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Design strategies of metal free-organic sensitizers for dye sensitized solar cells: Role of donor and acceptor monomers
Organic Electronics 15 (2014) 1205–1214
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Design strategies of metal free-organic sensitizers for dye sensitized solar cells: Role of donor and acceptor monomers Chieh-Yu Tseng, Fadlilatul Taufany, Santhanamoorthi Nachimuthu ⇑, Jyh-Chiang Jiang ⇑, Der-Jang Liaw Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 10 February 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: Organic dyes DFT/TDDFT Dual band Donor–acceptor Absorption spectra
a b s t r a c t A series of metal free organic sensitizers have been designed and their optoelectronic properties for DSSC applications have been systematically investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. The role of donor/acceptor monomers on the electron donating/withdrawing abilities has been discussed and promising donor–acceptor combinations are screened. Based on this screening, some of novel metal free sensitizers have been designed and their electronic and spectral properties have been investigated using DFT/TDDFT methods. Our results show that the designed molecules are promising candidates to provide good performances as sensitizers in the DSSC applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Considering the environmental issues and renewable resources, more and more attentions have been paid on the solar energy utilizations. The dye sensitized solar cells (DSSCs) have attracted considerable interest after the report of O’Regan and Gratzel [1] due to the high photonto-current conversion efficiency, easy production and cost-effective properties. In recent years, number of sensitizer molecules have been developed, which includes metal-free organic dyes [2–5], non-ruthenium metal dyes [6–9] and ruthenium (II)-polypyridly complexes [10–12] and reported their performances in the DSSC applications. At present, the state-of-the-art DSSCs are based on the ruthenium metal complexes, such as N3/N719 and black dye, which hold the record of the overall efficiencies up to 11.5% under standard (Global AM 1.5) irradiation [13– 15]. However, there are some snags of ruthenium(II)-based ⇑ Corresponding authors. Tel.: +886 2 27376653; fax: +886 2 27376644. E-mail addresses: [email protected] (S. Nachimuthu), [email protected] (J.-C. Jiang). http://dx.doi.org/10.1016/j.orgel.2014.03.022 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
sensitizers, for instance, containing expensive ruthenium metal, requires careful synthesis and tricky purification steps [16] which make researchers to find other possible solutions. Metal-free sensitizers are prepared rather inexpensively and compared to ruthenium-based complexes, these have larger molar extinction coefficient and higher energy absorption bands. Moreover, the major advantage of metal-free sensitizers is their tunable absorption and optoelectronic properties through suitable molecular design strategies [16]. Generally, the design of metal-free dyes is based on linking of electron donor/acceptor (D–A) systems through p-conjugated bridges (i.e., D–p–A molecular structure). Many novel D–p–A metal-free dyes with various donor moieties, such as coumarin [4,17], indoline [18], triphenylamine [3], phenothiazine [2], carbazole [19], and pyrrole [20], have been designed and used as efficient sensitizers for the DSSC applications. However, the photon-to-current conversion efficiency based on those sensitizers has been achieved up to 9.5% only, which is relatively low compared to the conventional Ru based sensitizers. In order to increase the efficiency of DSSCs, it is important to design
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the suitable dyes, which have reasonable optical properties. This could be possible if we design the sensitizer with the absorption spectrum covers visible-to-near-infrared range, have significant intra-molecular charge separation after absorbing sun light, and it should stably bind to the semiconductor surface so that electrons can inject to surface continuously [16,21,22]. Herein, we aimed to design a potential metal-free sensitizer which overcomes the flaws of existing dyes and to find an efficient way to tune optoelectronic properties. Nowadays, there are two ways in which the spectral properties of D–A or D–p–A molecules can be improved: changing the conjugation length of the sensitizer, and substituting different electron -rich and -deficient units to modify the spectrum. Previously it is reported that, some D–A oligomers have dual-band optical absorption spectra which provide broad absorption characteristics [23,24]. In general, dual-band absorption spectrum arises from two separate transitions, which lead to either charge transfer between the donor and acceptor units along with a p–p* transition (so-called intra-molecular charge transfer, ICT) [25,26], or reorganization of molecular orbitals to produce accessible low- and high-lying energy levels spread across both donor and acceptor units (usually named as p–p* transition to make a distinction from ICT) [27–29]. In the present study, we selected 12 monomers which are systematically varied by both in the donor and acceptor-units, to form the different p-conjugated D–A oligomers and evaluated their optoelectronic properties. The selected 12 monomers are thiophene (T), thienopyrazine (TP), dithienopyrazine (DTP), thiadiazolothienopyrazine (TDTP), 1,4-dihydro-1-phenylpyrazine (PPP), cyclopentadithiophene (CDT), dicyanomethylidene-cyclopentadithiophene (CDM), 9-phenylcyclopentadithiophene (TPAT), N,N-bis (4-methoxy-phenyl)-thiophene-2-amine (MPTA), 10-phenyl-10H-phenothiazine (PTAZ), triphenylamine (TPA), 4-methoxy-N-(4-methoxyphenyl)-N-phenyl- benzeneamine (MPBA), and 9-phenyl-9H-carbazole (PC). These monomers are derived from the several compounds [30], which are universally used in the design of D–A oligomers, namely thienopyrazine, thiophene, thiazine, carbazole, and phenylamine compounds, thus the resulting 66 D–A oligomers could be applied as a model system to represent the general classes of D–A oligomers.
2. Computational details All the calculations in this study were performed with Gaussian 09 package [31]. The geometries of neutral monomers and metal-free sensitizers were optimized using B3LYP exchange correlation functional [32] combined with the standard double-f plus polarization basis set, 6-31G(d) [33]. Different basis sets and functionals in gas phase did not affect the structural parameters much, but it influences the optoelectronic properties such as excitation energies and intensities. Hence, we have performed a benchmark calculation in order to find the most suitable method for simulating UV–Vis absorption. For this, we considered four DFT methods such as, PBE0, B3LYP, BHandHLYP and CAM-B3LYP to calculate the UV–Vis
absorption spectra for TC-1 [34], T2-1 [2] and I-1 [35] molecules of D–A type and L1 [36], TA–St–CA [3] and MK2 [19] molecules of D–p–A type. We used 6-31G (d) basis set for all the benchmark calculations and the calculated excitation energies of D–A type and D–p–A type molecules are shown in Table 1. Compared with the experimental values of D–A and D– p–A types of molecules, the calculated results show that the absorption energy from B3LYP was more accurate for D–A system and BHandHLYP method show perfect agreements for D–p–A system which contains longer conjugated length. Considering that D–p–A backbone molecules are main structures in real DSSCs, therefore, we choose BHandHLYP functional to calculate the optoelectronic properties of D–p–A molecules including 20-lowest excitation energies and intensities of all the metal-free sensitizers considered here. The optoelectronic properties were transformed, using the SWizard program [37,38], into simulated spectra as described before, using Gauss functions with half-widths of 4000 cm1, as shown in following equation:
X fI ðx x1 Þ2 eðxÞ ¼ c1 exp 2:773 D 1=2;I D21=2;I 1
! ð1Þ
where e is the molar extinction coefficient given M1 cm1 in unit; the energy x of all the allowed transitions included in Eq. (1), is expressed in cm1; fI and D1/2 are the oscillator strength and the half-bandwidths, respectively. 3. Results and discussion 3.1. The classification of donor- and acceptor-monomers The molecular structures of 12 selected monomers considered in this study are summarized in Fig. 1. These selected monomers are then classified whether they belong to donor- or acceptor-monomer and then, every possible combination of different donor- and acceptor-unit is combined together to form 66 D–A oligomers. This classification is made on the basis of their vertical ionization potential (IPv), vertical electron affinity (EAv), and HOMO/ LUMO energy levels. As described by Dixon et al. [39], stronger donor has smaller IPv (easy to lose an electron) and higher HOMO level that would lose electrons easily; and stronger acceptor has more negative value of EAv (easily to gain an electron) and lower LUMO level which would accept electrons strongly. Therefore, we have calculated
Table 1 The calculated UV–Vis absorption energies (in nm) for D–A and D–p–A sensitizers using different DFT methods.
