optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application

Accepted Manuscript Title: Optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application Authors: Aziz El alamy, Abdelkrim El-Ghayoury, Amina Amine, Mohammed Bouachrine PII: DOI: Reference: S1658-3655(17)30015-8 http://dx.doi.org/doi:10.1016/j.jtusci.2016.10.008 JTUSCI 360 To appear in: Received date: Revised date: Accepted date: 5-6-2016 20-10-2016 22-10-2016 Please cite this article as: Aziz El alamy, Abdelkrim El-Ghayoury, Amina Amine, Mohammed Bouachrine, Optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application, http://dx.doi.org/10.1016/j.jtusci.2016.10.008 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 proof before it is published in its final 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 Optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application Aziz El alamy*1,3, Abdelkrim El-Ghayoury2, Amina Amine1, Mohammed Bouachrine3 LCBAE/CMMBA, Faculty of Science, Moulay Ismail University, Meknes, Morocco Laboratoire MOLTECH Anjou, Université d’Angers, UFR Sciences, UMR 6200, CNRS, Bât K, Bd Lavoisier, 49045 Angers Cedex, France ESTM, Moulay Ismail University, Meknes, Morocco *Corresponding author E-mail: elalamyaziz@gmail.com, Tel.: +21200773252 Abstract Thanks to their optoelectronic properties and potential applications in a wide range of electronic and optoelectronic devices such as organic solar cells, the research in the organic -conjugated materials encompassing both polymers and oligomers have been widely studied over the last years In this work, a theoretical study using the DFT method on four azobenzene based iminopyridine is reported The theoretical ground-state geometry, electronic structure and the optoelectronic parameters (the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, the open-circuit voltage (Voc) and the oscillator strengths (O.S)) of the studied molecules were obtained by density functional theory (DFT) and time-dependent (TDDFT) approaches The effects of the structure length and the substituents on the geometries and optoelectronic properties of these materials are discussed to investigate the relationship between molecular structure and optoelectronic properties The results of this study are in agreement with the experimental ones and suggest these materials as good candidates for use in photovoltaic devices Keywords: -conjugated materials, azobenzene, optoelectronic properties, DFT calculations, HOMOLUMO gap Introduction In the last decade, organic electronic devices represent an important part of the electronic research; these electronic devices need special polymers and molecules with specific and adapted properties Π conjugated materials have much attention for optoelectronic and photovoltaic applications such as batteries [1,2], electroluminescent devices like light-emitting diodes (LEDs) [3], organic field-effect transistors [4] and organic photovoltaic cells (OPCs) [5-16] Small pi-conjugated materials short-chain have attracted much attention by many researchers the organic electronic field because they are not amorphous, can be synthesized as well-defined structures [17], to their unique electronic properties, to their high photoluminescence quantum efficiency and thermal stability [18] Azobenzene and its derivatives [19-20] have attracted attention as materials for organic electronic and photovoltaic applications [21-26] because of their optoelectronic, photochromic and absorption properties [27] and were easily synthesized In this work, theoretical study by using density functional theory (DFT) and time-dependent (TD-DFT) methods on four conjugated compounds of azobenzene based iminopyridine ligands M1, M2, M3 and M4 (Fig 1), which were easily synthesized by a condensation reaction between N,N-Dimethyl4,4’azodianiline or 4-(4-nitrophenylazo)aniline and 2-pyridinecarboxaldéhyde or 2,6pyridinedicarboxaldehyde [28].The geometry structures of neutral and doped forms, electronic properties and spectroscopic characteristics of these compounds have been predicted using DFT method with