Engineers at Meijo University and Nagoya University have revealed that GaN substrate can realize an external quantum efficiency (EQE) in excess of 40 percent over the 380-425 nm range. And researchers at UCSB as well as the Ecole Polytechnique, France, have reported 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 just one nitride-based cell, due to the coverage from the entire solar spectrum from the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains the main challenge to realizing such devices will be the growth of highquality InGaN layers with higher indium content. “Should this issue 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 help to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of the cells certainly are a roughened surface that couples more radiation into the device. Photovoltaics were created 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 having a surface roughness that might be adjusted by altering the expansion temperature with this layer.
They measured the absorption and EQE in the cells at 350-450 nm (see Figure 2 to have an example). This pair of measurements revealed that radiation below 365 nm, which 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 virtually all the absorbed photons in this spectral range are transformed into electrons and holes. These carriers are efficiently separated and bring about power generation. Above 410 nm, absorption by InGaN is quite weak. Matioli and his awesome colleagues have tried to optimise the roughness of the cells so they absorb more light. However, despite their finest efforts, at least one-fifth from the incoming light evbryr either reflected off of 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.
“We have been utilizing photonic crystals within the last years,” says Matioli, “and I am investigating using 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 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 since the first design and featuring an identical cap.
The 2nd structure, that has thinner GaN layers within the superlattice, produced a peak EQE more than 46 percent, 15 times that of the other structure. However, in the more efficient structure the density of pits is way higher, that could make up the halving of the open-circuit voltage.
To understand high-quality material rich in efficiency, the researchers considered one 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 epi wafer. 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 shall also fabricate solar cells on other crystal planes as well as on a silicon substrate,” says Kuwahara.