Searching n-type high-performance organic thermoelectric material with good air-stability and high transparency remains a big challenge. Here, we report an all-transparent n-type charge transfer complex [ZnBr2(Br-C6H4-NH2)2] with an ultra-wide band gap of 4.25 eV (~ 163 kBT). This material exhibits an ultrahigh electrical conductivity of ~ 2,936 S cm− 1 and a high Seebeck coefficient of − 114 µV K‒1, leading to an extraordinarily high power factor of ~ 3,797 µW m‒1 K‒2 at room temperature, which is a record value for organic thermoelectric materials. Remarkably, figure-of-merit (ZT) values of 0.23 at 298 K and 0.45 at 473 K were achieved, respectively. The ZT values are not only the state-of-the-art performance for n-type organic thermoelectric materials, but also better than those of some typical inorganic thermoelectric materials at near room-temperature range. The extraordinarily high thermoelectric performance is attributed to the electron transfer induced n-type heavily doped characteristic, high valley band degeneracy and heavy effective mass. Owing to its exceptional thermoelectric performance and excellent air-stability, [ZnBr2(Br-C6H4-NH2)2] can be considered as a milestone in the development of organic thermoelectric materials.
Article
Transparent Charge Transfer Complex with High Thermoelectric Performance
https://doi.org/10.21203/rs.3.rs-2582017/v1
This work is licensed under a CC BY 4.0 License
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Charge transfer
complex
n-type
gas pump
thermoelectric
metal-organic
air-stability
Thermoelectric energy recovery in near room-temperature range (300 K < T < 600 K) has attracted a huge interest due to the usages of various waste heat sources such as consumer electronics, home heating, solar cells and wearable devices on human bodies.1 The energy conversion efficiency of thermoelectric technology is expressed by the dimensionless figure-of-merit ZT = S2σT/ktol, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, ktol is the total thermal conductivity, and the product (S2σ) is power factor (PF).2 Traditional inorganic thermoelectric materials (Bi2Te3, GeTe, and PbTe) usually exhibit good thermoelectric performance, but do not have flexible properties.3 Compared with their inorganic counterparts, organic thermoelectric materials exhibit unique advantages in the form of mechanical flexibility, light weight, low-cost synthesis, low toxicity, and solution processability over large areas.4,5 Thus, organic thermoelectric materials are considered to be promising candidates in near room-temperature range for wearable electronics and lightweight powering elements of various Internet of Things.
Over the past decade, various p-type high-performance materials have been developed, especially those based on poly(3,4-ethylenedioxythiophene) (PEDOT).6–9 Recently, Durand et al.10 reported a record PF (2,900 µW m− 1 K− 2) in oriented thin films, which can be regarded as a milestone in the history of p-type organic thermoelectric materials. Overall, progress on n-type organic thermoelectric materials is lagging behind that of p-type organic thermoelectric materials.11 Nowadays, the scientific community has turned its focus to the more challenging n-type counterparts because both p-type and n-type materials are needed for thermoelectric module. Most n-type organic thermoelectric materials show much lower performance, which is largely due to inferior PF.4,12 To date, there are few n-type materials with reported PFs over 500 µW m− 1 K− 2.11−14 Thus, the overall ZT values are limited. For n-type organic thermoelectric materials, most of them are short-lived in the presence of oxygen and water, hampering their large-scale application.5 Furthermore, optically transparent thermoelectric elements are pretty attractive due to their potential new applications such as smart windows (or screens) with energy harvesting, temperature sensing functionalities, and quick on-chip cooling and power recovery for fully transparent electronic devices.15 In fact, most developed thermoelectric materials are optically opaque due to their small bandgap (Eg < < 2 eV), such as the (Bi,Sb)2Te3 family.16 Thus, alternative n-type organic materials with good air-stability and high transparency are highly desired if their performance can be very competitive with that of inorganics in near room-temperature range.
