Group III-nitride Materials for High Efficiency Photoelectrochemical Cells
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Group III-nitride materials for high efficiency photoelectrochemical cells J. W. Ager III, W. Walukiewicz, K. M. Yu, W. Shan, J. Denlinger,1 and J Wu2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 1 Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2 Current address: Chemistry Department, Harvard University, Cambridge, MA 02138 ABSTRACT Two ternary alloys based on III-nitride semiconductor alloys are explored as potential components of photoelectrochemical cells (PECs) for the direct generation of hydrogen using solar energy. For In1-xGaxN, it will be shown using prior measurements of band offsets that spontaneous water splitting can occur for x up to 0.2 and potentially higher. Flat band potential and photocurrent measurements from an n-type epilayer with x = 0.37 will be presented. This initial data appears to indicate that the flat band potential lies just below the H+/H2 from pH 0 – 14. In the case of GaAsxN1-x we will demonstrate that the replacement of a few percent of As in N sublattice drives the bandgap down from the GaN value (3.4 eV) into a range that is attractive for PEC cells [1]. This band gap reduction is explained by the valence band anticrossing that pushes the valence band maximum up initially by 0.5 eV. From the point of view of a PEC cell, this reduces the gap (desirable for efficiency) without compromising the desired H+/H2 overpotential. INTRODUCTION Photoelectrochemcial (PEC) cells, which use solar energy to split water into hydrogen and oxygen, were discovered in the early 1970’s [2]. To date, prototype PEC cells have not been sufficiently efficient or durable for practical uses. However, this has been primarily due to materials limitations. It has been established that with better materials, PEC cells could be used as a method of large scale solar production of hydrogen and as a key component of a carbon-free economy based on renewable sources of energy [3,4]. Practical realization of technologies for direct photochemical production of hydrogen, whether based on organic (e.g. photosynthesis) or inorganic materials, depend critically on combining acceptable solar response and efficiency and sufficient service lifetime for energy payback. Most inorganic semiconductors which have the potential for efficient photo-electrochemical (PEC) hydrogen generation due to their favorable bandgaps have unacceptably high corrosion rates for practical purposes; GaInP is one example [5]. On the other hand, more stable materials, such as metal oxides (TiO2), do not capture a sufficient portion of the solar spectrum and thus have rather low efficiencies [6]. In this context, two new ternary semiconductor alloys in the group III-nitride family– In1-xGaxN and GaAsxN1-x –represent new opportunities for the development of inorganic photoelectrochemical (PEC) cells for water splitting and hydrogen generation. That is, they should satisfy three essential requirements for PEC use. 1. Their conduction and valence band edges can b
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