Engineers at Meijo University and Nagoya University have demostrated that Gallium Nitride can realize an external quantum efficiency (EQE) of over 40 percent 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 integrated into a regular multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the best approach is a single nitride-based cell, due to the coverage of 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 rich in indium content. “Should this challenge be solved, a single nitride solar cell makes perfect sense.”
Matioli and 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 to limit conversion efficiency. Another novel feature with 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 manufactured from InGaN along with a p-type GaN cap having a surface roughness that could be adjusted by altering the development temperature of the layer.
The researchers measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for an example). This pair of measurements stated that radiation below 365 nm, that is absorbed by GaN wafer, 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 almost all the absorbed photons within this spectral range are converted into electrons and holes. These carriers are efficiently separated and play a role in power generation. Above 410 nm, absorption by InGaN is very weak. Matioli and his colleagues have tried to optimise the roughness of their cells so they absorb more light. However, despite having their very best efforts, a minumum of one-fifth from the incoming light evbryr either reflected off the top surface or passes directly from the cell. Two options for addressing these shortcomings are 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 actually have been utilizing photonic crystals for the past years,” says Matioli, “and I am investigating the use of 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 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 as the first design and featuring an identical cap.
The 2nd structure, which has thinner GaN layers within the superlattice, produced a peak EQE greater than 46 percent, 15 times those of one other structure. However, in the more efficient structure the density of pits is far higher, which could account for the halving from the open-circuit voltage.
To understand high-quality material rich in efficiency, the researchers turned to 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 epitaxial wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The team is looking to now build structures with higher indium content. “We shall also fabricate solar panels on other crystal planes as well as on a silicon substrate,” says Kuwahara.