Exploring photophysical properties of metal-free
coumarin sensitizers: an e
fficient strategy to
improve the performance of dye-sensitized solar
cells
†
Jinghui Wang,aMing Li,abDan Qi,aWei Shen,aRongxing He*aband Sheng Hsien Linc An efficient strategy was provided by adopting different numbers of electron-deficient units (pyrimidyl and quinolyl) into parent coumarin sensitizers to obtain excellent absorption in the short-wavelength region (B2 band), which eventually improves the performance of DSSCs. Density functional theory calculations were performed on both free dyes and dye–TiO2 complexes. As expected, introducing a single electron-deficient unit results in a positive influence on the power conversion efficiency (h) of DSSCs because of the larger short-circuit current density (Jscis proportional to optical absorption (4LHE), charge separation, dye regeneration (4reg) and electron injection (4inject)) and the higher open circuit voltage (Voc). The introduction of more pyrimidine facilitates charge separation and favors effective electron injection, whereas the second quinoline displays the opposite effect. The results give guidance to design promising candidates for future DSSCs applications.
1.
Introduction
Dye-sensitized solar cells (DSSCs) have received immense attention due to high power conversion efficiency (h) and low cost as compared to conventional silicon-based solar cells.1–3So
far Ru complexes are one of the most promising dyes for DSSCs withh > 11%, but the usage of rare and expensive ruthenium severely limits their application.2Therefore, metal-free organic
dyes (facile molecular design and high molar extinction coeffi-cient) are now being proposed as an alternative to Ru-based sensitizers because they are cheaper, more abundant and less toxic.4–6
Coumarin 343 (C343) and its derivatives2,7,8are one of the
most promising families of organic dyes because they undergo efficient and fast electron injection successfully used in DSSCs, but their performance is limited by narrow photoresponse range in the whole visible region, unwelcomep-stacked aggre-gation and undesirable charge recombination.9,10 For these
reasons, the h of DSSCs using coumarin sensitizers is much
lower than that based on Ru and Zn complexes.11Therefore,
Hara et al.12 expanded the p-conjugation by inserting more
vinylene units (–CH]CH–) or p-conjugated rings13,14(such as
thiophene, benzene, furan or pyrrole) into thep-spacers and Rociosa et al.15 introduced –NH
2 group into skeleton of
coumarin to solve these problems. Despite these two strategies are helpful to broaden the absorption bands of coumarin sensitizers in long-wavelength region (Q band), the relatively weak and narrow absorption bands in short-wavelength region still severely attenuate the light capture capability of sensitizers and further restrict the efficiency of DSSCs.2,16 How to put
forward appropriate countermeasures to improve the absorp-tion properties (including the absorpabsorp-tion range and light har-vesting efficiency) in the whole visible region has become a topic much worthy of discussion. In this work, thus, different electron-decient units were incorporated into p-spacers of dyes to improve the absorption intensity and broaden the absorption range of coumarin sensitizers in short-wavelength region. Experimentally, Wong et al.17 testied that the
triphenylamine-based sensitizers bearing a pyrimidine unit display enhanced spectral responses in the red portion of the solar spectrum and exhibit high h. Subsequently, He et al.18
theoretically explored how the electron-decient pyrimidine affected the performance of the porphyrin sensitizers, and claried that the electron-decient units would lead to redshi absorption and efficient charge separation. Most recently, He et al.19further investigated the role of varied-length spacers in
electron transfer, especially the inuence of p-spacer modied by electron-decient units on electron transfer. They found that
aSchool of Chemistry and Chemical Engineering, Southwest University, Chongqing
400715, China. E-mail: [email protected]
b
Education Ministry Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, China
cDepartment of Applied Chemistry, Institute of Molecular Science and Center for
Interdisciplinary Molecular Science, National Chiao-Tung University, Hsinchu 300, Taiwan
† Electronic supplementary information (ESI) available: Detailed discussion and calculated results obtained by different functionals (BELYP, PBE0 or M062X). See DOI: 10.1039/c4ra07904c
Cite this: RSC Adv., 2014, 4, 53927
Received 31st July 2014 Accepted 15th October 2014 DOI: 10.1039/c4ra07904c www.rsc.org/advances
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electron injection driving force of model compounds gradually decreases as the increase of p-spacer length, whereas the introduction of electron-decient pyrimidinyl breaks the dependence ofp-bridge length. With all the information, we believe that the adoption of electron-decient units into the p-spacer is a feasible strategy to improve h of DSSCs. On the basis of the theoretical interpretation together with the experi-mental results, two questions are worthy of considering. Firstly, it is found that the performance of DSSCs is improved when electron-decient unit is adopted in p-bridge of metal organic dyes. So we are very curious to know whether the metal-free coumarin sensitizers that introduce electron-decient units can also improve the light-harvesting efficiency and further raise the performance of DSSCs. Secondly, how do the different electron-decient units and their amount inuence absorption spectra, intrinsic characters of electron transition and different electron transfer processes of free sensitizers? It is not sufficient to assess the performance of DSSCs with the related characters of free dyes. Thus, the characters of dye–TiO2complexes should be probed to understand the electron transfer process from the dyes to TiO2.8,10
Herein, four model coumarin-derived chromophores (NKX-2311,10,12 NKX-2677,20–22 NKX-270023 and NKX-288313,24 were
investigated to conrm the signicant role of p-spacer in promoting theh of DSSCs (see ESI† for detailed discussion). Then, different numbers of electron-decient units (pyrimi-dine17and quinoline25) were adopted into NKX-2677, NKX-2700
and NKX-2883 to discuss the two questions mentioned above. The structures of all investigated dyes were illustrated in Fig. 1. To further rationalize the strategy presented in this work, eight
dye–TiO2 complexes were designed to investigate their
geometrical and electronic structures, absorption spectra, electron injection efficiency.8We believe that the present effort
to address the requirements of highly efficient DSSCs is a helpful exploration.
