Transparent Ti-doped In 2 O 3 Films Grown by Linear Facing Target Sputtering for Organic Solar Cells

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1 Journal of the Korean Physical Society, Vol. 63, No. 6, September 2013, pp Transparent Ti-doped In 2 O 3 Films Grown by Linear Facing Target Sputtering for Organic Solar Cells Ju-Hyun Lee and Han-Ki Kim Department of Advanced Materials Engineering for Information and Electronics, Kyung-Hee University, Yongin , Korea (Received 29 April 2013, in final form 20 May 2013) The electrical, optical, and structural properties of Ti-doped In 2O 3 (TIO) films grown by linear facing target sputtering (LFTS) were investigated as a function of the DC power and the annealing temperature for use as a transparent anode in organic solar cells (OSCs). By rapid thermal annealing at 600 C, we obtained optimized TIO anodes with a sheet resistance of Ohm/square and an optical transmittance of 84.88%, which are acceptable for OSC fabrication. The effective Ti dopant activation and the crystallization of the TIO film led to a reduced resistivity of the TIO films. The OSC fabricated on the optimized TIO film showed a higher power conversion efficiency of 2.576% compared to the OSC with as-deposited TIO anodes (1.645%) because the fill factors of the OSCs critically depend on the sheet resistance of the anode. Successful fabrication of OSCs with TIO anodes indicates that the LFTS-grown TIO film is a promising transparent anode for OSCs. PACS numbers: Dq, v, f Keywords: Linear facing target sputtering, Ti-doped In 2 O 3, Organic solar cells, Transparent anodes DOI: /jkps I. INTRODUCTION Bulk heterojunction organic solar cells (OSCs) have been widely researched as cost-effective next-generation energy storage devices following the Si-based thin film solar cells [1 6]. Because OSCs possess several interesting merits, such as light weight, simple structure, superior flexibility, and simple printing-based processing, they have been intensively studied as a substitute for Si thin film solar cells. Recently a high power conversion efficiency near 10% has been shown using an OSC, which is comparable to that for thin film Si solar cells, suggesting that the mass production of OSC will be possible in the near future [7]. In general, conventional OSCs consist of a transparent anode, organic buffer layer, organic active layer, and metal cathode layer. To improve the performance of OSCs, most researchers have developed new active organic materials or buffer layers in spite of the importance of transparent anodes. Although Sn-doped In 2 O 3 (ITO) films have been used as a transparent anode in OSCs, development of a new transparent anode is necessary to realize better OSC performance. For this reason, several transparent conducting oxide films, including In-Zn-O, In-Ge-O, Ti-In-Sn-O, In-Zn-Sn-O, Nb-Ti-O, Ga-Zn-O, In-Mo-O, In-Si-O, Al-Zn-O, and In-W-O films, have been suggested as promising transparent alternatives to the imdlhkkim@khu.ac.kr; Fax: conventional ITO anode [8 18]. In addition, a Ti-doped In 2 O 3 film (TIO) has been considered as a high-mobility transparent conducting oxide (HMTCO) [19 22]. Hest et al. reported that Ti ( at %)-doped In 2 O 3 films yielded excellent TCOs due to their high mobility of 80 cm 2 /V-s and high transmittance of 85% over a wide spectra range, including the near-infrared (NIR) region [19]. Heo et al. also reported that RF-sputtered TIO films had the lowest resistivity of Ohm-cm and a fairly high transmittance of 80% at 450 nm, which is appropriate for dye-sensitized solar cells [20]. Recently, Parthiban et al. reported that 2 at% Ti doped In 2 O 3 film prepared by using spray pyrolysis had a low resistivity of Ohm-cm and a high transmittance of 83% within the 400 and 2,500 nm wavelength region [21]. They suggested that a TIO electrode with high NIR transmittance could be utilized to improve solar cell efficiency. Although the potential of TIO films prepared by using RF/DC sputtering or the pyrolysis method has been reported, detailed investigation of TIO films prepared by linear facing target sputtering (LFTS) are still lacking. The LFTS is known to be a plasma damage-free sputtering technique due to its cathode and substrate geometry [23 26]. In addition, applications of TIO films in OSCs as a transparent anode have not yet been reported. In this work, we report on the characteristics of TIO films prepared by using LFTS for OSC applications as transparent anodes. By optimizing the DC power and the rapid thermal annealing (RTA) temperature, we obtained a TIO film with a resistivity of Ohm

