Polarization effect of MoO3 increases the thermoelectric properties

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Polarization effect of MoO3 increases the thermoelectric properties base on the PbS quantum-dots doped P3HT devices Lin Sun, Guo Xie, Ping Wu, Yan Xiong, and Ling Xu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00078 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Polymer Materials

Polarization Effect of MoO3 Increases the Thermoelectric Properties Base on the PbS Quantum-dots Doped P3HT Devices

Lin Sun 1,ϯ, Guo Xie1,ϯ, Ping Wu1, Yan Xiong1, Ling Xu 1,*

1

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,

Wu Han 430074, China ϯ

Authors Lin Sun and Guo Xie contributed equally to this article

*E-mail address: [email protected]. 1

ACS Paragon Plus Environment

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In this article, we fabricated PbS quantum-dots doped P3HT thin films, and their thermoelectric properties were enhanced by introducing the MoO3 interface layer into the ITO/PbS–doped P3HT/Al device. We found that the electrical conductivity of P3HT thin film increased after only doping PbS quantum-dots, while its Seebeck coefficient decreased. After introducing the MoO3 interface layer into the device forming the ITO/P3HT/MoO3/Al structure, its electrical conductivity and the Seebeck coefficient are increased simultaneously. Since the dielectric constant of MoO3 increases with rising temperature, we considered that the MoO3 interface layer brings a polarization effect with temperature changes. A polarization difference occurs between the MoO3 layer and the electron-phonon coupling in the P3HT material. The polarization difference and the entropy difference together drive the carriers to transport from the high temperature to the low temperature. The capacitance-frequency (C-F) characteristic results further confirmed that the polarization effect of MoO3 increases as temperature rises up. The polarization effect promotes the enhancement of thermoelectric properties in the ITO/P3HT/MoO3/Al device, leading to a big Power Factor (PF) of 0.203 μW/mK2 at 90 oC. We presuppose that the introduction of a similar metal oxide interface layer may be an effective enhancement for the thermoelectric properties of materials and devices.

Key words: PbS quantum-dots, P3HT thermoelectric properties, MoO3 interface layer, Surface polarization, Electron-phonon coupling

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1. INTRODUCTION Low-dimensional materials have a different electronic density of states compared with the three-dimensional bulk materials. When a low-dimensional material doped into the threedimensional material, since the wavelength of phonons is dozens of nanometers, while the wavelength of electrons is only a few nanometers, the inserted nanoparticle can only effectively scatter phonons, and cannot scatter electrons. The lattice thermal conductivity will reduce, meanwhile, the electrical conductivity still maintains the same value. 1 In the field of organic semiconductor thermoelectric materials, composite thermoelectric materials composed of organic materials and low-dimensional inorganic nanomaterials have become one of the research hotspots. 2-5 The P3HT (Poly(3-hexylthiophene-2,5-diyl)) possesses many excellent properties as an organic semiconductor material, such as high mobility, good solubility, small bang gap (HOMO and LUMO are -4.76 eV，-2.74 eV, respectively ), easy preparation, and flexible et al. 6-8 Furthermore, the P3HT has better thermal stability as a hole transport material, and it is also widely studied as an organic thermoelectric semiconductor due to the excellent solution processing capability, chemical stability, and high field-effect mobility.9-13 The electrical conductivity of P3HT can be controlled in the range of 10 −8 ~ 105 S•m-1 by doping with suitable dopants (e.g., FeCl 3, I2, HClO4, etc.).14, 15 For example, the FeCl3 doped materials have a small electrical conductivity, in the 0.1~0.0001 S•cm-1 range due to partial de-esterification of the isolated polymer. 13 Lim and co-workers enhanced the electrical conductivity of P3HT by doping F4TCNQ (2,3,5,6-Tetrafluoro-7,7,8,8tetracyanoquinodimethane) with vapor-phase infiltration method. 16

