RSL Energy promises to deliver on multi-band ZnTe-based photovoltaics

There aren t many plausible contenders for large-scale renewable energy sources. Solar power is one of them, but to make it practical in regions other than those with the sunniest climates it needs to be efficient and, perhaps more important, cheap. Silicon solar cells, the most readily produced photovoltaic devices by far, satisfy the latter of these criteria. But their poor power conversion from sunlight to electricity means it is unlikely that they will instigate any widespread adoption to solar power – at the time of writing, the silicon record lies at 24.7%.

Unfortunately, even if silicon devices were perfect they wouldn t get much past 30%. The reason is that no single bandgap can absorb all of the Sun s useful radiation, which includes potentially exploitable energy across the infrared, visible and ultraviolet.

Compound semiconductors such as GaAs have overcome this limitation by employing different alloys stacked in series, one on top of another, to cater for particular portions of the spectrum. But while these multi-junction cells have now attained efficiencies of up to 39% using three junctions, the extremely high cost (up to $40/cm2) of their complex structure has precluded their use in anything except space satellites and subsidized terrestrial projects.

Now, however, RSL Energy in Arizona, US – a joint venture between RoseStreet Labs and Sumitomo Chemical – is pioneering a different breed of solar cell. Based on oxygen-implanted ZnMnTe or ZnTe, these cells can achieve the same efficiency as a multi-junction cell but at a significantly lower cost, claim their proponents. Rather than using stacked semiconductor junctions, these new "multi-band" cells generate three individual bandgaps by having abnormally large quantities of mismatched atoms implanted into a single material. This, according to RSL Energy CTO Wladek Walukiewicz, avoids the need for expensive lattice matching and convoluted fabrication techniques. "Multi-junction cells can be a nightmare. Multi-band cells are much simpler, because they have just one junction."

RSL Energy s multi-band cells build upon a concept first proposed in the 1980s. Normally when semiconductors are alloyed, their properties are changed continuously and in a controlled manner. But some alloys are particularly averse to being formed. This is the case if one wants to replace some of the tellurium in ZnTe with oxygen: tellurium atoms are much larger and less electronegative than oxygen atoms, so ardently resist all attempts to be replaced.

In the 1990s, however, scientists discovered that they could force the substitution by using non-equilibrium processes. Walukiewicz was one of those scientists. He discovered that by accelerating oxygen atoms to high energies and firing them at a p-type ZnTe substrate, large amounts could be crammed into the dogged structure. The trouble with this heavy-handed approach, however, is that the crystallinity of the material is damaged during the bombardment. The crystal can be repaired by annealing, but in normal circumstances this process leaves the oxygen ample time to force its way out again.

"We have to use a process called pulsed-laser melting," explains Walukiewicz. "Oxygen does everything it can to avoid being a substitute, so you have to do the annealing fast and at a high temperature. One single pulse melts the layer, which then grows again very rapidly."

Walukiewicz replaces up to 5% of the tellurium with oxygen, of which about 3% is "active", meaning that it contributes to the modification of the bandgaps. In small amounts, the oxygen atoms only form localized energy levels. But as more is added, these energy levels eventually overlap to form a continuous narrow band, shifting up ZnTe s original conduction band slightly in the process. The result is a "split" band structure and three available absorption energies for incident photons: 0.8, 1.8 and 2.6 eV (see figure "Conventional III-V cells").

"These [energies] are close to what you need for optimal coverage of the solar spectrum," says Walukiewicz. "The oxygen band provides a stepping-stone for near-IR photons to excite electrons from the valence band to the conduction band." This, Walukiewicz adds, enables a theoretical efficiency of 56%, although in practice the devices are expected to demonstrate practical efficiencies closer to 48%. "A good rule of thumb is that if you get 80% of the theoretical maximum, you re doing well."

The potential for RSL Energy s multi-band solar cells will probably lie where multi-junctions cost demands have proved broadly untenable – for domestic use on Earth. One possibility is that a multi-band layer could be added ad hoc to existing silicon solar cells to incrementally improve their efficiency, which could help to extend the lifetime of a rapidly maturing silicon industry.

However, on their own the high power conversion of multi-band photovoltaics would allow them, for example, to be used in residential applications where space comes at a premium. While companies such as Emcore and Spectrolab are targeting utility-scale power plants with their multi-junction technology, they acknowledge that residential or roof-top use is unlikely.

RSL Energy s approach would also circumvent the need for "concentrator" optics that multi-junction cells require to generate power at high efficiencies. Currently, numerous concentrators are required to focus the Sun s rays onto a relatively small area of III-V cells, but Walukiewicz claims that in the long run his multi-band cells could be made cheaply enough to make concentrators completely unnecessary.

All of which could be good news for tackling the world s mounting carbon dioxide emissions. The US, one of the greatest consumers of electricity per capita, still obtains more than four-fifths of it from burning fossil fuels, such as oil. Solar cells account for less than a thousandth (US Department of Energy statistics). But RSL Energy isn t solely aiming for the terrestrial market – it is now applying the ZnTe approach to phosphorus-implanted GaAsN for satellite power supplies, which have to withstand full-strength solar radiation. "II-VIs are not radiation-hard materials like III-Vs," says Walukiewicz. "GaAsN could work in the same way and can even be grown by MBE very easily."