Study of Electrical Conductivity of PEDOT:PSS at

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Abstract—Temperature dependence of DC conductivity. σDC(T) of spin-coated poly (3,4 ethylenedioxythiophene): poly(4- styrenesulfonate) (PEDOT:PSS) films ...

2015 12th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE), Mexico, City. Mexico

Study of Electrical Conductivity of PEDOT:PSS at Temperatures >300 K for Hybrid Photovoltaic Applications A. Olivares1, I. Cosme1,2*, S. Mansurova1, A. Kosarev1, H. E. Martinez1 1

2

Instituto Nacional de Astrofísica, Óptica y Electrónica Catedrático en el Consejo Nacional de Ciencia y Tecnología (CONACyT) Puebla, Mexico [email protected]

Abstract—Temperature dependence of DC conductivity σDC(T) of spin-coated poly (3,4 ethylenedioxythiophene): poly(4styrenesulfonate) (PEDOT:PSS) films has been studied. σDC(T) was measured in films deposited from mixtures of PEDOT/PSS: H2O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1). Room temperature conductivity σRT of PEDOT/PSS films is enhanced from 4.46x10-5 S/cm to 6.07x10-4 S/cm after dilutions with H2O and IPA (1:1), respectively. Experimental data is fitted with two different transport models: thermal activated conduction and variable range hopping (VRH) model. It is found that σDC(T) of PEDOT/PSS:IPA (1:1) sample can be described by the one-dimensional (1D) VRH model with an pre-exponential factor σ0=28.5 S/cm and material-dependent parameter T0=39,204 K. Keywords—PEDOT/PSS; dilution; water, IPA; conductivity; temperature

I. INTRODUCTION Conducting polymers, as Poly (3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS), are very attractive organic materials for electronic and photovoltaic devices applications because of their high conductivity and their optical transparency. These materials have been used mainly in pure organic electronics devices [1] and photovoltaic devices [2]. On the other hand, organic-inorganic hybrid devices have recently attracted considerable attention [3]. Thus, due to the low cost solution processing in organic semiconductors and the well-known transport charge mobility in inorganic silicon-based materials. Particularly, it has been found that the use of spin-coated PEDOT:PSS as p-type layer and intrinsic silicon (crystalline or amorphous) is suitable for the formation of p-i junction in photovoltaic structures [4-6]. Despite the large number of applications for PEDOT:PSS, there is not a study of the electrical conductivity and charge transport for PEDOT:PSS films at high temperature (>300K). This temperature is important due to fabrication and operational temperatures of photovoltaic devices. Typically, 978-1-4673-7839-0/15/$31.00 ©2015 IEEE

the DC conductivity σDC(T) of PEDOT:PSS films has been reported at temperatures below room temperature (from 77° K to 300° K) in order to identify the main charge transport process in this material [7-10]. Nevertheless, the charge transport process in PEDOT:PSS have been difficult to interpreted in experimental data. II. EXPERIMENTAL PEDOT:PSS used have an ID#M121 from “Ossila Inc.” with a 1:6 weight ratio (PEDOT:PSS). The mixture used for the reference sample was prepared without dilution. Three different mixtures were prepared by dilution of different volume ratio as PEDOT/PSS:H2O (1:0.5), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1). Mixed solutions were filtered with a PVDF filter with pore sizes of 0.45 µm. Then, solutions were stirred for 24 hours at room temperature. PEDOT:PSS films were deposited in N2 ambient by spin coating on glass substrates with two planar titanium electrodes. The PEDOT:PSS film thicknesses were 40-45 nm. The conductivity measurements were carried out with a “Keithley 6517A” electrometer in vacuum thermostat from “Janis Inc.” at pressure P300K), experimental data is also fitted and compared to the thermal activated conduction (TAC) model described in [11] as follow:

⎡ ⎛ E a ⎞⎤ ⎟⎥ ⎣ ⎝ kT ⎠⎦

σ DC (T ) = σ 0 exp ⎢− ⎜

(2)

Where Ea is the thermal activation energy and k is the Boltzmann constant. III. RESULTS AND DISCUSSION Fig. 1 shows σDC(T) in the range from T= 300K to 450K for PEDOT/PSS:H2O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1) samples. Room temperature conductivity values are σRT=4.46x10-5 S/cm (H2O), 1.4x10-5 S/cm (without dilution), 6.07x10-5 S/cm (IPA 1:0.5) and 6.41x10-5 S/cm (IPA 1:1). In relation to the reference sample (without dilution), we observe a decrease of σDC(T) for the PEDOT/PSS:H2O sample. On the other hand, the samples prepared from IPA shows an increment of σDC(T) from 1:0.5 to 1:1 dilution.

