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Exploring the Photovoltaic Properties of Metal Bipyridine Complexes (Metal = Fe, Zn, Cr, and Ru) by Density Functional Theory

  • Ahmad Irfan EMAIL logo and Ghulam Abbas
Published/Copyright: February 15, 2018
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Zeitschrift für Naturforschung A
From the journal Zeitschrift für Naturforschung A Volume 73 Issue 4

Abstract

The synthesis and characterisation of mononuclear Fe complexes were carried out by using bipyridine (Compound 1) at ambient conditions. Additionally, three more derivatives were designed by substituting the central Fe metal with Zn, Cr, and Ru (Compound 2, Compound 3, and Compound 4), respectively. The ground state geometry calculations were carried out by using density functional theory (DFT) at B3LYP/6-31G** (LANL2DZ) level of theory. We shed light on the frontier molecular orbitals, electronic properties, photovoltaic parameters, and structure–property relationship. The open-circuit voltage is a promising parameter that considerably affects the photovoltaic performance; thus, we have estimated its value by considering the complexes as donors whereas TiO2 and/or Si were used as acceptors. The solar cell performance behaviour was also studied by shedding light on the band alignment and energy level offset.

Keywords: Density Functional Theory; Metal Bipyridine Complexes; Photovoltaics; Structure–Property Relationship; Synthesis

1 Introduction

Bipyridine (bipy) is a well-known bidentate ligand that has been extensively used for the coordination of a variety of metal ions. Bipyridine is a versatile ligand that can be easily functionalised to yield more interesting chelating ligands with interesting properties. Bipyridine complexes of ruthenium and osmium are particularly interesting because they are thermally and photochemically stable and often exhibit fluorescence [1], [2]. Luminescent ruthenium bipyridine complexes containing mercapto alkyl chains have been synthesised and are known to exhibit good photochemical properties [3]. Ruthenium and iron complexes of bipyridine have also found applications as photosensitisers [4], [5]. Protein surface recognition has been achieved by ruthenium bipyridine derivatives. Such interactions affect cytochrome C (cyt c) binding [6].

Much of the interest in bipyridine metal complex research lies in their applications in polymers and solar cells owing to their facile coordinating and unique photochemical properties. Accordingly, many metal complexes of bipyridine have found applications in solar cells [7], [8], [9], biosensors [10], and in the synthesis of molecular wires [11]. Herein, we report density functional theory (DFT) studies to analyse in detail the structural, electronic, and photovoltaic properties of iron bipyridine complex Fe(bipy)3 (Compound 1) and its designed derivatives, i.e. zinc bipyridine complex Zn(bipy)3 (Compound 2), chromium bipyridine complex Cr(bipy)3 (Compound 3), and ruthenium bipyridine complex Ru(bipy)3 (Compound 4) by replacing the central Fe metal with Zn, Cr, and Ru, respectively (see Fig. 1). We shed light on various solar cell parameters, band gap alignments, and open-circuit voltage (VOC) of Compounds 1–4 by first-principles study.

Figure 1: The structural presentation of complexes studied here: the central metal atom “1” is Fe, Zn, Cr, and Ru in Compounds 1–4, respectively, along with their numbering scheme.
Figure 1:

The structural presentation of complexes studied here: the central metal atom “1” is Fe, Zn, Cr, and Ru in Compounds 1–4, respectively, along with their numbering scheme.

2 Experimental Details

2.1 Materials and Measurement

All the chemicals and reagents were purchased from Across, Alfa Aesar, and Merck, and were used without further purification. FT-IR spectrum was taken on a Bruker spectrophotometer. Single-crystal X-ray diffraction (XRD) data was collected on an Oxford diffraction diffractometer, equipped with a CCD area detector and a graphite monochromator utilising Mo Ka radiation (k=0.71073 Å).

2.2 Synthesis

Fe(bipy)3 (Compound 1) was synthesised according to following procedure:

A solution of 2,2′-bipyridine (0.39 g, 0.24 mmol) in MeCN (20 mL) was added dropwise over a period of 20 min to a stirred solution of FeCl3 (0.04 g, 0.24 mmol) and triethanolamine (0.037 g, 0.25 mmol) in MeCN (20 mL). The reaction mixture was heated under reflux for 1 h, after which it was cooled to room temperature and then allowed to stand undisturbed in a sealed vial. Red coloured cubic crystals of Fe(bipy)3 suitable for X-ray crystallography [12], [13] were obtained after 2 days. The crystals were maintained in the mother liquor for X-ray crystallography or collected by filtration, washed with MeCN, and dried in vacuo; yield: 45 % Anal. Calc. for (Compound 1) C30H24FeN6: C, 68.71; H, 4.61; N, 16.02 Found: C, 68.67; H, 4.59; N, 15.97 FT-IR (KBr 599 cm− (C=N), 1551 (HCC), 1497 (HCC), 1457 (HCH).

