Lucas Cuadra

General equivalent circuit for intermediate band devices: Potentials, currents and electroluminescence

A general model to describe the operation of intermediate band solar cells (IBSCs), incorporating a significant number of physical effects such as radiative coupling between bands, and impact ionization and Auger recombination mechanisms, is presented in equivalent circuit form. The model is applied to IBSC prototypes fabricated from InAs quantum dots structures to determine the value of the circuit elements involved. The analysis shows evidence of splitting between the conduction and intermediate band quasi-Fermi levels, one of the fundamental working hypotheses on which operation of the IBSC depends. The model is also used to discuss the limitations and potential of this type of cell.

Experimental analysis of the quasi-Fermi level split in quantum dot intermediate-band solar cells

The intermediate-band solar cell (IBSC) has been proposed as a device whose conversion efficiency can exceed the 40.7% limiting value of single-gap cells. It utilizes the so-called intermediate-band material, characterized by the existence of a band that splits an otherwise conventional semiconductor bandgap into two sub-bandgaps. Two important criteria for its operation are that the carrier populations in the conduction, valence, and intermediate-bands are each described by their own quasi-Fermi levels, and that photocurrent is produced when the cell is illuminated with below-bandgap-energy photons. IBSC prototypes have been manufactured from InAs quantum dot structures and analyzed by electroluminescence and quantum efficiency measurements. We present evidence to show that the two main operating principles required of the IBSC are fulfilled.

Partial filling of a quantum dot intermediate band for solar cells

This paper describes how to partially fill the intermediate band formed by the confined states of quantum dots with electrons. Efficiencies of up to 63.2% have been calculated in ideal cases for solar cells with this intermediate band. In order to achieve this, the barrier region is n-doped so that the electrons delivered by the donors fall into the otherwise empty intermediate band states. This method produces a fully space-charged structure whose electrostatic properties are studied in this paper, thus confirming the feasibility of the proposed method. Partial filling of the intermediate band is necessary to provide strong absorption in transitions from it to both the valence and the conduction bands.

Quantum dot intermediate band solar cell

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 /spl Aring/ of radius, spaced in the range of 100 /spl Aring/ 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.

Influence of the overlap between the absorption coefficients on the efficiency of the intermediate band solar cell

This paper studies the effects of the overlap between the three absorption coefficients involved in the operation of the intermediate band solar cell on the performance of this novel photovoltaic converter. Although an arbitrary overlap, in general, reduces the limiting efficiency of the intermediate band solar cell (for example from 57.29% to 32.56% under 1000 suns), its impact can be minimized and almost suppressed when the absorption coefficients increase with energy, and light confinement is used. Another important consequence derived from the overlap between the absorption coefficients is that the thickness of the intermediate band solar cell, even using detailed balance arguments and in contrast to the detailed balance model of the conventional single-gap solar cell, now becomes a parameter to be optimized.

Quasi-drift diffusion model for the quantum dot intermediate band solar cell

This paper describes the application of the drift-diffusion model in order to illustrate the operation of the quantum dot intermediate band solar cell (QD-IBSC) and its validity limits. The main particularities of the model arise from the fact that the intermediate band solar cell (IBSC) is a two-minority carrier device. The role of the current transport in the IB is discussed, providing the beneficial conditions in which this current approaches zero. The electric field is also related to the current density in the intermediate band. The conditions in which the contribution of the electron and hole drift currents is small when compared to the total current are discussed. The description of the operation of the cell is aided by means of a numerical example.

Design constraints of the quantum-dot intermediate band solar cell

Abstract The design constraints of the quantum-dot intermediate band solar cell are reviewed. They involve the determination of the size of the dots, their spacing, regularity, doping and the materials themselves for their manufacture. The design constraints derived from the elimination of the photon recycling and the assumption of infinite mobility for describing the ideal operation of this cell are specifically analyzed in this paper. The suppression of the hypothesis of photon recycling, that causes a typical drop in the limiting efficiency from 63.2% to 58.3%, causes the thickness of the solar cell to become a parameter to be optimized. The value of the mobility required to cause an internal voltage drop lower than kT / e is discussed and a link is established with the density of states at the conduction and valence bands. It is also determined that the recombination at the barrier region can cause a dramatic drop in the limiting efficiency of the cell (up to 46.0% for an electron filling factor of the unit cell in the dot array of 52%) if the wave function of the electrons in the dots does not significantly penetrate the barrier region. In general, the optimum gaps of the cell must be also recalculated when a new working hypothesis is introduced.

Thermodynamic consistency of sub-bandgap absorbing solar cell proposals

This paper proves, using thermodynamic arguments, that ideal multiple quantum well (MQW) solar cells cannot exceed the efficiency of ideal ordinary solar cells unless carriers in the well are pumped by the absorption of a second photon. In this case, the cells behave identically to the intermediate band (IB) solar cell that allows for sub-band photon absorption. It is also proven that the IB solar cell complies with the second law of thermodynamics and, in this way, the theoretical potential of this cell for achieving efficiencies of 63.2% as compared to the 40.7% of ordinary cells, is confirmed.

Impact-ionization-assisted intermediate band solar cell

The effect on the efficiency of the intermediate band solar cell of certain types of impact ionization mechanisms is studied. It turns out to have several interesting advantages. For example, this mechanism may lead to cells with an efficiency that, although similar to that of the multijunction cells, would be otherwise very insensitive to the spectral mismatch. The efficiency of structures of p, n, or intrinsic type is also discussed in addition to the metallic intermediate band analyzed in earlier works.

Type II broken band heterostructure quantum dot to obtain a material for the intermediate band solar cell

Abstract This paper introduces the idea of achieving an intermediate band solar cell using a three-dimensional array of type II broken gap quantum dots. The determining factor is that electrons and holes must be confined in different and adjacent quantum dots. The overlap between bounded states of the adjacent quantum dots induces a half-filled intermediate band and three separate quasi-Fermi levels for describing, in non-equilibrium conditions, the carrier concentration in the valence, intermediate and conduction bands. InAs/GaSb could be an adequate material system for manufacturing the array of dots.