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Silver Nanowires Binding with Sputtered ZnO to Fabricate Highly Conductive and Thermally Stable Transparent Electrode for Solar Cell Applications Manjeet Singh, Tanka Raj Rana, SeongYeon Kim, Kihwan Kim, Jae Ho Yun, and Junho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01506 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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ACS Applied Materials & Interfaces

Silver Nanowires Binding with Sputtered ZnO to Fabricate Highly Conductive and Thermally Stable Transparent Electrode for Solar Cell Applications

Manjeet Singha†, Tanka R. Ranaa†, SeongYeon Kima, Kihwan Kimb, Jae Ho Yunb, and JunHo Kim*a a

Department of Physics, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon

22012, Republic of Korea b

Photovoltaic Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro,

Yuseong-gu, Daejeon 34129, Republic of Korea ABSTRACT: Silver nanowire (AgNW) film has been demonstrated as excellent and low cost transparent electrode in organic solar cells as an alternative to replace scarce and expensive indium tin oxide (ITO). However, the low contact area and weak adhesion with low-lying surface as well as junction resistance between nanowires have limited the applications of AgNW film to thin film solar cells. To resolve this problem, we fabricated AgNW film as transparent conductive electrode (TCE) by binding with a thin layer of sputtered ZnO (40 nm) which not only increased contact area with low-lying surface in thin film solar cell but also improved conductivity by connecting AgNWs at the junction. The TCE thus fabricated exhibited transparency and sheet resistance of 92% and 20Ω/□, respectively. Conductive atomic force microscopy (C-AFM) study revealed the enhancement of current collection vertically and laterally through AgNWs after coating with ZnO thin film. The CuInGaSe2 solar cell with TCE of our AgNW(ZnO) demonstrated the maximum power conversion efficiency of 13.5% with improved parameters in comparison to solar cell fabricated with conventional ITO as TCE.

KEYWORDS: thin film solar cell, Ag nanowires, transparent conducting electrode, CIGS, flexible electrode

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1. INTRODUCTION The transparent conductive electrode (TCE) materials have attracted the researchers due to their wide applications in photo-electronic devices like display, touch screen and photovoltaic.1,2 Currently, indium tin oxide (ITO) has been commercialized for most of the applications as TCE.3 Due to the scarcity and toxic nature of In based materials, the application of ITO film has been restricted. Further, the brittle nature of ITO film has limited its use in flexible opto-electronic devices.4 These limitations of ITO have circumscribed its use in flexible optoelectronics and low cost solar cell fabrications. The inherent drawbacks of ITO have induced the invention of alternative TCE materials. The alternative TCE must fulfill the criteria of transparent electrode selection, which must have sheet resistance below 100 Ω/sq and more than 80% transmittance.5 In order to replace ITO, various TCEs have been developed such as carbon nanotube,6,7 graphene,8,9 metal grid10 and metal nanowire network.11 The carbon based TCE which is cheap and promising, but percolated network of carbon nanotube does not fulfill the criteria of TCE selection: low sheet resistance and high optical transparency simultaneously. The metal grid based TCE requires batch-based processing which is expensive and unsuitable for large area solar cell fabrication. Comparatively, the random metal nanowire network, especially silver nanowires (AgNWs), possesses high transparency and good conductivity as well as good flexibility, and thus is found to be most befitting candidate for TCE applications. In addition, the solution processed AgNW films enables low cost, fast, and scalable roll-toroll fabrication of TCE. Despite these merits, the bare AgNW films suffer from high porosity, weak adhesion with low-lying surface, less contact area and high roughness.12 Several attempts have been made to resolve these issues. Embedding AgNWs in polymer can improve the surface roughness, but 2