In this study, we report a metal-organic charge transfer complex [ZnBr2(Br-C6H4-NH2)2], bis (4-bromoaniline) dibromido zinc. This work is inspired by our understanding of this emergent charge transfer complexes [MX2(R-C6H4-NH2)2] system (M is transition metals, X represents halogens, and R–C6H4–NH2 represents aromatic amines)——a new family of high-performance organic thermoelectric materials. Based on previous study17,18 and our recognition, manipulating the degree of charge transfer between the metal ions and ligands should be a good way for the discovery of new materials with higher thermoelectric performance. We infer that the electronegativity of transition metals will influence the degree of charge transfer. The electronegativity (Pauling scale) of Co, Ni, Cu, and Zn are 1.88, 1.91, 1.90, and 1.65, respectively. Zn element has the smallest electronegativity, which might reduce the degree of charge transfer in [MX2(R-C6H4-NH2)2]. Besides, Zn element is a non-toxic and naturally abundant element. The PF of the [ZnBr2(Br-C6H4-NH2)2] film is up to ~ 3,797 µW m‒1 K‒2 at 298 K, which represents the state-of-the-art value for organic thermoelectric materials. The ZT values of [ZnBr2(Br-C6H4-NH2)2] not only exceed all of current state-of-the-art n-type organic thermoelectric materials in near room-temperature range but also is comparable with that of common inorganic materials.
Film fabrication, micro-structures, crystal structure, and phase stability
The schematic fabrication of transparent metal-organic charge transfer complex thin films in ambient air via a facile gas pump method is depicted in Fig. 1A. One of the biggest advantages for gas pump method is its superb repeatability in thin film deposition, which is demonstrated in previously published studies.19–21 The whole process can be divided into three stages. First, liquid film is deposited via spin-coating or other solution deposition methods (for example, blade coating, slot-die coating, spray coating, and inkjet printing). Second, the liquid film is then placed for a few seconds into a low-pressure vacuum chamber to remove most of the solvents rapidly. Finally, a transparent [ZnBr2(Br-C6H4-NH2)2] thin film is obtained after heating on the hotplate (Fig. 1B). From the low-magnification scanning electron microscopy (SEM) image, we can observe that there are many radial surface structures (Figure S1a). In high-magnification SEM images, the thin film shows needle-like surface morphology (Fig. 1C, Figure S1b, c and d). The energy-dispersive X-ray (EDX) spectrum is presented in Figure S2, wherein the distribution of Zn, Br, and C elements in the [ZnBr2(Br-C6H4-NH2)2] film is homogeneous. A cross-sectional SEM image of the [ZnBr2(Br-C6H4-NH2)2] film prepared by gas pump treatment is given in Figure S3. The lattice fringes of samples display interplanar spacings of 0.2622 nm and 0.3167 nm, which match well respectively with those of the (811) and (11—6) crystal planes (Fig. 1D). The lattice fringes of samples exhibit interplanar spacings of 0.4509 nm, 0.3363 nm, and 0.2936 nm, which match well respectively with those of the (111), (512̅), and (309̅) crystal planes (Figure S4).
The [ZnBr2(Br-C6H4-NH2)2] compound is synthesized via the combination reaction between 4-bromoaniline (Br-C6H4-NH2) and zinc bromide (ZnBr2) (Figure S5). According to the concept of charge transfer complex, the [ZnBr2(Br-C6H4-NH2)2] material can be classified as a charge transfer complex. On the basis of Lewis acid-base theory, Br-C6H4-NH2 and ZnBr2 can act as Lewis base (electron donor) and Lewis acid (electron acceptor), respectively. Thus, the electron initially on the N-donor ligand (Br-C6H4-NH2) will be shared with the Zn metal ions, and then coordinate covalent bonds are formed. In this case, the negative ends of polar ligand molecules, which contain an unshared electron pair, are directed toward the Zn metal ion. So, the attractive interaction is the ion–dipole type.
