Electrochemical and Solid-State Letters, 13 共5兲 J39-J42 共2010兲

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1099-0062/2010/13共5兲/J39/4/$28.00 © The Electrochemical Society

Stacking Sequence Effect on the Electrical and Optical Properties of Multistacked Flexible IZO–Ag–IZO Electrodes Yong-Seok Park,a Kwang-Hyuk Choi,a Han-Ki Kim,a,z and Jae-Wook Kangb a

Department of Advanced Materials for Information and Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, South Korea Department of Material Processing, Hybrid Coating Group, Korea Institute of Material Science, Changwon 641-831, South Korea

b

We investigated the stacking sequence effect of indium zinc oxide 共IZO兲–Ag and IZO–Ag–IZO on the characteristics of multistacked flexible transparent electrodes for organic photovoltaics. In spite of the similar sheet resistances of the IZO–Ag and IZO–Ag–IZO stacked electrodes, the optical transparency of the latter is much higher than that of the former in the wavelength region of 400–800 nm due to the effective antireflection in the symmetric oxide–Ag–oxide structure. Furthermore, the flexible organic solar cells 共OSCs兲 fabricated on the IZO–Ag–IZO-stacked multilayer electrode showed a higher power conversion efficiency than those fabricated on the IZO–Ag–stacked multilayer electrode due to the higher optical transparency of the former electrode in the main absorption region of the poly共3-hexylthiophere兲 and 1-共3-methoxycarbonyl兲-propyl-1-phenyl-共6,6兲 C61 layers. This indicates that the multistacked electrode with the IZO–Ag–IZO sequence is more beneficial than that with the IZO–Ag sequence for low resistance and high transparency electrodes in flexible OSCs. © 2010 The Electrochemical Society. 关DOI: 10.1149/1.3312755兴 All rights reserved. Manuscript submitted December 9, 2009; revised manuscript received January 13, 2010. Published February 18, 2010.

Flexible organic photovoltaics have recently attracted much attention as eco-friendly and cost-efficient energy converting devices.1-3 In flexible organic solar cells 共OSCs兲, the flexible transparent conducting oxide 共TCO兲 electrode with a low sheet resistance and high optical transmittance is one of the most important components influencing their performance because the light transmission into the active layer and the carrier correction are critically affected by the optical and electrical properties of the TCO layer. In particular, flexible TCO electrodes with a very low sheet resistance comparable to that of metallic electrodes are required in large-area flexible OSCs to reduce their resistance.4,5 Until now, indium tin oxide 共ITO兲-coated flexible substrate has been employed as flexible anode materials in the roll-to-roll-based fabrication of large-area polymer solar cells even though it has a fairly high sheet resistance due to the absence of high quality TCO electrodes.6,7 For these reasons, several strategies, such as the proper design of the electrode and the use of metallic strips and TCO/Ag/TCO multilayer electrodes, have been investigated to lower the resistance of the transparent electrodes.8-11 In our previous works, we also suggested that Sn-doped In2O3 共ITO兲/Ag/ITO, Zn-doped In2O3 共IZO兲/Ag/IZO, Zn, Sn co-doped In2O3 共IZTO兲/Ag/IZTO, Ga-doped ZnO 共GZO兲/Ag/GZO, and Aldoped ZnO 共AZO兲/Ag/AZO multilayer electrodes would be promising low resistance electrodes for OSCs.12-14 Although the possibility of using a TCO/Ag/TCO electrode in OSCs is well documented, the effect of the metal oxide stacking sequence on the electrical and optical properties of the multistacked flexible transparent electrode in flexible OSCs has not yet been investigated in detail. In this work, we compared the electrical and optical properties of multistacked flexible electrodes with difference in stacking sequences, viz., IZO/Ag/IZO/Ag/IZO and IZO/Ag/IZO/IZO/Ag/IZO, for use in flexible OSCs. The optical transmittance of the multistacked flexible electrodes critically depends on the stacking sequence in spite of their identical sheet resistance and resistivity. The performance of the flexible OSCs is mainly affected by the transmittance of the multistacked flexible electrode because the stacking sequence mainly affects their optical properties. The multistacked flexible and transparent electrodes were continuously deposited on flexible polyethersulfone 共PES兲 substrates using a roll-to-roll sputtering system with multicathodes at room temperature.15 The PES substrate with a thickness of 200 ␮m was passed repeatedly over the cooling drum by the motion of the unwind and rewind rollers for the continuous deposition of the multistacked flexible electrodes. According to the stacking sequence of

z

E-mail: [email protected]

