In2Se3 Nanowire Growth and Physical Characterization for Photovoltaic Use

Anna DubovitskayaNorth Carolina School of Science and Mathematics

Introduction: Why Nanowires?

The interest in the use of nanowires for photovoltaic cells stems from their unique properties:

• Higher efficiencies due to increased absorption• Higher tolerance for defects• Shorter diffusion lengths, which reduce trapping or recombination

of photoexcited charge carriers• Nanowires of the same high crystalline quality and excellent charge

transport as bulk/thin films can be made at a lower cost• Better resistance to photodegradation compared to existing

photovoltaics.

Goals and Hypothesis

The objective for this project was to grow In2Se3 nanowires and document the parameter ranges for optimal growth.

Hypothesis: there exist nanowire growth parameters that produce ideal or near-ideal nanowires suitable for further characterization.

Method: Growing NanowiresWe used the vapor-liquid-solid (VLS) method:

1. supersaturate a substrate with vaporized In2Se3 powder

2. Gold catalyzes the growth3. Solid deposition results in nanowire growth

The following diagram shows this process with silicon nanowires:

Garnett et al. Nanowire Solar Cells

Method: Growing NanowiresThe ideal temperature and atomic percentage parameter for nanowire growth can be approximated using a binary phase diagram, where the shaded area represents ideal silicon nanowire growth:

Garnett et al. Nanowire Solar Cells

ProcedureThe nanowires were grown using In2Se3 powder (mixed with graphite in some experiments)

1. Silicon substrates cleaned using RCA-12. Poly-l-lysine solution, left for 10 minutes (positive surface)3. Rinsed with deionized water and dried. 4. Nanoparticles (10-50nm) placed on wafer5. Horizontal tube furnace • Temperature• Pressure• Ratio

6. Analyzed with SEM and EDX

Procedure: Horizontal Tube FurnaceThe growth of the nanowires occurred in a horizontal tube furnace:

• A controlled atmosphere:• Evacuate the tube furnace to 1x10-3 torr • Backfill with Argon to the growth pressure

• gas flow is controlled with a mass flow controller, • constant pressure is maintained by a throttle valve connected to a

capacitance manometer.

Best Results

Figure 1: Graphite addition. There is clear forest-like axial growth, with some branched growth as well [Powder Temperature 900°C; Substrate Temperature 528.7-165.9°C; Pressure 0.85 Torr; Flow Rate 60 SCCM; Time 4 Hours]

Figure 2: The first instance of uniform, nonradial nanowire growth seen in the project [Powder Temperature 900°C; Substrate Temperature 650-400°C; Pressure 1.50 Torr; Flow Rate 50 SCCM; Time 0.5 Hours]

Figures 3&4: Domain sectioning [Same as Figure 2]

Best Results

Figure 5: This is the best result, with crystalline growth and uniform dimensionality [Powder Temperature 900°C; Substrate Temperature 650-400°C; Pressure 1.50 Torr; Flow Rate 50 SCCM; Time 0.5 Hours]

Figure 6: Same substrate as Figure 5, but a different temperature zone. The growth direction is more random and the domains seen in Figures 3&4 are absent [Same as Figure 2]

Best Results: EDXThe previous results showed SEM images of the best results. For figure 5, the EDX analysis graph is below:

This shows In:Se ratios of around 1:1 or 3:4

Conclusions

It was found that the ideal parameters include:• Pressure: 1.0~1.5 torr• Flow rate: 27~40 SCCM• higher flow rates can impede the pressure, placing a limit on it

due to the pump’s inability to deal with the flow rate of the incoming gas

• Substrate Temperature: 640°C• manipulation of the Furnace Distance vs Temperature graph.

• Powder Temperature: 825°C• Gold Particle Diameter : 50nm• double coating.

Conclusions: Observations

The following additional observations were made:

• Graphite addition does not have as much impact as first thought with regard to NW growth

• Figure 5, the most ideal out of the 6 figures, had nanowire diameter ranges from 45-55nm using 50nm gold nanoparticle catalysts

• Ordered nanowires (the opposite of the sectioning effect in Figure 4) may be better for light trapping

• EDX results for Figures 2-6 showed no leftover presence of gold nanoparticles, which may indicate non-VLS growth

• High powder temperatures result in increased uncontrollable deposition (2D growth) as well as deposition before the start of the attempt

• Using gold particles that are spatially far apart or small in size results in greater 2D excess deposition

Future Work

Having physically characterized the In2Se3 nanowires, the next step will be to optically characterize them.

Since these nanowires were made with the intention of use in photovoltaic solar cells, their optical properties are crucial in determining their usefulness and maximum efficiency.

In particular, we want to test the time it takes for recombination of electron-hole pairs, and the absorption efficiency of In2Se3 nanowires.

AcknowledgementsDr. Jonathan Bennett

Department of Physics

North Carolina School of Science and Mathematics

Dr. Todd Roberts

Chancellor

North Carolina School of Science and Math

Dr. Marvin Wu,

Department of Physics

North Carolina Central University

Funding provided by: NCSSM Foundation Summer Physical Science Research Program, National Science Foundation, National Aeronautics and Space Administration