Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

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Summary

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

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Lee, S., Cho, K. S., Song, S., Kim, K., Eo, Y. J., Yun, J. H., Gwak, J., Chung, C. H. Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells. J. Vis. Exp. (149), e59909, doi:10.3791/59909 (2019).

Abstract

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Introduction

Silver nanowire (AgNW) networks have been extensively studied as an alternative to indium tin oxide (ITO) transparent conducting thin films due to their advantages over conventional transparent conducting oxides (TCOs) in terms of lower processing cost and better mechanical flexibility. Solution-processed AgNW network transparent conducting electrodes (TCEs) have thus been employed in Cu(In,Ga)Se2 (CIGS) thin-film solar cells1,2,3,4,5,6. Solution-processed AgNW TCEs are normally fabricated in the form of embedded-AgNW or sandwich-AgNW structures in a conductive matrix such as PEDOT:PSS, ITO, ZnO, etc.7,8,9,10,11 The matrix layers can enhance that the collection of the charge carriers present in the empty spaces of the AgNW network.

However, the matrix layers can generate interfacial defects between the matrix layer and underlying CdS buffer layer in CIGS thin-film solar cells12,13. The interfacial defects often cause a kink in the current density-voltage (J-V) curve, resulting in a low fill factor (FF) in the cell, which is detrimental to solar cell performance. We previously reported a method to resolve this issue by selectively depositing an additional thin CdS layer (2nd CdS layer) between the AgNWs and the CdS buffer layer14. The incorporation of an additional CdS layer enhanced the contact properties in the junction between the AgNW and CdS layers. Consequently, the carrier collection in the AgNW network was greatly improved, and the cell performance was enhanced. In this protocol, we describe the experimental procedure to fabricate robust electrical contact between the AgNW network and the CdS buffer layer using a 2nd CdS layer in a CIGS thin-film solar cell.

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Protocol

1. Preparation of Mo-coated glass by DC magnetron sputtering

  1. Load cleaned glass substrates into a DC magnetron and pump down to below 4 x 10-6 Torr.
  2. Flow Ar gas and set the working pressure to 20 mTorr.
  3. Turn on plasma and increase the DC output power to 3 kW.
  4. After pre-sputtering of 3 min for target cleaning, begin the Mo deposition until the Mo film thickness reaches approximately 350 nm.
  5. Set the working pressure to 15 mTorr while maintaining the same output power (i.e., 3 kW).
  6. Resume the Mo deposition until the total thickness of Mo reaches approximately 750 nm.

2. CIGS absorber layer deposition by means of a three-stage coevaporation

  1. Load Mo-coated glass into a preheated co-evaporator under a vacuum lower than 5 x 10-6 Torr.
  2. Set the temperatures of In, Ga, and Se effusion cells yielding deposition rates of 2.5 Å/s, 1.3 Å/s, and 15 Å/s, respectively.
    1. Check the deposition rates using the quartz crystal microbalance (QCM) technique. The deposition rates are dependent on the set temperature of effusion cells and the amount of materials in the effusion cells.
  3. Begin to supply In, Ga and Se onto the Mo-coated glass to form a 1 μm-thick (In,Ga)xSey precursor layer at the substrate temperature of 450 °C. The deposition time is 15 min (namely, 1st stage).
  4. Stop the In and Ga supplies and increase the substrate temperature to 550 °C.
  5. Begin to supply Cu (deposition rate: 1.5 Å/s) onto the (In,Ga)xSey precursor and continue until the Cu/(In + Ga) compositional ratio of the film reaches 1.15. Note that the Se deposition rate is maintained at 15 Å/s through the 2nd stage (namely, 2nd stage).
  6. Stop supplying Cu and evaporate In and Ga again with the same deposition rates as the 1st stage to finally form an approximately 2 μm-thick CIGS film with Cu/(In+Ga) compositional ratio of 0.9. Maintain the Se deposition rate and substrate temperature at 15 Å/s and 550 °C, respectively. The deposition time of this stage is 4 min (namely 3rd stage).
  7. In order to ensure a complete reaction, anneal the deposited CIGS film under ambient Se (15 Å/s) for 5 min at the substrate temperature of 550 °C.
  8. Cool down the substrate temperature to 450 °C under ambient Se (15 Å/s) and then unload the CIGS-deposited substrate when the substrate temperature is below 250 °C.

