The effects of macrocycle and anchoring group replacements on the performance of porphyrin based sensitizer: DFT and TD-DFT study

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The effects of macrocycle and anchoring group replacements on the performance of porphyrin based sensitizer: DFT and TD-DFT study A.S. Shalabi, A.M. El Mahdy, H.O. Taha, K.A. Soliman

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S0022-3697(14)00192-9 http://dx.doi.org/10.1016/j.jpcs.2014.08.002 PCS7363

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Journal of Physics and Chemistry of Solids

Received date: 23 February 2014 Revised date: 19 July 2014 Accepted date: 1 August 2014 Cite this article as: A.S. Shalabi, A.M. El Mahdy, H.O. Taha, K.A. Soliman, The effects of macrocycle and anchoring group replacements on the performance of porphyrin based sensitizer: DFT and TD-DFT study, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2014.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effects of macrocycle and anchoring group replacements on the performance of porphyrin based sensitizer : DFT and TD-DFT study A.S. Shalabi1*, A.M. El Mahdy2, H.O. Taha2, K.A. Soliman1 1

Department of Chemistry, Faculty of Science, Benha University, Benha, P.O.Box 13518, Egypt. Department of Physics, Faculty of Education, Ain Shams University, Cairo, Egypt.

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Abstract __________________________________________________________________ Density functional theory and time dependent-density functional theory calculations have been carried out in an attempt to design new phthalocycanine based sensitizers that could be expected to improve the performance of the porphyrin based sensitizer YD2-o-C8. This was done through replacing the porphyrin macrocycle and carboxylic acid anchoring group of YD2-o-C8 by phthalocyanine macrocycle and cyanoacrylic acid anchoring group, respectively. The performances of the suggested cells could be expected to improve the efficiency of the reference dye YD2-o-C8 with Ti38O76, (TiO2)60, SiC, and SrTiO3 semiconductors. Macrocycle replacement assists in promoting the efficiency in the red shoulder of the spectrum more effectively than that of the anchoring group. The effects of the former structural modifications on cell performance are confirmed in terms of frontier molecular orbitals, energy gaps, semiconductor valence and conduction band edges, density of states, molecular electrostatic potentials, non linear optical performances, reorganization energies, UV-vis. absorption and emission, life times of excited states, light harvesting efficiency, injection efficiency, charge collection, and free energy of regeneration.

Key words: porphyrins; phthalocyanines; anchoring groups; DSSCs; TD-DFT. 1 * Corresponding author: [email protected] (A.S.Shalabi).

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Introduction Porphyrins and phthalocyanines are perfectly suited for their integration in dye sensitized solar cells (DSSCs) [1]. Porphyrins (Pors) are highly conjugated heteroatomic compounds with a very intense absorption in the visible region (a strong Soret band in the 400-500 nm and a moderate Q band in the 550-650 nm). Porphyrins are among the most relevant types of photosensitizers for dye sensitized solar cells DSSCs [2]. However, their insufficient light harvesting ability in the visible region has been pointed out. The extinction coefficients of porphyrin Q bands are not very large. On the other hand, phthalocyanines (Pcs) have proved to fill this gap and are established among the benchmark dyes in the field due to their improved light harvesting properties in the far red- and near IR spectral region [3]. Additional advantages of Pcs are: (i) excellent p-type semiconducting behavior (ii) additional propensity to adopt horizontal orientations on the substrate (iii) longer excitation diffusion length relative to Pors. However, their efficiency values are in general below those made of their Por counterparts. With the design of push-pull zinc porphyrins containing electron and withdrawing groups to improve the directionality of the excited state and bulky groups to reduce aggregation, efficiencies have increased notably. Most recent advances on the development of a porphyrin sensitizer YD2-o-C8 with co-sensitization of an organic dye Y123 using a cobalt-based electrolyte AY1 attained power conversion efficiencies 12.5% at light intensity Pin (mW/cm2) =9.4, 12.7% at Pin=51.2, and 11.9% at Pin=99.5 of AM1.5 solar light, Yella, et al. [4] which are superior to those reported based on ruthenium complexes, represent the highest DSSC efficiency recorded and become a new milestone in this area. This success could be attributed to a good compromise between suppression of dye aggregation, extension of the π-conjugated system and enhancement of the charge transfer directionality in the extended state by introduction of an electron donating group opposite to the anchoring group, thus optimizing the system performance. However, in contrast to the various zinc porphyrin dyes applied in DSSCs, only few zinc phthalocyanine dyes have been reported. Studies on phthalocyanine for DSSC applications reveal that carboxyl (or other) anchoring group need to be electronically connected to the zinc phthalocyanine

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to facilitate the charge transfer from the LUMO of the dye to the 3d orbital of the metal. It was predicted that the improvement of visible light harvesting efficiency in porphyrin and phthalocyanine by altering the anchoring groups dyes would lead to better photovoltaic performance. T. Ripolles-Sanchis, et al. [5] designed a new set of alkoxy-wrapped push-pull porphyrins with carboxylic acid and cyanoacrylic acid anchoring groups for DSSCs. M. Garcia-Iglesias, et al.[6] investigated the effect of anchoring groups in Zn-phthalocyanine on the DSSC performance and stability. H. He, et al. [7] presented a simple acrylic acid functionalized Zn-porphyrin for cost effective DSSCs. Maria-Eleni Ragoussi, et al. [8] introduced carboxyethynyl anchoring ligands to improve the efficiency of phthalocyanine sensitized solar cells. The effects of various anchoring groups on optical and electronic properties of dyes in DSSCs were also examined by Li-Na Yang, et al. [9]. More recently, we examined the combined effects of metal modification of the macrocyle and extending the πelongation near the anchoring group on cell performance [10]. The former results provide guide lines for the design of new sensitizers based on porphyrin and phthalocyanine macrocycles. Since red shifting of Soret and Q bands provides a straightforward approach to increasing the efficiency of dyes as sensitizers, we focused our attention on the effects of structural modifications on narrowing the band gaps. By comparing dyes to a dye with known solar cell efficiency, we could identify candidate(s) that are ideal for DSSCs. We have therefore used DFT and TDDFT methodologies to design three new sensitizers, taking into account YD2-o-C8, which has the highest record efficiency of (11.9% -12.7%) as the reference sensitizer. We replaced the porphyrine macrocycle cavity and carboxylic acid anchoring group of YD2-o-C8 by a phthalocyanine macrocycle cavity and cyanoacrylic acid anchoring group. We further address the issues of co-sensitization, and coupling with a semiconductor electrode which enabled us to investigate the effect of the dye structure on the important light harvesting property, and consequently the photo-tocurrent conversion efficiency. The former structural modification effects were then confirmed in terms of several photo-physical and photo-voltaic properties, such as frontier molecular orbitals (FMOs), energy gaps, semiconductor valence band (VB) and conduction band (CB) edges, coupling with TiO2 anatase unit cell, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) performance, UV-vis. electronic absorption and emission, life times of the excited

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states, light harvesting efficiency (ΦLHE), electron injection efficiency (Φinject.), charge collection efficiency (ΦCC), free energy of regeneration (ΔGregen.), Stokes shifts, cosensitization, and 1H-NMR chemical shifts.