The Zn atom is tetracoordinated by two Br-anions and two N atoms in a slightly distorted tetrahedral geometry (Fig. 1E). This material crystallizes in the monoclinic system, with the space group P 21/n and the following cell parameters: a = 25.5601 (12) Å, b = 4.6619(2) Å, c = 27.7601(13) Å, α = 90°, β = 92.330(2)°, γ = 90°, V = 3,305.1 (3) Å3, and Z = 8 (Table S1). The [ZnBr2(Br-C6H4-NH2)2] single crystal for crystal structure determination was grown by the simplest slow solvent evaporation method (Figure S6). The detailed crystal data, bond lengths, and bond angles are listed in Table S1, S2, S3, and S4. A view of the molecular structure of the [ZnBr2(Br-C6H4-NH2)2] compound with atoms labelling is shown in Figure S7. The density of [ZnBr2(Br-C6H4-NH2)2] is 2.288 g/cm3 (see Table S1), which is much lower than that of typical thermoelectric materials (SnSe: 6.180 g/cm³, PbTe: 8.164 g/cm³, and Bi2Te3: 7.859 g/cm3). The crystal packing is based on well-ordered alternating ZnBr2 inorganic layers and Br-C6H4-NH2 organic layers. The layered characteristic of the [ZnBr2(Br-C6H4-NH2)2] structure was clearly shown in Figure S8, and the distance between adjacent layers (the interlayer distance, d) is measured to be 15.039 Å. The interaction between two adjacent layers along the c axis should be weak Van der Waals interactions. The [ZnBr2(Br-C6H4-NH2)2] molecules in the layer along the a axis are interconnected through relatively weak hydrogen bonds,22,23 the corresponding distances for this connection (N-H···Br) are 2.831 Å, 3.481 Å, and 3.063 Å (Figure S9). The interactions between the benzene ring group (C།H···C) are made on the well-known π interaction modelling a slipped stacking (π-staking22) (Figure S10).
As shown in Fig. 1F, no peaks related to possible reactants of ZnBr2 and Br-C6H4-NH2 can be observed in the X-ray diffraction (XRD) pattern of [ZnBr2(Br-C6H4-NH2)2]. Besides, the XRD peak intensity for the (002) and (004) planes is very strong, which is coincide with the preferred orientation growth displayed in SEM images (Figure S1). The experimental powder XRD data is highly consistent with the calculated powder XRD result (Figure S11).
Enhancing air-stability is one of the biggest challenges for n-doped organic thermoelectric materials.11,14 The main reason is that the n-type dopant is easy to react with the water or oxygen molecules in air.5 To explore the phase stability of [ZnBr2(Br-C6H4-NH2)2], the fresh samples without any encapsulation were exposed to ambient air continuously for a long time and recorded their transmittance spectra and XRD patterns. Surprisingly, their transmittance spectra are almost same before and after exposure to ambient air continuously for 17,500 h (see Figure S12). At the same time, the XRD patterns do not show additional reflections after exposure to ambient air continuously for 17,500 h (see Figure S13). The above-mentioned results proved that this metal-organic charge transfer complex [ZnBr2(Br-C6H4-NH2)2] is extremely phase-stable in air.
We wonder why this material can be so phase-stable in air? To obtain an in-depth understanding of the air-stability mechanism, ab initio molecular dynamic (AIMD) simulations for the interaction between H2O (O2) molecules and [ZnBr2(Br-C6H4-NH2)2] crystal were performed. As we all know, H2O molecules are easy to form hydrogen-bonded network. Thus, only four H2O molecules were considered on the [ZnBr2(Br-C6H4-NH2)2] (10_1) surface to avoid the interactions between H2O molecules, which can make the effect of H2O molecules on the [ZnBr2(Br-C6H4-NH2)2] surface clear. The H2O molecules initially locate in different positions of [ZnBr2(Br-C6H4-NH2)2] slab because of the preheating of pre-equilibration (see Figure S14). The crystal structure of [ZnBr2(Br-C6H4-NH2)2] slab was not damaged during AIMD simulation (Figure S14). The free energy of the system maintains at ‒1,513.2 eV/cell, suggesting the system is in thermal equilibrium during AIMD simulation (Figure S15). The results obtained via AIMD simulations are consistent with the experimental observations. Moreover, the bond strength of the polar covalent bond between N and Zn atoms in [ZnBr2(Br-C6H4-NH2)2] crystal is larger than that of the hydrogen bonds between H2O molecule and the —NH2 group. Therefore, the covalent bond between N and Zn atoms is difficult to be disrupted by H2O molecules. The crystal structure of [ZnBr2(Br-C6H4-NH2)2] slab was not damaged during the AIMD simulation for the interaction between O2 molecules and [ZnBr2(Br-C6H4-NH2)2] crystal (Figure S16). The AIMD total energy of the system during the heating process at 300 K from its initial configuration remains at ‒1,522.6 eV/cell (Figure S17). The AIMD simulation results revealed the reason why this material can be so phase-stable in air.
