Theoretical design of organoimido-substituted hexamolybdates with different electron donors for dye-sensitized solar cells

Dyes and Pigments 102 (2014) 6e12

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Theoretical design of organoimido-substituted hexamolybdates with different electron donors for dye-sensitized solar cellsq Yonghuai Wei, Ting Zhang, Zhongling Lang, Likai Yan*, Zhongmin Su** Institute of Functional Material Chemistry, Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2013 Received in revised form 9 October 2013 Accepted 22 October 2013 Available online 1 November 2013

Novel organoimido-substituted hexamolybdates dyes were designed by introducing 3,4-ethylenedioxythiophene (EDOT) or thienothiophene (TT) unit as electron donor based on [Mo6O18(MBTH)]2
. The electronic structures, absorption spectra and transition natures of designed systems have been theoretically investigated according to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Compared with dye 1, the absorption spectra of these designed organoimido-substituted hexamolybdates dyes exhibit both strong and broad absorptions from 400 to 800 nm, as well as remarkably red shift owing to the long p-conjugated bridge and high delocalization. Especially for dye 6, which contains a biTT unit, it has the largest maximum absorption wavelength (lmax) at 733 nm and may show a higher short-circuit current density (Jsc) as it possesses higher light harvesting efficiency (LHE) and reasonable driving force (DERP). Our work reveals that the designed molecule 6 is promising candidate for high performance solar cell materials. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar cell Hexamolybdates Electron donors Density functional theory Absorption spectra Jsc

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted increasing attention for their abilities, which can convert solar light to electricity as well as their easy preparation, inexpensive and efficient advantages since O’Regan and Grätzel reported their pioneering work in 1991 [1]. Photovoltaic (PV) cell is a promising renewable energy technology with broad potential to solve the future energy problem. Until now most investigations in this field have focused on n-type DSSCs (n-DSSCs), and the conversion efficiencies of around 11% have been reported [2e4]. In 1999, Lindquist and co-workers first described the preparation and characterization of a p-type DSSC (pDSSC) based on a nanostructured nickel oxide electrode sensitized by a freebase porphyrin or erythrosin B [6]. But the overall photoconvertion efficiency was very low (0.0076%) with a maximum incident-photo-to-current-conversion (IPCE) of 3.44% under AM 1.5. Recently, satisfactory progress have been made in enhancing the

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Fax: þ86 431 5684009. ** Corresponding author. E-mail addresses: [email protected] (L. Yan), [email protected] (Z. Su).

photovoltaic performance of NiO-based p-DSSCs [7e13], making it promising to assemble highly efficient tandem DSSCs in which both electrodes are photoactive. Unlike the extensively studied on nDSSCs, p-DSSCs still suffer from low efficiencies and thus limit the overall efficiency of the resulting tandem DSSCs. Therefore, improving the efficiency of p-DSSCs is of significant importance [5]. In p-DSSCs, visible light absorption by the dye is followed by electron transfer from the valence band (VB) of the semiconductor to dye. The reduced dye then can be regenerated by the redox mediator (I
3 ) in electrolyte. The injected holes pass into the external circuit and move to the counter electrode where the redox mediator is oxidized back to its original state (Fig. 1). The pattern of most organic sensitizers used in p-DSSC is an electron donor (D), a bridge (B, typically a p-spacer), and an electron acceptor (A), which are usually combined as a D-p-A rod-like configuration in order to improve the efficiency of the UV/Visible (UV/Vis) photoinduced intramolecular charge transfer (ICT) [14]. In the past decades, charge-transfer hybrids involving organic donors and inorganic acceptors have been noticed [15,16]. Recent discoveries indicate that such hybrids could result in highly efficient photovoltaic cell materials [17,18]. Among various inorganic clusters, polyoxometalates (POMs) are most attractive not only for their structural versatility and rich optoelectronic properties but also for their discrete molecular structures, which allow surface functionalization in a controlled and rational version [19]. POMs, a class of

0143-7208/$ e see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.10.034

Y. Wei et al. / Dyes and Pigments 102 (2014) 6e12

7

the Jsc, the open-circuit voltage (Voc), and the fill factor (ff). The ff of a solar cell is defined as the maximum power output (Jmax  Vmax) divided by the product of Jsc and Voc. This connection could be expressed as following:

h ¼ ffJsc Voc =Is

(1)

ff ¼ Jmax Vmax =Jsc Voc

(2)

where Is is the energy of incident sunlight. According to Eq. (1), improving Jsc and Voc is effective means to enhance h. Based on the principle, the maximum open-circuit voltage (VOCmax) in DSSCs can be determined by Eq. (3): [34]