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At present, researches on P3HT thermoelectric properties are mainly focused on the improvement by doping inorganic low-dimensional materials, 17-20 and many literatures have reported their results. For example, Hong and co-workers obtained high-performance and flexible thermoelectric CNT (carbon nanotube)/P3HT nanocomposite films by doping CNT into P3HT.21 The PbS is a traditional thermoelectric material with good thermoelectric properties,22 but the thermoelectric properties of P3HT with PbS quantum-dots have not been reported. The introduction of an interface layer between the organic/inorganic interfaces to filtrate low energy charge carriers can also enhance the P3HT thermoelectric properties. For example, He and co-workers found that the P3HT/Bi2Te3 interface can effectively enhance the P3HT thermoelectric properties via the energy-filtering effect.23 In addition, the photoexcitation and polarization methods can also improve the thermoelectric properties of P3HT thin films, but the reason why the Seebeck effect enhances as temperature increases is unclear.24-26 Recently, the method which increases the optoelectronic properties of devices by utilizing the interface layer has been widely used in solar cells, organic light emitting diodes and other fields. Therefore, the metallic oxide has attracted much attention because it can be an important interface layer material. Among many metallic oxide materials, MoO 3 materials possess excellent properties, such as good stability. Besides, the MoO3 work function (-5.4 eV) matches the HOMO energy level of P3HT, which facilities charge transportation. Especially, the dielectric constant of MoO 3 increases with increasing temperature 27, so the MoO3 can produce a strong polarization effect when the temperature rises up. 4


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In this article, we designed a vertical structure organic/inorganic hybrid P3HT thin film device. We adopted the PbS quantum-dots doping and a MoO 3 interface layer introduction method to enhance the thermoelectric properties of thin film devices and analyzed how they affect devices’ Seebeck coefficient and electrical conductivity.

2. EXPERIMENTAL SECTION 2.1 PbS quantum-dots preparation The solution method was adopted to prepare the PbS quantum-dots. Firstly, we mixed the 10 mL oleylamine and 3 mol/L PbCl 2 1 mL. The mixed solution stored in the vacuum condition with 80 °C, then kept 30 mins with 140 °C heating in the nitrogen condition. Afterward, the suspension liquid was cooled to 30 °C for the standby application. Secondly, 210 uL hexamethyldisilathiane (TMS2S) dissolved into 2 mL oleylamine, then the mixed solution was put into a flask, kept stirring and heated in a water bath to a high temperature of quantum-dots growth. Thirdly, when quantum-dots reached the appropriate size, the mixed solution was stopped the water bath and added the normal hexane and ethyl alcohol. Afterward, the original solution was centrifuged in the normal hexane to separate PbS quantum-dots, and the above method repeated three times. Then 6 mL oleic acid was added into the purified PbS solution, stewed 24 hours to make the unreacted PbCl 2 sediment, and filtered the remnant solution with the 200 nm aperture filter. Finally, PbS quantum-dots washed by using the normal hexane and ethyl alcohol, three times. After that, the sample dissolved in the normal hexane to prepare the required concentration. 2.2 Device preparation Firstly, 20 mg P3HT and 1 mL chloroform were placed into a 10 mL bottle with

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continuous stirring, and the specific mass ratio PbS quantum-dots solution also added. Then, the thin film obtained by spin coating the P3HT and PbS mixture solution on an ITO glass substrate. The coating speed is 1500 r/min with 50 seconds. The uniform amaranth thin film was prepared, then it annealed at 120 oC for 40 mins. The thickness of the P3HT layer we obtained was 350 nm. After that, both MoO3 dielectric layer and aluminum (Al) electrode were directly deposited onto the P3HT thin film by the evaporation method with the vacuum degree 4×10-4 Pa, and the evaporation speed was 0.5 Å/s. The thickness of the dielectric layer and the Al electrode were 10~30 nm and 100 nm, respectively, forming an ITO/PbS-doped P3HT/MoO3/Al device. 2.3 Characterization of materials and devices In our experiments, we measured the absorption spectrum characteristic of intrinsic and doped P3HT thin films by using an ultraviolet-visible spectrophotometer (UV3600, Shimadzu), and the band gap calculated by using the Tauc equation 28. The X-ray diffraction (XRD) patterns of pristine and doped P3HT films were measured by the X-ray diffractometer (D8 advance, Bruker). The surface topography and surface potential of intrinsic and doped P3HT thin film were characterized by the atomic force microscope (Dimension 3100, Veeco). The capacitance-voltage (C-V) and the capacitance-frequency (C-F) characteristic curves were measured by the impedance analyzer (4294A, Agilent), and the alternating current (AC) impedance spectrum of devices was also measured and used to calculate resistance and electrical conductivity. 2.4 Measuring the Seebeck coefficient The Seebeck coefficient testing device for thermoelectric samples mainly includes the 6