The stimulated absorption spectra of NKX-2677,22
NKX-270023 and NKX-288324 reproduce the experimental results
very well for therst time, indicating that the present theoret-ical methods are reliable. Compared with the three parent coumarin dyes, the new designed sensitizers have more outstanding performance in optical absorption intensity and range. Generally, the strong spectral absorption does not always facilitate effectual charge separation, which means the perfor-mance of sensitizers is not only relied on the extrinsic spectral absorption intensity but also on the intrinsic character of electron movement upon photoexcitation.18To assess the
effi-ciency of charge separation, we apply a novel approach proposed by Le Bahers26,27to quantify the charge transfer (CT)
distance (L), the overlap (U) between the zones of density depletion and increment and the amount of transferred charge (De). The three parameters are evaluated only from the ground state (GS) and excited state (ES) total electronic densities. The central idea of this method is to compute the barycenters of the regions of increased/decreased of electronic densities. The results show that thorough charge-separation between the donor and the acceptor of sensitizers, and valid electron injec-tion and regenerainjec-tion can be gained by introducing
electron-decient units. Although the coumarin derivatives trigger intensive interest due to their application in DSSCs, as far as we know, theoretical studies on their dye–TiO2 complexes still remain rather limited. Moreover, the electron injection mech-anism will be inuenced strongly by the electronic coupling (V) between dye molecule and semiconductor conduction band.2,15
Thus, it is necessary to give a deeper insight into the perfor-mance of dye–TiO2complexes, aiming to make sure the charge transfer and dye–TiO2 interaction occurring at the semi-conductor interface. In order to demonstrate how the electron-decient modied p-bridge improve the performance of the DSSCs, comprehensive investigation, including electronic structures, absorption spectra and electron transfer for isolated coumarin dyes and dye–TiO2 complexes, will be presented in the present work.
2.
Computational details
All calculations were implemented using Gaussian 09
program.28Firstly, we paid close attention to the selection of the
most appropriate functional to get the vertical excitation ener-gies and suitable HOMO and LUMO energy levels (HOMO is the
Fig. 1 Chemical structure of twelve coumarin-based dyes.
highest occupied molecular orbitals and LUMO denotes the lowest unoccupied molecular orbitals) of dyes.3The HOMO and
LUMO levels of sensitizers were used to assess dye regeneration efficiency and electron injection efficiency. The ground-state geometry optimizations of four model compounds (NKX-2311, NKX-2677, NKX-2700 and NKX-2883) were performed in solvent ethanol using B3LYP/6-31G(d,p) level. The vertical excitation energies and energy levels of the four model systems were computationally explored using different functionals (including B3LYP,29PBE0,30CAM-B3LYP,31LC-WPBE,32M062X19
and WB97XD33) and the results were compared with
experi-ments. Calculations showed that the most suitable excitation energies were obtained by the M062X functional (listed in Table 1S†) and the mosttting HOMO energy levels were got by the PBE0 functional (shown in Table 2S† along with the experi-mental data). Besides, the dependencies of HOMO level and transition energy on molecular geometry were explored using the six functionals mentioned above, and it showed that the best results (listed in Table 2S and Table 3S†) were obtained when ground-state geometries were optimized by the B3LYP functional. Therefore, for the eight newly designed coumarin dyes and the corresponding dye–TiO2complexes, their UV/VIS spectra and HOMO/LUMO levels were respectively calculated by the M062X and PBE0 functionals based on the B3LYP opti-mized geometries. Solvation is essential for accurate represen-tation of molecules' excirepresen-tation energies in the realistic environment, thus the solvent effect (in ethanol) was taken into account using the polarized continuum model (PCM) for all calculations.34,35
Then, the geometrical and electronic features of the free dyes were studied in detail. To investigate the efficiency of electron injection, both the compositions of those orbitals participated in the main electronic transitions and the driving force for electron injection were computed. The driving force for electron injection is related to the redox potentials which can be obtained by the Born–Haber cycle.36The Born–Haber cycle is
connected with the geometrical optimization of the mono-valent and bi-mono-valent oxidized coumarin sensitizers. All redox potentials involved in our study were obtained versus the normal hydrogen electrode (NHE). Furthermore, to assess the efficiency of charge separation, we computed the electron density difference of electron transitions. The electron densities of all orbitals related to the electron transitions were calculated with code Multwfn 2.5.37The extent of charge separation was
quantied by the three parameters mentioned above, that is, the charge transfer distance (L), the overlap (U) between the zones of density depletion and increment, and the amount of transferred charge (De), which were calculated by DctViaCube soware suit.26
In order to study the dye–titania complexes, we selected a neutral, stoichiometric cluster of Ti16O32, by exposing anatase 101 surface reported in Persson's work,38which is enough large
to reproduce the electronic absorption spectra of the complexes. In this case a mixed basis set (standard LANL2DZ basis set for Ti atoms and 6-31G(d,p) basis set for C, N, S, H and O atoms) was used to perform geometry optimizations and TD-DFT calcula-tions. Moreover, to evaluate the coupling interaction between
dye and TiO2surface, the partial and total densities of states (PDOS and DOS) for the optimized complexes were investigated at the same theoretical level.
3.