VOCmax ¼ E I3
=I

EVB ðp
SCÞ

Fig. 1. Schematic diagram illustrating the key processes in a p-type DSSC.

metal-oxygen cluster compounds, are well-known electron acceptors [20] and have been widely applied in preparing charge-transfer materials with organic donors [21]. Indeed, great efforts have been devoted in the functionalization of POM clusters, particularly the hexamolybdate ion, [Mo6O19]2
[22e24]. In the past few years, Peng and co-workers have reported both main chain and side chain POMcontaining conjugated polymers. They have demonstrated that POM clusters as electron acceptors, in conjunction with organic p-conjugated segments as electron donors, may have applications as novel PV materials [25,26]. However, the PV performances of the hybrid conjugated polymers are not spectacular. Later they synthesized a rod-coil diblock copolymer (DCP) with POM clusters linked to the coil block, such a hybrid DCP possesses significantly improved PV performance over their previously reported hybrid conjugated polymers [27]. In addition, Kamal et al. have successfully coupled Nmethylbenzothiazole hydrazone (MBTH-H2) to the [Mo6O19]2
isopolyanion [28]. The resulting organiceinorganic hybrids, which have been fully characterized, are promising candidates for the electrosynthesis of molecular materials with strong interactions between the anionic and the cationic components [28]. The electronic properties of [Mo6O18(MBTH)]2
have been studied by UV/Vis absorption measurements, and the absorption only exhibits features in the UV part of the spectrum. Quantum chemical (QM) techniques are efficient and very attractive tools for the interpretation of the experimental phenomenon. Theoretical calculations based on a variety of QM techniques have provided insights into the relationship between structures and the optical properties [29e32]. The availability of huge CPU resources now allows studying the absorption spectra of large molecular species at correlated levels of approximation [33]. In order to obtain novel dye sensitizers with broad and intense absorption in the Vis region of the solar spectrum, we design six novel POM-based organic-inorganic dyes for p-DSSCs in this work. Based on [Mo6O18(MBTH)]2
mentioned above, we construct molecules 1e6 by introducing EDOT or TT units as electron donor to select dyes with improved performance. And the geometries, electronic properties, absorption spectra and transition nature of designed dyes have been theoretically investigated by DFT and TDDFT calculations.

(3)

where EVB is the VB potential of the semiconductor. The Jsc in DSSCs is determined by the following equation:

Z Jsc ¼

LHEðlÞFinject hcollect dl

(4)

l

where LHE(l) is the light harvesting efficiency at a given wavelength, Finject is the electron injection efficiency, and hcollect is the charge collection efficiency. As a result, the enhancement of Jsc should focus on improving the LHE and Finject. The LHE is closely to the oscillator strength f and it can be expressed as:

LHE ¼ 1
10
f

(5)

2.2. Computational details In this work, all calculations were carried out by the Amsterdam Density Functional (ADF) 2009.01 program [35e37]. The local density approximation (LDA) characterized by the VoskoeWillke Nusair (VWN) [38] parameterization for correlation was used. The generalized-gradient approximation (GGA) was employed in the geometry optimizations by using the Becke [39] and Perdew [40] exchangeecorrelation (XC) functional. The zero-order regular approximation (ZORA) was adopted in all the calculations. Solvent effects were considered by conductor-like screening model (COSMO) [41] continuum method with the parameters of DMSO solvent (3 ¼ 46.7). The atomic radii for the atoms which actually define the size of the solvent cavity were chosen to be 2.09, 1.40, 1.70, 1.41, 1.08 and 1.82  A for Mo, O, C, N, H and S, respectively. To describe the electrons, we used Slater-type basis sets. Triple-z plus polarization basis sets were used to describe the valence electrons of all the atoms, whereas for transition metal molybdenum atom, a frozen core composed of 1s to 3spd shells was described by means of single Slater functions. Moreover, the value of the numerical integration parameter used to determine the precision of numerical integrals was 6.0. TDDFT calculations were performed using statistical average of orbital potentials (SAOP) by Gritsenko, Baerends et al. [42]. Geometrical optimizations of all systems under Cs symmetry constraints were carried out. 3. Results and discussion

