Theoretical design of porphyrazine derivatives as promising sensitizers for dye-sensitized solar cells

Abstract

Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been carried out on the electronic structure and optical properties of a set of heterocycle-fused zinc porphyrazine (ZnPz) derivatives, aiming at screening efficient sensitizers for dye-sensitized solar cells (DSSCs). Our results show that the absorption spectra of the designed dyes shift to longer wavelengths and the light harvesting efficiencies are much higher than isolated ZnPz. Moreover, the designed dyes have larger contributions of the anchoring group to the lowest unoccupied molecular orbitals (LUMOs) compared with the currently best sensitizer YD2-o-C8, indicating enhanced electron injection ability from the sensitizer to the semi-conductor. Furthermore, the designed dyes exhibit good performance in terms of the charge transfer characteristics, the driving force of electron injection and dye regeneration, and the excited-state lifetime. Overall, the designed dyes, especially indigo blue fused ZnPz and acridine fused ZnPz, are revealed to be promising sensitizers for high-efficiency DSSCs.

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1. Introduction

Considering global warming and increasing energy demands, more and more attention has been paid to the development of renewable energy resources.^1,2 In this respect, dye-sensitized solar cells (DSSCs) represent a highly promising approach for the direct conversion of light into electrical energy at low cost and with relatively high efficiency.^3,4 The heart of a DSSC is the dye, which operates much like chlorophyll in a photosynthetic plant cell. When a photon hits the dye, an electron is excited from the ground state to the excited state, leaving behind a hole, as thus initiates the primary steps of photon absorption and the subsequent electron transfer process. The most developed DSSCs dyes are based on ruthenium (Ru)–polypyridyl complexes which have provided the highest performances, with solar energy-to-electricity conversion efficiency over 11%.⁵ However, this system is toxic and high cost due to the scarcity of Ru metal. In this context, Ru-free dyes, such as triphenylamine-based organic dyes, have been applied as sensitizers in DSSCs, and the efficiency is more than 10%.^6–8

Recently, porphyrins (Por) form an important class of chromophores that are attractive for application in DSSCs, because of their photochemical and electrochemical stabilities, high molar extinction coefficients, low toxicity, ease of modification and potential low-cost.^9–14 Nevertheless, the insufficient light-harvesting properties of Por result in inferior performance compared with the Ru complexes applied in DSSCs. The absorption spectra of Por are characterized by two bands: a strong and intense Soret or B band at 400–450 nm and a moderate intense Q-band at 500–650 nm. Broadening and red-shifting of absorption bands together with an increasing intensity of the Q bands compared to that of the Soret band are promising strategies to solve the above problem.^15,16 Here we have paid attention to a new class of intermediate macrocycles called porphyrazines (Pz),^17–21 one of the large sensitizer families of Por-like, which contains four tetrapyrroles and nitrogens in meso position. Pz complexes exhibit a significant red shift and an intensification of the Q band and a more complicated Soret band due to additional n → π* transition introduced by azamethine groups. Though firstly synthesized more than 50 years ago,²² Pz have received relatively little attention compared with Por; however new synthetic strategies have paved the way for further exploration of their physical properties and potential applications.^21,23 There has been growing interest in studying the Pz owing to their high flexibility, rich coordination chemistry, and excellent stability against light, heat, and chemical substances.^24–29 It is known that the functional groups attached directly at the β-positions of rings in the Pz core can have stronger coupling to the macrocycle core than those from the identical groups but attached to the fused benzo rings of Por and, therefore, exert more significant effects on the physical properties of the compound.³⁰ The Pz have specific optical properties because they have symmetrical rich 18 π-electron aromatic macrocycle, which can play as a host to different metal ions in its central cavity.³¹ In experiments, both peripherally functionalized Pz and metallo-Pz have been extensively studied because of their interesting electron transfer, photosensitizing properties, along with magnetic and thermal characteristics.^29,32,33 Furthermore, Pz have also been of interest to theoretical chemists.^34–37 Lelj et al. investigated the electronic structure of peripherally unhindered Pz using a density functional approach and showed that peripheral substituents yield modifications to the “core” of the macrocycle and to the energy levels, changing σ and π interactions.³⁴ Arratia-Pérez et al. studied several Ti(IV) complexes of Pz and one phthalocyanine using density functional theory and time dependent density functional theory calculations at the level of LDA/BP86/TZ2P.³⁶ They suggested that three Ti–Pz complexes could act as light-harvesting sensitizers for DSSCs. Recently, Su et al. have performed a detailed TDDFT analysis on the ground-state structures and the absorption spectra of tetrathiafulvalene-annulated ZnPz, and suggested it as a candidate for DSSCs due to the broad and intense red-shifted Q band.³⁷ However, to the best of our knowledge, the influences of the fused aromatic units to the photophysical properties of ZnPz have not been considered experimentally or theoretically.

