Engineers at Meijo University and Nagoya University have demostrated that GaN on GaN 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 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, as a result of 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 expansion of highquality InGaN layers with high indium content. “Should this challenge be solved, one particular 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 assist to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of the cells 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 products featured a 60 nm thick active layer made of InGaN along with a p-type GaN cap having a surface roughness that may be adjusted by altering the growth temperature of this layer.
They measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for the example). This pair of measurements said that radiation below 365 nm, that is absorbed by GaN wafer, 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 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 very weak. Matioli and his awesome colleagues have attempted to optimise the roughness with their cells so they absorb more light. However, despite their finest efforts, at least one-fifth from the incoming light evbryr either reflected off 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 in the top and bottom surfaces, or even to trap the incoming radiation with photonic crystal structures.
“We have been dealing with photonic crystals over the past years,” says Matioli, “and i also am investigating the usage of 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 from a 2.5 µm-thick n-doped buffer layer on the GaN substrate along with 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, which includes thinner GaN layers in the superlattice, produced a peak EQE in excess of 46 percent, 15 times those of the other structure. However, inside the better structure the density of pits is way higher, which may make up the halving of the open-circuit voltage.
To comprehend high-quality material rich in efficiency, they turned to another 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 LED epitaxial wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The group is hoping to now build structures with higher indium content. “We will also fabricate solar cells on other crystal planes and also on a silicon substrate,” says Kuwahara.