Engineers at Meijo University and Nagoya University have revealed that Gallium Nitride can realize an external quantum efficiency (EQE) in excess of 40 percent over the 380-425 nm range. And researchers at UCSB and also the Ecole Polytechnique, France, have documented a peak EQE of 72 percent at 380 nm. Both cells have the potential to be included in a conventional multi-junction device to reap the high-energy region of the solar spectrum.
“However, the ultimate approach is one particular nitride-based cell, due to the coverage in the entire solar spectrum by the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that this main challenge to realizing such devices will be the development of highquality InGaN layers rich in indium content. “Should this problem be solved, a single nitride solar cell makes perfect sense.”
Matioli and 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 with 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 with a surface roughness that might be adjusted by altering the expansion temperature of this layer.
They measured the absorption and EQE of the cells at 350-450 nm (see Figure 2 for the example). This pair of measurements stated that radiation below 365 nm, which can be absorbed by GaN wafer, will 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 in this spectral range are changed into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is very weak. Matioli and his colleagues have made an effort to optimise the roughness of the cells so they absorb more light. However, despite their very best efforts, at least one-fifth in the incoming light evbryr either reflected off the top surface or passes directly with 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 to trap the incoming radiation with photonic crystal structures.
“I actually have been working with photonic crystals within the last years,” says Matioli, “and that i 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 form 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 a GaN substrate and 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 since the first design and featuring the same cap.
The 2nd structure, which has thinner GaN layers inside the superlattice, produced a peak EQE more than 46 percent, 15 times that relating to one other structure. However, inside the more efficient structure the density of pits is way higher, which could take into account the halving from the open-circuit voltage.
To comprehend high-quality material rich in efficiency, they looked to one third structure that combined 50 pairs of three nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of 3 nm thick Ga0.83In0.17N and .6 nm thick GaN LED. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
They 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.