Hydrogen generation through water splitting by n-InGaN working electrodes with bias generated from GaAs solar cell was studied. Instead of using an external bias provided by power supply, a GaAs-based solar cell was used as the driving force to increase the rate of hydrogen production. The water-splitting system was tuned using different approaches to set the operating points to the maximum power point of the GaAs solar cell. The approaches included changing the electrolytes, varying the light intensity, and introducing the immersed ITO ohmic contacts on the working electrodes. As a result, the hybrid system comprising both InGaN-based working electrodes and GaAs solar cells operating under concentrated illumination could possibly facilitate efficient water splitting.
© 2013 Optical Society of America
Hydrogen produced from a renewable source such as solar energy is considered a main energy carrier and has an important role in green economy. Hydrogen generation through photo-assisted electrolysis (photoelectrolysis) of water using a photoelectrochemical (PEC) cell has elicited considerable attention since Fujishima and Honda produced H2 and O2 using titanium dioxide (TiO2) photoanodes under UV light irradiation in 1972 . For practical application, photoelectrodes have to be able to absorb visible light rather than only UV light in the solar spectrum. A number of studies have reported on PEC cells operating under visible light illumination for water splitting [2, 3]. PEC cells with photoelectrodes containing InxGa1-xN-based materials have drawn much attention because the band-edge potentials of InxGa1-xN-based materials can satisfy water splitting conditions [4, 5]. Additionally, these photoelectrodes are potentially resistant to corrosion in aqueous solutions . The band gap of InxGa1-xN can vary from 0.7 eV to 3.4 eV by varying the content of In to fit most terrestrial solar spectra [5–10]. Fujii et al. indicated that the saturated photocurrent increased as the In composition of InxGa1-xN increased due to the decrease in bandgap energy; thus, more photons can be absorbed . Similar results have been reported by Li et al. . Although photocurrent density theoretically increases with increase in In content in InxGa1-xN-based working electrodes, the increase in In content causes material quality to degrade markedly. Thus, the working electrode made of high-quality material is a key factor for water photoelectrolysis. If an assisted bias is applied to the working electrode, the separation of photogenerated electron-hole pairs is improved and the charge transfer becomes faster at the semiconductor/electrolyte interface during the PEC water-splitting process. The assisted bias can also cause the photogenerated carriers to transit faster in the semiconductor, thereby reducing the probability of recombination with charge defects. In 1998, a monolithic photovoltaic–photoelectrochemical device for hydrogen production via water splitting was reported by Khaselev et al. . In their report, the PEC cell was voltage biased with an integrated photovoltaic device and split water directly under illumination. The incident light was the only energy input. Numerous practical approaches for solar cells combined either with a photocathode or with a photoanode for water splitting have been proposed . When a PEC cell is combined with a photovoltaic device, the hybrid system must work at the optimum operating point to split water efficiently . GaAs-based solar cells (SCs) have been used to provide the driving force for increasing the rate of hydrogen production instead of the generally used electricity from the power supply. Increasing the photocurrent requires more solar cells in series to oxidize water into oxygen because of the significant overpotential. A photoactive anode and/or a photoactive cathode can be used to reduce the number of solar cells in a series. In this study, InxGa1-xN working electrodes with assisted bias generated from GaAs SCs for water splitting are demonstrated. A meaningful hydrogen gas generation rate can be reached by using only one GaAs SC to applied voltage bias between 0.6 and 0.8 V. The effective resistances are tuned using different schemes to set the operating points to approach the Pmax point of the GaAs SC to improve current matching. The hybrid system could be a promising approach to efficiently split water via photoetrolysis because both InGaN-based PEC devices and GaAs SC could work under concentrated illumination to boost the gas generation rate.
The InxGa1-xN epitaxial layers used in this study were grown on c-face (0001) double-polished sapphire (Al2O3) substrates through metal organic vapor phase epitaxy. First, low-temperature GaN nucleation layer with a thickness of 30 nm was deposited on the sapphire substrates, followed by a 2 µm-thick unintentionally doped GaN (u-GaN) buffer layer and a 2 µm-thick Si-doped GaN (n-GaN) epitaxial layer. Finally, the 0.2 µm-thick Si-doped InxGa1-xN(n-InxGa1-xN) layer was deposited on the n+-GaN layer. The In composition of the n-InGaN was about 15%. Hall measurements showed that carrier concentration, mobility, and sheet resistivity of the samples were –8.8 × 1018 cm−3, 126 cm2/V-s, and 28.1 Ω/□, respectively. Typical transmittance taken from the n-In0.15Ga0.85N/n-GaN heterostructure exhibited two absorption edges at around 430 and 365 nm. Figure 1 shows the schematic structure of the n-In0.15Ga0.85N/GaN working electrodes. Figure 1(a) shows the top-view schematic structure where the bilayer Cr (50 nm)/Au (250 nm) is partly deposited on the n-type n-In0.15Ga0.85N layer to serve as ohmic contact . Figure 1(b) shows the cross-section view of the structure of the working electrode. The working electrodes with the structure shown in Figs. 1(a) and 1(b) were labeled as n-InGaN_pad in the following paragraphs. The working electrodes with finger-type ITO ohmic contacts were labeled as n-InGaN_finger, as shown in Figs. 1(c) and 1(d) [15, 16].
