Electrochemically deposited nano polyaniline films as hole transporting layers in organic solar cells

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Electrochemically deposited nano polyaniline

ﬁlms as hole

transporting layers in organic solar cells

Yu-Kai Han

a,n

, Mei-Ying Chang

b

, Ko-Shan Ho

a

, Tar-Hwa Hsieh

a

, Jeng-Liang Tsai

b

,

Pei-Chen Huang

a

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, Republic of China

b

Department of Photonics, National Sun Yat-sen University, Kaohsiung 804, Taiwan, Republic of China

a r t i c l e i n f o

Article history: Received 1 July 2013 Received in revised form 3 March 2014

Accepted 27 April 2014 Available online 5 June 2014 Keywords:

Polyaniline Buffer layer

Power conversion efﬁciency Transporting layer PEDOT:PSS primary doping

a b s t r a c t

In this study, we prepared organic solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61

-butyric acid methyl ester (PCBM) with electrochemically deposited nano polyaniline (PANI) buffer layers (either PANI– 0.1 M or PANI – 0.3 M) as hole transporting layers and compared their performances with those of solar cells lacking the buffer layer (i.e., bare ITO) or featuring buffer layers of poly(3,4-ethylenedioxythio-phene):polystyrenesulfonate (PEDOT:PSS) or PEDOT:PSS-covered PANI. The power conversion efﬁciency of the device featuring PANI/PEDOT:PSS as the buffer layer (2.76%) was greater than those of devices featuring bare ITO (0.75%) or the PANI– 0.1 M (1.33%), PANI – 0.3 M (1.78%), or PEDOT:PSS (2.30%) layers. We suspect that the increased conductivity of the PANI/PEDOT:PSS composite, caused by interactions between the PANI nitrogen atoms and the functional groups of PSS, led to additional doping of PANI. This primary doping effect by PSS toward PANI lowered the series resistance of the PANI/PEDOT:PSS buffer layer and, thereby, increased the photocurrent of the device. As a result, electrochemically deposited PANI buffer layers, with or without PEDOT: PSS, appear to be promising hole transporting layers for organic electronic devices that require different candidate materials (other than PEDOT:PSS conductors) and/or different processing conditions (other than spin-coating).

1. Introduction

Although poly(3,4-ethylenedioxythiophene):polystyrenesulfonate

(PEDOT:PSS) is insoluble in organic solvents, it is often employed as

a transparent hole-transporting material in organic cells because of its

high conductivity, simple processing, and suitable work function

[1]

.

The use of PEDOT:PSS has several limitations: it degrades under UV

illumination, it introduces water into the devices' active layer, and it

retains a degree of acidity that can damage indium tin oxide (ITO) at

high temperatures, thereby affecting the long-term stability of its

devices

[2]

. Therefore, several highly conductive materials, including

carbon nanotubes (CNTs)

[3]

, polyhexylthiophene

[4]

, and polyaniline

(PANI)

[5

–7]

, have been studied as possible replacements for PEDOT:

PSS as the hole injection or transporting layer.

Doping PEDOT with PSS allows it to become soluble in aqueous

media, thereby facilitating its deposition onto ITO surfaces through

spin-coating

—the most common method employed in the fabrication

of organic devices. Even though spin-coating is an inexpensive, rapid,

and simple means of depositing liquid materials, it can waste a lot of

material, making it less attractive for manufacturing processes.

Several methods for the deposition of hole transporting (e.g.,

PEDOT:PSS, polyaniline (PANI)] and active layers

—including inkjet

printing

[8]

, spray deposition

[9]

, roll-to-roll gravure printing

[10]

,

brush painting

[11]

, and screen printing

[12]

—have been

devel-oped to enhance the manufacturing process with the goal of mass

production of organic electronics. The presence of PANI-coated ITO

electrodes can decrease the operating voltage, increase the

elec-troluminescence quantum ef

ﬁciency, and improved device

relia-bility of organic light emitting diodes (OLED) because they can

lower the injection barrier between the anode and the

hole-transporting emissive layer

[13

–21]

. Notably, the performance of

ITO/PANI-based devices can be similar to that of corresponding

ITO/PEDOT:PSS-based devices

[20

–22]

.

Recently, electrochemical polymerization was reported as an

alternative means of depositing conductive polymers onto ITO

surfaces for organic device applications

[23

–25]

. Zhau et al.

[23]

electrochemically deposited PEDOT:PSS onto an ITO surface during

the formation of a device having the con

ﬁguration ITO/PEDOT:PSS/

ZnO:MDMO-PPV/Al and obtained a power conversion ef

ﬁciency

(

η

) of 0.33% under AM 1.5 illumination. Mello et al.

