ITO hybrid transparent conducting electrode for organic photovoltaics

Solar Energy Materials & Solar Cells 115 (2013) 71–78

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Flexible PEDOT: PSS/ITO hybrid transparent conducting electrode for organic photovoltaics Kyounga Lim a, Sunghoon Jung a, Jong-Kuk Kim a, Jae-Wook Kang b, Joo-Hyun Kim c, Sung-Hoon Choa d,1, Do-Geun Kim a,n a

Plasma Coating Research Group, Korea Institute of Materials Science (KIMS), 797, Changwondaero, Changwon, Gyeongnam 641-831, Republic of Korea Professional Graduate School of Flexible and Printable Electronics,Department of Flexible and Printable Electronics, Chonbuk National University, Jeonju 561-756, Republic of Korea c School of Mechanical & Automotive Engineering, Kookmin University, Jeongneung-Ro 77, Republic of Korea d Graduate School of NID Fusion Technology, Seoul National University of Science and Technology, Gongneun-Dong, Nowon-Gu, Seoul 139-743, Republic of Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 28 November 2012 Received in revised form 25 March 2013 Accepted 25 March 2013 Available online 21 April 2013

In order to improve the performance and mechanical flexibility of the transparent conducting electrodes (TCEs) for organic photovoltaics, we proposed a flexible PEDOT:PSS/ITO hybrid TCE. A conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was introduced as a buffer layer beneath the ITO film. This PEDOT:PSS/ITO hybrid electrode showed a sheet resistance of 85 Ω/sq, which was much better than that of a single-layer ITO film. The mechanical integrity of the flexible PEDOT:PSS/ITO hybrid electrode was investigated by bending and fragmentation tests. The failure bending radius was lower in bending tests of the PEDOT:PSS/ITO electrode than for the ITO electrode due to buffering effect of PEDOT:PSS film. A fragmentation test showed superior stretchability and better interfacial adhesion strength for the PEDOT:PSS/ITO hybrid electrode on a PET substrate than for the ITO electrode. We have prepared flexible organic photovoltaic (OPV) devices using the PEDOT: PSS/ITO hybrid TCE. The flexible OPV device fabricated using the PEDOT:PSS/ITO hybrid anode exhibited a power conversion efficiency of 3.21% (FF¼ 0.45, Voc ¼ 0.72 V), which is comparable with that using an ITO anode. A bending test of the OPV device also showed better mechanical bending and crack-resistance performances for the OPV device with the PEDOT:PSS/ITO hybrid anode than for the ITO anode. The comparable performance and superior flexibility of flexible OPVs with an PEDOT:PSS/ITO hybrid anode indicates that the PEDOT:PSS/ITO hybrid electrode is a promising flexible electrode scheme for next generation flexible photovoltaic devices. & 2013 Elsevier B.V. All rights reserved.

Keywords: ITO PEDOT:PSS Buffer layer Flexibility

1. Introduction In last two decades, increasing attention has been paid to organic electronics such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and organic thin film transistors (OTFTs) because of their potential for fabrication of flexible devices, and extensive progress has been made in their efficiency, lifetime and manufacturing process [1–4]. Indium–tin-oxide (ITO) is the most widely used material in transparent conducting electrodes (TCEs) for OPVs because it has better transmittance and conductivity than any other TCEs. Even though ITO shows superior electrical and optical properties, its application in flexible electronic devices n

Corresponding author. Tel.: +82 55 280 3507; fax: +82 55 280 3570. E-mail addresses: [email protected] (.-H. Choa), [email protected] (D.-G. Kim). 1 Tel.: +82 2 970 6593; fax: +82 2 972 2202. 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.03.028

remains limited. It is inherently brittle under tension and its adhesion to polymeric substrates is poor due to the low surface energy of polymers, which significantly limits the flexibility of the substrates. In addition, the high temperature during deposition or post annealing processes at more than 150 1C are essential for obtaining high quality ITO film; however, this high temperature process damages the plastic substrates. Thus, the flexibility and non-damaging fabrication process associated with TCEs are extremely demanded on the construction of flexible devices for high performance. For these reasons, various flexible electrodes, such as silver nanowires, thin metal films, conductive polymers, carbon nanotubes (CNTs), and graphene, have been suggested as alternatives to flexible ITO electrodes [5–11]. Kim et al. introduced an amorphous ITO (a-ITO) film on a polyethylene terephthalate (PET) substrate, deposited by a magnetron sputtering method with superimposing DC power on radio frequency (RF) power, to improve the flexibility

