Electronic structure of QD arrays: Application to intermediate-band solar cells

Intermediate band solar cells (IBSC) have been proposed as a potential design for the next generation of highly efficient photo-voltaic devices. Quantum nanostructures, such as quantum dots (QD), arranged in super-lattice arrays produce a mini-band (IB) that is separated by a region of zero density of states from other states in the conduction band. Additional absorbtion from the valence band to the IB and IB to the conduction band allows two photons with energies below the energy gap to be harvested in generating one electron-hole pair. We present a theoretical study of the electronic and optical properties of the IB formed by an InAs/GaAs QD array. The calculations are based on an 8-band k-p Hamiltonian, incorporating mixing between valence and conduction states, strain and piezoelectric fields. Theoretical results of the mini-band width variation with the period of the QD array in the z direction are presented. For one particular spacer distance, d_z = 4 nm, we report detailed variation of the optical dipole matrix elements through the mini-band and identify the character of the states involved. This approach captures the essential physics of the absorption processes in a realistic model of the IBSC structure and will be used to provide input parameters for predictive modelling of transport properties.