[24]

used

Contents lists available at

ScienceDirect

journal homepage:

www.elsevier.com/locate/solmat

Solar Energy Materials & Solar Cells

http://dx.doi.org/10.1016/j.solmat.2014.04.031

n_{Corresponding author.}

E-mail address:[email protected](Y.-K. Han).

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electrochemical

polymerization

to

deposit

sulfonated

PANI

(SPANI) and then prepared a device having the structure

TO/SPANI/poly(3-methylthiophene)/Al that exhibited a value of

η

of 0.8% under monochromatic irradiation (

λ

¼580 nm; 0.8 W/m

2

_).

Qu et al.

[25]

electrodeposited PANI as the anode buffer layer

(thickness: 120 nm) in the device structure ITO/PANI/MDMO-PPV:

PCBM/ZnO/Al and obtained a power conversion ef

ﬁciency of 0.65%.

Although electrochemically deposited PANI

ﬁlms are promising

materials for use in organic optoelectronics, the effects of the

morphologies of these

_{ﬁlms on the photovoltaic performance of poly}

(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C

61

-butyric acid methyl ester

(PCBM)

–based organic solar cells (OSCs) have seldom been reported.

It is well established

[25

–26]

that the surface morphology of a

ﬁlm of

an organic material can help or hinder the transport of carriers across

various interfaces. In a previous report

[26]

, we demonstrated that a

tube-like PANI nanomaterial increased the photovoltaic performance

of P3HT:PCBM-based devices when it acted as a hole-transport layer

(HTL) between PEDOT:PSS and P3HT:PCBM layers; in other words, the

morphology of the hole-transporting material played a key role

affecting the photovoltaic properties of that device.

In this study, we prepared OSCs featuring HTLs with various

surface morphologies

—bare ITO, ITO presenting an

electrochemi-cally deposited PANI thin

_{ﬁlm, and a PANI – 0.3 M thin ﬁlm}

spin-coated with PEDOT:PSS

—and compared their photovoltaic

proper-ties [open-circuit voltage (V

oc

); series resistance (R

S

); short-circuit

current density (J

SC

);

η

] with respect to their different surface

morphologies.

2. Experimental

Aniline (ANI) was doubly distilled prior to use and stored at

5 1C. Electrochemical experiments are performed in a

three-electrode cell; patterned ITO glass, a platinum wire, and Ag/AgCl

functioned as the working electrode, counter electrode, and

reference electrode, respectively. Cyclic voltammetry (CV) was

performed using an AUTO-LAB apparatus. The PANI

– 0.1 M and

PANI

– 0.3 M ﬁlms were obtained after electro-polymerization of

aqueous solutions containing 0.1 and 0.3 M ANI monomer,

respec-tively, and 1 M H

2

SO

4

, applying a sequential linear potential scan

rate of 0.01 V/s between

0.2 and þ0.9 V versus the Ag/AgCl

electrode. The PANI/PEDOT:PSS

ﬁlm was obtained by spin-coating

a PEDOT:PSS (AI 4083) solution onto the ITO/PANI

– 0.3 M surface

and then drying at 150

_{1C for 20 min. Field emission scanning}

electron microscopy (FE-SEM) images of water- or hexane-diluted

dispersions dried on cover glasses were recorded using a Hitachi

FE-2000 apparatus. The surface morphologies of the

ﬁlms were

characterized using a Ben-Yuan CSPM4000 scanning probe

micro-scope and an atomic force micromicro-scope operated in the tapping

mode. The

ﬁlm thickness was measured using a Dektak 6M stylus

pro

ﬁlometer. Optical spectra were recorded using a UV–vis

spec-trophotometer; the work functions of the

ﬁlms were measured

using a Riken Keiki AC-2 surface analyzer photoelectron

spectro-meter. The current density

–voltage (J–V) characteristics were

measured using a Keithley 2400 source meter while illuminating

the devices with white light (100 mW/cm

2

_{) from a halogen lamp.}

A single-crystalline silicon solar cell was used as a reference cell to

con

ﬁrm the stability of the light source; the mismatch factor was

not taken into account.

3. Results and discussion

3.1. Surface morphologies of devices

Smooth surface morphologies are required for anodes used in

organic optoelectronic devices because

“spikes” can cause

break-down and/or shorting, thereby affecting the performance

[27]

.

Fig. 1. (a)–(c) SEM and (d)–(f) AFM images of (a, d) ITO, (b, e) PANI – 0.1 M, and (c, f) PANI – 0.3 M ﬁlms.

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Acknowledgment

We thank the National Science Council of Taiwan (grant

NSC-101

–2221-E-151-037) for ﬁnancial support.