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the presence of carbon content in organic polymer can adversely affect the current extraction through the electrode.13-15 Other attempts have also been made to decrease the contact resistance and to increase the conduction of AgNW films by employing other materials such as composite with graphene, graphene oxide, sol gel metal oxide and nanoparticles.16-21 It has also been demonstrated that light irradiation of different intensity can also interconnect the nanowires junction making highly conductive film.22 These all reports16-21 rely on the fact that interconnecting the nanowire network at the junction and filling up the pores of the nanowires improves the conductivity for current collection without affecting transmittance of nanowire network to fabricate highly transparent and conductive electrode. There are some reports on composite electrodes of the metal oxide and AgNW.23-25 However, the deliberate arrangements of AgNW transparent composite films with low-lying buffer/absorber layer for high efficiency solar cell has not been thoroughly explored. Recently, we demonstrated solution processed Cu(In,Ga)Se2 (CIGS) solar cell11 employing fully solution-processed ZnO/AgNW/ZnO as TCE. However, due to weak adhesion between buffer and electrode interface and large contact resistance between AgNWs and solutionprocessed ZnO, the performance of the solar cell was very low. Kim et al.18 also demonstrated solution-processed TCE structured as AZO/AgNW/AZO/ZnO. They deposited ZnO and Al-doped ZnO (AZO) films by sol−gel reaction combustion at 200 °C. But the performance of fully solution-processed transparent electrode may be affected adversely due to decomposition26 of AgNWs at 200 oC. In above reports11, 18 the AgNW network was fully covered by a thick ZnO by multiple coating of ZnO sol-gel solution, where ZnO is thicker than nanowire diameter. Thick overlayer of ZnO increases the thermal stability of AgNW film, but at the same time impose the adverse effect on conductivity of the film. Further, the solution-processed ZnO and AZO may result in high resistant layer due to the inter-layer 3



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resistance. In this report, we demonstrated AgNW film binding with a very thin overlayer (40 nm) of sputtered ZnO as TCE and applied the TCE to CIGS solar cell. We used the ZnO overlayer to increase adhesion between AgNW and low-lying ZnO/buffer layer and to interconnect the AgNWs junctions. The fabricated TCE exhibited physical properties comparable to ITO and demonstrated the highest efficiency of CIGS solar cell compared to champion devices fabricated under similar condition.

2. EXPERIMENTAL SECTION Transparent Electrode Fabrication

All the chemicals used in the experiment were reagent grade from Sigma Aldrich. AgNWs were synthesized by the well known polyol process as reported earlier.27,28 Briefly, PVP (M.W. 55000, 0.2 g) was mixed with 25 mL ethylene glycol, and the solution was stirred at room temperature. After complete dissolution of PVP, 0.25 g of AgNO3 was added in the same solution and was stirred to dissolve AgNO3. After dissolving AgNO3, 3.5 g of FeCl3 solution (600 µM) was added in the same solution. Finally, the solution was transferred in preheated heating mantle at 130 oC and the reaction was continued for 4 h. After completion of reaction, the obtained product was washed using acetone and ethanol and centrifuged to collect the pure AgNWs. Finally, the AgNWs were dispersed in ethanol to prepare 2 wt% AgNW ink. The TCE was deposited by spin-coating AgNW ink, four times at 1000 rpm for 10 s and drying at 100 oC after each coating. The overlayer ZnO was deposited on spin-coated AgNW film using rf-sputtering for 15 mins with applied rf power of 100 W. The thickness of the ZnO film deposited for 15 mins was about 40 nm as measured by using FE-SEM. Solar cell device fabrication 4


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For fabrication of solar cell, co-evaporated CIGS film was used as absorber layer.29 Briefly, CIGS absorber layer was deposited by a three-stage process involving the co-evaporation of In, Ga, Cu and Se. In the first stage, approximately ~1 µm thick (In,Ga)2Se3 precursor was deposited by co-evaporation of In, Ga, and Se at the substrate temperature of 350 oC. Then, Cu and Se were evaporated to form Cu-rich Cu(In,Ga)Se2 films (i.e. 2nd stage) at 550 oC. Finally, In, Ga, and Se were again added into the films to convert from Cu-rich Cu(In,Ga)Se2 films to Cu-poor Cu(In,Ga)Se2 films at 550 oC (i.e. 3rd stage). The final compositions of films were adjusted to Cu/(In+Ga) = 0.9 and Ga/(In+Ga) = 0.35, which were determined by EDS. The details of the device fabrication are available in our previous study.29 The CdS film was deposited by chemical bath deposition as buffer layer and annealed in air at 200 oC after deposition.30After CdS deposition, ZnO (~50nm) was deposited by rf-sputtering. Finally, TCE was deposited as described above. The solar cells thus fabricated were scribed with knife to make the effective area of 0.12 cm2. Characterization