For the reaction between Br-C6H4-NH2 and ZnBr2 (Figure S5), a difference in energy (ΔE), ‒1.267 eV, was computed through density functional theory (DFT) calculation according to Eq. (1). Therefore, this is an exothermic reaction, indicating that [ZnBr2(Br-C6H4-NH2)2] is thermodynamically stable.
ΔE = E(product) − E(reactant) (1)
Where E(product) means the energy of product and E(reactant) represents the energy of reactant.
Thermoelectric Performance As A Function Of Temperature
Thermogravimetric analysis suggests the beginning of weight loss is at 477.15 K (Figure S18). So, a suitable temperature range for thermoelectric properties measurement is decided as from 298 K to 473 K. The method for thermoelectric parameters test is described in experimental procedures (Supporting Information, Figure S19). The temperature dependence on the in-plane total electrical conductivity and carrier concentration (n) of the [ZnBr2(Br-C6H4-NH2)2] film are shown in Fig. 2A. The total electrical conductivity decreased from 2,936 S cm‒1 to 2,108 S cm‒1 when the temperature increased from 298 K to 473 K. On the one hand, the film showed metallic-like temperature dependence of the conductivity, that is, the electrical conductivity decreases with increasing temperature. On the other hand, the film presented an increased carrier concentration when the temperature rises, varying from 6.57 × 1020 cm‒3 at 298 K to 1.00×1021 cm‒3 at 473 K, which is a typical semiconductor behavior.
As depicted in Figure S20, the transmittance of the film is high than 85% over the visible range (380–780 nm). The Eg is estimated to be about 4.25 eV (Figure S21). Such an ultra-wide Eg allows for the full transparency of [ZnBr2(Br-C6H4-NH2)2] in the visible spectral region. We compared the Eg and ZT value at room-temperature for typical thermoelectric materials in Fig. 3A (see detailed data in Table S5). Among all-transparent thermoelectric materials (Eg > 3 eV), n-type [ZnBr2(Br-C6H4-NH2)2] film exhibits the largest Eg and the highest room-temperature ZT.
Indium tin oxide (In:SnO2, ITO) is one of the most widely used transparent conducting oxides (TCO) due to its two important properties: high electrical conductivity (commercially available glass substrates covered by ITO layers, ~103–104 S/cm) and good optical transmittance (> 80%).34 Here, we note that this Zn-based metal-organic complex is an all-transparent material with high electrical conductivity (~ 2,936 S cm− 1 at 298 K, Fig. 2A). Combined with the merit of low-cost process, good solution processability, and non-toxic metal element, this all-transparent [ZnBr2(Br-C6H4-NH2)2] material with high electrical conductivity might be a competitive material for transparent electronics,35 such as touch screen displays, organic light-emitting diodes (OLEDs), and electronic paper (e-paper).
The temperature coefficient of resistance (α) is generally defined as the change in electrical resistance of a substance with respect to per degree change in temperature. As shown in Figure S22, the temperature coefficient of resistance of the [ZnBr2(Br-C6H4-NH2)2] film was calculated to be 0.00221°C‒1. This value is slightly smaller than those of commercial platinum (0.00393°C‒1) and copper (0.00386°C‒1), indicating that this material might be a potential candidate for flexible temperature sensors.
The Seebeck coefficient and mobility (µ) of the [ZnBr2(Br-C6H4-NH2)2] film as a function of temperature are displayed in Fig. 2B. The majority carriers in this compound are electrons, which can be inferred from the negative Seebeck coefficient of the [ZnBr2(Br-C6H4-NH2)2] film. The absolute value of Seebeck coefficient showed an increased trend when the temperature rises, exhibiting a typical characteristic for degenerate semiconductor24,29 where the Fermi level is inside the energy band. As shown in Fig. 2B, the electron mobility exhibited a decreased trend when the temperature rises. The vibration of the atoms within the [ZnBr2(Br-C6H4-NH2)2] lattice increases with increasing temperature, leading to the number of collisions between the electrons and the lattices is increased. Therefore, the electron mobility decreases with increasing temperature. As depicted in Figure S23, the electron mobility (27.87 cm2 V‒1 s‒1) at room temperature is slightly higher than that of tetrachloro derivative (4Cl-TAP) with a maximum value of 27.80 cm2 V‒1 s‒1 — a record value for n-channel organic field-effect transistors.36 Consequently, this new material with high electron mobility and good transparency might be used into transparent, high-performance n-channel field-effect transistors.