References

[1]J.S. Huang, P.F. Miller, J.S. Wilson, A.J. de Mello, J.C. de Mello, D.D.C. Bradley, Investigation of the effects of doping and post-deposition treatments on the conductivity, morphology, and work function of poly(3,4-ethylenedioxythio-phene)/poly(styrene sulfonate)ﬁlms, Adv. Funct. Mater. 15 (2005) 290–296. [2]Y.H. Kim, S.H. Lee, J. Noh, S.H. Han, Performance and stability of

electro-luminescent device with self-assembled layers of poly(3,4-ethylenedioxythio-phene)-poly(styrenesulfonate) and polyelectrolytes, Thin Solid Films 510 (2006) 305–310.

[3]S. Rait, S. Kashyap, P.K. Bhatnagar, P.C. Mathur, S.K. Sengupta, J. Kumar, Improving power conversion efﬁciency in polythiophene/fullerene-based bulk heterojunction solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 757–763. [4]J. Ohshita, Y. Tada, A. Kunai, Y. Harima, A. Kohno, Y. Kunugi, Effects of

annealing of poly(3-hexylthiophene) ﬁlm on the performance of double-layered EL devices of ITO/polymer/Alq3/Mg–Ag, Synth. Met. 157 (2007) 104–108.

[5]Y. Yang, A.J. Heeger, Effect of substrate materials in the growth of ZnSe on GaAs and GaP substrates, Appl. Phys. Lett. 64 (1988) 1245–1248.

[6]A.J. Heeger, I.D. Parker, Y. Yang, Carrier injection into semiconducting poly-mers: Fowler–Nordheim ﬁeld-emission tunneling, Synth. Met. 67 (1994) 23–29.

[7]S. Karg, J.C. Scott, J.R. Salem, M. Angelopoulos, Increased brightness and lifetime of polymer light-emitting diodes with polyaniline anodes, Synth. Met. 80 (1996) 111–117.

[8]F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412.

[9]K.J. Kim, Y.S. Kim, W.S. Kang, B.H. Kang, S.H. Yeom, D.E. Kim, J.H. Kim, S.W. Kang, Inspection of substrate-heated modiﬁed PEDOT:PSS morphology for all spray deposited organic photovoltaics, Sol. Energy Mater. Sol. Cells 94 (2010) 1303–1306.

[10]P. Kopola, T. Aernouts, S. Guillerez, H. Jin, M. Tuomikoski, A. Maaninen, J. Hast, High efﬁcient plastic solar cells fabricated with a high-throughput gravure printing method, Sol. Energy Mater. Sol. Cells 94 (2010) 1673–1680. [11]S.S. Kim, S.I. Na, S.J. Kang, D.Y. Kim, Annealing-free fabrication of P3HT:PCBM

solar cells via simple brush painting, Sol. Energy Mater. Sol. Cells 94 (2010) 171–175.

[12]F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475.

[13]B.S. Chuah, D.H. Hwang, S.T. Kim, S.C. Moratti, A.B. Holmes, J.C. deMello, R.H. Friend, New luminescent polymers for LEDs, Synth. Met. 91 (1997) 279–282.

[14]R.O. Garay, B. Mayer, F.E. Karasz, R.W. Lenz, Synthesis and characterization of poly[2,5-bis(triethoxy)-1,4-phenylene vinylene], J. Polym. Sci., Part A: Polym. Chem. 33 (1995) 525–531.

[15]B. Winkler, L. Dai, A.W.H. Mau, Novel poly(p-phenylene vinylene) derivatives with oligo(ethylene oxide) side chains: synthesis and pattern formation, Chem. Mater. 11 (1999) 704–711.

[16]G. Yu, High performance photonic devices made with semiconducting poly-mers, Synth. Met. 80 (1996) 143–150.

[17]C. Liedenbaum, Y. Croonen, P. s, J. Vleggaar, H. Schoo, Low voltage operation of large area polymer LEDs, Synth. Met. 91 (1997) 109–111.

[18]H.L. Wang, A.G. MacDiarmid, Y.Z. Wang, D.D. Gebler, A.J. Epstein, Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices, Synth. Met. 78 (1996) 33–37.

[19]P.K.H. Ho, M. Granström, R.H. Friend, N.C. Greenham, Ultrathin self-assembled layers at the ITO interface to control charge injection and electroluminescence efﬁciency in polymer light-emitting diodes, Adv. Mater. 10 (1998) 769–774. [20] S.A. Carter, M. Angelopoulos, S. Karg, P.J. Brock, Polymeric anodes for improved

polymer light-emitting diode performance, Appl. Phys. Lett. 70 (1997) 2067–2069.

