were compared in Fig. 5 followed the value order of I. In general, the DE between Tþ and I are stronger than Br except for T4. The lowest DE is consistently in T1′ of both I and Br. The highest DE is in T4 for I and T3 for Br. However, there are differences between interaction energies and reaction free energies in the arrangement of values. The lowest and highest DG for Br are still in T1′ and T4, respectively. As for I, lowest and highest DG are in T1 and T4. It is worth noting that the DG of Br are lower than that of I in T3 and T3′. Depend on above energy analysis, The T4 configuration can be firstly eliminated to be the most possible intermediate form for both I and Br. It is easy to confirm the T1′ is the most possible intermediate form for Br, because both interaction energy and reaction free energy of T1′ the lowest. For another electrolyte ion I, the lowest DE appears in T1′ and next in T1, the results is on the contrary for DG. Although the lowest DE and DG for I belong to different interadiate configurations (T1′ and T1), the energy values do not appear to be much different from the second lowest ones (T1 and T1′). We thus speculate both of T1 and T1′ all could be the intermediate forms for I. In addition, Fig. 6 shows the structural parameters of intermediates. In T1, the IeN distance is 3.87 Å, which is similar to the van der Waals radii [47] of bonding atoms. T1′ configuration for I is structural similar to the {L0þ,I} which is found by Nyhlen [31], but the distances between I and some near atoms in T1′ are smaller. Moreover, the distances between Br and some near atoms in T1′ are even smaller, this result may be explained by the size of ionic radius. In this configuration, electrolyte ion may interact with reactive region of adjacent benzene rings.
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Fig. 7. Interaction energy and reaction free energy of COþ in all obtained configurations; all values are sorted from small to large order for I.
3.2. Searching for dye regeneration intermediates of coumarin dye Similar analyses were employed on coumarin dye cations (CO). The NPA charges of COþ show that the positive atoms are primarily the N atom and the benzene ring which contains C6, C7 and C8. The corresponding spin density shows that the single electron is scarcely distributed on the two hexatomic rings contained N atom but distributed evenly on benzopyran ring. As can be seen from Fig. 4, the positive regions of ESP are around N atom and O atom of coumarin and the maxima of ESP almost located in these regions. Two additional maxima of ESP are near the acceptor part. Furthermore, the minima of ALIE all distribute on benzopyran ring. Overall, all criteria point toward the similar result: the benzopyran ring is the reactive region of coumarin. Electrolyte ions were sited close the reactive region from different directions, the obtained stable structures and the naming of them are listed in Figure S3. The results confirmed what we speculated. Seen from Figure S2, all electrolyte ions are in the reactive region and around N atom. It is noticeable that I and Br influence the geometry of CO dye in CO3 significantly, but this phenomenon is not obvious in CO1, CO2 and CO2′. The interaction energies (DE) together with reaction free energies (DG) of these stable structures are illustrated in Fig. 7. The numeral values of I series DE are similar to that of Br series in any configurations. As for DG, the numeral values of Br series are smallar than that of I series except for CO1 configuration. The CO3 configuration can still be eliminated to be the possible intermediate form for both I and Br. Owing to the relatively high DG of CO1, expecially for Br, this configuration is unlikely the most stable intermediat form despite it poccesses the lowest DE. Comparing CO2 with CO2′, the DE and DG of CO2′ are lower. Thus, CO2′ may be the most possible intermediate form and its structural parameters were showed in Fig. 8. In intermediate configuration, I and Br situate below C7 and the distances between Br and CO cation are still smaller. This location provides enough space for electrolyte ion to attack the N atom and surrounding C atoms. 3.3. Searching for dye regeneration intermediates of carbazole dye For simply structural carbazole dye cation CA: i) positive charges are concentrated on N atom and C1, C7, C9 and C12; ii) the spin density is relatively even in hole molecule, but a little more in the pyrrole unit; iii) large positive values of ESP mainly distribute
in two regions: around the N atom and bond NeC13, beside C8 and C9. The maxima of ESP mostly appear near N, C13, C11 and C8 atoms; iv) the two benzene rings all have the distribution of ALIE minima. In the one which do not directly connect to acceptor parts, values of minima are smaller. The stable structures of CA throughout optimization and frequency calculations are much more than that of T and CO. From Figure S4, it can be seen that all situations of electrolyte ions are beside molecule plan. It is supposed that good planarity of CA cation results in the decreasing of steric hindrance so that the attacking of electrolyte ions is multifaceted. The interaction energies (DE) and reaction free energies (DG) of these stable structures are illustrated in Fig. 9. Different from the previous energy results of T and CO, the DE and DG for all combined Br configurations are higher than that for I. It is easy to distinguish that CA5′ is the intermediate form for both Br and I because both of two energy values indicate CA5′ is the lowest energetic configuration. Fig. 10 shows the structural parameters of CA5′ for Br and I. There is big space superiority of electrolyte ions to attack dye cation in CA5′. Electrolyte ions may fully interact with the benzene ring which not directly connects to the acceptor part. In addition, a similar conclusion can