3. Growth of the CdS buffer layer on the CIGS absorber layer using a chemical bath deposition (CBD) method

  1. Prepare the CdS reaction bath solution in a 250 mL beaker by adding 97 mL of DI water, 0.079 g of Cd(CH3COO)2·2H2O, 0.041 g of NH2CSNH2, and 0.155 g of CH3COONH4. Stir the solution for several minutes to mix. Make sure that all added solutes are completely dissolved.
  2. Add 3 mL of NH4OH (28% NH3) into the bath solution and stir the solution for 2 min. Figure 1 shows the experimental setup of CBD for CdS.
  3. Put the CIGS sample into the reaction bath solution using a Teflon sample holder.
  4. Put the reaction bath into the water-heat bath maintained at 65 °C and stir the reaction bath solution at 200 rpm using a magnetic bar during the deposition process.
  5. React for 20 min to generate an approximately 70 to 80 nm CdS buffer layer on the CIGS.
  6. After the reaction, remove the sample from the reaction bath, wash with a flow of DI water, and dry with N2 gas.
  7. Anneal the sample at 120 °C for 30 min on a hot plate.

4. Fabrication of the AgNW TCE network

  1. Prepare a diluted AgNW dispersion (1 mg/mL) by mixing 19 mL of ethanol with 1 mL of a purchased ethanol-based AgNW dispersion (20 mg/mL).
  2. Pour 0.2 mL of the diluted AgNW dispersion onto a CdS/CIGS sample (2.5 cm x 2.5 cm) to cover the whole surface of the sample and rotate the sample with 1,000 rpm for 30 s.
  3. Repeat step 4.2 as needed to achieve the desired optical and electrical properties. Spin-coat the AgNWs 3x. A scanning electron microscopy (SEM) image of spin-coated AgNW TCE is shown in Figure 2.
  4. After spin-coating, anneal the sample at 120 °C for 5 min on a hot plate.

5. Deposition of the 2nd CdS layer

  1. Prepare a new CdS reaction bath solution as described in step 3.1.
  2. Deposit CdS as in section 3, except change the reaction time as necessary.
    NOTE: We optimized the reaction time, and 10 min resulted in the CIGS device with the best performance. The effect of 2nd CdS deposition time on a CIGS thin film solar cell device performance can be found in our previous work14.

6. Characterization techniques

  1. Characterize the surface and cross-sectional morphology of AgNWs and CdS-coated AgNWs by field emission SEM and transmission electron microscopy (TEM).
  2. Measure solar cell performance using a current-voltage source equipped with a solar simulator (1,000 W/m2, AM1.5G).

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Representative Results

The layer structures of the CIGS solar cells with (a) standard ZnO:Al/i-ZnO and (b) AgNW TCE are shown in Figure 3. The surface morphology of CIGS is rough, and a nanoscale gap can form between the AgNW layer and the underlying CdS buffer layer. As highlighted in Figure 3A, the 2nd CdS layer can be selectively deposited onto the nanoscale gap to create a stable electrical contact. The detailed explanation on the formation of electrical contact and enhancement of electrical properties and device performance can be found in the reference 14. The structural analysis of AgNW and CdS junction including cross sectional SEM and TEM, and corresponding elemental mapping can also be found in the reference 14.

Figure 4 shows the cross-sectional TEM images (a) along the 2nd CdS layer deposited on the AgNW network on the CdS/CIGS structure and (b) across the 2nd CdS layer deposited on the AgNW network. The CdS/CIGS structure shows a rugged surface morphology due to the granular structure of CIGS. Hence, bare AgNWs are suspended in air, and stable electrical contact with the CdS buffer layer cannot be expected. The 2nd CdS layer is uniformly deposited on the surface of the AgNWs, and the CdS layer on the core-shell AgNW (Ag@CdS NW) structure is produced as shown in Figure 4B. Furthermore, the 2nd CdS layer fills the air gaps between the CdS buffer layer and the AgNW layer, as shown in the inset of Figure 4A, and stable electrical contact is achieved.

