An n ‐ CdSe / SnO2 / n ‐ Si Tandem Electrochemical Solar Cell

Abstract

We demonstrate a new type of monolithic tandem junction solar cell. The cell is made up of a bottom (low energy gap) Schottky barrier and a top (high energy gap) semiconductor-electrolyte junction. By using silicon as the low gap and cadmium selenide as the high gap semiconductor we chose a pair of semiconductors which produces equal photocurrents when stacked on top of each other. We have observed open-circuit voltages up to 1.21V which demonstrate addition of the output voltage of the two cells. Short-circuit currents are low, reflecting the poor quality of the CdSe film.The energy bandgap of the absorbing semiconductor chosen for a solar cell represents a compromise for high photocurrent (suggesting a small gap) and high photovoltage (suggesting a large gap) (1). Tandemand higher multiple-junction solar cells improve this compromise by employing in series two or more absorbing semiconductors that cover a range of energy gaps (2). To maximize efficiency and to provide a single output voltage such cells are arranged in electrical series. Tandem cell configurations comprise either two physically separate cells which receive separate portions of the solar spectrum from a dichroic mirror (a spectrum-splitting filter) (3), or a monolithic structure whose top cell absorbs the short wavelength portion of sunlight and transmits the long wavelength portion to the lower cell (4).Series connection of the two cells requires balancing of the photocurrents produced by each individual cell. This requirement makes the two energy gaps depend on each other. The use of semiconductor alloys (3) provides flexibility in the choice of energy gaps, but requires expensive fabrication technology which can be justified only for high performance concentrators.The theoretical principles of tandem solar cells have been analyzed by several authors (5, 6). However, few experimental tests of tandem cells have been reported. It is difficult to find and to combine semiconductors with acceptable photovoltaic properties and whose bandgaps are well matched.In this paper we demonstrate a tandem cell with CdSe (Eg : 1.73 eV) and Si (Eg ~_ 1.12 eV). At Air Mass 1 insolation, the photon flux with hv ~ 1.73 eV is 1.3 • 1017 cm-~ sec-1 and with h~ ~ 1.12 eV, 2.7 • 10 l~ em -2 sec -1. Thus the photon currents absorbed in the two partners of a monolithic CdSe/Si structure are nearly equal, Therefore, the CdSe/Si combination lies close to the point of maximum attainable efficiency for monolithic tandem cells. This semiconductor combination provides a near-ideal vehicle for a highly efficient tandem cell without the expense of alloy semiconductors. Since polycrystalline n-CdSe photoanodes have" been demonstrated with promising efficiency (7), the n-CdSe/n-Si tandem photoanode is an interesting candidate for use in an efficient regenerative photoelectrochemical cell. The observed high output voltage also suggests application in a photoelectrochemical storage cell (8); the voltages developed by most photoelectrochemical cells are insufficient to drive the desi...

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