M. Ochoa

Advances towards 4J lattice-matched including dilute nitride subcell for terrestrial and space applications

Recent advances on the development of a 4J lattice-matched dilute nitride solar cell for terrestrial and space applications are described. Modeling of the solar cell is carried out using a drift-diffusion model and material parameters extracted from ad hoc electro-optical characterization resulting in an efficiency prediction of 47% for concentrations of 1000 suns AM1.5d G173 spectrum and 33% for 1× AM0. First experimental solar cell results of a dual-junction GaNAsSb/Ge solar cell and a triple-junction GaInP/Ga(In)As/GaNAsSb components of the full 4-Junction are shown.

Energy yield determination of concentrator solar cells using laboratory measurements

The annual energy conversion efficiency is calculated for a four junction inverted metamorphic solar cell that has been completely characterized in the laboratory at room temperature using measurements fit to a comprehensive optoelectronic model of the multijunction solar cells. A simple model of the temperature dependence is used predict the performance of the solar cell under varying temperature and spectra characteristic of Golden, CO for an entire year. The annual energy conversion efficiency is calculated by integrating the predicted cell performance over the entire year. The effects of geometric concentration, CPV system thermal characteristics, and luminescent coupling are highlighted.

On the thermal degradation of tunnel diodes in multijunction solar cells

Tunnel junctions are essential components of multijunction solar cells. These highly doped p/n junctions provide the electrical interconnect between the subcells that constitute a multijunction solar cell device. The conductivity and the peak tunneling current of tunnel diodes are known to be severely affected by thermal load. This is a general phenomenon observed in tunnel junctions despite the materials used, the dopants employed or the growth technique applied. Despite this generality, the explanations for this thermal degradation tend to be quite material/dopant specific. On the contrary, in this work we apply the amphoteric native defect model to explain this issue. In this context, the degradation can be explained as a consequence of the net loss of free carrier concentration produced by the creation of native compensating defects in the highly doped layers of the tunnel junction. Experiments carried out on n++ GaAs agree well with the model.

Modelling of lattice matched dilute nitride 4-junction concentrator solar cells on Ge substrates

Technology Computer Aided Design modeling is used to examine the performance under light concentration of a 4-J solar cell Ge-based that includes a 1-eV MBE-grown dilute nitride subcell. The 1-eV solar cell is modeled and examined by using material parameters extracted from detailed electro-optical characterization prior to be included into a multijunction structure. The modelling reveals the impact of the electric field-assisted collection in the performance of single junction solar cells and its effect when included in a 4-Junction solar cells. This effect is responsible for the lower FF (~15% lower) in the 4J when including the dilute nitride subcell, especially if it limits the photocurrent. Finally, an optimization procedure based on dilute nitrides with higher material quality is performed resulting in a 4-Junction solar cell with an efficiency of 47% for concentrations between 1000-2000 suns direct terrestrial spectrum.

Lowering perimeter recombination losses in micro-concentrator solar cells: A simulation study

A device simulation study regarding perimeter recombination losses in micro-concentrator solar cells is presented. Perimeter losses for traditional and heterojunction designs were computed under dark and illuminated conditions for different solar cell sizes and surface state densities. A 2D device simulation model based on SRH recombination has shown that the use of heterojunction designs could help mitigate perimeter recombination for micro-CPV solar cells.A device simulation study regarding perimeter recombination losses in micro-concentrator solar cells is presented. Perimeter losses for traditional and heterojunction designs were computed under dark and illuminated conditions for different solar cell sizes and surface state densities. A 2D device simulation model based on SRH recombination has shown that the use of heterojunction designs could help mitigate perimeter recombination for micro-CPV solar cells.

Preliminary analysis of annealing impact on 1 eV GaNAsSb solar cells

Impact of annealing temperature, time and process gas on 1 eV GaNAsSb solar cells is assessed. In situ and ex situ annealings are carried out in order to analyze their effect on the solar cell performance. Ex situ annealings on as grown samples using N2 as process gas increase the external quantum efficiency while using H2 as process gas degrades the solar cell performance. Best performance is obtained by in situ annealing in a molecular beam epitaxy reactor. External quantum efficiency increases in the short wavelength range (≤ 700 nm) and decreases in the long wavelength range (≥ 700 nm) after ex situ annealing using N2 as process gas on in situ annealed samples (in a molecular beam epitaxy reactor).