Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

Summary

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

Abstract

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (µc-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated µc-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

Introduction

There has been a long-standing demand for the application of functional nanostructures in a broad range of technological field. One of the expectations for this trend is to open new design of device architectures leading to improved or innovative performances. In the field of solar cells, for example, the use of metal nanostructures has been actively explored because of their intriguing optical (i.e., plasmonic) properties,¹ potentially beneficial to construct effective light trapping systems.^2,3 Indeed, some theoretical studies^4-6 have suggested that such plasmonic light trapping could achieve effects exceeding the conventional ray optics (texturing)-based light trapping limit.⁷ As a result, developing strategies to integrate desired metal nanostructures with solar cells has become increasingly important in order to realize these theoretical predictions.

A number of strategies have been proposed to meet this challenge.^8-24 These include, for instance, simple (low-cost) thermal annealing of metal films^8,9 or dispersion of pre-synthesized metal nanoparticles,^10,11 both of which resulted in successful demonstrations of plasmonic light trapping. However, it should be pointed out that the metal nanostructures fabricated by these approaches are usually challenging to match to the theoretical models. In contrast, the traditional nanofabrication techniques in semiconductor industries, such as photolithography and electron beam lithography,^12,13 can control structures well below the sub-100 nm level, but they are often too expensive and time-consuming to apply to solar cells, where large-area capability with low cost is essential. In order to fulfill the low-cost, high-throughput, and large-area requirements with nanoscale controllability, methods such as nanoimprint lithography,^14-16 soft lithography,^17,18 nanosphere lithography,^19-21 and hole-mask colloidal lithography^22-24 would be promising. Among these choices, we have developed a soft lithographic, advanced transfer printing technique.²⁵ Using a nanostructured poly(dimethylsiloxane) (PDMS) stamps and block copolymer-based adhesive layers, patterning of ordered metal nanostructures could be readily achieved on a number of technologically relevant materials, including the ones for solar cells.

The focus of this article is to describe the detailed procedure of our transfer printing approach to incorporate effective light trapping plasmonic nanostructures in existing solar cell structures. As a demonstrative case, Ag nanodisks and thin-film hydrogenated microcrystalline Si (µc-Si:H) solar cells were selected in this study (Figure 1),²⁶ although other types of metals and solar cells are compatible with this approach. Together with its process simplicity, the approach would be of interest to diverse researchers as a handy tool to integrate functional metal nanostructures with devices.

Protocol

1. Preparation of PDMS Stamps

1. Set a nanohole mold (nanoimprinted cyclo olefin polymer plastic film, size: 50 mm × 50 mm) in a polytetrafluoroethylene (PTFE) container.
2. Weigh vinylmethylsiloxane-dimethylsiloxane copolymer (0.76 g for the 50 mm × 50 mm mold) in a disposable glass bottle and mix it with Pt-divinyltetramethyldisiloxane complex (6 µl, using a digital micro pipette with a disposable polypropylene tip) and 2,4,6,8-tetramethyltetra-vinylcyclotetrasiloxane (24 µl, using a digital micro pipette with a disposable polypropylene tip).
3. Add methylhydrosiloxane-dimethylsiloxane copolymer (0.24 ml, using a digital micro pipette with a disposable polypropylene tip) in the glass bottle and mix it quickly using a disposable glass pipette. After the surface of the mold set in the PTFE container is blown by N₂, pour the resulting mixture (“hard” PDMS prepolymer) on the mold and start spin-coating at 1,000 rpm for 40 sec to achieve the layer thickness of ~40 µm.
4. Place the spin-coated sample in a hot chamber preheated at 65 °C for 30 min to briefly cross-link the hard PDMS.
5. During the heating, weigh silicone (6 g) and mix it with catalyst (0.6 g) in a disposable glass bottle. Place the glass bottle in a vacuum desiccator and apply vacuum (~133 Pa) for 15 min to remove the air trapped in the silicone mixture (“soft” PDMS prepolymer).
6. Take out the mold and soft PDMS prepolymer from the heating chamber and vacuum desiccator, respectively, and quickly pour the soft PDMS prepolymer onto the heated mold. The thickness of soft PDMS layer is ~3 mm.
7. Place the resulting sample in the vacuum desiccator again for further degassing at ~133 Pa at least for 1 hr.
8. Transfer the degassed sample to the heating chamber and start heating gradually up to 80 °C (heating rate ~3 °C/min). Keep this temperature for 5 hr to cross-link the hard and soft PDMS completely.
9. After cooling the sample down to RT, peel off the PDMS stamp carefully from the mold. Reuse the mold to prepare the second (or more) stamps, if necessary. Note: The same mold can be used at least five times without degradation of the stamp quality.
10. Cut the resulting nanopillar stamp (double-layered hard/soft PDMS composite)²⁷ into pieces of desired sizes (typically 7 mm × 7 mm for our solar cells) using a knife and store them under air until use.

