Engineers at Meijo University and Nagoya University have shown that GaN on GaN can realize an external quantum efficiency (EQE) of over 40 % over the 380-425 nm range. And researchers at UCSB and 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 regular multi-junction device to reap the high-energy region of the solar spectrum.
“However, the ultimate approach is the one about just one nitride-based cell, due to the 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 definitely the growth of highquality InGaN layers with higher indium content. “Should this issue be solved, a single 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 assist to screen polarization related charges at hetero-interfaces that limit conversion efficiency. Another novel feature of 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 devices featured a 60 nm thick active layer manufactured from InGaN as well as a p-type GaN cap having a surface roughness that could be adjusted by altering the development temperature of the layer.
They measured the absorption and EQE of the cells at 350-450 nm (see Figure 2 for an example). This kind of measurements stated that radiation below 365 nm, that is absorbed by InGaN, will not 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 virtually all the absorbed photons in this spectral range are transformed into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is extremely weak. Matioli along with his colleagues have attempted to optimise the roughness of their cells to make sure they absorb more light. However, even with their finest efforts, at least one-fifth from the incoming light evbryr either reflected off of the top surface or passes directly through the cell. Two alternatives for addressing these shortcomings are going to introduce anti-reflecting and highly reflecting coatings inside the top and bottom surfaces, or even to trap the incoming radiation with photonic crystal structures.
“I 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 researchers have been fabricating devices with higher indium content layers by switching to 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; and 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 2nd structure, that has thinner GaN layers inside the superlattice, produced a peak EQE greater than 46 percent, 15 times those of the other structure. However, within the more effective structure the density of pits is significantly higher, which could take into account the halving from the open-circuit voltage.
To realize high-quality material with high efficiency, the researchers turned to a 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 GaN LED. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The team is aiming to now build structures with higher indium content. “We will also fabricate solar panels on other crystal planes and on a silicon substrate,” says Kuwahara.