Engineers at Meijo University and Nagoya University have revealed that Gallium Nitride can realize an external quantum efficiency (EQE) of over 40 % over the 380-425 nm range. And researchers at UCSB and also 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 traditional multi-junction device to reap 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 through the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains the main challenge to realizing such devices is the expansion of highquality InGaN layers with higher indium content. “Should this issue be solved, just one 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 which help to screen polarization related charges at hetero-interfaces that limit conversion efficiency. Another novel feature with their 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 products featured a 60 nm thick active layer made from InGaN and a p-type GaN cap using a surface roughness that might be adjusted by altering the expansion temperature with this layer.
They measured the absorption and EQE from 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 particular 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 very weak. Matioli and his colleagues have tried to optimise the roughness with their cells so they absorb more light. However, despite their best efforts, one or more-fifth in the incoming light evbryr either reflected off the top surface or passes directly through the cell. Two options for addressing these shortcomings are 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 actually have been dealing with photonic crystals for the past years,” says Matioli, “and i also am investigating the usage of photonic crystals to nitride solar panels.” Meanwhile, Japanese scientific study has been fabricating devices with higher indium content layers by switching to 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; along with 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 because the first design and featuring an identical cap.
The next structure, that has thinner GaN layers within the superlattice, produced a peak EQE in excess of 46 percent, 15 times that of the other structure. However, within the more efficient structure the density of pits is far higher, that could account for the halving of the open-circuit voltage.
To understand high-quality material rich in efficiency, they turned to another 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 LED epitaxial wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The group 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.