a-SiC:H Films used as Photoelectrodes in a Hybrid, Thin-film Silicon

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a-SiC:H Films used as Photoelectrodes in a Hybrid, Thin-film Silicon Photoelectrochemical (PEC) Cell for progress toward 10% Solar-to Hydrogen Efficiency
Feng Zhua,b, Jian Hua, Augusto Kunratha , Ilvydas Matulionisb, Bjorn Marsenc, Brian Colec, Eric Millerc, and Arun Madana,b a: MVSystems, Inc., 500 Corporate Circle, Suite L, Golden, CO, USA 80401, www.mvsystems.info, fzhu@mvsystemsinc.com, Ph: (303) 271-9907, Fax: (303) 271-9771 b: Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO c: Hawaii Natural Energy Institute, University of Hawaii at Manoa, 1680 East-West Road, POST 109, Honolulu, HI 96822, Ph: (808) 956-8890, Fax: (808) 956-2336, Email: hnei@hawaii.edu

ABSTRACT
In this paper we describe the fabrication of amorphous SiC:H materials and its use as a photoelectrode in a photoelectrochemical (PEC) cell. With the increase of CH4 flow (in SiH4 gas mixture) during growth, the bandgap, Eg, increases from ~ 1.8eV, corresponding to amorphous silicon materail (a-SiH), to ~2.1eV, while the photoconductivity decreases from ~10-5 S/cm to ~10-8 S/cm. This high-quality a-SiC:H materials with Eg of 2.0eV when included into a solar cell configuration led to a conversion efficiency,η ~7% on textured Asahi U type SnO2 coated substrates, with the i-layer thickness of ~300nm. For a reduced i-layer thickness of ~100 nm, a current density, Jsc ~8.45mA/cm2 has been achieved, Immersing the a-Si:B:H/a-SiC:H structure in 0.33M H3PO4 electrolytes, produced a photocurrent of ~7mA/cm2. With a further optimization we expect that the photocurrent could exceed 9 mA/cm2. With the use of this configuration, it may therefore be possible to increase the solar-to-hydrogen (STH) efficiencies to beyond 10%.

Keywords: amorphous silicon carbon, photoelectrochemical, hydrogen, solar-to-hydrogen 1. INTRODUCTION
Hydrogen is emerging as an alternative energy carrier to fossil fuels. There are many advantages of hydrogen as a universal energy medium. For example, it is non-toxic and its combustion results in the formation of steam and liquid water. It is possible to produce from the most abundant material on earth: water. Hydrogen can be obtained electrolytically, photoelectrochemically, thermochemically, by direct thermal decomposition or biochemically from water. Hence the cleanliness and renewable aspect of the energy source is of critical importance. While a hydrogenoxygen fuel cell operates without generating harmful emissions, just steam, hydrogen production can give rise to significant greenhouse gases and other harmful byproducts, like direct electrolysis, steam-methane reformation, thermochemical decomposition of water and so on. Photochemical hydrogen production is similar to a thermo-chemical system, in that it also employs a system of chemical reactants, which leads to the splitting of water. However, the driving force is not thermal energy but light, i.e solar irradiation. In this sense, this system is similar to the photosynthetic system present in green plants. In its simplest form, a photoelectrochemical (PEC) hydrogen cell consists of a semiconductor electrode and a metal counter electrode immersed in an aqueous electrolyte. 1,2,3 Photoelectrolysis of water, first reported in the early 1970’s4, has recently received renewed interest since it offers a renewable, non-polluting approach to hydrogen production. At the 2006 SPIE Conference on Solar Hydrogen we

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reported that the photoelectrochemical (PEC) device performance which would lead to ~3% STH efficiency5. The device configuration of our “hybrid” PEC cells consisted of substrate/amorphous silicon (ni2pni1p)/ZnO/WO3, where substrate/amorphous silicon (ni2pni1p)/ZnO used an amorphous silicon (a-Si) tandem solar cell structure. Light enters the WO3 film, where it is partially absorbed and the residual of the spectra is absorbed in the a-Si photovoltaic device structure. The efficiency of the device in this configuration was limited by the bandgap (2.6-2.8eV) of the WO3 film, which can absorb only a small fraction of the Global AM1.5 spectrum and hence produced a very small current. As shown in Fig.1, under Global AM1.5 spectrum the available current is a function of Eg of the absorbing layer. Since the current of WO3 is far lower than that of a-Si:H devices, the current of the PEC cells is thus limited by WO3 leading to a low STH efficiency ~3%.