In the present work, a series of ZnPz dyes with different fused heterocycles have been investigated using DFT and TDDFT approach in order to design good sensitizers for DSSCs. The electronic structures and photophysical properties of the designed dyes have been compared with the parent ZnPz molecule and the efficient sensitizer YD2-o-C8. By incorporating the YD2-o-C8 dye and cosensitizing with Y123, the DSSC with a traditional liquid electrolyte has achieved a 12.3% efficiency recorder in 2011.¹² Finally, considered the balance in various properties required for the exploration of high-efficiency sensitizers, the indigo blue and acridine fused ZnPz dyes have been recommended as potential sensitizers for high efficiency DSSCs.

2. Computational details

All calculations were implemented in the Gaussian 09 package.³⁸ The ground-state geometries of all ZnPz dyes were optimized with 6-31G(d) basis set for C, H, O, N, S and LANL2DZ basis set for zinc atom using hybrid functional B3LYP.^39–41 Frequency calculations were performed at the same level of theory as the optimizations to confirm the nature of the minima (the structures were given in the ESI†). Based on the optimized ground-state geometries, the UV-vis spectra were obtained by performing single-point TDDFT calculations with the long-range corrected CAM-B3LYP functional.⁴² The obtained TDDFT results were submitted into the SWizard program (http://www.sg-chem.net/swizard/)^43,44 to generate the absorption spectra using the Gaussian model with a half-bandwidth of 3000 cm⁻¹. Solvation effects were included during the optimizations and the TDDFT calculations using a polarizable continuum model according to the CPCM method.^45–48 The chloroform solvent was employed throughout this investigation, because this solvent is widely used in experiments of Por and Pz compounds.^49,50

The lifetime of the excited state (τ) is an important factor for considering the efficiency of charge transfer (CT) of dyes. In the present investigation, the τ value was approximately set to the lifetime of spontaneous luminescence, which is estimated by⁵¹

where A_k,k′ is Einstein coefficient for spontaneous emission, e is the elementary charge, ħ is the reduced Planck's constant, and c is the speed of light in vacuum. Moreover, ΔE_k′,k and r_k,k′ represent the transition energy and transition dipole moment from states k to k′, respectively. This formula has been successfully applied to the investigation of several organic dyes.^52–54

3. Results and discussion

3.1 Calibration of the theoretical method

To validate the methodology employed in this work, the vertical excitation energies of ZnPz (see Fig. 1) were computed and compared with the experimental data. The Q band of ZnPz is calculated to lie at 2.18 eV, which is in excellent agreement with the experimental maximum absorption in chloroform solvent at 2.13 eV.⁵⁵ Besides, the B band with oscillator strength of 0.163 is predicted to appear at 3.61 eV, which is in accordance with the experimental value of 3.73 eV.⁵⁵ The good agreement between theory and experimental indicates that the used methodology mentioned in the computational details part is appropriate for such types of calculations.

Fig. 1 Molecular structures of ZnPz, YD2-o-C8, and the designed dyes (L1–L9).

