Rune Strandberg

Photofilling of intermediate bands

A detailed balance model for the intermediate band (IB) solar cell has been developed. The model allows the electron concentration in the IB to vary and assumes a linear relation between this concentration and the absorption coefficients related to transitions over the subband gaps. Numerical results show that for IBs with densities of states typical for quantum dot-superlattices it is possible to sustain a useful population of photogenerated electrons in the IB when the cell is exposed to concentrated light. For unconcentrated light the IB must be partially filled by means of doping to achieve high efficiencies within reasonable optical path lengths. The filling of the IB is shown to vary with light intensity, cell voltage, density of IB-states, and the positioning of the IB in the main band gap both for cells that are partially filled by doping and for photofilled cells.

Theoretical efficiency limits for thermoradiative energy conversion

A new method to produce electricity from heat called thermoradiative energy conversion is analyzed. The method is based on sustaining a difference in the chemical potential for electron populations above and below an energy gap and let this difference drive a current through an electric circuit. The difference in chemical potential originates from an imbalance in the excitation and de-excitation of electrons across the energy gap. The method has similarities to thermophotovoltaics and conventional photovoltaics. While photovoltaic cells absorb thermal radiation from a body with higher temperature than the cell itself, thermoradiative cells are hot during operation and emit a net outflow of photons to colder surroundings. A thermoradiative cell with an energy gap of 0.25 eV at a temperature of 500 K in surroundings at 300 K is found to have a theoretical efficiency limit of 33.2%. For a high-temperature thermoradiative cell with an energy gap of 0.4 eV, a theoretical efficiency close to 50% is found while ...

Limiting efficiency of intermediate band solar cells with spectrally selective reflectors

So far, theoretical efficiency limits for the intermediate band solar cell have been calculated under the assumption that the absorptivity of the solar cell is 1 for all photon energies larger than the smallest subband gap. In the present work, efficiency limits have been calculated under the assumption that the cell is covered by spectrally selective reflectors. The efficiency limit for the 1 sun 6000 K black body spectrum is found to increase from 46.8% to 48.5% and the limit for the AM1.5G spectrum (as defined by ASTM G173–03) is found to increase from 49.4% to 52.0%.

Heat to electricity conversion by cold carrier emissive energy harvesters

This paper suggests a method to convert heat to electricity by the use of devices called cold carrier emissive energy harvesters (cold carrier EEHs). The working principle of such converters is explained and theoretical power densities and efficiencies are calculated for ideal devices. Cold carrier EEHs are based on the same device structure as hot carriersolar cells, but works in an opposite way. Whereas a hot carriersolar cell receives net radiation from the sun and converts some of this radiative heat flow into electricity, a cold carrier EEH sustains a net outflux of radiation to the surroundings while converting some of the energy supplied to it into electricity. It is shown that the most basic type of cold carrier EEHs have the same theoretical efficiency as the ideal emissive energy harvesters described earlier by Byrnes et al. In the present work, it is also shown that if the emission from the cold carrier EEH originates from electron transitions across an energy gap where a difference in the chemical potential of the electrons above and below the energy gap is sustained, power densities slightly higher than those given by Byrnes et al. can be achieved.

Optimal Filling of the Intermediate Band in Idealized Intermediate-Band Solar Cells

The optimal filling of the intermediate band (IB) of an IB solar cell is investigated. Using models based on detailed balance principles, it is shown that the optimal filling varies with the size of the subband gaps, the absorptivity of the cell, and the degree of the overlap between the absorption coefficients as well as the mutual sizes of the absorption cross sections for transitions over the subband gaps. The results of calculations that show how nonoptimal filling affects the cell efficiency are also presented. In several cases, a deviation from the optimal filling will only result in small changes in the efficiency. However, cases where the efficiency is reduced dramatically due to nonoptimal filling are also identified. For some cases, the negative impact of nonoptimal filling can be reduced by increasing the absorptivity of the cell or, when the effect of photofilling is significant, by increasing the light concentration.

