Influence of Nitrogen Doping on Tungsten Oxide Thin Films for Photoelectrochemical Water Splitting
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Influence of Nitrogen Doping on Tungsten Oxide Thin Films for Photoelectrochemical Water Splitting Brian Cole, Bjorn Marsen, and Eric Miller Hawaii Natural Energy Institute, University of Hawaii, 1680 East West Road, POST 109, Honolulu, HI, 96822
ABSTRACT Thin films of tungsten oxide were investigated for use as a top junction in a hybrid photoelectrode. To increase the solar to hydrogen efficiency, the tungsten oxide requires a bandgap reduction to a range of 2.2 to 2.4 eV. Nitrogen doping of WO3 films was employed to reduce the bandgap via valence band modification. For low levels of doping, the bandgap was observed to increase, an effect attributed to decreased size of the polycrystals in the film. The photoelectrochemical efficiency was found to decrease from 80% for pure WO3 to 56% for films deposited at a nitrogen partial pressure of 1.5 mTorr. For even higher doping levels (to 5 mTorr N2), the bandgap was shown to decrease to a value of ~1.9 eV, but the structural data indicates that significant disorder had been introduced. This disorder is consistent with recurrent dislocations, an effect that is common for the tungsten oxide material.
INTRODUCTION Due to rising energy costs and environmental concerns, there is a renewed interest for applications using photoelectrochemical (PEC) cells for the solar production of hydrogen [1]. Within this class of materials, tungsten trioxide (WO3) has been shown to have properties favorable for PEC, including suitable electronic properties and good corrosion resistance in aqueous electrolytes [2,3]. Although much research is conducted towards identifying materials that can produce unassisted splitting of water, the approach here is to use a multijunction “hybrid” photoelectrode (HPE) [4]. The hybrid photoelectrode is a monolithic thin film device designed to provide solar conversion of water to hydrogen and oxygen. Figure 1 illustrates the conceptual design for a solar watersplitting system based on the HPE technology. The HPE device has a multiple junction structure where embedded under the photo-active catalyst is a PV solar cell. The solar cell is designed such that it converts the unused photons transmitted by the top junction, thereby providing the voltage boost required to enable the photoelectrochemical splitting of the H2O molecule.
2H2
Electrolyte (acidic)
O2
4H+
4e-
4H+ + 4e- → 2H2
Hydrogen catalyst
2H2O → O2 + 4H+ + 4e-
Multijunction Solar cell
Oxygen photocatalyst
Figure 1: Concept of hybrid photoelectrode – an integrated device for solar-powered photoelectrolysis of water. In principle, the tungsten oxide layer can be simply sputtered onto the top contact of the underlying solar cell. Treating this device as a series circuit (i.e. conditions requiring current matching in the stacked layers), the overall performance can be modeled using modified “loadline” analysis [5]. As illustrated in Figure 2, which shows modeling for individual optimized junctions in an amorphous-silicon/tungsten-oxide tandem device, the predicted integrated perf
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