be also obtained: Br is closer to CAþ than I. 3.4. Searching for dye regeneration intermediates of quinoline dye As for quinoline dye cation (Q): i) the positively charged atoms are dispersed in benzene ring and hexatomic ring, there isn't internal connection among them possibly; ii) the single electron is active around the benzene ring and N atom, inactive in three methyls connected to the hexatomic ring; iii) large positive values of ESP mainly distribute in three areas: around N atom, the methyl connected to N and the hexatomic ring. The maxima of ESP almost appear around N atom and inside the hexatomic ring, they scarcely locate in benzene ring; iv) The minima of ALIE all situate beside the benzene ring. It is a little bit complicated in this system because the four different criteria point to different situations. In order to seeking the intermediate form widely, electrolyte ions were sited on each capable location, and five possible intermediate configurations were obtained. (Figure S5). The analysis of interaction energies (DE) and reaction free energies (DG) were also done to screen out the most possible intermediate configurations. As can be seen Fig. 11, the DE values of Br
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3.5. Assessment the electrolyte effects and deprotonation effects for intermediates
Fig. 8. The intermediate forms with structural parameters of [COþI¡] and [COþBr-]. All distances are in Å.
and I are similar for all configurations. The DG values of I are lower than DG values of Br in Q2′ and Q4, and the situation is on contrary in other configurations. The highest value for both DE and DG are obtained in Q4, so it is the impossible configuration. Q1 configuration have the lowest DE for Br and I, and third lowest DG. The lowest DG for Br and I appear in Q2 and Q2′,respectively. The corresponding DE of Q2 and Q2′ are secondary. This observation well confirmed that Q2 and Q2′ are the possible intermediate forms for Br and I, and their structural parameters are presented in Fig. 12. Br locates on the opposite position comparing with I, and Br is still closer dye cation than I.
In all discovered configurations, electrolyte ions situated near N atom of donors except for CA. It is probably because a): N atom and the surrounding atoms act as a positive center of donor; b) planar CA cation gains the stronger space superiority, other areas of donor can attract electrolyte ions easily beside N neighborhood. As a consequence, the number of stable configurations is the most in CA. Under ideal condition, electrolyte only reduce oxidized dye and do not recombinate the charge from semiconductor. Planar donor group can interact with electrolyte easily but hardly keep off electrolyte from semiconductor. However, TPA may relatively balance this two aspects. It will be very useful to examine the deprotonation effects on the intermediates to estimate the interact behavior between deprotonated dye and electrolyte ions. The geometry optimization, interaction energy and reaction free energy of the intermediates with the proton removal of the cyanoacrylate acid were calculated. Some structural results are illustrated in Fig. 13 and the energetic results are summarized in Supporting Information. In the deprotonation situation, the analogous intermediate configurations with previous mentioned strategy were obtained. Some of them are not stable and few intermediate configurations exhibited slight different from the corresponding structures without deprotonation. For easy identification and comparison, we labeled the deprotonated configurations as added ‘d’ following the used names to distinguish the deprotonated ones. In deprotonated T system, I-dT1, I-dT1′ and I-dT2 are more stable configurations in I-based intermediates. I-dT1′ is the dominate one among three configurations in both interaction energy and reaction free energy. Stable Br-dT3 and Br-dT4 are not obtained in the calculation. The interaction energy and reaction free energy of other Br-based intermediate configurations do not correlate very well. Br-dT1 and Br-dT1′ have their respective advantages. Fig. 13 shows the structural fluctuation before and after deprotonation of the selected intermediate forms. The configurations of selected intermediate do not change much. Besides the distortion of the cyanoacrylate acid and two outward benzenes of TPA, the structural fluctuation also reflected in the similar movement of I and Br from their original positions to the one benzene ring of TPA. All corresponding deprotonated intermediate configurations were obtained in CO system. The numeral orders of interaction energy and reaction free energy for I and Br are correlated well. We can easily identify that dCO2′ is the most stable configuration for both I and Br in deprotonation situation. It seems like that the selecting of the most appropriate intermediate form is not effected by deprotonation. From the perspective of structural changes before and after deprotonation, whole configuration maintains well, the acceptor part warps slight and both I and Br shift to where is more close to N atom of donor. In deprotonated Q system, different initial structures (Q2 and Q3) were optimized to the similar final structure. dQ2 and dQ3 in deprotonation situation can be regarded as the same configurations of Q2 and Q3 approximately. dQ2 possesses the lowest interaction energy and reaction free energy for both I and Br, leading to the highest stability of itself. By looking at the structural fluctuation in Fig. 13, we can summarize: 1) besides the distortion of cyanoacrylate acid, the deformation of Q dye under depronotation is slight; 2) I and Br are all more close to N atom of donor against two different initial states. Similar to CO system, the energy values of deprotonated Br-based intermediate are lower than deprotonated I-based intermediate.
