Engineers at Meijo University and Nagoya University have shown that Gallium Nitride can realize an external quantum efficiency (EQE) of over forty percent over the 380-425 nm range. And researchers at UCSB and the Ecole Polytechnique, France, have claimed a peak EQE of 72 percent at 380 nm. Both cells have the potential to be incorporated into a regular multi-junction device to reap the high-energy region of the solar spectrum.
“However, the best approach is the one about just one nitride-based cell, as a result of coverage of the entire solar spectrum through the direct bandgap of InGaN,” says UCSB’s Elison Matioli.
He explains that the main challenge to realizing such devices is the development of highquality InGaN layers with higher indium content. “Should this issue be solved, just one nitride solar cell makes perfect sense.”
Matioli and 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 the cells are a roughened surface that couples more radiation into 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 might be adjusted by altering the growth temperature of the layer.
They measured the absorption and EQE in the cells at 350-450 nm (see Figure 2 for the example). This set of measurements said that radiation below 365 nm, which is absorbed by GaN on sapphire, does 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 nearly all the absorbed photons within this spectral range are changed into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is quite weak. Matioli and his colleagues have made an effort to optimise the roughness of their cells so they absorb more light. However, even with their best efforts, one or more-fifth from the incoming light evbryr either reflected off of the top surface or passes directly from the cell. Two choices for addressing these shortcomings will be to introduce anti-reflecting and highly reflecting coatings in the top and bottom surfaces, or to trap the incoming radiation with photonic crystal structures.
“I actually have been working with photonic crystals over the past years,” says Matioli, “and I am investigating using photonic crystals to nitride solar panels.” Meanwhile, Japanese scientific study has 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 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 2nd structure, which includes thinner GaN layers in the superlattice, produced a peak EQE in excess of 46 percent, 15 times those of the other structure. However, within the more effective structure the density of pits is significantly higher, which may account for the halving from the open-circuit voltage.
To understand high-quality material with high efficiency, they turned to another 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 InGaN 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 shall also fabricate solar panels on other crystal planes and on a silicon substrate,” says Kuwahara.