Results and discussion
The overall power conversion efficiency (h) can be characterized by the short-circuit current density (Jsc), the open circuit voltage (Voc), and thell factor of cell (FF), the h can be expressed as:2
h ¼Jsc$Voc$FF Pinc
(1) where Pincis the total solar power incident light on the solar cell, 100 mW cm2for AM 1.5. Correspondingly, Jscis determined by the light-harvesting efficiency of dyes (4LHE), the efficiency of charge injection (4inject), the efficiencies of charge collection (4cc) and dye regeneration (4reg), it can be described by the following equation:39
Jscf 4LHE$4cc$4inject$4reg (2)
In present work, we started discussion about the two ques-tion menques-tioned above from two aspects (Jsc(including4LHE,4cc, 4injectand4reg) and Voc).
3.1 How do different electron-decient units and their number affect the performance of isolated dyes?
3.1.1 Adoption of single pyrimidine or quinoline unit. Firstly, single pyrimidine and quinoline units were adopted into model dyes to explore whether different electron-decient units would have same effect on the efficiency of DSSCs. By comparing the performance of pyrimidyl-bridged and quinolyl-bridged dyes, we except to identify which electron-decient unit is more benecial to improving h of coumarin-based DSSCs.
a. Absorption spectra. The simulated UV/VIS spectra of new designed dyes are presented in Fig. 2, and the related electronic transition data of all dyes are given in Table 4S.† Apparently, all dyes exhibit two major absorption bands appearing at 400–600 nm (B1 band) and 200–400 nm (B2 band), respectively. The absorption band at 200–400 nm is ascribed to a localized p–p* transition and the prominent band at 400–600 nm is attributed to the intramolecular charge transfer (ICT) excitation from donor to acceptor.
Calculated results shown that all the newly designed dyes with single electron-decient unit exhibit a wider and stronger B2 (200–400 nm) band compared with the model sensitizers. The maximum absorption peaks (lmax) and light capturing ability of dyes are almost unchanged with one electron-decient unit adopted. Taking NKX-2677 and NKX-2677-P1 as example, for B1 band, thelmaxof NKX-2677-P1 is slightly blue-shied by about 19 nm as compared with the reference dye NKX-2677. Moreover, light-harvesting efficiency of NKX-2677 and NKX-2677-P1 are 0.986 and 0.991, respectively. Obviously, the slight blue shi and the almost unchanged light capturing ability indicate the pho-tophysical properties in B1 band for coumarin dyes could be maintained with one electron-decient unit adopted. For B2 band, the location of absorption peak (l*) for NKX-2677 is
computed to be about 199 nm with oscillator strength f¼ 0.320. In comparison with NKX-2677, the NKX-2677-P1 displays a quite stronger absorption band at 199 nm with f¼ 0.557. Remarkably, the adoption of pyrimidine unit signicantly enhances the absorption intensity of B2 band, and further increases the light-harvesting efficiency, 4LHE (0.521 for NKX-2677 and 0.723 for NKX-2677-P1, as listed in Table 5S†). Accordingly, the adoption of electron-decient unit greatly improves absorption properties in short-wavelength region with maintaining the intense absorption in long-wavelength region, which further increases the overall 4LHEof dyes. Moreover, owing to stronger electron withdrawing ability, pyrimidine unit is more favorable tolling the optical absorption vacancy in short-wavelength region of coumarin sensitizers relative to quinoline unit.
b. Electronic structures and photoinduced charge transfer character. A favorable charge separation will facilitate electronic injection from excited dye to semiconductor surface, and then promote the photogenerated charge to be transported to the electrodes.2 Consequently, effective electron collection
effi-ciency (4cc) requires a good intramolecular charge separation,40 so we evaluated 4cc with molecular orbital distribution and three parameters (L,U and De), respectively.26
Generally, the sensitizers with push–pull structure are composed of an electron donor and an electron acceptor con-nected with conjugated p-spacer. In these systems, a photon absorption induces an electron transfer from the D part to the A
part to form a D+–p–A excited state. Ideally, the electron completely transfers from the donor and localizes on the acceptor upon excitation. But in most real cases, the transferred charge is delocalized from the zone in the vicinity of the donor to region nearby the acceptor, which makes the extent of charge separation is different. For DSSCs, the h would be decreased due to the incomplete charge separation, so quantifying the length and magnitude of the CT is of much concern to evaluate the performance of sensitizer. In this work, three parameters of CT mentioned above are used to quantify the degree of electron transfer. The total electron densities of initial andnal states and their centroids should be calculated.
The CT distance (L) is dened by two barycenters of the electronic density depletion (R) and increment (R+) zones upon excitation.
L ¼ |R+ R| (3)
The barycenters of electronic density depletion (R) region and the electronic density increment (R+) region associated with electronic transition should be dened by r+(r) andr(r):
Rþ¼xþ; yþ; zþ¼ ð rrþðrÞdr ð rþðrÞdr (4)
Fig. 2 Calculated UV/Vis absorption spectra four series sensitizers in ethanol solution using M062X functional and the 6-31G(d,p) basis set (a) NKX-2677, NKX-2677-P1 and NKX-2677-P2; (b) NKX-2700, NKX-2700-P1 and NKX-2700-P2; (c) NKX-2883, NKX-2883-P1 and NKX-2883-P2; (d) NKX-2883, NKX-2883-Q1 and NKX-2883-Q2.
and R¼ ðx; y; zÞ ¼ ð rrðrÞdr ð rðrÞdr (5)
To calculate the two barycenters, the increaser+(r)/decrease r(r) of the density due to the electronic transition is expressed as: rþðrÞ ¼ DrðrÞ if DrðrÞ . 0 0 if DrðrÞ\0 (6) and rðrÞ ¼ DrðrÞ if DrðrÞ\0 0 if DrðrÞ . 0 (7)
In eqn (6) and (7),Dr(r) is dened as: Dr(r) ¼ rES(r) rGS(r)
(8) whererES(r) andrGS(r) are the electronic densities associated to excited and ground states. The fraction of electron exchange can be expressed as: De ¼ ð rþðrÞdr ¼ ð rðrÞdr (9)
The overlap (U) between the zones of density depletion and increment is written as:
U f (L H) (10)
H is dened as half of the sum of the centroids axis (s) along electron transfer direction.