the IZO and Ag layers, two types of multistacked transparent electrodes were fabricated at room temperature. The first type consists of a repeating IZO/Ag/IZO stack, such as the two stacked IZO/Ag/ IZO/IZO/Ag/IZO and three stacked IZO/Ag/IZO/IZO/Ag/IZO/IZO/ Ag/IZO electrodes. The second type consists of alternating IZO and Ag layers, such as IZO/Ag/IZO/Ag/IZO and IZO/Ag/IZO/Ag/IZO/ Ag/IZO. For simplicity, the unit sequence of the IZO–Ag–IZO or the IZO–Ag layer is referred to herein as IAI or IA, respectively. In the multistacked transparent electrode, the IZO layer with a thickness of 40 nm was deposited at a constant dc power of 800 W, an Ar/O2 flow ratio of 30/1 sccm, and a working pressure of 3 mTorr using an IZO target with a composition of 10 wt % ZnO + 90 wt % In2O3. In addition, a 12 nm thick Ag layer was sputtered onto the IZO layer at a constant dc power of 800 W, an Ar/O2 flow ratio of 30/1 sccm, and a working pressure of 3 mTorr using a metallic Ag target. Before the sputtering of the bottom IZO layer, the surface of the PES substrate was pretreated by an Ar ion beam at a dc pulsed power of 200 W to enhance the adhesion between the bottom IZO layer and the PES substrate and to remove the carbonrelated contamination layer. The resistivity and optical transmittance of the multistacked electrodes with different stacking sequences were measured by Hall measurements and UV/visible spectroscopy, respectively. X-ray diffraction 共XRD兲 and transmittance electron microscopy 共TEM兲 were employed to investigate the structure of the multistacked electrodes. Finally, flexible OSCs were fabricated on the various types of multistacked electrodes to correlate the electrode properties and performance of the flexible OSCs. The detailed fabrication process of the flexible OSCs was explained in our previous works.12-15 Figure 1 shows the cross-sectional TEM images of the multistacked electrodes with different stacking sequences and the high resolution electron microscopy 共HREM兲 images at the interface. The IAIAI 共Fig. 1a兲 and IAIAIAI 共Fig. 1b兲 electrodes clearly revealed well-defined IZO and Ag layers without the formation of an interface layer. All the IZO layers in the IAIAI and IAIAIAI electrodes exhibit a uniform contrast, indicating that the roll-to-roll sputtergrown IZO layers have an amorphous structure, as confirmed by the HREM images in Fig. 1c. However, the Ag layer sandwiched between the IZO layers showed a crystalline structure with a 共111兲 preferred orientation, as expected from the XRD result 共not shown here兲, even though it was sputtered at room temperature. Regardless of their position in the multistacked electrode, all of the IZO and Ag layers had completely amorphous and crystalline structures, respectively. The IZO and Ag layers in the IAIIAI and IAIIAIIAI electrodes also had similar amorphous and crystalline structures, as shown in Fig. 1d and e, respectively, as confirmed by the XRD

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Electrochemical and Solid-State Letters, 13 共5兲 J39-J42 共2010兲

Ag

(d) Ag

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IZO IZO

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Stacked layer number Figure 2. 共Color online兲 Sheet resistance and resistivity of the single IAI electrode and multistacked flexible electrode grown by roll-to-roll sputtering on the PES substrate with increasing stacking number.

IZO Ag

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Figure 1. Cross-sectional TEM images of the 共a兲 IAIAI, 共b兲 IAIAIAI, 共d兲 IAIIAI, and 共e兲 IAIIAIIAI electrodes. 共c兲 and 共f兲 are the cross-sectional HREM images obtained from the interface regions of the IAIIAI and IAIAI electrodes, respectively.

examination. In addition, the sharp interfaces in Fig. 1c and f indicate that there is no interface reaction or formation of an interfacial oxide layer between the IZO and Ag layers due to the use of a continuous and low temperature roll-to-roll sputtering process without breaking the vacuum. Figure 2 shows the sheet resistances and resistivities of the multistacked flexible electrodes with increasing number of stacking layers. In the single flexible IAI layer, it shows a low sheet resistance and resistivity of 6.15 ⍀/䊐 and 5.53 ⫻ 10−5 ⍀ cm, respectively, due to the existence of the low resistivity Ag layer, even though it was sputtered at room temperature. With increasing stacking number, the samples showed monotonically decreased sheet resistance and resistivity values regardless of the stacking sequence. In spite of their different stacking sequences, both the flexible IAIAI and IAIIAI electrodes showed similar sheet resistances of 2.52 ⍀/䊐 and resistivities of 4.54 ⫻ 10−5 ⍀ cm due to the additional insertion of two Ag layers. Further increasing the stacking number to produce the structures, IAIAIAI and IAIIAIIAI, led to a decrease in the sheet resistance and resistivity to 1.62 ⍀/䊐 and 4.6 ⫻ 10−5 ⍀ cm, respectively. Despite the process temperature and total thickness, the sheet resistances of the flexible IAIAIAI and IAIIAIIAI electrodes were much lower than that of the conventional amorphous ITO electrode. Considering the ohmic loss through the TCO layer in the large-area OSCs, the TCO has a low sheet resistance of less than 1 ⍀/䊐 for high efficient flexible OSCs. Figure 3 shows the dependence of the optical transmittance of the multistacked flexible electrode on the stacking number with insets of pictures showing a comparison of the optical transmittance. The optical transmittance of the single flexible IAI layer at wavelengths in the range of 400–600 nm corresponding to the absorption