3.2 The ground state properties

The molecular structures of the investigated systems, ZnPz, YD2-o-C8, and the designed dyes (L1–L9), are shown in Fig. 1. The designed dyes originate from ZnPz, where an acceptor/anchor group is added on one parrole ring while the opposite parrole ring is extended with different benzoheterocycles. The selected acceptor/anchor group is cyanoacrylic acid, because it has been widely used in D–π–A dyes thanks to its strong electron-withdrawing capacity and the steady interaction with the semiconductor surface. The designed dyes are distinguished by the fused benzoheterocycles: benzofuran (L1), benzothiophene (L2), indigo blue (L3), isoquinoline (L4), acridine (L5), phthalazine (L6), phenanthroline (L7), benzopyrimidine (L8), and coumarin (L9).

Based on the optimized structures of the molecules in Fig. 1, the electronic and optical properties, were computed and listed in Table 1. Schematic representations of the energy levels for the studied molecules are given in Fig. 2.

Table 1 The calculated absorption wavelength (cal. λ_abs), oscillator strength (f_L), calculated excitation energy for the lowest excited state (ΔE_L), molecular dipole moment (μ), the amount of transferred charge (q_CT), the charge-transfer distance (d_CT), transition dipole moment (r_k′,k), the first excited states lifetimes (τ) and the dominant electronic transition configurations for zinc porphyrazine derivatives, ZnPz and YD2-o-C8 in chloroform.^a H and L signify HOMO and LUMO, respectively

Dye	Cal. λabs (nm)	fL	ΔEL (eV)	EH^b (eV)	EL^c (eV)	ΔEHL^d (eV)	μ (D)	qCT (e)	dCT (Å)	rk′,k	τ (× 10^−9 s)	MO character (coefficient)
a All properties were calculated at the CAM-B3LYP/6-31G(d) (LANL2DZ) level.b The energy of the highest occupied molecular orbital (HOMO).c The energy of the lowest unoccupied molecular orbital (LUMO).d Energy gap between HOMO and LUMO.e Data taken in THF medium, from ref. 12.f The calculated absorption wavelength, HOMO, LUMO, and energy gap taken from ref. 16.g Calculated data taken in THF medium.
L1	625	1.118	1.98	−5.58	−3.34	2.24	15.79	0.498	0.554	22.99	5.3	H → L (0.69)
	332	0.682
L2	632	1.191	1.96	−5.56	−3.36	2.20	15.41	0.508	0.519	24.77	5.1	H → L (0.69)
	332	0.800
L3	675	1.580	1.84	−5.40	−3.50	1.90	15.69	0.511	0.316	35.10	4.3	H → L (0.67)
	308	0.834
L4	633	1.172	1.96	−5.62	−3.42	2.20	11.75	0.499	0.701	24.42	5.1	H → L (0.69)
	335	0.581
L5	630	1.355	1.97	−5.51	−3.34	2.17	16.82	0.516	0.762	28.11	4.4	H → L (0.69)
	316	1.161
L6	638	1.133	1.94	−5.75	−3.52	2.23	6.42	0.492	0.593	23.78	5.4	H → L (0.69)
	325	0.828
L7	659	1.042	1.88	−5.80	−3.58	2.22	7.25	0.467	0.295	22.60	6.3	H → L (0.69)
	340	0.635
L8	634	1.115	1.96	−5.70	−3.47	2.23	9.40	0.494	0.718	23.28	5.4	H → L (0.69)
	337	0.555
L9	639	1.088	1.94	−5.73	−3.49	2.24	8.16	0.488	0.671	22.89	5.6	H → L (0.69)
	320	0.922
ZnPz	570	0.444	2.18 (2.13)exp	−5.48	−2.87	2.61	0.83	0.272	0.030	8.34	10.9	H → L (0.68)
	344	0.163	3.61 (3.73)exp
YD2-o-C8	655^f (645^e)exp	—	—	−4.84^g	−2.61^g	2.23^g	4.43^g	0.403^g	1.271^g	9.92	11.7	H → L (0.62)
	437^f (448^e)exp			4.99^f	2.70^f	2.29^f

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Fig. 2 Orbital energy levels of the HOMO and LUMO and the HOMO–LUMO gaps of zinc porphyrazine derivatives and ZnPz. For comparison, orbital energy levels of the CB of TiO₂ (−4.0 eV vs. vacuum) and the I⁻/I₃⁻ redox couple (−4.8 eV vs. vacuum) are also provided by the dotted lines.