a
Sensitizers
PBE0
B3LYP
BHandHLYP
CAM-B3LYP
Exptl.a
TC-1 T2-1 I-1
451.7 441.6 435.9
401.7 461.7 453.3
354.3 367.2 374.4
359.0 370.2 381.6
400 452 483
L1 TA–St–CA MK2
482.4 504.5 575.2
506.9 534.2 612.8
405.8 409.3 481.5
404.3 403.0 472.6
405 410 480
Taken from Refs. [2,3,19,34–36].
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Fig. 1. Sketch of molecular structures of thienopyrazine, thiophene, thiazine, carbazole, and phenylamine-based derivative monomers.
the correlations between IPv vs. HOMO and EAv vs. LUMO, which are shown in Fig. S1. From Fig. S1, we found a linear relationship between the calculated IPv and HOMO and EAv with LUMO. From this relationship, we are able to identify the order of donor- and acceptor-monomers’ strengths. The strength of donor from strong to weak is: PPP > MPBA > MPTA > PTAZ > TPA > PC > CDT > DTP > TPAT > CDM > TDTP > TP > T and the order of acceptor strength is TDTP > CDM > DTP > TP > CDT > TPAT > PC > PTAZ > TPA > MPTA > MPBA > PPP > T. 3.2. Roles of donor and acceptor monomers in optoelectronic properties Next, we designed a series of p-conjugated D–A oligomers based on the order of electron donating- and electron withdrawing-abilities. One way to design these D–A oligomers is to distinguish what combination of donor and acceptor could have suitable UV–Vis absorption for the practical applications. Accordingly, the present study care-
fully evaluates the impact of the variation in strength of donor- and acceptor-units on the optoelectronic properties, such as energy gaps and the absorption spectra with their associated HOMOs-to-LUMOs electronic transitions. First, we begin with nine representative D–A oligomers such as, the strong donor PPP with five different-strength of acceptors (TDTP, DTP, CDT, PTAZ and MPBA) and the strong acceptor TDTP with different strength of donormonomers (PPP, MPTA, TPA, PC and CDM) and investigated their optoelectronic properties using TDDFT methods. Fig. 2 illustrates the electron distribution plots of the designed D–A oligomers along with their respective HOMO–LUMO energy gaps, respectively. As mentioned above, the HOMO/LUMO energy levels are interrelated to the corresponding electron donating and withdrawing abilities. Thus, in Fig. 2(b), a dramatic increase of LUMO energy levels is found as weakening the acceptor strength, which results in an increase in energy gaps. Also, the electron distribution plot (Fig. 2) indicates that the HOMOto-LUMO transitions vary from intra-molecular charge
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Fig. 2. The roles of (a) donor and (b) acceptor in governing the energy gap preferences of the selected p-conjugated D–A oligomers. In figures (a) and (b), the strength of donor- and acceptor-units, is varied from strong (left) to weak (right).
transfer (ICT) to p–p* transition while the strengths of acceptors are weaken. Contrarily, the HOMO levels gradually decrease while the strength of the donor becomes weaker, but the influence is not obvious as like acceptors and all the HOMO-to-LUMO transitions are having ICT character. These results indicate that, if two monomers are combined together to form an oligomer, the HOMO– LUMO energy gaps are more dominated by the acceptor counterpart. Although energy gaps are important indices when estimating the optoelectronic properties, UV–Vis absorption spectra are more reliable and direct data to evaluate how these sensitizers will perform in the solar cells. Thus, we have simulated the UV–Vis absorption of the designed D– A molecules and the spectra with corresponding transition assignments are shown in Fig. 3 and Table 2. As can be seen from the Fig. 3(a), the variation of donor strengths, i.e., from weak (CDM) to strong (PPP), does not affect the absorption spectra appreciably; the spectra remain dualband character, one absorption band is located at nearinfrared region, while the other is in the range of UV–Vis. Increasing the strength of donors makes the first absorption band red shifted and increases the intensities very little, (see Table 2) which are due to an increase in the degree
of charge separation. It has been observed for CDM–TDTP, TPA–TDTP, PPP–TDTP molecules, the transition characters below 400 nm are mainly contributed from p–p* transitions, whereas, PPP–TDTP has a low-lying satellite band which corresponds to ICT transition (k = 412 nm). The dual-band characteristic makes the absorption broad, which is beneficial for light-harvesting if the absorption spectrum covers visible region. On the other hand, the variation on the acceptor-monomers’ strength from weak-to-strong cases, the absorption band characteristics are transformed from mono- to dualband. The finding of this dual-band absorption characteristic is vital for the various applications in light harvesting and light-emitting devices. The strong donor (PPP) with different acceptors’ strengths (MPBA, CDT and TDTP) has been used for this illustration (Fig. 3(b)). From Table 2, the absorption of strong donor-weak acceptor (PPP–MPBA) oligomer is found only at UV region and it is mono-band characteristic which is due to p–p* transitions. However, when the strength of the acceptor counterpart increased, as represented in the case of PPP–TDTP molecule, a strong absorption band is noticed in the near-IR region (k = 1205.6 nm; originated from a ICT transition (Table. 2)) and also it has absorption band in the UV region as well
Fig. 3. The roles of (a) donor and (b) acceptor-units in governing the UV–Vis absorption spectra of the selected p-conjugated D–A oligomers. In figures (a) and (b), the strength of donor and acceptor -units, is varied from strong (top) to weak (bottom).
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Table 2 The calculated absorption energies (k), oscillator strengths (f), light harvesting efficiency (LHE) and transition characters of five representative D–A oligomers: CDM–TDTP, TPA–TDTP, PPP–TDTP, PPP–CDT and PPP–MPBA molecules. The states with f > 0.1 are shown. D–A molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
CDM–TDTP
S0 ? S1 S0 ? S7 S0 ? S8
1011.7 388.5 377
0.13 0.48 0.11
0.259 0.669 0.224
S0 ? S12 S0 ? S13
341.7 336.1
0.33 0.33
0.532 0.532
H?L (+100%)a H ? L + 2(+75%)b H-4 ? L (+64%)a H ? L + 2(+13%)b H-2 ? L + 1(+74%)b H-5 ? L (+49%)a
TPA–TDTP
PPP–TDTP
H-4 ? L (18%)a H-5 ? L (+20%)a
H-4 ? L + 1(12%)b
a
S0 ? S1 S0 ? S6 S0 ? S11 S0 ? S13
1058.7 388.4 332 317.2
0.15 0.58 0.29 0.27
0.292 0.737 0.487 0.463
H ? L (+100%) H ? L + 1(+92%)a H-8 ? L (+47%)b H ? L + 3(+94%)b
S0 ? S1 S0 ? S4 S0 ? S9 S0 ? S11
1205.6 412.3 333.6 321.9
0.19 0.47 0.27 0.25
0.354 0.661 0.463 0.438
H ? L (+100%)a H ? L + 1(+92%)a H-5 ? L (+67%)b H ? L + 4(+57%)b
H-7 ? L (37%)a
H ? L + 3(38%)b
a
PPP–CDT
S0 ? S1 S0 ? S3
435.9 339.4
0.81 0.44
0.845 0.637
H ? L (+100%) H-1 ? L (+86%)b
PPP–MPBA
S0 ? S2 S0 ? S7
370.6 321.6
1.05 0.11
0.911 0.224
H ? L (+92%)b H ? L + 3(+75%)a
‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p* Transition.