region of the active layer 关poly共3-hexylthiophere兲 共P3HT兲:1-共3methoxycarbonyl兲-propyl-1-phenyl-共6,6兲 C61 共PCBM兲兴 is very high due to the antireflection effect on the Ag layer sandwiched between the IZO layers.14 However, the variation in the optical transparency of the multistacked flexible electrode with the stacking number depends on the stacking sequence. In the IAI stacking sequence in Fig. 3a, the electrodes show a slight decrease in their optical transparency with increasing stacking number, especially in the wavelength range of 400–600 nm, as listed in Table I. The flexible IAIIAIIAI electrode showed an average transmittance of 65.08% in the wavelength range of 400–600 nm and a specific transmittance of 75.59% at 550 nm. However, the IAIAI and IAIAIAI electrodes with the IA stacking sequence showed much lower optical transmittances than the single IAI electrode, as shown in Fig. 3b. The flexible IAIAIAI electrode showed the minimum average optical transmittance of 49.65%, which is a much lower value than that of the IAIIAIIAI electrode even though it has a low sheet resistance. Furthermore, the multistacked electrodes in both Fig. 3a and b showed a shift in transmittance in the IR region with increasing stacking number, which is well matched with the simulation results 共not shown here兲. This shift at the IR end of the spectrum in the multistacked flexible electrodes is attributed to the plasma oscillation of the free carriers that screen the incident electromagnetic wave visible intraband transitions within the conduction band.16 The optical transmittance drops sharply near the plasma resonance frequency, which is dependent on the carrier density of the electrode. Using the sheet resistance 共Rsh兲 and optical transmittance 共T兲, the figure of merit values of the multistacked flexible electrodes were calculated using the equation 共T10 /Rsh兲 suggested by Haacke and are listed in Table I to correlate them with the performances of the OSCs.17 Due to the much higher optical transmittance of the IAI-stacked electrodes, they showed a much higher figure of merit values than the IAstacked electrodes. To compare the performances of the flexible OSCs fabricated on the multistacked flexible electrodes with different stacking sequences, large active area 共1 ⫻ 1 cm兲 bulk-heterojunction OSCs based on P3HT and PCBM were fabricated, as shown in the inset of Fig. 4. After the conventional electrode cleaning, poly共3,4ethylenedioxythiophene兲:poly共styrenesulfonate兲 共PEDOT:PSS兲 was

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Electrochemical and Solid-State Letters, 13 共5兲 J39-J42 共2010兲

IZO/Ag/IZO IZO/Ag/IZO/IZO/Ag/IZO IZO/Ag/IZO/IZO/Ag/IZO/IZO/Ag/IZO

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Figure 4. 共Color online兲 Current density–voltage characteristics of flexible OSCs fabricated on multistacked flexible electrodes with 共a兲 IAI and 共b兲 IA stacking sequences with the inset picture showing the flexibility of the OSCs.

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IZO/Ag/IZO IZO/Ag/IZO/Ag/IZO IZO/Ag/IZO/Ag/IZO/Ag/IZO

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IZO/Ag/IZO IZO/Ag/IZO/IZO/Ag/IZO IZO/Ag/IZO/IZO/Ag/IZO/IZO/Ag/IZO

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Wavelength [nm] Figure 3. 共Color online兲 Optical transmittance of the multistacked flexible electrode with 共a兲 IAI and 共b兲 IA stacking sequences with the insets showing the sample pictures. Table I. Characteristics of the single IAI electrode and multistacked flexible electrodes with different stacking sequences and detailed performance of flexible OSCs fabricated on the single and multistacked flexible electrodes. [1␾TC and 2␾TC correspond to the figure of merit values calculated from the average transmittance T (400–600 nm) and the transmittance at a wavelength of 550 nm, respectively.] Single

Rsh 共⍀/䊐兲 Average T 共%兲 at 400–600 nm T 共%兲 at 550 nm 1 ␾TC 共10−3 ⍀−1兲 2 ␾TC 共10−3 ⍀−1兲 Voc 共V兲 Jsc 共mA/cm2兲 FF 共%兲 PCE 共%兲

