Photon-enhanced thermionic emission (PETE) is a relatively new concept for high efficiency solar cells that utilize not only the energy of electrons excited across the band gap by photons, as in conventional photovoltaic solar cells, but also the energy usual lost to thermalization of the excited electrons. Efficiencies above 60% have been predicted theoretically for high solar concentration systems. Silicon is an interesting absorber material for high efficiency PETE solar cells, partly due to

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... its mechanical and thermal properties and partly due to its electrical properties, including a close to ideal band gap. The work function of silicon is, however, too high for practical PETE implementations. A well-known method for lowering the work function of silicon (and other materials) is to apply approximately a monolayer of cesium to the silicon surface. We present the first measurements of PETE in cesiated p-type and n-type silicon. It is shown that PETE in average can increase the thermionic emission current by more than an order of magnitude.