As shown in Fig. 2C, the in-plane total thermal conductivity decreased from 4.93 W K‒1 m‒1 to 3.66 W K‒1 m‒1 when the temperature increased from 298 K to 473 K. The total thermal conductivity (k) usually contains two parts, k = ke+ kL, where ke is the electronic part of thermal conductivity and kL is the lattice thermal conductivity.37 The ke can be obtained by the Wiedemann–Franz law, ke = LσT, where L is the Lorenz number.37 The Lorenz number that calculated by two different methods were nearly same (Figure S24) and the ke is in the range of 1.61–1.83 W K‒1 m‒1 (Fig. 2C).
The PF of the [ZnBr2(Br-C6H4-NH2)2] film is up to 3,797 µW m‒1 K‒2 at 298 K (Fig. 2D). This value is not only the highest PF ever reported for organic thermoelectric materials, but also even much higher than those of well-known inorganic thermoelectric materials, including PbTe (bulk sample), SnSe (single crystal), and others (Fig. 2D and 3B). Remarkably, ZT values of 0.23 at 298 K and 0.45 at 473 K were achieved, respectively (see Fig. 2E). As shown in Fig. 2E, the ZT values of metal-organic charge transfer complex [ZnBr2(Br-C6H4-NH2)2] are not only the state-of-the-art performance for n-type organic thermoelectric materials, but also even better than those of some typical inorganic thermoelectric materials at near room-temperature range. Additionally, the thermoelectric properties of thin film sample fabricated from two batches were presented in Figure S25.
Electron Transfer Induced N-type Self-doped Mechanism
The high ZT values are mainly caused by the ultrahigh PF, which comes from a high Seebeck coefficient and an extremely high electrical conductivity. From the point of view of physics, the transparency and conductivity of matter are a pair of contradictions. Here, we wonder why this wide bandgap material can shows such high electrical conductivity (~ 2,936 S cm− 1 at 298 K)? One key reason is the extremely high electron concentration (6.57 × 1020 cm‒3 at 298 K) in this material. To study the origin of the extremely high electron concentration in this metal-organic complex, a charge transfer induced n-type self-doped mechanism is proposed. The concept of “doping” in conductive polymer is different from inorganic semiconductor in microscopic details. For instance, chemical “n-doping” usually denotes the partial reduction of the polymer. We think that [ZnBr2(Br-C6H4-NH2)2] is a charge transfer complex which is also named as electron-donor-acceptor complex. We assumed that a fraction of electron in Br-C6H4-NH2 (function as n-type molecular dopants) is transferred into the host inorganic ZnBr2.
To reveal the electron transfer induced n-type self-doped mechanism, we studied the energy band structures of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2] by ultraviolet photoelectron spectroscopy (UPS) and UV-Vis spectroscopy. The Eg values of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2] are 3.19, 3.73, and 4.22 eV, respectively (Figure S26, S27, and S28). As shown in the left sides of Fig. 4A-C, the Fermi energy (EF) level values of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2] were respectively estimated to be − 3.67 eV, − 4.71 eV, and − 3.11 eV, acquired by the equation EF = 21.21 eV (He I) – Ecutoff (cutoff energy). The linear extrapolation values in the right sides of Fig. 4A-C represent the value of Ev – EF. So, the valence band maximum (EV) energy level values of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2] were calculated to be − 6.59 eV, − 8.32 eV, and − 9.71 eV, respectively (Fig. 4D). The conduction band minimum (Ec) energy level values of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2], obtained via adding the Eg to Ev, were estimated to be − 3.40 eV, − 4.59 eV, and − 5.49 eV (Fig. 4D).