2. Computational details Molecular electronic structure calculations were performed by using the density functional theory (DFT) and time dependent-density functional theory (TD-DFT). The electrons in DFT and TD-DFT methodologies are described quantummechanically, while the much heavier nuclei are treated classically [11]. DFT and TDDFT methodologies provide a modern and versatile means to investigate molecular and solid state structures, reaction pathways, thermo chemistry, dipole moments, spectroscopic response, and many other properties [12]. The DFT calculations were performed by using Becke’s three-parameter exchange functional (B3) with Lee-Yang-Parr (LYP) correlation functional [13]. The B3LYP hybrid functional has been chosen since it provides a rather accurate description of metal interactions. Hybrid functional such as B3LYP provides a fair indication of the relative energies, and in some cases the resulting differences between the experimental values and the calculated ones can be considered a systematic error [14]. B3LYP correctly reproduces the thermo chemistry of many compounds including transition metal atoms. Several successful applications using hybrid functionals have been reported [15]. Full geometry optimizations of the dyes were carried out at the B3LYP level of theory using an all electron double zeta basis set with one polarization function on heavier atoms 6-31g(d) with the following thresholds: maximum force:0.002500, RMS force: 0.001667, maximum displacements: 0.010000, and RMS displacement: 0.006667. The optimal geometries obtained were then employed to calculate all of the photo-physical and photo-voltaic properties at the B3LYP/6-31g(d) level of theory. UV.vis electronic absorption and the related property of life times of excites states were calculated by using the range separated hybrid functional CAM-B3LYP/631g(d). The contributions of singly excited state configurations to each electronic

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transition of the dyes were calculated with the time dependent density functional theory (TD-DFT). The density of states (DOS) were calculated by using the Gauss Sum 2.2.5 program [16]. All other calculations were carried out by using Gaussian 09 system [17] and the following SCF convergence criteria were considered (i) convergence on RMS density matrix=1.00D-07 within 128 cycles (ii) convergence on MAX density matrix=1.00D05 (iii) convergence on energy=1.00D-05. The optimal geometries of molecular skeletons and molecular orbital densities were visualized by using the corresponding Gauss View software.

3. Results and discussion Despite the complexity of processes involved in DSSCs, there are two main ways in which the efficiency of a dye solar cell can be improved: (i) extending the light harvesting region into the near-infrared (NIR) (ii) lowering the redox potential of the electrolyte to increase Voc [18]. However, the improvement of a DSSC performance has been basically controlled by the light harvesting capability of the dye, particularly the strong absorption in the NIR region where the solar flux of photons is maximal. In the present study, an attempt has been made to improve the highest record efficiency (11.9% -12.7%) of the synthesized cell YD2-o-C8 sensitizer [4] by designing structurally related candidates. We are therefore interested in the relative measures of the photo-physical and photo-voltaic properties rather than in the absolute measures. It is conceivable that the comparison of the properties obtained should give useful indications to the performances of the designed dyes. We have focused on the effects of structural modifications on narrowing the energy gaps, and consequently the red shifted absorption bands. The modifications of the dye structure were performed by replacing the porphyrin macrocycle by phthalocyanine macrocycle , and the carboxylic acid anchoring groups at the macrocycle periphery by cyanoacrylic acid anchoring group to tune the energy gaps. The present study helped us to figure out the integrated features of porphyrin and phthalocyanine sensitizers and the performances of the corresponding DSSCs. It also brought to light how the photophysical and photo-voltaic properties play a key role in the DSSC efficiency. The molecular structures and optimized geometries of the dyes Zn-Por-1 (YD2-o-C8), Zn-Por-2, Zn-Pc-1, and Zn-Pc-2 are presented in Fig.1.

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3.1 Frontier molecular orbitals The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are frequently referred to as frontier molecular orbitals (FMOs). Since charge separated states is one of the main factors affecting solar cell efficiency, qualitative predictions of the efficiency of sensitizers could be made using HOMO, LUMO, and HOMO-LUMO energy gaps. HOMOs must be localized on the donor and LUMOs on the acceptor to create an efficient charge separated states [19]. To clarify the donor-pi-acceptor (D-π-A) nature of the present dyes, the HOMOs and LUMOs were calculated, and the frontier orbital isosurface plots are presented in Fig.2. From the frontier molecular orbitals illustrated, we could see that in Zn-Por-1 and Zn-Por-2 HOMOs are localized on the diphenylamine donor groups, LUMOs on the carboxylic acid and cyanoacrylic acid acceptor groups, and the porphyrin chromophors constitute the π-bridges. In Zn-Pc-1 and Zn-Pz-2, HOMOs are localized on the phthalocyanine macrocycle donors, LUMOs on the carboxylic acid and cyanoacrylic acid acceptor groups, and the phthalocyanine chromophors constitute the π-bridges. The present structures are therefore (D-π-A) type donors with electron-rich (donor) and electron-poor (acceptor) sections connected through a conjugated (π) bridge. The electron poor section is functionalized with an acidic binding group that couples the molecule to an electrode surface, such as TiO2 anatase, to form an integrated DSSC in the presence of a redox mediator. The DFT calculations show coplanar conformation between the macrocycle moiety and the π-spacer connecting anchoring group, Fig.2. The coplanar push-pull system is favorable to the interaction between the electron donor group and the conjugated πspacer. Furthermore, it is beneficial to the electron injection due to the modified energy levels arising from the introduction of the π-conjugated spacer. As shown in Fig.2 the π- electron density distributions of HOMOs and LUMOs localize mainly at the conjugated macrocyle cavities. Interestingly, the anchoring groups possess nearly exclusive electron density distributions at LUMO+1 of Zn-Por-2, Zn-Pc-1, and ZnPc-2 dyes suggesting that electron injection from higher excited states involving LUMO+1 should be more efficient than the lower ones involving LUMO.

