Electrical and Optical Properties of Asymmetric Dielectric/Metal/Dielectric (D/M/D) Multilayer Electrode Prepared by Radio-Frequency Sputtering for Solar Cells

http://dx.doi.org/10.5369/JSST.2015.24.1.15 pISSN 1225-5475/eISSN 2093-7563

Electrical and Optical Properties of Asymmetric Dielectric/Metal/Dielectric (D/M/D) Multilayer Electrode Prepared by Radio-Frequency Sputtering for Solar Cells

Rina Pandey

, Ju Won Lim

, Keun Yong Lim

, Do kyung Hwang

, and Won Kook Choi

Abstract

Transparent and conductive multilayer thin films consisting of three alternating layers FZTO/Ag/WO

have been fabricated by radio- frequency (RF) sputtering for the applications as transparent conducting oxides and the structural and optical properties of the resulting films were carefully studied. The single layer fluorine doped zinc tin oxide (FZTO) and tungsten oxide (WO

) films grown at room tem- perature are found to have an amorphous structure. Multilayer structured electrode with a few nm Ag layer embedded in FZTO/Ag/

WO

(FAW) was fabricated and showed the optical transmittance of 87.60 % in the visible range (λ = 380~770 nm), quite low electrical resistivity of ~ 10

Ω cm and the corresponding figure of merit (T

/R

) is equivalent to 3.0×10

Ω

. The resultant power conversion efficiency of 2.50% of the multilayer based OPV is lower than that of the reference commercial ITO. Asymmetric D/M/D multilayer is a promising transparent conducting electrode material due to its low resistivity, high transmittance, low temperature deposition and low cost components.

Keywords: Transparent conductive oxide, RF Sputtering, Structural and optical properties, Bulk hetero-junction organic pho- tovoltaic’s cells (BHJ-OPVs), Power conversion efficiency

1. INTRODUCTION

Transparent conducting oxides (TCO) are the integral part of the present day electro-optic devices as transparent conducting electrode (TCE). The inherent properties, transmittance and electrical conductivity of the TCOs are important factors which make them potential candidates for TCE applications such as plasma display panels, flat panel displays, touch panels, solar cells, organic light emitting diode, and gas sensors [1]. Different metal oxide semiconductors like In ₂ O ₃ , SnO ₂ , ZnO, and TiO ₂ have been extensively employed to fabricate TCO thin films [2]. The most common TCO consists of large band gap semiconducting metal oxides such as indium, tin, cadmium and zinc oxide doped with group III (Al [3-7], B [8], Ga [9-11] or group VII (F [12], Cl

[13] elements to reduce their resistivity while retaining high transparency in the visible range (λ=380~770 nm). Because of high conductivity, transparency, and the possibility to generate very flat films with good reproducibility, indium tin oxide (ITO) is one of the most employed TCO [14]. Due to the high cost and scarcity of indium in TCO, there is urgent need of an alternative material with low cost and similar properties [2]. In these days, researchers are dedicated to find new transparent conductive electrodes such as nanotubes, graphene, metal nanowires, dielectric-metal-dielectric (D/M/D) and related structures. D/M/D materials have been suggested as a candidate to overcome the limits of both the electrical and optical properties of single layer TCOs. It allows both the overall carrier concentration and the mobility to be increased, prevailing to some extent the limitation imposed by ionized impurity scattering in metal oxide single layers. Sandwiching a thin metal layer between two dielectric layers D/M/D has been presented as an alternative approach to obtain the combined benefits of high transmission as well as the excellent conductivity of metals. Studies on a variety of multilayer electrodes [15-22] have been investigated for indium free transparent conducting oxides for organic photovoltaic’s cells.

Silver was found to perform the best as the middle metal layer in sandwiched D/M/D structure. A pure Ag metal film has the lowest resistivity of all metals and exhibits relatively low absorption in the visible region. In our study, Fluorine doped zinc tin oxide

1

Interface Control Research Center, Korea Institute of Science and Technology (KIST), Seongbuk Gu, Hwarangro 14 Gil 5, Seoul 136-791, Korea

2

Department of Nanomaterials Science and Engineering, Korea University of Science and Technology (KUST), Gajeongro 217, Yuseong-gu, Daejeon 305- 350, Korea

+

Corresponding author: [email protected] (Received : Dec. 19, 2014, Revised : Jan. 20, 2015)

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/

licenses/bync/3.0) which permits unrestricted non-commercial use, distribution,

and reproduction in any medium, provided the original work is properly cited.

