PEDOT:PSS Thin Film as Transparent Electrode in

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Abstract. Two types of polar solvent materials namely glycerol and ethylene glycol (EG) were used in this study as dopants for PEDOT:PSS. The 2 to 10% of ...

Advanced Materials Research Vol. 501 (2012) pp 252-256 Online available since 2012/Apr/12 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.501.252

PEDOT:PSS Thin Film as Transparent Electrode in ITO-Free Organic Solar Cell Zurianti A. Rahman1,2,a,*,Khaulah Sulaiman1,b, Ahmad Shuhaimi1,c and Mohamad Rusop3,d 1

Low Dimensional Materials Research Centre, Department of Physics, Faculty Of Science, University of Malaya, 50603, Kuala Lumpur, Malaysia 2

Faculty of Applied Science, University of Technology MARA 40450, Shah Alam, Selangor, Malaysia.

3

NANO-SciTech Centre, Institute of Science, University Of Technology MARA 40450, Shah Alam, Selangor. Malaysia a,*

[email protected], [email protected] [email protected], [email protected]

c

Keywords: Free ITO-organic solar cell; alternative anode material; modified PEDOT:PSS; polar solvent.

Abstract. Two types of polar solvent materials namely glycerol and ethylene glycol (EG) were used in this study as dopants for PEDOT:PSS. The 2 to 10% of doped PEDOT:PSS were synthesized using sol-gel technique and were spin-coated onto glass substrate. The optical, conductivity and morphological characteristics of the doped PEDOT:PSS thin films were measured via UV-VIS spectrometer, two-point probes technique and AFM measurement, respectively. All films showed direct band gap behavior and compared to the pristine thin film, the doped PEDOT:PSS show higher transparency in visible range. Furthermore, the conductivity of glycerol and EG doped PEDOT:PSS thin films were also improved due to the changes in molecule alignment and interchain interaction in the thin films. Introduction Indium tin oxide (ITO) is the common transparent conductive oxide (TCO) material used in optoelectronic device, however since the issues upstretched in the limited supply of indium metal and chemically unstable reaction which results a growth in the cost of manufacturing process have urged the studies to seek replacement for ITO as anode layer in PSC [1]. There are effective studies to replace ITO with poly(ethylenedioxythiopene): poly(styrenesulfonate) (PEDOT:PSS) due to several reasons. PEDOT:PSS has good conductivity, transparency and has high work function, about 5.1 eV and additionally, it possess electron blocking characteristic that prevent recombination of free charges in PSC. The high value of work function is responsible in reducing the barrier height between anode and polymer interface which results in better holes and electron separations [2]. As a replacement to ITO, conductivity of PEDOT:PSS could be improved even more to promise efficient function as anode layer in PSC. Application of metallic Ag grid to PEDOT:PSS and thermal deposition of Au lines to PEDOT:PSS by thermal evaporation and soft lithography are alternatives to increase the conductivity of PEDOT:PSS [3, 4]. However these two methods involve a vacuum technique and high cost processing. Hence, expensive cost process has channeled the research to focus on a simple and cheaper process such as sol-gel method. This procedure involve the modification of PEDOT:PSS with the addition of high boiling point polar solvent material such as glycerol, dimethyl sulfoxide (DMSO), ethylene glycol (EG) and sorbitol [5]. In this work, two polar solvents of glycerol and EG which have high boiling point of 290oC and 197.3oC, respectively were used as dopant material into PEDOT:PSS. Glycerol, one of the plyol compound group has been successfully enhanced the efficiency of polymeric light emitting diode (PLED) [6] and photodiode device [7]. The second material, EG, is an organic compound and reported to produce a good interaction with PEDOT:PSS led to the improvement of its electrical All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.185.75.247-27/04/12,04:06:37)

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properties [8]. Both solvents are alike in physical structure which is colourless and odorless liquids. Furthermore, they have almost identical in their molecular structure with equal chemical component groups (Fig. 1) where EG and glycerol has 2 and 3 hydroxyl group, respectively. The comparison between their optical, electrical and morphology properties as dopant into PEDOT:PSS were the main focus in this study. PEDOT:PSS Ethylene Glycol (EG) Glycerol

