A review of the novel concepts in photovoltaics through their experimental achievements

The intermediate band solar cell (IBSC), the multiple exciton generation solar cell (MEGSC) and the hot carrier solar cell (HCSC) are three novel concepts in photovoltaics which aim to achieve high efficiency devices. In this paper we assess to what extent their physical principles of operation have been experimentally verified. It is found that there is experimental evidence supporting the underlying theory for all three

Review of experimental results related to the operation of intermediate band solar cells

The intermediate band solar cell (IBSC) has drawn the attention of the scientific community as a means to achieve high-efficiency solar cells. Complete IBSC devices have been manufactured using quantum dots, highly mismatched alloys, or bulk materials with deep-level impurities. Characterization of these devices has led, among other experimental results, to the demonstration of the two operating principles of an IBSC: the production of the photocurrent from the absorption of two below bandgap energy photons and the preservation of the output voltage of the solar cell. This study offers a thorough compilation of the most relevant reported results for the variety of technologies investigated and provides the reader with an updated record of IBSC experimental achievements. A table condensing the reported experimental results is presented, which provides information at a glance about achievements, as well as pending results, for every studied technology

Raising the Efficiency Limit of the GaAs-based Intermediate Band Solar Cell Through the Implementation of a Mololithic Tandem with an AlGaAs top Cell.

The high efficiency limit of the intermediate band solar cell (IBSC) corresponds to the case of using as intermediate band (IB) host material a semiconductor with gap in the range of 2 eV. Traditional photovoltaic materials, such as Si and GaAs, are not appropriate to produce IB devices because their gaps are too narrow. To overcome this problem, we propose the implementation of a multi-junction device consisting of an IBSC combined with a single gap cell. We calculate the efficiency limits using the detailed balance model and conclude that they are very high (> 60% under maximum concentration) for any fundamental bandgap from 0.7 to 3.6 eV in the IBSC inserted in the tandem. In particular, the two-terminal tandem of a GaAs-based IBSC current matched to an optimized AlGaAs top cell has an efficiency limit as high as 64%

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller

Modelling and Characterization of Multiple Level Intermediate Band Solar Cell

Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockley Queisser efficiency limit for conventional solar cells. This novel technology requires the use of a new type of material named intermediate band (IB) material which makes a better use of the solar spectrum thanks to the existence of a collection of electronic levels within the band gap of the semiconductor. Quantum Dots (QDs) remain as a feasible technology to implement IB materials. InAs/GaAs QD-IBSCs were manufactured in order to test the validity of the concept, although their real size and shape are far from the optimum. This causes extra electron levels to appear within the nanostructure confining potential, degrading the performance of the device. In this paper, the effect of these extra levels will be studied through a multiple level IBSC model based on the detailed balance, but modified so a term accounting for the non-radioactive recombination (NRR) is also included. The model is completed with constant fitting parameters so the concentration JL-VOC curves (which do not incorporate series resistance effects) can be fitted. Several QD-IBSCs where manufactured, measured and fitted with this model, rendering relevant information about the recombination nature of the QD-IBSC

Interband absorption of photons by extended states in intermediate band solar cells

This paper considers sub-bandgap photon absorption in an InAs/GaAs quantum dot matrix. Absorption coefficients are calculated for transitions from the extended states in the valence band to confined states in the conduction band. This completes a previous body of work in which transitions between bound states were calculated. The calculations are based on the empirical k·p Hamiltonian considering the quantum dots as parallelepipeds. The extended states may be only partially extended?in one or two dimensions?or extended in all three dimensions. It is found that extended-to-bound transitions are, in general, weaker than bound-to-bound transitions, and that the former are weaker when the initial state is extended in more coordinates. This study is of direct application to the research of intermediate band solar cells and other semiconductor devices based on light absorption in semiconductors nanostructured with quantum dots

Modeling and characterization of multiple level intermediate band solar cell

Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockley Queisser efficiency limit for conventional solar cells. This novel technology requires the use of a new type of material named intermediate band (IB) material which makes a better use of the solar spectrum thanks to the existence of a collection of electronic levels within the band gap of the semiconductor. Quantum Dots (QDs) remain as a feasible technology to implement IB materials. InAs/GaAs QD-IBSCs were manufactured in order to test the validity of the concept, although their real size and shape are far from the optimum. This causes extra electron levels to appear within the nanostructure confining potential, degrading the performance of the device. In this paper, the effect of these extra levels will be studied through a multiple level IBSC model based on the detailed balance, but modified so a term accounting for the non-radiative recombination (NRR) is also included. The model is completed with constant fitting parameters so the concentration JL-VOC curves (which do not incorporate series resistance effects) can be fitted. Several QD-IBSCs where manufactured, measured and fitted with this model, rendering relevant information about the recombination nature of the QD-IBSCs

A puzzling solar cell structure: an exercise to get insight on intermediate band solar cells

We introduce one trivial but puzzling solar cell structure. It consists of a high bandgap pn junction (top cell) grown on a substrate of lower bandgap. Let us assume, for example, that the bandgap of the top cell is 1.85 eV (Al 0.3Ga 0.7As) and the bandgap of the substrate is 1.42 eV (GaAs). Is the open-circuit of the top cell limited to 1.42 V or to 1.85 V? If the answer is ldquo1.85 Vrdquo we could then make the mind experiment in which we illuminate the cell with 1.5 eV photons (notice these photons would only be absorbed in the substrate). If we admit that these photons can generate photocurrent, then because we have also admitted that the voltage is limited to 1.85 V, it might be possible that the electron-hole pairs generated by these photons were extracted at 1.6 V for example. However, if we do so, the principles of thermodynamics could be violated because we would be extracting more energy from the photon than the energy it initially had. How can we then solve this puzzle

Virtual-bound, filamentary and layered states in a box-shaped quantum dot of square potential form the exact numerical solution of the effective mass Schrodinger equation

The effective mass Schrodinger equation of a QD of parallelepipedic shape with a square potential well is solved by diagonalizing the exact Hamiltonian matrix developed in a basis of separation-of-variables wavefunctions. The expected below bandgap bound states are found not to differ very much from the former approximate calculations. In addition, the presence of bound states within the conduction band is confirmed. Furthermore, filamentary states bounded in two dimensions and extended in one dimension and layered states with only one dimension bounded, all within the conduction band which are similar to those originated in quantum wires and quantum wells coexist with the ordinary continuum spectrum of plane waves. All these subtleties are absent in spherically shaped quantum dots, often used for modeling

Wide-bandgap InAs/InGaP quantum-dot intermediate band solar cells

Current prototypes of quantum-dot intermediate band solar cells suffer from voltage reduction due to the existence of carrier thermal escape. An enlarged sub-bandgap EL would not only minimize this problem, but would also lead to a bandgap distribution that exploits more efficiently the solar spectrum. In this work we demonstrate InAs/InGaP QD-IBSC prototypes with the following bandgap distribution: EG = 1.88 eV, EH = 1.26 eV and EL > 0.4 eV. We have measured, for the first time in this material, both the interband and intraband transitions by means of photocurrent experiments. The activation energy of the carrier thermal escape in our devices has also been measured. It is found that its value, compared to InAs/GaAs-based prototypes, does not follow the increase in EL. The benefits of using thin AlGaAs barriers before and after the quantum-dot layers are analyzed