(Fig. 3(b)). The appearance of the absorption band at both UV and visible to near-IR region, thus justifies the dualband absorption characteristics. From the above results, it has been noticed that the influence of acceptors is more significant in determining the optoelectronic properties of designed D–A oligomers and also the overall spectra can be red shifted by increasing the strength of the donor. However, the simulated UV–Vis absorption spectra of the designed molecules are not enough to use as efficient sensitizer in DSSCs since, for efficient sensitizer, the absorption band should be broader and cover the whole visible region. Thus, we try to employ the molecules which have only dual-band characteristic to broaden the absorption spectrum in the visible region.
3.3. Design strategy based on dual-band characteristics Here, we proposed a design strategy on the basis of the requirements of high-efficiency metal-free sensitizer, including significant intra-molecular charge transfer, broad UV–Vis absorption spectra with high molar extinction coefficients and appropriate excitation energies, and proper geometries. In order to increase the degree of charge transfer when electronic excitation occurs, it is preferable to choose strong donors and strong acceptors counterparts for efficient sensitizers; for instance, the combinations of strong donors (PPP, MPBA and TPA) and strong acceptors (TDTP, CDM and TP). However, from the results shown in Fig. 3(b), even though the strongest acceptor, TDTP possess dual-band character and broad absorption
Fig. 4. UV–Vis absorption spectra of (a) MPBA–CDM based and (b) MPBA–TP based oligomers. The black solid line represents donor and acceptor system with anchoring group (D–A*CN), blue dash one is for donor and acceptor system with p-linker (D–p–A), and the red dash line is D–p–A*CN system.
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Table 3 Absorption energy (k), oscillator strength (f), light harvesting efficiency (LHE) and transition character of the singlet excited states of MPBA–CDM series calculated by BHandHLYP/6-31G(d) level of theory. The states with k > 300 nm and f > 0.1 are shown. Molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
MPBA–CDM*CN
S0 ? S1 S0 ? S2 S0 ? S3 S0 ? S5
733.3 472.9 405 323.6
0.22 0.12 1.25 0.20
0.397 0.241 0.944 0.369
S0 ? S6
312.6
0.64
0.771
H-1 ? L (+28%)a H ? L (30%)a H-1 ? L+1(+17%)a H-3 ? L (+25%)a H-1 ? L+1(10%)a H-6 ? L (+20%)a H ? L+2(+13%)b H-2 ? L (+17%)a
S0 ? S7
305.3
0.22
0.397
H ? L (+68%)a H-1 ? L (+58%)a H ? L+1(+71%)a H-6 ? L (+32%)a H-5 ? L (11%)a H-1 ? L+1(+21%)a H-3 ? L (20%)a H ? L+2(+45%)b
MPBA–T–CDM
S0 ? S1 S0 ? S3 S0 ? S5
729.3 374 309.2
0.09 1.52 0.54
0.187 0.970 0.712
H ? L (+46%)a H ? L+1(+79%)a H-6 ? L (+87%)a
H-1 ? L (+45%)a
MPBA–T–CDM*CN
S0 ? S1 S0 ? S3 S0 ? S6
760.7 429.2 340.9
0.29 1.65 0.42
0.487 0.978 0.620
H-1 ? L (+39%)a H-1 ? L+1(+20%)a H-1 ? L+1(30%)a
S0 ? S7
313.9
0.45
0.645
H ? L (+55%)a H ? L+1(+60%)a H ? L+1(+33%)a H ? L+2(20%)b H-8 ? L (+45%)a
H-7 ? L (+18%)a
Excitations in bold are assigned to important transitions. ‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p*.
spectrum, it’s long-wavelength absorption band observed at 1200 nm, which is far away from the visible region. Whereas, the acceptor, CDT which is not so strong, its first absorption peak shifted to visible region and also exhibits charge transfer character which is necessary for DSSCs (Table 2). According to these observations, we suggest design strategies as follows; it is better to consider the acceptor which has electron withdrawing ability between TDTP and CDT (i.e. CDM, DTP, and TP), substitute p-conjugated linker between electron-rich and electron-deficient groups and decorate acceptors with an anchoring group. Moreover, it is easy to tune the spectrum of CDT-based molecules by employing stronger donor. As discussed in previous section, the overall spectra of D–A oligomers have bathochromic shift when increasing the strength of donor counterpart (shown in Fig. 3(a)). In this work, thiophene is employed as a p linker to extent the conjugation, which results in high absorption intensities. Thiophene is among the most frequently used p-spacer in organic dyes due to its high structural stability, effective for the controlling intramolecular charge separation and optical properties [40,41]. Recent studies report the better performances of thiophene based dyes for DSSC application [42,43]. The 2-cyanoacrylic acid is commonly used as anchoring group for metal-free DSSCs and is a well-known electron acceptor which can help charge separation and bind strongly on TiO2 surface [5,44–46]. In brief, we selected strong donors, like PPP, MPBA and TPA, and strong acceptors, CDM, TP and CDT, with thiophene (p-bridge) and 2-cyanoacrylic acid (anchoring group), to design new sensitizers and investigated their optoelectronic properties.