The TCE deposited on the glass was characterized by using FE-SEM (JSM-7001F, Jeol) for morphological measurement, and with UV-Visible (UV−Vis) spectrometer (Lambda 40, Perkin Elmer) for transmittance measurement. To get highly magnified image of cross section, we carried out cross-section polishing with commercial cross section polisher (IB-09020CP, Jeol). The cross-sectional plane of completed device was polished by Ar ion beam milling at lowest acceleration voltage, 4 kV, to avoid possible thermal damage to the films. The electrical properties of TCE were investigated by Hall effect measurement. Hall effect measurement was done by using HMS-3000 (Ecopia) with applied magnetic field of 0.55 T and with the Van der Pauw method. Conductive AFM (C-AFM) (Nanoscope, Bruker) was used to characterize current extraction nature of TCE under 0.1 V of bias voltage in contact mode. 5



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The current density-voltage (J-V) curve of the CIGS solar cell was measured by using source meter (Keithley 2400) under AM 1.5G filtered illumination of 1000 Wm−2 Xe lamp (Abet Technology),which was calibrated with Si reference cell. The external quantum efficiency (EQE) was measured by using Xe light source and monochromator combined with light chopper and lock-in amplifier (McScience Inc.). 3. RESULTS AND DISCUSSION Transparent Conductive Electrode (TCE) study

We fabricated AgNW network with ZnO overlayer, which is denoted as AgNW(ZnO) in the subsequent part of the paper. Figure 1(a) shows the pristine AgNWs synthesized by using polyol method. The as-synthesized AgNWs showed diameter and length as 60 nm and 30~40 µm respectively. Figure 1 (b) presents the AgNW(ZnO) film prepared as TCE. Schematic presentation of the fabrication procedure of AgNW(ZnO) electrode is given in scheme 1. The coating of ZnO film (blue color film in scheme 1) with thickness of 40 nm increases the effective diameter of current path making a kind of core-shell structure with AgNWs and creates the close contact among them as well as with low-lying surface. It is to be noted that the actual diameter of AgNWs remains same, but conductive diameter increases due to the composite formation between AgNWs and ZnO. The increase in the effective diameter of AgNWs enhances the conductivity laterally and vertically. Since the thickness of ZnO is very less about 40 nm, it is difficult to distinguish pristine AgNWs and AgNW with ZnO coated on it.

The increase in current collection of the AgNW(ZnO) TCE with respect to bare

AgNW film is explained in the subsequent part of the paper using C-AFM analysis. Figure 1 (b) presents the AgNW(ZnO) film prepared as TCE. The as-synthesized AgNW showed diameter and length as 60 nm and 30~40 µm respectively. To measure optical and electrical properties of the AgNW(ZnO) TCE, UV-Vis transmittance and Hall effect measurements were carried out. To fabricate the most transparent and highly 6


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conductive TCE, we optimized the density of AgNWs by number of spin-coating cycles. The optical transmittance of AgNW(ZnO) TCE was compared with conventional ITO and bare AgNW film. Figure 2(a) presents the transmittance spectra of four times spin-coated AgNW(ZnO), ITO, ZnO and bare AgNW film prepared on the glass. It was observed that AgNW(ZnO) exhibited the transmittance of 92 % at 550 nm higher than the ITO, AgNW and ZnO films. The sheet resistance of the AgNW(ZnO) film with AgNWs spin-coated less than four times was found to be much higher. For four times spin-coated AgNW film, the sheet resistance as well as transmittance was observed in the acceptable range for fabrication of TCE. As we increased the spin-coating times, the sheet resistance was decreased, but the transmittance was reduced significantly as shown in Fig. 2(b). The optical transmittance, sheet resistance and electrical parameters of TCE prepared along with conventional ITO as TCE are summarized in Table 1. Table 1 Optical and electrical properties of AgNW(ZnO), AgNW, ITO and ZnO TCEs deposited on the glass TCE

Transmittance

Sheet resistance (Rs)