Detailed balance analysis of area de-coupled double tandem photovoltaic modules

This paper describes how layers of area de-coupled top and bottom cells in photovoltaic tandem modules can increase the efficiency of two-terminal tandem devices. The point of the area de-coupling is to allow the number of top cells to differ from the number of bottom cells. Within each of the layers, the cells can be horizontally series-connected and the layers can then be current- or voltage-matched with each other in a tandem module. Using detailed balance modeling, it is shown that two-terminal tandem modules of this type can achieve the same theoretical efficiency as stacks of independently operated cells, often referred to as four-terminal cells. Optimal ratios of the number of bottom cells to the number of top cells are calculated. Finally, it is shown that modules with a bottom layer consisting of 60 cells with a band gap of 1.11 eV, resembling standard silicon modules, offer sufficient resolution to optimize the number of top cells and achieve high efficiency over a large range of top cell band g...

Evaluation of a Selection of Intermediate Band Materials Based on Their Absorption Coefficients

The intermediate band materials BSSi₂₁₄, Cu₄CrGa₃S₈, Cu₄TiGa₃S₈, Mg₂In₃VS₈, S₃₂Zn₃₁Cr, and Te₃₂Zn₃₁Cr, as well as a certain configuration of InAs quantum dots in GaAs, are evaluated as candidates to implement highly efficient intermediate band solar cells. The evaluation implies calculating theoretical efficiencies by combining an existing mathematical model and the absorption coefficients for the investigated materials. The model takes into account the energy dependence and spectral overlaps of the absorption coefficients related to transitions between various pairs of electronic bands. The presented results represent theoretical efficiencies for flat-plate solar cells, without light-trapping schemes, based on absorption coefficients publicly available in scientific journals. Only BSSi₂₁₄ and InAs quantum dots in GaAs turn out to have theoretical efficiencies close to or above the detailed balance efficiency limit for single-bandgap cells. It appears unlikely that cells made of the other materials will be able to show efficiencies higher than single-bandgap cells either due to unfortunate absorption coefficients or due to bandgap combinations that are too far from the optimal. The results highlight the fact that materials have to be selected with great care when attempting to make IBSC prototypes with higher efficiency than conventional solar cells.

Spectral and temperature sensitivity of area de-coupled tandem modules

Area de-coupling is a recently suggested method for current- or voltage-matching two-terminal tandem modules. It has previously been shown that under standard conditions, area de-coupled modules have the same theoretical efficiency as four-terminal tandem cells for any combinations of band gaps. In this work, the spectral and temperature sensitivity of ideal area de-coupled modules is investigated by detailed balance modeling. Voltage-matched area de-coupled modules are found to be considerably less sensitive to changes in the spectrum than current-matched modules. Current-matched modules are, on the other hand, found to be less sensitive to changes in the temperature. Under normal conditions, the difference in temperature sensitivity has a negligible impact on the efficiency compared to the difference in spectral sensitivity, making voltage-matched modules the preferred choice. The difference in efficiency between an area de-coupled voltage-matched module and a four-terminal device is found to be too small to be of any practical consequence even under changing conditions. This finding is in agreement with earlier work by Lentine et al. on microsystem-enabled photovoltaic modules.

Theoretical efficiency of intermediate band solar cells with overlapping absorption coefficients for various combinations of band gaps

The most commonly used criteria for evaluation of the suitability of different intermediate band materials for use in intermediate band solar cells are their band gaps. One often sees that such an evaluation is made based on theoretical efficiency limits with non-overlapping absorption coefficients. In this work the theoretical efficiency limits for various degrees of overlap are calculated for relevant combinations of band gaps. It is found that the optimal position of the intermediate band moves towards the middle of the band gap when the overlap increases. It is also shown that overlap between the absorption coefficients can increase the theoretical efficiency for some band gap combinations. This work aims to serve as a rough guide for determining whether a combination of band gaps is promising for use in intermediate band solar cells or not.

Comparison of the sensitivity to spectral variation of voltage- and current-matched tandem devices with luminescent coupling and thickness optimization

Using models that do not take luminescent coupling into account, voltage-matched tandem devices have previously been shown to be less sensitive to spectral variation than their current-matched counterparts. The present paper compares the spectral sensitivity of voltage-matched and current-matched tandem devices by applying a detailed balance model that takes luminescent coupling into account. Current-matched stacks where the thickness of the top cell thickness has been optimized are also considered. The main finding is that luminescent coupling reduces the difference in spectral sensitivity between voltage-matched and current-matched devices. There is still a significant difference, however. It is also found that current-matched devices with a top cell thickness-optimized for the AM1.5 spectrum are more sensitive to variations in the incoming spectrum than comparable cells with a fully absorbing top cell.