3.4. The optoelectronic properties of designed D–p–A sensitizers The molecular structures of the selected MPBA–CDM, MPBA–TP, and MPBA–CDT oligomers are modified through
an introduction of p-linker, thiophene (T), and electron withdrawing substituent, 2-cyanoacrylic acid, (*CN) to form their subsequent analogue oligomers, termed as MPBA–T– CDM*CN, MPBA–T–TP*CN, and MPBA–T–CDT*CN, respectively. The schematic illustrations of the designed sensitizers are shown in Fig. S2 of Supplementary material. Fig. 4 shows the calculated UV–Vis absorption spectra of MPBA–CDM and MPBA–TP based series and their related transition data including absorption energies, oscillator strengths and transition assignments are summarized in Tables 3 and 4 (The results of MPBA–CDT series and other combinations of designed dyes are given in Tables S1–S2 and Figs. S3–S6 of Supplementary material). It has been observed that the UV–Vis absorption spectra of these two systems have dual band character. In MPBA–CDM series, long-wavelength band, named as Band 1, is located around 750 nm; and short-wavelength band, named as Band 2, is about 400 nm. Both absorption bands are in the visible region. But for MPBA–TP series, Band 1 and Band 2 are located at 530 nm and 350 nm, respectively. As can be seen from Fig. 4, the absorption intensity of the visible-region band is found to be increased due to the incorporation of the p-linker, which is in agreement with the previous experimental results [47,48]. Apart from this, it also red shifts the overall spectra of both the series significantly. On the other hand, considering the effect of electron withdrawing anchoring group, the intensities of Band 1 in both cases are found to be increased largely. Also, the overall absorption spectra of MPBA–CDM and MPBA–TP are bathochromic shift around 30 and 90 nm, respectively. It has been noted that the substitution of p-linker and electron withdrawing anchoring group with strong donors and strong acceptors can increase the molar extinction coefficients as well as red-shift the overall spectra. Also, we have calculated the light harvesting efficiency (LHE) of the designed dyes using the formula:
LHE ¼ 1—10f
ð2Þ
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Table 4 Absorption energy (k), oscillator strength (f), light harvesting efficiency (LHE) and transition character of the singlet excited states of MPBA–TP series calculated by BHandHLYP/6-31G(d) level of theory. The states with k > 300 nm and f > 0.1are shown. Molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
MPBA–TP*CN
S0 ? S1 S0 ? S4
514.2 326
0.99 0.36
0.898 0.563
H ? L (+92%)a H ? L+1(+72%)a a
MPBA–T–TP
S0 ? S1 S0 ? S2 S0 ? S4
472.6 349 319.3
0.63 0.12 0.65
0.766 0.241 0.776
H ? L (+77%) H-1 ? L (+64%)a H ? L+1(+73%)b
MPBA–T–TP*CN
S0 ? S1 S0 ? S4
563.6 335
1.24 0.54
0.942 0.712
S0 ? S5
320.4
0.21
0.383
H ? L (+84%)a H ? L+1(+46%)a H ? L+2(+16%)b H ? L+2(+69%)b
H-1 ? L+1(+18%)a H-1 ? L (+20%)a H ? L (17%)a
H-1 ? L+1(+27%)a H ? L+1(14%)a
Excitations in bold are assigned to important transitions. ‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p*.
where f is the oscillator strength of the designed dyes corresponds to the wavelength and the calculated values are given in the respective tables. As can be seen from those values, the LHE of the designed dyes are large, which is required to enhance the photocurrent response of the DSSCs. Figs. 5 and 6 show the high-lying and low-lying molecular orbitals of the most important excitations, which illustrate the either charge transfer or p–p* character, of MPBA–CDM and MPBA–TP based systems, respectively. For both the cases, the main assignment of Band 1 is due the transition from HOMO to LUMO, whereas for Band 2 is HOMO to LUMO+1. It can be seen from Fig. 5, all the transitions of MPBA–CDM series show the ICT character, which is required for the highly efficient sensitizers. However, in the HOMO to LUMO transition, the electrons are transferred from donor (MPBA) to acceptor counterpart (CDM) whereas, in the HOMO to LUMO+1 transition, the electrons are transferred to both acceptor and anchoring
groups, except in MPBA–T–CDM case. This indicates that, in MPBA–CDM*CN and MPBA–T–CDM*CN, the Band 2 is well charge separated and also it is in the visible region (405, 429 nm see Table 3), hence, it is expected that the excited electrons efficiently inject into the TiO2 surface. On the other hand, Band 1 of MPBA–CDM series is an ineffective for DSSC applications, since the electrons are localized in acceptor unit rather than the anchoring group after excitation. Compared with MPBA–CDM series, MPBA–TP series have better charge transfer characters because excited electrons are delocalized at both acceptor (TP) and anchoring group (shown in Fig. 6). This may be due to the electron withdrawing ability of CDM is relatively stronger than TP, which may compete the electron withdrawing ability of anchoring groups. This indicates that, the strength of the considered acceptor is too strong, some ineffective transitions may appear in the spectrum. It has been noticed that the calculated absorption spectra for the designed
Fig. 5. Representations of HOMO, LUMO and LUMO+1 corresponding to the important excitations of MPBA–CDM series shown in Table 3. Both HOMO to LUMO and HOMO to LUMO+1 reveal ICT character in all the cases.
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Fig. 6. Representations of HOMO, LUMO and LUMO+1 corresponding to the important excitations of MPBA–TP series shown in Table 4. Both HOMO to LUMO and HOMO to LUMO+1 reveal ICT character in all the cases.
sensitizers significantly red-shifted as compared to the conventional Ru based dyes (N3, C101, C106 and CYCB11) [49–52] which is ideal for the DSSCs. The results reveal that the optical properties of the metal free sensitizers can be tuned in accordance with the requirements by means of combining suitable donor and acceptor moieties. For instance, if we combine the strong acceptor and strong donor monomers, the long wavelength band may not be suitable for DSSC due to its less electron injection ability, but the short wavelength band shifted to visible region because of donor effect and it can be used to harvest the sun light. Also, the incorporation of thiophene and 2-cyanoacrylic acid enhances the intensity of the overall absorption spectra and making the spectra bathochromic shift through increasing conjugation length. 4. Conclusions In this contribution, we proposed a design strategy to improve the photon-to-current conversion efficiency of DSSCs based on the metal-free sensitizers using DFT calculations. From these theoretical results, we identified the electron -donating and-withdrawing abilities of selected monomers and investigated their roles in the optoelectronic properties. Further, we have systematically investigated the UV–Vis absorption spectra and electronic transition characters of selected D–p–A sensitizers and reported their spectral properties for DSSC applications. From our results, we found that the acceptor monomers play an important role in determining the optoelectronic properties. Further it is evident that the substitution of thiophene and 2-cyanoacrylic acid enhances the intensity of the absorption significantly and the overall absorption spectrum can be red shifted. The simulated spectra reveal that the characteristic of dual-band is present in the selected sensitizers, which is highly required for the efficient sensitizer. Particularly, in the MPBA–TP based series, the
long wavelength absorption band is due to the ICT character and importantly excited electrons not only localized on the acceptor, but also on the anchoring group: hence it is easy to inject the excited electrons into the TiO2 surface. In summary, we suggested some potential sensitizers which have suitable optoelectronic properties and we believe that these molecules can be used as an efficient sensitizes for the new generation of DSSCs. Acknowledgements We acknowledge the financial support from the National Science Council of Taiwan (NSC 101-2113-M-011004-MY3). We are also thankful to the National Center of High-Performance Computing (NCHC) for donating computer time and facilities. Appendix A. Supplementary material Supplementary material: the plots of vertical ionization potential (IPv), electron affinity (EAv), schematic illustrations of selected fragments and optoelectronic properties of MPBA–CDT based series are shown. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.orgel.2014.03.022. References [1] B. O’Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–739. [2] H.N. Tian, X.C. Yang, R.K. Chen, Y.Z. Pan, L. Li, A. Hagfeldt, L.C. Sun, Phenothiazine derivatives for efficient organic dye-sensitized solar cells, Chem. Commun. (2007) 3741–3743. [3] S. Hwang, J.H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.H. Lee, W. Lee, J. Park, K. Kim, N.G. Park, C. Kim, A highly efficient organic sensitizer for dye-sensitized solar cells, Chem. Commun. (2007) 4887–4889. [4] Z.S. Wang, Y. Cui, Y. Dan-Oh, C. Kasada, A. Shinpo, K. Hara, Thiophene-functionalized coumarin dye for efficient dye-sensitized
