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

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.

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 cells^1,2,3,4,5,6. 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 cells^12,13. 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 (2^nd 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 2^nd CdS layer in a CIGS thin-film solar cell.

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)_xSe_y precursor layer at the substrate temperature of 450 °C. The deposition time is 15 min (namely, 1^st 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)_xSe_y 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 2^nd stage (namely, 2^nd stage).
6. Stop supplying Cu and evaporate In and Ga again with the same deposition rates as the 1^st 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 3^rd 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 2^nd 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 2^nd 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).

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 2^nd CdS layer can be selectively deposited onto the nanoscale gap to create a stable electrical contact. The detailed explanation on the format...

Discussion

Note that the deposition time of the 2^nd CdS layer must be optimized to achieve the optimal cell performance. As the deposition time increases, the thickness of the 2^nd CdS layer increases, and consequently, the electrical contact will improve. However, further deposition of the 2^nd 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 2^nd CdS ...

Materials

List of materials used in this article

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