Room for improvement in solar cells

With conversion efficiency records claimed on a regular basis over recent months, the growing potential for energy production using III-V solar cells has rarely been out of the headlines. But, as the recent European Photovoltaics and Solar Energy Conference in Milan showed, there are still plenty of new ideas for further refinement of the technology.

Dwarfed by the existing market for silicon photovoltaics, III-V cells are only just becoming a commercial product for terrestrial applications. However, III-V research themes enjoyed a high profile at the Milan event s technical conference, right from the opening plenary session.

Spectrolab s Richard King kicked things off with a look at the recent breakthroughs leading to 40.7% efficiency in traditional triple-junction concentrator designs, before highlighting the potential offered by adding further junctions to deliver efficiencies heading towards the theoretical limit of 72%.

The problem with adding more junctions based on InGaN or GaAsN is not just the lattice mismatch with the existing epitaxial structure. The current generated by these subcells needs to be matched to the three junctions, while the changeable nature of the terrestrial solar spectrum (particularly in the early and late periods of the day) offers further complications.

King says that a four-junction cell could produce a maximum external quantum efficiency of 59%. Spectrolab s researchers have already made preliminary designs reaching 35%. These, and futuristic five or six-junction converters, are some way from commercial release, however, and King was keen to stress what could be produced in the III-V fab today. In 2008, Spectrolab will produce triple-junction cells with an average lot efficiency of 37%, rising to 40% in 2010.

While US-based Spectrolab and its compatriot Emcore are the current technological leaders in the sector, Matthias Meusel highlighted how the top European cell-maker, Azur Space Solar Power, is also pushing forward with designs for terrestrial applications. Much of Azur s work is focused on custom designs for its commercial partners, but the German company is also working on extra-thin, flexible cells (just 20 µm thick compared with the standard 145 µm).

Using a 12 × 4 inch Aixtron reactor for the wafer production of 35% efficiency cells, Azur is working on additional junctions, admitting that GaInNAs layer quality must be improved for this concept to be feasible. Lattice-mismatched structures are also in the pipeline, with Meusel acknowledging that bypass diodes are an indispensable element of the cell design.

Allen Barnett, University of Delaware, described the newly expanded DARPA program to reach 50%, showing how the combination of a GaInP/GaAs 31.7% tandem cell with a 6.2% GaInAsP and 5% silicon cell could be combined in a 42.9% module. Barnett predicts 50% efficient modules in around three years.

Of course, satellite power is still the workhorse application of the triple-junction cell, and, in a session dedicated to space technologies, Emcore s Paul Sharps discussed the firm s ZTJ cell, so-called because it represents the end of the line for improvements in GaInP/GaAs/Ge triple-junction designs. The ZTJ began space qualification tests in August.

Just three years ago, Sharps believed that 28.5% would be the ultimate limit for the average lot efficiency of these cells under a one-sun illumination. But, by tuning the aluminum composition of the InGaAlP layer of the ZTJ to improve current matching, Sharps and colleagues were able to push to 29.5%. He expects the benchmark efficiency of these cells in mass-production to reach 30% in 2008.

Getting beyond this figure will require a completely new design, however, with Sharps convinced that 30% really is the limit for lattice-matched triple-junctions in space. Emcore s new inverted metamorphic (IMM) cells, which the company also showed off at its industry exhibition booth, look like the key to future space applications. So thin that, upon first glance, one resembles a piece of discarded aluminum foil, the IMM offers the potential for super-light, rollable solar panels that could be unfurled by a satellite or spacecraft after launch.

Another US group looking at potential fourth-junction materials is Alex Freundlich s at the University of Houston. Freundlich described one candidate, GaAsN, as a "pathetic semiconductor". Problems with doping and a poor minority carrier lifetime are the key difficulties with this material, although multi-quantum-well (MQW) devices based on the dilute nitride could be possible. Using the rarely seen technique of chemical beam epitaxy, the Houston research team made a 15-period MQW stack of alternating GaAsN and GaAs layers and, although only a low current was generated, a 0.6 V photovoltage suggests that the approach may have some merit.

Alongside a plethora of research into novel cell materials, such as InGaN, polycrystalline germanium, quantum dots and ZnS nanoparticles, important work is now being directed towards proving the reliability of III-V cells in real-world conditions. David Faiman from the Ben Gurion University of the Negev in Israel appears to have put an array of GaAs cells through the toughest conditions yet. He used the world s largest solar dish, known as PETAL, to focus intense beams of sunlight onto the module over the course of a few months.

Encouragingly, Faiman found no signs of degradation, much to his surprise, despite exposing the unencapsulated array to 1000× sunlight and, inadvertently, to condensation. Here s hoping that the rigorous testing regimes required for long-term cell deployment in space have made the technology sufficiently robust and reliable for applications on Earth.