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The large sizes of the present dyes create barriers between holes in the redox mediator and electrons in the electrode surface, thereby inhibiting dye recombination. It is also noteworthy that the strain of a porphyrin ring is likely to cause fast radiative relaxation in the porphyrin singlet excited states [20]. The radiative relaxation would compete with electron injection to the conduction band (CB) of an oxide electrode, resulting in a decrease of the cell performance. As shown in Fig.2, the macrocycle cavities are not strained under the effect of replacing porphyrin macrocycle by phthalocyanine macrocyle or changing the type of the anchoring group, explaining the expected high cell performance of the four dyes. Moreover, for phthalocyanines, the additional four nitrogens are placed at the macrocycle planes indicating that the lone pairs participate in the π-conjugated systems at one hand, and explaining the expected higher cell performances of phthalocyanines relative to porphyrins at the other hand. The electron densities of the terminal anchoring groups at LUMOs of the four dyes are shown in Fig.2. The increased electron density of the cyanoacrylic acid anchoring group relative to that of the carboxylic acid anchoring group may be attributed to the excess nitrogen and the extended length of π-conjugation. The larger electron density lead to efficient electron injection from the dye singlet excited state to the CB of the electrode owing to the strong electronic coupling between the excited adsorbed dye and the 3d- orbitals, that make up the CB of the electrode surface through the anchoring group, resulting in the difference in the cell performances between the two systems. This gives a primary indication to the enhanced cell performance of the dyes containing cyanoacrylic acid anchoring groups relative to those containing carboxylic acid anchoring groups.

3.2 Energy gaps The energy difference between the HOMO and LUMO is termed the HOMOLUMO energy gap or band gap. The photovoltaic performance of a dye sensitized solar cell is closely associated with the energy gap of the dye, which correlates with the experimental parameters: maximum short current intensity (Jsc, mA/Cm2) and (E1/2(oxidation)-E1/2(reduction)/V) potentials. Developing efficient near infrared NIR dyes represents one of the basic strategies for improving the performance of the DSSCs. Frontier orbital energy levels and energy gaps of the dyes Zn-Por-nC and

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Zn-Pc-nC, where n=1-3, are given in Table 1. As can be seen, replacing the porphyrin ring by phthalocyanine ring (and/or) replacing the carboxylic acid anchoring group by cyanocarboxylic acid anchoring group lead to narrowing the band gap of the highest record efficient dye YD2-o-C8 (Zn-Por-1). This nominates ZnPor-2, Zn-Pc-1, and Zn-Pc-2 dyes to be better candidates for DSSCs by virtue of harvesting more light in the solar spectrum with expected percent efficiencies exceeding 11.9% to 12.7%. The expected order of performance is Zn-Por-1 < ZnPor-2 < Zn-Pc-1 < Zn-Pc-2. As shown in Table 1, while the addition of cyanocacrylic acid anchoring group to either porphyrin or phthalocyanine macrocycle periphery marginally decreases the corresponding band gap, the replacement of porphyrin macrocycle by phthalocyanine macrocycle sharply decreases the corresponding band gap. The decrease in HOMOLUMO energy gap mainly comes from destabilizing (raising) the HOMO. The macrocycle (and/or) anchoring group replacement negligibly affect the level of LUMO. The effect of ring replacement on cell performance is therefore more significant than that of the anchoring group replacement. In other words, ring replacement leads to harvesting more light in the domains of UV.vis and NIR. Furthermore, the molecular structures of the present dyes with elongated peripheral side chains, and anchoring group moieties help in suppressing the macrocycle aggregations on the electrode surface. When the sensitizer Zn-Por-1 is compared with Zn-Por-2 and Zn-Pc-1 with ZnPc-2, we will observe that they have similar macrocycle cavities but different anchoring groups. On the other hand, when the sensitizer Zn-Por-1 is compared with Zn-Pc-2 and Zn-Por-2 with Zn-Pc-2, we will observe that they have similar anchoring groups but different macrocycle cavities. Since the band gaps follow the order Zn-Por-1> Zn-Por-2> Zn-Pc-1> Zn-Pc-2, it can be concluded that either replacing porhyrin macrocycle cavity by phthalocyanine, or carboxylic acid anchoring group by cyanoacrylic improves the light harvesting property and consequently the performance of Zn-Por-1 (YD2-o-C8). It also follows that the combined effect of macrocycle and anchoring group replacements in the sensitizer Zn-Pc-2 has the greatest impact on the light harvesting property and performance of Zn-Por-1 (YD2o-C8). Moreover, the forecasted optimal performance of the present sensitizer ZnPc-2 relative to the other three sensitizers should be attributed to (i) the extra four

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nitrogens of the phthalocyanine macrocycle cavity (ii) the strong-pulling power of CN and the extended π-conjugation of the cyanoacrylic acid anchoring group. To calibrate the present calculations with the available experimental and theoretical results, we considered the HOMO-LUMO energy gaps. The band gap (2.325 eV) of the present Zn-Por-1 dye obtained from full geometry optimization at the B3LYP/631g(d) level of theory is very close to the band gap (2.334 eV) of the identical YD2-oC8 dye calculated at the same B3LYP/6-31g(d) level of theory by Yella, et al. [4]. The band gap (2.334 eV) of the present Zn-Por-1 dye may also be correlated with the experimental value [E1/2(oxidation)-E1/2(reduction)=2.690 V] of YD2-o-C8 reported by Yella, et al. [4] where [E1/2(oxidation)/V] is related to the irreversible process Epa. It is worth noting here that electron volt (eV) is a unit of energy, and volt (V) is a unit of electrical potential difference. They are not the same thing but there is a connection. The energy gained (or lost) by a charge q (in Coulombs) moving through a potential difference V is [1eV=1V*q]. 1eV is the energy gained (or lost) by an electron moving through a potential difference of 1 volt. We may also note that the concept of molecular design with the cyanoacrylic acid acceptor has been widely applied in highly efficient organic dyes [22]. While such an approach does not work well for the porphyrin sensitizers as demonstrated by Ripolles-Sanchis, et al. [5] it does work well for both of the porphyrin and phthalocyanine sensitizers as demonstrated here.

3.3 Mesoporous surface The present DSSCs are based on p-type organic dye donors and n-type inorganic semiconductor acceptors, namely, DSSCs using semiconductor electrodes and redox mediators. In solid-liquid junction solar cells, the voltage is attributed to the energy gap between the Fermi level (near conduction-band level for n-type semiconductor) of the semiconductor electrode and the redox potential of the mediator in the electrolyte. The HOMOs, LUMOs, and energy gaps of Zn-Por-1, Zn-Por-2, Zn-Pc1, and Zn-Pc-2 dyes as well as (TiO2)40 , Ti35O76 , SiC, and SrTiO2 semiconductor electrodes [23] are presented in Fig.3. An efficient electron injection into an electrode surface is required for enhancing photo-to-current conversion efficiency of a DSSC. Inspection of Fig.3, reveals that the LUMOs of the dyes are higher than the lower edges of the conduction bands (CBs) of the semiconductor electrodes, and HOMOs are noticeably higher than the upper edges of valence bands (VBs) of the