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of ITO film at low temperatures [12]. A metal oxide/thin metal/ metal oxide (OMO) structure is another way to improve flexibility. Choa et al. reported that an indium–zinc-oxide/Ag/indium–zincoxide (IZO/Ag/IZO) structure exhibited a failure bending radius of 6 mm in the outer bending test and a sheet resistance of 5.37 Ω/sq [13]. Graphene guarantees high flexibility as well; however, it is too difficult to obtain high quality graphene and the manufacturing process is complicated. A percolation network of Ag nanowires (NWs) was recently suggested as a promising transparent electrode to replace ITO films because Ag NWs have an inherent low resistivity, a high specular transmittance, and superior flexibility [14,15]. However, Ag NW electrodes have problems with respect to the irregular morphology of the film and poor surface adhesion to the substrate. Formally, the conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as a hole injection layer to enhance performances of organic solar cells (OPV) and organic light emitting devices (OLED) [16–18]. PEDOT: PSS is also used as a light extraction layer for OLED because PEDOT:PSS has lower reflective index than ITO [19,20]. Both of cases, PEDOT:PSS layer is formed onto ITO layer and the usages of PEDOT:PSS are not to improve flexibility of devices. Lately, PEDOT: PSS is used as not only ITO assistance but also TCE itself since the quality of PEDOT:PSS has been developed. Among PEDOT:PSSs, PH-series shows outstanding performances for TCEs such as high transparency, good conductivity, and “excellent flexibility”. Especially, the conductivity of PH 1000 (Clevios) could be obtained more than 1000 S/cm with solvent additives and post treatment [21–26]. The other reason which PEDOT:PSS has spotlighted is that PEDOT:PSS has highly flexibility thus it could be adopted in roll to roll process thereby mass production is possible. [27–29]. However, organic photovoltaic devices with only PEDOT:PSS electrodes do not provide sufficiently high efficiency for large area devices due to the limited conductivity of the PEDOT:PSS even though PEDOT:PSS has good flexibility [30]. Therefore to overcome limitation of PEDOT:PSS performance, many researchers have tried to adopt metal grids under PEDOT:PSS layer for enhancing the performances of the devices [31–33]. In this work, we introduced a hybrid TCE which consisted of ITO and a PEDOT:PSS layer to obtain both advantages of good flexibility and high conductivity. The conductive polymer PEDOT:PSS was inserted as a buffer layer between the ITO film and PET substrate. This PEDOT:PSS buffer layer substantially enhanced the electrical property of the PEDOT:PSS/ITO hybrid electrode compared to the single-layered ITO electrode. We used lab-made outer and fragmentation test systems to examine the flexibility of the PEDOT:PSS/ ITO hybrid electrode grown on a PET substrate. The PEDOT:PSS/ITO hybrid electrode showed superior flexibility and good adhesion strength. The flexible organic photovoltaic devices were fabricated in two structures of PET/PEDOT:PSS/ITO-P/PBDTTT-C:PCBM/Al and PET/PEDOT:PSS/ITO/ PEDOT:PSS-P/PBDTTT-C:PCBM/Al. The device of using PEDOT:PSS/ITO hybrid anode showed similar performance to that of using ITO anode; however, the device with hybrid anode had greater flexibility and crack resistance.