2. Preparation of Block Copolymer Solutions

1. Weigh the powder of polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP) in a glass bottle and mix it with o-xylene in the ratio of 3 mg/ml (PS-b-P2VP/o-xylene).
2. Stir the mixture using a PTFE-coated magnetic stirring bar at 70 °C for 1 hr.
3. Keep the resulting solution for more than 24 h at RT without stirring to complete the formation of self-assembled micelles. Tightly seal the solution and store it under ambient conditions. Note: The quality of the solution is unchanged even one year after preparation.

3. Preparation of µc-Si:H Substrates

1. Wash glasses covered with SnO₂:F (denote glass/SnO₂:F, hereafter) with H₂O (500 ml), detergent (500 ml), and H₂O (500 ml) at RT using an ultrasonic bath (15 min for each). Dry them by blowing N₂.
2. Load the cleaned glass/SnO₂:F substrates in a substrate holder and deposit ZnO:Ga (20 nm) using a direct current (DC) sputtering system with the conditions specified in Table 2.
3. Load the glass/SnO₂:F/ZnO:Ga substrates in another substrate holder and deposit µc-Si:H p (10 nm), i (500 nm), and n (40 nm) layers using a plasma-enhanced chemical vapor deposition (PECVD) system with the conditions specified in Table 2.
4. Store the resulting µc-Si:H (glass/SnO₂:F/ZnO:Ga/µc-Si:H p-i-n) substrates under vacuum or N₂ until the transfer-printing step.

4. Ag-coating of PDMS Stamps

1. Wash the PDMS stamps (prepared in step 1) with EtOH (30 ml) using an ultrasonic bath for 15 min and dry by blowing N_2.
2. Load the cleaned PDMS stamps on a sample holder using double-sided adhesive tape and deposit an Ag film (10-80 nm) using an electron beam (EB) evaporation system with following conditions: deposition rate = 5-10 Å/sec, pressure = ~1.5 × 10^-4 Pa.
3. Take out the Ag-coated stamps from the EB evaporation system and use them immediately in the following transfer printing step.

5. Transfer Printing of Ag Nanodisks on Thin-film Si Surfaces

1. Take out the thin-film Si substrates stored under vacuum or N₂ and spin-coat with the PS-b-P2VP solution (0.3 ml for 50 mm × 50 mm sample, using a digital micro pipette with a disposable polypropylene tip) at 5,000 rpm for 40 sec.
2. Wet the PS-b-P2VP-coated surface with EtOH using a digital micro pipette (5 µm/cell area) and apply the Ag-coated PDMS stamp softly to the EtOH-wet surface. Do not press the stamp.
3. Place the thin-film Si substrate with the stamp in a vacuum chamber and apply vacuum (~133 Pa).
4. After 5 min, fill the vacuum chamber with air and take out the thin-film Si substrate.
5. Remove the stamp from the thin-film Si substrate by holding the both side of the stamp with tweezers to transfer-print Ag nanodisks. Note: If successful, the trace of stamping is visible as a greenish spot.
6. Rinse the transfer-printed thin-film Si substrate with a continuous flow of EtOH for 15 sec (~30 ml) and dry by blowing N₂.
7. Remove the PS-b-P2VP coating using an Ar plasma system.
   1. Place the transfer-printed thin-film Si substrate in the process chamber of the Ar plasma system.
   2. Pump out the air in the process chamber for ~5 min (pressure ~20 Pa).
   3. Open the valve of an Ar gas line and manually adjust the flow rate to 4 sccm. Wait for ~5 min to stabilize the pressure to 40 Pa.
   4. Generate Ar plasma for 108 sec.
   5. Close the valve of the Ar gas line, stop pumping, and fill air into the process chamber to take out the plasma-cleaned, transfer-printed thin-film Si substrates.

6. Completion of Thin-film Si Solar Cell Fabrication

1. Attach metal masks to the transfer-printed thin-film Si substrates after the Ar plasma treatment using polyimide tapes.
2. Load the masked substrates in a substrate holder of a DC sputtering system and deposit ZnO:Ga (100 nm), Ag (250 nm), and ZnO:Ga (40 nm) sequentially with the conditions specified in Table 2.
3. Detach the metal masks from the substrates and remove the masked thin-film Si layers (i.e., the area where ZnO:Ga and Ag were not deposited) using a reactive ion etching (RIE) system.
   1. Place the samples in the process chamber of the RIE system.
   2. Pump out the air in the process chamber following manufacturer’s instructions.
   3. Set the process conditions as follows following the manufacturer’s instruction: SF₆/O₂ flow rate =100/20 sccm, pressure = 20 Pa, power 100 W, time = 1 min 20 sec.
   4. Open the SF₆ and O₂ gas lines, stabilize the pressure, and generate plasma.
   5. Close the valve of the SF₆ and O₂ gas line, stop pumping, and fill N₂ into the process chamber to take out the samples.
4. Put the samples in a vacuum-annealing chamber and start heating gradually up to 175 °C under vacuum (~133 Pa). Keep this temperature for 2 hr, and then allow to cool to RT. Fill air in the chamber and take out samples, which now can be called cells.
5. Solder Sn-Zn-based alloy on the front transparent electrode (glass/SnO₂:F/ZnO:Ga, the exposed part by the RIE treatment) using an ultrasonic soldering device.