Fig. 1 Molecular structure of PEDOT:PSS glycerol and EG, the polar solvents/ additives to PEDOT:PSS Experimental Procedures Glass substrates were cleaned using distilled (DI) water, acetone and alcohol after being inserted in an ultrasonic bath for 20 min. The solution of PEDOT:PSS and polar solvent consists of glycerol and ethylene glycol (EG) was filtered using 0.45 µm syringe before starting the mixing procedure. The modified PEDOT:PSS were prepared by sol-gel technique where the polar solvent was added to PEDOT:PSS with different percentages from 2 to 10% of volume based, in order to seek for the optimum amount of each polar solvent involved in this work. Subsequently, the solutions were stirred for some time to improve the mixture and to assure that the polar solvent were completely dissolved before deposition process. The modified PEDOT:PSS was deposited on the cleaned and dried glass substrate via spin coating technique at 1000 rpm of spin speed for 60 s and dried on hot plate at 120°C for 20 min. To study the optical and morphology characteristics of the samples, measurements using several systems were implemented. A surface profiler Kosaka ET-3000i was utilized to measure the films thickness. The optical characterization of the doped PEDOT:PSS thin films were obtained from the UV-VIS transmission spectra using Perkin-Elmer Lambda 12 UV/Vis spectrophotometer. Then, absorption coefficient (α) was calculated using (1), whereas energy gap, Eg was determined by Tauc relationship in (2) [9]:

α=

ଶ.ଷ଴ଷ

‫ܣ‬

(1)

αℎν = α௢ (ℎν − ‫ܧ‬௚ )௡

(2)



Whilst, conductivity measurement of the thin films was performed using a two-point probes and power supply system (Advantest R6243) and the morphology and surface roughness of the film were measured using an atomic force microscope (AFM) of Park Systems XE-100. Results and Discussion The optical transmission spectra for the samples were measured in order to study the transparent behavior for each modified PEDOT:PSS thin films with respect to the different additive dissolved. Fig. 2(a) shows the transmission spectra of modified PEDOT:PSS with the respective additives compared to the pristine. All samples showed good transparency in visible range and a relatively small rise of about 10% in transmittance within wavelength of 400 to 500 nm, of the modified PEDOT:PSS films either with glycerol or EG compared to the pristine sample.

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Despite the transparency characteristic, the transmission spectra were also useful in determining of the Eg of the thin films, by exploiting the plot of the Tauc’s relation (2), as shown in Fig. 2(b). The extrapolation of the linear line as indicated in the plot give an estimated value of Eg for each sample. The results indicated that the estimated Eg of the doped PEDOT:PSS film with EG has shifted to lower energy compared to the undoped sample. The variation in the value of the optical band energy may be attributed to the changes in the amount of carrier concentration in the film based on the Burstein-Moss (B.M) effect [10]. (a)

84

(αE)2 X 1012

Transmission (%)

88

80 76 pristine Glycerol EG

72 68 400

500

600

700

800

Wavelength (nm)

(b)

45 40 35 30 25 20 15 10 5 0

Glycerol EG pristine

3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 E(eV)

900 1000

Fig. 2 (a) Transmission spectra for pristine PEDOT:PSS compared to modified PEDOT:PSS with additives, Glycerol and EG,(b) Graph of (αE)2 versus hν, the extrapolation of the linear curve gave the value of Eg. Hence, in this case, the decrease in Eg for modified PEDOT:PSS with EG solvent indicates the reduction in carrier concentration of PEDOT:PSS with the modification of EG molecules. Subsequently, Tauc relationship has been rearranged with natural logarithm and derivation to plot a graph as shown in Fig. 3(a) with variation of energy. Then, utilizing a particular energy value at the peak (Fig. 3(a)), ln(αhν) versus ln(hν-Eg) was plotted as presented in Fig. 3(b). Consequently, n value can be determined from the slope, which then might be used to determine the type of energy gap; either direct (n = 0.5) or indirect (n = 2) [10]. The n value of approximately 0.5 (0.489) indicated a direct energy gap of modified PEDOT:PSS thin films. This direct energy gap would be beneficial factor for any PEDOT:PSS when it used as transparent anode for solar cells in future studies [9]. 6