-3

σDC (ohms-cm)-1

10

The curves in Fig. 2 show the highest correlation coefficient for the VRH model in the PEDOT/PSS:IPA 1:1 sample with a maximum at 1. In relation to this, researchers have found agreements with the experimental data and the theoretical values of β around ½, ¼ and 1 for 1D VRH [7], 3D VRH and nearest neighbor Hopping (nn-H) [8], respectively at temperature range T=77K to 300K. We believe that at higher temperatures (>300K) the charge transport process in PEDOT:PSS system is dominated by two different mechanism: hopping process and thermal activated process. As it is shown in Fig. 2 there is no a maximum value of the correlation coefficient near to the theoretical values (β= ½, ¼ or 1) for the samples PEDOT/PSS: H2O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: IPA (1:0.5) at the temperature range T=300K to 480K. Table 1 shows the correlation coefficient and the relative errors of the σ0, T0 and Ea for the TAC, 1D VRH and 3D VRH fitting for all samples. The highest correlation coefficient (β=0.998) is found for the PEDOT/PSS:IPA (1:1) sample from TAC and 1D-VRH models.

-4

10

PEDOT/PSS:H2O (1:0.5) PEDOT/PSS PEDOT/PSS:IPA (1:0.5) PEDOT/PSS:IPA (1:1)

-5

10

Fig. 2. Correlation coefficient of experimental data fit according to VRH model versus β exponent for PEDOT/PSS: H2O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS:IPA (1:0.5) and PEDOT/PSS:IPA (1:1) over the temperature range 300K.

300

320

340 360 380 400 TEMPERATURE (K)

420

440

460

Fig. 1. DC conductivity, σDC(T), for the spin-coated films: PEDOT/PSS: H2O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1) from 300 K to 440K.

Despite the differences in the interpretation of the experimental data about the charge transport in the PEDOT:PSS films, it is believed that the dominating charge transport in this system is realized via hopping mechanisms [46]. The fitting correlation coefficients for VRH and TAC models are analyzed. In Fig. 2 the correlation coefficients of the VRH model fit according to (1) are shown.

TABLE I.

CORRELATION COEFFICIENT AND RELATIVE ERROR FOR ADJUSTED PARAMETERS IN THE RANGE T=300K TO 430K

Model Sample/Parameter PEDOT/PSS:IPA (1:1) PEDOT/PSS:IPA (1:0.5) PEDOT:PSS PEDOT/PSS: H2O (1:0.5) Model Sample/Parameter PEDOT/PSS:IPA (1:1) PEDOT/PSS:IPA (1:0.5) PEDOT:PSS PEDOT/PSS: H2O (1:0.5) Model Sample/Parameter PEDOT/PSS:IPA (1:1) PEDOT/PSS:IPA (1:0.5) PEDOT:PSS PEDOT/PSS: H2O (1:0.5)

Thermal Activated Conduction CCR σ0_RE Ea_RE 0.998 1.6% 0.6% 0.997 1.4% 0.6% 0.985 0.57% 2.2% 0.968 0.83% 3.2% 1D-VRH β=1/2 T0_RE CCR σ0_RE 0.998 2.2% 0.7% 0.994 7% 1% 0.990 1.4% 1.7% 0.976 1.8% 2.8% 3D-VRH β=1/4 T0_RE CCR σ0_RE 0.997 1.3% 0.8% 0.992 2.6% 1.5% 0.993 12.3% 1.5% 0.980 10.4% 2%

Correlation coefficient (a.u)

1.000 0.998

The Fig. 3(a)-(b) shows the fit for the experimental σDC(T) data to the thermal activated conduction and 1D-VRH models versus 1/kT and T-β, respectively. The slope in Fig. 3(a) can be interpreted as the activation energy Ea from (2). On the contrary, the slope in the Figure 3(b) can be interpreted as T0 for an exponent β=1/2 in the 1D-VRH model from (1).

