Engineers at Meijo University and Nagoya University have demostrated that GaN substrate can realize an external quantum efficiency (EQE) of more than 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 conventional multi-junction device to reap the high-energy region of the solar spectrum.
“However, the best approach is just one nitride-based cell, because of 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 development of highquality InGaN layers rich in indium content. “Should this issue be solved, one particular 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 into 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 might be adjusted by altering the development temperature with this layer.
The researchers measured the absorption and EQE in the cells at 350-450 nm (see Figure 2 to have an example). This set of measurements stated that radiation below 365 nm, which can be absorbed by GaN on sapphire, 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 virtually all the absorbed photons within this spectral range are changed into electrons and holes. These carriers are efficiently separated and bring about power generation. Above 410 nm, absorption by InGaN is very weak. Matioli and his awesome colleagues have attempted to optimise the roughness of the cells so that they absorb more light. However, even with their finest efforts, one or more-fifth from the incoming light evbryr either reflected off the top surface or passes directly through the cell. Two options for addressing these shortcomings are going to introduce anti-reflecting and highly reflecting coatings inside the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“I have been utilizing photonic crystals within the last years,” says Matioli, “and that 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 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 over a GaN substrate as well as a 100 nm p-type cap; as well as a 50 pair superlattice with alternating layers of 3 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 next structure, which has 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 more effective structure the density of pits is significantly higher, which may take into account the halving in the open-circuit voltage.
To comprehend high-quality material with higher efficiency, the researchers considered a third structure that combined 50 pairs of three nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of three nm thick Ga0.83In0.17N and .6 nm thick InGaN LED. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
They is looking to now build structures with higher indium content. “We are going to also fabricate solar cells on other crystal planes and on a silicon substrate,” says Kuwahara.