2. Experimental We prepared two kinds of TCEs, which were a single-layered ITO electrode and an PEDOT:PSS/ITO hybrid electrode on PET substrate. The surface energy or the wettability of PEDOT:PSS film on PET was improved by O2 plasma pre-treatment of the PET substrate for 5 min under 500 W of RF power. The conductivity of PEDOT:PSS was increased by addition of 5 wt% of dimethyl sulfoxide (DMSO) to the PEDOT:PSS solution. The addition of DMSO increased the conductivity of PEDOT:PSS by 2 orders of magnitude

(around 900 S/cm). A spray coating technique was used to form the PEDOT:PSS film (150 nm) on 125 μm PET substrates. After formation, the PEDOT:PSS film was post-annealed for 15 min at 120 1C on a hotplate. Subsequently, the ITO film was deposited onto the PEDOT:PSS layer. In particular, the mechanical properties of the ITO film were improved by depositing the ITO film via a sputtering method that superimposed RF power with DC power [12]. During the sputtering process, the base pressure was maintained at 2.4  10−6 Torr and the working pressure was kept at 1.1  10−3 Torr. We used Ar as a carrier gas, with a flow rate of 30 sccm and O2 as a reactive gas with a flow rate of 0.3 sccm. We applied 50 W of RF power and 0.5 A of DC current during the deposition. The thicknesses of PEDOT:PSS films and the ITO films were measured by an alpha-step surface profiler (Tencor P-11, Surface Profiler). The electrical and optical properties of films were examined using a four-point probe and a UV/visible spectrometer (Cary 5000). The microstructural properties of ITO films were analyzed by X-ray diffraction (XRD, X-pert pro). The thickness dependence of the ITO films was investigated by depositing ITO films of various thicknesses (20–150 nm) on 125 μm PET substrates. The mechanical integrity, such as the bendability, stretchability, and adhesion strength of the PEDOT:PSS/ITO hybrid electrode, was evaluated by the bending test and the uniaxial tension test, also known as the fragmentation test. Lab-made test machines were designed to conduct bending and fragmentation tests. The fourpoint probe station was used to measure changes in electrical resistance, and an optical microscope was mounted on the probe station to observe cracks in the films. For the bending test, the bending radius is calculated using the following Eq. (1) [34]. BendingRadiusðRÞ ¼

L qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2π ðdL=LÞ−ðπ 2 hs =12L2 Þ

ð1Þ

where L, dL/L, and hs denote the initial length, the applied strain, and the substrate thickness, respectively. The nominal bending strain can also be calculated using the following Eq. (2): Strain ¼

hs 2R

ð2Þ

The fragmentation test is a well established methodology for determining the tensile cracking resistance and the interfacial adhesion strength of ITO on a polymer substrate [35,36]. Uniaxial stretching of the samples results in the initiation of cracks in the coating at defect sites and these begin to propagate perpendicularly to the loading direction at a critical strain. The progressive cracking of the coating was analyzed in terms of crack density (CD), defined as the inverse of the average fragment length l, and calculated from the average of the number of cracks, Ni, counted on k micrographs of with W, at strain ε, as CD ¼ ð1 þ εÞ∑ki ¼ 1 N i =kW The factor (1+ε) corrects for crack opening to a first approximation. The detailed fragmentation test procedures and CD calculation method have been well described previously [35,37]. When the PET substrate is further stretched, compressive strain is induced in the ITO along the direction perpendicular to the tension, since Poisson's ratio of PET is larger than for an ITO film. This is referred to as Poisson's contraction [38]. During the test, a brittle channel cracking that propagates perpendicular to the direction of tension tends to precede buckling delamination. Buckling delamination occurs as the channel crack density becomes almost saturated [39]. The buckling delaminations initiated by strains between substrates and films are the indication of the interfacial adhesion strength of ITO on PET substrate. The size of the mechanical bending and fragmentation test sample was 25 mm  25 mm.