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semiconductor surfaces. This indicates that all of the present dyes should be capable of injecting electrons into the present semiconductor electrodes upon excitation. Furthermore, all HOMOs of the dyes fall within the (HOMO - LUMO) energy gaps of the semiconductor electrodes. The LUMOs of the dyes are close, implying that the electron injection processes are comparable. If the difference between the lower edge of the CB of a semiconductor electrode and the redox potential of an electrolyte is taken as a measure of the open circuit voltage (Voc), the electron injection processes should be most favorable into Ti35O76 oxide semiconductor, since lowest edge of CBs is assigned to this oxide. In this context, it may be noted that the open circuit voltage (Voc) is obtained at present only by experiment, where the relationship between this quantity and the electronic structure of dye is still unknown. However, the analytical relationship between Voc and ELUMO may exist, Kumar, et al. [24]. According to the sensitization mechanism and single electron and single state approximation, there is an energy relationship eVoc = ELUMO-ECB where ECB is the energy of the lower edge of the conduction band, and e is the electron charge. So the Voc may be obtained by applying the following formula Voc = [ELUMO-ECB]/e The formula indicates that the higher the ELUMO , the larger the Voc . The results of the present dye sensitizers prove this tendency for Zn-Pc-1 and Zn-Pc-2 i.e. under the effect of replacing porphyrin macrocycle by phthalocyanine macrocycle. We also note that Ning, et al. [25] speculated that the increased ELUMO might produce deep electron injection for retarded charge recombination. Solvent effects should also be taken into account. Cahen, et al. [26] reported that the maximum for the lower edge of the CB in an oxide semiconductor electrode surface, such as TiO2, using saturation photo voltage measurements (-4.45 eV) shifts to (-3.90 eV) when immersed in a solvent with an electrolyte. To explain the relation between band gap reduction due to a particular modification of the dye and conversion efficiency of the solar cell, suppose that the situation, in which HOMO of the sensitizer is lower enough than the redox potential

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of the electrolyte and the LUMO is higher enough than the conduction band of the semiconductor electrode, will not be influenced by the HOMO, LUMO variations. In this case, as the HOMO-LUMO gap decreases, more photons at the longerwavelength side would be absorbed to excite the electrons into the unoccupied molecular orbital, which increases the short current intensity Jsc and further the conversion efficiency η of the solar cell. The former data therefore suggest the sensitized mechanism of the present dye-semiconductor couple to be an interfacial electron transfer between the semiconductor electrode and the dye, such that electron injection process takes place from the excited dye to the semiconductor conduction band. This is a kind of typical interfacial electron transfer reaction [28]. To minimize solar energy technology limitations, all solid state dye sensitized solar cells (ss-DSSCs) have been developed. In ss-DSSC, a thin-film compound, such as spiro-OMeTAD and CuSCN, replaces the entire liquid electrolyte. The cell uses both n-type and p-type semiconductors and a monolayer dye molecule serving as the junction between the two. Each nearly spherical nanoparticle, made of titanium dioxide, is an n-type semiconductor. The sunlight-absorbing dye, where photons are converted into electricity, lies right between the two semiconductors. The thin film plays an additional role in the operation of the cell that is not played by the liquid electrolyte couple, and that role is light absorption. However, the conversion efficiencies of ss-DSSCs are still lower than liquid junction cells. Now, if the present dye sensitizers are applicable, i.e. the redox potential of the solid electrolyte was higher than the HOMOs of the dyes, one could expect a series of highly efficient ss-DSSCs without the problematic liquid electrolytes.

3.4 Density of states and molecular electrostatic potentials As shown in Table 1, the band gaps are narrowed under the effect of replacing the porphyrin macrocycle by phthalocyanine (and/or) replacing the carboxylic acid anchoring group by the cyanoacrylic acid anchoring group. This explains the expected higher solar cell efficiencies of Zn-Pc-n dyes relative to Zn-Por-n dyes, and implies that the density of states (DOS) become more abundant near Fermi levels of Zn-Pcn sensitizers. Consequently, explanation of the differences in photo-to-current conversion efficiency due to the former replacements, in particular macrocycle

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replacement, is sought in the density of states. The density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. A high DOS at a specific energy level means that there are many states available for occupation, and therefore scales linearly with the photovoltaic conversion efficiency. The calculated DOS are presented in Fig.4 to clarify the effects of macrocyle and anchoring group replacements. As shown, DOS becomes more abundant near Fermi levels in both cases, where the overall features of DOS change, and deep vales develop on both sides nearby Fermi levels. . The molecular electrostatic potential (MEP) contours or surfaces have been established extensively as a guide to the interpretation and prediction of molecular behavior [29]. It has been shown to be a useful tool in studying both electrophilic and nucleophilic processes, in particular the “recognition” of one molecule by another [30]. Molecular electrostatic potentials are either negative, low potentials that are characterized by an abundance of electrons and reactive with electrophiles, or positive, high potentials that are characterized by an absence of electrons and reactive with nucleophiles. We denote the former by a deep red color, and the later with a deep blue color. The molecular electrostatic potential surfaces are given in Fig.5. As shown, the intensity of the positive and negative MEP contours are affected by the macrocycle and anchoring group replacements. The negative (red) low potentials grow up in the regions of the macrocycle cavities and extend to the (octyloxy) side chains. Simultaneously, the positive (blue) high potentials are significantly reduced in these regions but extend to the regions of the anchoring groups. In other words, while the macrocyle regions are readily available for electrophilic processes, the regions of the anchoring groups are readily available for nucleophilic processes. This distribution ensures the issues of charge separated states and unidirectional charge transfer which contribute to the enhanced photovoltaic performances of the relevant sensitizers.

3.5 Non linear optical performance The molecular polarizability, anisotropy of polarizability, and electronic dipole moment are calculated. The non linear optical (NLO) properties, such as polarizability and anisotropy of polarizability, characterize the response of a system in an applied electric field, which in turn determines the intramolecular charge delocalization based on asymmetric polarization induced by electrons, and donor and acceptor groups in πelectron conjugated molecules. They determine the strength of molecular interactions

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as well as the cross sections of different scattering and collision processes. The potential of semiconducting organic materials to transport electric current and to absorb light in the ultraviolet-visible part of the solar spectrum is due to the sp2hybridization of carbon atoms. The electron in the pz-orbital of each sp2-hybridized carbon atom will form π-bonds with the neighboring pz- electrons. These π-electrons are of a delocalized nature, resulting in high electronic polarizability. Another important potential of semiconducting organic materials to transport electric current and to absorb light in the ultraviolet-visible part of the solar spectrum is the relatively small diffusion length of the excitons, which are an important intermediates in the solar energy conversion process. Usually strong electric fields are required to dissociate excitons into free charge carriers, which are the desired final products for photovoltaic conversion. The significance of polarizability of molecular systems is dependent on the efficiency of electronic communication between acceptor and the donor groups as that will be the key to intramolecular charge transfer [31]. Good candidates for sensitizers in DSSCs are due to its larger molecular polarizabilities, better light harvesting efficiencies, proper matching of the groundstate oxidation potentials with respect to the redox couple, and higher dipole moments of the adsorbed analogues, Lee, et al. [32]. The higher values of molecular polarizabilities and dipole moments are important for more active NLO performance. The NLO phenomena occur at sufficiently intense fields. As the applied field strength increases, the polarization response of the medium is no longer linear and the induced polarization (P) becomes a function of the applied field [33].