[21]H. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H. Schenk, R. Demandt, H. Schoo, Soluble phenyl-substituted PPVs: new materials for highly efﬁcient polymer LEDs, Adv. Mater. 10 (1998) 1340–1343.

[22] J.C. Scott, S.A. Carter, S. Karg, M. Angelopoulos, Polymeric anodes for organic light-emitting diodes, Synth. Met. 85 (1997) 1197–1200.

[23] J. Yan, C.H. Sun, F.U. Tan, X.J. Hu, P. Chen, S.C. Qu, S.Y. Zhou, J.K. Xu, Electropolymerized poly(3,4-ethylenedioxythiophene):poly(styrene sulfo-nate) (PEDOT:PSS)ﬁlm on ITO glass and its application in photovoltaic device, Sol. Energy Mater. Sol Cells 94 (2010) 390–394.

[24] R. Valaski, F. Muchenski, R.M.Q. Mello, L. Micaroni, L.S. Roman, I.A. Hummelgen, Sulfonated polyaniline/poly(3-methylthiophene)-based photovoltaic devices, J. Solid State Electrochem. 10 (2006) 24–27.

[25] F. Tan, S. Qu, J. Wu, Z. Wang, L. Jin, Y. Bi, J. Cao, K. Liu, J. Zhang, Z. Wang, Electrodeposited polyanilineﬁlms decorated with nano-islands: characteriza-tion and applicacharacteriza-tion as anode buffer layers in solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 440–445.

[26] M.Y. Chang, C.S. Wu, Y.F. Chen, B.Z. Hsieh, W.Y. Huang, K.S. Ho, T.H. Hsieh, Y.K. Han, Polymer solar cells incorporating one-dimensional polyaniline nanotubes, Org. Electron. 9 (2008) 1136–1139.

[27] Y.K. Kim, A.M. Ballantyne, J. Nelson, D.D.C. Bradley, Effects of thickness and thermal annealing of the PEDOT:PSS layer on the performance of polymer solar cells, Org. Electron. 10 (2009) 205–209.

[28] D.J. Milliron, I.G. Hill, C. Shen, A. Kahn, J. Schwartz, Surface oxidation activates indium tin oxide for hole injection, J. Appl. Phys. 87 (2000) 572–576. [29] J.H. Choi, E.S. Lee, S.H. Choi, H.K. Baik, K.M. Song, Y.S. Lim, Work function

increase of indium–tin–oxide surfaces by atmospheric air plasma treatment with steady-state airﬂow, J. Vac. Sci. Technol. A 23 (2005) 1479–1483. [30] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efﬁcient polymer

solar cells with nanoscale control of the interpenetrating network morphol-ogy, Adv. Funct. Mater. 15 (2005) 1617–1622.

[31]G. Xue, S. Uchida, B.P. Rand, S.R. Forrest, 4.2% Efﬁcient organic photovoltaic cells with low series resistances, Appl. Phys. Lett. 84 (2004) 3013–3015. [32] F.L. Zhang, A. Gadisa, O. Inganas, M. Svensson, M.R. Andersson, Inﬂuence of

buffer layers on the performance of polymer solar cells, Appl. Phys. Lett. 84 (2004) 3906–3908.

[33] S. Kirchmeyer, K. Reuter, Scientiﬁc importance properties and growing applications of poly(3,4-ethylenedioxythiophene), J. Mater. Chem. 15 (2005) 2077–2088.

[34] A. Elschner, F. Brudera, H.-W. Heuera, F. Jonasa, A. Karbacha, S. Kirchmeyerb, S. Thurma, R. Wehrmanna, PEDT/PSS for efﬁcient hole-injection in hybrid organic light-emitting diodes, Synth. Met. 111–112 (2000) 139–143. [35] H. Bejbouji, L. Vignau, J.L. Miane, M.T. Dang, E.M. Oualim, M. Harmouchi,

A. Mouhsen, Polyaniline as a hole injection layer on organic photovoltaic cells, Sol. Energy Mater. Sol. Cells 94 (2010) 176–181.

[36] R. Ulbricht, S.B. Lee, X. Jiang, K. Inoue, M. Zhang, S. Fang, R.H. Baughman, A.A. Zakhidov, Transparent carbon nanotube sheets as 3-D charge collectors in organic solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 416–419. [37]O.P. Dimitriev, Cooperative doping in polyaniline-poly(ethylene-3,4-dioxy

thiophene):poly(styrenesulfonic acid) composite system, J. Polym. Res. 18 (2011) 2435–2440.

[38] J. Kim, S. Park, N.F. Scherer, Ultrafast dynamics of polarons in conductive polyaniline: comparison of primary and secondary doped forms, J. Phys. Chem. B 112 (2008) 1557s6–15587.