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Fig. 9. Interaction energy and reaction free energy of CAþ in all obtained configurations; all values are sorted from small to large order for I.
The situation of CA system is most complicated. Firstly, IdCA6′ and Br-dCA1′ were not obtained by the optimization. Secondly, interaction energy and reaction free energy for I and Br are not correlate and the numeral trends are unordered except for dCA5 and dCA5′. Thirdly, the deprotonated configurations of dCA2′ and dCA3′, dCA5 and dCA5′ are almost identical. We can easily found that dCA5 or dCA5’ is the most possible intermediate form in deprotonation situation. As can be seen in Fig. 13, the location of electrolyte ions is quite different from the original position from both sides of CA dye. The electrolyte ions in dCA5 located beside the H atom and parallel with molecular plane of CA dye. 4. Conclusion In this study, we have performed theoretical prediction on the intermediate configurations for four organic dyes with different typical donors in DSSC. Four dye cations have been analyzed from a wide variety of aspects, including geometry, charge distribution, spin density, electrostatic potential and average local ionization energy. Based on these judgements, we designed and optimized possible intermediate forms then verified their
stability by frequency calculations. In addition, interaction energies and reaction free energies of these intermediate structures were calculated and analyzed in detail. The most appropriate intermediate forms of four dye cations have been flitered throughout the comparison of the two kinds of energy values of all possible configurations. In compared processes, Br is sometimes superior to I as a result of the lower value of interaction energy and reaction free energy. In the appropriate intermediate forms, Br is always closer to dye cations than I. To our best knowledge, Br/Br 3 was not commonly used owing to the high redox potential. Few dye possess such large energy gap to match the redox potential (Eredox) and reduction potential of the conduction band of semiconductor (ECB) simultaneously although high redox potential generate large open-circuit voltage (Voc). If the newly designed dye would accommodate the high redox potential, Br =Br 3 is a good redox couple. The obtained intermediate forms show that electrolyte ions interact with dye cations by attacking N atoms and the groups with strong space superiority. This is a reason why planar CA dye cation can provide more reactive sites to attract electrolyte ions than other dyes. However, to avoid the recombination of electrons from semiconductor with intermediate state, electrolyte should stay away
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Fig. 12. The intermediate forms with structural parameters of [QþI] and [QþBr]. All distances are in Å.
Fig. 10. The intermediate forms with structural parameters of [CAþI¡] and [CAþBr¡]. All distances are in Å.
from semiconductor. Planar dyes with strong space superiority could not fix the electrolyte ions around donor groups only, so it seems to balance the dye regeneration and recombination hardly. We suggest that TPA donor group can not only interact with
electrolyte but also keep electrolyte away from semiconductor. Moreover, the deprotonation effects upon the interact behavior between dyes and electrolyte ions were probed. The results indicate that values of interaction energy and reaction free energy reduce significantly and the intermediate configurations shift slightly. The investigation of the reactivity of dye cation is an effective pathway to predict the dye regeneration mechanism, further studies are required to confirm the transition states and reaction pathways.
Fig. 11. Interaction energy and reaction free energy of Qþ in all obtained configurations; all values are sorted from small to large order for I.
