N. Nelson and co-workers describe on page 2988 micrometre-thick plant photosystem I crystals of up to 1000 layers of serially arranged protein chlorophyll complexes. In these proteins, photosynthesis efficiently converts light to electricity, generating unprecedented photovoltages (>49 V). The crystals are self-assembled from nanometre-sized protein structures. The successive layers form serially photoinduced dipoles in the crystal that give rise to a large electric field. The millimetre-size crystals are stable, function in a dry environment, and can be used in micro-optoelectronic devices.