Engineers at Meijo University and Nagoya University have revealed that Gallium Nitride can realize an external quantum efficiency (EQE) of more than forty percent over the 380-425 nm range. And researchers at UCSB and also the Ecole Polytechnique, France, have reported a peak EQE of 72 percent at 380 nm. Both cells have the potential to be integrated into a regular multi-junction device to reap the high-energy region of the solar spectrum.
“However, the best approach is that of just one nitride-based cell, as a result of coverage of the entire solar spectrum from the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains the main challenge to realizing such devices is definitely the growth of highquality InGaN layers with higher indium content. “Should this challenge be solved, one particular nitride solar cell makes perfect sense.”
Matioli and his co-workers have built devices with highly doped n-type and p-type GaN regions which help to screen polarization related charges at hetero-interfaces that limit conversion efficiency. Another novel feature of their cells are a roughened surface that couples more radiation into the device. Photovoltaics were created by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These products featured a 60 nm thick active layer made from InGaN along with a p-type GaN cap using a surface roughness that might be adjusted by altering the growth temperature with this layer.
They 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, which can be absorbed by GaN on sapphire, fails to 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 almost all the absorbed photons in this 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 along with his colleagues have made an effort to optimise the roughness of their cells to make sure they absorb more light. However, despite their finest efforts, at least one-fifth of the incoming light evbryr either reflected off the top surface or passes directly with the cell. Two choices for addressing these shortcomings are to introduce anti-reflecting and highly reflecting coatings in the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“I actually have been utilizing photonic crystals for the past years,” says Matioli, “and i also am investigating using photonic crystals to nitride solar panels.” Meanwhile, Japanese scientific study has been fabricating devices with higher indium content layers by switching to 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 from a 2.5 µm-thick n-doped buffer layer on the 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 second structure, that has thinner GaN layers in the superlattice, produced a peak EQE more than 46 percent, 15 times that of the other structure. However, within the more effective structure the density of pits is far higher, which may make up the halving in the open-circuit voltage.
To comprehend high-quality material rich in efficiency, they considered one 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 epitaxial wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
They is hoping to now build structures with higher indium content. “We will also fabricate solar panels on other crystal planes and on a silicon substrate,” says Kuwahara.