Multistacked flexible electrode

IAI

IAIIAI

IAIIAIIAI

IAIAI

IAIAIAI

6.15 81.58 86.80 21.23 39.47 0.54 7.95 58 2.52

2.52 72.79 81.21 15.70 46.91 0.54 7.50 57 2.31

1.62 65.08 75.59 8.52 38.09 0.54 6.32 61 2.10

2.68 56.32 59.26 1.21 2.01 0.52 5.53 60 1.75

1.64 53.45 49.65 1.16 0.55 0.54 4.97 63 1.69

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Electrochemical and Solid-State Letters, 13 共5兲 J39-J42 共2010兲

spin-coated on the multistacked electrodes followed by drying at 150°C for 10 min in N2 ambient. A blend solution consisting of 20 mg of P3HT and 20 mg of PCBM in 1 mL of 1,2-dichlorobenzene was spin-coated on top of the PEDOT:PSS layers in a N2 atmosphere. Then, a solvent-annealing process was performed in a Petridish, forming an active layer with a thickness of ⬃230 nm. To enhance the crystallinity of the active layer, a thermal annealing process was performed using a hot plate in a glove box at 150°C for 20 min. Finally, a LiF共1.2nm兲/Al共100 nm兲 cathode was patterned on the P3HT:PCBM active layer using a thermal evaporator system with a metal shadow mask, as shown in the inset of Fig. 4. The inset pictures show the flexible OSCs fabricated on the multistacked electrodes 共IAIIAI兲. Figure 4a and b shows the photocurrent density– voltage 共J-V兲 curves of the flexible OSCs measured under 100 mW/cm2 illumination with a AM 1.5 G condition. The detailed performance of the flexible OSCs is summarized in Table I. In the flexible OSCs fabricated on the IAI, IAIIAI, and IAIIAIIAI electrodes, their power conversion efficiency 共PCE兲 is mainly dependent on the optical transmittance. In spite of the IAIIAIIAI electrode showing the highest fill factor, due to its having the lowest sheet resistance, the OSCs with this electrode showed a lower PEC than that with the single IAI electrode due to the low optical transmittance of the former electrode. Therefore, it is obvious that matching the transmittance of the multistacked electrode with the absorption region of the active materials is a more important factor than the sheet resistance in determining the performance of the flexible OSCs. Due to the dominance of the optical transmittance of the multistacked electrode, the PCE of the flexible OSCs decreased in proportion to the figure of merit value of the electrodes, as shown in Table I. The performances of the flexible OSCs fabricated on the IAI, IAIAI, and IAIAIAI electrodes also showed a similar dependence on the optical transmittance of the multistacked electrode, as shown in Fig. 4b. Compared to the PCE 共2.52%兲 of the flexible OSC fabricated on the single IAI electrode, those of flexible OSCs fabricated on the IAIAI and IAIAIAI electrodes abruptly showed the decreased PCE values of 1.75 and 1.69%, respectively. As expected from Fig. 3b, the abruptly increased optical transmittance of the multistacked electrode caused by the additional insertion of a Ag layer was responsible for the decreased PCE of the OSCs. Although the IAIIAIIAI 共1.62 ⍀/䊐兲 and IAIAIAI 共1.64 ⍀/䊐兲 electrodes showed similar sheet resistances, the PCE 共2.10%兲 of the OSC fabricated on the IAIIAIIAI electrode was higher than that 共1.69%兲 of the OSC fabricated on the IAIAI electrode probably due to the large difference in their optical transparency. Therefore, it is concluded that the IAI stacking sequence 共IAIIAI and IAIIAIIAI兲 is a better choice for use in the multistacked flexible electrodes than the IA

stacking sequence 共IAIAI and IAIAIAI兲 to obtain a low sheet resistance without a large loss of the optical transmittance for large-area flexible OSCs. In summary, we report the effect of different stacking sequences, viz., IAI and IA, on the electrical and optical properties of multistacked electrodes for large active area OSCs. In spite of the sheet resistances of the IAIIAI and IAIIAIIAI electrodes being similar to those of the IAIAI and IAIAIAI electrodes, respectively, the optical transparency of the IAI-stacked electrode is much higher than that of the IA-stacked electrode in the wavelength region of 400–80 nm due to the effective antireflection in the symmetric oxide–Ag–oxide structure. Due to its higher optical transmittance, the OSC fabricated on the IAI multistacked electrode showed a higher PCE than that fabricated on the IA multistacked electrode. This indicates that the multistacked electrode with the IAI sequence is more beneficial than that with the IA sequence to obtain a low sheet resistance for largearea OSCs. Acknowledgments This work was supported by the Korea Foundation for International Cooperation of Science and Technology 共KICOS兲 through a grant provided by the Korean Ministry of Education, Science and Technology 共MEST兲 in K20701010289-07B0100-07511. Kyung Hee University assisted in meeting the publication costs of this article.

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