20

(a)

(b)

5 15

ln(αhν)

dln(αhν)/dhν

4 3 2 1 0

10

y = 0.4892x + 15.409 5 0

3.3

3.5

3.7

3.9

E(eV)

4.1

4.3

4.5

-4

-3

-2

-1

ln(hν-Eg)

0

1

2

Fig. 3 Graphs of (a) dln(αhν)/dhν versus hν and (b) ln(αhν) versus ln(hν-Eg) were plotted to determine

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The morphology of pristine-PEDOT:PSS and modified PEDOT:PSS thin films are as shown in AFM images (Fig. 4). The AFM image of pristine PEDOT:PSS clearly shows that the morphology of the un-doped sample is very smooth with granular structure which attributes to the existing of polymer nanoparticles from aqueous dispersion [11]. However, upon the addition of 4% glycerol into PEDOT:PSS, an obvious change in the morphology occurs with increasing in the surface roughness. The thickness, rms value of AFM surface roughness and conductivity of investigated films (Table 1). Glycerol probably act as a plasticizer upon addition to PEDOT:PSS which results in the engorged colloidal particles as shown which in return, improved the conductivity between the conductive region of the films [12]. On the other hand, with EG additive, the granular structure becomes smaller with a sharper engorgement results in the highest roughness value. This variation was claimed to be related to the conformational modification which responsible in raising the interchain interaction[13]. Based on the values in Table 1, it is agreeable that the addition of polar solvent in PEDOT:PSS has affected the morphology and conductivity of PEDOT:PSS [14].

pristine

With 4% glycerol

6% Ethylene Glycol

Fig. 4 AFM images of the pristine PEDOT:PSS and modified PEDOT:PSS with glycerol and EG (The size of AFM image is 5 x 5 µm) In addition, by comparing the annealed and non-annealed thin films of glycerol dopedPEDOT:PSS, the annealed film has a higher roughness value than the non-annealed film. This is attributed to the fact that heat treatment affected the colloidal particle size in the film to become larger and leading to the increase in surface roughness. Owing to the fact that glycerol is under polyol group, heat treatment to the film with glycerol association could gradually increase the conductivity of the film [7]. Table 1 Thickness, surface roughness and conductivity values for pristine and modified PEDOT:PSS measurements. PEDOT:PSS Thickness (nm) rms roughness (nm) Conductivity (S/cm) Pristine 126 1.12 1.0 4 % Glycerol doped annealed 134 1.81 12.8 8% Glycerol doped non128 1.04 1.2 annealed EG 6% annealed 112 4.23 1.6 Conclusion All films of PEDOT:PSS showed high transparency in visible range and the doped PEDOT:PSS thin films are increased compared to the pristine film. Although all films showed direct band gap behavior, the thin films with additive of EG demonstrated small changes in Eg which correspond to the decrease in carrier concentration with EG association compared to the pristine film and glycerol doped PEDOT:PSS. The conductivity of PEDOT:PSS thin film was steadily enhanced due to the well molecule alignment with the presence of glycerol and the increase in interchain interaction in PEDOT:PSS film with EG doped. Moreover, the conductivity results seem to be reasonably consistent with the surface roughness occurs from the AFM images.

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Acknowledgement The authors would like to thank University of Malaya for PPP grant, SLAB scholarship from Ministry of High Education (MOHE) and University of Technology MARA for their finance support and NANO-SciTech Centre, Institute of Science, (UiTM) for UV-VIS measurements. References [1]

[2]

[3]

[4] [5] [6]

[7] [8]

[9]

[10] [11]

[12]

[13]

[14]

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Solid State Science and Technology XXVI 10.4028/www.scientific.net/AMR.501

PEDOT:PSS Thin Film as Transparent Electrode in ITO-Free Organic Solar Cell 10.4028/www.scientific.net/AMR.501.252

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