2 Transparent Ti-doped In 2O 3 Films Grown by Ju-Hyun Lee and Han-Ki Kim Fig. 1. (Color online) (a) An actual state picture of the LFTS system for depositing TIO films. (b) Schematics diagram of the LFTS system for depositing TIO films. (c) Picture of a high- density plasma, which is effectively confined between TIO targets. cm and an optical transparency of 84.88%, which are acceptable values for transparent anodes in OSCs. Successful fabrication of an OSC on annealed TIO films indicates that the TIO film is a promising anode to replace the conventional ITO anode in OSCs. II. EXPERIMENTS The 200 nm thick Ti-doped In 2 O 3 (TIO) films were sputtered on glass substrates by using an especiallydesigned LFTS system equipped with rotatable facing cathodes at room temperature at various of DC powers. Using commercial TIO (3 wt% Ti-doped In 2 O 3 : ANP Ltd) targets, TIO films were sputtered in the LFTS system, in which both TIO targets were parallel and faced each other at a target-to-target distance (TTD) of 650 mm, as shown in Fig. 1(a). In the LFTS system, the ladder-type magnet arrays in the linear cathodes created uniform and strong magnetic fields between the TIO targets, as described in Fig. 1(b). As we previously reported, the LFTS has several advantages due to its facing target geometry and substrate location, which prevent the direct irradiation of the plasma on the substrate and effectively reduce plasma damage on the films [23 26]. Using the facing TIO targets, the TIO films were sputtered on a glass substrate with dimensions of mm 2 at a constant Ar/O 2 flow ratio of 20/0.2 sccm and a working pressure of 1.2 mtorr for various DC powers applied to both TIO targets. Figure 1(c) shows a picture of an effectively-confined plasma between TIO targets, in which negative voltage was applied to the TIO targets. During the TIO film sputtering, the TTD and target to substrate distance (TSD) were maintained at 65 and 30 mm, respectively. The TIO films grown at an optimized DC power were rapidly thermally annealed at various temperatures for 1 min under vacuum (35 mtorr). The electrical properties of the TIO films were analyzed by using Hall effect measurements (HL5500PC, Accent Optical Technology). The optical transmittance of the TIO films was measured with a UV/visible spectrometer (UV 540, Unicam). The structure of the TIO films was investigated by using X-ray diffraction (XRD: M18XHF-SRA) for various DC powers and RTA temperatures. To investigate the feasibility of using the optimized TIO film as a transparent anode for OSCs, we fabricated conventional P3HT:PCBM-based bulk heterojunction OSCs were fabricated on TIO films. After the TIO films had been cleaned, the PEDOT:PSS (Clevios PH 510, H. C. Starck) was spin-coated onto the TIO anodes and subsequently annealed at 120 C for 10 min. A blend solution of 50-mg P3HT (Rieke Metals) and 50 mg PCBM (Nano-C) in 2 ml of o-dichlorobenzene (o-dcb) was spin-coated onto the TIO anode in a nitrogen environment. A solvent annealing treatment was then performed for 120 min while keeping the photoactive films inside a covered glass jar. Each sample was subsequently annealed at 110 C for 10 min. Finally, a Ca/Al (20/100 nm) cathode with an area of 4.66 mm 2 was deposited on the photoactive layer by using thermal evaporation. The cathode layer was patterned by using a shadow metal mask. The photocurrent density-voltage (J-V ) curves of the OSCs fabricated on the optimized TIO anodes were measured with a Keithley 1200 measurement unit under 100-mW/cm 2 illumination and AM 1.5 G conditions. High resolution transmission electron microscopy (HRTEM) was also employed to investigate both the microstructure and the interface between the