Corresponding data of highest occupied molecular orbital (HOMO)s, lowest unoccupied molecular orbital (LUMO)s, and HOMO–LUMO gaps are listed in Table 1. The HOMO–LUMO gap is basically the energy that must be fed to the molecule to kick it from the ground state to an excited state. The smaller energy gaps make the electrons more easily to be excited and are beneficial for absorbing the light at longer wavelength region. Hence, more photons can be absorbed at the same time, which may contribute to obtain a higher short circuit current density (J_sc) and overall power conversion efficiency (η).⁵⁶

As shown in Fig. 2, the frontier molecular orbital energy gaps are significantly affected by those different heterocycles fused on ZnPz. The smallest energy gap between HOMO and LUMO is 1.90 eV for L3 among all the designed dyes. The energy gaps of L1–L9 are smaller than that of ZnPz, with the smallest value of 1.90 eV in L3. The order of the energy gaps is L3 (1.90 eV) < L5 (2.17 eV) < L2 = L4 (2.20 eV) < L7 (2.22 eV) < L8 = L6 (2.23 eV) < L1 = L9 (2.24 eV) < ZnPz (2.61 eV) (see Table 1). Previous studies⁵⁷ indicate that to some extent, the smaller the HOMO–LUMO gap of the sensitizer, the higher the efficiency of corresponding solar cells. The calculated energy gap of L3 (1.90 eV) is lower than that of dye YD2-o-C8 (2.23 eV) which is the so far best sensitizer.¹² Moreover, we can see from Fig. 2, the decrease in HOMO–LUMO energy gap mainly comes from both the raise of HOMO and the decline of LUMO. Note that the HOMO and LUMO energies of the L3 were calculated to be −5.40 and −3.50 eV, respectively (see Table 1). Furthermore, by comparing the computed HOMO and LUMO energies of L1–L9 with the edge of the CB of TiO₂ and the potential of the most used of the redox couple I⁻/I₃⁻, Fig. 2 shows that they all have more negative HOMO energies than the I⁻/I₃⁻ redox couple (−4.8 eV vs. vacuum),⁴⁵ which implies a fast regeneration of the oxidized dyes, while more positive LUMO energies than the CB of TiO₂ (−4.0 eV vs. vacuum)⁵⁸ which could ensure an effective injection of excited electrons. Therefore, the designed ZnPz derivatives L1–L9, especially L3, can be used as promising candidates as broad-spectrum dyes for high-efficiency DSSCs.

To get more insights about the electronic structure, we further investigate spatial distribution and the composition of the frontier orbitals, which are related to the charge transport character as suggested by Mizuseki et al.⁵⁹ The orbital spatial distributions of HOMO and LUMO for L3 are shown in Fig. 3 and other designed dyes are shown in Fig. S1.† The molecular orbital composition of the HOMO and LUMO for designed dyes are listed in Table 2 calculated using the Multiwfn program,⁶⁰ which shows that the HOMO and LUMO are mainly contributed from the porphyrazine ring except L3. It is well known that a good sensitizer requires the HOMO localizing on the donor part and the LUMO on the acceptor, as it is facile for charge separation. From Table 2 and Fig. 3, it can be seen that the HOMO mainly originates from indigo blue unit, whereas the LUMO mainly comes from porphyrazine ring for L3, which indicates that L3 is a good charge-separated sensitizer. Jiang et al.⁶¹ designed a series of porphyrin sensitizers with different electron-donating and withdrawing substituents and calculated the molecular orbital compositions of YD2-o-C8 dye. The compositions of the HOMO and LUMO for YD2-o-C8 dye are also listed in Table 2. It is known that the contributions from the anchoring group of the LUMO strongly influence the electronic coupling between the excited adsorbed dye and the 3d orbital of TiO₂.⁶² The contribution of anchoring groups of YD2-o-C8 to LUMO is only 2%, whereas it is more than 15% in the designed molecules L1–L9. As a consequence, the designed dyes may exhibit stronger more electronic coupling with the semiconductor surface and probably would be more favorable for electron injection into the TiO₂ surface than YD2-o-C8 dye. The excitation from the ground state (S₀) to the first excited state (S₁) for L1–L9 is dominated by the transition from HOMO to LUMO (see Table 1). From Table 1, it can be observed that most of the changes of substituents have small effect on the percentages of the transitions in the construction of the first excited state.