Fig 1. Current available in the AM1.5 spectrum as a function of bandgap of the absorbing layer5

In order to achieve 10% STH efficiency, an operating point around 8.1mA/cm2 is needed in the PEC device5. Therefore, the photoactive electrode top layer in the hybrid PEC device needs to generate at least this much current, and hence an Eg of ~2.3 eV or narrower (as shown in Fig.1) is required. Among the candidates for water splitting materials, we are exploring the use of a-SiC:H and SiNx materials, which can be routinely grown using the PECVD ( plasma enhanced chemical vapor deposition) technique. It is also possible that these types of materials could be more stable physically and chemically than Si, and whose Eg can be tailored into the ideal range by the control of stoichiometry. Prior studies on aSiC:H have shown the ability to generate photocurrent, which enabled solar cells with Jsc = 10.6mA/cm2 and open circuit voltage, Voc = 0.92V with an absorber Eg of 2.0eV6. It is possible, therefore, that in addition to generating the photocurrent that is necessary for >10% STH efficiency, the a-SiC:H photoactive layer, when in contact with the electrolyte, could also produce a significant photovoltage. This photovoltage would reduce the voltage that is needed in the photovoltaic junction(s) in order to split water. In this paper, we describe the preparation of a-SiC:H materials, its inclusion into devices and its performance in electrolyte and we will share our vision of the path forward to >10% STH efficiency, that could make these devices commercially viable.

2. EXPERIMENTAL
A-SiC:H films were fabricated in a PECVD cluster tool system specifically designed for the thin film semiconductor market and manufactured by MVSystems, Inc. The samples were prepared on a 30 x 40 cm2 substrate carrier situated on the anode side of the RF electrode assembly. The intrinsic a-SiC:H films were deposited using CH4 and SiH4 at 200°C. The flow rates were varied from 0 to 12 sccm for CH4. Prior to deposition, the system was pumped to 10-6 Torr. During the deposition, the pressure was kept at 550 mTorr. The substrates used in this experiment were Corning type 1737 glass, textured Asahi U type SnO2 coated glass and crystalline Si wafers. Asahi U type substrates were used in solar cells and photoelectrolysis experiments at HNEI, based on a three-electrode system. For solar cells, the p-layer used was aSiC:H:B and was deposited from SiH4, CH4, and B2H6 gas mixtures while the n-layer was prepared using SiH4 and PH3

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gas mixture. The 1737 glass substrates were used for determining the optoelectronic properties, such as Eg and conductivity, and the Si wafer was used to obtain IR (infra red spectroscopy) data. All films were examined under Golbal AM 1.5 light source using a Xenon lamp with an intensity of ~100mW/cm2 for conductivity and photocurrent measurements. Eg was determined using Tauc’s plot.

3. RESULTS AND DISCUSSION
3.1 a-SiC:H materials Fig.2 shows the bandgap, photoconductivity, σph and γ as function of CH4 flow rate used during growth in SiH4 gas mixtures. With the increase of CH4 flow (0-12sccm) Eg increases from ~1.8eV to ~2.06eV(Fig.2(a)) while the dark conductivity, σd of the films decreases to <10-12 S/cm, which is the limit of the sensitivity of our measurement technique. We also note that σph (as measured under an approximate white light Global AM1.5 illumination source) decreases from 10-6 to 10-8 S/cm when CH4 flow rate is increased in the gas mixture (Fig.2(b)). Here, a parameter, gamma (γ), is defined from σph∝ Fγ, where σph is the photoconductivity and F is the illumination intensity; we infer the DOS (density of defect states) of the amorphous semiconductor from this measurement7. Device quality a-Si materials generally exhibit γ >0.9. As shown in Fig.2 (b), at CH4 flow rate of ~12 sccm, γ decreases to a low value of ~0.7, indicative of a material with high defect states. For CH4 flow rate < 8 sccm, γ > 0.9, which indicates the DOS in materials is low.
p h o to c o n d u c tiv ity (s /c m )
2. 08 2. 06 2. 04

1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09
0 2 4 6 8 10 12 14

1.2 1 0.8 0.6 0.4 0.2 0 0 2 4 6 8 10 12 14 CH4 flow (sccm) Gamma

bandgap ( eV)

2. 02 2 1. 98 1. 96 1. 94 1. 92 1. 9 1. 88

CH4 f l ow ( sccm )

(a) Bandgap vs. CH4 flow rate.

(b) Photoconductivity, σph and γ vs. CH4 flow rate.

