Engineers at Meijo University and Nagoya University have demostrated that GaN substrate can realize an external quantum efficiency (EQE) in excess of 40 percent over the 380-425 nm range. And researchers at UCSB and also 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 conventional multi-junction device to harvest the high-energy region of the solar spectrum.
“However, the ultimate approach is that of a single nitride-based cell, because of 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 the expansion of highquality InGaN layers with high indium content. “Should this problem 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 that assist to screen polarization related charges at hetero-interfaces to limit conversion efficiency. Another novel feature of the cells are a roughened surface that couples more radiation to the device. Photovoltaics were made by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These products featured a 60 nm thick active layer made from InGaN as well as a p-type GaN cap with a surface roughness that could be adjusted by altering the growth temperature of 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 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 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 the cells so that they absorb more light. However, even with their best efforts, at least one-fifth from the incoming light evbryr either reflected off of the top surface or passes directly through the cell. Two options for addressing these shortcomings are to introduce anti-reflecting and highly reflecting coatings within the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“We have been dealing with photonic crystals over the past years,” says Matioli, “and i also am investigating the usage of photonic crystals to nitride solar panels.” 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 between a 2.5 µm-thick n-doped buffer layer on a GaN substrate as well as a 100 nm p-type cap; as well as 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 the same cap.
The second structure, which includes thinner GaN layers in the superlattice, produced a peak EQE more than 46 percent, 15 times that relating to the other structure. However, inside the more efficient structure the density of pits is significantly higher, which could account for the halving in the open-circuit voltage.
To understand high-quality material with higher efficiency, the researchers considered 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.
They 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.