3 Computational Details

The DFT is a good and reliable theory that is implemented in the Gaussian 09 package [14]. The DFT [15] was treated according to Becke’s three-parameter gradient-corrected exchange potential and the Lee–Yang–Parr gradient-corrected correlation potential (B3LYP) [16], [17], and all calculations were performed by using the 6-31G** basis set [18], [19], [20], and the LANL2DZ basis set was applied for Fe, which was found to be precise for metal-containing systems [21]. The structures investigated in the present study were optimised in the ground state (S0) at the B3LYP/6-31G** (LANL2DZ) level of theory. The density of states was convoluted using GaussSum 2.1 software [22].

4 Results and Discussion

4.1 Structural Parameters

The geometrical parameters, i.e. bond lengths and bond angles, have been tabulated in Table 1. The B3LYP/6-31G** (LANL2DZ) is a reasonable choice that reproduced the experimental geometrical parameters (see atom numbering scheme in Fig. 1). Recently, Fan and co-workers computed the ground state geometry of Fe complex by using LDA BP86/TZP (TZ2P for Fe) in ADF software. Our computed data is also in good agreement with the findings of Fan et al. [23]. The B3LYP/6-31G** (LANL2DZ) level is a reliable approach and thus the ground state geometries of designed derivatives (Compounds 2–4) have been optimised at the same level of theory.

Table 1:

Experimental (Exp) and ground state optimised (Opt) bond length (Å) and bond angles (degree) of Compound 1 at B3LYP/6-31G*(LANL2DZ) level of theory.

Bond lengthsOptExpBond anglesOptExp
Fe1–N22.0041.978N2–Fe1–N5181.7981.63
Fe1–N522.0041.978N12–Fe1–N3281.8081.63
Fe1–N122.2101.978N22–Fe1–N4281.7981.68
Fe1–N322.2421.978N2–Fe1–N32174.19172.68
Fe1–N221.8931.978N12–Fe1–N42174.17172.68
Fe1–N422.0201.978N22–Fe1–N52174.15172.62

4.2 Electronic Properties

The frontier molecular orbitals (FMO), i.e. highest occupied molecular orbitals (HOMOs), and lowest unoccupied molecular orbitals (LUMOs) energy levels are noteworthy to regulate various electrochemical, optical properties, and photovoltaic performances. In Table 2, the energies of the eight FMOs, HOMO, first HOMO (HOMO-1), second HOMO (HOMO-2), third HOMO (HOMO-3), LUMO, first LUMO (LUMO+1), second LUMO (LUMO+2), and third LUMO (LUMO+3) are displayed. Figure 2 illustrates the charge density delocalization and localization patterns of these FMOs. In Compound 1, the HOMO is distributed on three ligands A–C, whereas Fe has no participation in the formation of HOMO. On the other hand, HOMO-1, HOMO-2, and HOMO-3 are distributed on the Fe. The LUMO is localised on ligands B and C (for ligand details, see Fig. 1), LUMO+1 on ligands A and B, LUMO+2 was distributed on three ligands, and LUMO+3 on ligands A and C. In Compound 2, the HOMO is distributed on three ligands A–C, the HOMO-1 at ligand C, HOMO-2 and HOMO-3 at ligands A and B. The LUMO is localised on ligands A and B, LUMO+1 on ligand C, whereas LUMO+2 and LUMO+3 distributed on A and B ligands. In Compound 3, HOMO, HOMO-1, and HOMO-2 are distributed in the central metal Cr whereas HOMO-3 is at ligand C. LUMO and LUMO+3 are localised on three ligands A–C, LUMO+1 on ligand B and LUMO+2 on A and C ligands. In Compound 4, the HOMO is distributed on three ligands A–C whereas the HOMO-1, HOMO-2, and HOMO-3 are distributed on the Ru. The LUMO is localised at A and C, LUMO+1 at A and B, and LUMO+2 are localised on three ligands A–C, LUMO+3 on ligand C with partial contribution from A and B ligands. Previously, it had been clarified that metal complexes such as iron–nitrogen–heterocyclic-carbene generate photoelectrons in the conduction band (CB) of TiO2 from metal-to-ligand charge transfer (MLCT) [24]. Similarly, we have also observed MLCT from HOMO-1 to LUMO in all studied derivatives except in Zn–bipyridine complex (Compound 2).