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following parts: heater, voltage testing module and temperature testing module, as illustrated in Figure 1. The sample was put into the test fixture on the top of the metal cermet heater (HT24S, ThorLabs), and the heater temperature was controlled by the voltage of directcurrent (DC) power. Voltage testing module is a voltmeter (PXIe-1073, National Instruments) with the sensitivity of nano-voltage which connects to the computer, and its two leads connect to the top and bottom of the sample for measuring the thermovoltage ΔV. Temperature testing module consists of a pair of K-type thermocouples and a data acquisition module which connects to the computer for measuring the temperature difference ΔT. All the voltage and temperature data saved by the computer. In the experiment, the bottom side of the device is the high-temperature side with temperature Th, and the top side is the low-temperature side with temperature Tc. So the temperature difference ΔT is the difference value Th - Tc, and the sample temperature T 0 is the average value of Th and Tc. Therefore, the Seebeck coefficient S is obtained by S = ΔV/ΔT. To reduce the experimental error, the average value of three groups ΔV/ΔT obtained as the average Seebeck coefficient value at the same sample temperature T 0. 3. RESULTS AND DISCUSSION 3.1 Characterization of pristine and PbS quantum-dots doped P3HT films Previous literature reports 1 indicate that nanostructures can effectively decrease the lattice thermal conductivity by scattering phonons at the interface, and it cannot scatter the electrons. Therefore, the low-dimensional PbS quantum-dots doped P3HT film may have reduced lattice thermal conductivity and constant electrical properties, thereby boosting the

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P3HT thermoelectric properties. We prepared the pristine and doped P3HT thin films and characterized their properties. Figure 2(a) shows the absorption spectrum curves of the pristine and PbS quantum-dots doped thin films. The absorption peak appears gradually red shift as the doping concentration of PbS quantum-dots increases. When the absorption wavelength reaches 600 nm, a small peak associated with the light absorption of PbS quantum-dots appears. According to the Tauc plot 22 (Details described in Supporting Information S1), we obtained the energy band gap Eg of the pristine P3HT thin film, and the calculated E g value of 2.07 eV is close to the literature report.29 Similarly, Eg values of thin films with different PbS: P3HT mass ratio of 1: 8, 1: 4, and 1:2 were calculated, which are 2.05 eV, 2.04 eV, and 2.03 eV, respectively. It shows that doping PbS quantum-dots can reduce the band gap of P3HT thin film, and the higher doping concentration obtains the smaller bandgap. Figure 2(b) presents the P3HT films’ XRD patterns. The main diffraction peaks of (100), (200), and (300) are observed. This result indicates that P3HT is an ordered, self-organized lamellae structure with an interlayer spacing. The Scherrer equation (D =0.9λ/βcosθ) with the full width at half maximum (FWHM) of the (100) diffraction peaks reveals that the crystal size of doped P3HT is 308 Ȧ, and that of the pristine is 221 Ȧ. It indicates that the pristine P3HT has lower crystallinity than the PbS doped P3HT. Figure.3 shows the surface topography and surface potential of the pure and doped P3HT thin films. The pure P3HT thin film possesses lower roughness, and its surface potential is between 1.1 eV~1.2 eV. However, the incorporation of PbS quantum-dots increases the roughness and the surface potential. In doped thin films with PbS:P3HT=1:8 8