3 Journal of the Korean Physical Society, Vol. 63, No. 6, September 2013 Fig. 2. (Color online) (a) Sheet resistance and resistivity, and (b) mobility and carrier concentration of as-deposited TIO films grown by using LFTS for various DC powers. (c) Optical transmittance of as-deposited TIO films as a function of the DC power. (d) Figure of merit value of as-deposited TIO films calculated from the transmittance and the sheet resistance. TIO and P3HT:PCBM active layer. III. RESULTS AND DISCUSSION To determine the optimum DC power for the asdeposited TIO films, we measured their electrical and optical properties. Figure 2(a) shows the sheet resistance and resistivity of an as-deposited TIO film for increasing DC power. At low DC powers below 450 W, the TIO films showed a fairly high sheet resistance and resistivity. However, an increase in DC power abruptly decreased the sheet resistance and resistivity. At a DC power of 550 W, the TIO film showed the lowest sheet resistance of Ohm/square and a resistivity of Ohm-cm. Due to absence of substrate heating, the as-deposited TIO films showed a higher sheet resistance and a higher resistivity than previously values reported for the TIO films [19 22]. The decreased resistivity of the TIO films with increasing DC power could be explained by the increased carrier mobility, as shown in Fig. 2(b). The increase in the DC power resulted in an increase in the carrier mobility from 2.47 to 8.49 cm 2 /V-s. In general, an increase of DC power during sputtering is well known to yield an increase in the film s density, which is closely related to the carrier mobility. Therefore, the low resistivity of the as-deposited TIO film prepared at 550 W could be attributed to the increase in the carrier mobility caused by improvement of film density. Figure 2(c) demonstrates the optical transmittance of the as-deposited TIO films with increasing DC power. The upper panels show pictures of TIO films grown on glass substrates with increasing DC power from 250 to 600 W. All as-deposited TIO films showed a similar optical transmittance near 80% at wavelengths between 400 and 800 nm. As shown in Fig. 2(d), the TIO film prepared at a DC power of 350 W showed the highest average optical transmittance of 82.61% between 400 and 800 nm, which corresponds to the absorption wavelength of the P3HT:PCBM active layer. However, a further increase in DC power led to a slightly decreased optical transmittance of the TIO film, which is consistent with the TIO sample images in the upper panel of Fig. 2(c). Figure 2(d) shows the figure of merit value (T 10 /R sh ) for the as-deposited TIO films on calculated from the

4 Transparent Ti-doped In 2O 3 Films Grown by Ju-Hyun Lee and Han-Ki Kim Fig. 3. (Color online) (a) Sheet resistance and resistivity, and (b) mobility and carrier concentration of LFTS grown TIO films with increasing RTA temperature. (c) Optical transmittance of TIO films as a function of RTA temperature. (d) Figure of merit value of annealed TIO films calculated from the transmittance and the sheet resistance. sheet resistance (R sh ) and the transmittance (T) for increasing DC power. The better quality TCO films showed a higher figure of merit value due to their high optical transmittance and low sheet resistance [27]. Therefore, it is generally possible to determine the optimum condition of TCO via the highest figure of merit value. In the case of the as-deposited TIO film, an increase in the DC power to 450 W resulted in a higher figure of merit value (4.16 Ohm 1 ). Further increases in the DC power, however, reduced the figure of merit value due to the decreased optical transmittance. Therefore, we determined the optimum DC power to prepare a TIO film as 450 W. To decrease the sheet resistance and increase the optical transmittance of the TIO film grown at a DC power of 450 W, we carried out post annealing at various RTA temperatures. Figure 3(a) shows the sheet resistance and the resistivity of a TIO film for increasing RTA temperature. Like other TCO films, the TIO film showed reduced sheet resistance and resistivity with increasing RTA temperature. At a RTA temperature of 600 C, the TIO film showed the lowest sheet resistance of Ohm/square and the lowest resistivity of Ohm-cm, which are comparable to those of ITO films. The decreased resistivity of the annealed TIO films could