Engineers at Meijo University and Nagoya University have demostrated that GaN on GaN can realize an external quantum efficiency (EQE) of over forty percent 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 included in a regular multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the greatest approach is one particular nitride-based cell, due to the 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 is definitely the expansion of highquality InGaN layers rich in indium content. “Should this problem 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 which help to screen polarization related charges at hetero-interfaces that limit conversion efficiency. Another novel feature with their cells really are a roughened surface that couples more radiation in to the device. Photovoltaics were made by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These products featured a 60 nm thick active layer manufactured from InGaN and a p-type GaN cap with a surface roughness that may 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 said that radiation below 365 nm, which is absorbed by GaN on sapphire, 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 transformed 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 and his awesome colleagues have attempted to optimise the roughness of their cells so that they absorb more light. However, even with their very best efforts, a minumum of one-fifth in the incoming light evbryr either reflected from the top surface or passes directly from the cell. Two choices for addressing these shortcomings will be to introduce anti-reflecting and highly reflecting coatings inside the top and bottom surfaces, or trap the incoming radiation with photonic crystal structures.
“I actually have been dealing with photonic crystals for the past years,” says Matioli, “and i also 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 on a GaN substrate as well as 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 2nd structure, which includes thinner GaN layers inside the superlattice, produced a peak EQE greater than 46 percent, 15 times that of one other structure. However, in the more effective structure the density of pits is significantly higher, which could take into account the halving from the open-circuit voltage.
To comprehend high-quality material with higher efficiency, the researchers considered a 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 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.