Metal-oxide bandgap engineering for photo- electrochemical water splitting

Photo-electrochemical (PEC) systems based on transition metal oxides such as titanium dioxide, zinc oxide (ZnO), and tungsten trioxide (WO₃) have received extensive attention because of their potential applications in hydrogen production from aqueous solutions. However, most metal oxides that are currently studied have bandgaps that are too wide to efficiently absorb a major fraction of the solar spectrum. Therefore, bandgap reduction is a critical issue for improving PEC performance. It has been suggested that doping with carbon, sulfur, and nitrogen (N) could reduce the bandgap of most wide-bandgap metal oxides, although a significant amount of N can only be incorporated into ZnO and WO₃ at low temperatures. However, N-doped films grown at low temperatures usually exhibit very poor crystallinity, which is detrimental to PEC performance. A possible cause for this are uncompensated, charged N atoms. To addres s these problems, we developed a charge-compensated donor-acceptor co-incorporation approach.¹ This method helped to enhance dopant solubility, reduce defect recombination, and improve material quality. We used ZnO as a prototype material to demonstrate the benefits of passive co-incorporation of gallium (Ga) and N dopants in transition metal oxides for PEC applications. In addition, a good photoresponse depends on an optimum carrier concentration. We demonstrated how donor-acceptor co-incorporation can tune carrier concentration and lead to enhanced PEC performance.²

Figure 1. (a) Optical-absorption coefficients—(αhν)², where hν corresponds to the photon energy—of two nitrogen (N)-doped zinc oxide films—ZnO:N(1) and ZnO:N(2)—and a gallium (Ga)/N co-doped ZnO sample, ZnO:(Ga,N), respectively. Inset: Optical-absorbance curves. (b) Current-voltage curves under illumination, using UV/IR and green filters. Ag: Silver. AgCl: Silver chloride.

Figure 1(a) shows the optical-absorption coefficients of doped samples of ZnO grown under three different conditions.¹ Samples ZnO:N(1) and ZnO:N(2) were doped with N and grown at 25 and 500°C, respectively, while the ZnO:(Ga,N) sample was co-doped with Ga and N and grown at 500°C. The bandgap of the ZnO:N(2) film was 3.27eV, which is the same as for pure ZnO. On the other hand, because of the incorporation of N, the ZnO:N(1) film exhibited a significantly decreased bandgap of 1.75eV. The ZnO:(Ga,N) film had a bandgap of 3.19eV, which is smaller than that of the ZnO:N(2) film but still close to the bulk-ZnO bandgap. Incorporation of N into ZnO modifies its electronic structure by generating an impurity band above the valence band. Absorption from this impurity band cannot be characterized by direct band transitions. It typically results in an absorption tail in the optical-absorption-coefficients curve: see Figure 1(a). This tail can be considered as a further bandgap reduction, which enables light harvesting in much longer-wavelength regions than for the ZnO:N(2) film. The enhanced PEC performance achieved by Ga and N co-incorporation is demonstrated by the measured current-voltage curves under illumination using UV/IR and green filters: see Figure 1(b). Because of its wide bandgap, the ZnO:N(2) film exhibited no clear photoresponse and showed a negligible photocurrent across the full potential range. On the contrary, and despite having smaller light absorbance, the ZnO:(Ga,N) film exhibited a larger photocurrent than the ZnO:N(1) sample. This indicates that a high recombination rate of photogenerated electrons and holes occurs in the ZnO:N(1) film, which might be due to its inferior crystallinity. Moreover, the co-incorporated ZnO:(Ga,N) film exhibited a remarkably enhanced photoresponse compared to the ZnO:N(1) film in long-wavelength regions.

Figure 2. Current-voltage curves under continuous illumination (arrows) and dark conditions (black curve) using a UV/IR filter, measured from copper (Cu)-doped ZnO (ZnO:Cu) and two Cu/Ga co-doped ZnO films, ZnO:(Cu,Ga)0.001 and ZnO:(Cu,Ga)0.002. The 0.001 and 0.002 values indicate the amount of Ga used during film deposition.

To study the effect of co-incorporation in the carrier concentration we doped ZnO with copper (Cu) and Ga and showed how this optimizes the carrier concentration (and hence the PEC performance) of Cu-doped ZnO films (ZnO:Cu).² The ZnO films doped with Cu and Ga were deposited while keeping the amount of Cu constant and varying the amount of Ga. Figure 2 shows measured current-voltage curves under illumination with the UV/IR filter and dark currents for ZnO:Cu, ZnO:(Cu,Ga)0.001, and ZnO:(Cu,Ga)0.002. (The 0.001 and 0.002 values indicate the amount of Ga used during film deposition.) Both ZnO:(Cu,Ga) films showed p-type conductivities, indicating that the role of Ga is to reduce the hole concentration generated by Cu incorporation. Reduction of the hole concentration increases the depletion width, so that more photon-generated electron-hole pairs can be collected. Thus, the ZnO:(Cu,Ga)0.001 and ZnO:(Cu,Ga)0.002 films showed larger photocurrents than the Ga-free ZnO:Cu film. In particular, the ZnO:(Cu,Ga)0.002 sample exhibited lower photocurrents than ZnO:(Cu,Ga)0.001, indicating that an optimum carrier concentration exists. If the carrier concentration is too low, the film's resistance increases and the photocurrent begins to decrease again.

In summary, we have demonstrated the viability of the donor-acceptor co-incorporation approach. We co-incorporated Cu, Ga, and N into ZnO, thus improving its crystallinity, carrier concentration, and—consequently—PEC performance. We continue to aim for further PEC performance improvements.