be attributed to the increased carrier concentration and mobility, as shown in Fig. 3(b). Because the RTA process resulted in the activation of Ti dopants and the crystallization of the TIO films, the 600 C-annealed TIO film had a higher carrier concentration ( cm 3 )and mobility (30.3 cm 2 /V-s) compared to the as-deposited TIO film. Figure 3(c) demonstrates the optical transmittance of the TIO film as a function of RTA temperature. The upper panels also showed the transparency of the annealed TIO films. Compared to the as-deposited TIO sample in Fig. 2(c), the annealed TIO film demonstrates a higher optical transmittance, regardless of the RTA temperature. In particular, the annealed TIO films show a much higher transmittance than conventional ITO films in the NIR region. In general, ITO films showed a fairly low optical transmittance in the NIR region due to the high density of free carrier [28]. However, it was noteworthy that the TIO film showed a high NIR transmittance

5 Journal of the Korean Physical Society, Vol. 63, No. 6, September 2013 Fig. 4. (Color online) (a) XRD plots of the as-deposited TIO films for various DC powers applied to the TIO targets. (b) XRD plots of TIO films for various value of the RTA temperature. even though it had a resistivity similar to these of the ITO films. To determine the optimum RTA temperature, we also calculated the figure of merit value for the TIO films with increasing RTA temperature as shown in Fig. 3(d). Due to the high transparency of the TIO films, the figure of merit value for the TIO film was critically affected by the sheet resistance. Therefore, the TIO film annealed at 600 C exhibited the highest figure of merit value of 7.44 Ohm 1. Thus, the 600 C-annealed TIO film prepared at a DC power of 450 W was found to be as the optimized TIO sample. Figure 4(a) shows the XRD plot of as-deposited TIO films for various of DC powers applied in TIO targets. All XRD plots showed a broad halo pattern, indicating that the LFTS-grown TIO films had a completely amorphous structure. During the LFTS process, the TIO film was not directly exposed to the plasma, and the substrate temperature was maintained at temperatures Fig. 5. (Color online) (a) Schematic fabrication process of OSCs with optimized TIO anodes. (b) Cross-sectional TEM images of OSCs with optimized TIO anodes. (c) Current density - voltage characteristics of OSCs fabricated with asdeposited and optimized TIO anodes. below 50 C. Thus, all as-deposited TIO films shows an amorphous feature even though the sample was grown at a high DC power. Figure 4(b) showed XRD plots of the LFTS-grown TIO film at a DC power of 450W for various RTA temperatures. All annealed TIO films showed similar XRD plots, regardless of the RTA temperature. The XRD plots of the annealed TIO films exhibited several peaks, 2θ =30.78 (222), (400), (440), and (622), indicative of the annealed TIO films having bixbyite structures. All TIO films can be seen to have had a (222) preferred orientation, regardless of the RTA temperature, as is the case with conventional ITO films. As discussed by Thilakan et al., the addition of oxygen gas to the argon plasma during ITO deposition could enhance the (222) plane due to restoration of the ITO stoichiometry [29]. To investigate the feasibility of using TIO films as transparent anodes for OSCs, we fabricated conventional P3HT:PCBM-based OSCs by using the optimized TIO films with a sheet resistance of Ohm/square and an average transmittance of 84.88%. Figure 5(a) shows a schematic of the fabrication process of an OSC on a TIO

6 Transparent Ti-doped In 2O 3 Films Grown by Ju-Hyun Lee and Han-Ki Kim Table 1. Performances of OSCs fabricated with asdeposited and 600 C-annealed TIO anodes. FF J sc V oc PCE (%) (ma/cm 2 ) (V) (%) As-deposited TIO Annealed TIO anode. Firstly, the TIO anode was patterned by using a thin shadow mask during the LFTS process. After patterning of the TIO anode, the PEDOT:PSS and the organic active (P3HT:PCBM) layers were spin-coated on the patterned TIO anodes. After removing the edges of the organic layers with a DCB solution to achieve electrical contact with the TIO anode, a Ca/Al multilayer cathodewithanareaof4.66mm 2 was patterned on