Project description
The project, N-IBCell, aims to fabricate high efficiency intermediate band solar cells (IBSCs). These are solar cells design to absorb also below bandgap energy photons by means of an electronic energy band that is located within the host semiconductor bandgap, producing thus enhanced photocurrent while ideally maintaining the photovoltage given by the host photovoltaic material bandgap.
IBSCs have a theoretical efficiency limit of 63.2%, 50% higher than the limit for standard solar cells. Another advantage of such IBSCs is their simplicity, and hence their reduced production costs, as they require a single p-n junction only. The first IBSCs were fabricated only 10 years ago, and a few different approaches have so far been suggested and attempted. The most successful one relies on quantum dots (QDs) based on III-As semiconductors.
Promising results have been achieved, although the optimum materials have not been identified and realized yet. The realization of an ideal QD-IBSC requires a periodical three-dimensional (3D) superlattice of relatively small QDs with a narrow size distribution. A confined energy level of electrons in the QDs form the desired IB, and ideally only a single confined level exists. The most popular fabrication method is to take advantage of the spontaneous self-assembly of coherent 3D islands in lattice mismatched epitaxy known as Stranski-Krastanov (SK) growth.
An array of closely spaced QD layers will complete the 3D QD superlattice. The QD approach is the only so far demonstrating both the absorption of below bandgap photons and the photo-voltage preservation in IBSCs. However, the photo-generated current in QD-IBSC has been too small to lead to the expected high conversion efficiencies. This is mainly due accumulation of QD lattice-mismatch strain, which tends to degrade the QDs, increase their size inhomogeneity, reduce their surface density and limits the number of absorbing QD layers in the superlattice (to avoid misfit dislocations).
In this respect, the proposed N-IBCell project deals with strain compensation and/or mediation of highly strained III-N based materials, namely In(Ga)N/InGaN QDs arrays, by insertion of thin BInGaN layers in the QDs proximate vicinity, in an attempt to improve both the QDs size uniformity and surface density, as well as increasing the number of coherently strained QDs array layers.
Another strategy for implementing the IBSC concept is based on highly mismatched semiconductor alloys (HMAs), where the IB arises from the insertion into a semiconductor compound host material of a very small amount (usually in dilution limit) of elemental impurity, whose orbitals perturb the conduction band (CB) states. The perturbation results in a splitting of the CB into two bands, where the lowest will take the role of the IB.
IBSCs based on HMAs have several advantages over QD materials: HMAs are simpler since they consist of a single layer (not a QD stack) and the absorption coefficient for the IB transitions are comparable to that of the valence band to CB transition. Dilute nitride alloys, especially Ga(In)NAs, have recently shown viable capabilities for IBSCs demonstrating remarkable photocurrent enhancement, however accompanied with a drop in photovoltage.
Ga(In)NAs has been studied for more than a decade for use in telecom applications and recently as 1-eV bandgap materials in multi-junction solar cells, and a major challenge is short lifetime of their charge carriers due to N-related non-radiative and scattering centres.
These centres can be reduced by annealing the samples, and by introducing less nitrogen, but this leads to an increase of the material bandgap. This increase is a problem for the multi-junction cells and telecom applications, but is an advantage for the use in an IBSC. In the proposed N-IBCell project we thus aim to further develop dilute nitrides as IBSC materials, with focus on improving their electronic and optical properties.
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