Fig 2. Bandgap, photoconductivity, σph and γ are plotted as function of the CH4 flow rate used during the PECVD fabrication of a-SiC:H material. Evidence of carbon incorporation in the films can be discerned from the use of Infrared (IR) spectroscopy. As shown in Fig 3, the peak at 2000 cm-1 shifts towards ~2080 cm-1 once CH4 flow rate increases to 4 sccm. This shift of Si-H stretching vibration mode is mainly caused by incorporation of C atoms, and probably due to the back-bonding of the Si atoms to carbon.

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0.2 0.18 IR absorption (a.u.) 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0
no CH4 2sccm 4sccm 5sccm 6sccm 7sccm 8sccm 9sccm

CH4 flow increase

1800 1850 1900 1950 2000 2050 2100 2150 2200

wave number (cm-1)

Fig.3 IR spectra of a-SiC:H deposited with different CH4 flow rates. As shown in Fig. 2(b), σph decreases with increasing CH4 flow rate implying an increase of the recombination centers in the material and is in contrast with γ exceeding >0.9. However σph, as measured under white light illumination, does not take into account the increase of Eg and a concomitant reduction in the absorption coefficient. In order to evaluate this further, an appropriate way is to measure the photocurrent, Ip, at a long wavelength, λ, (600nm), where there should be uniform bulk absorption. The photocurrent can be expressed as, Ip = e.Nph (λ) (1-Rλ) (1 – exp ( –αλ d))ηĩ/tt, Where Nph(λ) is the photon flux, Rλ is the reflection coefficient, αλ is the absorption coefficient, d is the film thickness, η is the quantum efficiency of photo generation, ĩ is the recombination lifetime and tt is the transit time. Assuming that η, ĩ, tt and (1-Rλ) are constant for different films (i.e. different Eg), then to a first order approximation, X= Ip /(1– exp(– αλd)), can account for the changes in the absorption coefficient as Eg increases. This is shown in Fig.4 and it is noted that the normalized current, X, does not change significantly as Eg increases. This is in contrast to the decrease in σph with Eg shown in Fig.2. This then suggests that the DOS has remained low and is consistent with γ ~1 (low DOS) throughout the range and up to Eg~2.0eV.

1.0E-08 normalized ip Normalized Current (Amps) Eg 1.0E-09

2.4 2.2 2 Eg (eV) 1.8 1.6

1.0E-10

1.4 1.2

1.0E-11 0 2 4 6 8 10 12 14 CH4 flow rate (sccm)

1

Fig.4. Normalized photocurrent Ip and Eg as function of the CH4 flow rate.

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3.2 Solar cells using a-SiC:H material as the absorber. The previous results suggest that high quality a-SiC:H materials can be fabricated with Eg of ~2.0eV. To test the viability of the material in a junction, we have incorporated the material into p-i-n structure and constructed as normal solar cells in the configuration, glass/Asahi U Type SnO2 substrate/p-a-SiC:B:H/i-a-SiC:H/n-a-Si/Ag. The Ag top contact defines the device area as to be 0.25cm2. The thickness of the i-layer is ~300nm. Figs.5(a) and (b) show the J-V and QE (quantum efficiency) curves, respectively. Under AM1.5 white light illumination, Voc = 0.91V, Jsc=11.64mA/ cm2, FF (fill factor)=0.657, FF under blue (400nm) illumination and FF under red (600nm) illumination both exhibit 0.7, which indicates this is a good device and that a-SiC:H material is of high-quality. Compared with the normal a-Si:H devices ( Eg~1.8eV), the QE response peak shifts towards a shorter wavelength; whereas at long wavelength the QE response is reduced due to the increase of Eg. Jsc of ~8.45 mA/cm2 has been obtained with reduced a-SiC:H intrinsic layer thickness (~100nm). This shows that it is possible to use amorphous SiC:H as a photoelectrode in PEC cells.
14 12 10 Jsc (mA/cm2) QE 8 6 4 2 0 0. 00 0. 20 0. 40 0. 60 V ( v) 0. 80 1. 00 Voc=0.91v Jsc=11.64mA/cm2 FF=0.657, efficiency=6.96 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 400 450 500 550 600 650 700 750 800 850 900 Wavelength (nm ) a-SiC a-Si