Figure 5 and Table 1 show the device performance of a CIGS thin-film solar cell with bare AgNW and Ag@CdS NW TCEs. Due to unstable electrical contact, the cell with bare AgNWs has poor device performance. Deposition of a 2nd CdS layer greatly enhances the cell performance, as shown in the J-V characteristics in Figure 5. The cell with the Ag@CdS NW TCE showed a greater than 50% increase in device efficiency and FF compared to the bare AgNW TCE.

Figure 1
Figure 1: Chemical bath deposition setup. An image of the experimental setup for chemical bath deposition of CdS on CIGS. Please click here to view a larger version of this figure.

Figure 2
Figure 2: An SEM image of the AgNW TCE. The SEM image shows the spin-coated AgNW TCE on the CdS/CIGS/Mo structure. Please click here to view a larger version of this figure.

Figure 3
Figure 3: Schematic diagram of CIGS thin-film solar cells. Layer structure of a CIGS thin-film solar cell with (A) ZnO:Al/i-ZnO TCO and (B) AgNW TCE with a 2nd CdS layer. Please click here to view a larger version of this figure.

Figure 4
Figure 4: Structural analysis of Ag@CdS NW. (A) Cross-sectional TEM image along a Ag@CdS NW on a CdS/CIGS structure and (B) high-resolution TEM image across a Ag@CdS NW. Please click here to view a larger version of this figure.

Figure 5
Figure 5: Device performance comparison. J-V characteristics of CIGS thin-film solar cells with bare AgNW and Ag@CdS NW TCEs. Please click here to view a larger version of this figure.

Cell VOC (V) JSC (mA/cm2) Efficiency (%) FF (%)
Bare AgNW TCE 0.60 29.5 7.9 44
Ag@CdS TCE 0.65 32.3 14.2 67.2

Table 1: Device performance data. A summary of the device performance derived from the J-V curves.

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Discussion

Note that the deposition time of the 2nd CdS layer must be optimized to achieve the optimal cell performance. As the deposition time increases, the thickness of the 2nd CdS layer increases, and consequently, the electrical contact will improve. However, further deposition of the 2nd CdS layer will result in a thicker layer that reduces light absorption, and the device efficiency will decrease. We achieved the best cell performance with 10 min of deposition time for the 2nd CdS layer and determined that the cell efficiency decreased with longer deposition times.

To evaluate our method, we compared the device performance of the Ag@CdS NW-based CIGS solar cell with that of a standard CIGS solar cell with a sputtered ZnO:Al/i-ZnOl TCO, as described in Figure 3A14. The J-V characteristics were nearly equal, and the overall device performances were very similar. This result proves that our solution process method can produce a high-performance thin-film solar cell.

Various methods have been applied to enhance the electrical properties of AgNW TCE including the incorporation of conducive matrix. The method described in this protocol is simple and effective to enhance the electrical contact property between AgNWs and underlying CdS buffer layer in CIGS thin film solar cell. Due to the enhanced contact property, the solar cell performance is greatly improved. The method is designed to apply to the CdS/CIGS system but is not limited to the CdS/CIGS system. When an appropriate CBD method is created, our method can be applied to create high-quality electrical contact between AgNWs and the buffer layer in chalcogenide thin-film solar cells.

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Disclosures

The authors declare that they have no competing financial interests.

Acknowledgments

This research was supported by the In-House Research and Development Program of the Korea Institute of Energy Research (KIER) (B9-2411) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2016R1D1A1B03934840).

Materials

Name Company Catalog Number Comments
Mo Materion Purity: 3N5 Mo sputtering
Cu 5N Plus Purity: 4N7 CIGS deposition
In 5N Plus Purity: 5N CIGS deposition
Ga 5N Plus Purity: 5N CIGS deposition
Se 5N Plus Purity: 5N CIGS deposition
Ammonium acetate Alfa Aesar 11599 CdS reaction solution
Ammonium hydroxide Alfa Aesar L13168 CdS reaction solution
Cadmium acetate dihydrate Sigma-Aldrich 289159 CdS reaction solution
Thiourea Sigma-Aldrich T8656 CdS reaction solution
Silver Nanowire ACSMaterial AgNW-L30 AgNW dispersion

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References

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  8. Singh, M., Jiu, J., Sugahara, T., Suganuma, K. Thin-film copper indium gallium selenide solar cell based on low-temperature all-printing process. ACS Applied Materials and Interfaces. 6, (18), 16297-16303 (2014).
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