The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. Nanostructured materials and certain alloys have been employed in the practical implementation of these devices. This Progress Article reviews the range of different approaches and discusses how to resolve the remaining technical issues.

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Understanding intermediate-band solar cells

The intermediate band (IB) solar cell has been proposed to increase the current of solar cells while at the same time preserving the output voltage in order to produce an efficiency that ideally is above the limit established by Shockley and Queisser in 1961. The concept is described and the present realizations and acquired understanding are explained. Quantum dots are used to make the cells but the efficiencies that have been achieved so far are not yet satisfactory. Possible ways to overcome the issues involved are depicted. Alternatively, and against early predictions, IB alloys have been prepared and cells that undoubtedly display the IB behavior have been fabricated, although their efficiency is still low. Full development of this concept is not trivial but it is expected that once the development of IB solar cells is fully mastered, IB solar cells should be able to operate in tandem in concentrators with very high efficiencies or as thin cells at low cost with efficiencies above the present ones.

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The Intermediate Band Solar Cell: Progress Toward the Realization of an Attractive Concept

There is a practical interest in developing semiconductors with levels situated within their band gap while preventing the non-radiative recombination that these levels promote. In this paper, the physical causes of this non-radiative recombination are analyzed and the increase in the density of the impurities responsible for the mid-gap levels to the point of forming bands is suggested as the means of suppressing the recombination. Simple models supporting this recommendation and helping in its quantification are presented.

Intermediate bands versus levels in non-radiative recombination

Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of multiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the con fined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400006928

Solar Cells Based on Quantum Dots: Multiple Exciton Generation and Intermediate Bands

Understanding how to engineer nanomaterials for targeted solar-cell applications is the key to improving their efficiency and could lead to breakthroughs in their design. Proposed mechanisms for the conversion of solar energy to electricity are those exploiting the particle nature of light in conventional photovoltaic cells, and those using the collective electromagnetic nature, where light is captured by antennas and rectified. In both cases, engineered nanomaterials form the crucial components. Examples include arrays of semiconductor nanostructures as an intermediate band (so called intermediate band solar cells), semiconductor nanocrystals for multiple exciton generation, or, in antenna-rectifier cells, nanomaterials for effective optical frequency rectification. Here, we discuss the state of the art in p-n junction, intermediate band, multiple exciton generation, and antenna-rectifier solar cells. We provide a summary of how engineered nanomaterials have been used in these systems and a discussion of the open questions.

https://www.univie.ac.at/photovoltaik/vorlesung/ss2015/unit9/review_nanomaterials.pdf

Engineered nanomaterials for solar energy conversion.

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.

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

The intermediate band solar cell is a novel solar cell with the potential of achieving a limiting efficiency of 63.2 % on the basis of the absorption of two sub-bandgap photons to create one electron-hole pair. The path towards its practical implementation has started following three strategies: a) Engineering the IB material through quantum dot technology, b) Direct synthesis of the IB material and c) creation of a localized absorber layer within a highly porous large bandgap semiconductor.

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Progress towards the practical implementation of the intermediate band solar cell

First-Principles Calculation to N-type Beryllium Related Co-doping and Beryllium Doping in Diamond

In this paper, we systematically investigated the adsorption characteristics, electronic structure(DOS), band structure and thermal stability of diamond surface with Hydrogen terminals. We found that the most stable adsorption performance may occur on (100) surface. The adsorption stability of hydrogen atom on plane (110) is the second, and the worst on plane (111). A very shallow acceptor level is introduced through Hydrogen termination, explaining the ideal p-type diamond characteristics. The stability of the hydrogen terminal structure decreases as temperature rises. This structure has deteriorated significantly since 400 K, and the instability of the hydrogen-terminated structure on the surface is the root cause of the decrease in the hole concentration of hydrogen-terminated diamond at high temperature. 

D. Zhou

Papers

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

Progress towards the practical implementation of the intermediate band solar cell

First-Principles Calculation to N-type Beryllium Related Co-doping and Beryllium Doping in Diamond

Adsorption and Thermal Stability of Hydrogen Terminationṇ on Diamond Surface: A First-Principles Study