K. Lim et al. / Solar Energy Materials & Solar Cells 115 (2013) 71–78

as a PEDOT:PSS layer (150 nm thickness) is inserted, the sheet resistance of the ITO(20 nm)/PEDOT:PSS hybrid electrode substantially decreases to 85 Ω/sq. A similar tendency is also shown for a 50 nm thickness of ITO. The sheet resistance of a 50 nm-thick

ITO PET

Sheet Resistance (Ω/sq)

240

100

200 80 160 60

120 80

Transmittance (%)

We compared the properties of the PEDOT:PSS/ITO hybrid TCE with those of ITO electrode by fabricating flexible OPV devices using the PEDOT:PSS/ITO hybrid TCE and ITO electrodes as an anodes. The fabricated devices had the conventional structure of an OPV device. For the OPV device using an ITO electrode, the thickness of the ITO anode was 150 nm. For the OPV device using an PEDOT:PSS/ITO hybrid TCE, the thickness of the PEDOT:PSS film was 150 nm, and the 80 nm-thick ITO anode was deposited on top of the PEDOT:PSS film. The PEDOT:PSS-P (Clevios P), as a hole injection layer, was then spin-coated onto each ITO anode and PEDOT:PSS/ITO hybrid anode. A 100 nm-thick PBDTTT-C:PCBM (poly[4,8-bis-substituted-benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b] thiophene-2,6-diyl] (PBDTTT-C): [6,6]-phenyl C61-butyric acid methylester) was then coated as an active layer onto the PEDOT: PSS-P. The top Al electrode (120 nm) was deposited using a thermal evaporator for both devices. Fig. 1 shows the schematic structure layouts of the fabricated OPV devices. The photocurrent density–voltage (J–V) characteristics were measured using a Keithley 2400 source measurement unit. The cell performance was evaluated under AM 1.5 simulated illumination with an intensity of 100 mW/cm2 (Pecell Technologies Inc., PEC-L11 model). The flexibility of the fabricated OPV devices was also evaluated by the bending tests. During bending tests, the J–V curves and existence of the cracks were continuously monitored.

73

40 Sheet Resistance Transmittance

40

20 0

20

40

60 80 100 120 Thickness of ITO (nm)

140

160

Fig. 2. Sheet resistance and transmittance of an ITO electrode as a function of various thicknesses of the ITO electrode.

3. Results and discussion PEDOT

Sheet Resistivity (Ω/sq)

PET

250

100

200

80

150

60

100

40

50

20

0 50

100

150

200

250

Transmittance (%)

Fig. 2 shows the sheet resistance and averaged optical transmittance for wavelengths between 430 nm and 600 nm for the ITO electrode on a 125 μm PET substrate. As the thickness of ITO electrode increases from 20 nm to 100 nm, the sheet resistance of ITO electrode substantially decreases from 230 Ω/sq to 47 Ω/sq and then saturates at around 42 Ω/sq over a 150 nm thickness. The transmittance of the ITO electrode remained between 87% and 96% throughout the entire thicknesses. Fig. 3 shows the sheet resistance and averaged transmittance of the PEDOT:PSS film. As the thickness of the PEDOT:PSS film increases, the sheet resistance decreases; in particular, the sheet resistance dramatically drops from 204 Ω/sq to 62 Ω/sq when the film thickness increases from 100 nm to 150 nm. At the same time, the transmittance is gradually reduced as the thickness of the PEDOT:PSS film increases. Table 1 summarizes the sheet resistance and transmittance of the ITO electrode and PEDOT:PSS/ITO hybrid electrode on PET substrates for various thicknesses of the ITO film. The sheet resistance of a 20 nm-thick ITO electrode is 230 Ω/sq. However,

0 300

Thickness (nm) Fig. 3. Sheet resistance and transmittance of PEDOT:PSS films as a function of various thicknesses of PEDOT:PSS films.

Fig. 1. Schematic illustration of organic photovoltaic device (a) using an PEDOT:PSS (150 nm)/ITO(80 nm) anode, (b) using an ITO (150 nm) anode.

K. Lim et al. / Solar Energy Materials & Solar Cells 115 (2013) 71–78

5 ITO 20nm ITO 50nm ITO 100nm ITO 150nm 5

4

3

4 3

2

2 1

1

0 7.0

6.5

6.0

5.5

5.0

4.5

4.0

0 40

30 20 Bending Radius (mm)

10

ITO 150nm

ITO 100nm

ITO 50nm

ITO 20nm

20

30

40

50

60

2θ(degree) Fig. 5. XRD analysis as a function of various thicknesses of ITO electrodes based on 125 μm PET.

