Engineers at Meijo University and Nagoya University have revealed that Gallium Nitride can realize an external quantum efficiency (EQE) of over 40 % over the 380-425 nm range. And researchers at UCSB as well as 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 traditional multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the greatest approach is the one about a single nitride-based cell, due to the coverage of the entire solar spectrum through the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains the main challenge to realizing such devices is the growth of highquality InGaN layers rich in indium content. “Should this problem be solved, one particular 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 that help to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of their cells certainly are a roughened surface that couples more radiation into 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 may be adjusted by altering the growth temperature with this layer.
They measured the absorption and EQE in the cells at 350-450 nm (see Figure 2 for an example). This kind of measurements revealed that radiation below 365 nm, which can be absorbed by GaN wafer, 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 nearly all the absorbed photons in this spectral range are converted into electrons and holes. These carriers are efficiently separated and play a role in power generation. Above 410 nm, absorption by InGaN is extremely weak. Matioli and his awesome colleagues have attempted to optimise the roughness of the cells so that they absorb more light. However, even with their very best efforts, one or more-fifth from the incoming light evbryr either reflected off of the top surface or passes directly from the cell. Two choices for addressing these shortcomings are 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 also am investigating using photonic crystals to nitride solar cells.” Meanwhile, Japanese scientific study has 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 from a 2.5 µm-thick n-doped buffer layer over 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 since the first design and featuring an identical cap.
The second structure, which has thinner GaN layers within the superlattice, produced a peak EQE greater than 46 percent, 15 times those of one other structure. However, within the more efficient structure the density of pits is far higher, which may account for the halving in the open-circuit voltage.
To comprehend high-quality material rich in efficiency, they looked to a third structure that combined 50 pairs of three nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of three nm thick Ga0.83In0.17N and .6 nm thick LED 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 are going to also fabricate solar panels on other crystal planes and on a silicon substrate,” says Kuwahara.