Bulk

Mobility

Figure of

(T%) at 550 nm

Ω/□

concentration

(cm2/Vs)

merit (ɸ x 10-3)

( cm-3)

AgNW(ZnO)

92

20

1.4x1020

18

21

AgNW

91

18

1.1 x 1020

15

21

ITO

93

56

2.1 x 1020

49

9

ZnO

90

4.1 x 109

1.8 x 1011

8

8 x 10-11

As it is shown in Table 1, the AgNW(ZnO) prepared as TCE shows improved optical and electrical properties. The sheet resistance of 20 Ω/□ and 92 % transmittance is acceptable value for TCE selection. The carrier concentration and mobility of AgNW(ZnO) were measured to be 1.4 x 1020 cm-3 and 18 cm2V-1s-1, respectively, which were comparable to our 7



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best ITO film. The high values of bulk concentration and mobility of AgNW(ZnO) may be due to increased metal concentration in the film.31 The values of mobility and bulk concentration of our AgNW(ZnO) TCE are almost equal to those of other TCEs which are being developed to replace ITO.32,33 To define superiority of TCE, a parameter known as figure of merit (ɸ) has been introduced.34 Figure of merit is defined as φ = T 10 / Rs , where T is transmittance at 550 nm and Rs is sheet resistance of the TCE. The higher value of ɸ indicates the better TCE. The calculated values of ɸ for different TCEs are given in Table 1, where the ɸ of AgNW(ZnO) shows highest value of 21. The ɸ of AgNW(ZnO) is close to that of bare AgNW film. However, for bare AgNW film, poor adhesion between AgNWs and lowlying surface that develop higher junction resistance were found not to be suitable for application in solar cell. As we increased the spin-coating cycle, the value of ɸ was found to be decreased due to the decrease in the transmittance as shown in Fig. 2(b). Thus, we concluded that the four times spin-coating of AgNW provided the optimum density of AgNW network of TCE. We have also studied the thickness of ZnO below 40 nm to make TCE with AgNWs. There was not much change in the transmittance and mobility of AgNW and ZnO composite TCE by decreasing the ZnO thickness while the TCE performance in solar cell was found to be degraded gradually due to insufficient thickness of ZnO which is necessary for binding the AgNWs with each other and low-lying surface. We found that 40 nm thickness of ZnO is minimum requirement to fabricate AgNWs and ZnO composite film for promising TCE in solar cell applications. To study characteristics of current collection, we carried out C-AFM study of TCE films. Figure 3 shows the topography and corresponding C-AFM current image of three different TCEs. Smooth surface and no current signal are observed in the surface topography and CAFM image of ZnO (Figs. 3(a) and 3(b)). Figures 3(c) and 3(d) show surface topography and 8


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corresponding current flow for the bare AgNW film, where maximum current of 10 nA (difference between highest and lowest currents) under 0.1 V bias is observed. After binding the AgNW film with thin overlayer of sputtered ZnO, the AgNW(ZnO) film shows small decrease in surface roughness in topography image (Fig. 3(e)). The corresponding current image of AgNW(ZnO) film (Fig. 3(f)) shows the more enhanced current flow of 19 nA in comparison to the bare AgNW film. In order to give perception of the current flow for each AgNW, we measured current profiles for AgNW and AgNW(ZnO) films (Fig. S1). As it is shown in Fig. S1, the current through AgNW in AgNW(ZnO) film is observed to be higher compared to AgNW film. High currents more than 12 nA are observed in AgNW(ZnO) film, while currents are observed to be lower than 6 nA in AgNW film. The bare AgNW film is supposed to have large number of empty pores between the AgNWs which hinders the efficient collection of charge carriers and thereby induces decreased current in C-AFM.35,36 After coating a thin layer of ZnO on AgNW film the effective conducting diameter of the AgNWs increases a little bit, which is visible in FE-SEM and AFM images of AgNW(ZnO) (Figs. 1(b), 3(e) and 5(b)).The increased effective conducting diameter of AgNWs increases the lateral current collection around the AgNWs and also the current along the AgNWs. Thus, increase in current collection for AgNW(ZnO) compared to bare AgNW film indicates that ZnO coating over AgNWs not only improves roughness and conductivity by filling up the pores between AgNWs and but also increases the electron collection capability with enlarged contact area (Scheme 1). For application of AgNW(ZnO) TCE to thin film solar cell especially for CIGS solar cell, it should remain thermally stable up to certain temperature. It is reported that AgNWs remain thermally stable at below 200 oC, and above 200 oC AgNWs are decomposed into smaller particles.26 We studied the thermal stability of the AgNW(ZnO) film by open air heating. 9



