Engineers at Meijo University and Nagoya University have demostrated that GaN substrate can realize an external quantum efficiency (EQE) of over 40 percent 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 traditional multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the best approach is that of a single nitride-based cell, due to the coverage of the entire solar spectrum through the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that this main challenge to realizing such devices is definitely the development of highquality InGaN layers with high indium content. “Should this problem be solved, a single 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 that limit conversion efficiency. Another novel feature with their cells 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 as well as a p-type GaN cap with a surface roughness that may be adjusted by altering the growth temperature of this layer.
The researchers measured the absorption and EQE from the cells at 350-450 nm (see Figure 2 for an example). This set of measurements stated that radiation below 365 nm, which can be 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 almost all the absorbed photons in this particular spectral range are converted into electrons and holes. These carriers are efficiently separated and bring about power generation. Above 410 nm, absorption by InGaN is very weak. Matioli along with his colleagues have tried to optimise the roughness of their cells to make sure they absorb more light. However, despite their best efforts, at least one-fifth from the incoming light evbryr either reflected off 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 trap the incoming radiation with photonic crystal structures.
“I have been working with photonic crystals for the past years,” says Matioli, “and I 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 turning 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 over a GaN substrate as well as 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 the same cap.
The 2nd structure, that has thinner GaN layers in the superlattice, produced a peak EQE greater than 46 percent, 15 times that of one other structure. However, in the more efficient structure the density of pits is way higher, that could take into account the halving of the open-circuit voltage.
To understand high-quality material with higher efficiency, the researchers looked to one third structure that combined 50 pairs of 3 nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of three 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.
They is hoping to now build structures with higher indium content. “We will also fabricate solar cells on other crystal planes and on a silicon substrate,” says Kuwahara.