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

Silver nanowire networks are an emerging technology to replace traditional transparent conductive oxides in the thin-film solar cell application. However, the electrical contact to the underlying layer has been a problem. Our protocol is a simple process method to enhance electrical contact property between silver nanowire network and the underlying CdS buffer layer in CIGS thin-film solar cells.

Our method is a very simple, reproducible, and cheap solution-based process. It is also comparable to the existing solution-based process, to fabricate CIGS thin-film solar cells. First, load cleaned glass substrates into a DC magnetron, and pump down to blow four times 10 to the minus six torr.

Flow argon gas, and set the working pressure to 20 millitorr. Turn on the plasma, and increase the DC output power to three kilowatts. After pre-sputtering three minutes for target cleaning, begin the molybdenum deposition until the molybdenum film thickness reaches approximately 350 nanometers.

Next, set the working pressure to 15 millitorr, while maintaining the output power at 3 kilowatts. Resume the molybdenum deposition until the total thickness of molybdenum reaches approximately 750 nanometers. Load the molybdenum-coated glass into a preheated co-evaporator under a vacuum lower than 5 times 10 to the minus 6 torr.

Set the temperatures of the indium, gallium, and selenium effusion cells yielding deposition rates of 2.5, 1.3, and 15 angstroms per second, respectively. Check the deposition rates using the quartz crystal microbalance technique. Begin to supply indium, gallium, and selenium onto the molybdenum-coated glass to form a one-micrometer-thick indium-gallium-selenium precursor layer at a substrate temperature of 450 degrees Celsius.

After 15 minutes, stop the indium and gallium supplies and increase the substrate temperature to 550 degrees Celsius. Next, begin to supply copper onto the indium-gallium-selenium precursor and continue until the copper-to-indium+gallium compositional ratio of the film reaches 1.15. Stop supplying copper and evaporate the indium and gallium again with the same deposition rates as the first stage to form an approximately 2-micrometer-thick CIGS film with a copper-to-indium+gallium compositional ratio of 0.9.

Maintain the selenium deposition rate and substrate temperature at 15 angstroms per second and 550 degrees Celsius, respectively. In order to ensure a complete reaction, anneal the deposited CIGS film under ambient selenium for 5 minutes at a substrate temperature of 550 degrees Celsius. Decrease the substrate temperature to 450 degrees Celsius under ambient selenium, and then unload the CIGS deposited substrate when the substrate temperature is below 250 degrees Celsius.

Prepare a cadmium sulfide reaction bath solution in a 250 milliliter beaker, by adding deionized water, cadmium acetate dihydrate, thiourea, and ammonium acetate. Stir the solution for several minutes until homogeneous. Add 3 milliliters of ammonium hydroxide to the bath solution and stir for 2 minutes.

Next, place the CIGS sample into the reaction bath solution using a Teflon sample holder. Place the reaction bath into a water heat bath, maintained at 65 degrees Celsius. Stir the reaction bath solution at 200 RPM during the deposition process, allowing the reaction to proceed for 20 minutes to generate an approximately 70-80 nanometer cadmium sulfide buffer layer on the CIGS.

After the reaction, remove the sample from the reaction bath, wash with a flow of deionized water, and dry with nitrogen gas. Anneal the sample at 120 degrees Celsius for 30 minutes on a preheated hotplate. Prepare a 1-milligram-per-milliliter silver nanowire dispersion by mixing 90 milliliters of ethanol with 1 milliliter of a 20-milligram-per-milliliter ethanol-based silver nanowire dispersion.

Pour 0.2 milliliters of the diluted silver nanowire dispersion onto a cadmium sulfide CIGS sample, to cover the whole surface, and rotate the sample at 1000 RPM for 30 seconds. Following this, spin-coat the silver nanowires 3 times. After spin-coating, anneal the sample at 120 degrees Celsius for 5 minutes on a preheated hotplate.

Prepare a new cadmium sulfide reaction bath solution as previously described. Deposit cadmium sulfide as previously described, except change the reaction time, as necessary. Now, characterize the surface morphology of cadmium sulfide-coated silver nanowires by optical microscopy.

Measure solar cell performance using a current voltage source equipped with a solar simulator. The layer structures of the CIGS solar cells with standard aluminum-doped zinc oxide on intrinsic zinc oxide and silver nanowire network transparent conducting electrodes are shown here. The second cadmium sulfide layer can be selectively deposited onto the nanoscale gap to create a stable electrical contact.

Cross-sectional transmission electron microscopy images, along the second cadmium sulfide layer, deposited on the silver nanowire network on the cadmium sulfide CIGS structure, and across the second cadmium sulfide layer deposited on the silver nanowire network, are shown here. The second cadmium sulfide layer is uniformly deposited on the surface of the silver nanowires and the cadmium layer on the core-shell silver nanowire structure is produced. The second cadmium sulfide layer fills the air gaps between the cadmium buffer and silver nanowire layers, and stable electrical contact is achieved.

The device performance of a CIGS thin-film solar cell with bare silver nanowires, and the cadmium sulfide layer on the core-shell silver nanowire transparent conducting electrodes is shown here. Due to unstable electrical contact the cell with bare silver nanowires has poor device performance. Deposition of a second cadmium layer greatly enhances the cell performance.

The most important step in our protocol is to fabricate a second CdS layer on the silver nanowire network. The divergent time can be optimized by measuring the device performance of the CIGS thin-film solar cell. We suggest a method to fabricate robust nano-scale electrical contact in the CIGS system.

We believe our method can be applied to other solar cell systems, which require enhancement of electrical contact properties.