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Fig. 13. The structural fluctuation of the intermediate before (transparent) and after (solid) deprotonation.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21173096) and the State Key Development Program for Basic Research of China (Grant No.2013CB834801) and the Jilin Provincial Natural Science Foundation (Grant No. 201215031) and The Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20110061110018) and Research Fund for the Doctoral Program of Higher Education (Grant No.20130061120025) Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.03.026. References [1] O’regan B, Grfitzeli M. A low-cost, high-efficiency solar cell based on dyesensitized. Nature 1991;353:24. [2] Kalyanasundaram K, Gr€ atzel M. Applications of functionalized transition metal complexes in photonic and optoelectronic devices. Coord Chem Rev 1998;177(1):347e414. €tzel M. Molecular photovoltaics. Acc Chem Res 2000;33(5): [3] Hagfeldt A, Gra 269e77. €tzel M. Photoelectrochemical cells. Nature 2001;414(6861):338e44. [4] Gra €tzel M. Dye-sensitized solar cells. J Photochem Photobiol C Photochem Rev [5] Gra 2003;4(2):145e53. [6] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nanowire dye-sensitized solar cells. Nat Mater 2005;4(6):455e9. [7] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev 2010;110(11):6595e663. [8] Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. A high-light-harvesting-efficiency coumarin dye for stable dye-sensitized solar cells. Adv Mater 2007;19(8):1138e41. [9] Chen CY, Wu SJ, Wu CG, Chen JG, Ho KC. A ruthenium complex with superhigh light-harvesting capacity for dye-sensitized solar cells. Angew Chem 2006;118(35):5954e7. [10] Mozer AJ, Griffith MJ, Tsekouras G, Wagner P, Wallace GG, Mori S, et al. ZnZn porphyrin dimer-sensitized solar cells: toward 3-D light harvesting. J Am Chem Soc 2009;131(43):15621e3. [11] Koops SE, O'Regan BC, Barnes PR, Durrant JR. Parameters influencing the efficiency of electron injection in dye-sensitized solar cells. J Am Chem Soc 2009;131(13):4808e18. [12] Lü X, Mou X, Wu J, Zhang D, Zhang L, Huang F, et al. Improved-performance dye-sensitized solar cells using Nb-doped TiO2 electrodes: efficient electron injection and transfer. Adv Funct Mater 2010;20(3):509e15. [13] Martinson AB, Elam JW, Liu J, Pellin MJ, Marks TJ, Hupp JT. Radial electron collection in dye-sensitized solar cells. Nano Lett 2008;8(9):2862e6.
[14] Dang X, Yi H, Ham M-H, Qi J, Yun DS, Ladewski R, et al. Virus-templated selfassembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nat Nanotechnol 2011;6(6):377e84. [15] Feldt SM, Lohse PW, Kessler F, Nazeeruddin MK, Gratzel M, Boschloo G, et al. Regeneration and recombination kinetics in cobalt polypyridine based dyesensitized solar cells, explained using Marcus theory. Phys Chem Chem Phys 2013;15(19):7087e97. [16] Farnum BH, Jou JJ, Meyer GJ. Visible light generation of I-I bonds by Rutris(diimine) excited states. Proc Natl Acad Sci U S A 2012;109(39):15628e33. [17] Privalov T, Boschloo G, Hagfeldt A, Svensson PH, Kloo L. A study of the interactions between I-/I-3 redox mediators and organometallic sensitizing dyes in solar cells. J Phys Chem C 2009;113(2):783e90. [18] Hu C-H, Asaduzzaman AM, Schreckenbach G. Computational studies of the interaction between ruthenium dyes and X and X 2 , X¼Br, I, at. Implications for dye-sensitized solar cells. J Phys Chem C 2010;114(35):15165e73. [19] Asaduzzaman AM, Chappellaz GA, Schreckenbach G. Relationship between dyeeiodine binding and cell voltage in dye-sensitized solar cells: a quantummechanical look. J Comput Chem 2012;33(31):2492e7. [20] Asaduzzaman AM, Schreckenbach G. Interactions of the N3 dye with the iodide redox shuttle: quantum chemical mechanistic studies of the dye regeneration in the dye-sensitized solar cell. Phys Chem Chem Phys 2011;13(33):15148e57. [21] Schiffmann F, VandeVondele J, Hutter J, Urakawa A, Wirz R, Baiker A. An atomistic picture of the regeneration process in dye sensitized solar cells. Proc Natl Acad Sci U S A 2010;107(11):4830e3. [22] Clifford JN, Palomares