Engineers at Meijo University and Nagoya University have revealed that GaN on GaN can realize an external quantum efficiency (EQE) in excess of 40 % over the 380-425 nm range. And researchers at UCSB and 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 traditional multi-junction device to reap the high-energy region of the solar spectrum.
“However, the best approach is that of one particular nitride-based cell, as a result of coverage from the entire solar spectrum by the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains the main challenge to realizing such devices is the growth of highquality InGaN layers with higher indium content. “Should this problem 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 that limit conversion efficiency. Another novel feature of their cells really 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 made of InGaN as well as a p-type GaN cap with a surface roughness that could be adjusted by altering the expansion temperature of this layer.
They measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for the example). This kind of measurements said that radiation below 365 nm, which is absorbed by GaN wafer, fails to contribute to 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 transformed into electrons and holes. These carriers are efficiently separated and play a role in power generation. Above 410 nm, absorption by InGaN is extremely weak. Matioli and his awesome colleagues have made an effort to optimise the roughness of the cells so that they absorb more light. However, despite having their finest efforts, a minumum of one-fifth from the incoming light evbryr either reflected off the top surface or passes directly from the cell. Two choices for addressing these shortcomings are 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 have been working with photonic crystals for the past years,” says Matioli, “and i also am investigating the use of photonic crystals to nitride solar panels.” Meanwhile, Japanese researchers have been fabricating devices with higher indium content layers by embracing superlattice architectures. Initially, the engineers fabricated two type 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 and 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 since the first design and featuring an identical cap.
The next structure, which has thinner GaN layers inside the superlattice, produced a peak EQE in excess of 46 percent, 15 times those of the other structure. However, inside the more efficient structure the density of pits is significantly higher, which may make up the halving from the open-circuit voltage.
To realize high-quality material with high efficiency, they looked 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 wafer. 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 will also fabricate solar cells on other crystal planes and also on a silicon substrate,” says Kuwahara.