Fig. 3 Frontier molecular orbitals of L3 in chloroform solvent.

Table 2 Molecular orbital composition (in %) of the HOMO and LUMO orbitals of the designed dyes and YD2-o-C8 dye

Molecule	Orbital	Zinc porphyrazine	Heterocycles	Anchoring ligand
a Values of YD2-o-C8 of molecular orbital composition are obtained from ref. 61.
L1	HOMO	80	14	6
	LUMO	73	11	16
L2	HOMO	76	18	5
	LUMO	73	10	17
L3	HOMO	30	68	2
	LUMO	59	29	12
L4	HOMO	79	16	6
	LUMO	73	11	16
L5	HOMO	70	25	5
	LUMO	73	10	17
L6	HOMO	83	11	6
	LUMO	72	12	16
L7	HOMO	85	10	5
	LUMO	71	14	15
L8	HOMO	83	11	6
	LUMO	73	11	16
L9	HOMO	85	9	6
	LUMO	72	12	16

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	Orbital	Porphyrin	π-linker	Anchoring ligand
YD2-o-C8	HOMO	70^a	6^a	0^a
	LUMO	80^a	11^a	2^a

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3.3 Absorption spectra

TDDFT calculations show that the maximum absorption maxima (λ_max) for designed dyes follow the order of L3 (675 nm) > L7 (659 nm) > L9 (639 nm) ≈ L6 (638 nm) > L8 (634 nm) ≈ L4 (633 nm) ≈ L2 (632 nm) > L5 (630 nm) > L1 (625 nm) > ZnPz (570 nm) (see Table 1 and Fig. 4).

Fig. 4 Electronic absorption spectra of zinc porphyrazine derivatives and ZnPz.

This indicates that the introducing of cyanoacrylic acid and mono heterocycle substituent fused ZnPz derivatives leads to red shift in absorption spectra. The λ_max of L3 (675 nm) is red-shifted compared to those of YD2-o-C8 (655 nm,¹⁶ 645 nm (ref. 12)). The combination of added π-conjugation and decreased molecular symmetry manifests itself as a red shift in the absorption spectrum of the molecule.⁶³ From Fig. 4, it is clearly observed that L3 shows the best absorption properties in UV-vis range, i.e. greatly red-shifted λ_max (∼100 nm longer than that of ZnPz) and broader absorption spectrum from 300 to 800 nm for all the designed dyes. Meanwhile, L1 shows the smallest λ_max, which blue-shifts of ∼50 nm compared to L3. In addition, another finding is that the designed dyes have bigger molecular dipole moments (Table 1) compared to ZnPz. Mono heterocycle in ZnPz poses a noticeable influence on molecular dipole moment and can lead to bigger dipole moment. Charge-transfer plays a key role in DSSCs. The amount of transferred charge (q_CT) and the charge-transfer distance (d_CT), based only on the computed electronic density for the ground and excited states of designed dyes are listed in Table 1 calculated using the program by Ciofini et al.^64,65 It is obvious that both q_CT and d_CT for designed dyes increase compared to the ZnPz (0.272 e, 0.030 Å). Meanwhile, it is noted that the optimal CT performance is obtained for the introduction of acridine, L5, which allows the transfer of 0.516 e over 0.762 Å. The introduction of phenanthroline, L7, has the worst performance with 0.467 e only transferred 0.295 Å, which implies a lower efficiency of charge transfer. With the introduction of mono heterocycle of ZnPz, the transition from the relevant ground to excited-states should induce a stronger CT than that of ZnPz. From Table 1, we can also see that the calculated q_CT of all designed dyes are larger than YD2-o-C8 dye (0.403 e), but the calculated d_CT are all smaller than YD2-o-C8 dye (1.271 Å). From the obtained q_CT and d_CT data, we can see that the designed dyes have good charge-transfer character, which is better than ZnPz and comparable with YD2-o-C8 dye.

