Understanding the operation of quantum dot intermediate band solar cells (original) (raw)
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Elements of the design and analysis of quantum-dot intermediate band solar cells
Thin Solid Films, 2008
We have demonstrated recently that two below bandgap energy photons can lead to the creation of one electron-hole pair in a quantum-dot intermediate band solar cell (QD-IBSC). To be effective, the devices used in the experiments were designed to a) half-fill the intermediate band with electrons; b) to allocate the quantum dots in a flat-band potential region, and c) to prevent tunnelling from the n emitter into the intermediate band. QD-IBSCs have also shown degradation in their open-circuit voltage when compared with their counterparts without quantum dots. This loss is due to the presence of the intermediate band (IB) together with the incapacity of the quantum dots to absorb sufficient below bandgap light as to contribute significantly to the photogenerated current. It is predicted, nevertheless, that this voltage loss will diminish if concentration light is used leading to devices with efficiency higher than single gap solar cells. A circuit model that includes additional recombination levels to the ones introduced by the IB is described to support this discussion.
Elements of the design and analysis band solar of quantum-dot intermediate cells
Thin Solid Films, 2008
We have demonstrated recently that two below bandgap energy photons can lead to the creation of one electron-hole pair in a quantum-dot intermediate band solar cell (QD-IBSC). To be effective, the devices used in the experiments were designed to a) half-fill the intermediate band with electrons; b) to allocate the quantum dots in a flat-band potential region, and c) to prevent tunnelling from the n emitter into the intermediate band. QD-IBSCs have also shown degradation in their open-circuit voltage when compared with their counterparts without quantum dots. This loss is due to the presence of the intermediate band (IB) together with the incapacity of the quantum dots to absorb sufficient below bandgap light as to contribute significantly to the photogenerated current. It is predicted, nevertheless, that this voltage loss will diminish if concentration light is used leading to devices with efficiency higher than single gap solar cells. A circuit model that includes additional recombination levels to the ones introduced by the IB is described to support this discussion.
Progress in quantum-dot intermediate band solar cell research
The intermediate band solar cell concept can be implemented in practice by means of quantum dots (QDs), but a number of challenges must be solved in order to make progress. This paper describes some of them: a) the problem of having the quantum dots embedded in the space charge region, b) the identification of the energy levels involved in the QD system and c) the weak absorption provided by the dots. Regarding the first, the inclusion of semiconductor dumping field layers sandwiching the region containing the stack of QDs is suggested as a way to drive the QDs into a flat band potential region. Concerning "b", the intermediate band is found to be separated from the conduction band by only 0.2 eV, far from the optimum value. Finally, the weak light absorption provided by the dots is discussed as a factor, together with the low intermediate band to conduction band bandgap that prevents a significant quasi-Fermi level split between the IB and the CB under normal illumination conditions.
Theoretical Study of One-Intermediate Band Quantum Dot Solar Cell
International Journal of Photoenergy, 2014
The intermediate bands (IBs) between the valence and conduction bands play an important role in solar cells. Because the smaller energy photons than the bandgap energy can be used to promote charge carriers transfer to the conduction band and thereby the total output current increases while maintaining a large open circuit voltage. In this paper, the influence of the new band on the power conversion efficiency for the structure of the quantum dots intermediate band solar cell (QDIBSC) is theoretically investigated and studied. The time-independent Schrödinger equation is used to determine the optimum width and location of the intermediate band. Accordingly, achievement of maximum efficiency by changing the width of quantum dots and barrier distances is studied. Theoretical determination of the power conversion efficiency under the two different ranges of QD width is presented. From the obtained results, the maximum power conversion efficiency is about 70.42% for simple cubic quantum...
Optical Characterization of Quantum Dot Intermediate Band Solar Cells
2008
In this paper we present an optical characterization for quantum dot intermediate band solar cells (QDIBSCs). The cells were developed by growing a stack of ten InAs/GaAs QDs layers between p and n doped GaAs conventional emitters. Electroluminescence, EL, photoreflectance, PR, and transmission electron microscopy, TEM, were applied to the samples in order to test and characterize them optically. The results, derived from the application of the different techniques, showed a good correlation. TEM images revealed a very good structural quality of the QDs, which seem to evolve in shape-strain from the bottom to the top of the stack. Corresponding to the quality observed by TEM, strong signals from EL and PR resolved unambiguously the energy band diagram of the QDIBSCs. By fitting PR data we were able to indentify the coexistence of bands and discrete energy levels coming from the IB material. The PR data evidenced also a strong electric field over the dots, attributed to the space cha...
Quantum dot intermediate band solar cell
2000
This paper discusses the possibility of manufacturing the intermediate band solar cell (IBSC), a cell with the potential of achieving 63.2% of efficiency under concentrated sunlight, using quantum dot technology. The 0-dimensionality nature of the dots avoids electron thermalisation between bands enhancing the possibilities for radiative recombination between bands and making possible the existence of three quasi-fermi levels, some of the pivots the theory of the IBSC is sustained on. In this sense, it is suggested that an InGaAs/AlGaAs system could be used for band engineering the optimum bandgaps of the IBSC cell (0.71 and 1.24 eV). Dots should be about 40 Å of radius, spaced in the range of 100 Å and distributed in a three dimensional array. The Stranski and Krastanow method is proposed as a technology for achieving this goal. The possibility of n-doping the dots is also discussed
Physical Review Applied, 2020
So far physics of quantum electronic transport has not tackled the problems raised by quantum dot intermediate-band solar cells. Our study shows that this physics imposes design rules for the inter-subband transition. We developed an analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport occurring in this transition. Our results clearly indicate that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor. For that, we propose to assume the dot in a tunnel-shell whose main characteristics can be obtained by a simple analytical formula. Moreover, we show that in a realistic case, the energy transition only needs to be larger than 0.28 eV to obtain a quasi Fermi-level splitting. This quite small value designates the quantum dot solar cell as a serious candidate to be an efficient intermediate-band solar cell. This work gives a framework to design efficient inter-subband transitions and then opens new opportunities for quantum dot intermediate-band solar cells.
Progress in Photovoltaics: Research and Applications, 2013
The reported experimental evidence for the quasi-Fermi level split in quantum-dot intermediate-band solar cells is carefully examined. It is shown that the separation of the quasi-Fermi level of the intermediate band from that of the conduction band is not consistent with the experimental results of the quantum efficiency and the luminescence intensity of the InAs/GaAs cells. The fact that the electroluminescence spectrum is too wide, extending much further than we expect on the basis of the measured quantum efficiency in the direction of increasing photon energies, indicates that the temperature of the optically active regions of the cell during the electroluminescence measurements is considerably higher than room temperature. The best agreement with the experimental results is achieved with a temperature of about 525 K. This temperature rise is probably a result of the heating effect of the relatively high forward current used in the luminescence experiments. It is argued that the lack of a quasi-Fermi level split in this case is associated with the absence of a gap in the emission/absorption spectrum of sub-bandgap photons.