To create an efficient charge-separated state, HOMO should be localized on the donor part, and LUMO should be localized on the acceptor.2 From Table 1, the introduction of single
electron-decient unit changes the charge population of HOMOs from entire molecules to donor groups of sensitizers, which leads to effective charge-separated state for coumarin sensitizer. To illustrate visually the electron transition, the electron density difference plots of S0/ S1excitation and the three parameters were calculated (Table 1). In electron density difference plots, the direction of electron transfer is from the green area to the blue one. Clearly, the adoption of single pyrimidine or quinoline promotes more charge transfer to the acceptor part of dyes, and further increases the values ofDe and L, such as 1.100e (NKX-2883) < 1.050e (NKX-2883-Q1) < 1.120e(NKX-2883-P1) and 9.403 ˚A (NKX-2883) < 10.405 ˚A (NKX-2883-P1) < 11.782 ˚A (NKX-2883-Q1). It indicates that adding one pyrimidine or quinoline unit into dye contributes to the formation of effective charge-separated state. In addition, the overlap (U) between the zones of density depletion and incre-ment is also an important parameter to determine whether an effective charge separation between the donor and acceptor occurs or not. The weak overlap is accompanied by a good
charge separation. As shown in Table 1, with one pyrimidine or quinoline adopted, the HOMO electron density of single electron-decient unit tailored dyes is mainly localized on the coumarin 343 moiety (the donor part), which decreases the values of overlap (U), the calculated overlap (U) complies with the order ofUNKX-2883-P1(0.192) <UNKX-2883-Q1(0.216) <UNKX-2883 (0.350). Again, the present theoretical results suggested that the adoption of single electron-decient unit (especially pyrimi-dine) is advantageous to the effective charge separation because of the weaker overlap.
c. Electron injection and regeneration for sensitizers. It is well known that the charge injection is one of the key electron transfer processes and is important for the performance of DSSCs devices.2In present work, we estimate the efficiency of
electron injection from three aspects, that is, the energy levels of the acceptor fragment (conjugated p-spacer and anchoring group) and the donor of sensitizer, the electronic coupling between chromophores and TiO2surface, and driving force of electron injection.19Also, dye regeneration is one of the most
central electron transfer processes for the overall DSSCs performance in terms of both the reuse and long life expec-tancies of sensitizers. The regeneration efficiency can be eval-uated by the driver force of dye regeneration. The driving forces depend strongly on the ground state oxidation potential of sensitizers (Eox(dye)).19
Eox(dye)z 3HOMO+ 4.44 eV (11)
In this paper, all calculated results are gained by the PBE0 functional. The calculated (Table 2S†) for NKX-2677, NKX-2700 and NKX-2883 are 0.900 eV, 0.810 eV and 1.022 eV, respectively. These results are well in agreement with the experimental values (0.91 eV for NKX-2677,220.82 eV for NKX-270023and 0.97
eV for NKX-2883).24The small discrepancy between theory and
experiment (<0.1 eV) suggests that the present theoretical method is reliable.
In general, the electron transfer is described as that electron is supposed to move from the LUMO of coumarin to the LUMO of semiconductor tunneling through the LUMO ofp-spacer.18
To estimate the efficiency of electron injection from dye to TiO2 surface (4inject), the energy levels of donor fragment (coumarin 343) and acceptor moiety (conjugatedp-spacer and anchoring group) were calculated (Table 2). Obviously, the LUMO energy levels of all of the acceptor groups are located below the LUMO energy level of C343 (1.503 eV), which leads the energy drops of LUMOdonor–LUMOacceptorfor pyrimidine-bridged or one-quinoline-bridged dyes be larger than those of their parent analogues. It indicates that the introduction of electron-decient unit is favorable for the charge transfer from C343 to acceptor (the p-spacer and anchoring group), and further increase the contributions from the anchoring group to LUMO of dyes (Table 3). As reported by Troisi et al.,41 the large
contribution from anchoring group of dye to LUMO will effec-tively strengthen electronic coupling between sensitizer and
conduction band of semiconductor. Strong electronic
coupling42,43promotes the electron injection from dye to titania.
From Table 3, our calculated results show that the contributions
from the anchoring group of new designed dyes with single electron-decient unit are comparatively lager than the corre-sponding reference dyes, which means the electronic coupling between dye and TiO2is effectively enhanced by modifying the p-spacer of coumarin dyes with electron-decient units for efficient electron injection.Additionally, the thermodynamic driving force (DG0) for electron injection is an important parameter to characterize the efficiency of electron injection, which provides an evidence to screen promising sensitizers for DSSCs. The driving force can evaluate the electron injection efficiency related to the corresponding vertical excitation:18
4injectf DG0 (12)
andDG0can be calculated by the following equation:36
DG0¼ E
CB DEver Eredox (13)
whereDEver represents the vertical excitation energy obtained from TD-DFT calculations. The value of ECBis the conduction band edge of TiO2(0.44V vs. NHE). DEredoxis the ground-state redox potentials of the studied system, which can be given by the Nernst equation.
