State-of-the-art efficiency can be coaxed out of commercial silicon solar cells using a method pioneered in Australia, writes Professor Stuart Wenham of UNSW.

Silicon solar cell technology has developed at an unprecedented rate in recent years, both in terms of improving cell performance and also in lowering costs. The cells have market share well over 90%.

Of particular importance is the Passivated Emitter and Rear Cell (PERC), a recently commercialised cell technology which originated from the University of New South Wales (UNSW) in Australia. The market share for the PERC cell is increasing rapidly and it is expected to dominate internationally within several years.

The technology was invented at UNSW in the 1980s and developed during the 1990s by a research team led by professors Martin Green and myself, Stuart Wenham. The success of the technology in the lab can be clearly seen in the chart below, with the red diamonds showing successful performance world records achieved by this technology, taking silicon cell efficiencies from about 18% up to about 25% during this period.

A schematic of a PERC cell shows the use of dielectric layers covering most of the front and rear surfaces to passivate the silicon and the use of heavy doping beneath the metal contacts to reduce losses associated with extracting current from the solar cell.

Interestingly, while the PERC cells were achieving 25% efficiency in the lab, commercial silicon solar cells were only achieving about 16% efficiency in production. About 20 years later, commercial silicon solar cells are finally successfully adopting the PERC technology, leading to commercial cell efficiencies rapidly closing in on the 25% efficiency target set in the lab 20 years earlier.

Perhaps even more importantly than the whopping 50% performance increase over this period, economies of scale, manufacturing innovation and new technology have collectively led to price reductions of more than 90% to the point where silicon photovoltaics can now generate electricity in most locations world-wide more cheaply than conventional energy sources.

Cost conundrum

One of the main reasons for its slow introduction commercially has been the excessive cost of the high-quality silicon wafers used for making the record-efficiency PERC solar cells in the lab. To succeed commercially, wafer costs had to be reduced to less than 1% of those used in achieving the record 25% efficient lab cells. Initially this cost reduction was achieved only by significantly compromising the quality of the wafers, making the pursuit of the high-efficiency attributes of the PERC technology pointless.

Without good-quality wafers, the well-passivated front and back surfaces and the well-passivated metal/silicon interfaces of the PERC technology provide little improvement in performance.

This is where the new advanced hydrogenation technology developed by the same UNSW team has had a major impact. This technology, based on the use of lasers to control the charge state of hydrogen atoms within the silicon wafer, has been heralded as a “breakthrough for silicon photovoltaics” by the UK Institution of Engineering and Technology when it awarded me the 2013 A.F. Harvey Engineering Prize.

Atomic-scale fix

It has been known for decades that hydrogen atoms within silicon can be positively, neutrally or negatively charged and that each defect or imperfection within the silicon can potentially be fixed or healed by hydrogen atoms provided they are in the correct charge state.

The breakthrough, however, has only come in recent years when the UNSW team developed new technology for controlling the charge state of the hydrogen atoms, making it possible for the hydrogen to move up to 100,000 times more easily within the silicon to find defects, while also being more reactive to more easily passivate or fix such defects.

The power of this technology to improve the quality of low-cost silicon wafers is still being explored and exploited, with wafers already being improved in quality by the hydrogen by as much as 100 times.

A reject screen-printed solar cell receives advanced hydrogenation treatments to control the charge state of the hydrogen atoms in different ways to improve the cell efficiency from 15.4% to 18.5%. The PL images show the progressive improvement in silicon quality with blue/black representing very bad quality and yellow/white being very good quality.

This potential is demonstrated above, where a low-quality upgraded metallurgical-grade (UMG) silicon wafer receives successive advanced hydrogenation processes, each controlling the hydrogen atoms in different ways to passivate different types of imperfections, thereby transforming the silicon wafer into one of more than 100 times higher quality.

The photoluminescence (PL) image for the wafer after each process is shown, with the blue/black colours representing very poor quality silicon while the yellow represents very good quality silicon that is able to make high efficiency solar cells.

By applying the same advanced hydrogenation treatments to finished reject solar cells of very low efficiency, significant improvements in the silicon wafer quality can still be achieved, with significant corresponding improvements in cell efficiency achieved.

This laser-based prototype industrial tool from DR Laser in China uses the advanced hydrogenation technology for passivating defects and contaminants in low cost silicon.

Cheap upgrades

Perhaps more importantly, for the first time low-cost commercial wafers are able to receive this advanced hydrogenation treatment to transform their quality into approaching that of the wafers used by UNSW 20 years earlier when achieving record 25% efficient PERC cells in the lab, but at less than 1% of the cost!

This allows commercial wafers to have a quality compatible with the high-efficiency attributes of the PERC technology with efficiencies of 22-23% being recently achieved in production by industry partners of UNSW.

The university now has relations with more than 10 of the world’s largest solar cell manufacturers, having signed licences and/or agreements for the large-scale manufacturing of this advanced hydrogenation technology, mostly in conjunction with their PERC cells. These include companies from Germany, Singapore, China, Taiwan, Korea and India, with a further five companies in advanced stages of negotiating equivalent contracts.

Holding steady

An important contribution made by the hydrogenation is not only to improve the material quality and solar cell efficiencies, but also to eliminate many of the instabilities in commercial solar cells that lead to their degradation during operation in the field.

In the same way as hydrogen is able to repair or fix imperfections in the low-cost silicon material during manufacturing, it is also able to repair the damage that would normally occur during operation in the field, therefore minimising the corresponding degradation previously experienced by most commercial solar cells.

The university has also worked with five equipment manufacturers in Germany, Switzerland, Taiwan and China to design and develop new industrial tools for the implementation of the first generation of the UNSW advanced hydrogenation technology, with these tools now commercially available for large-scale manufacturing. An example is the tool shown here, manufactured by DR Laser in China, where a laser is used to control the charge state of the hydrogen atoms, facilitating greatly enhanced hydrogen passivation and improvement in the silicon quality.

Development of the technology has been generously funded by the Australian Renewable Energy Agency, leading to the corresponding IP portfolio being Australian-owned and independently valued in the billions.

Stuart Wenham is Scientia Professor at ARC Photovoltaics Centre of Excellence, School of Photovoltaic and Renewable Energy Engineering at the University of NSW.