2. Methods

3.1. Molecular structure

2.1. Theoretical background

Based on [Mo6O18(MBTH)]2
, we design dyes 1e6 by changing the electron donors with TT or EDOT units, which are easily synthesized and widely used for high-efficiency photovoltaic devices. The molecular structures of dyes 1e6 are shown in Fig. 2. Dye 1 is

The energy conversion efficiency (h) is an important parameter to evaluate the performance of a solar cell. It is closely connected to

8

Y. Wei et al. / Dyes and Pigments 102 (2014) 6e12

Fig. 2. Molecular structures of the organic-polyoxometalates dyes 1e6.

designed by adding a cyanoacrylic acid as the anchoring group on parent system for strongly adsorbing onto the semiconductor surface. To obtain highly effective dyes for DSSCs, dyes 2 and 3 are designed by introducing an EDOT and a TT unit as the electron donor, respectively. In order to lower the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and further red shift the maximum absorption of UVeVis spectrum, we prefer to enhance the ability of the electron donor as well as the p conjugated linker. So dye 4 is designed by introducing an EDOT and a TT units simultaneously, while dyes 5 and 6 are designed by adding a biEDOT and a biTT unit, respectively. The optimized geometrical structures of dyes 1e6 are shown in Fig. 3. 3.2. Electronic structure The frontier molecular orbital (FMO) distributions of dyes 1e 6 are shown in Fig. 4. Obviously, the HOMOs localize over the

organic groups whereas the LUMOs localize over the POM moiety, which are far away from the attaching group and are beneficial for a p-type DSSC. As expected, the LUMO energies and distributions of dyes 1e6 are similar due to the same acceptor (POM). While the HOMO energies of dyes 1e6 are obviously different owing to the different electron-donating abilities of the electron donors, which increase in the order of 1 < 3 < 2 < 4 < 6 < 5. As depicted in Table 1, by adding an EDOT or a TT unit, the HOMO levels of dyes 2 and 3 are slightly larger than that of dye 1 by 0.24 eV and 0.22 eV, respectively. However, the HOMO levels of dyes 4, 5 and 6 are significant promoted by enhancing the p conjugated linkers with stronger electrondonating units. The results indicate that stronger electrondonating units with longer p conjugated linkers lead to smaller HOMO and LUMO gaps. In addition, the HOMO energies of dyes 1e6 are lower than 0.1 eV (the VB of NiO) [43], which ensure an effective injection of holes. Simultaneously, the LUMO energies of dyes 1e6 are more positive than I
/I
3 redox couple, which is responsible for an effective injection of excited electrons. 3.3. Absorption properties TDDFT calculations are performed to investigate the influence of the electron-donating units on the electronic absorption spectra of dyes 1e6. The simulated electronic absorption spectra of dyes 1e6 are presented in Fig. 5. And the lmax of dyes 1e6 are in the order: 1 < 2 < 3 < 5 < 4 < 6. It can be clearly seen that the absorption spectra of dyes 1, 2 and 3 display two distinct absorption bands in the visible and near-infrared region of the solar spectrum. Compared with dye 1, the maximum absorption wavelengths of dyes 2 and 3 red shift by 51 and 93 nm, respectively. As for dyes 4, 5 and 6, the maximum absorption wavelengths are further red shifted due to the introduction of stronger electron-donating units. In addition, dyes 4, 5 and 6 exhibit both strong and broad absorptions between 500 and 800 nm in the Vis region. It is worthy to mention that dye 6 containing a biTT unit, has the largest lmax at 733 nm. The absorption spectra of dyes 1e6 reveal that the stronger electron-

Fig. 3. Optimized molecular structures of dyes 1e6. (Mo, O, N, C, S, and H are shown in blue, red, dark blue, gray, yellow, and white, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Y. Wei et al. / Dyes and Pigments 102 (2014) 6e12

9

Fig. 4. The frontier molecular orbitals of dyes 1e6.

