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A: New Tools and Methods in Experiment and Theory
Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells Zahra Parsa, S. Shahab Naghavi, and Nasser Safari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03668 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018
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The Journal of Physical Chemistry
Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells Z. Parsa,† S. Shahab Naghavi,‡ and N. Safari∗,† †Department of Chemistry, Shahid Beheshti University, G.C., Evin, 1983969411 Tehran, Iran ‡Department of Physical and Computational Chemistry, Shahid Beheshti University, G.C., Evin, 1983969411 Tehran, Iran E-mail:
[email protected] Abstract
Introduction
Over the past decade, tremendous effort has been made to improve the lightharvesting ability of push-pull porphyrin dyes. Despite notable success achieved in this direction, push-pull porphyrin dyes still suffer from a poor light-harvesting efficiency owing to the lack of absorption between the Soret- and Q-bands. To tackle this issue, here we design a series of push-pull porphyrin dyes with anchoring groups either at meso- or β-position using calculations based on first-principles time-dependent density functional theory. In contrast to the common perception, we find that porphyrin dyes bearing an electron-donor at meso-position and an electron-acceptor at β-position produce an additional extended band between the Soret and Q-bands appeared at around 500 nm due to S0 → S3 excitation leading to a much higher light harvesting performances compared to meso- and betadisubstituted ones. In addition, changing the π-conjugated linker at the acceptor site from ethylene-linker (C=C) to acetylene-linker (C≡C) further improves the light harvesting ability of meso-β porphyrin dyes, making them promising candidates for dye-sensitized solar cell application.
Dye-sensitized solar cells (DSSCs) have attracted much attention due to their excellent photovoltaic performance, low production costs, and environmentally-friendly features. 1–7 The original form of DSSCs was realized in the ruthenium (II) bipyridyl complexes 8 where a solar-to-electric power conversion (PCE) of about 12% has been reported. 9,10 Despite a high PCE and tunability of electronic properties, 11 their extensive application has been limited because of the rarity, requirement for careful synthesis, and environmental issues associated with ruthenium metal. 12,13 From this perspective, organic dyes with no metal or inexpensive metal complexes have become the central focus of DSSCs due to low-cost production and facile modulation of their electronic structure to improve their light-harvesting efficiency. 14 An elegant strategy to improve the light-harvesting ability of the sensitizers is to use an electrondonor (D, push), and electron-acceptor (A, pull) group, connected via a π-conjugated bridge. 2,15 Separating donor from acceptor in the so-called donor-π-acceptor, or push-pull dyes allows the spatial separation of electrons and holes and thus decreasing their recombination rate (i.e., increasing exciton lifetimes). Among organic dyes, 16 push-pull porphyrin dyes have demonstrated their potential because of the high light absorption in Soret- and Q-bands. 17 However,
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the lack of significant absorption between the Soret- and Q-bands limits their light-harvesting ability. 18–21 Therefore, broadening of the Soretand Q-bands, which leads to a broader absorption over the entire solar spectrum, has been one of the primary goals in this field. 22–24 Porphyrin dyes have a large molecular panel with 12 active sites: eight β (those on the pyrrolic rings) and four Meso (methine bridges) positions. Substitution of different push and pull groups on each site allows fine tuning of their optical and redox properties. 25–29 The first porphyrin sensitizer appeared in 1993 for βsubstituted chlorophyl derivatives with a maximum PCE of 2.6%. 30 Since then the efficiency of porphyrin dyes has been continuously increasing by the use of different push/pull groups and molecular engineering of porphyrin sensitizers, 31–38 where the efficiency of 7–8% for βsubstituted porphyrins has been reported. 