(FZTO) and tungsten oxide (WO ₃ ) were chosen as a dielectric material in the asymmetric D/M/D structure. Fluorine can be a promising anion doping candidate with its ionic radius similar to that of oxygen (F ^- =1.31 Å, O ₂ ^- =1.38 Å) and WO ₃ is a high refractive index material (with n~2.0 in the visible range). Multilayer thin films consisting of three alternating layers FZTO/Ag/WO ₃ (FAW) have been fabricated by radio-frequency (RF) sputtering for the applications as transparent conducting oxides. The structural and optical properties of single layer (FZTO and WO ₃ ) and multilayer FZTO/Ag/WO ₃ (FAW) thin films are characterized. Studies on metal-oxide hole transport layers (HTLs) i.e. WO ₃ [23], NiO _x [24], V ₂ O ₅ and MoO ₃ [25] as an alternative to PEDOT: PSS have been studied. In this work, the resulting device structure consists of LiF/

Al/P3HT:PC60BM/PEDOT:PSS/FZTO/Ag/WO ₃ /Glass. It should be noted here that the WO ₃ has not been used as a HTLs in an attempt to replace PEDOT: PSS. There was no report with the asymmetric D/M/D structure. So, we studied the above mentioned device structure for the organic photovoltaic’s cells. Bulk hetero- junction organic photovoltaic’s (BHJ-OPVs) devices with a normal structure were fabricated on FZTO/Ag/WO ₃ (FAW) multilayer electrodes at room temperature in which the PEDOT:

PSS was used as a hole transparent layer (HTL). PEDOT: PSS is a p-type semiconductor, a good hole transport material and assures better hole collection from the polymer into the electrode (commercial ITO or multilayer). These devices were then characterized in terms of the open circuit voltage (V _oc ), current density (J _sc ), fill factor (FF) and power conversion efficiency (PCE) which were compared to OPV devices with an ITO electrode as a reference.

2. EXPERIMENTAL

Highly transparent and conducting FZTO/Ag/WO ₃ thin films were prepared on glass substrates (Corning Eagle XG 0.7 mm, Alkaline Earth Boro-Aluminosilicate) for the fabrication of the OPVs device by radio frequency sputtering (rf, 13.56 MHz). The substrate was ultrasonically precleaned in acetone, methanol and deionized water for 10 min each respectively in order to remove impurities on the substrate surface. The bottom WO ₃ layer was sputtered using WO ₃ ceramic target at an RF power of 100 W and working pressure of 0.27 Pa. Silver (Ag) (RND, Korea, Ag 99.99%) intermediate metal layer was deposited by RF sputtering at 50 W under an Ar plasma gas pressure of 0.27 Pa. Finally the top FZTO layer was sputtered using ZTO ceramic target (composited with 30 wt% ZnO and 70 wt% SnO ₂ and also by

introducing a mixed gas of pure Ar, CF ₄ , and O ₂ forming gas into the sputtering chamber while sputtering ZTO target) at an RF power, working pressure of 100 W and 0.27 Pa. For the uniformity of the thin films, the substrate was constantly rotated at a rate of 7 rpm during sputtering process. The target to substrate was fixed at 70 mm during the sputtering process. The optimized FAW multilayer and commercial ITO electrodes were specially designed laser patterned for the fabrication of the conventional bulk hetero-junction OPVs. PEDOT: PSS layer was spin coated at 4000 rpm for 40 s onto the electrode substrates, annealed at 130 ^o C for 30 min on a hot plate. Photoactive layer was spin coated from P3HT: PC60BM blends dissolved in dichlorobenzene at 2500 rpm for 40 s, which corresponds to a layer thickness of ~ 100 nm. The annealing process was performed at 150 ^o C for 10 min inside glove box. After the evaporation of the solvent, the sample was loaded into a vacuum chamber and LiF/Al thin film (0.6/100 nm) was deposited as a cathode electrode. Schematic diagrams depicting the overall device structure of the OPVs are illustrated in Fig. 5(a).

X-ray diffraction (XRD) analysis (Rigaku Dmax 2500/server) with CuKα radiation (wavelength=1.5418 Å) was performed to investigate the crystallographic structure of the FZTO and WO ₃ films. The surface microstructure of the Ag thin films deposited on the WO ₃ /glass substrate with the variation of Ag thickness was observed by using scanning electron microscopy (NOVA Nano SEM 200). Electrical resistivity, carrier concentration, and hall mobility were characterized using Hall-effect measurement.