Table 1 The frontier molecular orbital energies of dyes 1e6. Dye

1

2

3

4

5

6

LUMO (eV) HOMO (eV)


4.09
5.19


4.08
4.95


4.08
4.97


4.07
4.66


4.08
4.62


4.07
4.76

donating units play a crucial role to red shift the absorption spectrum. Meanwhile, the results prove that the influence of TT unit in dye 2 is stronger than EDOT unit in dye 3 for red shifting the absorption spectrum. Similarly, the influence of biTT unit in dye 5 is

stronger than biEDOT unit in dye 6. Therefore, we conclude that it is an effective way to introduce electron-donating units, particularly biTT unit, to enhance the absorption in the visible region of the solar spectrum. To clarify the electronic transition natures for absorptions of dyes 1e6, the molecular orbitals involved in the dominant electron transitions are calculated and shown in Fig. 6. As all systems have similar electronic transition nature, the absorption spectrum of dye 6 is analyzed as a representative on the basis of the relevant absorption wavelength, the main transitions as well as the oscillator strength (f). As shown in Fig. 5, the absorption spectrum of dye 6

Fig. 5. Absorption spectra of the organic-polyoxometalates dyes 1e6 in DMSO.

10

Y. Wei et al. / Dyes and Pigments 102 (2014) 6e12

Fig. 6. The frontier molecular orbitals for main configuration of dyes 1e6 (assignment: H ¼ HOMO, L ¼ LUMO, Lþ1 ¼ LUMOþ1, H-1 ¼ HOMO-1).

consists of two absorption peaks at 479 and 733 nm with oscillator strengths of 0.63 and 0.99, respectively. The maximum absorption at 733 nm corresponds to the promotion of an electron from HOMO to LUMOþ4 and LUMOþ3. Fig. 6 shows that HOMO is mainly contributed from the organic fragment, while LUMOþ4 and LUMOþ3 are mainly contributed from the biTT fragment and POM cluster, respectively. As a consequence, the absorption actually originates from the mixing of two types of electron transitions: the ICT from the organic donor fragment to the POM and the pep* transition within the organic donor fragment. The electron transition nature shows that increasing the p-conjugated bridge length and the delocalization is an effective way to improve the absorption property of system in Vis region. 3.4. Photovoltaic performance based on dyes 1e6: factors influencing Jsc As discussed above, LHE and Finject are the two main influencing factors on Jsc. The high LHE could enhance the Jsc and finally increase h of DSSCs according to Eqs. (1) and (4). Therefore, we simulate the UV/Vis absorption spectra of the dyes in DMSO solution to provide insight into the LHE. The calculated maximum absorption wavelengths, dominant molecular orbital transitions, oscillator strengths, and LHE are summarized in Table 2. The LHE of dyes 1e6 increase as: 1 < 2 < 3 < 4 < 5 < 6. As shown in Table 2, the introduction of both EDOT and TT units plays a crucial role in improving the LHE of systems. The LHE of dyes 2 and 3 are larger than that of dye 1 by 0.14 and 0.19, respectively. By introducing stronger electron-donating units in the p conjugated linker, the LHE of dyes 4, 5 and 6 increase evidently. The results may be interpreted as the increase of p conjugated linker, which leads to high delocalization and the effective electron transitions from organic fragments to POM. Among all these dyes, dye 6 has the largest LHE, and thus the dye 6 is expected to possess a higher Jsc. According to Eq. (4), another way for enhancing Jsc is to improve Finject, which is related to the driving force (DERP). The injection

driving force can be formally expressed within Koopmans’ approximation as: [18]

h

dye dye dye NiO DERP ¼ E0
0
2EOX þ ERED þ EVB

i

(6)

dye

NiO is the NiO VB edge. E where EVB 0
0 is the electronic vertical transition energy responding to lmax. More quantitatively for a closeddye shell system, ELUMO corresponds to the reduction potential of the dye dye (ERED ), whereas the HOMO energy is related to the potential of dye dye dye dye dye first oxidation (i.e.,
EHOMO ¼ EOX ). The calculated EOX , ERED , E0
0 and DERP are listed in Table 3. The results displayed in Table 3 reveal that the electron donor dye dye significantly influences on EOX , while slightly affects ERED , which leads to a smaller driving force. The
DERP of dyes 1e6 increase as: 5 < 4 < 2 < 6 < 3 < 1. Thus, judge from
DERP related to Jsc, we propose that the cells based on dyes 2, 4 or 5 may not suitable for obtaining a higher Jsc. Although dye 3 has similar driving force with dye 1, while the LHE of dye 3 is smaller than dye 1 as discussed above. So dye 3 is not suitable for obtaining a higher Jsc. The dye 6

