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Superior tunnel junctions for ultra-high concentration CPV

Substituting the GaAs-based cathode with a GaInP alloy in the tunnel-junction results in a 1.3 percent increase in solar cell efficiency in the triple junction cell and allows operation at 15,000 suns

Scientists from the Solar Energy Institute of the Technical University of Madrid have developed a high bandgap tunnel junction (TJ) for multi-junction cells that sets a new benchmark for current handling capability.

The team believes that these TJs, which also deliver increased transparency, will aid their quest to develop high efficiency solar cells operation at ultra-high concentrations.

Grown by MOCVD, the p++-AlGaAs/n++-GaInP tunnel junction (TJ) produced by the researchers displays an outstanding electrical performance.

The TJ is intended to interconnect the top (GaInP) and middle (GaInAs) subcells of concentrator triple junction solar cells. In as-grown tunnel diodes, an average peak tunnelling current density (Jp) of 996 A/cm2 and a specific resistance of 7 x 10-5 Ω cm2 were obtained, whilst record-performing devices exhibited Jp above 1050 A/cm2.

When introduced in multi-junction solar cell structures, tunnel diodes are known to suffer significant degradation. This results from the thermal load associated with the growth of the rest of the solar cell structure, which is deposited at much higher temperatures (for details, see I. Garcia et al in the Journal of Physics D: Applied Physics).

In order to simulate this effect, the researchers from Madrid annealed the samples. After the thermal load, these devices still exhibited high performance, although the average Jp reduced to 235 A/cm2 and specific resistance increased to 1 x 10-4 Ω cm2. Despite this drop, these values are still significantly high enough to allow the TJ to operate in the ohmic region up to about 15,000 suns with a negligible voltage drop (see figure below).

Left: Average J-V measurements of p++-AlGaAs:C/n++-GaInP : Te TJs fabricated with the as-grown structures (black circles) and after thermal annealing at 675oC for 30 min (red triangles)

Right: Magnification of left figure for low voltages where current densities equivalent to operation at 1000, 5000 and 10000 suns are drawn as references (dotted lines)

The design and manufacture of multi-junction solar cells - and accordingly tunnel junctions - for operation at ultra high concentration (i.e. several thousand suns) is one of the research goals of the III-V Semiconductor Group of the Solar Energy Institute in Madrid.

Progress by the III-V Semiconductor Group is seen by comparing work from 2008 to now.  Back then, they reported in Applied Physics Letters a GaInP/GaAs dual-junction solar cell with a record efficiency of 32.6 percent at 1026 suns. The TJ employed in that case (a p++-AlGaAs:C/n++-GaAs:Te) showed peak current densities of 2000 A/cm2 and 300 A/cm2 for the as-grown and annealed devices, respectively.

The motivation for introducing the new TJ is to increase transparency.  Success on this front also results in a gain in current around 0.56 A/cm2, implying an improvement of 1.3 percent (absolute) in the triple junction solar cell efficiency.

The details of the work described here can be found in the paper,“Highly conductive p + + -AlGaAs/n + + -GaInP tunnel junctions for ultra-high concentrator solar cells,” by E. Barrigón et al in Progress in Photovoltaics: Research and Applications, 22, 4, pp.399 - 404. (April 2014).DOI: 10.1002/pip.2476

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