(a) J-V curve (b) QE curve Fig.5 The J-V and QE curve of a-SiC solar cell. 3.3 Photocurrent and stability measurement of a-SiC:H film in phosphoric acid (H3PO4). In order to use a-SiC:H as the photoelectrode, the photocurrent and stability of a-SiC:H films in electrolyte are important issues. Fig.7 shows an a-SiC:H photoelectrode consisted of a-SiC:H(p)/a-SiC:H(i) structure, prepared on Asahi U type SnO2 substrates. The contact to the Asahi U type SnO2 substrate is made by soldering leads to the uncoated SnO2 bottom, followed by a-SiC:H intrinsic layer, which is the layer in contact with the electrolyte. We suspect that the electrolyte acts like the n-layer in a solar cell, with the result that the semiconductor/electrolyte behaves like a solid-state a-SiC:H p-i-n junction. The intrinsic layer provides a low defect layer where the photo-generated carriers drift without recombination, under the influence of the built-in field made possible by the p and the n layers. The photoelectrodes were subjected to current-vs-potential scans in a three-electrode setup, using a Pt sheet counter-electrode and a saturated calomel electrode (SCE) as the reference, in 0.33M H3PO4 electrolytes. Current-vs-potential scans were recorded in the dark and under simulated AM1.5 Global light produced by an Oriel 1-kW solar simulator.

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0 photocurrent @ 1.5Vsce (mA/cm2)
photocurrent @ a.5Vsce (mA/cm2)

0.00

a-SiC:H intrinsic layer: 300nm -1 -2 -3 -4 -5 -6 2.50

-1.00 -2.00 -3.00 -4.00 -5.00 -6.00 -7.00 -8.00 0 100 200

B2H6 flow : 3.5 sccm

3.00

3.50 B2H6 f low (sccom)

4.00

4.50

300

400

500

a-SiC:H intrinsic layer (nm)

(a) Photocurrent vs. B2H6 doping

(b) Photocurrent vs. Thickness of i-layer

Fig.6 Effects of B2H6 doping in p-layer and i-layer thickness on photocurrent. Fig.6 shows the effect of B2H6 doping in p-layer and i-layer thickness on the photocurrent. In Fig.6(a), B2H6 doping flow varies from 2.7sccm to 4sccm in the reactant gas, while the p-layer and the a-SiC:H intrinsic layer thickness is kept at 30nm and 300nm respectively. Photocurrent increases at first, and then decreases as B2H6 flow rate increases. The effect of a-SiC:H intrinsic layer thickness on photocurrent is shown in Fig.6 (b), where p-layer B2H6 doping flow is 3.5sccm. As i-layer thickness reduces, photocurrent increases. At thickness of 100nm, photocurrent is ~ 7mA/cm2, which is in contrary to that in PIN structure device, where current increases with thickness. At present we are trying to understand this behavior. Fig.7 shows the image of samples after this test. No corrosion could be noted and hence a-SiC:H film may be stable.

Fig.7 Optical scan images of a a-SiC::H(p)/a-SiC:H(i) structure electrode after photoelectrochemical testing in phosphoric acid (H3PO4). 3.4 Pathway to 10% solar-to-hydrogen efficiency (STH) in PEC. Fig.8 shows the band diagram for a PEC cell, which differs that when WO3 is used as the photolectrode5. Instead of oxygen evolution at the photoelectrode surface, hydrogen evolution occurs. This is because a-SiC:H acts as a p-type semiconductor. Similar to the use of WO3 photolectrodes, the use of a-SiC:H behaves very much like a triple junction solar cell.

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2H2O +4h+→4H+ +O2↑ 4H++4e-→2H2 ↑

Fig.8 Diagram of band diagram for a PEC cell with a p-type a-SiC photoelectrode. A-SiC:H is used routinely as a window layer in a-Si solar cells since its Eg increases with increasing C incorporation (Eg ranges from ~1.85eV to >2.0eV). Generally the electronic quality of a-SiC:H improves at lower values of Eg (less than ~2.1eV) since the C-C bond length differs significantly from the Si-Si bond length leading to increased disorder7. N-type and p-type doping can be achieved with good control of the Fermi level position, EF8,9. As shown in Fig.1, in order to reach 10% STH efficiency, the photocurrent of 8 mA/cm2 per cell must be achieved. It is uncertain at this time exactly how the a-SiC:H interacts with the electrolyte, ie. the flat band energy of the a-SiC:H with respect to the hydrogen evolution energy. However there is some evidence that the conduction and valence bands align well with the H2/H+ and OH-/O2 redox potentials 10. Table1. The simulation results of PEC devices Jsc Jsc obtained photovolta Voc from photoavailable to date Voc from possible achieved Jsc Jsc obtained ic layer PV Photo- electrode (filtered by (filtered photoelect STH STH to available to date configurati layer(s) electrode bandgap top layer) by top rode (V) (mA/cm2) (mA/cm2) (%) date(%) on (V) (eV) 2 (mA/cm ) layer) (mA/cm2) WO3 a-SiC:H a-SiC:H a-SiC:H a-SiC:H a-SiC:H a-SiC:H 2.6-2.8 2 2 2 2 2 2 4.3-3.3 14.4 14.4 14.4 14.4 14.4 14.4 2.5 11.64 (300nm) 8 .45 (100nm) ~7 (100nm) 9 (100nm) 9 (100nm) 12 a-Si/a-Si a-Si/a-Si a-Si/a-Si a-Si/a-Si a-Si/a-Si a-Si/nc-Si a-Si/nc-Si 8.0 4.0 8.0 8.0 >9.0 >15.0 >12.0 6.5 1.68 1.68 1.68 1.68 1.68 1.35 1.35 0.55 0.55 0.55 0.55 0.55 0.55 0.55 3.25 4.9 9.84 8.61 11.07 11.07 14.76 3.15