The experimental setup of water splitting via the n-InGaN photoanode with assisted bias generated from GaAs SC is shown in Fig. 2. Figure 2 shows the positive and negative electrodes of GaAs SC, which are connected to the n-InGaN working electrode and the potentiostat, respectively. When the GaAs SC was placed under illumination, the SC applied bias between the working electrode and platinum counterelectrode. Another 300 W Xe lamp was used as light source for illuminating the n-InGaN working electrode. The setup was a two-electrode device associated with potentiostat as the current meter. Therefore, the driving force of carriers for the hybrid system was provided by the GaAs SC and band bending at the semiconductor/electrolyte interface.
3. Results and discussions
The effective resistances (Reff) of PEC cells were tuned using different schemes to set the operating points to the Pmax point of GaAs SC. In this study, the schemes included changing the electrolytes, varying the light intensity, and using the immersed finger-type ITO ohmic contacts. First, the n-InGaN working electrodes associated with different electrolytes served as PEC cells. The electrolytes including 1 M NaCl, 1 M NaOH, and 1 M HCl were used as electrolytes to tune the Reff of PEC cells for water splitting. At the same time, a potentiostat was used to measure the photocurrent of the hybrid (PV/PEC) system, as shown in Fig. 2.
The measured photocurrents were marked on the current-characteristic (I–V) curve of GaAs SC under illumination, as shown in Fig. 3(a). The marks on the I–V curve refer to the operating points of the hybrid system. Figure 3(b) shows a typical spectral response take from the PEC cells with working electrode made of n-In0.15Ga0.85N/n-GaN heterostructure. Cutoff wavelength shown on the spectral response was consistent with the absorption edge of transmittance taken from the n-In0.15Ga0.85N/n-GaN epitaxial heterostructure. The relatively larger photocurrent was obtained when the electrolyte of 1 M HCl was used in the PEC cells, indicating that the n-InGaN working electrodes associated with 1 M HCl electrolyte were more capable of reaching the Pmax point of the GaAs SC than when 1 M NaCl and 1 M NaOH were used. The different photocurrents indicated that PEC cells had different Reff, and that Reff could be tuned effectively by varying electrolytes. Thus, the most suitable electrolyte for the hybrid system to set the operating points to approach the Pmax point of the GaAs SC can be chosen. Since each electrolyte has its corresponding electrochemical potential, the PEC using different electrolytes could result in different magnitudes of band bending in the n-InGaN working electrode, indicating that the driving force for the separation of photogenerated electron-hole pairs in the n-InGaN working electrode was relatively higher than that of the other two electrolytes when 1 M HCl was used as electrolyte. In addition to changing electrolytes, one can vary the incident light intensity to tune the Reff of PEC cells when both the InGaN-based working electrode and GaAs SC work under concentrated illumination. Using a Fresnel lens to focus the light on the working electrode can generate more charge carriers and thereby reduce the Reff of the semiconductor because of the photo-doping effect. Figure 4 shows that the photocurrents increased with the increase in incident light intensity on the working electrodes. The operating point could move closer to the Pmax point of the GaAs SC when the incident light was concentrated by a Fresnel lens. The concentrated light intensity was estimated to be around 2 W/cm2.
On the other hand, our previous studies have shown that immersed finger-type ITOohmic contacts on n-GaN working electrodes enhanced photocurrent density (i.e., gas generation rate) compared with working electrodes without immersed ITO ohmic contacts [16, 17]. The enhancement in photocurrent density can be attributed to the small distance between neighboring ITO ohmic contacts, which was sufficient to alleviate carrier recombination, thereby increasing collection efficiency of the photogenerated carriers by the ITO ohmic contacts [16, 17]. In the present study, the immersed ITO contacts were used to decrease the Reff of the working electrode, pushing the operating point to the Pmax point of GaAs SC. The Reff of hybrid PEC cells can be reduced by changing the layout of immersed ITO contacts.
In Fig. 5, the legends of n-InGaN_pad and n-InGaN_finger indicate the n-InGaN working electrodes without and with immersed ITO finger contacts, respectively. The photocurrents of the PEC cells with immersed ITO finger contacts were higher than those of PEC cells without immersed ITO fingers. The operating points of the PEC cells were further pushed to the Pmax point of GaAs SC when the working electrodes had immersed ITO finger contacts. The operating point at the triangle symbol, which is shown in Fig. 5, corresponded to gas generation of hydrogen with a rate of about 3mL/hr.cm2 when the n-InGaN with immersed finger-type ITO ohmic contacts served as the working electrode associated with GaAs SC in series and the electrolyte of 1 M HCl was used. Assuming that all the photogenerated electrons and holes are used for the water splitting reaction, the overall photo-conversion efficiency in this study ranged from 0.18% to 0.23%.
Different approaches, including changing the electrolytes, varying the light intensity and introducing immersed ITO ohmic contacts on the working electrodes, were performed to tune the operating points to the maximum power point of GaAs SC. From the results, one can expect that increasing the area of the working electrode to tune the operating point could be an alternative. However, a problem for this approach could arise due to the current matching issue, namely, the photocurrent of the hybrid system may be limited by the short-circuit current of GaAs SCs. Fortunately, this issue can be addressed by using concentrated light illumination on GaAs SCs to tune the current matching. Therefore, the hybrid system composed of both InGaN-based working electrodes and GaAs SCs operating under concentrated illumination could be a promising solution to the need for efficient water splitting.
This work was supported from National Science Council for the financial support under contract Nos. NSC-101-2221-E-218-012-MY3, NSC-101-2221-E-006-171-MY3, NSC-100-2112-M-006-011-MY3 and NSC-100-3113-E-006-015-.
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