Calculated driving forces of electron injection43 for all
sensitizers are also calculated and listed in Table 4. Intro-ducing single pyrimidine or quinoline into dyes can increase the driving force, and further make for electron injection. For
2677 and NKX-2677-P1, the value of DG0 for NKX-2677-P1
Table 1 Frontier molecular orbital spatial distribution and electron density difference plots of electronic transition S0/ S1for each coumarin dyes performed in ethanol solution using the M062X functional with 6-31G(d,p) basis set. L is the electron transfer distance (˚A); De is the fraction of electron exchange (|e|),U is overlaps between the regions of density depletion and increment (isovalue: 4 104e au3)
Molecule HOMO LUMO Picture f L (Ang) De U
NKX-2311 1.774 5.164 1.024 0.376 NKX-2677 1.851 9.748 1.118 0.136 NKX-2677-P1 2.067 10.449 1.149 0.110 NKX-2677-P2 1.789 11.24 1.197 0.059 NKX-2700 2.347 10.995 1.060 0.143 NKX-2700-P1 2.194 11.841 1.166 0.076 NKX-2700-P2 2.178 12.812 1.155 0.052 NKX-2883 2.255 9.403 1.100 0.350 NKX-2883-P1 2.001 10.405 1.120 0.192 NKX-2883-P2 2.024 11.182 1.144 0.168 NKX-2883-Q1 2.270 11.782 1.050 0.216 NKX-2883-Q2 2.117 12.934 1.073 0.241
(2.543 eV) is more negative than that of NKX-2677 (2.290 eV), it suggests that introducing single pyrimidine into dyes can increase the driving force for electron injection, and further make for electron injection. As shown in Table 4, all the newly designed sensitizers with single electron-decient units have more negative DG0 than their corre-sponding parent compounds. Especially, both the electron injection driving forces of NKX-2883-P1 and NKX-2883-Q1 are more negative than that of NKX-2883 (the order is NKX-2883 (2.210 eV) > NKX-2883-Q1 (2.219 eV) > NKX-2883-P1 (2.510 eV)). All results imply the introduction of single
electron-decient unit (especially pyrimidine unit) is conductive to improving the4injectof dyes.
The regeneration of sensitizers is also considered in the present research, and it can be estimated by the driver force of dye regeneration (DGreg).18
DGreg¼ Eox(dye)+ 0.3 (14)
As listed in Table 4, the regeneration driving forces are NKX-2677 (0.600 eV) > NKX-NKX-2677-P1 (0.665 eV) and NKX-2700 (0.510 eV) > NKX-2700-P1 (0.596 eV), indicating that the adoption of single pyrimidine unit is benecial to the dye regeneration. For NKX-2883-P1 and 2883-Q1, the values of regeneration driving forces also become more negative with the introduction of electron-decient unit and the order of driving force for dye regeneration is NKX-2883 (0.722 eV) > NKX-2883-Q1 (0.746 eV) > NKX-2883-P2 (0.754 eV). All calculated results imply the driving forces for dye regeneration should be more negative by adopting electron-decient unit (see Table 4). d. The open circuit voltage. For our investigated dyes, the adoption of electron-decient unit slightly affects the shi of conduction band edge of semiconductor, so the number of injected electron in the conduction band (nc) is the main factor that inuence the values of Voc. As reported by Kusama et al.,44 injected electron in conduction band of TiO2could recombine with electron acceptor (I2) and oxidized sensitizers leading to small nc. It is accepted that I2preferred to bind with N of CN, S of thiophene forming dye–iodine complexes available for serious charge recombination, which is the main route of photovoltage losses.16,45In this work, the absence of sulfur atom
in electron-decient unit (pyrimidine and quinoline) for NKX-2677-P1, NKX-2700-P1, NKX-2883-P1 and NKX-2883-Q1 could be benecial to suppress charge recombination, which improves the ncfor high Voc.
3.1.2 Introduction of a couple of pyrimidine or quinoline units. Secondly, a second pyrimidine or quinoline unit was adopted into single electron-decient unit tailored dyes to explore the quantity of electron-decient unit how to affect the propertied of dyes.
a. Absorption spectra. Similarly, the UV-VIS absorption spectra of 2677-P2, 2700-P2, 2883-P2 and NKX-2883-Q2 also exhibit two major absorption (B1 and B2) bands. The maximum absorption peak (lmax) of NKX-2677-P2 adopted the second pyrimidine unit blue-shis to 441 nm from 489 nm of NKX-2677-P1 (see Table 4S†) which could be explained by the fact that the dihedral angle formed between thep-spacer and coumarin 343 of NKX-2677-P2 is more than 10, whereas the dihedral angle of NKX-2677-P1 is less than 3. NKX-2677-P2 displays a stronger and red-shied B2 absorption band (l*) comparing with NKX-2677-P1. As shown in Table 4S,† the M062X functional simulated B2 band is red-shied with inserting a second pyrimidine moiety from NKX-2677-P1 to NKX-2677-P2 by 102 nm, and their corresponding oscillation strengths are 0.557 and 0.678, respectively. Thus, the calculated 4LHEof B2 band increases with the growing number of pyrim-idine unit (0.723 for NKX-2677-P1 and 0.790 for NKX-2677-P2). Obviously, the two pyrimidine or quinoline units in these four
Table 2 Calculated HOMO, LUMO energy levels (eV) of the donor (coumarin 343), the acceptor (conjugated p-spacer and anchoring group) fragments and LUMOdonor–LUMOacceptorof all sensitizer