3.4 Excited-state properties

The light harvesting efficiencies (LHE) of the dye should be as high as possible to increase the photocurrent if the excited processes have CT character. LHE can be calculated as:⁶⁶

LHE = 1 − 10^−f (2)

where f is the oscillator strength of the dye associated to the λ_max.

The electron injection from the excited dyes to the semiconductor conduction band and the dye regeneration process can be described as a CT reaction. In terms of Marcus theory for electron transfer,⁶⁷ the CT rate constants can be affected by the free energy change related to the reaction. The free energy change for electron injection (ΔG^inject) determines the electron injection rate and therefore the photocurrent in DSSCs and it can be viewed as the electron injection driving force. A larger driving force is desirable for more rapid electron injection rate and then higher overall efficiency of DSSCs.⁶⁸ Preat et al.⁶⁶ have proposed a theoretical scheme to quantify the electron injection onto a TiO₂ surface. The ΔG^inject can be expressed by the following equation,

ΔG^inject = E^dye*_OX − E^SC_CB (3)

where E^dye*_OX is the oxidation potential of the dye in the excited state, and E^SC_CB is the reduction potential of the conduction band (CB) of the semiconductor. As we know that the CB of semiconductor is sensitive to the conditions, and it is difficult to determine accurately, the TiO₂ (E^SC_CB = −4.0 eV vs. vacuum)⁵⁸ is adopted in this work. The E^dye*_OX can be calculated as follows:

E^dye*_OX = E^dye_OX − λ_max (4)

where E^dye_OX is the redox potential of the ground state, and λ_max is the lowest absorption energy associated with the photoinduced intramolecular CT. Whereas the free energy change of regeneration (ΔG^regen) can affect the rate constant of redox process between the oxidized dyes and electrolyte. The ΔG^regen can be calculated as:

ΔG^regen = E^electrolyte_redox − E^dye_OX (5)

where E^electrolyte_redox is the redox potential of electrolyte. The E^electrolyte_redox of commonly used redox couple I⁻/I₃⁻ is about −4.8 eV vs. vacuum.⁴⁵ The E^dye_OX is obtained by the computational method which is proposed by De Angelis et al.⁶⁹ The computational details are described in the ESI.† The calculated E^dye_OX, E^dye*_OX, LHE, ΔG^inject and ΔG^regen for designed dyes are listed in Table 3. As can be seen in Table 3, the calculated LHE of the designed dyes and ZnPz at λ_max are 0.92, 0.94, 0.97, 0.93, 0.96, 0.93, 0.91, 0.92, 0.92 and 0.64 for L1, L2, L3, L4, L5, L6, L7, L8, L9 and ZnPz, respectively. Note that LHE of L3 is the highest of the present designed dyes, which means L3 has the most efficient light harvesting capability among the designed dyes. The calculated results show that the ΔG^inject of the ZnPz derivatives are negative, which means that the excited state with intramolecular CT character lies above the TiO₂ conduction band edge. For the designed dyes, the ΔG^inject (minimal value – 0.51 eV) of fusing acridine (L5) is larger than that of other dyes, and the ΔG^inject (maximal value – 0.18 eV) of fusing phenanthroline (L7) is the lowest of the dyes. This suggests L5 has the largest while L7 has the smallest driving force of electron injection in all designed dyes. It is worth noting that the driving force of electron injection of ZnPz (ΔG^inject = −0.69 eV) is larger than the designed dyes. As shown in Table 3, the ΔG^regen (0.90 eV) of L7 is larger than that of other designed dyes. This suggests that L7 has the largest rate constant for regeneration. Hence, the electron injection of L5 might be faster than that of other designed dyes, while the dye regeneration rate of L7 might be faster than that of other designed dyes.