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and 1:4, their surface potentials are between 1.7 eV~2.1 eV and 2.6 eV~3.4 eV, respectively, showing the typical characteristics of the p-type organic semiconductor material. Compared with the pristine P3HT thin film, the doped film has a higher surface potential and surface charge energy. 3.2 Thermoelectric properties of PbS quantum-dots doped P3HT The doped film devices with PbS: P3HT mass ratios of 1:8 and 1:4 fabricated by the spin-coating method. Their Seebeck coefficient and the electrical conductivity were measured, and PF values were calculated. All the results are shown in Figure 4. Figure 4(a) shows the Seebeck coefficient curves of pristine and doped P3HT devices under different temperatures. In the process from room temperature to 100 oC, the Seebeck coefficient of the pristine device ITO/P3HT/Al changes from 28 μV/K to the 93 μV/K, meanwhile Seebeck coefficients of doped devices ITO/PbS: P3HT(1:8)/Al and ITO/PbS: P3HT(1:4)/Al change from 27 μV/K to 86 μV/K and from 25 μV/K to the 78 μV/K, respectively. From Figure 4(a), we found that the Seebeck coefficient increased with the increasing temperature due to the entropy difference effect. However, when the doping concentration of PbS quantum-dots increases, the Seebeck coefficient is decreasing. The reason is that doping PbS quantum-dots caused the carrier concentration change. The more PbS quantum-dots are doped, the higher carrier concentration obtained, resulting in a decrease in the Seebeck coefficient. The P3HT electrical conductivity of the pristine and doped devices was measured by the two electrode method, and their values indicate that the electrical conductivity increased after doping PbS quantum-dots as shown in Figure 4(b). The electrical conductivity of the pristine P3HT device is 0.012 S/cm at room temperature, while the PbS: P3HT=1:8 doped 9



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device is 0.015 S/cm, and the PbS: P3HT=1:4 doped is 0.036 S/cm. The higher electrical conductivity obtained by increasing the doping concentration within limits, but the electrical conduction decreases with increasing temperature because of the scattering effect of electrons and phonons. In general, the increase of carrier concentration and electrical conductivity results in a decrease in the Seebeck coefficient, so doped P3HT devices have a smaller Seebeck coefficient than the pristine P3HT device. The Seebeck effect is generated by the entropy difference caused by the temperature difference that drives the carriers transported from the high-temperature to the low-temperature. Therefore, the Seebeck coefficient can be expressed by the equation: 30 𝑺=

𝒌𝑩 𝒆

𝑵

(𝐥𝐧 ( 𝒄 ) + 𝑨) 𝒏

(1)

Where 𝒌𝑩 is the Boltzmann constant, 𝑵𝒄 is the effective density of states, 𝒆 is electron charge, n is the charge carrier concentration, and A is the charge carriers’ kinetic energy normalized to kBT. From the equation (1), we know that the carrier concentration and the Seebeck coefficient are related to the density of states near the Fermi energy level, and there exists a coupling relationship between the electrical conductivity and the Seebeck coefficient. Therefore, the electrical conductivity increases while the Seebeck coefficient decreases in the doped ITO/P3HT/Al device. The PF values were calculated from the electrical conductivity and Seebeck coefficient, and results are shown in Figure 4(c). The 1:4 doped device has the best PF value of 0.01 μW/mK2 at the 90 °C. Besides, the doped P3HT devices have a higher PF value than the pristine P3HT because the Seebeck coefficient curves of doped devices change more rapidly 10