B3LYP/6-31G(d) calculation, the HOMO, and LUMO level energies were exanimated and the gap energy is evaluated as the difference between the HOMO and LUMO energies (E gap = EHOMO – ELUMO) The calculations were carried out using the Gaussian09 program Thus, and based on the optimized molecule structures; the ground state energies and oscillator strengths were investigated using the TDDFT/ B3LYP/6-31G(d,p) calculations The effects of the substituents and chain length on the geometries and optoelectronic properties of these materials were investigated and discussed The computed results are in good agreement with the experiment ones and the TD-CAM-B3LYP method gives a good prediction and describes well the optical properties of these compounds in their excited states M1 M2 M3 M4 Fig.1 Chemical structures of the studied molecules Computational studies All computations were performed using the Gaussian09 package [29] The density functional theory (DFT) with Becke’s three-parameter functional and Lee–Yang–Parr functional (B3LYP) [30] and 631G (d, p) basis set [31] was used to investigate the geometry and electronic properties (optimization structures, HOMO, LUMO, and gap (HOMO – LUMO) energies) of all compounds To obtain the charged (doped) structures of the studied molecules, we start from the optimized structures of the neutral form The study of the electronic transitions (Vertical electronic excitation spectra, including wavelengths, oscillators strengths (OS), and main configuration assignment) was carried out by means of dependent density functional theory (TD-DFT) [32-35] calculations with B3LYP and CAM-B3LYP functional on the corresponding DFT-optimized structure of the ground state and employing the 631G(d,p) basis set In fact, these calculation methods have been successfully applied to other conjugated materials [36] Results and discussion 3.1 Geometric properties The optimized structures of the four organic compounds in their neutral forms in a vacuum without symmetry constraints at B3LYP/6-31G(d,p) level are illustrated in Fig M1 M2 M3 M4 Fig.2: optimized geometries obtained by B3LYP/6-31G(d, p) of the studied molecules Fig 3: The scheme of the bond di (i=1-14) lengths and dihedral angles θi (i=1-8) The selected dihedral angle θi (i=1-8) and bond distance parameters di (i=1-14) in neutral and doped forms are collected in Table 1,2 and Fig On the one hand, the dihedral angles θ1, θ2, θ4, θ5, θ7 and θ8 of all molecules are similar and are 180° except θ3 and θ6 have a slight torsion and are in the range of 142-147°, which can due to the repulsion interaction between the hydrogen atom related to the carbon of the fragment C=N and the hydrogen atom of the adjacent phenyl ring On the other hand, the simple bond lengths C-C; d1, d2, d4, d5, d7, d8, d10, d11, d13 and d14 for all compounds in neutral forms are all within 1.37 –1.47 Å, which is shorter by ~ 0.1 Å than that of ethane (1.54 Å) This is partly caused by the p-bonding interaction and results in the partial double-bond character of the bridge bond, thereby strengthening and shortening the bridge bond Moreover, and going from the neutral structures to the excited ones and from M1 to M8, we found that the simple bond lengths (d1, d2, d4, d5, d7, d8, d10, d11, d12, d13 and d14) in the neutral form decrease passing from (M2 ≈ M4) to (M1 ≈ M3) and going from the neutral structures to the excited ones While, the double bond lengths (d3, d6, d9 and d12) become longer than that of the ethylene (1.34 Å), moreover, during the doping process the double bond lengths (d3, d6, d9 and d12) become longer and increase in the following order: (M2 ≈ M4) < (M1 ≈ M3) This can be explaining by the introduction of the substituents N,dimethyl (for M1 and M3) and nitro (for M2 and M4) The results of the geometric properties improve the intermolecular charge transfer (ICT) within the studied molecules Table 1: Optimized selected bond lengths (Å) of the studied molecules in neutral and doped forms obtained by B3LYP/6-31G (d, p) level L1 L2 L3 L4 di Neutral Doped Neutral Doped Neutral Doped Neutral Doped d1 1.3788 1.3527 1.4698 