Reduced internal electric fields, rougher surfaces and the introduction of superlattices are helping to drive up the efficiencies of nitride solar cells COMPARED to lasers, transistors and LEDs, solar cells based on nitrides tend to deliver poor performances. But these materials can yield efficient photovoltaics, according to research by both a pair of Japanese Universities and by a team led by the University of California, Santa Barbara (UCSB).

Engineers at Meijo University and Nagoya University have shown that InGaN/GaN superlattice structures 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 incorporated into a traditional 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, due to the coverage of the entire solar spectrum by the direct bandgap of InGaN," says UCSB’s Elison Matioli.

He explains that the main challenge to realizing such devices is the growth of highquality InGaN layers with high indium content. "Should this problem be solved, a single 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 help to screen polarizationrelated charges at hetero-interfaces that limit conversion efficiency. Another novel feature of their cells 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 nmthick active layer made of InGaN and a p-type GaN cap with a surface roughness that could be adjusted by altering the growth temperature of this layer .

The researchers measured the absorption and EQE of the cells at 350-450 nm (see Figure 2 for an example). This pair of measurements revealed that radiation below 365 nm, which is absorbed by GaN, does 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 nearly all the absorbed photons in this spectral range are converted into electrons and holes. These carriers are efficiently separated and contribute to power generation. Above 410 nm, absorption by InGaN is very weak. Matioli and his colleagues have tried to optimise the roughness of their cells so that they absorb more light. However, even with their best efforts, at least one-fifth of the incoming light is either reflected off the top surface or passes directly through the cell. Two options for addressing these shortcomings are 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 have been working with photonic crystals for the past 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 type 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 and a 100 nm p-type cap; and a 50 pair superlattice with alternating layers of 3 nm thick Ga0.83In0.17N and 0.6 nm-thick GaN, deposited on the same substrate and buffer as the first design and featuring an identical cap.

The second structure, which has thinner GaN layers in the superlattice, produced a peak EQE in excess of 46 percent, 15 times that of the other structure. However, in the more efficient structure the density of pits is far higher, which could account for the halving of the open-circuit voltage.

To realize high-quality material with high efficiency, the researchers turned to a third structure that combined 50 pairs of 3 nmthick layers of Ga0.83In0.17N and GaN with 10 pairs of 3 nm thick Ga0.83In0.17N and 0.6 nmthick GaN. Pit density plummeted to below 106 cm-2 and peak EQE hit 59 percent.

The team is aiming 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.

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