P = χ(1) E + χ(2) E2 + χ(3) E3 + ..... where the (2) and (3) coefficients represent susceptibilities of the medium respectively. At the molecular level eq. (1) is expressed as [34]

the

second

(1) and

third

order

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P = α ij E + γ ijkl E 3 + ..... β ijk E 2 + j j
(2)

ijk = first hyperpolarizability (second order effects),

ij = polarizability,

ijkl =second hyperpolarizability (third order effects), and i,j,k,l corresponds to the molecular coordinates. The definition [35] for the mean polarizability is

1 α = (α xx + α yy + α zz ) 3

(3)

and the anisotropy of polarizability is Δα =

2 1 + α xx - α yy 2

α yy - α zz

2

+

α zz - α xx

2

+ 6α

2 xz +

6α

2 xy +

6α

2 yz

1 2

(4)

The total dipole moment can be calculated by using the following equation

μ tot = μ where

i

i x,y,z

2 + x

μ

2 + y

μ

2 z

1 2

(5)

is the electronic dipole moment.

The Gaussian polarizability tensors (αxx, αxy, αyy, αxz, αyz, αzz ) can be obtained by finite field, sum over states method, and coupled perturbed Hartree-Fock method. However, the use of the previous methods with large sized basis sets and molecules are too expensive. Here, we compute the polarizability and hyperpolarizability tensors as numerical derivatives of the dipole moment obtained by a frequency job output file of at the B3LYP/6-31G(d) level of theory. The calculated parameters described above for Zn-Por-n and Zn-Pc-n dyes are given in Table 2 to clarify the effects of macrocycle and anchoring group replacements. Several facts emerge from Table 2: (i) the values of the polarizability α increase with macrocycle (and/or) anchoring group replacements and are dominated by the term αxx . All other terms are less affected by these replacements (ii) the

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anisotropy of polarizability increase with macrocycle (and/or) anchoring group replacements (iii) the values of the resultant dipole moment μ increase with macrocycle (and/or) anchoring group replacements, and are dominated by the term μx. All other terms are less affected by these replacements. While the effects of anchoring group replacements on polarizability α anisotropy of polarizability Δα are more significant than those of macrocycle replacements, the effects of anchoring group replacements on total dipole moments μ are less significant than those of macrocycle replacements. The largest values, and consequently the highest field responses, are assigned to the dye Zn-Pc-2 due to the combined effects of macrocycle and anchoring group replacements. To conclude, the reported values of polarizability α, anisotropy of polarizability , and resultant dipole moment μ suggest that replacing porphyrin macrocycle by phthalocyanine, and carboxylic acid anchoring group by cyanoacrylic lead to more active NLO performances, stronger responses to the external electric field, and induce higher photo-to-current conversion efficiency.

3.6 Reorganization energy The efficiency of a DSSC depends sensitively on the charger transfer character of the lowest excited state in the dye. The charge transport in a donor-pi-acceptor (D-π-A) sensitizer can be described as a sequence of uncorrelated hops. In the case of self exchange reaction, i.e. electron transfer reaction from a charged species to an adjacent neutral species, the rate of charge transfer can be approximately described as [36]

K CT A e

4 KBT

(6)

where A is a prefactor that depends on the strength of the electronic coupling between adjacent units, λ is the reorganization energy, KB is the Boltzmann constant, and T is the temperature. The reorganization energy is the energy required for all needed structural adjustments in order to adopt the configuration of the compound that is necessary for the electron transfer process [37]. The reorganization energy λ can be divided into two terms, the external and internal reorganization energies. The external reorganization energy can be derived from the polarization changes in the

16

dielectric solvent environment, whereas the internal reorganization energy is a measure of structural changes between ionic and neutral states [38]. Baesd on time resolved studies [39], the recombination rate was found to be insensitive to the nature of the solvent that surrounds the sensitizer and electrolyte composition. Therefore, the external reorganization energy does not have a significant influence on the electron transfer dynamics. Thus the reorganization energy is mainly influenced by the internal electron transfer processes which can be defined as follows [40].

λhole = (E

E

M

λelectron = (E

M

M

) + (E M

E

M

E

) + (E M

M

(7)

)

E

M

)

(8)

where E+, E, and E- represent the energies of the cation, neutral, and anion species based on their lowest energy geometries, respectively, while M+, M, and M- denote their optimized structures at corresponding ion states. For example,

E

M

is the energy

of a cation/anion calculated with the optimized structure of the neutral species M. It was observed that the trend for the total reorganization energies λtotal was inversely proportional to the short current intensity (Jsc), i.e. as λtotal decreases, the Jsc value increases; which was consistent with equations 7 and 8 that minimizing λ leads to a high electron transfer rate. Furthermore, the magnitudes of the charge carrier transport rates should be balanced in order to attain a high efficiency. Since the band gaps of Zn-Pc-n dyes are smaller than Zn-Por-n counterparts, Table 1, Jsc values are expected to be larger. We have therefore compared between λtotal values of Zn-Pc-1 and Zn-Pc-2. As shown in Table 3, λtotal of Zn-Pc-2 is smaller than that of Zn-Pc-1 indicating that Zn-Pc-2 has a more balanced charge carrier transport rate. This indicates the increasing probability of confined excitons in the emitting layer due to replacing the carboxylic acid anchoring group by cyanoacrylic acid of the present phthalocyanines.

3.7 UV-vis. absorption In order to understand the electronic transitions of the present dyes, time dependent density functional theory (TD-DFT) calculations of the electronic absorption spectra in vacuum were performed. The 15 lowest spin-allowed singlet-

