A quantum-well superlattice solar cell for enhanced current output and minimized drop in open-circuit voltage under sunlight concentration

摘要

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (V_(oc)) and degrade the fill factor; the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In_(0.13)Ga_(0.86)As (4.7 nm)/GaAs_(0.57)P_(0.43) (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0mAcm~(?2)) with a minimal drop in V_(oc)(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron-hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in V_(oc) as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of V_(oc) for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in V_(oc), photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state.