1.4837 1.3788 1.3643 1.4704 1.4790 d2 1.4028 1.3688 1.4172 1.3896 1.4027 1.3877 1.4173 1.4024 d3 1.2676 1.2960 1.2639 1.2511 1.2676 1.2787 1.2634 1.2626 d4 1.4124 1.3713 1.4082 1.3566 1.4123 1.3946 1.4092 1.3913 d5 1.4027 1.3688 1.3997 1.3496 1.4027 1.3920 1.4005 1.3827 d6 1.2811 1.2852 1.2810 1.2836 1.2809 1.2828 1.2804 1.2851 d7 1.4726 1.4634 1.4722 1.4596 1.4728 1.4697 1.4729 1.4660 d8 - - - - 1.4728 1.4697 1.4729 1.4660 d9 - - - - 1.2809 1.2828 1.2804 1.2851 d10 - - - - 1.4027 1.3920 1.4005 1.3827 d11 - - - - 1.4123 1.3946 1.4092 1.3913 d12 - - - - 1.2676 1.2787 1.2634 1.2626 d13 - - - - 1.4027 1.3877 1.4173 1.4024 d14 - - - - 1.3788 1.3643 1.4704 1.4790 Table 2: Dihedral angle (°) values in neutral forms obtained by DFT/B3YP/6-31G (d,p) calculation Mi θ1 θ2 θ3 θ4 θ5 θ6 θ7 θ8 M1 -179.90 -179.67 -147.29 -179.60 - - - - M2 -179.85 -179.70 -142.79 -179.75 - - - - M3 -179.62 -179.31 -147.47 -179.66 -179.66 -147.47 -179.33 -179.63 M4 179.99 -179.86 -143.89 -179.96 -179.97 -143.88 -179.85 -179.99 3.2 Frontier molecular orbital and electronic properties 3.2.1 Frontier molecular orbital The study of the frontier molecular orbitals (FMO) leads to give us an indication about the intramolecular charge transfer (ICT) in the pi-conjugated organic molecule and can inform us about excitation properties Figure illustrates the frontier orbital density surfaces of HOMO and LUMO orbitals of the studied compounds As shown in Fig.4, the HOMO and LUMO are localized on the entire studied molecules, with much localization on the donor part for HOMO, and on the acceptor part for LUMO Moreover, the HOMO in the neutral form of all compounds possesses a p-anti-bonding character between two adjacent fragments and p-bonding character within subunit, whereas the LUMO exhibits a p-bonding character between the subunits HOMO LUMO M1 M2 M3 M4 Fig.4: The contour plots of HOMO and LUMO orbital’s of the studied compounds 3.2.2 HOMO–LUMO energy gap The conjugated molecules are characterized by a highest occupied molecular (HOMO) orbital and a lowest unoccupied molecular orbital (LUMO) , these parameters are particularly very interesting since they are involved in the electron transition, in which the photoinduced electron transfers from the excited-state compound to the acceptor PCBM The energy gap (Eg) for M1, M2, M3 and M4 was obtained by the differences of HOMO and LUMO energy levels (ΔHOMO-LUMO) using B3LYP/6-31G(d,p) and the results are listed in Table The calculated HOMO/LUMO (in eV) energy values of M1, M2, M3 and M4 are -5.02/-2.06 eV, -6.27/3.03eV, -5.01/-2.13eV and -6.37/-3.17eV respectively We found a destabilization of the HOMO and LUMO energies passing from M1 to M3 and from M2 to M4, this can be explained by a π-conjugated length in M3 and M4 compared with M1 and M2 The HOMO–LUMO energy gap values are 2.96 eV (M1), 3.23 eV (M2), 2.88 eV (M3) and 3.20 eV (M4) , these values are increased in the following order M3 < M1 < M4 < M2 This may be attributed to the increase of mesomeric effect in the NO2 acceptor group (comparing M1 with M2 and M3 with M4) and to the conjugated length system in these molecules (comparing M1/M2 with M3/M4) Therefore, the obtained band gap values (2.88 - 3.23 eV) is sufficient to consider applications of this oligomer in optoelectronic and photovoltaic devices In addition, the higher dipole moments are beneficial to facilitate efficient intramolecular photoinduced electron transfer The calculated dipole moment of M1-M4 in their neutral and doped forms, as shown in Table the dipole moment values of these compounds are in the range 3.18-8.53 D/neutral forms and 6.96-10.78D/doped forms We can remark that the dipole moment increases in the following order M1 < M3 < M4 < M2 and this factor increases passing from the neutral to the doped forms Table 3: Theoretical electronic properties parameters (HOMO, LUMO, Gap) by (eV) obtained by B3LYP/6-31G(d,p) of the studied molecules Egap (eV) µ (Debye) Compounds EHOMO (eV) ELUMO (eV) Neutral Doped Neutral Doped M1 -5.02 -2.06 2.96 2.84 3.18 10.78 M2 -6.27 -3.03 3.23 2.81 8.53 8.78 M3 -5.01 -2.13 