Engineers at Meijo University and Nagoya University have demostrated that Gallium Nitride can realize an external quantum efficiency (EQE) of over forty percent over the 380-425 nm range. And researchers at UCSB and the Ecole Polytechnique, France, have reported 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 greatest approach is the one about one particular nitride-based cell, due to the coverage from 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 with high indium content. “Should this issue be solved, a single 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 really are a roughened surface that couples more radiation in to 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 and a p-type GaN cap with a surface roughness that could 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 to have an example). This pair of measurements said that radiation below 365 nm, which is absorbed by InGaN, does not 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 within 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 very weak. Matioli and his awesome colleagues have made an effort to optimise the roughness of the cells so they absorb more light. However, despite having their finest efforts, a minumum of one-fifth from the incoming light evbryr either reflected off of the top surface or passes directly through 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 even to trap the incoming radiation with photonic crystal structures.
“We have been utilizing photonic crystals for the past years,” says Matioli, “and I am investigating the use of photonic crystals to nitride solar panels.” Meanwhile, Japanese scientific study has been fabricating devices with higher indium content layers by turning 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 from a 2.5 µm-thick n-doped buffer layer on the GaN substrate as well as 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 as the first design and featuring the same cap.
The second structure, that has thinner GaN layers within the superlattice, produced a peak EQE greater than 46 percent, 15 times that of one other structure. However, within the better structure the density of pits is way higher, which may account for the halving in the open-circuit voltage.
To understand 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 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.
The group is aiming to now build structures with higher indium content. “We shall also fabricate solar cells on other crystal planes and also on a silicon substrate,” says Kuwahara.