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| Fig. 1: a) General principle of operation of a semiconductor-based PEC cell. Incident light generates electron-hole pairs that particpate in a redox reaction b) Specific case for a water-splitting PEC cell, where an n-type semiconductor facilitates water electrolysis into hydrogen and oxygen. |
Semiconductor photoelectrochemical (PEC) cells represent a class of devices where electron-hole pair creation due to incident photons drives a reduction-oxidation (redox) reaction.
Illumination of a semiconductor with light having photon energies in excess of the semiconductor's band gap drives valence band electrons into excited energy states, the conduction band, leaving behind an electron vacancy that we treat as a positively charged quasi-particle called a 'hole'. Placing semiconductor materials in contact with metals or other semiconductors creates electric potentials in thermal equilibrium due to differences in the materials' Fermi energies. Careful design of such a semiconductor junction allows manipulation of electron and hole densities via these "built-in" electric potentials.
Redox reactions describe a class of chemical reactions in which a species is oxidized (donates electrons) and another species is reduced (receives electrons) simultaneously. For a redox reaction to proceed, energetic requirements dictated by the reaction potentials must be satisfied. These energetic requirements typically involve constraints on the allowed energies of the electrons and holes participating in the reaction.
By proper choice of a semiconductor material, we can control the energies of photon-created electron-hole pairs to facilitate a desired electrochemical redox reaction. Coupled with a semiconductor junction, this technique forms the basis for PEC cells. A schematic of a prototypical semiconductor PEC cell is provided in Fig. 1a. The "band bending" in the semiconductor due to a junction allows conduction band electrons to be swept away from the semiconductor-liquid interface, leaving only holes to participate in the reaction. Electrons for the reduction reaction are supplied by a "counter-electrode," electrically connected to the semiconductor forming a complete circuit.
The overall water splitting reaction is given by
where hν represents an incident photon. This reaction can be broken down into the following two half-reactions:
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| Fig. 2: Schematic of a tandem cell in a Z-scheme. The electrolysis of water occurs in a two-step process involving two semiconductor materials and a dye-sensitizer. |
The first half-reaction (oxidation) involves a hole contributed by the semiconductor and the second half-reaction (reduction) involves an electron from the counter-electrode. Fig. 1b provides a schematic of a semiconductor-based water splitting PEC.
Theoretical efficiencies of water splitting semiconductor PECs can be increased by switching to a tandem cell design. A tandem cell refers to two or more photosystems connected in series. A modern tandem cell design is schematically illustrated in Fig. 2. In this design, two semiconductors with different band gaps are used to more efficiently harness sunlight. A wide band gap (Eg = 2.6 eV) semiconductor absorbs the blue portion of the solar spectrum and the holes generated drive the oxidation half-reaction. A second narrow band gap (Eg = 1.6 eV) semiconductor absorbs the red fraction of the solar spectrum, providing electrons for the reduction half-reaction.
Replacing a single semiconductor material with two allows improved efficiency since it is a challenge to identify a single semiconductor with energy levels ideal for both half-reactions simultaneously.
© Alok Vasudev. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Gratzel, "Photoelectrochemical Cells," Nature 414, 338 (2001).
[2] J. Nowotny et al., "Solar-Hydrogen: Environmentally Safe Fuel For the Future," Intl. J. Hydrogen Energy 30, 521 (2005).