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Design strategies of metal free-organic sensitizers for dye sensitized solar cells: Role of donor and acceptor monomers Chieh-Yu Tseng, Fadlilatul Taufany, Santhanamoorthi Nachimuthu ⇑, Jyh-Chiang Jiang ⇑, Der-Jang Liaw Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 10 February 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: Organic dyes DFT/TDDFT Dual band Donor–acceptor Absorption spectra
a b s t r a c t A series of metal free organic sensitizers have been designed and their optoelectronic properties for DSSC applications have been systematically investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. The role of donor/acceptor monomers on the electron donating/withdrawing abilities has been discussed and promising donor–acceptor combinations are screened. Based on this screening, some of novel metal free sensitizers have been designed and their electronic and spectral properties have been investigated using DFT/TDDFT methods. Our results show that the designed molecules are promising candidates to provide good performances as sensitizers in the DSSC applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Considering the environmental issues and renewable resources, more and more attentions have been paid on the solar energy utilizations. The dye sensitized solar cells (DSSCs) have attracted considerable interest after the report of O’Regan and Gratzel [1] due to the high photonto-current conversion efficiency, easy production and cost-effective properties. In recent years, number of sensitizer molecules have been developed, which includes metal-free organic dyes [2–5], non-ruthenium metal dyes [6–9] and ruthenium (II)-polypyridly complexes [10–12] and reported their performances in the DSSC applications. At present, the state-of-the-art DSSCs are based on the ruthenium metal complexes, such as N3/N719 and black dye, which hold the record of the overall efficiencies up to 11.5% under standard (Global AM 1.5) irradiation [13– 15]. However, there are some snags of ruthenium(II)-based ⇑ Corresponding authors. Tel.: +886 2 27376653; fax: +886 2 27376644. E-mail addresses: [email protected] (S. Nachimuthu), [email protected] (J.-C. Jiang). http://dx.doi.org/10.1016/j.orgel.2014.03.022 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
sensitizers, for instance, containing expensive ruthenium metal, requires careful synthesis and tricky purification steps [16] which make researchers to find other possible solutions. Metal-free sensitizers are prepared rather inexpensively and compared to ruthenium-based complexes, these have larger molar extinction coefficient and higher energy absorption bands. Moreover, the major advantage of metal-free sensitizers is their tunable absorption and optoelectronic properties through suitable molecular design strategies [16]. Generally, the design of metal-free dyes is based on linking of electron donor/acceptor (D–A) systems through p-conjugated bridges (i.e., D–p–A molecular structure). Many novel D–p–A metal-free dyes with various donor moieties, such as coumarin [4,17], indoline [18], triphenylamine [3], phenothiazine [2], carbazole [19], and pyrrole [20], have been designed and used as efficient sensitizers for the DSSC applications. However, the photon-to-current conversion efficiency based on those sensitizers has been achieved up to 9.5% only, which is relatively low compared to the conventional Ru based sensitizers. In order to increase the efficiency of DSSCs, it is important to design
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the suitable dyes, which have reasonable optical properties. This could be possible if we design the sensitizer with the absorption spectrum covers visible-to-near-infrared range, have significant intra-molecular charge separation after absorbing sun light, and it should stably bind to the semiconductor surface so that electrons can inject to surface continuously [16,21,22]. Herein, we aimed to design a potential metal-free sensitizer which overcomes the flaws of existing dyes and to find an efficient way to tune optoelectronic properties. Nowadays, there are two ways in which the spectral properties of D–A or D–p–A molecules can be improved: changing the conjugation length of the sensitizer, and substituting different electron -rich and -deficient units to modify the spectrum. Previously it is reported that, some D–A oligomers have dual-band optical absorption spectra which provide broad absorption characteristics [23,24]. In general, dual-band absorption spectrum arises from two separate transitions, which lead to either charge transfer between the donor and acceptor units along with a p–p* transition (so-called intra-molecular charge transfer, ICT) [25,26], or reorganization of molecular orbitals to produce accessible low- and high-lying energy levels spread across both donor and acceptor units (usually named as p–p* transition to make a distinction from ICT) [27–29]. In the present study, we selected 12 monomers which are systematically varied by both in the donor and acceptor-units, to form the different p-conjugated D–A oligomers and evaluated their optoelectronic properties. The selected 12 monomers are thiophene (T), thienopyrazine (TP), dithienopyrazine (DTP), thiadiazolothienopyrazine (TDTP), 1,4-dihydro-1-phenylpyrazine (PPP), cyclopentadithiophene (CDT), dicyanomethylidene-cyclopentadithiophene (CDM), 9-phenylcyclopentadithiophene (TPAT), N,N-bis (4-methoxy-phenyl)-thiophene-2-amine (MPTA), 10-phenyl-10H-phenothiazine (PTAZ), triphenylamine (TPA), 4-methoxy-N-(4-methoxyphenyl)-N-phenyl- benzeneamine (MPBA), and 9-phenyl-9H-carbazole (PC). These monomers are derived from the several compounds [30], which are universally used in the design of D–A oligomers, namely thienopyrazine, thiophene, thiazine, carbazole, and phenylamine compounds, thus the resulting 66 D–A oligomers could be applied as a model system to represent the general classes of D–A oligomers.
2. Computational details All the calculations in this study were performed with Gaussian 09 package [31]. The geometries of neutral monomers and metal-free sensitizers were optimized using B3LYP exchange correlation functional [32] combined with the standard double-f plus polarization basis set, 6-31G(d) [33]. Different basis sets and functionals in gas phase did not affect the structural parameters much, but it influences the optoelectronic properties such as excitation energies and intensities. Hence, we have performed a benchmark calculation in order to find the most suitable method for simulating UV–Vis absorption. For this, we considered four DFT methods such as, PBE0, B3LYP, BHandHLYP and CAM-B3LYP to calculate the UV–Vis
absorption spectra for TC-1 [34], T2-1 [2] and I-1 [35] molecules of D–A type and L1 [36], TA–St–CA [3] and MK2 [19] molecules of D–p–A type. We used 6-31G (d) basis set for all the benchmark calculations and the calculated excitation energies of D–A type and D–p–A type molecules are shown in Table 1. Compared with the experimental values of D–A and D– p–A types of molecules, the calculated results show that the absorption energy from B3LYP was more accurate for D–A system and BHandHLYP method show perfect agreements for D–p–A system which contains longer conjugated length. Considering that D–p–A backbone molecules are main structures in real DSSCs, therefore, we choose BHandHLYP functional to calculate the optoelectronic properties of D–p–A molecules including 20-lowest excitation energies and intensities of all the metal-free sensitizers considered here. The optoelectronic properties were transformed, using the SWizard program [37,38], into simulated spectra as described before, using Gauss functions with half-widths of 4000 cm1, as shown in following equation:
X fI ðx x1 Þ2 eðxÞ ¼ c1 exp 2:773 D 1=2;I D21=2;I 1
! ð1Þ
where e is the molar extinction coefficient given M1 cm1 in unit; the energy x of all the allowed transitions included in Eq. (1), is expressed in cm1; fI and D1/2 are the oscillator strength and the half-bandwidths, respectively. 3. Results and discussion 3.1. The classification of donor- and acceptor-monomers The molecular structures of 12 selected monomers considered in this study are summarized in Fig. 1. These selected monomers are then classified whether they belong to donor- or acceptor-monomer and then, every possible combination of different donor- and acceptor-unit is combined together to form 66 D–A oligomers. This classification is made on the basis of their vertical ionization potential (IPv), vertical electron affinity (EAv), and HOMO/ LUMO energy levels. As described by Dixon et al. [39], stronger donor has smaller IPv (easy to lose an electron) and higher HOMO level that would lose electrons easily; and stronger acceptor has more negative value of EAv (easily to gain an electron) and lower LUMO level which would accept electrons strongly. Therefore, we have calculated
Table 1 The calculated UV–Vis absorption energies (in nm) for D–A and D–p–A sensitizers using different DFT methods.
