Semiconductor nanostructure quantum ratchet for high efficiency solar cells

Conventional solar cell efficiencies are capped by the similar to 31% Shockley-Queisser limit because, even with an optimally chosen bandgap, some red photons will go unabsorbed and the excess energy of the blue photons is wasted as heat. Here we demonstrate a quantum ratchet device that avoids this limitation by inserting a pair of linked states that form a metastable photoelectron trap in the bandgap. It is designed both to reduce non-radiative recombination, and to break the Shockley-Queisser limit by introducing an additional sequential two photon absorption (STPA) excitation channel across the bandgap. We realise the quantum ratchet concept with a semiconductor nanostructure. It raises the electron lifetime in the metastable trap by similar to 104, and gives a STPA channel that increases the photocurrent by a factor of similar to 50%. This result illustrates a new paradigm for designing ultra-efficient photovoltaic devices.