Engineers at Meijo University and Nagoya University have revealed that GaN on GaN can realize an external quantum efficiency (EQE) of over 40 percent over the 380-425 nm range. And researchers at UCSB as well as the Ecole Polytechnique, France, have documented a peak EQE of 72 percent at 380 nm. Both cells have the potential to be included in a conventional multi-junction device to reap the high-energy region of the solar spectrum.
“However, the best approach is the one about a single nitride-based cell, due to the coverage in the entire solar spectrum from the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that the main challenge to realizing such devices will be the growth of highquality InGaN layers rich in indium content. “Should this challenge be solved, just one nitride solar cell makes perfect sense.”
Matioli and his awesome 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 that limit conversion efficiency. Another novel feature of the cells really are a roughened surface that couples more radiation in to the device. Photovoltaics were produced by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These products featured a 60 nm thick active layer made of InGaN as well as a p-type GaN cap using a surface roughness that may be adjusted by altering the expansion temperature of the layer.
The researchers measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for an example). This kind of measurements said that radiation below 365 nm, that is absorbed by GaN on sapphire, fails to play a role in 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 almost all the absorbed photons within this spectral range are converted 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 colleagues have made an effort to optimise the roughness of their cells to make sure they absorb more light. However, despite having their finest efforts, at least one-fifth in the incoming light evbryr either reflected off of the top surface or passes directly with the cell. Two options for addressing these shortcomings will be to introduce anti-reflecting and highly reflecting coatings inside 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 scientific study has been fabricating devices with higher indium content layers by switching to superlattice architectures. Initially, the engineers fabricated two form of device: a 50 pair superlattice with alternating 3 nm-thick layers of Ga0.83In0.17N and GaN, sandwiched from a 2.5 µm-thick n-doped buffer layer on a GaN substrate and a 100 nm p-type cap; along with 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 because the first design and featuring the same cap.
The second structure, which includes thinner GaN layers in the superlattice, produced a peak EQE in excess of 46 percent, 15 times those of one other structure. However, in the more effective structure the density of pits is way higher, that could account for the halving in the open-circuit voltage.
To comprehend high-quality material rich in efficiency, they turned to a third structure that combined 50 pairs of 3 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.
They is aiming to now build structures with higher indium content. “We will also fabricate solar panels on other crystal planes as well as on a silicon substrate,” says Kuwahara.