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Figures 4(a) and 4(b) present FE-SEM images of bare AgNW film heated at 250 oC and AgNW(ZnO) film at 300 oC on hot plate, respectively. It is clear that from the FE-SEM images AgNW(ZnO) film is thermally stable up to 300 oC without losing the AgNWs morphology, while bare AgNW film was found to decompose into smaller particles after heating just at 250 oC (Fig. 4a). We checked thermal stability by measuring sheet resistance with temperature (Fig. S2). The AgNW(ZnO) shows stable sheet resistance up to 300~350 oC, while AgNW shows huge increase of sheet resistance above ~200 oC. These results indicate that ZnO binds AgNWs well and increases the stability towards any damage and loss even upon heating at 300 oC. Application of AgNW(ZnO) in solar cell device fabrication To evaluate the performance of AgNWs based TCE, we fabricated the CIGS solar cell with AgNW(ZnO) TCE. The cross section FE-SEM image of CIGS solar cell device is shown in Fig. 5. Figure 5(a) shows clearly different layers in the completed solar cell, and Fig. 5(b) shows the high magnification FE-SEM image of AgNW(ZnO) TCE after cross-sectional polishing. The AgNWs can be clearly seen at the surface of AgNW(ZnO) and appear little bit thicker than as-prepared AgNWs due to top coating of sputtered ZnO. The AgNWs covered by ZnO layer are shown with white dotted-circles in Fig. 5(b). The absence of voids between the AgNW(ZnO) and low-lying surface indicates that AgNW(ZnO) is in good contact with low-lying ZnO/CdS/CIGS, and the absence of voids between AgNWs also indicates that ZnO overlayer fills well the junction space of AgNWS. The EDS mapping for the elements Zn and Ag in the cross section image of CIGS solar cell is presented in Fig. S3 to clearly locate the AgNWs in the AgNW(ZnO) TCE. We observed the bright spots for AgNWs in the cross section image which are marked by arrows in Fig. S3.

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Table 2 Parameters of the CIGS solar cells fabricated with different transparent electrodes AgNWs, ITO and AgNW(ZnO)

TCE

Jsc (mA/cm2)

Voc (V)

FF (%)

Eff. (%)

Rs (Ωcm-2)

Rsh (Ωcm-2)

AgNWs ITO AgNW(ZnO)

0.2 30.7 33.7

0.54 0.65 0.64

33 65 62

0.05 13.04 13.50

-0.92 0.65

-147 299

Figure 6(a) shows the J-V characteristic curves of the CIGS solar cells fabricated with different TCEs of AgNWs, ITO and AgNW(ZnO). The device parameters of CIGS solar cell are tabulated in Table 2. It was observed that the CIGS solar cell with AgNW(ZnO) as TCE exhibited the efficiency of 13.5% with short circuit current density (Jsc) of 33.7 mA/cm2, open circuit voltage (Voc) of 0.64 V and fill factor (FF) of 62%, which was comparable to the CIGS solar cell efficiency of 13.04% with Jsc of 30.7 mA/cm2, Voc of 0.65 V and FF of 64% with conventional ITO as TCE. Performance of the solar cell with only AgNW film was very low with 0.05% efficiency. All the three types of solar cell devices showed different performance although they have high transparency and lower sheet resistance of TCEs. The CIGS solar cell with AgNW(ZnO) TCE exhibited highest efficiency with increased Jsc compared to solar cell made with ITO TCE and is higher than the champion CIGS solar cell device fabricated with AgNWs-based TCE.18 As it is expected, the Voc values of both the devices with AgNW(ZnO) and ITO are about same, which indicate that recombination losses via structural defects and impurities are very small in the solar cell with AgNW(ZnO). Dramatic decrease in solar cell performance using only AgNW film as TCE is believed to be 11



