Engineers at Meijo University and Nagoya University have shown that GaN substrate can realize an external quantum efficiency (EQE) of more than 40 percent over the 380-425 nm range. And researchers at UCSB and the Ecole Polytechnique, France, have reported 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 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 expansion of highquality InGaN layers rich in indium content. “Should this challenge be solved, one particular 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 of the cells really are a roughened surface that couples more radiation into the device. Photovoltaics were produced by depositing GaN/InGaN p-i-n structures on sapphire by MOCVD. These devices featured a 60 nm thick active layer made of 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 in the cells at 350-450 nm (see Figure 2 for an example). This set of measurements said that radiation below 365 nm, which is absorbed by InGaN, fails to 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 in this particular spectral range are transformed 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 awesome colleagues have tried to optimise the roughness with their cells so that they absorb more light. However, despite having their finest efforts, one or more-fifth of the incoming light evbryr either reflected off the top surface or passes directly from the cell. Two options for addressing these shortcomings will be 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 working with photonic crystals for the past years,” says Matioli, “and i also am investigating the usage of photonic crystals to nitride solar cells.” 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 from a 2.5 µm-thick n-doped buffer layer over a GaN substrate and 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 because 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 that of the other structure. However, within the more efficient structure the density of pits is far higher, which may make up the halving of the open-circuit voltage.
To realize high-quality material rich in efficiency, they turned 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 aiming to now build structures with higher indium content. “We are going to also fabricate solar cells on other crystal planes and also on a silicon substrate,” says Kuwahara.