Solar cell with charged quantum dots: Optimization for high efficiency

Most of investigations of quantum dot photovoltaic devices are aimed at the development of the intermediate band solar cell. To form the intermediate band by quantum dot electron levels, the dots should be placed close to one to another. This leads to strain accumulation and defects, which increase the photocarrier recombination, and recombination losses. To avoid the nanostructuring-induced recombination, we proposed and studied an alternative approach, which is based on the separation of quantum dots (QDs) or QD clusters from the conducting channels by potential barriers created by quantum dots with built-in charge (Q-BIC). Charging of QDs improves the performance of QD solar cells due to the following factors: Negative dot charging increases electron coupling to sub-bandgap photons and provides effective harvesting of IR energy. Because of the strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs act as deep traps for electrons, but they are shallow traps for holes. Thus, the holes trapped in QDs may be excited by thermal phonons, while excitation of localized QDs electrons requires IR radiation or the interaction with hot electrons. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Charging of QDs is also an effective tool for managing the potential profile at micro- and nanoscales. Filling QDs predominantly from dopants in the QD medium allows one to maintain the macroscale profile analogous to that in the best conventional single-junction solar cells.