Table 2:

Computed energies of eight FMOs: HOMO-3, HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2, and LUMO+3 of metal bipyridine complexes at B3LYP/6-31G** (LANL2DZ) level of theory.

ComplexesHOMO-3HOMO-2HOMO-1HOMOLUMOLUMO+1LUMO+2LUMO+3
Compound 1−5.23−5.23−5.14−2.17−1.55−1.55−0.55−0.30
Compound 2−6.07−6.05−5.99−2.13−1.61−1.47−0.48−0.36
Compound 3−6.01−3.14−3.10−3.08−1.57−0.98−0.98−0.54
Compound 4−4.99−4.99−4.82−2.26−1.53−1.53−0.58−0.29
Figure 2: Distribution pattern of the important FMOs of Compounds 1–4 at B3LYP/6-31G** (LANL2DZ) level of theory.
Figure 2:

Distribution pattern of the important FMOs of Compounds 1–4 at B3LYP/6-31G** (LANL2DZ) level of theory.

For Compound 1, the calculated energies of HOMO, HOMO-1, HOMO-2, and HOMO-3 are −2.18, −5.14, −5.23, and −5.23 eV, respectively. The calculated energies of LUMO, LUMO+1, LUMO+2, and LUMO+3 are −1.55, −1.55, −0.55, and −0.30 eV, respectively. The HOMO1-LUMO energy gap of 3.59 revealed that it would be easy to transfer the electrons from occupied to unoccuped molecular orbitals. For Compound 2, the calculated energies of HOMO, HOMO-1, HOMO-2, and HOMO-3 are −2.13, −5.99, −6.05, and −6.07 eV, respectively. The calculated energies of LUMO, LUMO+1, LUMO+2, and LUMO+3 are −1.61, −1.47, −0.48, and −0.36 eV, respectively. A HOMO1-LUMO energy gap of 4.38 was observed. For Compound 3, the calculated energies of HOMO, HOMO-1, HOMO-2, and HOMO-3 are −3.08, −3.10, −3.14, and −6.01 eV, respectively. The almost similar energy values of HOMO, HOMO-1, and HOMO-2 reveal that these are almost emerged in each other. The calculated energies of LUMO, LUMO+1, LUMO+2, and LUMO+3 are −1.57, −0.98, −0.98, and −0.54 eV, respectively. The energies of LUMO+1 and LUMO+2 are the same, which elaborates that the electron transfer from occupied to LUMO+1 or LUMO+2 would have almost similar chances. The HOMO3-LUMO energy gap has been observed at 4.44 eV. For Compound 4, the calculated energies of HOMO, HOMO-1, HOMO-2, and HOMO-3 are −2.26, −4.82, −4.99, and −4.99 eV, respectively. The calculated energies of LUMO, LUMO+1, LUMO+2, and LUMO+3 are −1.53, −1.53, −0.58, and −0.29 eV, respectively. The HOMO1-LUMO energy gap of 3.29 revealed that it would be easy to transfer the electrons from occupied to unoccupied molecular orbitals. The energies of HOMO-2 and HOMO-3 from Compounds 1, 2, and 4 reveal that these are emerged in each other. The energies of LUMO and LUMO+1 from Compounds 1 and 4 are the same, which elaborates that the electron transfer from HOMO to LUMO or LUMO+1 would have almost similar chances (see Tab. 2).

4.3 Electron Injection Barrier

The hole and electron injection energies for Compound 1 were computed. The electron injection energies of Compound 1, Compound 2, Compound 3, and Compound 4 were calculated as [(=−ELUMO−(−work function of metal)]. The work function of Al is −4.08 eV. The electron injection energy was observed as 2.53 eV [=−1.55−(−4.08)], 2.47 eV [=−1.61−(−4.08)], 2.51 eV [=−1.57−(−4.08)], and 2.55 eV [=−1.53−(−4.08)] from the Fe, Zn, Cr, and Ru complexes to Al electrode. The hole injection energy of Fe complex is 1.06 eV [=−4.08−(−5.14)], 1.91 eV [=−4.08−(−5.99)], −0.98 eV [=−4.08−(−3.10)], and 0.74 eV [=−4.08−(−4.82)] from Fe, Zn, Cr, and Ru complexes to the Al electrode by considering the HOMO-1 of the compounds and work function of Al. The hole injection energy of Cr complex was found to be positive by considering the energy of HOMO-3 and work function of Al as 1.93 eV [=−4.08−(−6.01)].

