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

Abstract

We present a theoretical analysis of intraband optical transitions from the intermediate pseudo-band of confined states to the conduction band in a finite, inhomogeneous stack of self-assembled semiconductor quantum dots. The chain is modeled with an effective Hamiltonian including nearest-neighbor tunnel couplings and the absorption under illumination with both coherent (laser) and thermal radiation is discussed. We show that the absorption spectrum already for a few coupled dots differs from that of a single dot and develops a structure with additional maxima at higher energies. We find out that this leads to an enhancement of the overall transition rate under solar illumination by up to several per cent which grows with the number of QDs but saturates already for a few QDs in the chain. The decisive role of the strength of inter-dot coupling for the stability of this enhancement against QD stack inhomogeneity and temperature is revealed.One of the ways to improve the efficiency of solar cells is to introduce an intermediate band in the energy spectrum of a photovoltaic structure 1,2 . In this way, electrons can be sequentially promoted from the valence band to the intermediate band and then to the conduction band by absorbing photons with energies below the band gap which are not converted into useful electrochemical energy in a standard structure. As an implementation of this concept, a stack of quantum dots (QDs) in the intrinsic region of a p-i-n junction solar cell has been proposed 3 . This idea has indeed gained some experimental support in recent years 4-12 . Quantum-dot-embedded p-i-n solar cells show higher quantum efficiency in near infrared range but their overall efficiency still is lower than the efficiency of similar devices without QDs 4-12 . On the theory side, models involving a single QD were formulated to describe the kinetics of transitions from and into the intermediate levels 13,14 . 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)4-12 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.In this paper, we study the intraband optical absorption associated with the electron transition from the states confined in a finite stack of quantum dots to the conduction band. (Fig. 1) We propose a relatively simple and computation-effective model which, however, includes all the essential features of the system, in particular the inhomogeneity of the e...

In‐plane coupling effect on absorption coefficients of InAs/GaAs quantum dots arrays for intermediate band solar cell

“…The electronic structure of a QD array is characterised by a BZ determined by the QD array dimensions (Figure ) . To calculate the electronic structure, the only modification required to the basis set is to replace the reciprocal lattice vectors in the PW expansion with those shifted because of the QD superlattice: In the case of vertically aligned QDs in a columnar QD array, with L x and L y chosen to be large enough to prevent the lateral coupling; In the case of laterally aligned QDs in an in‐plane QD matrix, where k | | = k x x ̂ + k y ŷ , K | | = K x x ̂ + K y ŷ and r | | = xx ̂ + yŷ , and L z is chosen to be large enough to prevent the vertical coupling.…”

Effect of electron-hole separation on optical properties of individual Cd(Se,Te) quantum dots

“…The QDs for which the QD and the barrier materials have no common atom are by far less investigated, despite of several advantages they offer. In the case of such QDs with type-I confinement one should mention here larger band offsets providing higher confining potentials for both, e and h. Such mixed structures may lead also to type-II confinement, with a decreased wavefunction overlap of the confined e and h. Such increased e-h spatial separation and resulting enhancement of confined carriers lifetimes is highly attractive for studies of magnetic polaron in semimagnetic QDs 1,2 or Aharonov-Bohm effect, 3 as well as for implementation of QDs in photovoltaic devices [4][5][6] . The intermediate case between type-I and type-II confinement, as in the present study, offers a chance of investigation of the impact of e and h wavefunctions overlap on properties of excitonic QDs emission.…”

Matrix elements of intraband transitions in quantum dot intermediate band solar cells: the influence of quantum dot presence on the extended-state electron wave-functions

“…Intraband transitions, which are indispensable for the realization of QD IBSCs, have been theoretically investigated by several groups [14][15][16][17][18] for structures consisting of QDs embedded in a matrix (a QD/matrix structure), and also periodic arrays of QDs. In these structures, the electron envelope functions (wave functions) of states that lie above the matrix conduction band minimum (CBM) are expected to be affected by the presence of the QDs.…”