17

singlet transitions were calculated. The absorption in visible and near-UV regions are the most important regions for photo-to-current conversion, so only the singlet to singlet transitions of the absorption bands with modulus configuration interaction coefficients greater than 0.10, wave lengths longer than 300 nm, and oscillator strengths greater than 0.10 were considered. The early theoretical interpretations of the spectra of various metalloporphyrins have been made by Gouterman [41] who initially used the four orbital model to interpret the Q and B bands, which assumes that the HOMO and HOMO-1 are almost degenerate in energy and well separated from the other levels, and a similar assumption is made for the LUMO and LUMO+1. As shown in Table 1, non negligible separations between the two highest occupied orbitals and the two lowest unoccupied orbitals result in the fact that the four orbital model no longer holds for these molecules. As shown in Table 3, the maximum absorption bands are red shifted under the effect of macrocycle replacement (and/or) anchoring group replacement. The effect of macrocycle replacement on light harvesting is more significant than that of anchoring group replacement. The maximum absorption bands result from the electronic transitions which take place from the initial states that are mainly contributed by HOMOs to the final states that are mainly contributed by LUMOs. The other absorption bands result from the electronic transitions which take place from the initial states that are contributed by HOMOs-n to the final states that are contributed by the LUMOs+n. These absorption bands in the visible region are typical π-π* transitions. The maximum absorption bands are red shifted by 315 and 285 nm under the effect of macrocycle replacements, and by 60 and 31 nm under the effect of anchoring group replacemen. Usually, if the absorption bands are close to infrared region, it is expected that the dyes have higher photo-to-current efficiency. On this basis, the dyes Zn-Por2, Zn-Pc-1, and Zn-Pc-2 are expected to have higher photo-to-current conversion efficiencies than the dye Zn-Por-1 (YD2-o-C8) which is characterized by (11.9-12.7) percent efficiencies. Fig.6 ............................................................. A measurement of the performance of a DSSC is called incident photon-to-current conversion efficiency (IPCE). Simply, it is a function of light harvesting efficiency (ΦLHE), electron injection efficiency (Φinject), and charge collection efficiency (ΦCC) and can be expressed as [42]: IPCE = ΦLHE Φinject ΦCC

(9)

18

The efficiency of the electron density movement is related to both optical absorption intensity and available electron transition and could be estimated from the light harvesting efficiency (ΦLHE) and the coefficient of effectual electron transition. The equation used for the calculation of ΦLHE is [43]: ΦLHE = 1- 10-f

(10)

where f is the oscillator of the dye associated to the λ. The calculated ΦLHE values of the present dyes at λmax are listed in Table 4. Obviously macrocycle (and/or) anchoring group replacements lead to increasing ΦLHE of Zn-Por-1 (YD2-o-C8) sensitizer. The electron injection efficiency (Φinject) is closely related to the driving force (ΔGingect.) of the electron injection from the photo induced excited states of the sensitizer to the CB of the semiconductor. A larger driving force is desirable for more rapid and efficient electron injection and then higher photocurrent of DSSCs. Preat, et al. [44] have proposed a theoretical scheme to quantify the electron injection onto a TiO2 surface. The free energy change (ΔGingect.) in eV for the electron injection can be expressed by the following equation [45]: dye* ΔGinject. = EOX -

where

E

dye* OX

E

sc

(11)

CB

is the oxidation potential of the dye in the excited state, and

E

sc CB

is the

reduction potential of the semiconductor CB ( 2.77, 2.97, 3.00, and 3.00 eV for Ti35O76, (TiO2)60, SiC, and SrTiO respectively). Assume that the electron injection occurs from the unrelaxed excited state,

E Where

E

dye OX

dye* OX

=

E

E dye OX

dye* OX

-

can be expressed as

E

dye

(12)

exc.

is the redox potential of the ground state, and

transion energy (in eV) at λmax. The calculated

E

dye exc.

,

E

dye OX

,

E

E dye* OX

dye exc.

is the vertical

, and ΔGingect for

Ti35O76, (TiO2)60, SiC, and SrTiO3, semiconductor electrodes are also listed in Table 4. As can be seen, while ΔGingect increases (more negative) with macrocycle

19

replacement, it decreases (less negative) with anchoring group replacement, implying more favorable electron injections from the excited-states of Zn-Pc-n dyes to the semiconductors CBs. The results also indicate that the efficiency of electron injection depends on the type of the semiconductor electrode, and may be ordered as follow: SrTiO3 > SiC > (TiO2)60 > Ti35O70. Moreover, the product (ΦLHE x Φinject ) of Zn-Pcn is greater than that of Zn-Por-n implying greater contribution to IPCE. The efficiency of dye regeneration or the free energy change of dye regeneration ΔGregen can affect the rate constant of redox process between the oxidized dyes and electrolyte. Taking into account the ideal redox potential (3.5 eV vs vacuum), ΔGregen can be calculated from the relation dye ΔGregen = E OX

-

E

electrolyte redox

(13)

electrolyte

Where Eredox is the redox potential of the electrolyte. The the free energy changes of the present dyes are also listed in Table 4. As shown, replacing the porphyrin macrocycle by phthalocyanine macrocycle generates a considerable reduction of ΔGregen. Overall, the given data reveal that the UV-vis. absorptions are photo induced electron transfer processes. This in turn implies that the corresponding excitations generate charge separated states, and contribute to the sensitization of photo-tocurrent conversion processes. In other words, macrocycle (and/or) anchoring group replacements shift the absorption bands to longer wavelengths, and make the dyes Zn-Por-n (n=2) and Zn-Pc-n (n=1-2) good potential candidates for harvesting more light in the UV-vis. region of the solar spectrum for photovoltaic applications relative to the highest efficient synthesized cell Zn-Por-1 (YD2-o-C8).

3.8 The life times of excited states The life time of excited state is an important factor for considering the efficiency of charge transfer of dyes. A dye with a longer life time in the excited state is expected to be more facile for charge transfer. An accurate prediction of an excited state life time needs to account for all deactivation channels of the excited states. To roughly estimate the life time of the excited state, an assumption was made by Yang, et al. [46] that spontaneous deactivation is the main competing deactivation process with electron injection and other deactivation channels can be omitted. Although the

20

assumption is very speculative and it could not be expected to obtain accurate life times which are close to the real values, it is conceivable that the comparison of the life times obtained with the same assumption should give a useful clue to the performances of dyes at the relative measure. The life time of an excited state was estimated by using the formula [47]: 2 2 3 4e E r 1 kk kk A kk = 4 3 3 c A kk (14) where charge,

A

kk

is Einstein coefficient for spontaneous emission, e is the elementary

is the reduced Planck’s constant, c is the speed of light in vacuum, and rkk

represent the transition energy and dipole moment from k to k respectively. This formula has been successfully applied to the investigation of several organic dyes [48]. The life time of the first excited state was employed to predict the efficiency of electron injection. The calculated life time of the first excited state (S1) along with the related excitation energy and oscillator strength for Zn-Por-n (n=1-2) and Zn-Pc-n (n=1-2) are given in Table 5. As shown, the life times of the excited states increase with replacing the porphyrin macrocyle by phthalocyanine. This reveals that the phthalocyanine dyes Zn-Pc-n (n=1-2) are more efficient in electron injection relative to the porphyrin dyes Zn-Por-n (n=1-2) in consistence with ΔGinject values of Table 4. On the other hand, the life times of the excited states decrease with replacing the carboxylic acid anchoring group by cyanoacrylic acid, again in consistence with ΔGinject values of Table 4. This could be attributed to the stronger pulling power of the cyanoacrylic acid. Cyanoacrylic acid is an efficient acceptor group but its dual role of both acceptor and anchor group could favor fast charge recombination thus explaining the lower injection efficiency. With this, it can be concluded that replacing the porphyrin macrocycle by phthalocyanine macrocycle is promising in the design of dyes with higher photo-to-current conversion efficiencies relative to the highest record reference cell YD2-o-C8.