a
Sensitizers
PBE0
B3LYP
BHandHLYP
CAM-B3LYP
Exptl.a
TC-1 T2-1 I-1
451.7 441.6 435.9
401.7 461.7 453.3
354.3 367.2 374.4
359.0 370.2 381.6
400 452 483
L1 TA–St–CA MK2
482.4 504.5 575.2
506.9 534.2 612.8
405.8 409.3 481.5
404.3 403.0 472.6
405 410 480
Taken from Refs. [2,3,19,34–36].
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Fig. 1. Sketch of molecular structures of thienopyrazine, thiophene, thiazine, carbazole, and phenylamine-based derivative monomers.
the correlations between IPv vs. HOMO and EAv vs. LUMO, which are shown in Fig. S1. From Fig. S1, we found a linear relationship between the calculated IPv and HOMO and EAv with LUMO. From this relationship, we are able to identify the order of donor- and acceptor-monomers’ strengths. The strength of donor from strong to weak is: PPP > MPBA > MPTA > PTAZ > TPA > PC > CDT > DTP > TPAT > CDM > TDTP > TP > T and the order of acceptor strength is TDTP > CDM > DTP > TP > CDT > TPAT > PC > PTAZ > TPA > MPTA > MPBA > PPP > T. 3.2. Roles of donor and acceptor monomers in optoelectronic properties Next, we designed a series of p-conjugated D–A oligomers based on the order of electron donating- and electron withdrawing-abilities. One way to design these D–A oligomers is to distinguish what combination of donor and acceptor could have suitable UV–Vis absorption for the practical applications. Accordingly, the present study care-
fully evaluates the impact of the variation in strength of donor- and acceptor-units on the optoelectronic properties, such as energy gaps and the absorption spectra with their associated HOMOs-to-LUMOs electronic transitions. First, we begin with nine representative D–A oligomers such as, the strong donor PPP with five different-strength of acceptors (TDTP, DTP, CDT, PTAZ and MPBA) and the strong acceptor TDTP with different strength of donormonomers (PPP, MPTA, TPA, PC and CDM) and investigated their optoelectronic properties using TDDFT methods. Fig. 2 illustrates the electron distribution plots of the designed D–A oligomers along with their respective HOMO–LUMO energy gaps, respectively. As mentioned above, the HOMO/LUMO energy levels are interrelated to the corresponding electron donating and withdrawing abilities. Thus, in Fig. 2(b), a dramatic increase of LUMO energy levels is found as weakening the acceptor strength, which results in an increase in energy gaps. Also, the electron distribution plot (Fig. 2) indicates that the HOMOto-LUMO transitions vary from intra-molecular charge
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Fig. 2. The roles of (a) donor and (b) acceptor in governing the energy gap preferences of the selected p-conjugated D–A oligomers. In figures (a) and (b), the strength of donor- and acceptor-units, is varied from strong (left) to weak (right).
transfer (ICT) to p–p* transition while the strengths of acceptors are weaken. Contrarily, the HOMO levels gradually decrease while the strength of the donor becomes weaker, but the influence is not obvious as like acceptors and all the HOMO-to-LUMO transitions are having ICT character. These results indicate that, if two monomers are combined together to form an oligomer, the HOMO– LUMO energy gaps are more dominated by the acceptor counterpart. Although energy gaps are important indices when estimating the optoelectronic properties, UV–Vis absorption spectra are more reliable and direct data to evaluate how these sensitizers will perform in the solar cells. Thus, we have simulated the UV–Vis absorption of the designed D– A molecules and the spectra with corresponding transition assignments are shown in Fig. 3 and Table 2. As can be seen from the Fig. 3(a), the variation of donor strengths, i.e., from weak (CDM) to strong (PPP), does not affect the absorption spectra appreciably; the spectra remain dualband character, one absorption band is located at nearinfrared region, while the other is in the range of UV–Vis. Increasing the strength of donors makes the first absorption band red shifted and increases the intensities very little, (see Table 2) which are due to an increase in the degree
of charge separation. It has been observed for CDM–TDTP, TPA–TDTP, PPP–TDTP molecules, the transition characters below 400 nm are mainly contributed from p–p* transitions, whereas, PPP–TDTP has a low-lying satellite band which corresponds to ICT transition (k = 412 nm). The dual-band characteristic makes the absorption broad, which is beneficial for light-harvesting if the absorption spectrum covers visible region. On the other hand, the variation on the acceptor-monomers’ strength from weak-to-strong cases, the absorption band characteristics are transformed from mono- to dualband. The finding of this dual-band absorption characteristic is vital for the various applications in light harvesting and light-emitting devices. The strong donor (PPP) with different acceptors’ strengths (MPBA, CDT and TDTP) has been used for this illustration (Fig. 3(b)). From Table 2, the absorption of strong donor-weak acceptor (PPP–MPBA) oligomer is found only at UV region and it is mono-band characteristic which is due to p–p* transitions. However, when the strength of the acceptor counterpart increased, as represented in the case of PPP–TDTP molecule, a strong absorption band is noticed in the near-IR region (k = 1205.6 nm; originated from a ICT transition (Table. 2)) and also it has absorption band in the UV region as well
Fig. 3. The roles of (a) donor and (b) acceptor-units in governing the UV–Vis absorption spectra of the selected p-conjugated D–A oligomers. In figures (a) and (b), the strength of donor and acceptor -units, is varied from strong (top) to weak (bottom).
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Table 2 The calculated absorption energies (k), oscillator strengths (f), light harvesting efficiency (LHE) and transition characters of five representative D–A oligomers: CDM–TDTP, TPA–TDTP, PPP–TDTP, PPP–CDT and PPP–MPBA molecules. The states with f > 0.1 are shown. D–A molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
CDM–TDTP
S0 ? S1 S0 ? S7 S0 ? S8
1011.7 388.5 377
0.13 0.48 0.11
0.259 0.669 0.224
S0 ? S12 S0 ? S13
341.7 336.1
0.33 0.33
0.532 0.532
H?L (+100%)a H ? L + 2(+75%)b H-4 ? L (+64%)a H ? L + 2(+13%)b H-2 ? L + 1(+74%)b H-5 ? L (+49%)a
TPA–TDTP
PPP–TDTP
H-4 ? L (18%)a H-5 ? L (+20%)a
H-4 ? L + 1(12%)b
a
S0 ? S1 S0 ? S6 S0 ? S11 S0 ? S13
1058.7 388.4 332 317.2
0.15 0.58 0.29 0.27
0.292 0.737 0.487 0.463
H ? L (+100%) H ? L + 1(+92%)a H-8 ? L (+47%)b H ? L + 3(+94%)b
S0 ? S1 S0 ? S4 S0 ? S9 S0 ? S11
1205.6 412.3 333.6 321.9
0.19 0.47 0.27 0.25
0.354 0.661 0.463 0.438
H ? L (+100%)a H ? L + 1(+92%)a H-5 ? L (+67%)b H ? L + 4(+57%)b
H-7 ? L (37%)a
H ? L + 3(38%)b
a
PPP–CDT
S0 ? S1 S0 ? S3
435.9 339.4
0.81 0.44
0.845 0.637
H ? L (+100%) H-1 ? L (+86%)b
PPP–MPBA
S0 ? S2 S0 ? S7
370.6 321.6
1.05 0.11
0.911 0.224
H ? L (+92%)b H ? L + 3(+75%)a
‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p* Transition.
