Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

We report for the first time a successful fabrication and operation of an InAs/GaAs quantum dot based intermediate band solar cell concentrator photovoltaic (QD-IBSC-CPV) module to the IEC62108 standard with recorded power conversion efficiency of 15.3%. Combining the measured experimental results at Underwriters Laboratory (UL®) licensed testing laboratory with theoretical simulations, we confirmed that the operational characteristics of the QD-IBSC-CPV module are a consequence of the carrier dynamics via the intermediate-band at room temperature.

We report, for the first time, about an intermediate band solar cell implemented with InAs/AlGaAs quantum dots whose photoresponse expands from 250 to ∼6000 nm. To our knowledge, this is the broadest quantum efficiency reported to date for a solar cell and demonstrates that the intermediate band solar cell is capable of producing photocurrent when illuminated with photons whose energy equals the energy of the lowest band gap. We show experimental evidence indicating that this result is in agreement with the theory of the intermediate band solar cell, according to which the generation recombination between the intermediate band and the valence band makes this photocurrent detectable. DOI: 10.1103/PhysRevLett.114.157701 PACS numbers: 84.60.Jt, 72.40.+w, 73.50.Pz, 85.60.Dw Intermediate band solar cells (IBSC) were proposed [1] as a means to exceed the efficiency of single gap solar cells thanks to the absorption of below-band gap energy photons. Intermediate band materials are characterized by the existence of a set of electronic states, named intermediate band (IB), within the semiconductor band gap E G , splitting this in two sub-band gaps, E L and E H , with E L the subband gap with the lowest energy. Because of this IB, belowband gap energy photons can pump electrons from the valence band (VB) to the IB, photons labeled 2 in Fig. 1(a), and from the IB to the conduction band (CB), photons labeled 3 in Fig. 1(a). The conversion efficiency limit of these cells is 63.2% and can be extended beyond 80% if incorporated in a multi-IB structure [2].Since the concept emerged, several approaches have been proposed for its implementation. These approaches can be divided into those based in (a) quantum dots (QD IBSC) [3] [see Fig. 1(b)], (b) highly mistmached alloys [4], and (c) the insertion of selected impurities at high concentrations [5]. An exhaustive review detailing the experimental achievements by each of the approaches has been recently published [6]. These achievements, emphasizing the ones that are relevant for the discussion of this Letter and belonging to the QD IBSC category, can be summarized as follows.Quantum efficiency measurements soon demonstrated that below-band gap energy photons, with energy enough to pump electrons from the VB to the IB, were able to produce photocurrent [7]. However, this was not enough proof that QD IBSCs had the potential to exceed the efficiency of single gap solar cells, the reason relying on the mechanism promoting an electron from the IB to the CB. If this mechanism was thermal pumping, the additional subband gap photocurrent did not imply additional electrical work [8]. On the contrary, if this mechanism was due to the absorption of a second photon, additional electrical work was possible. It was in 2006 [9] that, when illuminating QD IBSC's simultaneously with low energy photons, capable only of pumping electrons from the IB to the CB, and photons with energy capable of pumping electrons from the VB to the IB, it was demonstrated that electrons could be pumped from the ...

Coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC), because of their ability to facilitate the formation of minibands. The quantum coupling effect that exists between states in QDs in an array influences the electronic and optical properties of such structures. So far, great experimental and theoretical efforts have been devoted to study the vertically coupled QD arrays. We present here a method based on multi-band k p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs-based lateral QD arrays. Formation of the intermediate band (IB) in all cases was achieved via delocalisation of the electron ground state (e0). We show that the IB in a laterally coupled QD-IBSC is more robust against external electric field from the solar cell's pn junction than that in a vertically coupled arrangement. Because of symmetry of the QD array lattice and QD states itself, which are C 2v for the zinc blend QDs, the electronic and absorption structures were obtained via sampling throughout the reciprocal space in the first Brillouin zone of QD arrays.the IB, and the second photon subsequently pumps another electron from the IB to the conduction band (CB). To this end, it is necessary that the IB is half filled with electrons so that it can supply electrons to the CB as well as receive them from the VB. The electron-hole pairs generated in this way add up to the conventionally generated ones by the absorption of a single photon, the third one, pumping an electron from the VB directly to the CB. Therefore, if the quasi-Fermi level separations between bands are sustainable upon external excitation, the photocurrent of the SC, and ultimately its efficiency, is enhanced. According to this concept, increase in photocurrent in IBSC occurs without degradation of the output voltage of the cell. The output voltage is given by the split between the CB's electron and

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