Table 1 Sheet resistances and transmittances of the ITO electrodes and PEDOT:PSS/ITO hybrid electrodes as a function of ITO thickness.

ITO Thickness (nm)

20 50 100 150

PET/ITO

0

Fig. 4. Bending reliability tests with decreasing bending radius as a function of various thicknesses of ITO electrodes based on 125 μm PET.

Intensity (a.u.)

ITO electrode is 85 Ω/sq, while that of an ITO(50 nm)/PEDOT:PSS hybrid electrode decreases to 51 Ω/sq. It is noteworthy that a highly conductive PEDOT:PSS layer is more effective in reducing the sheet resistance of the PEDOT:PSS/ITO hybrid electrode when a thinner ITO film of 20 nm is used, whereas it has little influence on the sheet resistance of an ITO electrode above 100 nm in thickness. The sheet resistance is known to be affected by the electrical property of the underlayer; therefore, due to the effect of the PEDOT:PSS layer, the resistivity is much lower for the PEDOT:PSS/ ITO hybrid electrode than for a single-layered ITO electrode. The increase in ITO thickness for PEDOT:PSS/ITO hybrid electrode produces little improvement in electrical resistance when the ITO film is sufficiently thick. When a PEDOT:PSS layer is inserted, the optical transmittances is reduced by approximately 10%; the average optical transmittance of the PEDOT:PSS/ITO hybrid electrode is 84.8%. Fig. 4 shows the results of the outer bending test for the ITO electrode on a PET substrate with a decreasing outer bending radius as a function of ITO electrode thickness. The change in resistance of the ITO electrode is expressed as ΔR(¼R−–R0)/R0, where R0 is the initially measured resistance and R is the measured value after substrate bending. The bending test results show that, at the initial stage, the electrical resistance of the bent ITO electrodes is unchanged with decreasing bending radius. However, at a specific bending radius, the electrical resistance sharply increases due to the initiation of cracks on the ITO films. This point is defined as the critical bending radius. As shown in Fig. 4, the critical bending radii of the ITO electrodes change with the film thickness. The critical bending radii of 20, 50, 100, and 150 nm thick ITO electrodes are 4.5, 4.5, 6, and 6 mm, respectively. Note that the critical bending radius increases as the thicknesses of the ITO electrodes increases, which indicates that an increase in ITO film thickness reduces flexibility. This result is also estimated from Eq.(2); that is, the strain increases with increases in film thickness, so that a thick film fails earlier during a bending test. In addition, it is very interesting that the critical bending radii of a 20 nm thick ITO electrode and a 50 nm thick ITO are the same (4.5 mm), and those of a 100 nm thick ITO electrode and a 150 nm thick ITO are also identical (6 mm). To understand this phenomenon, we have examined the crystallinity of ITO films. The microstructure of thin film, such as the crystallinity or surface roughness, is well known to affect the mechanical property and flexibility [40,41]. Films containing multiple crystallinities have been reported to show poor mechanical flexibility [42,43]. Fig. 5 shows the XRD results for different thicknesses of ITO electrodes. The XRD pattern indicates that the amorphous ITO structure grows below 100 nm in thickness, and then a (222) crystalline form of

Resistance Change (ΔΩ/Ω ΔΩ/Ω)

74

PET/PEDOT/ITO

Rsheet (Ω/sq)

T(%)

Rsheet (Ω/sq)

T(%)

230 7 5 857 2 477 1 427 1

96 83 87 92

857 5 517 2 377 2 367 2

87 78 82 84

K. Lim et al. / Solar Energy Materials & Solar Cells 115 (2013) 71–78

75

delamination, we can observe transverse surface cracks in the electrodes running perpendicularly to the stretching direction after the stretching test. The transverse cracking and buckling delamination that occurred during the fragmentation test were initiated at about 6% strain for ITO electrode and about 9% strain for PEDOT:PSS/ITO hybrid electrode. The development of these cracks is due to the lateral contraction of the sample, which results from Poisson effects [38]. The crack density of the PEDOT:PSS/ITO hybrid electrode then gradually increases, and almost saturates at a strain of 10%. These results indicate that a better cracking resistance and interfacial adhesion strength on the PET substrate was achieved for the PEDOT:PSS/ITO electrode than for the ITO electrode. We investigated the possibility of using an PEDOT:PSS/ITO hybrid TCE as a transparent conducting anode for flexible OPV devices by fabricating two kinds of flexible organic photovoltaic devices using an PEDOT:PSS/ITO hybrid electrode and an ITO

