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Solar & Alternative Energy

Building radial junction thin-film solar cells on silicon nanowires

Mature silicon thin-film deposition techniques combined with advanced radial junction designs could yield a new generation of photovoltaics that compete on cost with national power grid infrastructures.
10 May 2012, SPIE Newsroom. DOI: 10.1117/2.1201204.004224

Radial junction thin-film solar cells grown over silicon nanowires (SiNWs) offer an opportunity to decouple the path of light absorption and carrier separation. Silicon thin-film deposition is an industrially proven and environmentally friendly technology that has the potential to deliver terawatt-scale solar energy without presenting a material availability issue. Combining advanced radial junction designs with mature Si thin-film deposition techniques could lead to a new generation of photovoltaics capable of bridging the gap toward grid parity.

One critical challenge is to grow the SiNWs using a standard industrial technique: ideally, using plasma-enhanced chemical vapor deposition (PECVD) systems, on low-cost heterogeneous substrates, and at low process temperatures. High throughputs of SiNWs can be produced on top of inexpensive glass or metal surfaces via a metal-nanoparticle-catalyzed vapor-liquid-solid method. However contamination from the metal catalyst during growth can reduce carrier lifetime in the material. Commonly used gold (Au) catalysts are known to cause mid-gap recombination centers. As a consequence, Au-catalyzed SiNWs require ex situ high-temperature oxidation and chemical cleaning steps before being incorporated into photovoltaic devices. Prototype solar cells built on untreated Au-catalyzed SiNWs have so far suffered from low Voc (<0.3V) and poor fill factors. To address this issue, we have been working on a group of low-melting-point alternative catalysts, including indium, tin, and gallium to catalyze SiNW growth in a PECVD system on low-cost substrates.1–5 Besides the immediate benefit of achieving growth at temperatures as low as 240°C,3 these catalysts introduce shallow levels in the c-Si (crystalline silicon) bandgap and show evidence of removal from the SiNWs by in situ hydrogen plasma treatment.4–7

Figure 1. (a and b): Plasma-assisted in situ catalyst formation and vapor-liquid-solid (VLS) silicon nanowire (SiNW) growth mechanism in a standard plasma-enhanced chemical vapor deposition reactor. (c) Scanning electron microscope (SEM) images of indium catalyst drops formed on top of zinc oxide (ZnO)/aluminum-coated glass at various temperatures during a hydrogen plasma process, and of SiNWs.8 PECVD: Plasma-enhanced chemical vapor deposition.

To incorporate SiNWs into Si thin films deposited by PECVD, we have established an in situ catalyst formation control to tailor the density and size distribution of the SiNWs. Catalyst drops several tens of nanometers wide can be formed by hydrogen plasma treatment of a ZnO/Al (zinc oxide/aluminum)-glass substrate coated with a thin ITO (indium tin oxide) or SnO2 (tin oxide) layer: see Figure 1(a). The distribution, density, and size dispersion of the catalyst particles can be controlled by appropriate plasma conditions: see Figure 1(c). This control allows us to engineer the SiNW arrangements and achieve the best trade-off between light trapping and uniform coverage.

We have also made SiNWs by a vapor-liquid-solid method mediated by low-melting-point/low-surface-tension catalysts. Having investigated their growth mechanism, we resolved the paradox concerning the growth stability of SiNWs mediated by the low-surface-energy catalyst.6, 7 As shown in Figure 1(b), the growth of p-type SiNWs can be triggered by introducing a mixture of H2 (hydrogen gas), SiH4 (silicon hydride gas), and trimethylboron at 240–500°C. After the SiNW growth, the cata-lyst drops can be removed in a H2 plasma environment, as observed in the scanning electron microscope (SEM) images presented in Figure 1(c).

Figure 2. (a) Schematic of the multilayer configuration of a single radial junction thin-film solar cell unit. (b and d) Corresponding SEM images of the radial junction structure. (c) Photograph of a 5×5cm2SiNW solar cell sample.8

We then carried out a systematic and broad parametric study on the construction of radial junction a-Si:H (amorphous silicon hydride) thin-film cells over the SiNWs, and explored the impact of doping, length, density of the SiNWs, and the absorber thickness on the performance of our devices.8–12 Figure 2(a) and (b) shows a schematic illustration and SEM image of a complete a-Si:H radial junction p-i-n SiNW solar cell. The final fabrication step is an ITO contact layer (∼120nm thick) on the top of the junctions that makes it possible to shed light through the cell surface. Figure 2(c) shows a photograph of the SiNW radial junction solar cell on a 5×5cm2 substrate (with a 40nm-thick i-layer). Based on the effective density control in catalyst formation, we are able to fabricate solar cells with SiNW density ranging from 2.3×107/cm2 to 6.3×108/cm2—see Figure 3(a–c)—and evaluate the direct impact of nanowire density on the density-voltage characteristics and external quantum efficiency spectrum of the devices. Recent advances on Sn-catalyzed SiNW solar cells have led to efficiencies of up to 5.6%.8–10 These results set an encouraging benchmark for further optimization of radial junction thin-film photovoltaics.