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due to higher resistance in collection of charge carriers from porous nature of AgNW films and poor adhesion with low-lying ZnO/CdS/CIGS layer.37 Therefore, the gaps between the wires and gaps between wires and low-lying ZnO/CdS/CIGS film should be filled to increase the contact area as well as to decrease the contact resistance of the AgNWs. These gaps can be filled by using thin layer of high band gap ZnO nanoparticles on the AgNW film, which can bind the AgNWs at the junction to decrease the contact resistance36,38 and also provide large-area contact with low-lying ZnO/CdS/CIGS layer. Silver metal film sandwiched between ZnO has also been demonstrated to increase the conductivity and carrier transport of ZnO layer,31 and the similar mechanism may also operate in the case of AgNWs film coated with sputtered ZnO. Sputtered ZnO thin film increases the lateral and vertical photocurrent collection by increasing the effective conductive diameter and decreasing porosity of AgNW(ZnO) film, as is also visible in C-AFM image of AgNW(ZnO) (Fig. 3(f)). This is the reason of increased Jsc of 33.7 mA/cm2 than 30.7 mA/cm2 for ITO and 0.2 mA/cm2 for bare AgNW film. We compare series resistance (Rs) values of the solar cell devices with different TCEs. The Rs values for solar cells with AgNW(ZnO) and ITO as TCE are measured to be 0.65 Ωcm2 and 0.92 Ωcm2, respectively. A slight decrease in Rs in AgNW(ZnO) indicates more conductive AgNW(ZnO) film compared to the ITO film. The lower value of Rs is also confirmed by absence of cross-over in illuminated and dark J-V curves.39 The value of Rs of solar cell with AgNW(ZnO) is about same as one of the champion CIGS solar cell device.40 The Rsh values for AgNW(ZnO) and ITO are calculated to be 299 and 147 Ωcm2, respectively. The higher Rsh value indicates less shunt path for charge carriers and the superiority of our AgNW(ZnO) as TCE. We present statistical analysis of CIGS solar cell parameters obtained from 15 solar cells 12


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fabricated with AgNW(ZnO) and ITO as TCEs in Table S1 and Fig. S4. CIGS solar cells with AgNW(ZnO) TCEs was found to have higher average efficiency (12.18 %) and have smaller standard deviation (0.674), while the solar cells with ITO TCEs was observed to have average efficiency (10.59 %) and have larger standard deviation (1.275). The higher efficiency was mainly due to the higher average FF (63.55 %) and average Jsc (30.65 mA/cm2) for AgNW(ZnO) TCE, while average Voc was similar in AgNW(ZnO) and ITO. Thus, statistical analysis concluded that the higher efficiency was mainly due to increase in Jsc and FF for AgNW(ZnO) TCE. The lower value of Rs and higher value of Rsh of the CIGS solar cell with AgNW(ZnO) TCE support the higher FF and result in high efficiency solar cell. To further confirm the increase of Jsc for CIGS solar cell with AgNW(ZnO) TCE compared to ITO TCE, we carried out EQE measurements for both devices. Figure 6(b) presents the EQE curves for solar cell devices with AgNW(ZnO) and ITO as TCE. Our CIGS solar cell with AgNW(ZnO) showed higher Jsc (33.7 mA/cm2) than solar cell with ITO (30.7 mA/cm2). The EQE value was found to be higher in the wavelength range of 500~1050 nm for solar cell with AgNW(ZnO) than that with ITO. The maximum EQE was 78% at 560 nm in comparison to the 72% at 740 nm for ITO. The high EQE in the lower wavelength range around 550 nm corresponds to the decrease in absorption losses in buffer and window layers, by which increase in Jsc was obtained.41 The high EQE in the range of 500 ~ 600 nm is also consistent with higher transmittance of AgNW(ZnO) film as in optical transmittance result of Fig. 2. The EQE value was also higher in the higher wavelength region from 800 to 1000 nm. The improved EQE value in higher wavelength region (marked with arrow) for our AgNW(ZnO) is attributed to lower current loss at the CdS/CIGS and at interface of AgNW(ZnO) window and ZnO/CdS/CIGS compared to ITO as TCE .41

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Table 3 Device parameters of CIGS solar cells with different AgNWs-based TCEs AgNWs-based TCE

Jsc (mA/cm2)

Voc (V)

FF (%)

Eff. (%)

Ref.