21

3.10 Co-sensitization In an effort to further enhance the conversion efficiency, the co-sensitization approach is suggested to make up the weak absorption of porphyrin between 450 and 550 nm. One of the strategies for improving this conversion efficiency is to reduce the energy gap of the dyes so that more light in the spectral range 650-940 nm can be absorbed. Using a dye that absorbs further into the NIR while still managing to generate and collect the charge carriers efficiently, could increase the power conversion efficiency beyond 14%. However, finding one dye that absorbs strongly all the way from 350-940 nm is extremely difficult. The data given in Table 3, indicate that the combinations (ZnPor-n+Zn-Pc-n) can be employed as co-sensitizers to extend light harvesting beyond the capability of each individual dye. For instance, the combination (Zn-Por-1+Zn-Pc-2) should absorb fairly strong from 532 to 881 nm.

4. Conclusions We have performed an extensive set of theoretical calculations employing the state of the art DFT and TD-DFT methodologies in order to design new phthalocyanine sensitizers capable of exceeding the highest record efficiencies of porphyrin sensitizers. While the concept for molecular design with the cyanoacrylic acid anchoring group alone has been widely applied in highly efficient organic dyes [22], such an approach does not work well for the porphyrin sensitizers [5]. Herein, we demonstrate that the concept for molecular design with the cyanoacrylic acid anchoring group and phthalocyanine sensitizers can be applied in highly efficient organic dyes. By comparing dyes to a dye with known solar cell efficiency, we could identify candidate(s) that are ideal for DSSCs. It is conceivable that the comparison of the photo physical and photo voltaic properties obtained with the same assumption should give a useful clue to the performances of dyes at the relative measure. Several photo voltaic and photo physical properties were calculated to provide the electronic information about the roles played in dominating the performances. We focused on macrocycle and anchoring group replacements, in addition to testing several

22

semiconductor electrodes and interpreted the reasons that the suggested sensitizers could be expected to improve the performance of the synthesized YD2-o-C8 cell. The expected high efficiencies are ascribed to rich charge separation, unidirectional charge transfer, narrower band gaps, increase of DOS nearby Fermi levels, active NLO performance, increasing the probability of confined excitons in the emitting layer, delocalization of the negative charges near the anchoring groups, efficient light harvesting, electron injection and charge collection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The structural modifications, in particular the replacement of porphyrin by phthalocyanine, lead to desirable properties related to increasing the photo-to-current conversion efficiency. The additions of four nitrogens and four phenyl groups to the porphyrin macrocycle (the phthalocyanine macrocycle derivative) could create potential good sensitizers for DSSCs. However, to exert the latent ability of porphyrin, it is also necessary to incorporate the cyanoacrylic acid anchoring group and a suitable redox electrolyte. We may note that the addition of four nitrogens in the phthalocyanine donor moiety participates in the construction of HOMO with sp2 hybridized character, and plays an important role in assisting charge transfer and cell performance relative to the porphyrin counterparts. The present results provide structural guide lines for improving the performances of a porphyrin based sensitizer, provide a nontraditional theoretical model, in which the porphyrin ring of the dye is replaced by a phthalocyanine ring, and lead us to suggest that an optimization of both the donor-π-acceptor (D-π-A) structures of the dyes and the semiconductor electrodes could lead to DSSCs with yet improved efficiencies. The present methods and techniques are applied equally well to both of the reference and designed sensitizers, so that they can be well predictive. Further experimental investigations to substantiate the present theoretical modeling are necessary.

23

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[43] J Preat, C Michaux , D Jacquemin, E A. Perpete, Enhanced Efficiency of Organic Dye- Sensitized Solar Cells: Triphenylamine Derivatives, J. Phys. Chem. C, 113(2009)16821 –16833. [44] J Preat, D Jacquemin, C Michaux , E A. Perpete, Improvement of the efficiency of thiophene-bridged compounds for dye-sensitized solar cells, Chem. Phys. 376 (2010)56–68. [45]R Katoh, A Furube, T Yoshihara,K Hara,G Fujihashi,S Takano,S Murata, H Arakawa, M. Tachiya, Efficiencies of Electron Injection from Excited N3 Dye into Nanocrystalline Semiconductor (ZrO2, TiO2, ZnO, Nb2O5, SnO2,In2O3) Films, J. Phys. Chem. B, 108 (2004) 4818-4822. [46]L-N Yang, Z-Z Sun, S-L Chen, Z-S Li, The effects of various anchoring groups on optical and electronic properties of dyes in dye-sensitized solar cells, Dyes and Pigments 99 (2013) 29-35. [47]Einstein A. On the quantum theory of radiation., Phys Z 18(1917)121-128.

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[48]Chen SL, Yang LN, Li ZS, How to design more efficient organic dyes for dyesensitized solar cells? Adding more sp2-hybridized nitrogen in the triphenylamine donor. J Power Sources 223(2013)86-93.

  

Table 1: Frontier orbital energy levels (HOMO and LUMO) and energy gaps (E.G) of the dyes Zn- Por –n and Zn- Pc -n (n=1-2) calculated at the B3LYP/631g(d) level of theory. All energies are given in eV. Values between round brackets were obtained from the TD analysis at CAM-B3LYP/6-31g(d) level of theory. 



Zn- Por - 1

Zn- Pc- 1

-1.922 -2.269 -4.594 -4.961 2.325 (2.329)

-1.189 -2.034 -3.512 -3.595 1.478 (1.459)

Zn- Por - 2 Zn- Pc - 2



LUMO+1 LUMO HOMO HOMO-1 E.G.

-2.029 -2.650 -4.674 -5.048 2.024 (2.093)

-1.962 -2.366 -3.661 -3.744 1.295 (1.406)

31



Table 2: Polarizabilities (), anisotropy of polarizability (), and dipole moments () of the dyes Zn- Por -n and Zn- Pc-n (n=1-2) calculated at the B3LYP/6-31g(d) level of theory.