Optical transmittance of the films was measured using a UV- Visible spectrometer (Perkin Elmer UV/Vis spectrometer Lambda 18) in the wavelength range from 200~900 nm. The active area of the fabricated device was 0.09 cm ² _. The electrical properties of the OPV devices were recorded using a Keithely 2400 source- measure unit under ambient conditions. The photocurrent was obtained under illumination from a Thermal Oriel solar simulator (Am 1.5 G, 100 mA/cm ² ). The light intensity was calibrated using Si photovoltaic’s (PV) solar cells.

3. RESULTS AND DISCUSSIONS

3.1 Single WO

and Ag Thin Film

Fig. 1(a) shows XRD spectra of the as-deposited WO ₃ thin

films along with the samples annealed at three different

temperatures of 200, 300, and 400 ^o C are plotted in 2 Θ ranges

20~80 ^o . The WO ₃ thin films deposited at room temperature are

found to be amorphous. Even after the as-deposited WO ₃ films

were annealed up to 300 ^o C, they still exhibit the amorphous structure. However, it is noticeable that new diffraction peaks for the WO ₃ annealed at 400 ^o C appear in XRD spectra at 2Θ = 23.15, 24.28, and 55.90 ^o respectively corresponding to the (002), (200), and (420) planes of WO ₃ [26]. This means the amorphous WO ₃ films are converted into polycrystalline structure through vacuum annealing at 400 ^o C. Very recently, our group reported FZTO thin films and their structural, electrical and optical properties with the variations of relative partial pressure of oxygen to CF ₄ and post annealing temperature [27]. Fig. 1(b) shows the optical transmittance spectra of the WO ₃ thin films as deposited and annealed at various temperatures in the wavelength range of 200~900 nm. The entire films exhibit an average optical transmittance (λ=380~770 nm) of higher than 80% and a sharp fundamental absorption band edge are found in the vicinity of ~ 380 nm for all specimens. Increasing annealing temperature did not significantly change the total transmittance of films but did cause a shift in the absorption edges to lower energy ranges and thus band-gap decreases. Using the

fundamental absorption edge the band-gap energies are approximately estimated by extrapolation of the linear portion of the (αhυ) ² ~A (hυ-E _g ) ^m , where m is a constant which determines the type of optical transition (m=1/2 for allowed direct transitions and m=2 for allowed indirect transitions), α is the absorption coefficient, hn is the photon energy, A is a constant and E _g is the optical band gap. The absorption edge shifts toward lower photon energy side and the optical band-gap of the WO ₃ thin film decreases as annealing temperature increases [26].

The surface morphology of Ag thin film deposited on the WO ₃ bottom layer as a function of thickness is shown in Fig. 2. With increasing the Ag thickness (7~8 nm), surface morphology of the film shows the island structure. Smooth surface was observed at the Ag thickness of (9~10 nm). Islands seemed to play a role of interrupting the electron movement as a resistance, which deteriorated electronic property as can be observed from Fig. 2.

3.2 FZTO/Ag/WO

Multilayer Electrode

Asymmetric designs consisting of dielectric layer stacks around a mid layer of a metal thin film can produce very high transmittance in the visible spectral range. A multilayer thin film structure with maximum transmittance can be designed using the Macleod simulation software. The thickness of the Ag was fixed 9 nm (as observed from Fig. 2). The transmittance of the FAW multilayer electrode as a function of thickness of the WO ₃ films and commercial ITO are shown in Fig. 3(a). The sheet resistance and average transmittance T ave.(380~770 nm) of the multilayer electrode FZTO (40 nm)/Ag (9 nm)/WO ₃ (45 nm) is 87.60% and 9.06 Ω/sq.

Fig. 1. (a) XRD patterns of WO

thin films as deposited and annealed at various temperatures and (b) optical transmittance of the WO

thin films as deposited and annealed at various temperatures.

Fig. 2. Surface microstructure of the Ag layer on WO

/glass with

increasing thickness.

The commercial ITO shows the average transmittance of 86.60%

and sheet resistance 12.20 Ω/sq. The sheet resistance of the FAW multilayer electrode as a function of WO ₃ thickness 9~10 Ω/sq.