Molecule 3LUMO 3HOMO 3LUMOdonor–LUMOacceptor
Coumarin 343 1.503 5.512 Frag-2311 2.665 7.620 1.162 Frag-2677 2.807 6.266 1.304 Frag-2677-P1 2.985 6.842 1.482 Frag-2677-P2 3.110 6.978 1.607 Frag-2700 2.839 5.927 1.336 Frag-2700-P1 3.021 6.364 1.518 Frag-2700-P2 3.108 6.938 1.605 Frag-2883 3.004 6.259 1.501 Frag-2883-P1 3.151 6.757 1.648 Frag-2883-P2 3.229 7.375 1.726 Frag-2883-Q1 3.022 6.490 1.519 Frag-2883-Q2 2.966 6.589 1.463
Table 3 Molecular orbital composition (in %) of the highest occupied and the lowest unoccupied molecular orbital of the twelve coumarin sensitizers performed in ethanol solvent using the M062X functional and the 6-31G(d,p) basis set
System Orbital Anchoring ligand p-Linker C343
NKX-2311 HOMO 11 12 77 LUMO 35 20 46 NKX-2677 HOMO 4 34 62 LUMO 39 49 12 NKX-2677-P1 HOMO 2 28 70 LUMO 44 46 10 NKX-2677-P2 HOMO 0 8 92 LUMO 48 47 4 NKX-2700 HOMO 3 43 54 LUMO 35 56 9 NKX-2700-P1 HOMO 3 33 64 LUMO 42 49 6 NKX-2700-P2 HOMO 0 10 90 LUMO 45 55 3 NKX-2883 HOMO 3 39 58 LUMO 24 56 19 NKX-2883-P1 HOMO 1 31 68 LUMO 35 54 11 NKX-2883-P2 HOMO 0 19 80 LUMO 41 52 7 NKX-2883-Q1 HOMO 1 34 65 LUMO 34 56 10 NKX-2883-Q2 HOMO 0 24 76 LUMO 42 54 4
dyes destroy the planarity of molecular backbone, which results in a signicant blue-shied B1 band. However, the situation of B2 band is just opposite to that of B1 band, that is, the intro-duction of a couple of pyrimidine or quinoline units effectively shis the absorption peaks (l*) of B2 band to longer wavelength region and signicantly enhances the corresponding absorp-tion intensity. Comprehensively, we considered that increasing the amount of electron-decient units is unfavorable for improving the4LHEof coumarin dye due to the obviously blue shi of lmax.
b. Electronic structures and photoinduced charge transfer character. As shown in Table 1, the electron density distribu-tions of HOMOs of the sensitizers with two electron-decient groups are more centralized on the coumarin 343 than that of the dyes with single electron-decient units, whereas all the sensitizers' LUMOs are localized on the linker and acceptor moieties. This reveals that the introduction of two electron-decient units changes the charge population of HOMOs and induces the better charge separation. Also, the three important parameters (L, De and U) of CT are calculated and used to discuss the property of charge transfer for the new sensitizers. Our calculated results show that with increase of the number of electron-decient pyrimidine the values of De and L are raised, whereas the value of the overlap is decreased. This means that the increase of pyrimidine would be helpful to improve the efficiency of charge separation. However, the overlap of NKX-2883-Q2 is larger than that of NKX-2883-Q1 (although they are still smaller than that of NKX-2883, the order is UNKX-2883-Q1 (0.2162) < UNKX-2883-Q2 (0.2410) < UNKX-2883 (0.3495), which indicates the adoption of single quinoline favors charge sepa-ration, but the introduction of couple of quinoline units could not further improve the efficiency of charge separation. Thus, the increasing number of quinoline is not suitable to improve the performance of coumarin-based DSSCs. The reason that pyrimidine unit motivates more effective charge separation
than quinoline group may come from the following facts: better coplanarity of pyrimidine with their neighboring groups and its stronger electron withdrawing ability.
c. Electron injection and regeneration for sensitizers. From Table 2, evidently, the LUMO energy levels of all the acceptor groups with two pyrimidine units are located below those of all the acceptor moieties with single pyrimidine, therefore the energy drop of LUMOdonor–LUMOacceptor would be effectively increased with more pyrimidine unit adopted. It suggests that the adoption of more pyrimidine units is favorable for charge separation between the C343 donor and the extended acceptor (thep-spacer and anchoring group) and further increases the contribution from the anchoring group to the LUMO of dye (Table 3). For example, the contributions for NKX-2883-P1 and NKX-2883-P2 respectively are 35% and 41%, which indicates that NKX-2883-P2 has stronger electronic coupling between sensitizers and TiO2 surface for effective electron injection. Also, as shown in Table 4, the more the adopted pyrimidine units, the higher the driving force of electron injection would be. However, due to the weaker electron withdrawing ability of quinoline unit and poor coplanarity of multiple quinoline-bridged dyes, it should be pointed out that the energy drop of 2883-Q2 (1.436 eV) is slightly smaller than that of 2883-Q1 (1.459 eV) and the values ofDG0(dye)become more negative with more quinoline introduced into coumarin dyes (2.219 eV (NKX-2883-Q1) >2.068 eV (NKX-2883-Q2)), which implies the introduction of more quinoline units weakens the charge separation and is not conductive to the electron injection process.
