APPLIED PHYSICS LETTERS 98, 023102 共2011兲
Influence of surface roughness of aluminum-doped zinc oxide buffer layers on the performance of inverted organic solar cells Sung-Woo Cho,1 Young Tae Kim,1 Won Hyun Shim,1,2 Sun-Young Park,1 Kwang-Dae Kim,3 Hyun Ook Seo,3 Nilay Kumar Dey,3 Jae-Hong Lim,1 Yongsoo Jeong,1 Kyu Hwan Lee,1 Young Dok Kim,3,a兲 and Dong Chan Lim1,a兲 1
Materials Processing Division, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea 2 School of Materials Science and Engineering, Pusan National University, San 30 Jangjeon-dong Geumjeong-gu, Pusan 609-735, Republic of Korea 3 Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
共Received 10 November 2010; accepted 19 December 2010; published online 10 January 2011兲 Aluminum-doped zinc oxide 共AZO兲 films 共70 nm thick兲 with dissimilar surface roughness were created on indium tin oxide coated glass and were used as electrodes for inverted organic solar cells. The photovoltaic performance of the devices depended strongly on the surface roughness of the AZO films. Increases in the surface root-mean-square roughness of AZO films from 2.5 to 10.9 nm enhanced power conversion efficiency from 0.5% to 1.4% due to increased contact area between electrode and active layer. © 2011 American Institute of Physics. 关doi:10.1063/1.3537961兴 There exists significant interest in bulk heterojunction organic solar cells 共OSCs兲 due their cost-effectiveness and potential application in flexible devices.1–4 Much effort has been made for improving power conversion efficiency 共PCE兲 of OSCs, which has recently reached over 7%.5 A drawback of the conventional OSCs is its low stability due to easy 共photo-兲 degradation of various interfaces involved in the device.6,7 As an alternative, highly efficient OSCs using an inverted structure were demonstrated, in which the positions of the anode and cathode are reversed. In this structure, electrons created in the active layer are injected into the transparent conducting oxide, which in most cases is indium tin oxide 共ITO兲.8,9 Such an inverted structure significantly improves the air stability of the solar cell. Furthermore, vertical phase separation in metal oxide nanowires/polymer blends has proven to be advantageous in the inverted structure.8,9 Diverse approaches have been used for improving photovoltaic performance of OSC. Specifically, increases in the mobility of electrodes, improvements in electron-hole separation, and enhanced transmittance of light in electrodes have been achieved by using various functional materials.10,11 Recently, we have shown that the surface roughness control of electrodes can be a crucial factor for photovoltaic performance. By incorporation of single-walled carbon nanotubes 共SWCNT兲 in ZnO buffer layers on ITO, an enhancement in photovoltaic performance was found, and this result was partly attributed to the enhanced roughness of ZnO surfaces by SWCNTs.12 In the present work, we fabricated inverted organic solar cells 共IOSCs兲 consisting of aluminum-doped zinc oxide 共AZO兲 buffer layers between ITO and the P3HT active layer. The AZO surface roughness was controlled without varying the film thickness, and the influence of surface roughness of AZO on the photovoltaic performance of the IOSCs was investigated. The short-circuit current 共Jsc兲, open-circuit voltage 共Voc兲, fill factor 共FF兲, and PCE of IOSCs were shown to a兲
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increase as the surface roughness of AZO increased. Thus, the importance of electrode surface morphology control for obtaining better performance of photovoltaic devices is demonstrated. AZO films with thicknesses of 70, 90, 110, 140, and 330 nm, respectively, were deposited on an ITO 共10 ⍀ / square兲-coated glass substrate by rf magnetron sputtering. At a constant rf power of 50 W, an Ar flow rate of 20 cm3 / min and working pressure of 7 ⫻ 10−3 mbar, a 3 in. AZO target 共2 wt % Al2O3-doped ZnO兲 was sputtered at room temperature. After sputter-deposition of AZO buffer layers, AZO films were