(Fig. 3(b)). The appearance of the absorption band at both UV and visible to near-IR region, thus justifies the dualband absorption characteristics. From the above results, it has been noticed that the influence of acceptors is more significant in determining the optoelectronic properties of designed D–A oligomers and also the overall spectra can be red shifted by increasing the strength of the donor. However, the simulated UV–Vis absorption spectra of the designed molecules are not enough to use as efficient sensitizer in DSSCs since, for efficient sensitizer, the absorption band should be broader and cover the whole visible region. Thus, we try to employ the molecules which have only dual-band characteristic to broaden the absorption spectrum in the visible region.
3.3. Design strategy based on dual-band characteristics Here, we proposed a design strategy on the basis of the requirements of high-efficiency metal-free sensitizer, including significant intra-molecular charge transfer, broad UV–Vis absorption spectra with high molar extinction coefficients and appropriate excitation energies, and proper geometries. In order to increase the degree of charge transfer when electronic excitation occurs, it is preferable to choose strong donors and strong acceptors counterparts for efficient sensitizers; for instance, the combinations of strong donors (PPP, MPBA and TPA) and strong acceptors (TDTP, CDM and TP). However, from the results shown in Fig. 3(b), even though the strongest acceptor, TDTP possess dual-band character and broad absorption
Fig. 4. UV–Vis absorption spectra of (a) MPBA–CDM based and (b) MPBA–TP based oligomers. The black solid line represents donor and acceptor system with anchoring group (D–A*CN), blue dash one is for donor and acceptor system with p-linker (D–p–A), and the red dash line is D–p–A*CN system.
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Table 3 Absorption energy (k), oscillator strength (f), light harvesting efficiency (LHE) and transition character of the singlet excited states of MPBA–CDM series calculated by BHandHLYP/6-31G(d) level of theory. The states with k > 300 nm and f > 0.1 are shown. Molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
MPBA–CDM*CN
S0 ? S1 S0 ? S2 S0 ? S3 S0 ? S5
733.3 472.9 405 323.6
0.22 0.12 1.25 0.20
0.397 0.241 0.944 0.369
S0 ? S6
312.6
0.64
0.771
H-1 ? L (+28%)a H ? L (30%)a H-1 ? L+1(+17%)a H-3 ? L (+25%)a H-1 ? L+1(10%)a H-6 ? L (+20%)a H ? L+2(+13%)b H-2 ? L (+17%)a
S0 ? S7
305.3
0.22
0.397
H ? L (+68%)a H-1 ? L (+58%)a H ? L+1(+71%)a H-6 ? L (+32%)a H-5 ? L (11%)a H-1 ? L+1(+21%)a H-3 ? L (20%)a H ? L+2(+45%)b
MPBA–T–CDM
S0 ? S1 S0 ? S3 S0 ? S5
729.3 374 309.2
0.09 1.52 0.54
0.187 0.970 0.712
H ? L (+46%)a H ? L+1(+79%)a H-6 ? L (+87%)a
H-1 ? L (+45%)a
MPBA–T–CDM*CN
S0 ? S1 S0 ? S3 S0 ? S6
760.7 429.2 340.9
0.29 1.65 0.42
0.487 0.978 0.620
H-1 ? L (+39%)a H-1 ? L+1(+20%)a H-1 ? L+1(30%)a
S0 ? S7
313.9
0.45
0.645
H ? L (+55%)a H ? L+1(+60%)a H ? L+1(+33%)a H ? L+2(20%)b H-8 ? L (+45%)a
H-7 ? L (+18%)a
Excitations in bold are assigned to important transitions. ‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p*.
spectrum, it’s long-wavelength absorption band observed at 1200 nm, which is far away from the visible region. Whereas, the acceptor, CDT which is not so strong, its first absorption peak shifted to visible region and also exhibits charge transfer character which is necessary for DSSCs (Table 2). According to these observations, we suggest design strategies as follows; it is better to consider the acceptor which has electron withdrawing ability between TDTP and CDT (i.e. CDM, DTP, and TP), substitute p-conjugated linker between electron-rich and electron-deficient groups and decorate acceptors with an anchoring group. Moreover, it is easy to tune the spectrum of CDT-based molecules by employing stronger donor. As discussed in previous section, the overall spectra of D–A oligomers have bathochromic shift when increasing the strength of donor counterpart (shown in Fig. 3(a)). In this work, thiophene is employed as a p linker to extent the conjugation, which results in high absorption intensities. Thiophene is among the most frequently used p-spacer in organic dyes due to its high structural stability, effective for the controlling intramolecular charge separation and optical properties [40,41]. Recent studies report the better performances of thiophene based dyes for DSSC application [42,43]. The 2-cyanoacrylic acid is commonly used as anchoring group for metal-free DSSCs and is a well-known electron acceptor which can help charge separation and bind strongly on TiO2 surface [5,44–46]. In brief, we selected strong donors, like PPP, MPBA and TPA, and strong acceptors, CDM, TP and CDT, with thiophene (p-bridge) and 2-cyanoacrylic acid (anchoring group), to design new sensitizers and investigated their optoelectronic properties.
