Engineers at Meijo University and Nagoya University have revealed that GaN substrate can realize an external quantum efficiency (EQE) of more than forty percent over the 380-425 nm range. And researchers at UCSB and the Ecole Polytechnique, France, have claimed a peak EQE of 72 percent at 380 nm. Both cells have the potential to be integrated into a regular multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the ultimate approach is that of a single nitride-based cell, because of the coverage from the entire solar spectrum from the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that this main challenge to realizing such devices will be the expansion of highquality InGaN layers with high indium content. “Should this problem be solved, a single nitride solar cell makes perfect sense.”
Matioli and his co-workers have built devices with highly doped n-type and p-type GaN regions that assist to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of the cells really are a roughened surface that couples more radiation to the device. Photovoltaics were made by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These products featured a 60 nm thick active layer manufactured from InGaN as well as a p-type GaN cap using a surface roughness that might be adjusted by altering the development temperature of this layer.
The researchers measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for the example). This pair of measurements stated that radiation below 365 nm, which can be absorbed by GaN on sapphire, will not contribute to current generation – instead, the carriers recombine in p-type GaN.
Between 370 nm and 410 nm the absorption curve closely follows the plot of EQE, indicating that nearly all the absorbed photons in this particular spectral range are transformed into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is extremely weak. Matioli and his awesome colleagues have attempted to optimise the roughness with their cells so they absorb more light. However, despite having their very best efforts, at least one-fifth of the incoming light evbryr either reflected from the top surface or passes directly through the cell. Two alternatives for addressing these shortcomings are going to introduce anti-reflecting and highly reflecting coatings in the top and bottom surfaces, or trap the incoming radiation with photonic crystal structures.
“We have been working with photonic crystals over the past years,” says Matioli, “and that i am investigating the usage of photonic crystals to nitride solar cells.” Meanwhile, Japanese researchers have been fabricating devices with higher indium content layers by embracing superlattice architectures. Initially, the engineers fabricated two type of device: a 50 pair superlattice with alternating 3 nm-thick layers of Ga0.83In0.17N and GaN, sandwiched between a 2.5 µm-thick n-doped buffer layer on a GaN substrate and a 100 nm p-type cap; and a 50 pair superlattice with alternating layers of three nm thick Ga0.83In0.17N and .6 nm-thick GaN, deposited on the same substrate and buffer since the first design and featuring an identical cap.
The next structure, which has thinner GaN layers inside the superlattice, produced a peak EQE more than 46 percent, 15 times those of another structure. However, in the more effective structure the density of pits is way higher, which may make up the halving from the open-circuit voltage.
To comprehend high-quality material rich in efficiency, the researchers looked to one third structure that combined 50 pairs of three nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of 3 nm thick Ga0.83In0.17N and .6 nm thick LED epi wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The team is hoping to now build structures with higher indium content. “We shall also fabricate solar panels on other crystal planes as well as on a silicon substrate,” says Kuwahara.