Secondly, high-efficiency and high-quality battery technologies are becoming increasingly diversified. Domestic and international photovoltaic manufacturers, such as JA Solar, Trina Solar, Jinko Solar, Suntech Power, LONGi Solar, Aikon, Jineng, Zhonglai, Hanwha, Sunpower, Kyocera, and Sharp, are researching high-efficiency battery technologies, such as Passivated Emitter and Back Surface (PERC), Emitter Passivation and Full Back Diffusion (PERT), Metal Through-Through-Wrap (MWT), Heterojunction with Intrinsic Amorphous Layer (HIT), and Interdigitated Back Contact (IBC), are gradually forming different technological schools of thought.
The diversification of high-efficiency battery technologies directly impacts the supply structure of upstream raw materials. Conductive pastes, as crucial materials for improving the photoelectric conversion efficiency of solar cells and the power of photovoltaic modules, play a vital role in enhancing solar cell efficiency. In recent years, metal conductive paste suppliers, represented by DuPont, have closely followed the pace of battery manufacturers, developing conductive pastes that match their high-efficiency battery technologies.
PERC cells achieve commercial breakthrough; slurry is the key to improving efficiency.
While different high-efficiency battery technologies exist, photovoltaic manufacturers looking to expand production at this stage all need to consider one question: should they upgrade existing production lines or innovate? Among various high-efficiency battery technologies, PERC cells have significant performance/cost advantages. They are well-compatible with the production volumes of existing cells and modules, while achieving an efficiency improvement of about 1%, making them the first high-efficiency battery technology to achieve a large-scale commercial breakthrough.
Currently, PERC cell production exceeds 13.4GW. According to Asia Chemical Consulting, global PERC cell production will reach 25GW in 2017. In terms of efficiency, the production line efficiency of polycrystalline PERC cells produced by leading manufacturers has reached 19.5%, while the average efficiency of monocrystalline PERC production lines is at the 21% level, with the highest efficiency reaching 21.6%.
"Monocrystalline PERC products have both cost and performance advantages, making them the most competitive products currently available, and there is still a lot of room for efficiency improvement in the future," said Wang Wenjing, a researcher and director of the Solar Cell Technology Research Department at the Institute of Electrical Engineering, Chinese Academy of Sciences. She added that the efficiency improvement of PERC cells is closely related to the conductive paste, and the appropriate paste selection can maximize the advantages of this new cell structure and maximize its efficiency.
With the rapid development of PERC batteries, the market demand for dedicated metallic conductive pastes for PERC batteries is becoming increasingly strong. To meet the specific requirements of PERC batteries for pastes, paste suppliers have developed a series of dedicated pastes for PERC batteries, such as PERC front-side low-temperature silver paste, back-side aluminum paste, and PERC+ back-side burn-through silver paste.
"Key technologies for further improving the efficiency of PERC cells include: first, the optimization of PERC-related equipment and processes, using a front emitter structure with lower recombination (such as diffusion methods with lower surface concentration); second, factors related to silicon wafer quality, such as improving minority carrier lifetime; and third, factors related to pastes, such as matching the front side with excellent contact capabilities and ultra-fine line printing, and using aluminum-doped paste for back-side conductivity," said Cao Qian, Technical Manager at DuPont. He added that DuPont™ Solamet® has developed a complete set of integrated metallization solutions specifically for PERC, including the new generation PERC front silver paste PV20A, the PV56x series back silver paste, and the PV36x series back aluminum paste.
Cao Qian further explained that the front-side paste can further help improve the efficiency of PERC. If the front-side gate line width is narrowed by 10um, it can bring about an efficiency gain of about 0.1%. In addition to the gate line width, further reducing the contact resistance between the paste and the silicon wafer can also bring an efficiency gain of 0.1%. If the paste can achieve better contact at a lower surface concentration, the front-side emitter has room to be optimized towards low doping and high sheet resistance, thereby achieving further efficiency gains.
In short, key optimization directions for the front side are: fine-line printing, Ag-Si contact, and controlled emitter recombination. These are also the optimization directions of PV20A compared to the previous generation of front side silver paste. DuPont's new generation PERC front side silver paste PV20A can form good contact at lower sintering temperatures, thereby reducing the formation of voids on the back side and damage to the passivation layer. Compared with conventional front side silver, the efficiency improvement of the new generation PERC front side silver can exceed 0.1%.
In June 2017, Zhejiang Aiko, a subsidiary of Guangdong Aikon, announced a collaboration with DuPont, achieving a conversion efficiency of over 21.5% for its monocrystalline PERC cells, using DuPont™ Solamet® PV20A positive silver paste.
In addition to the customized improvements to the front silver paste, PERC cells also have higher requirements for the back conductive aluminum paste. Because the sintering temperature of PERC cells is lower than that of conventional cells, at the lower sintering temperature, conventional back silver paste exhibits increased contact resistance, resulting in a decrease in photoelectric conversion efficiency.
“To help battery companies solve their problems, DuPont™ Solamet® developed the PV36x series of back aluminum pastes, which, in addition to their excellent electrical properties, also exhibit excellent physical properties, with a smooth sintered surface, no powder shedding, low warpage of less than 200um, good hot water test results, and long-term reliable aging performance,” said Cao Qian.
Conductive paste is a raw material that, besides silicon wafers, affects both cost and efficiency, and plays an important role in the research and development of high-efficiency battery technology. Therefore, the choice of paste has a profound impact on battery manufacturing companies. Choosing a reliable paste supplier is beneficial to reducing the risk of failure and improving the return on investment.