Figure 3. (a–c) SEM images of the radial junction a-Si:H (amorphous silicon hydride) thin-film solar cells obtained on top of SiNWs with different densities. Corresponding cells after top indium tin oxide contact coating are shown in inset. (d and e) The evolution of density-voltage characteristics and external quantum efficiency (EQE).8

Our future work will aim at adopting atomic layer deposition for a high-quality conformal transparent conductive oxide coating layer. We will also explore depositing and optimizing hydrogenated microcrystalline silicon as an intrinsic absorber for a radial junction solar cell, investigate the benefits of thin-film tandem radial junction structures, and the eventual deployment of a new generation of high-performance and cost-effective SiNW thin-film solar cell.

Linwei Yu, Martin Foldyna, Benedict O'Donnell, Gennaro Picardi, Pere Roca i Cabarrocas
Laboratory for the Physics of Interfaces and Thin Films
École Polytechnique, CNRS
Palaiseau, France

Linwei Yu is a permanent CNRS researcher. His work focuses on the development of next-generation thin-film photovoltaics: the growth mechanism, transport, and optical properties of semiconductor nanostructures, thin-film transistors, sensors, flexible electronics, and single-electron logic.

1. I. Zardo, S. Conesa-Boj, S. Estradé, L. Yu, F. Peiro, P. Roca i Cabarrocas, J. Morante, J. Arbiol, A. Fontcuberta i Morral, Growth study of indium-catalyzed silicon nanowires by plasma enhanced chemical vapor deposition, Appl. Phys. A: Mater. Sci. Process. 100(1), p. 287-296, 2010.
2. I. Zardo, L. Yu, S. Conesa-Boj, S. Estrade, P. J. Alet, J. Rossler, M. Frimmer, P. Roca i Cabarrocas, F. Peiro, J. Arbiol, J. R. Morante, A. Fontcuberta i Morral, Gallium assisted plasma enhanced chemical vapor deposition of silicon nanowires, Nanotechnology 20(15), p. 155602, 2009.
3. L. Yu, B. O'Donnell, P.-J. Alet, S. Conesa-Boj, F. Peiro, J. Arbiol, P. Roca i Cabarrocas, Plasma-enhanced low temperature growth of silicon nanowires and hierarchical structures by using tin and indium catalysts, Nanotechnology 20(22), p. 225604, 2009.
4. L. Yu, P.-J. Alet, G. Picardi, I. Maurin, P. Roca i Cabarrocas, Synthesis, morphology, and compositional evolution of silicon nanowires directly grown on SnO2 substrates, Nanotechnology 19(48), p. 485605, 2008.
5. P.-J. Alet, L. Yu, G. Patriarche, S. Palacin, P. Roca i Cabarrocas, In situ generation of indium catalysts to grow crystalline silicon nanowires at low temperature on ITO, J. Mater. Chem. 18(43), p. 5187-5189, 2008.
6. L. Yu, F. Fortuna, B. O'Donnell, G. Patriache, P. Roca i Cabarrocas, Stability and evolution of low-surface-tension metal catalyzed growth of silicon nanowires, Appl. Phys. Lett. 98(12), p. 123113, 2011.
7. L. Yu, B. O'Donnell, J.-L. Maurice, P. Roca i Cabarrocas, Core-shell structure and unique faceting of Sn-catalyzed silicon nanowires, Appl. Phys. Lett. 97(2), p. 023107, 2010.
8. L. Yu, B. O'Donnell, M. Martin Foldyna, P. Roca i Cabarrocas, Radial junction amorphous silicon solar cells on PECVD grown silicon nanowires, Nanotechnology. In press.
9. B. O'Donnell, L. Yu, M. Foldyna, P. Roca i Cabarrocas, Silicon nanowire solar cells grown by PECVD, J. Non-Cryst. Solids, 2011. doi:10.1016/j.jnoncrysol.2011.11.026
10. J. Cho, B. O'Donnell, L. Yu, K.-H. Kim, I. Ngo, P. Roca i Cabarrocas, Sn-catalyzed silicon nanowire solar cells with 4.9% efficiency grown on glass, Prog. Photovolt. Res. Appl., 2012. In press. doi:10.1002/pip.1245
11. L. Yu, B. O'Donnell, P.-J. Alet, P. Roca i Cabarrocas, All-in-situ fabrication and characterization of silicon nanowires on TCO/glass substrates for photovoltaic application, Solar Energy Mater. Solar Cells 94(11), p. 1855-1859, 2010.
12. M. Foldyna, L. Yu, B. O'Donnell, P. Roca i Cabarrocas, Optical absorption in vertical silicon nanowires for solar cell applications, Proc. SPIE 8111, p. 811110, 2011. doi:10.1117/12.892690