Table 2 Calculated excitation energy (Ev), maximum absorption wavelength (lmax), major assignment, oscillator strength (f) and LHE (H ¼ HOMO and L ¼ LUMO). Dye

Ev/eV

lmax/nm

Major assignment

f

LHE

1

2.30

539

0.39

0.59

2

2.10

590

0.56

0.73

3

1.96

632

0.64

0.78

4

1.75

710

0.93

0.88

5

1.91

646

0.97

0.89

6

1.69

733

H / Lþ4 (71%) H / Lþ3 (14%) H / Lþ4 (55%) H-2 / Lþ3 (28%) H / Lþ4 (60%) H-1 / Lþ3 (15%) H / Lþ4 (60%) H-1 / Lþ3 (16%) H / Lþ4 (60%) H / Lþ7 (20%) H / Lþ4 (59%) H / Lþ3 (15%)

0.99

0.90

Y. Wei et al. / Dyes and Pigments 102 (2014) 6e12 Table 3 Key parameters for deducing DERP obtained for dyes 1e6. dye

dye

dye

Dye

EOX (eV)

ERED (eV)

E0
0 (eV)

DERP (eV)

1 2 3 4 5 6

5.19 4.95 4.97 4.66 4.62 4.76


4.09
4.08
4.08
4.07
4.08
4.07

2.30 2.10 1.96 1.75 1.92 1.69


4.09
3.82
4.00
3.60
3.34
3.86

has an improved LHE, similar driving force as well as a red-shifted absorption spectrum compared with dye 1. Therefore, we expect that the DSSC based on dye 6 will have a higher Jsc. As a consequence of the above results, dye 6 is the most promising candidate among these series of organic-polyoxometalates dyes. 4. Conclusions In summary, we have designed a series of organoimidosubstituted hexamolybdates containing different electron donors to obtain novel high performance dyes for p-DSSCs. The electronic structures, absorption spectra and electronic transition characteristics of designed systems have been discussed by using DFT and TDDFT methods. The results show that the designed systems are possible p-type DSSC dyes as their HOMO energies are lower than the VB of NiO. Owing to the long p-conjugated bridge and high delocalization, the absorption spectra of dyes 2e6 exhibit strong and broad absorptions in the UV/Vis region of the solar spectrum and remarkably red shift compared with dye 1. Further study shows that introducing a biTT unit to dyes is an effective way to enhance the absorption in the Vis region of the solar spectrum. The TDDFT calculations reveal that dye 6 is expected to be promising candidate for photovoltaic applications as it possesses narrow energy gap, appropriate FMO energy levels and exhibits stronger and broader absorption in the Vis region and higher LHE. A DSSC using dye 6 may show a higher Jsc comparing with the other dyes, and thus the overall energy conversion efficiency would be improved. The present work is expected to be helpful for the design of organicpolyoxometalates dyes with target properties to improve the performance of dye-sensitized solar cells. Acknowledgments The authors gratefully acknowledge financial support by NSFC (21073030 and 21131001), Program for New Century Excellent Talents in University (NCET-10-318), Doctoral Fund of Ministry of Education of China (20100043120007), and the Science and Technology Development Planning of Jilin Province (20100104 and 20100320). References [1] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 1991;353:737e40. [2] Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, et al. Enhance the optical absorptivity of nanocrystalline TiO2 film with high molar extinction coefficient ruthenium sensitizers for high performance dye-sensitized solar cells. J Am Chem Soc 2008;130:10720e8. [3] Mishra A, Fischer MKR, Bäuerle P. Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules. Angew Chem Int Ed 2009;48:2474e99. [4] Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han L. Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn J Appl Phys 2006;45:638e40. [5] Huang ZJ, Natu G, Ji ZQ, Hasin P, Wu YY. P-type dye-sensitized NiO solar cells: a study by electrochemical impedance spectroscopy. J Phys Chem C 2011;115: 25109e14. [6] He J, Lindström H, Hagfeldt A, Lindquist SE. Dye-sensitized nanostructured ptype nickel oxide film as a photocathode for a solar cell. J Phys Chem B 1999;103:8940e3.

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