The energy band diagram of Br-C6H4-NH2, ZnBr2, and [ZnBr2(Br-C6H4-NH2)2] are shown in Fig. 4D. According to semiconductor physics, the majority of the energy states below EF contain electrons and the majority of energy states above EF are empty of electrons. As indicated in Fig. 4D, electrons from Br-C6H4-NH2 will transfer to the lower energy states of ZnBr2 until thermal equilibrium is reached. The thermal-equilibrium electron concentration in the conduction band (n0) can be expressed as
Here, Nc is the effective density of states functions in the conduction band, kB is Boltzmann constant, mn* is the density of states effective mass of electron, and h is Planck constant. According to Eqs. (2) and (3), n0 will be very large due to the EF is much higher than that of Ec (Fig. 4E). Therefore, [ZnBr2(Br-C6H4-NH2)2] is an n-type degenerate semiconductor (Fig. 4E). We can also understand the phenomenon that the absolute value of S increased with the rising temperature (Fig. 2B).
X-ray photoelectron spectroscopy (XPS) was performed to study the electron transfer process. For the Zn 2p spectrum of ZnBr2 (Fig. 5A), the Zn 2p1/2 and 2p3/2 peaks shift to a lower value in comparison with that of [ZnBr2(Br-C6H4-NH2)2]. For the Br 3d spectrum of ZnBr2 (Fig. 5B), the Br 3d3/2 and Br 3d5/2 peaks shift to a lower value in comparison with that of [ZnBr2(Br-C6H4-NH2)2]. These chemical shift should be caused by the electron transfer from amine groups of Br-C6H4-NH2. As illustrated in Fig. 5C, the electron transfer from N to Zn atom, resulting in an increase in electron density of the Zn atom, and then the binding energy of Zn electrons is decreased. At the same time, the electronegativity (electron withdrawing power) of the Zn atom is reduced. Then, the electron density of the Br atom is improved. Therefore, the binding energy of Br electrons is decreased.
To further confirm the charge transfer from Br-C6H4-NH2 to ZnBr2, fourier transform infrared spectroscopy (FTIR) of Br-C6H4-NH2 and [ZnBr2(Br-C6H4-NH2)2] were performed. As shown in Fig. 5D, stretching vibration of -NH2 groups appeared at 3,482 and 3,385 cm− 1 for Br-C6H4NH2, which was shifted to 3,288 and 3,225 cm− 1 upon reacting Br-C6H4-NH2 with ZnBr2, respectively. This result indicated that the organic-metal complex [ZnBr2(Br-C6H4-NH2)2] is formed due to the interaction between Lewis base (Br-C6H4-NH2) and Lewis acid (ZnBr2). As illustrated in Figure S29, Br-C6H4-NH2 and [ZnBr2(Br-C6H4-NH2)2] show different electron paramagnetic resonance (EPR) signals. The value of charge transfer between Br-C6H4-NH2 and ZnBr2 is about 1.641×10− 5 e per [ZnBr2(Br-C6H4-NH2)2] molecule (Figure S30).
As shown in Fig. 5E, the sum of Bader charge of Br1 and Br2 in ZnBr2 are − 0.26 e, which is equal to the Bader charge of Zn atom. As shown in Fig. 5E, the Bader charge of Br1 and Br2 in [ZnBr2(Br-C6H4-NH2)2] are − 0.41 e and − 0.37 e, respectively, which originated from Zn atom. However, the Bader charge of Zn is + 0.74 e rather than + 0.77 e, which suggests that 0.04 e charge should be transferred from N1 and N2 to Zn. As displayed in Fig. 5F, the yellow and cyan isosurfaces show the charge gain and lost regions, respectively. The yellow regions marked with elliptic dotted line mean the sharing of electrons between Zn and N atoms. As a result, the chemical bond between Zn and N atoms should be covalent bond.
Origin Of High Seebeck Coefficient And Electron Mobility
To elucidate the high electrical transport properties in [ZnBr2(Br-C6H4-NH2)2] crystal, we performed DFT calculations to obtain the complex electronic band structures. As shown in Fig. 6A, the k-point path in the Brillouin zone of [ZnBr2(Br-C6H4-NH2)2] is Γ—C|C2—Y2—Γ—M2—D|D2—A—Γ|L2—Γ—V2. As illustrated in Fig. 6B, the band structure confirms the indirect Eg of the compound with valence band maximum (VBM) on the Y2, M2, L2, and V2 high-symmetry points and conduction band minimum (CBM) located at the Γ point of the Brillouin zone. According to Eq. (4), effective mass plays an important role in tuning Seebeck coefficient. In addition, high valley band degeneracy is beneficial for high thermoelectric properties. According to the Eqs. (4) and (5),37 the higher the density of states (DOS) effective mass md*, the higher the Seebeck coefficient.