ITO starts developing at thicknesses over 100 nm. In general, crystallinity is reported to evolve as thickness increases [44,45], and our results show good agreement with previously published results. Hence, the current findings clearly explain why 20 and 50 nm-thick ITO electrodes with amorphous structure have a similar bendability, and why 100 and 150 nm-thick ITO electrodes with crystalline structure have also similar mechanical bendability. Fig. 6 shows the outer bending test results for ITO(20 nm)/ PEDOT:PSS and ITO(100 nm)/PEDOT:PSS hybrid electrodes. The ITO(20 nm)/PEDOT:PSS hybrid electrode shows cracks beginning to initiate on the electrode surface at a bending radius of 3.5 mm, and the electrical resistance slightly increases at a bending radius of 3 mm. The ITO(100 nm)/PEDOT:PSS electrode shows cracks beginning to initiate at a bending radius of 4 mm, and then, at a bending radius of 3.5 mm, the electrical resistance sharply increases. Comparison of these bending test results to those of the single-layered ITO electrode, as shown in Fig. 4, indicates that the PEDOT:PSS/ITO hybrid electrode has better bendability than does the ITO electrode, where the cracks were generated at a bending radius of 4.5 mm and 6 mm for 20 nm and 100 nm thick ITO electrodes, respectively. Furthermore, a lower crack density is observed for ITO/PEDOT electrodes than for ITO electrodes. The robustness of the PEDOT:PSS/ITO hybrid electrode against severe bending indicates that the PEDOT:PSS/ITO hybrid electrode is a desirable flexible electrode material for flexible OPVs. The lower bendability of the ITO(100 nm)/PEDOT:PSS electrode compared with ITO(20 nm)/PEDOT:PSS electrode is inferred from the bending results of the ITO electrode in Fig. 4, which shows that the bendability of the ITO film decreases as the thicknesses of the ITO films increases. Fig. 7 summarizes the fragmentation test results of the ITO (20 nm) and ITO(20 nm)/PEDOT:PSS hybrid electrodes. This graph shows the increase in crack density (CD) versus strain when the substrate is stretched. The ITO electrode showed the first cracks at a PET substrate strain of 2.8% and the number of cracks increased rapidly. The PEDOT:PSS/ITO hybrid electrode showed cracks beginning at a strain of 4% and the electrical resistance of the PEDOT: PSS/ITO hybrid electrode also increased sharply. The PEDOT:PSS/ ITO hybrid electrode clearly shows superior stretchability compared to the ITO electrode. The pictures shown in the upper panel are the optical microscope (OM) images of the crack damage stages of each electrode, which are the crack initiation and buckling delamination initiation. In the initial stages of buckling

Crack initiation

Buckling delamination initiation

300

Crack Density [mm-1]

250 200 150 100 50

PET/PEDOT/ITO PET/ITO

0 0

2

4

6

8

10

12

14

16

Strain [%] Fig. 7. Crack density curve vs. strain for an ITO electrode (20 nm) and an PEDOT: PSS/ITO(20 nm) hybrid electrode under uniaxial loading.

PET/PEDOT/ITO(20 nm) 5

Resistance Change [ΔR/R [ 0]

PET/PEDOT/ITO(20nm) PET/PEDOT/ITO(100nm) 4 5 4

3

100 μm

3

2

2

Crack initiation at R = 3.5 mm

1

PET/PEDOTITO 100(nm)

0

1

7

6

5

4

3

0 45

40

35

30

25

20

15

Bending Radius [mm]

10

5

0

100 μm

Crack initiation at R = 4.0 mm Fig. 6. Bending reliability tests with decreasing bending radius for an PEDOT:PSS/ITO(20 nm) hybrid electrode and an PEDOT:PSS/ITO(100 nm) hybrid electrode. The optical microscope (OM) image shows crack initiation for the PEDOT:PSS/ITO(20 nm) and the PEDOT:PSS/ITO(100 nm) hybrid electrode at a bending radius of 3.5 mm and 4.0 mm, respectively.

