Engineers at Meijo University and Nagoya University have demostrated that GaN on GaN can realize an external quantum efficiency (EQE) in excess of 40 percent over the 380-425 nm range. And researchers at UCSB as well as 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 reap the high-energy region of the solar spectrum.
“However, the greatest 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 high indium content. “Should this challenge 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 to limit conversion efficiency. Another novel feature with their cells 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 might be adjusted by altering the development temperature of this layer.
They measured the absorption and EQE of the cells at 350-450 nm (see Figure 2 for the example). This pair of measurements said that radiation below 365 nm, that is absorbed by InGaN, will not play a role in 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 transformed into electrons and holes. These carriers are efficiently separated and bring about power generation. Above 410 nm, absorption by InGaN is quite weak. Matioli along with his colleagues have tried to optimise the roughness with their cells so that they absorb more light. However, despite having their best efforts, one or more-fifth from the incoming light evbryr either reflected off of the top surface or passes directly with the cell. Two options for addressing these shortcomings will be to introduce anti-reflecting and highly reflecting coatings within the top and bottom surfaces, or even to trap the incoming radiation with photonic crystal structures.
“I actually have been working with photonic crystals within the last 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 turning 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 the 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 second structure, which includes thinner GaN layers within the superlattice, produced a peak EQE in excess of 46 percent, 15 times that relating to one other structure. However, in the better structure the density of pits is far higher, that could account for the halving of the open-circuit voltage.
To realize high-quality material with higher efficiency, they turned to one third structure that combined 50 pairs of three nm thick layers of Ga0.83In0.17N and GaN with 10 pairs of 3 nm thick Ga0.83In0.17N and .6 nm thick LED epi wafer. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.
The group is hoping 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.