0.996 0.994 0.992 0.990 0.988 PEDOT/PSS:H2O (1:0.5)

0.986

PEDOT/PSS PEDOT/PSS:IPA (1:0.5) PEDOT/PSS:IPA (1:1)

0.984 0.0

0.2

0.4

0.6

0.8

β Exponent (a.u)

1.0

1.2

1.4

For PEDOT/PSS:IPA sample (1:1) fitted by the VRH model, characteristic values are T0=3.9x104 K and σ0=28.5 S/cm. This values are close to the theoretical values with a physical meaning. Similar values, T0=3.2x106 K and σ0=24.6 S/cm are reported in [8] for PEDOT:PSS films. Despite the

large range of σ0 values reported in literature from σ0= 10-6 to 102 for organic materials, there is not a consistent physical sense of these values [6-10]. The correlation coefficient results and the wide range of σ0 values show the difficulty of fitting the experimental data to a single model. For example, with similar correlation coefficient in the PEDOT/PSS:IPA (1:1) sample for the fit in both, 1D-VRH and TAC models, and values of Ea=165 eV and T0=3.9x102 K it is not possible to discriminate one of the two models without a consistent value of σ0. Finally, the discrepancies between the experimental data and the fitted σDC(T) in relation to the correlation coefficient for PEDOT/PSS: H2O (1:0.5), PEDOT:PSS (without dilution) or PEDOT/PSS:IPA (1:0.5) samples show that there is no single charge transport process taking place in PEDOT:PSS material at higher temperatures (T>300). Thus, additional studies are required to clarify the dominating process.

correlation coefficient analysis, , the experimental data was adjusted to the thermal activated conduction and 1D-VRH models without a maximum value near to the theoretical values. Thus, it is not possible to discriminate between a single 1D-VRH model and TAC model. We suggested the contribution of both transport process at temperatures < 300 K. However, it was found that the σDC(T) for PEDOT/PSS:IPA (1:1) sample is more consistent with literature and this is described by the 1D VRH model with σ0=28.5 S/cm and T0=39,204 K values even at higher temperature (>300K). ACKNOWLEDGMENT The authors acknowledge Adrian Itzmoyotl and Victor Aca from microelectronics laboratory for the assistance in fabrication process. Hiram Enrique Martinez and Antonio Olivares acknowledge support from CONACyT through the PhD scholarships with numbers 362152 and 363344, respectively.

ln (σDC), ohms-1-cm-1

-5.5 -6.0

PEDOT/PSS:IPA 1:1 Slope =164 εr=0.6%

-6.5

σ0=0.165 S/cm εr=1.6%

-7.0 -7.5

REFERENCES [1]

PEDOT/PSS:IPA 1:0.5 Slope 144 εr=0.6% -2

-8.0

σ0=5.9x10 S/cm εr=1.6%

-8.5

NO DILUTION

-9.0

Slope 62 εr=2.2% -4

σ0=9.96x10 S/cm εr=0.57%

-9.5

-10.0

WATER 1:1 Slope 61 εr=3.2%

-10.5

σ0=4.22x10 S/cm εr=0.83%

-4

26

28

30

32 1/kT

34

36

38

a) PEDOT/PSS:IPA 1:1 4 Slope 3.9x10 εr=0.7%

-6.0

ln (σDC), ohms-1-cm-1

-6.5

σ0=28.5 + 4 S/cm εr=2.2%

-7.0 -7.5 -8.0

PEDOT/PSS:IPA 1:0.5 4 Slope 3.0 x10 εr=1% σ0=5.4 + 1 S/cm εr=7% 3

-8.5

NO DILUTION

-9.0 -9.5

-10.0

Slope 5.0 x10 εr=1.7% -3

σ0=7x10 S/cm εr=1.4%

WATER 1:1 3 Slope 5.6 x10 εr=2.8% -3

σ0= 3x10 S/cm εr=1.8%

0.046

0.048

0.050

0.052 0.054 T^-1/2 (K)

0.056

0.058

b) Fig. 3. Experimental In |σDC(T)| versus a)1/kT and b) T-1/2. Straight lines are fits to the thermal activated conduction model (1) and 1D VRH model (2) respectively. Characteristic values are showed pre-exponential factor σ0 and activation energy (Ea) or material-dependent parameter.

IV. CONCLUSION We have studied the temperature dependence of σDC in H2O/IPA diluted PEDOT:PSS films. Room temperature conductivity increases from the lowest value of σRT=4.46x10-5 S/cm for PEDOT/PSS:H2O to the highest value of σRT=6.41x10-5 S/cm for PEDOT/PSS:IPA (1:1). From the

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