wet-etched using 0.1% of HCl solution 共etching rate: 1.55 nm/s兲 with different etching times 共0, 8, 16, 32, and 160 s兲 using a dipping method.13,14 It is worth pointing out that different initial film thickness combined with various etching resulted in the same film thickness with diverse roughness. The final thickness of all etched-AZO films was fixed at 70 nm, as determined by scanning electron microscopy and alpha-step profilometry. The mobility of etched-AZO共70nm兲/ITO共150 nm兲 films was determined by Hall measurement with van der Pauw geometry at room temperature by means of a four point probe. Transmittance of the etched-AZO/ITO films was measured in the wavelength range from 200 to 800 nm by a UV/visible spectrometer. In addition, the surface morphology of etched-AZO/ITO films was analyzed by atomic force microscopy 共AFM兲. For the optical characterization 共photoluminecence兲 of the samples, a QM-4/2500SE with a laser line of 280 nm was used at room temperature. IOSCs were composed of a stack of Ag/NiOx/ photoactive layer 共P3HT:PCBM兲/AZO/ITO coated glass. P3HT and PCBM were purchased from Rieke Metals, Inc. and Sigma-Aldrich, respectively. A mixture of P3HT:PCBM 共1:1兲 in 1,2-dichlorobenzene 共concentration: 50 mg/ml兲 was vigorously stirred overnight. Then, this solution was spincoated at 1000 rpm for 1 min onto the etched-AZO films on ITO coated glass substrates. After deposition of these layers, the samples were treated at 60 ° C under atmospheric conditions for 10 min. The fabrication of photovoltaic cells was
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© 2011 American Institute of Physics
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Appl. Phys. Lett. 98, 023102 共2011兲
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FIG. 1. 共Color online兲 共a兲 Sheet resistance and resistivity and 共b兲 optical transmittance of an etched-AZO and nonetched-AZO films as a function of etching time.
completed by additionally depositing a NiOx layer via spincoating 共2000 rpm兲. The NiOx layer was prepared by using NiO powder 共5 ⫻ 10−4M兲 with a mean diameter of 50 nm 共reagent-grade chemical兲 dissolved in isopropyl alcohol, which acts as an electron blocking and hole transporting layer into the metal electrode.15 Finally, a Ag electrode was deposited on the cell by the evaporation method under a low pressure of 10−6 mbar. It is worth emphasizing that, except for the electrode evaporation, all the fabrication procedures of the present work took place in solution under atmospheric pressure. The active area of the device was 0.38 cm2. The final structure of the photovoltaic cell was annealed at 150 ° C under atmospheric pressure for 10 min. Efficiencies of the solar cells were tested using a solar simulator illuminated with a photointensity of 100 mW/ cm2 共AM 1.5 spectrum兲. Figure 1 summarizes the sheet resistance and resistivity 共a兲 and transmittance of the etched-AZO films 共b兲 on ITOcoated glass with different etching times. Similar sheet resistances of about 10.7 ⍀ / square and resistivities of 2.37 ⫻ 10−4 ⍀ cm were found for all the samples studied here, Fig. 1共a兲. In addition, optical transmittance of differently etched-AZO films is also analogous with an average transmittance value of 77.5%–78.8% in the wavelength range of 550–650 nm. It is worth mentioning that the optical transmittance of the electrode in this wavelength range is crucial for the performance of OSCs.16,17 One can conclude that the differently etched-AZO films with the same final thickness showed similar electrical and optical properties regardless of etching time. Figure 2 shows AFM images of variously etched-AZO films on ITO-coated glass. It can be clearly seen that the surface roughness increased significantly as a function of etching time. When the etching time was increased from 0 to 160 s, the root-mean-square 共rms兲 roughness was increased
FIG. 2. 共Color online兲 AFM image of an etched-AZO and nonetched-AZO films as a function of etching time: 共a兲 nonetched, 共b兲 8 s, 共c兲 16 s, 共d兲 30 s, and 共e兲 160 s.