Fig. 3(b) and (c) shows the measured transmittances (T _s and T _p ) of the multilayer electrode FZTO (40 nm)/Ag (9 nm)/WO ₃ (45 nm)

as a functions of wavelength (λ=400~900 nm) and angle of incidence (Θ=0~70 ^o ). The transmittance of the multilayer electrodes (FZTO 40 nm/Ag 9 nm/WO ₃ 45 nm) goes on decreasing with increasing the angle of incidence (T _s transverse- electric TE). On the other hand, the transmittance goes on increasing with increasing the angle of incidence upto 50 ^o (T _p transverse-magnetic TM). The average transmittance of the multilayer electrode (T _p ) at angle of incidence 50 ^o is 90.04%

which is 3% higher than normal incidence 0 ^o . The reason behind this is due to the decrement of reflectivity for p-polarized light with increase in angle of incidence upto the Brewster angle, beyond which it increases again [28]. The rate of decrease in transmission with increasing angle of incidence is higher for the s- polarization as a result of the Fresnel reflection at the air-dielectric interface [29].

It is well known, resistivity is proportional to the reciprocal of the product of carrier concentration (n _e ) and mobility (μ). The resistivity of the multilayer FAW electrode was nearly saturated at ~10 ^-5 Ω cm. With increasing the WO 3 thickness, the resistivity slightly increase with decreasing the carrier concentration. The resistivity of FZTO (40 nm)/Ag (9 nm)/WO ₃ (45 nm) was determined to be 8.9×10 ^-5 Ω cm, which is quite low compared to the value of 1×10 ^-4 Ω cm of ITO which was used as a reference.

With increasing the WO ₃ thickness, the mobility increases from 16 cm ² V ^-1 s ^-1 to 22 cm ² V ^-1 s ^-1 as can be observed from Fig. 4.

3.3 OPVs FZTO/Ag/WO

Multilayer Electrode Schematic diagrams depicting the overall device structure of the OPVs are illustrated in Fig. 5(a). The thickness of the hole transport (PEDOT: PSS) and photo active layer (P3HT: PC60BM) layer were changed and evaluated using a finite-difference time-domain Fig. 3. (a) Optical transmittance of FZTO/Ag/WO

multilayer elec-

trodes as a function of FZTO thickness 40 nm and WO

thickness (30~45 nm). Optical transmittance for (b) s and (c) p polarized light incident at an angle Θ=0~70

.

Fig. 4. Electrical properties of the FZTO/Ag/WO

multilayer structure.

(FDTD) simulation [Supporting Information]. Conventional bulk hetero-junction organic photovoltaic’s (BHJ-OPV’s) was fabricated on the optimized FZTO/Ag/WO ₃ and commercial ITO electrode.

Fig. 5(b) and Table. 1 shows current density-voltage (J-V) curve of BHJ-OPV’s from the FZTO/Ag/WO ₃ multilayer electrode and the commercial ITO as a transparent electrode. The commercial ITO shows open circuit (V _oc ) of 0.64 V, short circuit current (J _sc ) of 8.31 mA/cm ² , fill factor (FF) of 0.55, and power conversion efficiency (PCE) of 3.0%. Whereas, FZTO (40 nm)/Ag (9 nm)/

WO ₃ (45 nm) multilayer electrode shows a V _oc of 0.63 V, J _sc of 6.76 mA/cm ² , FF of 0.57, and PCE of 2.50%. These values were lower than that of commercial ITO and D/M/D electrode [30,32,34], the efficiency was slightly higher value [18,31,33].

The absorption of the P3HT: PC60BM active layer mainly appeared at the wavelength of 400~600 nm. The average

transmittance at the wavelength of 400~600 nm of the multilayer FZTO/Ag/WO ₃ is lower than that of ITO 88.70%. Low PCE can be attributed with the factors of photon flux and difference in energy barrier between transparent electrode and hole transparent layer.

4. CONCLUSIONS

In this paper, we study about the structural and optical properties of single layer and multilayer thin films. Indium free FZTO (40 nm)/Ag (9 nm)/WO ₃ (45 nm) multilayer transparent conducting electrodes with sheet resistance low as 9.06 W cm and average transmittance high as 87.60%. The resultant power conversion efficiency of 2.50% of the multilayer based OPV is lower than that of the reference commercial ITO. The reason for the low value can be explained with the factors of photon flux and lower transmittance at wavelengths of 400~600 nm compared to that of commercial ITO electrode.

ACKNOWLEDGMENT

This work was partially supported by the Converging Research Centre Program through the Ministry of Science, ICT and Future Planning, Korea and Industrial Core Technology Development Program (2MR2010) and ATC program (10048659) from Ministry of Trade, Industry (MOTIE), Republic of Korea.

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