7. Measurement of External Quantum Efficiency (EQE)

1. Attach a light-shielding mask to a fabricated cell using polyimide tape and set the masked cell in a cell holder. Connect probes to the front soldered electrode (+) and back Ag/ZnO:Ga electrode (-).
2. Measure EQE spectra using an EQE measurement system following manufacturer’s instructions with a wavelength range and step of 300-1,100 nm and 5 nm, respectively.

8. Measurement of Photovoltaic Current-Voltage (J-V) Characteristics

1. Calibrate the light intensity of a J-V characteristics measurement system using an amorphous Si reference cell.
   1. Set the amorphous Si reference cell to a cell holder of the J-V characteristics measurement system, and illuminate the light.
   2. Read the photogenerated current using a digital multi-meter equipped in the J-V characteristics measurement system. Adjust the light intensity until the photogenerated current shows the right value for the reference cell (8.34 mA/cm²).
2. Attach a light-shielding mask to a cell and set the masked cell in a cell holder. Connect probes to the front soldered electrode (+) and back Ag/ZnO:Ga electrode (-).
3. Illuminate the calibrated light (100 mW/cm², 1 sun) on the cell and measure photogenerated currents using the J-V characteristics measurement system following manufacturer’s instructions with a voltage step of 0.02 V.

Results

Figure 2 outlines the general process for the transfer printing of Ag nanodisks on the surface of µc-Si:H (n layer). Briefly, an Ag film (thickness: 10-80 nm) is first deposited on the surface of a nanopillar PDMS stamp by electron beam evaporation. In parallel, a PS-b-P2VP solution is spin-coated on the surface of a freshly prepared µc-Si:H n layer. Subsequently, a droplet of EtOH is placed on the PS-b-P2VP-coated surface, and the Ag-deposited PDMS stamp is placed on ...

Discussion

In this article, a double-layered hard/soft PDMS composite was employed as stamp materials.²⁷ This combination was found to be essential to precisely replicate the parent nanostructure in the mold, which was a hexagonally close-packed round-hole array whose diameter of 230 nm, depth of 500 nm, and hole center-to-center spacing of 460 nm. When only soft PDMS was used, the stamp always resulted in a poorly nanostructured surface (for example, no sharp edge in the inverted pillar structure) due to the low Young&#...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nanohole mold	Scivax	FLH230/500-120
PTFE container	Eishin	n/a	Custom made
Hard-PDMS materials
Vinylmethylsiloxane-dimethylsiloxane copolymer	Gelest	VDT-731
Pt-divinyltetramethyldisiloxane complex	Gelest	SIP6831.1
Methylhydrosiloxane-dimethylsiloxane copolymer	Gelest	HMS-301
2,4,6,8-tetramethyltetra-vinylcyclotetrasiloxane	Sigma-Aldrich	396281	Additive for hard-PDMS
Soft-PDMS materials	Dow Corning	Sylgard-184	Silicone precursor
PS-b-P2VP	Polymer Source	P5742-S2VP	Mn × 103 = 133-b-132
Glass/SnO2:F substrates	Asahi Glass Co. Ltd.	Type VU	Chemical mechanical polished by D-process Inc. (http://d-process.jp/index.html) to flatten the surfaces
Detergent	Fruuchi Chemical Co. http://www.furuchi.co.jp/eng/main.htm	Semico-clean 56	Used for the cleaning of Glass/SnO2:F substrates
ZnO:Ga supputtering target	AGC Ceramics Co. Ltd.	5.7GZO
Ag supputtering target	Mitsubishi Materials Co.	4NAg
Double-sided adhesive tape	Nisshin EM Co.	732
Polyimide tape	Dupont	Kapton 650S#25
Sn-Zn-based Solder	Kuroda Techno Co., Ltd.	Cerasolzer AL-200
Digital micro pipette	Nichiryo	00-NPX2-20 00-NPX2-200 00-NPX2-1000
Heating chamber	Tokyo Rikakikai Co., Ltd.	VOS-201SD
Electron beam evaporator	Canon-Anelva	n/a	Custom made
Electron beam evaporator	Arios	n/a	Custom made
Sputtering system	Ulvac	SBR-2306
PECVD system	Shimadzu Emit Co. Ltd.	SLCM-13
Ar plasma system	Diner Electric Gmbh	Femto
RIE system	Samco Inc.	RIE-10NR
Ultrasonic soldering device	Colby-Eishin Enterprises, Inc.	SUNBONDER
EQE measurement system	Bunkoukeiki Co. Ltd.	CEP-25BXS
J-V characteristics measurement system	Bunkoukeiki Co. Ltd.	OTENTOSUN-5S-I/V
Amorphous Si reference cell	Bunkoukeiki Co. Ltd.	WPVS-NPB-S1	For light intensity calibration
Digital multi-meter	Keithley Instruments Inc.	2400