E, Nazeeruddin MK, Gratzel M, Durrant JR. Dye dependent regeneration dynamics in dye sensitized nanocrystalline solar cells: evidence for the formation of a ruthenium bipyridyl cation/iodide intermediate. J Phys Chem C 2007;111(17):6561e7. [23] Mosconi E, Yum J-H, Kessler F, Gomez Garcia CJ, Zuccaccia C, Cinti A, et al. Cobalt electrolyte/dye interactions in dye-sensitized solar cells: a combined computational and experimental study. J Am Chem Soc 2012;134(47): 19438e53. [24] Asaduzzaman AM, Schreckenbach G. Computational studies on the interactions among redox couples, additives and TiO2: implications for dyesensitized solar cells. Phys Chem Chem Phys 2010;12(43):14609e18. [25] Kislenko SA, Amirov RH, Samoylov IS. Effect of cations on the TiO2/acetonitrile interface structure: a molecular dynamics study. J Phys Chem C 2013;117(20): 10589e96. [26] Lobello MG, Fantacci S, De Angelis F. Computational spectroscopy characterization of the species involved in dye oxidation and regeneration processes in dye-sensitized solar cells. J Phys Chem C 2011;115(38): 18863e72. [27] Xie M, Chen J, Bai F-Q, Wei W, Zhang H-X. Theoretical studies on the interaction of ruthenium sensitizers and redox couple in different deprotonation situations. J Phys Chem A 2014;118(12):2244e52. [28] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized solar cells with an organic photosensitizer featuring orderly conjugated ethylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22(5): 1915e25. [29] Wang Z-S, Sayama K, Sugihara H. Efficient eosin Y dye-sensitized solar cell containing Br-/Br3-electrolyte. J Phys Chem B 2005;109(47):22449e55. [30] Teng C, Yang X, Li S, Cheng M, Hagfeldt A, Wu LZ, et al. Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br/Br3 electrolytes. Chem A Eur J 2010;16(44):13127e38. ́ ́
84
M. Xie et al. / Dyes and Pigments 120 (2015) 74e84
[31] Nyhlen J, Boschloo G, Hagfeldt A, Kloo L, Privalov T. Regeneration of oxidized organic photo-sensitizers in Gratzel solar cells: quantum-chemical portrait of a general mechanism. Chem Phys Chem 2010;11(9):1858e62. [32] Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phy 1993;98(7):5648e52. [33] Grimme S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006;27(15): 1787e99. [34] Grimme S. Accurate description of van der Waals complexes by density functional theory including empirical corrections. J Comput Chem 2004;25(12):1463e73. [35] Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 2010;132(15):154104. [36] Torres E, DiLabio GA. A (nearly) universally applicable method for modeling noncovalent interactions using B3LYP. J Phys Chem Lett 2012;3(13): 1738e44. [37] Hariharan PC, Pople JA. The influence of polarization functions on molecular orbital hydrogenation energies. Theor Chim Acta 1973;28(3): 213e22. [38] Igel-Mann G, Stoll H, Preuss H. Pseudopotentials for main group elements (IIIa through VIIa). Mol Phys 1988;65(6):1321e8.
[39] Barone V, Cossi M. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J Phys Chem A 1998;102(11):1995e2001. [40] Cossi M, Barone V, Cammi R, Tomasi J. Ab initio study of solvated molecules: a new implementation of the polarizable continuum model. Chem Phys Lett 1996;255(4):327e35. [41] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian, Inc., Wallingford CT, gaussian 09, Revision D.01. Wallingford CT: Gaussian, Inc; 2009. [42] Murray JS, Sen K. Molecular electrostatic potentials: concepts and applications, vol. 3. Elsevier; 1996. [43] Sjoberg P, Murray JS, Brinck T, Politzer P. Average local ionization energies on the molecular surfaces of aromatic systems as guides to chemical reactivity. Can J Chem 1990;68(8):1440e3. [44] Lu T, Chen F. Multiwfn: a multifunctional wavefunction analyzer. J Comput Chem 2012;33(5):580e92. [45] Humphrey W, Dalke A, Schulten K. VMD: visual molecular dynamics. J Mol Graph 1996;14(1):33e8. [46] Manzetti S, Lu T. The geometry and electronic structure of aristolochic acid: possible implications for a frozen resonance. J Phys Org Chem 2013;26: 473e83. [47] Bondi A. van der Waals volumes and radii. J Phys Chem 1964;68(3):441e51.