Engineers at Meijo University and Nagoya University have revealed that Gallium Nitride can realize an external quantum efficiency (EQE) of more than 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 incorporated into a conventional multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the greatest approach is that of a single nitride-based cell, because of the coverage in 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 the expansion of highquality InGaN layers rich in indium content. “Should this issue 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 that help to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of their cells 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 devices featured a 60 nm thick active layer manufactured from InGaN along with 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 set of measurements stated that radiation below 365 nm, that is absorbed by InGaN, will not 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 almost all the absorbed photons in this spectral range are transformed into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is extremely weak. Matioli and his colleagues have made an effort to optimise the roughness of their cells to make sure they absorb more light. However, even with their best efforts, a minumum of one-fifth of the incoming light evbryr either reflected off of the top surface or passes directly through 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.
“I actually have been working with photonic crystals over 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 embracing superlattice architectures. Initially, the engineers fabricated two form 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 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 since the first design and featuring an identical cap.
The second structure, which has thinner GaN layers in the superlattice, produced a peak EQE in excess of 46 percent, 15 times that relating to the other structure. However, within the more effective structure the density of pits is way higher, which may make up the halving from the open-circuit voltage.
To understand high-quality material with high efficiency, they looked to a third 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 epi wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
They 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.