XX ZnPor -1 ZnPc-1 ZnPor2 ZnPc2

XY

YY

XZ

YZ

ZZ





x

y

z



1147 1406 622 0.824 0.402 0.300 0.965 2274 4.48 1676 -1.93 221.27 1287 1746 861 2.347 0.398 0.139 2.385 1816 9.90 1254 -2.33

2179 9.45 1315 -5.76

92.22

84.92

1141 1545 963

1.964 0.944 0.172 2.186

2903 3.85 1694 28.51 247.17 1321 1972 1432 4.857 1.487 0.983 5.174



Table 3 : Computed excitation energies, electronic transition configurations, oscillator strengths (ƒ), and molar extinction coefficients () for the optical transitions of the absorption bands in the UV-vis. regions (involving HOMOs) of the dyes Zn- Por -n and Zn- Pc-n (n=1-2) in vacuum computed at the CAM-B3LYP/ 6-31G(d) level of theory. Zn- Por - 1 State Configuration composition with (corresponding transition orbital)  

0.1

Excitation energy (eV/nm)

ƒ

H/( M-1 Cm-1)

32



2

0.42(407 -> 410); 0.55 (408 -> 409)

2.3290 / 532

0.1172

6693

3

0.52 (406 -> 409); -0.13 (406 -> 411); 0.32 (407 -> 410); -0.28 (408 -> 409)

2.9573 /419

0.4250

17657

Excitation energy (eV/nm) 1.4590 /850 1.5363/ 807 2.7210/ 455

ƒ

HOMO is No. 408 orbital 





Zn- Pc - 1 State Configuration composition with 0.1 (corresponding transition orbital) 1 0.11 (461 -> 465); 0.69 (462 -> 463) 2 0.69 (461 -> 463); -0.12 (462 -> 465) 3 0.70 (460 -> 463)

H/(M-1 Cm-1)

0.5283 0.7518 0.1077

50618 51098 4367

ƒ

H/( M-1

HOMO is No. 462 orbital

Zn- Por - 2 State Configuration composition with 0.1 (corresponding transition orbital) 1 0.34 (420 -> 423); 0.54 (421 -> 422); -0.26 (421 -> 424) 3

-0.35 (419 -> 422); 0.30 (419 -> 424); -0.11 (419 -> 425); 0.39 (420 -> 423); -0.30 (421 -> 422)

Excitation energy (eV/nm) 2.0928 / 592 2.8490/ 435

Cm-1)

0.4273

18340

0.7249

29890

HOMO is No. 421 orbital

Zn- Pc - 2 State Configuration composition with 0.1 (corresponding transition orbital)  

Excitation energy (eV/nm)

ƒ

H/(M-1 Cm-1)

33



1 2 3

1.4064/881 1.4906/831 2.2221/557

0.69 (475 -> 476) 0.68 (474 -> 476); -0.11 (475 -> 478) 0.62 (474 -> 477); -0.26 (475 -> 477)

0.4431 0.9299 0.2877

54232 55247 13334

HOMO is No. 475 orbital

dye

dye*

, EOX , Gregen , and Gingect. calculated at the    B3LYP/6-31g(d) level of theory. All energies are given in eV. Table 4: LHE ,

 

Zn Po r1 Zn Pc -1 Zn Po r 



 dye

E

exc.



2.133 1 1.696 5 2.012 9

f

0.342 5 0.446 2 0.496 3

E

OX





LHE

E

0.545 5 0.642 1 0.681 1

 dye

OX



E

dye* OX



4.59 4 2.461 3.51 2

1.815

4.67 4 2.661

Grege n

1.094

0.012

1.174

Gingect. Ti35O7 (TiO2)6 6

-0.309

-0.954

-0.109

SiC SrTiO

0

3

-0.509

0.53 9

-0.739

-1.154

1.18 4

-1.384

-0.309

0.33 9

-0.539

34



2 Zn Pc -2

1.663 3

1.135 2

0.926 7

3.66 1 1.997

0.160

-0.773

-0.973

1.00 3

-1.203

Table 5: The lifetimes of the first excited states (S0) of the dyes Zn- Por -n and Zn- Pc-n (n=1-2)a. 



Zn- Por - 1 Zn- Pc - 1 Zn- Por - 2 Zn- Pc - 2     

E (eV)b

fc

rk,k'd

W(10-9 s)e

2.3290 1.4590 2.0928 1.4064

0.1172 0.5283 0.4273 0.4431

2.0546 14.7793 8.3336 12.8589

36.22 20.47 12.30 26.27

35

      

a

All properties were calculated at the CAM-B3LYP/ 6-31G(d) level of theory. Calculated excitation energy for the first excited state. c Oscillator strength. d Transition dipole moment. e Lifetime for the first excited state. b

> Theoretical non-traditional approach to improve the performance of porphyrin based DSSCs. > Phthalocyanine based sensitizers could be expected to exceed the highest record efficiency of YD2-o-C8. > Structural modifications include macrocycle and anchoring group replacements. > The expected high efficiencies are confirmed by DFT calculations of several photo-physical and photo-voltaic properties.

 

Figure

Zn- Por-2

Zn- Pc-1

Fig.1 : Skeletal and wireframe optimized structures of Zn- Por –n and Zn- Pc -n (n=1-2).

Zn- Por-1

Zn- Pc-2

HOMO

HOMO

HOMO

HOMO

LUMO

LUMO +1

Zn- Por-1 Zn- Por-2 Zn- Pc-1 Zn- Pc-2 Fig.2: Frontier orbital isosurface plots (isodensity contours=0.02 a.u) of Zn- Por –n and Zn- Pc -n (n=1-2).

LUMO

LUMO

LUMO

LUMO +1

LUMO +1

LUMO +1

-3.51

1.48

-2.03

-4.67

2.02

-2.65

-3.66

1.30

-2.36

Ti38O76

-6.55

3.78

-2.77

SiC

- 6.00

(TiO2)60

-7.52

4.55

3.00

SrTiO3

- 6.20

3.20

-2.97 -3.00 -3.00

Ti35O76, SiC, and SrTiO2 computed at the B3LYP/6-31G(d) level of theory. All energies are given in eV.

Fig. 3: HOMOS, LUMOs, and energy gaps of Zn- Por –n, Zn- Pc -n (n=1-2), and the semiconductor electrodes (TiO2)40,

Zn-Por-1 Zn-Pc-1 Zn-Por-2 Zn-Pc-2

-4.59

2.32

-2.27

DOS

DOS

Zn - Pc - 1

Energy (eV)

(-2.03)

Energy (eV)

(-3.52)

(-4.59)

(-2.27)

Zn - Pc - 2

Zn - Por - 2

(-2.65)

Energy (eV)

(-2.36)

Energy (eV)

(-3.66)

(-4.67)

Fig.4: HOMOs, LUMOs, and density of states (DOS) of Zn- Por –n and Zn- Pc -n (n=1-2).

Zn - Por - 1

DOS DOS

Zn - Pc- 1

Zn-Por -2

Zn - Pc- 2

Fig.5 : Molecular electrostatic potential (MEP) contours of Zn- Por –n and Zn- Pc -n (n=1-2).

Zn - Por - 1

Zn-Por-2

Zn-Por-1

Fig.6: UV.vis. spectra of of Zn- Por -n, Zn- Pc- n (n=1-2),

Zn-Pc-2

Zn-Pc-1