A photovoltaic device structure based on internal electron emission

There has been an active search for cost-effective photovoltaic devices since the development of the first solar cells in the 1950s (refs 1–3 ). In conventional solid-state solar cells, electron–hole pairs are created by light absorption in a semiconductor, with charge separation and collection accomplished under the influence of electric fields within the semiconductor. Here we report a multilayer photovoltaic device structure in which photon absorption instead occurs in photoreceptors deposited on the surface of an ultrathin metal–semiconductor junction Schottky diode. Photoexcited electrons are transferred to the metal and travel ballistically to—and over—the Schottky barrier, so providing the photocurrent output. Low-energy (∼1 eV) electrons have surprisingly long ballistic path lengths in noble metals 4 , 5 , allowing a large fraction of the electrons to be collected. Unlike conventional cells, the semiconductor in this device serves only for majority charge transport and separation. Devices fabricated using a fluorescein photoreceptor on an Au/TiO 2 /Ti multilayer structure had typical open-circuit photovoltages of 600–800 mV and short-circuit photocurrents of 10–18 µA cm -2 under 100 mW cm -2 visible band illumination: the internal quantum efficiency (electrons measured per photon absorbed) was 10 per cent. This alternative approach to photovoltaic energy conversion might provide the basis for durable low-cost solar cells using a variety of materials.