Making Energy While The Sun Shines

Portable power in the form of solar cells represents what could be described as the utopian solution to consumer consumption; power wherever you need it, whenever you need it (as long as there’s some light available).



In reality, the technical requirements for portable solar cells are often a mismatch for their actual characteristics; highly efficient, flexible, light and cheap. Today, the solutions targeting this application area fall short on probably all four. However, there is one company that believes it has developed a technology that puts a tick in all these boxes.



Alta Devices is using a technique that is seeing application in other areas of power semiconductors; growing a layer of Gallium Arsenide on top of a Silicon wafer. Forming GaAs in this was makes the substrate commercially viable without sacrificing its benefits in power electronics.



In the case of Alta Devices, the layer first grown and then removed using a flexible, plastic film. The result is a flexible substrate that can be further processed to create a photovoltaic device. By instantiating multiple cells on a single film, Alta Devices is able to develop solutions that can generate enough energy to power not only portable equipment but other forms of electrical devices, creating solar cells that it believes represent a viable portable power solution.



The Right Chemistry



The process overcomes one of the main problems with photovoltaic cells manufactured from GaAs; the cost of the substrate. Rather than each PV requiring its own GaAs wafer, Alta’s solution means the base wafer never actually gets any thinner; the GaAs layer — about 1um thick — is first grown on the wafer and then removed using the plastic film, leaving the wafer untouched and unchanged, ready for another growth phase.



Alta Devices calls this process ‘photo recycling’ and, not surprisingly, the company’s CEO, Chris Norris, cites it as one of its key differentiators.



The growth process actually uses two gasses; trimethylaluminium and arsenic, deposited using MOCVD (metal organic chemical vapour deposition), which promotes the growth of a single, perfect crystal to a thickness of about 1um, the plastic film is around 50um.



Norris says that photo recycling actually contributes around 2% of the cell’s efficiency, which itself far exceeds the industry average. Other competing flexible PV cells deliver around 5 to 10% efficiency, according to Norris, Alta Devices’ technology produces PVs with 29% efficiency today, using a single-junction (diode) device. The technology’s roadmap will extend that to 33% using dual-junction devices and ultimately as high as 37% using triple-junction devices.



As well as lowering the wafer coat, another large contributing factor to the technology’s commercial viability is in maximising the cell’s efficiency and Norris explained that part of the way this is achieved is by having a wider filter than most of its competitors, allowing it to make use of light at wavelengths of between 400 and 700nm.



Pilot(less) Programme



UAVs, or unmanned air vehicles, are growing in popularity within several sectors, not least the military. Typically these are small airborne devices that use a battery to drive an electric motor turning a propeller. Just as electronic components have reduced in size and power, so too have the devices they enable, such as surveillance sensors including cameras. Fitting cameras to UAVs provides a clear tactical advantage to military forces but it is also finding applications in industry, for site inspection or area monitoring. However, their main drawback is their limited flight time; the bigger the battery the more heavy the UAV, effectively self-limiting the maximum possible flight time.



Alta Devices’ technology has already been adopted by UAV manufacturers to extend the vehicles’ flight time, by applying the flexible PVs to the UAV’s wings they remain non-intrusive and contribute little to the overall weight, but deliver a considerable energy boost to the UAV’s battery and subsequent flight time.



Norris believes the potential market is so large that UAV manufacturers alone could keep a solar cell company in business. However, with their foot already in the door, Norris says Alta Devices is using sales in to this sector to fund the development of its own pilot manufacturing line.



According to Norris, the company’s MOVCD equipment forms an important part of its unique product offering; so much so that the equipment was developed in four separate parts using four different manufacturers, operating in isolation.



While the MOVCD process is critical to the PV efficiency, another crucial feature of the production process developed by Alta is its capacity; according to Norris it can process 850 wafers/hour, compared to just 15/hr using ‘off the shelf’ MOVCD equipment (as two of its competitors do). Also, the temperature coefficient of the resulting film is around one fifth of Silicon, meaning it performs better at high temperatures as well as low light. All together the company has filed over 77 patents covering the three key innovations.



The demand for portable power is growing; every new generation of smart phone offers more processing power but, as a result, consumes more energy. This is of course just one application, there will be many more.