76

K. Lim et al. / Solar Energy Materials & Solar Cells 115 (2013) 71–78

voltage (J–V) curves of flexible OPVs fabricated on a PEDOT:PSS/ITO and ITO anodes. The inset in Fig. 8 shows a photograph of a flexible OPV device fabricated on a flexible PET substrate using an PEDOT: PSS/ITO hybrid anode. The flexible OPV device fabricated on a PEDOT:PSS/ITO anode shows FF¼ 0.45, Voc ¼ 0.72 V, while the OPV device fabricated on a ITO shows FF¼0.48, Voc ¼ 0.74 V. The differences in FF and Voc values for each device are thought to be attributable to the differences in the sheet resistance and the change in the work functions for the PEDOT:PSS/ITO and the ITO anodes. A lower short circuit current density (Jsc) of the OPV device fabricated with the PEDOT:PSS/ITO anode could be explained by the lower transmittance of the PEDOT:PSS/ITO anode when compared to the ITO anode. The power conversion efficiencies (PCEs) of OPV devices fabricated using the PEDOT:PSS/ITO anode and the ITO anode are 3.21% and 3.80%, respectively. Fig. 9 shows the results of the outer bending tests for both devices. The PCE has been normalized in order to compare the mechanical properties. The PCEs of both devices show no significant changes until a bending radius of 9 mm. However, the PCE of an OPV device fabricated with the ITO anode drastically drops at a bending radius of 9 mm. The PCE value of an OPV device fabricated with the PEDOT:PSS/ITO anode gradually decreases until a bending radius of 4 mm. The PCE value of an OPV device with an PEDOT:PSS/ITO anode retains 60% of the initial PCE value even at a 4 mm bending radius. Comparisons of the outer bending results of OPV devices with the outer bending results of the ITO and PEDOT:PSS/ITO hybrid electrodes are shown in Fig. 4 and Fig. 6, the mechanical bendability results of the fabricated devices are slightly inferior to those of the electrodes themselves. For example, for a 150 nm-thick ITO electrode, cracks are generated at a bending radius of 6 mm. On the other hand, for an OPV device fabricated with a 150 nm-thick ITO anode, cracks are generated at a bending radius of 7 mm. This slight difference could be attributed to the damage generated during device fabrication process or due to handling process such as defects, scratches, and accumulated internal stress during deposition of the individual layers. Leterrier has suggested that cracks are formed under induced stress at microscopic defects such as pinholes in the films and surface defects on the underlying polymer substrates [46]. The cracks then propagate from these defects with an increase in stress. In addition, poor adhesion between individual films might induce cracking and delamination of the films. However, our opinion is that these issues can be resolved by optimization of the fabrication process. The left panel of Fig. 9 shows the OM images of the surface cracks of the OPV devices during the bending tests. For the OPV device fabricated with the ITO anode, the

electrode on a PET substrate. Both electrodes were used as anodes in the OPV devices. We also fabricated conventional OPVs with ITO/glass substrates as a reference and PEDOT:PSS electrode in the same manner, these consisted of a commercial ITO (5 Ω/sq) and PEDOT:PSS (65 Ω/sq) as an anode. Table 2 summarizes the performance of each OPV device. The sheet resistance of the PEDOT:PSS(150 nm)/ITO(80 nm) anode was 45 Ω/sq and that of the ITO (150 nm) anode was 35 Ω/ sq. Even though the ITO thickness of the PEDOT:PSS/ITO hybrid anode is 80 nm, the PEDOT:PSS/ITO(80 nm) anode shows a similar value for the sheet resistance to that obtained with a single-layered ITO anode of 150 nm thickness. Fig. 8 shows the current density– Table 2 Summary of the performance of the fabricated OPV devices.