When multiple energy bands contribute to the charge carrier transportation:
Here, Nv is band valley degeneracy, mb* is the band-mass of a single valley.
The band valley degeneracy comes from two parts, including orbital degeneracy (similar energy at the band extrema) and valley degeneracy (multiple degenerated carrier pockets in the Brillouin zone due to the crystal symmetry).38 More generally, energy bands may be treated as effectively converged when their energy separation is tiny (compared with kBT). We have studied the band valley degeneracy of [ZnBr2(Br-C6H4-NH2)2] and its effect on thermoelectric properties. Usually, high-symmetry thermoelectric materials benefit from high band degeneracy.39 As shown in Fig. 6B, the CBM of [ZnBr2(Br-C6H4-NH2)2] is located at the Γ point with an exceptionally high band degeneracy of Nv = 24 (6×1×4, six-fold orbital degeneracy and four-fold valley degeneracy), and the VBM lies at the Y2, M2, L2, and V2 point with an exceptionally high band degeneracy of Nv = 28 (4×2×1/2 + 4×4×1/2 + 4×4×1/2 + 4×4×1/2, four-fold orbital degeneracy and four-fold valley degeneracy). Here, there are six energy bands with similar energy (an energy range about 0.1 eV upon the CBM) at the band extrema (the upper inset in Fig. 6B). Thus, it has a total of six-fold orbital degeneracy for conduction band. Similarly, there are four energy bands with similar energy (an energy range about 0.1 eV upon the VBM) at the band extrema (the lower inset in Fig. 6B). So, the orbital degeneracy for conduction band is four-fold. The widely used thermoelectric material (Bi,Sb)2Te3 shows significant valley degeneracy, with Nv = 6 in both the conduction and valence bands.40 Therefore, the band valley degeneracy of [ZnBr2(Br-C6H4-NH2)2] is exceptionally high. On the basis of the DFT calculated band structure, the effective masses for electrons and holes were calculated, and the results are illustrated in Table 1. According to the Eq. (4), the experimental values of md* were calculated via experimental data of n, S, and T. As shown in Figure S31, the experimental values of md* are comparatively large, which might be attributed to the exceptionally high Nv according to the Eqs. (4) and (5). As mentioned above, the exceptionally high band degeneracy and heavy effective mass of [ZnBr2(Br-C6H4-NH2)2] are indeed responsible for the high Seebeck coefficient (− 114 µV K‒1 at 298 K) at such doping levels (6.57 × 1020 cm‒3 at 298 K).
|
Direction |
me* (m0) |
Direction |
mh* (m0) |
|---|---|---|---|
|
Γ—C |
1.455 |
Y2—C2 |
2.890 |
|
Γ—Y2 |
1.455 |
Y2—Γ |
2.891 |
|
Γ—M2 |
1.646 |
M2—Γ |
2.366 |
|
Γ—A |
6.383 |
M2—D |
1.928 |
|
Γ—L2 |
1.445 |
L2—Γ |
2.464 |
|
Γ—V2 |
1.445 |
V2—Γ |
1.772 |
We studied the density of states for [ZnBr2(Br-C6H4-NH2)2] by DFT calculations (Fig. 6C). From the computed density of states, it can be inferred that the VBM mainly results from the hybridization between Br 4p and C 2p orbitals, whereas the CBM is dominated by C 2p, Zn 4s, and Br 4p orbitals.
Figure 3D and 3E displayed the isosurface charge density at the VBM and the CBM of the [ZnBr2(Br-C6H4-NH2)2]. The size of the isosurface suggests that an electron at the CBM resides almost entirely in the side of C atoms, which is in agreement with the DOS at CBM edge as shown in Fig. 6C. A hole at resides mainly in the side of C, N, and Br atoms, which is consistent with the DOS at the VBM as depicted in Fig. 6C.