AZO/AgNW/AZO

28.54

0.35

59.2

6.0

42

AgNW/PEDOT:PSS

33.80

0.60

57.0

11.6

43

AgNW/ITO (nanoparticles) AZO /AgNW/ AZO /ZnO (all solution processed) AgNW(ZnO)

30.10 36.43

0.49 0.54

69.0 56.2

10.3 11.0

37 18

33.70

0.64

62

13.5

This work

To understand the better performance of our AgNW(ZnO) TCE in solar cell, we compared the and reviewed other CIGS solar cells fabricated using different AgNWs-based TCEs.42,43,37,18 Table 3 presents device parameters for other highest efficiency CIGS solar cells and our CIGS solar cell. All solar cells in Tables adopt same absorber and buffer, co-evaporated CIGS and chemical bath deposited CdS. Various AgNWs-composite TCEs have demonstrated different efficiencies. On flexible substrate, TCE of AgNWs sandwiched with Al-doped ZnO (AZO) demonstrated 6.04 % of efficiency and sustained the same efficiency for 1000 times bending cycle, whereas ITO could have only 5% of their original value.42 This result suggests that AgNWs-based TCE can have high level of flexibility without compromising efficiency of CIGS solar cell. However, the cell showed reduced Voc of 0.35 V which might be due to the carrier recombination at buffer/absorber interface. The AgNWs-based composite with polymer PEDOT:PSS

43

has been demonstrated conversion efficiency of 11.6% with Voc of

0.6 V, Jsc of 33.8 mA/cm2 and FF of 57%. This CIGS solar cell showed the comparable parameters with our solar cell except lower FF and lower Voc. The reason for lower FF may be the carbon rich polymer PDEOT:PSS

13-15

which generates highly resistive layer between 14


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AgNWs hindering the charge carrier collection and thereby increasing series resistance of the device and lowering the fill factor. The champion solar cell in Ref. 43 shows reduced Rs of 2.4 Ωcm-2 only after post-annealing at 300 oC, which is still not small. The Voc of 0.6 V in the same cell43 is achieved by reducing recombination loss in TCE/CdS interface by inserting hole blocking layer of CBD-processed Zn(O,S,OH). However, CBD-deposited Zn(O,S,OH) and post-annealing at 300 oC may not be so effective to get improved CdS/CIGS and Zn(O,S,OH)/CdS interfaces for higher Voc. The TCE fabricated by using AgNWs and ITO nanoparticles (ITO-NP) 37 demonstrated the efficiency of 10.3% with Jsc = 30.1 mA/cm2,Voc = 0.49 V and FF = 69 %. To get Voc of 0.49 V, AgNWS/ITO-NP electrode was deposited directly on the CdS buffer by using chemically benign solution, which may reduce the possible damage on CdS buffer and CuInSe2 interface. However, the Voc of 0.49 V implies that chemical deposition just on CdS induces some damages on CdS and CuInSe2, which causes Voc deficit. The solution processed TCE of AZO/AgNW/AZO/ZnO structure18 demonstrated conversion efficiency of 11.3% with Jsc= 36.43 mA/cm2,Voc = 0.54 V and FF = 56.21 %. Our solar cell exhibited the highest efficiency of 13.5% with Jsc = 33.7 mA/cm2, Voc = 0.64 V and FF = 63%, where Voc is the highest and FF is second highest value. The CIGS solar cell with solution processed AZO/AgNW/AZO/ZnO as a TCE also showed comparable value of Jsc to our device but demonstrated significantly lower Voc and FF. The lower FF and Voc could be ascribed to multiple coating of solution processed AZO and ZnO film which induces inter-layer traps. The traps generated between the layers provide the leakage current path for photo-generated charges and also enhance the possibility of charge carrier recombination, and thereby lower the Voc and FF. The solar cells and their device parameters in the Table 3 indicate that the conversion efficiency with TCE composite of AgNWs with AZO, PDEOT:PSS, ITO or solution processed AZO /AgNW/AZO/ZnO can be improved by 15