39–41 In this regard, a meso-disubstituted porphyrin called SM315 14 has recorded a high light absorption due to the broadening of Soret- and Q-bands, yielding a PCE of 13%. Despite having excellent light-harvesting properties, this dye still lacks absorption in the range of 480 to 630 nm. Since 2009, 15,42 meso-disubstitution push-pull porphyrin dyes have emerged as the most efficient ones. However, there is a limited number of studies that address the impact of the different positions of the push/pull groups on the solar absorption efficiency of porphyrin sensitizers. Masatoshi Ishida et al. showed that the efficiency of β-functionalized ZnEP1 is 5.9%, comparable to that of meso-functionalized YD2 (6.2%). 43 Furthermore, it was suggested that the presence of the C≡C triple-bond at the βpositions would facilitate efficient electron injection into the conduction band of the TiO2 . 43 Gabriele Di Carlo and co-workers reported that β-mono or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies compared with meso-substituted pushpull dyes when acting as sensitizers in DSSCs. 44 In a subsequent study in 2013, they reported higher energy conversion efficiencies for the βsubstituted ZnII porphyrin (6.1%) rather than the meso-substituted one (3.9%). 25 In their re-
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cent review, they also highlight the role of the β-substituted ZnII porphyrins for PV application, 39 challenging the common perception of the superior performance of mesodisubstitution porphyrin dyes. In this context, a systematic study on the effect of substitution on different sites, i.e., meso-meso, β-β, and meso-β, on the absorption efficiency of porphyrin dyes has not been explored. Acceptor
Donor
N
N C3 N3
Zn
N4
C3 N3
N2
β-βet
C3 N3
Zn
N4
m-βet
N4
m-mac
N
COOH
C3 N3 N
Zn
N2 N1 C1
COOH
C3 N3
N2 N1 C1
N2 N1 C1
β-βac
COOH
N
Zn
N4
N1 C1
COOH
N4
Zn
N2 N1 C1
m-βac
N2 C3 N3 Zn N4 N1 C
COOH
1
ZnTPP
Figure 1: Structure of porphyrin dyes studied in this work. The colored areas emphasize the donor and acceptor groups. Acceptor groups are connected to the porphyrin core by either ethylenic (C=C) or acetylenic (C≡C) πconjugated groups. In this work, we comparatively investigate a series of push-pull-type porphyrin sensitizers featured by structural arrangements based on a donor-π-acceptor (D-π-A) architecture. As seen in Figure 1, in the studied structures, p-Me2 NC6 H4 −C≡C− is the electron donating group, and 4-ethynyl benzoic acid (with acetylene-linked) together with 4-ethenyl benzoic acid (with ethylene-linked,trans isomer) are tested as acceptor groups. Using density functional theory (DFT), we study the effect of donor and acceptor positions as well as the πconjugate unit (ethylenic, et vs. acetylenic, ac) on the energy level alignment, energy gap, elec-
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The Journal of Physical Chemistry
tron localization and photophysical properties of porphyrin dyes. To simplify, we name the five studied porphyrin dyes as following: the donor at mesa position and the acceptor with ethylene and acetylenic group at β position are named m-βet and m-βac respectively. Following the same nomenclature other configurations studies here are β-βet ,β-βac , and m-mac . We find that these sensitizers have high molar extinction coefficients over a broad absorption region from 300 to 900 nm. In particular, m-βet and m-βac with two wide well-separated bands in the Soret region, are promising candidates for highly efficient DSSC applications. It is noteworthy that in m-βet , and m-βac dyes two possible isomers could be synthesized. Therefore we compared molecular orbitals and absorption properties of these isomers in supplementary information. Notice that the substituent position on the pyrrolic ring have negligible effect on the electronic properties of the dyes (Table S2 and Figure S4).
tron donor (D), carboxylic acid is the electron acceptor (A), and ethenyl and acetylene are πconjugated groups. The selected bond lengths are listed in Table 1. The significant changes in the Zn–N bond lengths of the porphyrin dyes mark the importance of substitution positions of push and pull groups on the geometry of porphyrin core. Table 1: Selected bond lengths of porphyrin dyes in the unit of angstrom, atomic enumerations are shown in Figure1. D–A denotes the distance between push and pull groups Dye ZnTPP β-βet β-βac m-βet m-βac m-mac
Zn-N1 Zn-N2 Zn-N3 Zn-N4 N1 -C1 N3 -C3
D–A
2.043 2.074 2.074 2.061 2.061 2.045
8.711 8.649 7.438 7.424 6.910
Computational details
2.043 2.020 2.022 2.036 2.038 2.045
β-βet