2.88 2.86 4.29 7.40 M4 -6.37 -3.17 3.20 3.14 6.68 6.96 Fig.5 Data of the absolute energy of the frontier orbitals HOMO and LUMO for the studied molecules and ITO, PCBM A, PCBM and the aluminum (Al) 3.3 Absorption properties To investigate the UV-vis absorption properties of these compounds, the vertical excitation energies (eV), wavelength absorption (λabs /nm), oscillator strengths (OS /eV) for electronic excitations and main transition contribution were carried out through The absorption spectra max of the compounds Mi by IEF-PCM/TD-CAM-B3LYP/6-31G(d, p) in dichloromethane The calculated results are summarized in Table 5, and the simulated absorption spectra are shown in Fig The calculated wavelength values were compared with experiment ones The absorption spectra show similar profile for M1/M2 and M3/M4 which present a main intense band at higher energies from 379 to 457 nm which was observed in the visible region and was assigned to the ICT transitions For the compounds, M1 and M2, the strongest absorption peaks arise from S0 to S2, which corresponds to the dominant promotion of an electron from HOMO to LUMO While the strongest absorption peaks arise from S0 to S3 for M3 and M4, these transitions correspond to the dominant promotion of an electron from HOMO to LUMO+1 Moreover, the adsorption maximum of M3 is centered at 457 nm, which is 21, 76 and 78 nm red-shifted compared to the λmax of M2 (436 nm), M4 (381 nm) and M1 (379 nm) respectively This bathochromic shift due to the electron-withdrawing effect of the introduction of the NO2 group (comparing M1,M3 with M2,M4) and of the chain length effect passing from M1,M2 to M3,M4 Therefore, the CAM-B3LYP was the functional of choice for UV/Vis absorption spectra calculation for these dyes because is in excellent agreement with the experiment results (Table 4) Table 4: Experimental (in dichloromethane 2.10-5 M) and theoretical absorption maxima λmax (nm) for all molecules calculated by; (a) ZINDOs, (b) TD-B3LYP/631G (d,p) and (c) TD CAM-B3LYP/6-31G(d, p) in dichloromethane a b c exp  max  max  max  max [28] M1 426.74 416.34 436.82 438 M2 416.34 430.72 379.37 380 M3 436.36 448.41 457.33 457 M4 423.72 429.27 381.12 380 Table 5: Absorption spectra data obtained by the IEF-PCM/TD-CAM-B3LYP/6-31G(d, p) methods for the studied compounds in dichloromethane Li Electronic transitions M1 M2 M3 M4 Wavenumber abs OS (Cm ) (nm) (eV) Transition -1 exp (nm) (%) S0 S1 22850.65 437.62 0.0007 HOMO-2LUMO (71%) S0 S2 24763.81 436.81 1.7598 HOMOLUMO (84%) S0 S3 32243.84 310.13 0.0126 HOMO-1LUMO (34%) S0 S4 36779.13 271.89 0.2165 HOMO-1LUMO (35%) S0 S5 36881.56 271.13 0.0022 HOMO LUMO+4 (24%) S0 S6 37217.90 268.68 0.0117 HOMO-4 LUMO (36%) S0 S1 21185.10 482.02 0.0002 HOMO-1LUMO (77%) S0 S2 26639.87 379.37 1.6699 HOMOLUMO (84%) S0 S3 31915.57 313.32 0.0000 HOMO-9LUMO (48%) S0 S4 33489.17 298.60 0.0221 HOMO LUMO+1 (32%) S0 S5 35394.27 282.53 0.0212 HOMO-3LUMO (70%) S0 S6 35559.61 281.21 0.0267 HOMO-5LUMO (79%) S0 S1 22821.61 478.18 0.0050 HOMO-4LUMO+1 (38%) S0 S2 22822.42 468.16 0.0004 HOMO-5LUMO+1 (37%) S0 S3 24311.33 457.33 3.0397 HOMOLUMO+1 (44%) S0 S4 25032.39 399.48 0.6317 HOMO LUMO (43%) S0 S5 31961.55 312.87 0.0197 HOMO-1LUMO+2 (17%) S0 S6 32093.02 311.59 0.0000 HOMO-2 LUMO (23%) S0 S1 21168.97 472.38 0.0002 HOMO-2LUMO+1 (38%) S0 S2 21168.97 472.38 0.0001 HOMO-3LUMO+1 (37%) S0 S3 26238.20 381.12 2.895 HOMOLUMO+1 (45%) S0 S4 27114.12 368.81 0.5920 HOMO LUMO (46%) S0 S5 33316.57 300.15 0.0425 HOMO-1LUMO+2 (22%) S0 S6 33350.44 299.84 0.0191 HOMO LUMO+2 (25%) [28] 438 380 457 380 Fig 6: The absorption spectra max of the compounds Li by: (a) IEF-PCM/TD-CAM-B3LYP/631G(d, p) in dichloro-methane (on the left) ; experimental absorption in dichloromethane (2·10-5 M) at room temperature (on the rigth) 3.4 Photovoltaic properties To investigate the photovoltaic properties of the studied molecules, it’s very important to examine the charge transfer from the donor (molecules Mi) to the acceptor PCBM (fullerene derivative) Herein, we studied the photovoltaic properties of four compounds M1-M4 as a donor, blended with [6,6]-phenylC61-butyric acid methyl ester (PCBM C60), which is the most largely used as an acceptor photovoltaic and active layer in solar cell devices As shown in Fig.5, we noted that the LUMO levels of all studied compounds are higher than that of PCBM (C60 and C70), and the difference between the LUMO energy levels of Li (i=1-4) and PCBM are: 1.64, 0.67, 1.57, 0.53 eV/PCBM C60 and 1.48, 0.51, 1.41, 0.37 eV /PCBM C70 respectively (Table 6) These latter values suggest that the electron transfers from the studied molecules to the conductive band of PCBM are possible Moreover, we note that the LUMO energy levels of the studied compounds are much higher than that of the ITO conduction band edge Thus, all studied molecules have an ability to inject electrons into ITO electrodes The photovoltaic efficiency performance data of the photovoltaic cell (the power conversion efficiency (η)) values was calculated according to the following equation (1) [37]: η = FF Voc Jsc (1) Pin Where Pin is the incident power density, Jsc is the short-circuit current, Voc is the open-circuit voltage, and FF denotes the fill factor The maximum open circuit voltage (Voc) of the bulk heterojunction (BHJ) solar cell is related to the difference between HOMO of the electron donor and LUMO of the electron acceptor, taking into account the energy lost during the photo-charge generation [38-39] The theoretical values of opencircuit voltage Voc have been calculated from the following expression (2) [40]: Voc = E − E − 0.3 (2) Based on the above equation, the calculated Voc values of the studied compounds are in the rage 1.012.37eV/PCBM C60 and 1.17-1.53eV/PCBM C70 respectively These values are higher (should be larger than 0.3 eV) which guarantees efficient exciton split and charge dissociation at the Donor/Acceptor interface [41-43] and suggest the molecules M1-M4 good candidates for photovoltaic application Table 6: Data of EHOMO, ELUMO , the Open Circuit Voltage Voc by eV and dipole moment (Debye) Molecules EHOMO ELUMO Voc (eV)/ (eV) (eV) PCBM C70 PCBM C60 PCBM C70 PCBM C60 M1 -5.02 -2.06 1.18 1.02 1.48 1.64 M2 -6.27 -3.03 2.43 2.27 0.51 0.67 M3 -5.01 -2.13 1.17 1.01 1.41 1.57 M4 -6.37 -3.17 1.53 2.37 0.37 0.53 PCBM C70 - -3.54 - - - - PCBM C60 -6.1 -3.70 - - - - i(eV)/ Conclusion Four conjugated compounds of azobenzene based iminopyridine ligands M1- M4 have been synthesized by A Ayadi et al [28], and investigated by theoretical study using density functional theory (DFT) at B3LYP level for geometric and electronic properties calculations, and time-dependent TD/CAMB3LYP was used to calculate the optoelectronic and absorption properties The results of the optimized structures for all studied compounds have similar confirmations (quasi-planar conformation) The calculated gap energies values of the studied molecules are in the range 2.86-3.23eV in their neutral forms The presence of nitro substituent group instead of N, dimethyl in the extreme phenyl and the chain length lead to a decrease in the band gap energy The Voc calculated values of M1-M4 range from 1.01 to 2.37eV /PCBM C60 and from 1.17 to 1.53eV /PCBM C70, these values are sufficient for a possible efficient electron injection Moreover, the UV–Vis absorption properties obtained by using TD/CAM-B3LYP/6-31G(d,p) calculations ranging from 379 to 457 nm, these values are in good agreement with the experiment ones From this study, the obtained results suggest these molecules as good materials for photovoltaic applications like organic solar cells Finally, the procedures of theoretical calculations are in agreement with the experiment results, and can be employed to predict and assume the optoelectronic and photovoltaic properties of the other materials Acknowledgments: This work has been supported by the project Volubilis AI n°: MA/11/248 We are grateful to the “Association Marocaine des Chimistes Théoriciens” (AMCT) for its pertinent help concerning the programs References [1] L Dou, Y Liu, Z Hong, G Li, Y Yang, Low-bandgap near-IR conjugated polymers/molecules for 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