Reference PET/ITO(150 nm) device PET/PEDOT:PSS/ITO(80 nm) device PET/PEDOT device

Rsheet (Ω/sq)

Jsc Voc (V) FF (mA/cm2)

PCE(%)

5 35 45 62

11.49 10.81 9.93 9.70

4.76 3.80 3.21 2.85

0.75 0.74 0.72 0.72

0.55 0.48 0.45 0.41

Current Density [mA/cm2]

5 Reference PET/PEDOT/80-ITO Device PET/150-ITO Device PET/PEDOT Device

0

-5

-10

-15 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Bias (V) Fig. 8. J–V characteristics of OPV devices using an ITO anode (150 nm), an PEDOT: PSS/ITO(80 nm)/hybrid anode, and PEDOT:PSS anode on PET substrate, and an ITO/ glass substrate under AM 1.5 illumination. The inset figure shows a photograph of a flexible OPV device fabricated on flexible PET substrate using an PEDOT:PSS/ITO hybrid anode.

Normalized PCE

1.0

0.8

0.6

0.4

0.2 ITO(150nm) device PEDOT:PSS/ITO(80nm) device

0.0 25

20

15

10

5

0

Bending Radius (mm) Fig. 9. Bending reliability tests of OPV devices using an ITO anode (150 nm) or an PEDOT:PSS/ITO(80 nm) hybrid anode as a function of the bending radius. An optical microscope (OM) image shows the surface crack initiation of the OPV devices during the bending test.

K. Lim et al. / Solar Energy Materials & Solar Cells 115 (2013) 71–78

transverse surface cracks are observed over the entire width of the OPV device, along a direction perpendicular to that of the loading stresses. These stresses are typically observed on brittle ITO films deposited on polymer substrates when the film is subjected to an external tensile strain. On the other hand, web-shaped cracks are shown for the OPV device with the PEDOT:PSS/ITO anode, and the crack length and crack density are also smaller than those seen for the OPV device with the ITO anode. These results confirm that the PEDOT:PSS layer provides an effective buffering of the brittle ITO film, thereby preventing the propagation of the cracks in the ITO film. In future work, we will study the effects of the barrier layer on flexibility of devices with flexible PEDOT:PSS/ITO hybrid electrode.

4. Conclusion We proposed a highly flexible, transparent, conductive electrode that consists of an ITO and PEDOT:PSS layer. The PEDOT:PSS layer was inserted between the ITO electrode and a PET substrate as a buffer layer in order to improve electrode flexibility without loss of its electrical and optical properties. A 20 nm-thick ITO electrode had a sheet resistance for the ITO electrode of 230 Ω/sq, while the ITO electrode with the PEDOT:PSS layer showed a decreased sheet resistance of up to 85 Ω/sq. The flexibility of the ITO films was proportional to their thicknesses. The mechanical flexibility of the ITO electrode was enormously improved by inserting the PEDOT:PSS layer between the ITO film and the PET substrate. The outer bending test results for the PEDOT:PSS/ITO hybrid electrode showed that the change in the electrical resistance was very small even when bent below 3.5 mm. The fragmentation test results also showed that the PEDOT:PSS/ITO hybrid electrode had a superior stretchability compared to the ITO electrode. No cracks were generated even though the substrate was stretched to a strain of 4%. The crack density, determined from the fragmentation tests, indicated that better cracking resistance and interfacial adhesion strength on a PET substrate was obtained with the PEDOT:PSS/ITO hybrid electrode than with the ITO electrode. The power conversion efficiency of OPV devices fabricated using the PEDOT:PSS/ITO anode was 3.21%, which was similar to that obtained using a single-layered ITO anode. The bending test of the OPV devices showed that better mechanical bending and crack-resistance performance was obtained for an OPV device with an PEDOT:PSS/ITO hybrid anode than with an ITO anode. In particular, the OPV device with the PEDOT:PSS/ ITO anode showed web-shaped cracks, which indicated that the PEDOT:PSS layer provided effective buffering of the brittle ITO film, thereby preventing the propagation of cracks in the ITO film.

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