from 2.524 to 10.903 nm. Summarizing the results in Figs. 1 and 2, we suggest that construction of OSCs using differently etched-AZO films can provide information about dependence of the surface morphology for photovoltaic performance with only minor influences from optical and electronic properties of AZO films. We investigated the dependence of the device performance on the surface morphology of AZO. PCE, FF, Jsc, and Voc of OSCs with various AZO buffer layers are summarized as a function of etching time in Fig. 3. When an etching of 8 s was used, the Voc was increased by more than 0.1 V with respect to the nonetched surfaces. However, no further increase in Voc was found by additional etching 共etching time was increased up to 160 s兲. Regarding the Jsc and FF, a short etching time 共8 s兲 resulted in a significant improvement with respect to the values of the nonetched counterpart. Increasing etching time from 8 to 30 s did not lead to any further improvement in the Jsc and FF values; however, when the etching time was increased to 160 s, Jsc and FF values became even higher. The Jsc and FF of the device consisting of 160 s etched-AZO films with a rms roughness of 10.930 nm were 6.78 mA cm−2 and 0.38, respectively, and these values are more than 50% higher than those of the devices with nonetched films. The PCE of the organic solar cells with nonetched-AZO was ⬍0.5%, and etching of the AZO film for 160 s resulted in a significant improvement in the device PCE 共1.36%兲. As shown in Fig. 3, the general trend from our data is that the photovoltaic performance is increased with increasing surface roughness of AZO films. In order to shed light on the origin of the enhanced photovoltaic performances of IOSCs by etching of AZO films on ITO, the PL spectra of P3HT layers on various AZO films
Appl. Phys. Lett. 98, 023102 共2011兲
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FIG. 3. 共Color online兲 Summary of Jsc, Voc, FF, and PCE of the devices with increased etching time and non etched-AZO films.
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were obtained 共Fig. 4兲. The PL peaks at 600–800 nm correspond to the -ⴱ transition of the P3HT backbone.18 The shape of the PL spectra was found to be almost identical for differently etched-AZO surfaces. However, a large change in absolute intensity of the PL spectrum could be found for different surface roughnesses; the etched surfaces showed a lower intensity in the PL spectra compared to the nonetched ones. Photoluminescence can result from the recombination of electron-hole pairs, which can be created by the absorption of light. A lower PL intensity can result from a more efficient charge-separation at the interface between P3HT and AZO. Based on the results in Fig. 4, we suggest that higher roughnesses of AZO film surfaces can result in a more efficient separation of electrons and holes created in P3HT, finally resulting in an enhanced photovoltaic performance.19 This is due to a larger contact area between P3HT and a rougher electrode with a higher surface area. It should be PL spectra of P3HT on AZO
Etched time: 0 sec Etched time: 30 sec Etched time: 160 sec
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650 700 750 800 Wavelength [nm]
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FIG. 4. 共Color online兲 Photoluminescence spectra of etched-AZO and nonetched-AZO films as a function of etching time.
mentioned that the photovoltaic performance of the OSC is not only dependent on the surface roughness rms value, but also on many other surface structure factors such as shape. This is probably the reason why the correlation between the roughness rms value and photovoltaic performance is not perfect. However, we clearly show that not only electronic and optical properties but also surface morphology control of the electrode is important for creating a high-performing photovoltaic devices. In summary, we prepared AZO films on ITO with various surface roughnesses with only minor changes in film thickness, electronic and optical properties. Use of these AZO films for fabricating IOSCs shed light on the influence of surface morphology of AZO films on photovoltaic performance. We demonstrate that an increase in the surface roughness of AZO films can lead to a significant improvement in the photovoltaic performance 共Voc, Jsc, FF, and PCE兲. We suggest that control of the surface roughness of electrodes is crucial for fabricating high-performing photovoltaic devices. This research was supported by the Korea Institute of Materials Science 共KIMS兲 and the Korea Foundation for International Cooperation of Science and Technology 共KICOS兲 through a grant provided by the Korean Ministry Science and Technology in K207010000284-07A0100. 1
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