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than the electrical conduction. In Supporting Information S3, we also measured and calculated the electrical conductivity, carrier concentration and mobility of PbS quantumdots doped devices by using the C-V measurement at room temperature, and results are shown in Figure.S3 (Supporting Information). Their electrical conductivity and carrier concentration increase, but the mobility decreases. Results are consistent with the decrease of Seebeck coefficient and the increase of electrical conductivity. 3.3 Further improve thermoelectric properties by introducing MoO3 interface layer Doping PbS quantum-dots boost the thermoelectric properties of P3HT by increasing the PF value. However, the Seebeck coefficient is reduced because of an increase in carrier concentration. To further enhance the thermoelectric properties of the doped P3HT devices, we introduced a MoO 3 interface layer into devices, measured their Seebeck coefficient and electrical conductivity, and calculated PF values, seeing Figure 5. In Figure 5(a), the Seebeck coefficient of the pristine P3HT device which has the structure ITO/P3HT/MoO 3/Al changes from 38 μV/K to 129 μV/K at the temperature range of room temperature to 100 oC. After doping PbS quantum-dots into the P3HT, the Seebeck coefficient of the doped device with ITO/PbS: P3HT(1:8)/MoO 3/Al structure changes from 37 μV/K to 115 μV/K, and that of the device with ITO/PbS: P3HT(1:4)/MoO 3/Al structure changes from 35 μV/K to 105 μV/K. Their Seebeck coefficients increase with rising temperature, and the pristine P3HT with MoO 3 layer still has the bigger Seebeck coefficient. As shown in Figure 5(b), the pristine device with the MoO 3 layer has the measured electrical conductivity of 0.014 S/cm, meanwhile, the electrical conductivity of 1:8 and 1:4 doping ratio devices with the MoO 3 layer are 0.038 S/cm and 0.11 S/cm, respectively. 11



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Compared with devices without the MoO3 interface layer, devices with MoO3 interface layer have the larger electrical conductivity and Seebeck coefficient. It indicates that introducing MoO3 interface layer can synchronously increase the Seebeck coefficient and electrical conductivity for P3HT thin film devices. Figure 5(c) is the calculated PF values for P3HT thin film devices with the MoO 3 interface layer. At 90 °C, the PF value of pristine P3HT device with ITO/P3HT/MoO 3/Al is 0.017 μW/mK2, and that of the doped device ITO/PbS: P3HT(1:8)/MoO 3/Al is 0.072 μW/mK2. Especially, the PF value of the doped device ITO/PbS: P3HT(1:4)/MoO 3/Al can reach up to 0.203 μW/mK 2 at 90 oC. The inserted histogram shows the maximum PF values of devices with and without MoO 3, it indicates that the MoO 3 interface layer can considerably improve the thermoelectric properties of the P3HT thin film. Showing in Figure.S4

(Supporting

Information

S4),

the

carrier

concentration

of

doped

ITO/P3HT/MoO3/Al devices increases as the doping concentration of PbS quantum-dots increases at room temperature, and their electrical conductivity increases too. As shown in Figure 5, as the temperature increases, adding a MoO3 interface layer in the device to form an ITO/P3HT/MoO 3/Al structure increases the electrical conductivity and Seebeck coefficient because of a reduction of the charge accumulation at the interface. Besides, this can be attributed to the result that the more carriers driven by the polarization effect which caused by the MoO3 interface layer. In our experiment, the dielectric constant of MoO3 increases with rising temperature as shown results in Figure.S5 (Supporting Information S5). Therefore, the temperature is crucial in improving the polarization

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characteristic of MoO3 materials, and the polarization effect of MoO 3 at the high temperature is stronger than that at the low temperature. In the ITO/P3HT/MoO 3/Al thin film devices, the coupling effect of electrons and phonons in P3HT is enhanced with the temperature change in the vertical direction, resulting in a surface polarization. There will produce a polarization difference in the temperature difference direction of the device.26 At a constant temperature difference, the polarization difference can enhance the built-in electrical filed of the device. The polarization difference, a new force, and entropy difference both drive carriers transported from high-temperature to low-temperature, enhancing the potential of the device, so the Seebeck effect enhanced. 3.4 Capacitance-frequency characterization further confirm the thermoelectric enhancement To further analyze the reason for the increase in Seebeck coefficient and electrical conductivity of the device, and the interaction between temperature and polarization effects, the capacitance-frequency (C-F) characteristic was gauged. Figure 6 shows the C-F characteristic curve of ITO/P3HT/Al, ITO/PbS-doped P3HT/Al, ITO/P3HT/MoO 3/Al and ITO/PbS-doped P3HT/MoO3/Al devices at the temperature of room temperature, 40 oC, 60 oC,

and 80 oC, respectively. Usually, the C-F curve is divided into a low-frequency area and a high-frequency area

according to the frequency range. 31 Since the interface charges of the material respond to the low-frequency AC signal, the capacitance signal in the low-frequency area reflects the interfacial information of the device, and the capacitance signal in the high-frequency area reflects the material internal information. 32