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increasing the contact area with low-lying CdS/CIGS, reducing the leakage current and recombination losses in buffer/absorber and TCE/buffer interfaces. In our experimental results, the low-lying sputtered ZnO film (~50nm) on CdS buffer prevent CdS/CIGS interface and CdS buffer from being degraded during TCE deposition process, which provides high Voc and high Jsc. Further, AgNW film coated with a very thin overlayer (40 nm) of sputtered ZnO showed improved parameters with respect to other four types of TCE film due to increase in contact area with lower lying ZnO/CdS/CIGS as well as compactness by connecting AgNWs at the junction and laterally, which enables high efficiency solar cell device with lower Rs and high FF. Thus, the TCE of AgNW(ZnO) showed the best efficiency among the CIGS solar cells fabricated using AgNWs-based TCE.

4. CONCLUSIONS We have demonstrated a new kind of AgNW(ZnO) TCE fabricated by binding AgNW film with a very thin sputtered ZnO film. The binding of AgNW film with ZnO has been found to increase the adhesion with lower-lying surfaces and connectivity between AgNWs, which resulted in increase the conductivity of the AgNW(ZnO) film. The increase of lateral and vertical conductivity of AgNW(ZnO) film has been confirmed by C-AFM measurement. The transmittance (92%) and sheet resistance (20 Ω/□) of AgNW(ZnO) TCE have been obtained better than the conventional ITO film. The CIGS solar cell fabricated using AgNW(ZnO) as TCE exhibited the power conversion efficiency of 13.5 % better than their ITO counterpart. Our AgNW(ZnO) as TCE, demonstrated higher efficiency ever reported for CIGS solar cell fabricated using AgNWs-based TCE. It is expected that AgNW(ZnO) TCEs can be applied to flexible thin film solar cells and other flexible devices with large area.

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ASSOCIATED CONTENT Supporting Information Fig. S1 describing the EDS elemental mapping for Ag and Zn in the cross section image of AgNW(ZnO) and Table S1 and Fig. S2 for statistical analysis for 15 CIGS solar cells parameters fabricated using ITO and AgNW(ZnO) as TCE. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *J.Kim. E-mail: [email protected]. Tel.: +82-32-835-8221. Author Contributions †These authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research work was financially supported by the National Research Foundation of Korea (NRF) funded by the Korean government (NRF-2014R1A2A1A11053109) and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government and Ministry of Trade, Industry and Energy (No. 20123010010130).

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Scheme 1

Schematic presentation of TCE employing AgNW(ZnO) AgNW(ZnO) for CIGS solar cell

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Figures

Fig. 1 FESEM images of (a) AgNWs synthesized by Polyol process and (b) AgNW film coated with 40nm thick sputtered ZnO. Scale bar is 1 µm.

Fig. 2 UV-Visible transmittance spectra for different types of transparent electrode (a) ZnO, ITO, AgNW and AgNW(ZnO) and (b) transmittance curves of AgNW(ZnO) films made with different spin coating times.

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Fig. 3 C-AFM topography and current images for different types of transparent electrodes. (a) Topography of ZnO , (b) C-AFM of ZnO, (c) topography of AgNW film, (d) C-AFM of bare AgNW film, (e) topography of AgNW(ZnO) and (f) C-AFM of AgNW(ZnO). Scale bar is 5 µm.

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Fig. 4 FE-SEM images of (a) bare AgNW film heated at hot plate at 250 oC (b) AgNW(ZnO) film heated at hot plate at 300 oC.

Fig. 5 Cross section FE-SEM images of the (a) CIGS solar cell device fabricated using AgNW(ZnO) TCE and (b) magnified images of AgNW(ZnO) TCE after cross-section polishing.

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Fig. 6 (a) J-V and (b) EQE curves of CIGS solar cells fabricated using AgNW(ZnO) (Eff.= 13.5 %) and ITO (Eff.= 13.04 %) as TCE.

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Table of Content

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Silver Nanowires Binding with Sputtered ZnO to ... - ACS Publications

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