Backsheets are often an afterthought when it comes to solar panels, since they’re not really made to be seen. Adhered to the back of modules, backsheets provide electrical insulation while also sealing the modules from outside forces like moisture and UV light. With the attention being paid to new module advances like higher voltages and unique cell designs, extra consideration must also be afforded to the backsheet. The same polymer-based backsheet can’t be slapped on every module.
Adapting backsheets for mainstream module updates, like 1,500-V models and bifacial panels, is of greatest importance right now. The backsheet community is split as to whether back-contact modules are a growing trend worth investing R&D into, but manufacturers are keeping an eye on it. Other panel advances are well into the future, but the more successful backsheet brands are staying on top of trends, keeping reliability as the core focus.
The solar industry is moving from 1,000-V to 1,500-V systems, and backsheets need to follow suit.
“The main requirement of the backsheet is electrical insulation. If you want to change from 1,000 to 1,500 volts, then of course you have to consider higher requirements for electrical insulation,” said Marco Jaeger, PV product manager at Dunmore, a backsheet manufacturer.
The International Electrotechnical Commission (IEC) has updated its backsheet requirements for higher voltage systems, now mandating that 1,500-V panels have thicker backsheets. Each backsheet has three layers, and the middle/thickest layer provides most of the insulation properties. By increasing that core layer on 1,500-V systems, these panels are better insulated from the higher voltage. A thicker backsheet also provides another benefit, said Mark Ma, global marketing manager for DuPont Photovoltaic Solutions—more panel stability.
“This also helps a little bit to prevent or reduce the microcracking of the cell,” Ma said. “You want to use a thicker backsheet because the wafer of the cell is getting thinner. By using a thicker backsheet, after some mechanical loading, the microcrack on the cell is less.”
While the bifacial module market—which exposes both sides of a solar cell—has largely turned to glass-glass designs, replacing the backside with a transparent backsheet instead can provide many advantages. DuPont and Dunmore have released backsheets for bifacial modules, both providing a transparent sheet with a white grid for extra reflectivity between the cells.
“It’s tough to compare glass/glass and glass/backsheet modules since they each have advantages and disadvantages,” Jaeger said. “With the bifacial backsheet design, the idea was to have some selective light management. We have higher power output because of the reflective stripes.”
A bit more power generation from those white stripes is not the only advantage of using a backsheet instead of glass on the back of a module. Not only are the modules lighter and easier to transport and install, limiting the use of glass also limits the opportunity for potential induced degradation (PID). Sodium ions in glass drift when exposed to voltage, allowing electrical current to leak out of a module that’s now not performing at its peak power. Bifacial modules with transparent backsheets reduce PID concerns by one-half over glass-glass models.
DuPont, which manufactures the popular polyvinyl fluoride (PVF) backsheet brand Tedlar, makes sure all its backsheet advances are tested to last. The company doesn’t just hop on a trend, as is evidenced by its lack of enthusiasm for back-contact modules.
“We get our inputs from the technical community,” said Kaushik Roy Choudhury, senior scientist and a project leader at DuPont. “We have had internal programs with the circuitry printed on the backsheet to make contact more feasible involving back-contact modules; however, as a market trend, I don’t think we have enough indicators that back-contact cells are going to be a major player in the market.”
Global science-based company DSM disagrees. The manufacturer recently launched a conductive backsheet for back-contact modules that the company claims can increase the panel’s energy output by 3%. Module manufacturer Silfab quickly bought into it, using the conductive backsheet on its new line of back-contact modules.
DSM’s design uses a network or pattern of metal foil attached to a regular backsheet to make a “conductive” polymer sheet. Coupled with the back-contact technology, which moves all the electrical contacts from the front of the cell to the back for maximum front-module energy capture, Silfab claims its conductive-backsheet modules outperform conventional modules by 30%.
“Long-term, our conductive backsheet has broad potential for all kinds of applications,” said Pascal de Sain, VP of DSM Advanced Solar, in a press release. “Right now, we focus on commercial and residential rooftops, as well as building-integrated solutions, where maximum power output and aesthetics are essential.”
Future module advances
Not all module designs may require advanced backsheets. For example, half-cell modules, which use half-cut cells for increased panel performance and durability, function the same as full-cell modules. The only difference is the half-cell layout may require larger backsheets, but their design doesn’t impact backsheet requirements.
DuPont is not just looking at new cell designs. It’s also exploring the basic science behind the individual cells.
“Beyond the p-type PERC cells, which we think will be highly adopted in 2019 and 2020, I think the next wave would be around n-type PERC, which will be the next-level cell technology,” Choudhury said.
More brands may be switching over their production lines to adapt to this trend. P-type cells are inherently more susceptible to degradation because of their chemical makeup, and DuPont has heard of more severe PID issues with p-type PERC modules. The company is trying to update its backsheet to reduce PID chances, but it’s also looking at how to better protect n-type designs, which have lower PID concerns.
“For n-type, we believe it is a little bit more sensitive to moisture,” DuPont’s Ma said. “If the backsheet could have a lower water vapor transmission rate, that could improve the performance of the n-type cell.”
Backsheet companies are staying vigilant to avoid missing any module design trends.
“We get a very relevant and accurate voice of the customer from the market and integrate that into our product development, leading to relevant products,” Choudhury said. “We keep pace with all the new technology, but we are always concerned about reliability.”