Table 3 The calculated oxidation potential in ground state (E^dye_OX, in eV), the oxidation potential in excited state (E^dye*_OX, in eV), light harvesting efficiencies (LHE), the free energy change for electron injection (ΔG^inject, in eV), the free energy change for regeneration (ΔG^regen, in eV) for designed dyes and zinc porphyrazine

Dye	LHE	E^dyeOX	E^dye*OX	ΔG^inject	ΔG^regen
L1	0.92	5.51	3.53	−0.47	0.71
L2	0.94	5.50	3.54	−0.46	0.70
L3	0.97	5.39	3.55	−0.45	0.59
L4	0.93	5.56	3.60	−0.40	0.76
L5	0.96	5.46	3.49	−0.51	0.66
L6	0.93	5.67	3.73	−0.27	0.87
L7	0.91	5.70	3.82	−0.18	0.90
L8	0.92	5.63	3.67	−0.33	0.83
L9	0.92	5.64	3.70	−0.30	0.84
ZnPz	0.64	5.49	3.31	−0.69	0.69

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The decay of S₁ to S₀ is an important competing process with the electron injection to the conducting band of semi-conductor. A dye with longer lifetime on S₁ state is expected to be more facile for electron injection. The calculated lifetime of S₁, along with the related transition dipole moment for the designed dyes L1–L9, ZnPz, and YD2-o-C8 are given in Table 1. The orders of the obtained excited state lifetimes (τ) are as following, 4.3 × 10⁻⁹ s (L3) < 4.4 × 10⁻⁹ s (L5) < 5.1 × 10⁻⁹ s (L2) = 5.1 × 10⁻⁹ s (L4) < 5.3 × 10⁻⁹ s (L1) < 5.4 × 10⁻⁹ s (L6) = 5.4 × 10⁻⁹ s (L8) < 5.6 × 10⁻⁹ s (L9) < 6.3 × 10⁻⁹ s (L7) < 10.9 × 10⁻⁹ s (ZnPz) < 11.7 × 10⁻⁹ s (YD2-o-C8). Although the lifetimes of L1–L9 are a little shorter than ZnPz or YD2-o-C8, all the lifetimes are at nanosecond (10⁻⁹ s) timescale. This suggests the excited states of L1–L9 will not decay to the ground state very quickly, and may behave similar to YD2-o-C8 in the electron injection process. It is worth to mention that the conjugation part of the designed dyes is better than ZnPz and YD2-o-C8, which might increase the possibility of aggregation among the sensitizers. The dye aggregation suppresses electron transport from the excited dye molecule to TiO₂ surface resulting in lower DSSC performance.⁷⁰ To prevent the aggregations, several ways have been proposed, but this is not the focus of this study. In addition, the light-to-electric-energy conversion is decided by not only the bare dye but also other factors, including electron injection to conduction band of semiconductor, dye regeneration, recombination, and electrolyte system and so on. Therefore, to confirm that these designed series can be the potential candidates for DSSCs applications, further experimental supports are highly requisite.

4. Conclusions

In this work, using DFT and TDDFT calculations, we have designed a series of heterocycle-fused zinc porphyrazine compounds as sensitizers for DSSCs. The designed dyes, L1–L9, exhibit smaller HOMO–LUMO energy gaps, stronger and red-shifted absorption in Q-band, evident charge transfer character, proper driving forces for electron injection and dye regeneration, and long excited-state lifetime. Especially to be noted indigo blue fused ZnPz (L3) shows the best optical property, and acridine fused ZnPz (L5) demonstrates the largest driving force for electron injection. In summary, our results show that the designed dyes L1–L9, especially L3 and L5, are promising candidates for high efficiency DSSCs.

Acknowledgements

This work is financially supported by the Major State Basic Research Development Programs of China (2011CBA00701, 2012CB721003), the National Natural Science Foundation of China (21303007), the Specialized Research Fund for the Doctoral Program of Higher Education (20131101120053), the Excellent Young Scholars Research Fund of Beijing Institute of Technology (2014Y1218), the Basic Research Fund of Beijing Institute of Technology (20121942011, 20131942008), and Beijing Key Laboratory for Chemical Power Source and Green Catalysis (2013CX02031).

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Footnote

† Electronic supplementary information (ESI) available: Cartesian coordinates of optimized structures, frontier molecular orbitals of zinc porphyrazine derivatives calculated at the B3LYP/6-31G(d) level in chloroform solution, and the computational details of E^dye_OX. See DOI: 10.1039/c4ra02204a

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