Engineers at Meijo University and Nagoya University have revealed that GaN substrate can realize an external quantum efficiency (EQE) of more than 40 % 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 incorporated into a conventional multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the best approach is the one about one particular 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 that the main challenge to realizing such devices will be the growth of highquality InGaN layers with higher indium content. “Should this issue 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 help to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of the cells certainly are a roughened surface that couples more radiation to the device. Photovoltaics were produced by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These units featured a 60 nm thick active layer made of InGaN as well as a p-type GaN cap having a surface roughness that could be adjusted by altering the expansion temperature with this layer.
They measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for an example). This pair of measurements stated that radiation below 365 nm, which can be absorbed by InGaN, fails to bring about 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 virtually all the absorbed photons in this spectral range are converted into electrons and holes. These carriers are efficiently separated and bring about power generation. Above 410 nm, absorption by InGaN is very weak. Matioli and his awesome colleagues have tried to optimise the roughness of the cells so that they absorb more light. However, despite their finest efforts, a minumum of one-fifth of the incoming light evbryr either reflected off of the top surface or passes directly with the cell. Two choices for addressing these shortcomings are going to introduce anti-reflecting and highly reflecting coatings inside the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“We have been working with photonic crystals for the past years,” says Matioli, “and I am investigating using 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 kind 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 over a GaN substrate and a 100 nm p-type cap; as well as 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 an identical cap.
The second structure, which includes thinner GaN layers within the superlattice, produced a peak EQE more than 46 percent, 15 times that of the other structure. However, in the more effective structure the density of pits is way higher, which may account for the halving in the open-circuit voltage.
To comprehend high-quality material rich in efficiency, the researchers turned to one third structure that combined 50 pairs of 3 nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of three nm thick Ga0.83In0.17N and .6 nm thick GaN LED. 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 shall also fabricate solar cells on other crystal planes and on a silicon substrate,” says Kuwahara.