Density functional theory (DFT) and timedependent density functional theory (TDDFT) calculations were carried out with the Gaussian 09 package. 45 We employed the B3LYP functional 46,47 with 6-31g(d) basis to relax the structures and subsequently calculate their vibrational frequencies. Lack of imaginary frequencies confirms that the selected porphyrin dyes are true minima on the potential energy surfaces. The TD-B3LYP/6-31g(d) level of theory was used to calculate optical properties and absorption spectra. The solvent effect was considered in the framework of the self-consistent reaction field polarizable continuum model (PCM). 48
2.043 2.072 2.067 2.058 2.054 2.044
2.043 2.021 2.022 2.027 2.028 2.044
1.377 1.379 1.375 1.380 1.377 —
1.377 1.377 1.377 — — —
m-βet
m-βac β-βac m-mac
Figure 2: Side view of the studied porphyrin dyes. The β − β-disubstituted porphyrin dyes (highlighted in red) show a deviation from planarity causing blue-shift at Q-band and reduction of the molar absorption coefficient at Soretband (see Figure 6) due to less overlapping of π-orbitals. In β-disubstitution porphyrins, N1 −Zn and N3 −Zn bond lengths are significantly (≈ 0.05 Å) larger than N2 −Zn and N4 −Zn ones.These bond length alterations resulted in out-of-plane displacements of the pyrrole rings, thus relieving the steric strain imposed by the β-substituent. 49 It should also be noted that, these structural changes in not only due to steric effects, but also due to changes in the electron density of the β-system. 49 hence such varying bond lengths between zinc and its
Result and Discussion Geometry Structure of the Dyes The optimized structure of ZnTPP and five porphyrin dyes, studied in the present work, are depicted in Figure 1. Dimethyl amine is the elec-
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The Journal of Physical Chemistry
neighboring nitrogen atoms lead to a large deviation of β-disubstituted porphyrin dyes from planarity. In meso-disubstituted and mesoβ-push-pull porphyrins the opposing bonds N1 −Zn and N3 −Zn are lengthened while bonds N2 −Zn and N4 −Zn are shortened, leading to relatively uniform Zn–N bonds, retaining the planarity of the porphyrin dyes. As seen in Figure2, β-βet and β-βac have large deviations from planarity, 39 m-βet , m-βac are nearly planar, and m-mac is planar. Later in this article, we will address the effect of non-planarity on the optical properties of β-disubstituted porphyrin dye.
for ZnTPP are -5.146 and -2.3 eV, respectively. All the studied push-pull porphyrin dyes have higher HOMO and lower LUMO compared to ZnTPP. Detailed analysis of the MOs reveals that the HOMO and LUMO are affected by two main factors: (i) position of the donor on the porphyrin ring, and (ii) type of the πconjugated linker. Donor group at the meso position significantly lowers the LUMO, increases the HOMO and thus reduces the HOMOLUMO gap of the porphyrin dyes. Therefore, compared to β-βet and β-βac ones, the LUMO energy levels of m-βet , and m-βac , and m-mac dyes become significantly closer to the CB of TiO2 , implying a stronger electronic coupling between the LUMO and 3d orbitals of TiO2 substrate and thus an efficient dye injection. The presence of a donor group at the meso position raise the position of the HOMO level. 52 As seen in Figure 3, substitution on meso and β positions mainly affect the energy levels of HOMO and LUMO leading to a greater splitting between HOMO with HOMO-1 and LUMO with LUMO+1 as happen for m-βet , m-βac and mmac .
Frontier Molecular Orbitals A proper alignment of the dye’s molecular levels with the TiO2 band edges is crucial for the operation of DSSCs. for an efficient dye injection, LUMO must be higher than the conduction band (CB) of TiO2 (−4.0). 50,51 As seen in Figure3, all the studied dyes satisfy the mentioned criteria. -1.5
Table 2: The LUMO and HOMO energies and HOMO–LUMO gaps of porphyrin dyes in THF and Acetonitrile
-2.0 -2.5 Energy (eV)
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-3.0
THF -3.5
2.85
2.35
2.30
2.23
2.18
2.08
-4.0 -4.5
Redox Couple
-5.0 -5.5
ZnTPP β-βet
β-βac
m-βet
m-βac
m-mac
Acetonitrile
Dye
EHOMO
ELUMO
Band Gap
EHOMO
ELUMO
Band Gap
ZnTPP β-βet β-βac m-βet m-βac m-βac
-5.146 -4.891 -4.912 -4.790 -4.810 -4.812
-2.3 -2.536 -2.611 -2.560 -2.628 -2.736
2.85 2.35 2.30 2.23 2.18 2.08
-4.919 -4.938 -4.805 -4.826 -4.829
-2.567 -2.645 -2.594 -2.662 -2.767
2.35 2.29 2.21 2.16 2.06
As already discussed, another important factor altering the energy level of frontier MOs is the π-conjugated linker. The acetylene as πconjugated linker stabilizes the LUMO of β-βac , m-βac and decreases their HOMO-LUMO gaps compared to those porphyrin dyes linked via ethylene.The calculated HOMO-LUMO gaps of the investigated porphyrin dyes are in the following order : m-mac < m-βac < m-βet < ββac