4.4 Photovoltaic Properties

In the first step, solar energy is harvested by a semiconductor. In various solar cell designs/fabrications, rare elements are being used but due to environmental issues and some limitations, their substituents are the main area of interest. Recently, Fe has been the focus of interest replacing ruthenium. In recent developments, Fe–nitrogen–heterocyclic–carbene complex was achieved and it was found that Fe complex generates photoelectrons in the CB of TiO2 from MLCT [24]. Similarly, in the present study, the ICT has been observed from Fe, Cr, and Ru of HOMO-1, HOMO-2, and HOMO-3 to the ligand(s) of LUMO, LUMO+1, LUMO+2, and LUMO+3. The photovoltaic efficiency can be calculated by using the following equation:

(1)η=VocJscFFPin

4.4.1 Open-Circuit Voltage

The VOC is a promising parameter that considerably affects the photovoltaic performance. Usually, VOC depends on materials and various factors such as light intensity, light source, energy levels, device temperature, work functions of the electrodes, external fluorescence efficiency and charge-carrier recombination [25]. Some experimental and computational work exhibited a relationship between VOC and donor (D) HOMO and acceptor (A) LUMO energy gaps [26], [27], [28], [29]. The relationship between the VOC and electronic structure is not yet clear. Some studies showed an analytical relationship between VOC and LUMO energy (ELUMO), which can be evaluated by the following [30]:

(2)VOC=ELUMO−ECB

where the energy of semiconductor’s CB edge is ECB. It is expected that higher ELUMO can lead to superior VOC. Hitherto, valance band (VB) and CB values for TiO2 were considered −7.40 and −4.20 eV, respectively [31]. According to Liu et al. [32], VB and CB of Si are −5.43 and −3.92 eV, respectively. The estimated VOC of Compounds 1–4 have, respectively, been observed as 2.65, 2.59, 2.63, and 2.67 eV by considering the TiO2 VB. The estimated VOC values of Compounds 1–4 have, respectively, been observed as 2.37, 2.31, 2.35, and 2.39 eV by considering the Si VB, which are higher than the previously studied numbers of compounds [33].

4.4.2 Band Gap Alignment

In 2016, Pan [34] stated that for the tuning of electro-optical properties, carrier mobility, and for better solar energy, harvesting type II band alignment is important. Likewise, it was also disclosed that type II band alignment could lead to superior photovoltaic efficiency, improved electron–hole separation, and light absorption. In the present study, type II band alignment was observed by considering the HOMO-1 and LUMO of Compound 1, Compound 2, Compound 4, and the VB and CB of TiO2. The type II band alignment was observed by considering the HOMO-3 and LUMO of Compound 3 and VB and CB of TiO2. The type II band alignment was also observed by considering the HOMO-1 and LUMO of Compound 1, Compound 4, and the VB and CB of Si (see Fig. 3). This preferred band alignment revealed that these compounds might be suitable for use in heterojunction solar cells.

Figure 3: Band alignment model of Compounds 1–4 with respect to TiO2 and Si [HOMO-1 and LUMO can be considered VB and CB in solid-state (HOMO-3 for Compound 3) to understand the band gap alignment].
Figure 3:

Band alignment model of Compounds 1–4 with respect to TiO2 and Si [HOMO-1 and LUMO can be considered VB and CB in solid-state (HOMO-3 for Compound 3) to understand the band gap alignment].

The average VB and CB values of Si/TiO2 are −6.14 and −4.06 eV. We have studied the band alignment by considering Compounds 1–4 as donors and the average VB and CB of TiO2 and Si as acceptors (see Fig. 4). Interestingly, TiO2 and Si as acceptors are favourable for type II band alignment.

Figure 4: Band alignment model of Compounds 1–4 with respect to the average value of VB and CB of TiO2 and Si [HOMO-1 and LUMO can be considered VB and CB in solid-state (HOMO-3 for Compound 3) to understand the band gap alignment].
Figure 4:

Band alignment model of Compounds 1–4 with respect to the average value of VB and CB of TiO2 and Si [HOMO-1 and LUMO can be considered VB and CB in solid-state (HOMO-3 for Compound 3) to understand the band gap alignment].

In hybrid solar cells, the donor’s exciton dissociation ensued at the (donor–acceptor) D–A interface. The force required to overcome the exciton binding energy is directed by the energy level offset (ELO) of the ELUMO (donor) and CB edge (acceptor) [35]. A small LUMO offset between D and A is favoured for a high VOC. The ELO is 2.65, 2.59, 2.63, and 2.67 eV for Compounds 1–4 with TiO2 acceptor, respectively, and it is 2.37, 2.31, 2.35, and 2.39 eV with Si to overcome the exciton binding energy showing that Compounds 1–4 with Si acceptor might determine better photovoltaic performance. The exciton dissociation in the acceptor is the ELO of the donor EHOMO1 and the VB of the acceptor. The ELO for Compound 1, Compound 2, and Compound 4 is 2.26, 1.41, and 2.58, respectively, with TiO2 acceptor. The ELO for Compound 1, Compound 2, and Compound 4 is 0.29, 0.56, and 0.61, respectively, with Si acceptor and unveiled that the energy requisite to dissociate excitons with TiO2 acceptor would be higher.

