Intermediate Band Formation and Intraband Absorption for Electrons in an Inhomogeneous Chain of Quantum Dots

Abstract

We study the electron states of a chain of non-identical, vertically stacked quantum dots. We discuss how the pseudo-band formed of the ground states confined in the quantum dots disintegrates upon increasing the inhomogeneity of the electron energies and analyze the impact of localization on the intraband absorption from the pseudo-band to extended (bulk) states. We describe also the dependence of the intraband absorption spectrum on the quantum dot size.

Intraband absorption in finite, inhomogeneous quantum dot stacks for intermediate band solar cells: Limitations and optimization

“…The additional absorption peaks that can be seen in Fig. 2 result from interference effects which are due to delocalization of the electron state and lead to preferred transitions to states with k z = 2πj/D, where j is an integer 21 . Disorder, which in our case has the form of inhomogeneity of the "on-site" energies ǫ n , destroys the coherently delocalized electron states and leads to localization.…”

“…We assume the wave function confinement sizes l = 4.5 nm, l z = 1.0 nm (see Ref. 21 for the discussion of the dependence on these parameters), E = −250 meV. Based on earlier k • p calculations 27 , the tunnel coupling t is given by the formula ln t t 0 = −KD,…”

“…On the other hand, modeling of the electron states and optical absorption in chains and arrays of QDs has been mostly limited to infinite, periodic superlattices of identical dots [15][16][17][18][19][20] . As we have shown recently 21 , enhanced absorption can appear also in finite chains of non-identical QDs but it is suppressed if the inhomogeneity of the QD chain (leading to non-identical electron ground state energies in the individual QDs) becomes too large. Since the actual QD chains are always finite (usually built of several to a few tens of QDs) [4][5][6][7][8][9][10][11][12] and unavoidably inhomogeneous it is of large practical importance for the optimal design of intermediate band photovoltaic devices to extend the theoretical analysis to such more realistic structures.…”

“…The states |n are coupled by nearest neighbor couplings. The eigenstates |ν and the corresponding energies E ν are thus obtained as the eigenstates of the effective chain Hamiltonian (assuming a single confined state in each dot) 21 ,…”