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In Figure 6, the capacitance of pristine and doped devices ITO/P3HT/Al decreases as the temperature increases in the low-frequency area, which means that the charge accumulation decreases with the temperature addition at the interface. When the MoO 3 interface layer is introduced into devices, their capacitance increase (Figure.6(c) and 6(d)), indicating an increase in charge accumulation at the interface, resulting in an augment in the interfacial density of states. So the interfacial density of states increases with the rising temperature in devices ITO/P3HT/MoO3/Al, and it controls the coupling intensity of electrons and phonons. The coupling intensity of electrons and phonons can be described by the following equation: 33 𝝀 = 𝑵(𝑬𝑭 )𝑩𝟐 /𝑴𝝎𝟐

(2)

where 𝑬𝑭 is the density of states which close to the Fermi Level, 𝑴 the atomic mass, the 𝝎 average phonon frequency, and 𝑩 is the matrix constant associated with the material structure. From the equation (2), we known that the coupling intensity of electrons and phonons will enhance as the interfacial density of states increases with rising temperature in the P3HT/MoO3 thin film device, so the surface polarization effect enhance. In addition, the dielectric constant change of MoO 3 also introduce the surface polarization effect at the same time, and it increases with the rising temperature. The capacitance characteristics of P3HT thin film devices with MoO3 layer indicates that the MoO3 interface layer can improve the temperature dependent polarization properties. Therefore, the MoO 3 interface layer can simultaneously increase the Seebeck coefficient and the electrical conductivity, enhancing the thermoelectric properties of the device. So the PF value of the device with doping and MoO 3 layer can reach up to 0.203 μW/mK 2 at 90 oC. 14


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4. CONCLUSIONS In this article, the thermoelectric properties of the P-type conducting polymer P3HT can be improved by doping PbS quantum-dots and introducing the MoO 3 interface layer. The following conclusions obtained: (1) PbS quantum-dots doping can efficiently increase the electrical conductivity but result in a decrease in the Seebeck coefficient. However, introducing the MoO 3 interface layer synchronously increase the Seebeck coefficient and electrical conductivity in P3HT devices. The P3HT device with 1:4 doping and with MoO 3 interface layer has a maximum PF value of 0.203 μW/mK2 at 90 oC. (2) The reason for the enhancement is due to the MoO 3 strong polarization effect. In the P3HT device, the MoO 3 layer will generate the temperature-dependent polarization effect, and it leads to an increase in built-in electric field and charge transport, so enhance the Seebeck effect. (3) Utilizing the capacitance-frequency

(C-F)

characteristic

properties,

the

temperature-dependent

polarization effect in devices of ITO/P3HT/Al and ITO/ P3HT/MoO 3/Al can be further proved. The capacitance of ITO/P3HT/MoO 3/Al device increased in the low-frequency area as temperature increases. This result indicates that the MoO 3 dielectric constant increases with the rising temperature, so it can enhance the polarization effect and promote the electron transport performance. We presupposed that introducing the metal oxide interface layer into devices is an effective method for enhancing their thermoelectric properties. It can greatly develop and promote the advancement of thermoelectric materials and devices.

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◼

ASSOCIATED CONTENT

*S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *** The Carrier concentration of intrinsic and doped device of ITO/P3HT/MoO3/Al, and dielectric constant measurement of MoO3 at different temperature and frequency. (PDF) ◼

AUTHOR INFORMATION

Corresponding Authors *Tel: +86(0) 13469960282, E-mail: [email protected]. ORCID: Ling Xu: 0000-0001-9676-5769 Notes The authors declare no competing financial interest

◼

ACKNOWLEDGMENTS

We acknowledge financial support from the Fundamental Research Funds for the Central Universities in Huazhong University of Science and Technology (Grant 2016YXMS033) and the Double first-class research funding with independent intellectual property of ICARE (Grant 3011187028).