The driving force of dye regeneration (DGreg) is also calcu-lated and is used to investigate the relationship between the amount of electron-decient units and the efficiency of dye regeneration. As displayed in Table 4, the absolute values of driving forces for dye regeneration are increased with more pyrimidine or quinoline unit adopted, which means that
Table 4 EstimatedDG(aq),DG0,DEver, Eox(dye),DGreg, f and LHE for all coumarin sensitizers. All calculation is obtained by PBE0 functional with b-31G(d,p) basis set
Scheme DG(aq)a DG0b Eredoxc DEverd Eox(dye)e Eox(dye*)f DGregg fh LHEi
NKX-2311 10.302 2.894 5.862 2.528 1.131 1.397 0.831 1.626/0.345 0.976/0.548 NKX-2677 9.216 2.290 4.776 2.047 0.900 1.147 0.600 1.407/0.614 0.961/0.757 NKX-2677-P1 9.510 2.543 5.070 2.087 0.965 1.123 0.665 1.472/0.742 0.966/0.819 NKX-2677-P2 10.087 3.070 5.647 2.138 1.078 1.059 0.778 0.911/0.933 0.877/0.883 NKX-2700 8.890 2.048 4.450 1.962 0.810 1.152 0.510 1.825/0.710 0.985/0.805 NKX-2700-P1 9.241 2.436 4.801 1.925 0.896 1.028 0.596 1.575/0.891 0.973/0.872 NKX-2700-P2 9.674 2.704 5.234 2.089 1.055 1.034 0.755 1.080/1.305 0.917/0.951 NKX-2883 9.089 2.210 4.649 1.998 1.022 0.976 0.722 1.900/0.361 0.987/0.564 NKX-2883-P1 9.380 2.510 4.940 1.990 1.054 0.936 0.754 1.568/0.501 0.973/0.685 NKX-2883-P2 9.891 2.889 5.451 2.122 1.179 0.944 0.879 1.371/0.653 0.957/0.778 NKX-2883-Q1 9.140 2.219 4.700 2.042 1.046 0.995 0.746 1.695/0.577 0.980/0.735 NKX-2883-Q2 9.192 2.068 4.752 2.244 1.099 1.145 0.799 1.430/0.668 0.963/0.785
aCalculated Gibbs free energy change (DG
(aq),in V) due to the oxidation of the dyes in aqueous solutions. bThe driving force (DG0, in V) electron injection related to the electronic transition in B1 band.cRedox potential (E
redoxvs. NHE, in V).dThe vertical excitation energy (DEver, eV) corresponding to the maximum wavelength of spectral absorption.eThe oxidized potential (vs. NHE, E
ox(dye), in eV) of ground state.fThe oxidized potential of therst excited state (Eox(dye*)) following unrelax path for dyes.gThe regeneration energy (DGreg, in V).hThe oscillation strength of the two major absorption bands (B1 and B2 bands).iThe light harvest efficiency (LHE) corresponding to B1/B2 bands. Here the NHE is declared to be zero at all temperatures and its Gibbs free energy change takes the standard value of4.44 eV.
increasing the amount of pyrimidine and quinoline units could promote the regeneration of oxidized dyes.46,47 The effective
regeneration of oxidized dyes is benecial to the unidirectional charge transport and avoiding the charge recombination.48
Alebbi49pointed out that inefficient regeneration could limit the
device photocurrent. Therefore, the present theoretical results reveal that the sensitizers combining two electron-decient groups should have a vantage of regeneration of oxidized dyes.
3.2 How do different electron-decient units and their number affect electronic structures and absorption spectra of dye–TiO2complexes?
So far, we have described how the electron-decient units and their number affect the electronic and structural properties of free coumarin sensitizers, but it is not sufficient to look at the electronic structures of these systems individually. Thus, the next step in this contribution was to investigate the dye–titania complexes2,15,50 (including their electronic structures,
absorp-tion spectra, and electronic coupling between dye and TiO2). FT-IR8,18 study suggested that the sensitizers are chemically
adsorbed on the surface of nanocrystalline TiO2in a bidentate carboxylate mode. In this bidentate structure, both the carboxylic oxygen atoms are bound to twove-fold coordinated Ti atoms, and the acid hydrogen atom (H) is bound to the bridging oxygen on the TiO2 surface. Therefore we adopted the bidentate bridging mode for adsorption of all of the sensi-tizers on the TiO2nanoparticle surface. The optimized geome-tries for the four dye–TiO2 complexes are given in Fig. 3. Generally, upon excitation, charge injection from sensitizer to semiconductor can occur through direct mechanism or indirect
mechanism.50,51The direct mechanism is a“one-step” electron
injection from the ground state of sensitizer to the conduction band of semiconductor. In the direct-mechanism case, a new charge-transfer band should appear in the absorption spec-trum, but it is not present in the spectra of isolated sensitizer and semiconductor. In indirect mechanism, the charge isrstly provoked from the ground state of sensitizer to excited state which is also localized on dye itself, and then the charge transfers from excited state into the conduction band. The mechanism of electron injection plays a vital role in deter-miningh of DSSCs. In the following discussion, we focused on theseve dye–TiO2complexes (including NKX-2883/TiO2, NKX-2883-P1/TiO2, NKX-2883-P2/TiO2, NKX-2883-Q1/TiO2and NKX-2883-Q2/TiO2 complexes) to probe how the different electron-decient units and their number affect the performance of these newly designed dye–titania complexes.
For all dye–TiO2 complexes, the absorption properties are closely related to those of their free dyes. As presented in Fig. 1S–3S,† it is found that a broad and enhanced B2 absorp-tion is obtained for all the dye–titania complexes with single electron-decient unit, which lls the optical absorption vacancy in short-wavelength region and thus improves the photo-to-electric efficiency of coumarin-based DSSCs. While for those dye–TiO2complexes with two electron-decient groups, the efficiency is not signicantly enhanced due to the seriously blue-shied absorption in long-wavelength region, although the oscillator strength of B2 band is also strengthened.
Several photo-excitations are involved in the formation of absorption spectra of these ve complexes. A few of these excitations showing high oscillator strengths are given in Table
Fig. 3 Optimized geometries for the eight dye–TiO2complexes and their HOMO and LUMO orbitals relevant during photoexcitation.