We investigated the charge density of [ZnBr2(Br-C6H4-NH2)2] to study the origins of the high electron mobility. The electron density maps in different planes for [ZnBr2(Br-C6H4-NH2)2] are shown in Fig. 6F and 6G. These results indicate that the electrons tend to delocalization along the plane that close to the benzene ring linked with Br and N atoms and the plane that constituted by N—Zn—N atoms. In other words, the electrons are delocalized in the whole [ZnBr2(Br-C6H4-NH2)2] molecule. A delocalized pi bond is a pi (π) bond that electrons can move freely between more than two nuclei. The form of delocalized pi bond is beneficial to increase the conductivity of the material. For example, the famous TTF-TCNQ exhibits metal-like electrical conductivity,41 which is attributed to the form of delocalized pi bond. π-stacking interactions, pervasive intermolecular interactions between conjugated molecules, are especially well suited for transferring electrons from molecule to molecule. The overlap between molecules will exponentially increase as the distance of π-stacking contacts is reduced. For this material, the distance of π-stacking contacts is extremely short (3.291 Å, 3.022 Å, and 2.928 Å, Figure S10). Thus, electrons tend to delocalization between molecules, result in a high electron mobility in this material.
In conclusion, this n-type transparent metal-organic charge transfer complex [ZnBr2(Br-C6H4-NH2)2] uniquely combines giant PF (3,797 µW m‒1 K‒2), high electrical conductivity (2,936 S cm‒1), exceptional air-stability, easy synthesis, as well as simple and cost-effective laboratory procedures. Most importantly, the PF of 3,797 µW m‒1 K‒2 represents the state-of-the-art value for organic thermoelectric materials at room-temperature. Remarkably, ZT values of 0.23 at 298 K and 0.45 at 473 K were achieved, respectively. The ZT values are not only the highest performance for n-type organic thermoelectric materials, but also better than those of some typical inorganic thermoelectric materials at near room-temperature range. We further demonstrated that the extraordinarily high thermoelectric performance arises from the electron transfer induced n-type heavily doped mechanism, high valley band degeneracy and heavy effective mass. Furthermore, we proved that the [ZnBr2(Br-C6H4-NH2)2] material is extremely air-stable via experiments and AIMD simulation. More importantly, the ability to modify chemical structure through control over metal–ligand interaction on a molecular level could directly impact the properties of the complex. So, we believe that a series of new materials with higher thermoelectric performance will be discovered. Our findings open very interesting perspectives for multifunctional technologies combing transparent electronics, flexible electronics and thermoelectricity.
Film fabrication. The [ZnBr2(Br-C6H4-NH2)2] precursor solution was dropped on pre-cleaned quartz glass substrate in ambient air and spin-coated at 1,200 rpm for 15 s. Subsequently, the sample was immediately transferred to a home-developed gas pump chamber with a chamber pressure of 1,500 Pa, pumping the sample for 1 min. All films were annealed at 60°C for 20 min.
Characterizations. Air-stability test: The [ZnBr2(Br-C6H4-NH2)2] films were stored in ambient air at room temperature without humidity control. The relative humidity of the local climate is normally 30 ~ 70%.
Acknowledgments
This work was financially supported by the National Key R&D Program of China (No. 2019YFB1503200) and National Program for Support of Top-notch Young Professionals. We thank Lu Bai and Chunfu Tan at the Instrument Analysis Center of Xi'an Jiaotong University for assistance with various characterization and measurements. The authors would like to thank Hai-Jiao Xie and Dong Li from shiyanjia lab for support of thermoelectric properties tests (www.shiyanjia.com).
Author contributions
X. L. L, G. Z, and X. Z contribute equally to this work. X. L. L conceived the idea and designed the experiments. G. Z. and X. L. L performed the majority of the sample fabrication and characterization. X. Z finished first-principle DFT calculations, molecular dynamics simulations and data analysis. X. L. L performed the majority of data analysis and wrote the original draft of the manuscript. G. J. Y. and C. L. W. supervised the project. W. T. Z. and G. L. conducted part characterizations. L. Z. Z. finished the data analysis of HRTEM. X. F. Z., Y. L., M. Q. W, H. J. W., and B. C. provided advices and expertise. All authors discussed the results and commented on the manuscript.
Competing interests
The authors declare no competing interests.
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There is NO Competing Interest.