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Figure 1. The schematic diagram of the Seebeck coefficient measurement.

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Abs.(a.u.)

1.0 (a)

P3HT 1:8 1:4 1:2

0.8 0.6 0.4 0.2 0.0 400

(b)

500 600 700 800 Wavelength (nm)

900

(100)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PbS:P3HT=1:4 Pure P3HT (200)

5

10

(300)

15

20

2 Theta (deg.)

25

Figure 2. (a) The absorption spectrogram of pristine and doped P3HT thin film. (b) XRD profiles of pristine P3HT and 1:4 doped-P3HT thin film.

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Figure 3. Surface topography (a,c,e) and surface potential (b,d,f) of pristine and doped P3HT thin film. Among them, (a) and (b) are the pristine P3HT, (c) and (d) are the doped P3HT with doping ration PbS: P3HT = 1: 8, (e) and (f) are the doped P3HT with doping ratio PbS: P3HT = 1: 4.

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PbS-doped P3HT(1:4)/Al PbS-doped P3HT(1:8)/Al P3HT/Al

S (V/K)

(a) 100 80 60 40 20

20

40

(b) 0.04

60 80 T (C)

100

 (S/cm)


0.03 0.02 0.01 30

12

(c)

10

PF (x10-3 W/mK2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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45

60 75 T(C)

90


8 6 4 2 0

30

45

60 T(C)

75

90

Figure 4. (a) Seebeck coefficient, (b) electrical conductivity, and (c) PF value of pristine and doped P3HT thin film device at the different temperature.

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S(V/K)

150 (a) 120

PbS-doped P3HT(1:4)/MoO3 PbS-doped P3HT(1:8)/MoO3 P3HT/MoO3 MoO3 (24nm)

90 60 30 20

40

0.3 (b) 0.2

60 80 T (C)

100

 (S/cm)

PbS-doped P3HT(1:4)/MoO3 PbS-doped P3HT(1:8)/MoO3 P3HT/MoO3

0.1 0.0 15

0.2

0.21 Maximum PF value

0.3 (c) PF (W/mK2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

45 60 T (C)

Undoped w/o MoO3 Undoped/MoO3 Doped w/o MoO3 Doped/MoO3

75

90

PbS:P3HT(1:4)/MoO3 PbS:P3HT(1:8)/MoO3 P3HT/MoO3

0.20 0.02 0.01

0.1

0.00

0.0 30

45

60 75 T (C)

90

Figure 5. (a) Seebeck coefficient, (b) electrical conductivity, and (c) PF values of pristine and doped P3HT thin film device with MoO3 layer. The thickness of MoO3 layer was 24 nm. (The inserted picture shows the maximum PF value of devices with and without MoO3 layer.)

21



ITO/P3HT/Al

RT 40 C 60 C 80 C

6.0n

8.0n (b) 6.0n

ITO/doped P3HT/Al RT 40 C 60 C 80 C

4.0n

4.0n

2.0n

2.0n 0.0 100

1k

10k 100k Frequency (Hz)

ITO/P3HT/MoO3/Al

20.0n (c) 16.0n 12.0n

1M

RT 40 C 60 C 80 C

8.0n

0.0 100

1k 10k 100k Frequency (Hz)

1M

30.0n ITO/doped P3HT/MoO /Al 3 (d) RT 24.0n

Capacitance (nF)

Capacitance (F)

(a)

Capacitance (F)

8.0n

Capacitance (nF)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40C 60C 80C

18.0n 12.0n

4.0n 0.0 100


1M

6.0n 100


1M

Figure 6. Capacitance-frequency relation of P3HT thin film devices at the different temperature, (a) ITO/P3HT/Al device, (b) ITO/PbS-doped P3HT/MoO3/Al device, (c) ITO/P3HT/MoO3/MoO3/Al device, and (d) ITO/PbS-doped P3HT/MoO3/Al device.

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◼

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Table of Contents Graphic

Enhance the surface polarization


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Polarization effect of MoO3 increases the thermoelectric properties

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