4S.† Most of these excitations are mainly originated from HOMO to HOMO-2. The unoccupied orbitals involved in the major excitations mainly come from LUMO (Table 4S†). For theseve complexes, the change in electronic structure upon adsorption is small, which results in the energy of isolated photoexcited dyes to reside near the energy of excited states of dye–titania complexes. Thus, LUMOs of dye–titania complexes are very similar to those of free dyes, and therefore localized mainly on dyes, which means weak electronic coupling between dyes and TiO2.2 As reported in previous literature,15 the elec-tronic coupling between semiconductor conduction band and dye molecule strongly inuences the electron injection mech-anism. The direct mechanism requires a strong electronic coupling, while weak electronic coupling favors the indirect mechanism. It suggests that in the present cases the indirect mechanism should be more possible. Besides, the different number of pyrimidine and quinoline units only has a small effect on the electronic structures of these dye–titania complexes.
Aer binding to titanium there is a redistribution of energy levels.39 The computed Kohn–Sham orbital energy levels for
isolated dyes, (TiO2)16cluster and dye–TiO2complexes are pre-sented in Fig. 4. As reported in previous studies, the higher the LUMO is located in the semiconductor conduction band, the more efficient the electron injection is. From Fig. 4, LUMOs of NKX-2883-P1/TiO2and NKX-2883-Q1/TiO2are localized higher than that of NKX-2883/TiO2, which implies the introduction of single pyrimidine or quinoline unit contributes to providing
more sufficient driving force for electron injection. However, compared with NKX-2883-P1/TiO2 complex, the location of
LUMO for NKX-2883-P2/TiO2 complex is almost unchanged,
revealing that they have almost the same electron injection efficiency. Introducing one more quinoline unit into NKX-2883-Q1/TiO2makes the location of LUMO for NKX-2883-Q2/TiO2is lower, and therefore decreases the efficiency of electron injec-tion, which accords with our discussed results mentioned above. To better predict the efficiency of electron injection, we should adopt the molecular dynamics of the ultrafast electron injection in our further research.52–54
The total and partial densities of states18 for dye–TiO 2 complexes were used to illustrate electronic coupling between the dye and cluster, which are showed in Fig. 4S.† For all dye– titania complexes, the electronic densities of LUMO orbitals are mainly localized on the dyes with slight contribution on cluster, it suggests weak electronic coupling between dye and TiO2. This small contribution plays a key role in heterogeneous electron transfer. From Fig. 4S,† we clearly observe that with single pyrimidine and quinoline units adopted into NKX-2883/TiO2 complex, this small contribution is increased slightly, which indicates more mixing between dye and cluster orbitals leading to slightly enhanced electron coupling between dye and TiO2. With one more pyrimidine or quinoline units adopted into
NKX-2883-P1/TiO2 and NKX-2883-Q1/TiO2 complexes, the
contributions from the cluster to unoccupied molecular orbitals is almost unaltered, it implies the interfacial interaction between dye and TiO2cluster would be insensitive by increasing the number of pyrimidine or quinoline units.
In the present work, only two pyrimidine and quinoline units are used to study the number of electron-decient units how to inuences the performance of sensitizers and DSSCs. We believe it is enough because it has been found that the intro-duction of two electron-decient units seems unfavorable for improving the photophysical properties of dye–TiO2 systems. Additionally, in our previous report,19we have proved that the
extension the length of the p-spacer is disadvantageous to increase the performance of DSSCs.
4.
Conclusions
In summary, we offered a comprehensive discussion about the inuence of different electron-decient units and their amount on absorption property, charge separation, electron injection and dye regeneration. Briey, the Jsc(4LHE,4cc,4injectand4reg) and Voccan be effectively improved by adopting single pyrimi-dine or quinoline unit into model coumarin dyes. Moreover, owing better coplanarity with the neighboring groups and its stronger electron withdrawing ability, pyrimidine should be more suitable to improve the performance of DSSCs relative to quinoline. Further, with a second electron-decient unit adop-ted, a red-shied and strengthened absorption in short-wavelength region and a blue-shied absorption in long-wavelength region are obtained. The introduction of more pyrimidine units facilitates the electron transfer from donor to acceptor and favors the effective electron injection, whereas the second quinoline displays opposite effect. Despite the
Fig. 4 Molecular orbital energy level (eV) diagram of (column 1) 2311/ TiO2 complex, (column 2) NKX-2311 dye, (column 3) 2677/TiO2 complex, (column 4) NKX-2677 dye, (column 5) 2700/TiO2complex, (column 6) NKX-2700, (column 7) 2883/TiO2complex, (column 8) NKX-2883 dye, (column 9) isolated TiO2, (column 10) NKX-2883-P1 dye, (column 11) 2883-P1/TiO2complex, (column 12) NKX-2883-P2 dye, (column 13) 2883-P2/TiO2complex, (column 14) NKX-2883-Q1 dye, (column 15) NKX-2883-Q1/TiO2 complex, (column 16) NKX-2883-Q2 dye and (column 17) NKX-2883-Q2/TiO2 complex in ethanol solution. Green represents localization of molecular orbital both on the dye and the surface. Blue represents localization of molecular orbital on TiO2. Red represents localization of molecular orbital on dyes.
introduction of electron-decient unit makes for electronic coupling between dyes and TiO2, but cannot alter the indirect mechanism of electron injection for these investigated systems. The present theoretical exploration shows that the performance of metal-free coumarin sensitizers and the efficiency of the DSSCs are sensitive to the amount and variety of electron-decient units. We believe it is a promising future research direction to broaden and strengthen the absorption in short-wavelength region (B band) of metal-free coumarin sensitizer and further improve the efficiency of the DSSCs.
Acknowledgements
We acknowledge generousnancial support from the National Natural Science Foundation of China (no. 21173169 and 20803059), Chongqing Municipal Natural Science Foundation (cstc2013jcyjA90015) and the Fundamental Research Funds for the Central Universities (no. DXJK2013A008).
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