3.4. The optoelectronic properties of designed D–p–A sensitizers The molecular structures of the selected MPBA–CDM, MPBA–TP, and MPBA–CDT oligomers are modified through
an introduction of p-linker, thiophene (T), and electron withdrawing substituent, 2-cyanoacrylic acid, (*CN) to form their subsequent analogue oligomers, termed as MPBA–T– CDM*CN, MPBA–T–TP*CN, and MPBA–T–CDT*CN, respectively. The schematic illustrations of the designed sensitizers are shown in Fig. S2 of Supplementary material. Fig. 4 shows the calculated UV–Vis absorption spectra of MPBA–CDM and MPBA–TP based series and their related transition data including absorption energies, oscillator strengths and transition assignments are summarized in Tables 3 and 4 (The results of MPBA–CDT series and other combinations of designed dyes are given in Tables S1–S2 and Figs. S3–S6 of Supplementary material). It has been observed that the UV–Vis absorption spectra of these two systems have dual band character. In MPBA–CDM series, long-wavelength band, named as Band 1, is located around 750 nm; and short-wavelength band, named as Band 2, is about 400 nm. Both absorption bands are in the visible region. But for MPBA–TP series, Band 1 and Band 2 are located at 530 nm and 350 nm, respectively. As can be seen from Fig. 4, the absorption intensity of the visible-region band is found to be increased due to the incorporation of the p-linker, which is in agreement with the previous experimental results [47,48]. Apart from this, it also red shifts the overall spectra of both the series significantly. On the other hand, considering the effect of electron withdrawing anchoring group, the intensities of Band 1 in both cases are found to be increased largely. Also, the overall absorption spectra of MPBA–CDM and MPBA–TP are bathochromic shift around 30 and 90 nm, respectively. It has been noted that the substitution of p-linker and electron withdrawing anchoring group with strong donors and strong acceptors can increase the molar extinction coefficients as well as red-shift the overall spectra. Also, we have calculated the light harvesting efficiency (LHE) of the designed dyes using the formula:
LHE ¼ 1—10f
ð2Þ
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Table 4 Absorption energy (k), oscillator strength (f), light harvesting efficiency (LHE) and transition character of the singlet excited states of MPBA–TP series calculated by BHandHLYP/6-31G(d) level of theory. The states with k > 300 nm and f > 0.1are shown. Molecule
Electronic transition
k (nm)
f (a.u)
LHE
Transition assignment
MPBA–TP*CN
S0 ? S1 S0 ? S4
514.2 326
0.99 0.36
0.898 0.563
H ? L (+92%)a H ? L+1(+72%)a a
MPBA–T–TP
S0 ? S1 S0 ? S2 S0 ? S4
472.6 349 319.3
0.63 0.12 0.65
0.766 0.241 0.776
H ? L (+77%) H-1 ? L (+64%)a H ? L+1(+73%)b
MPBA–T–TP*CN
S0 ? S1 S0 ? S4
563.6 335
1.24 0.54
0.942 0.712
S0 ? S5
320.4
0.21
0.383
H ? L (+84%)a H ? L+1(+46%)a H ? L+2(+16%)b H ? L+2(+69%)b
H-1 ? L+1(+18%)a H-1 ? L (+20%)a H ? L (17%)a
H-1 ? L+1(+27%)a H ? L+1(14%)a
Excitations in bold are assigned to important transitions. ‘‘H’’ represents HOMO and ‘‘L’’ represents LUMO. a ICT transition. b p ? p*.
where f is the oscillator strength of the designed dyes corresponds to the wavelength and the calculated values are given in the respective tables. As can be seen from those values, the LHE of the designed dyes are large, which is required to enhance the photocurrent response of the DSSCs. Figs. 5 and 6 show the high-lying and low-lying molecular orbitals of the most important excitations, which illustrate the either charge transfer or p–p* character, of MPBA–CDM and MPBA–TP based systems, respectively. For both the cases, the main assignment of Band 1 is due the transition from HOMO to LUMO, whereas for Band 2 is HOMO to LUMO+1. It can be seen from Fig. 5, all the transitions of MPBA–CDM series show the ICT character, which is required for the highly efficient sensitizers. However, in the HOMO to LUMO transition, the electrons are transferred from donor (MPBA) to acceptor counterpart (CDM) whereas, in the HOMO to LUMO+1 transition, the electrons are transferred to both acceptor and anchoring
groups, except in MPBA–T–CDM case. This indicates that, in MPBA–CDM*CN and MPBA–T–CDM*CN, the Band 2 is well charge separated and also it is in the visible region (405, 429 nm see Table 3), hence, it is expected that the excited electrons efficiently inject into the TiO2 surface. On the other hand, Band 1 of MPBA–CDM series is an ineffective for DSSC applications, since the electrons are localized in acceptor unit rather than the anchoring group after excitation. Compared with MPBA–CDM series, MPBA–TP series have better charge transfer characters because excited electrons are delocalized at both acceptor (TP) and anchoring group (shown in Fig. 6). This may be due to the electron withdrawing ability of CDM is relatively stronger than TP, which may compete the electron withdrawing ability of anchoring groups. This indicates that, the strength of the considered acceptor is too strong, some ineffective transitions may appear in the spectrum. It has been noticed that the calculated absorption spectra for the designed
Fig. 5. Representations of HOMO, LUMO and LUMO+1 corresponding to the important excitations of MPBA–CDM series shown in Table 3. Both HOMO to LUMO and HOMO to LUMO+1 reveal ICT character in all the cases.
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Fig. 6. Representations of HOMO, LUMO and LUMO+1 corresponding to the important excitations of MPBA–TP series shown in Table 4. Both HOMO to LUMO and HOMO to LUMO+1 reveal ICT character in all the cases.
sensitizers significantly red-shifted as compared to the conventional Ru based dyes (N3, C101, C106 and CYCB11) [49–52] which is ideal for the DSSCs. The results reveal that the optical properties of the metal free sensitizers can be tuned in accordance with the requirements by means of combining suitable donor and acceptor moieties. For instance, if we combine the strong acceptor and strong donor monomers, the long wavelength band may not be suitable for DSSC due to its less electron injection ability, but the short wavelength band shifted to visible region because of donor effect and it can be used to harvest the sun light. Also, the incorporation of thiophene and 2-cyanoacrylic acid enhances the intensity of the overall absorption spectra and making the spectra bathochromic shift through increasing conjugation length. 4. Conclusions In this contribution, we proposed a design strategy to improve the photon-to-current conversion efficiency of DSSCs based on the metal-free sensitizers using DFT calculations. From these theoretical results, we identified the electron -donating and-withdrawing abilities of selected monomers and investigated their roles in the optoelectronic properties. Further, we have systematically investigated the UV–Vis absorption spectra and electronic transition characters of selected D–p–A sensitizers and reported their spectral properties for DSSC applications. From our results, we found that the acceptor monomers play an important role in determining the optoelectronic properties. Further it is evident that the substitution of thiophene and 2-cyanoacrylic acid enhances the intensity of the absorption significantly and the overall absorption spectrum can be red shifted. The simulated spectra reveal that the characteristic of dual-band is present in the selected sensitizers, which is highly required for the efficient sensitizer. Particularly, in the MPBA–TP based series, the
long wavelength absorption band is due to the ICT character and importantly excited electrons not only localized on the acceptor, but also on the anchoring group: hence it is easy to inject the excited electrons into the TiO2 surface. In summary, we suggested some potential sensitizers which have suitable optoelectronic properties and we believe that these molecules can be used as an efficient sensitizes for the new generation of DSSCs. Acknowledgements We acknowledge the financial support from the National Science Council of Taiwan (NSC 101-2113-M-011004-MY3). We are also thankful to the National Center of High-Performance Computing (NCHC) for donating computer time and facilities. Appendix A. Supplementary material Supplementary material: the plots of vertical ionization potential (IPv), electron affinity (EAv), schematic illustrations of selected fragments and optoelectronic properties of MPBA–CDT based series are shown. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.orgel.2014.03.022. References [1] B. O’Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–739. [2] H.N. Tian, X.C. Yang, R.K. Chen, Y.Z. Pan, L. Li, A. Hagfeldt, L.C. Sun, Phenothiazine derivatives for efficient organic dye-sensitized solar cells, Chem. Commun. (2007) 3741–3743. [3] S. Hwang, J.H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.H. Lee, W. Lee, J. Park, K. Kim, N.G. Park, C. Kim, A highly efficient organic sensitizer for dye-sensitized solar cells, Chem. Commun. (2007) 4887–4889. [4] Z.S. Wang, Y. Cui, Y. Dan-Oh, C. Kasada, A. Shinpo, K. Hara, Thiophene-functionalized coumarin dye for efficient dye-sensitized
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