PERC technology: advantages and comparison with standard cells
PERC technology has for some years become the production standard of the main manufacturers of photovoltaic modules.
But what exactly is this technology and what are its benefits?
What does PERC mean?
Literally, it stands for "Passivated Emitter and Rear Cell, or passivated emitter and rear cell.
What is that?
PERC cell technology defines a solar cell architecture that differs from the standard cell architecture, which has been in use for three decades and is usually present in all photovoltaic manuals.
To date, the vast majority of crystalline solar cells produced follow the structure presented below.
In order, from the top to the back:
Silkscreened silver paste to form the contacts
Anti-reflective coating
Phosphorus-diffused, boron-doped silicon wafers forming the PN junction
Aluminum rear surface field (Al-BSF)
Silk-screened aluminum paste
The goal is to get the most out of electrons from solar cells, the PERC architecture essentially allows for improved light capture near the back surface and to optimize electron capture.
There is PERC and PERC
Under the acronym PERC, we actually also find the solar cell technologies called PERL, PERT, PERF. Although they were not initially considered in the PERC nomenclature, they are now generally considered to be part of the same family
Because it is an interesting technology
The main advantage of the PERC cell structure is that it allows manufacturers to achieve higher efficiencies than standard solar cells, which are reaching their physical limits. In the current state of technology, an absolute gain of up to 1% in efficiency can be achieved. Although there are multiple stages in the manufacturing process, the efficiency gains allow for cost savings, even at the system level. We must always keep in mind that the holy grail is improving efficiency while reducing costs at the same time. And improving the efficiency of solar cells contributes to cost reduction.
Hence this cell architecture is regarded as one of the best potential for producing highly efficient solar panels at competitive prices.
Is PERC technology new?
The PERC architecture is not really new at all. The first evocation of the technology dates back to the University of New South Wales in Australia in 1983 and the first paper [1] was published in 1989. As this cell concept offered the best potential for achieving high efficiency, UNSW it used to achieve its multiple world efficiency records which resulted in nearly 25% [2]. The other two competing technologies were the Back Contact technology, popularized by Sunpower, and the HIT technology marketed by Panasonic.
Why now?
Interestingly, the standard solar cell architecture has been in use since the mid-1980s. Since then, the technology has undergone incremental improvements, with better pastes to form face contacts, thinner contact fingers, optimized anti-reflective coating ... It took the industry nearly 30 years to achieve near-research-level efficiencies.
The chart below summarizes the solar industry's historic quest to improve its technology.
As stated earlier, there is always a gap between the performance achieved at the research level and what is achieved in mass production at the industrial level. So if PERC cell technology is emerging now, it is mainly for economic reasons. Indeed, the industry must always compromise to realize economically and technically feasible concepts. During 30 years, the continuous incremental improvements made to standard cell technology have been economically and technically feasible. Now that the standard concept reaches its limits and that technical know-how is available along the value chain to introduce PERC technology, it can constitute a viable new platform for producing high-power, high-efficiency solar panels.
As anticipated by ITRPV , a body that brings together a set of manufacturers in the different phases of the value chain and that works on technological trends, PERC technology will progressively take the largest market share. As this is the sense of history in the photovoltaic sector to improve more and more, sooner or later most of the installed panels will be equipped with this technology.
Does PERC technology inherently exhibit better performance in low light conditions?
In parallel with the development of PERC technology, we have seen topics related to improving performance in low light conditions appear on the datasheets. It is therefore legitimate to ask whether these two facts are related. While it is true that you will find PERC-based modules with improved low light performance, it has nothing inherently to do with PERC cell technology.
The technological challenge
As with any new technology, the challenge behind PERC technology is being able to augment the technology by controlling the process.
Among the challenges related to PERC technology, two are most likely to impact the owner of a panel equipped with this technology.
The first relates to light-induced degradation. LID is this effect that causes a module to lose a percentage of its power after the first exposure to light. It explains why manufacturers with linear warranties never guarantee 100% potency after the first year. Due to the higher doping levels commonly applied in PERC cells, the negative effect due to LID is increased with PERC technology compared to standard cells with Al-BSF.
There is also an argument about induced potential degradation. There have been numerous papers and articles that raise this issue, and in particular for the polycrystalline PERC. It is not trivial as this type of defect can completely damage the performance of a power plant. The best recommendation we can make regarding this issue is to make sure that the modules you supply are granted the certificate according to IEC TS 62804 for PID resistance and that there is confidence in the consistency that the manufacturer applies to the selection of materials and processes to ensure their production is PID free.
PHOTOVOLTAIC MODULES WITH PERC TECHNOLOGY ON UTOPIA
Sources:
[1]: AWBlakers, A.Wang, AMMilne, J.Zhao, MAGreen, 22.8% Efficient Silicon Solar Cell, Appl. Phys. Lett. 55 (1989) 1363-1365.
[2]: Zhao J; Wang A; Keevers MJ; Green MA, 2000, High efficiency PERT cells on SEH p-type Si substrates and PERT cells on SHE n-type Si substrates
[3]: MA Green, The Passivated Emitter and Rear Cell (PERC): From conception to mass production, Solar Energy Materials & Solar Cells 143 (2015) 190ā197
[4]: MA Green, Forty Years of Photovoltaic Research at UNSW, Journal and Proceedings of the Royal Society of New South Wales, vol. 148, nos. 455 & 456, pp. 2-14