Engineers at Meijo University and Nagoya University have shown that Gallium Nitride can realize an external quantum efficiency (EQE) of over 40 percent over the 380-425 nm range. And researchers at UCSB and 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 regular 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, because of the coverage in the entire solar spectrum through the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that the main challenge to realizing such devices is definitely the growth of highquality InGaN layers rich in indium content. “Should this issue be solved, just one nitride solar cell makes perfect sense.”
Matioli along with 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 to limit conversion efficiency. Another novel feature of their 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 units featured a 60 nm thick active layer manufactured from InGaN along with a p-type GaN cap having a surface roughness that could be adjusted by altering the development temperature of the layer.
The researchers measured the absorption and EQE of the cells at 350-450 nm (see Figure 2 to have an example). This kind of measurements stated that radiation below 365 nm, which can be absorbed by InGaN, 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 virtually all the absorbed photons within this spectral range are changed into electrons and holes. These carriers are efficiently separated and play a role in power generation. Above 410 nm, absorption by InGaN is quite weak. Matioli along with his colleagues have attempted to optimise the roughness of the cells so they absorb more light. However, even with their best efforts, at least one-fifth from the incoming light evbryr either reflected off of the top surface or passes directly from the cell. Two alternatives for addressing these shortcomings are going to introduce anti-reflecting and highly reflecting coatings within the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“We have been utilizing photonic crystals over the past years,” says Matioli, “and that i am investigating using photonic crystals to nitride solar panels.” Meanwhile, Japanese researchers have been fabricating devices with higher indium content layers by switching to 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 on a GaN substrate as well as 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 an identical cap.
The next structure, that has thinner GaN layers within the superlattice, produced a peak EQE in excess of 46 percent, 15 times that of one other structure. However, in the more efficient structure the density of pits is far higher, which may account for the halving of the open-circuit voltage.
To comprehend high-quality material with higher efficiency, the researchers turned to a 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 epitaxial wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The group is aiming to now build structures with higher indium content. “We are going to also fabricate solar cells on other crystal planes as well as on a silicon substrate,” says Kuwahara.