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(200nm) Also, it is likely that the carbon content in SiC will lead to increased corrosion resistance compared to the a-Si films. By understanding the interaction at the semiconductor-electrolyte interface of a-SiC:H, the film can be optimized to achieve greater than 9 mA/cm2. Solar cells comprised entirely of a-SiC:H have been fabricated by us, with Jsc=11.64mA/cm2 and Voc = 0.91V with an absorber Eg of 2.0 eV (Fig.5). It is possible, therefore, that in addition to generating the photocurrent necessary for STH efficiency >10%, the a-SiC:H photoactive layer, when in contact with the electrolyte, could produce a significant photovoltage, reducing the voltage that is needed from the photovoltaic junction(s) to split water. To achieve STH efficiency >10%, it is possible that ultimately a “micromorph” (a-Si and nano-crystalline Si) tandem cell will be needed. Table 1 shows the results from such simulations using a-SiC:H as the photoelectrode. If the thickness of a-SiC:H is too thick (e.g. 300nm), the total current of device will be limited by photovoltaic layer, leading to a small STH efficiency. By reducing the photoelectrode thickness to ~100 nm, photovoltaic layer current will increase, resulting in an increase of the STH efficiency to 10%. So far, in the electrolyte, using an a-SiC(p)/a-SiC(i) structure, the p/i structure current is about 7mA/cm2. By improving this structure, the total current could lead to > 9mA/cm2. With “micromorph” tandem devices instead of amorphous tandem cells, the STH efficiency is expected to be even higher, as shown in the Table1.

4. CONCLUSIONS
State-of-the-art a-SiC:H materials and solar cells for use in a PEC device for hydrogen production have been fabricated using the PECVD technique. Employing Asahi U type SnO2 substrate, the performance of a-SiC:H solar cells with thickness 300nm was Voc=0.91V, Jsc=11.64mA/cm2, FF=0.657 with an η~7%. The current density obtained at reduced device thickness of 100nm was ~8.45mA/cm2. The photocurrent of the a-SiC:B:H layer/a-SiC:H i-layer structure in 0.33M H3PO4 electrolyte was measured to be ~7mA/cm2. Preliminary results indicate that the a-SiC photoelectrode exhibite stability when immersed in the electrolyte. It is therefore possible, that the approach of using a-SiC as the photoelectrode could lead to a target STH efficiency of 10%.

ACKNOWLEDGEMENTS
This work was supported by the U.S. Department of Energy under Cooperative Agreement DE-FC36-00GO10538 and by the UNLV Research Foundation under subcontract # RF-05-SHGR-005. The authors would like to thank Ed Valentich for his help in sample preparation.

REFERENCES
1. R. Narayanan, B. Viswanathan, “Chemical and Electrochemical Energy Systems”, University Press, 1998. 2. Tokio Ohta, “Solar-Hydrogen Energy Systems”, Pergamon Press, 1979 3. Margaret K. Mann, “Sensitivity Study of the Economics of Photoelectrochemical Hydrogen Production”, 1999 4. A. Fujishima and K. Honda, Nature 238, 37 (1972). 5. A. Stavrides, A Kunrath, J, Hu, proceedings of the SPIE Conference on Solar Hydrogen and Nanotechnology, Vol. 6340 (San Diego), 2006 6. R. Hollingsworth, P. Bhat, and A. Madan, Proceedings of the 19th IEEE Photovoltaic Specialists Conference, New Orleans, p684, 1987 7. A. Madan and M.P. Shaw, The Physics and Applications of Amorphous Semiconductors, 1988. 8.DeMichelis et al, J. Appl. Phys., vol. 72, (1992), p. 1327-1333. 9. S. F. Yoon,a) R. Ji, and J. Ahn, J. Vac. Sci. Technol. A 15(1), Jan/Feb P21-28,1997 10. Nozik and Memming, J. Phys. Chem., vol. 100 (1996), p. 13061-13078.

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