It can be expected that the VOC of studied compounds with TiO2 acceptor might be higher as compared with Si (acceptor). Although smaller ELO with Si (acceptor) compared with TiO2 also revealed that prior acceptor would be a good competitor as well. Thus, by fabricating the heterojunction photovoltaic device, TiO2/Si as acceptor would be good strategy that can lead to higher VOC and smaller ELO, resulting in improved solar cell efficiency.

5 Conclusions

The geometrical parameters computed at DFT-B3LYP/6-31G** (LANL2DZ) level of theory are in good agreement with the experimental evidence. This level is the rational choice to predict the properties of interests for tris(bipyridyl)Fe, tris(bipyridyl)Zn, tris(bipyridyl)Cr, and tris(bipyridyl)Ru complexes. The HOMO-1, HOMO-2, and HOMO-3 are delocalised at metal whereas LUMO, LUMO+1, LUMO+2, and LUMO+3 are of antibonding character (π*) distributed on the ligands in all metal complexes except the Zn complex. The intramolecular MLCT has been observed from occupied to unoccupied molecular orbitals in tris(bipyridyl)Fe, tris(bipyridyl)Cr, and tris(bipyridyl)Ru complexes. The type II band alignment perceived that Compound 1, Compound 2, and Compound 4 might be appropriate for use in heterojunction solar cells. It is expected that the heterojunction photovoltaic device of Compound 1 and Compound 4 as donor and TiO2 and/or Si as acceptor would be a noble approach to enhance the efficiency by improving the VOC, lowering ELO, and shifting the band alignment to type II. Finally, it is also anticipated that considering the VB and CB of both the Si/TiO2 as acceptors with the studied metal bipyridine complexes might be favourable for type II band alignment.

Acknowledgments

The authors would like to express their gratitude to the Research Center for Advanced Materials Science, King Khalid University, Saudi Arabia, for support.

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Received: 2017-11-10
Accepted: 2018-01-18
Published Online: 2018-02-15
Published in Print: 2018-03-28

©2018 Walter de Gruyter GmbH, Berlin/Boston

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  8. Maxwell-Like Equations for Free Dirac Electrons
  9. Exploring the Photovoltaic Properties of Metal Bipyridine Complexes (Metal = Fe, Zn, Cr, and Ru) by Density Functional Theory
  10. The Successive Applications of Two Types of Gauge Transformations for the q-Deformed Modified Kadomtsev-Petviashvili Hierarchy
  11. Similarity Reduction and Exact Solutions of a Boussinesq-like Equation
  12. Conformal Cosmology with a Complex Scalar Field and a Gauge-Mediated Supersymmetric Breaking Potential
https://doi.org/10.1515/zna-2017-0406
Keywords for this article
Density Functional Theory; Metal Bipyridine Complexes; Photovoltaics; Structure–Property Relationship; Synthesis

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  1. Frontmatter
  2. Heun Polynomials and Exact Solutions for the Massless Dirac Particle in the C-Metric
  3. Effect of Varying Pnictogen Elements (Pn=N, P, As, Sb, Bi) on the Optoelectronic Properties of SrZn2Pn2
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  5. Intense Isolated Short Attosecond Pulse Generation from a Coherent Superposition State in a Spatially Inhomogeneous Field
  6. Optical Solitons for the Perturbed Nonlinear Schrödinger Equation with Kerr Law and Non-Kerr Law Nonlinearity
  7. The Artificial Hamiltonian, First Integrals, and Closed-Form Solutions of Dynamical Systems for Epidemics
  8. Maxwell-Like Equations for Free Dirac Electrons
  9. Exploring the Photovoltaic Properties of Metal Bipyridine Complexes (Metal = Fe, Zn, Cr, and Ru) by Density Functional Theory
  10. The Successive Applications of Two Types of Gauge Transformations for the q-Deformed Modified Kadomtsev-Petviashvili Hierarchy
  11. Similarity Reduction and Exact Solutions of a Boussinesq-like Equation
  12. Conformal Cosmology with a Complex Scalar Field and a Gauge-Mediated Supersymmetric Breaking Potential
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