the organic active layer. Figure 5(b) shows a cross-sectional TEM image of the OSC fabricated on the TIO anode. The cross-sectional image clearly demonstrates a well-defined TIO anode, organic active layer and Al:Ca cathode layer without interfacial reactions. As confirmed by XRD examination, the annealed TIO film showed a well-developed columnar structure with (222) preferred orientation. In addition, the sharp interface between the TIO anode and the PE- DOT:PSS layer indicates the stability of the TIO anode against the acidic PEDOT:PSS solution. Figure 5(c) shows the current density-voltage (J-V ) curves of an OSC fabricated on as-deposited and 600 C annealed TIO anode with the inset showing the OSC sample picture. Detailed information regarding the power conversion efficiency (PCE, η), short circuit current density (J sc ), open circuit voltage (V oc ), and fill factors (FF) were calculated from the J-V curves in Fig. 5(c). Detailed information regarding η, J sc,v oc,andff of the OSC with as-deposited and annealed TIO anodes is presented in Table 1. The OSC fabricated on the asdeposited TIO anode showed a V oc of 0.51 V, a J sc of ma/cm 2, a FF of 39.86% and a PCE of 1.64%. However, the OSC with the 600 C-annealed TIO anode exhibited better performance, i.e., av oc of 0.58 V, aj sc of ma/cm 2, a FF of 54.23% and a PCE of 2.576%. In our previous research, we reported that the sheet resistances of IZTO anodes with different thicknesses critically affected the performance of OSCs [30]. Like the IZTO anode, the sheet resistance of the TIO anode critically affected the performance of OSCs. The series resistance of OSCs critically affects the slope of the J-V curve at J=0 ma/cm 2. The higher slope for the OSC with an annealed TIO anode compared to that of the OSC with an as-deposited TIO anode indicates that the sheet resistance and the contact resistance between the organic layer and the TIO anode were reduced by means of the RTA process at 600 C. The FF and the J sc of the OSC critically depended on the series resistance and the transmittance of the transparent electrode. Therefore, the improvement of the FF and the J sc of the OSCs with the annealed TIO anode could be attributed to the low sheet resistance and the high transparency of the annealed TIO anode. IV. CONCLUSION The effects of the DC power and the RTA temperature on the properties of LFTS-grown TIO films were investigated for applications in OSCs as transparent anodes. We found that the 600 C annealing of the TIO anode resulted in a sheet resistance of Ohm/square, and an optical transmittance of 84.88%, which are comparable to the values for conventional ITO anodes. In particular, the TIO film shows a higher transmittance in the NIR region due to a low free-carrier concentration. In addition, the TIO film showed a fairly smooth surface regardless of RTA temperature, indicating stable surface properties. The OSCs fabricated on a 600 CannealedTIO anode showed better performance than the OSC with an as-deposited TIO anode due to the reduced sheet resistance and the increased optical transmittance. ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (no ) funded by the Ministry of Knowledge Economy (MKE), Korea. REFERENCES [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995). [2] C. J. Brabec, V. Dyakonov and U. Scherf, Organic Photovoltaics: Materials, Device Physics and Manufacturing Technologies, (Wiley-VCH, Weinheim, 2008). [3] K. Norman, M. V. Madsen, S. A. Gevorgyan and F. C. Krebs, Society 132, (2010). [4] S. R. Dupont, M. Oliver, F. C. Krebs and R. H. Dauskardt, Sol. Energy Mater. Sol. Cells 97, 171 (2012). [5] F. C. Krebs, Sol. Energy Mater. Solar Cells 93, 394 (2009). [6] T. Ameri, G. Dennler, C. Lungenschmied and C. J. Brabec, Energy Environ. Sci. 2, 347 (2009). [7] NEWS & ANALYSIS, Science 332, 293 (2011). [8] C.-K. Cho and H.-K. Kim, J. Nanosci. Nanotechnol. 12, 3346 (2012). [9] S.-B. Kang, J.-W. Lim, S. H. Lee, J.-J. Kim and H.-K. Kim,J.Phys.D:Appl.Phys.45, (2012). [10] J.-W. Lim, S. J. Kang, S. H. Lee, J.-J. Kim and H.-K. Kim, J. Appl. Phys. 112, (2012). [11] J. Kim, S.-I. Na and H.-K. Kim, Sol. Energy Mater. Sol. Cells 98, 424 (2012).

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