Good rear side passivation can effectively increase PERC cell internal reflection of light and increase c-Si solar cell response under long wave, and also can effectively reduce back surface recombination and increase life of minority carriers. PERC cell to rear side Al paste basic requirements are as below: 1) Good fill effect at film opening position; 2) Moderate damage to dielectric film; 3) Suitable continuous, uniformity and thickness LBSF; 4) Good reliability (adhesive property, EL, anti aging performance etc.)
Except essential features of traditional c-Si cell back Ag, which are good printability and relatively low Ag content, PERC cell rear side Ag paste also needs to have the following performances: 1) Low activity, non-contacting rear side Ag paste can reduce damage to rear side passivation layer, so that open circuit voltage of cell can be increased; 2) Relatively wide process window, fits low temperature sintering process; 3) Excellent adhesion and aging adhesion.
To PERC cell front side Ag paste, in order to cooperate with PERC technology to get higher transfer efficiency, except increase contact performance, fine wire printing to reduce grid shading area etc. conversional performances, it also needs to overlay double printing, distribution printing and multi bus bar technology etc. What is more, in order to assist PERC cell reducing light induced degradation, it also needs PERC front silver has wind sintering process window to fits low temperature sintering.
It is reported that, with assist of Heraeus PV high performance Ag paste, JinkoSolar and Longi Solar refresh world record of PERC cell efficiency for many times.
Bifacial PERC cell comes up with new challenges to Al paste
PERC is bifacial structure that is the simplest and has most cost efficient. PERC bifacial cell common manufacturing technology is basing on PERC mono-facial cell, changing printing technology of PERC cell, rear side changes from all Al layer to part Al layer, in hence, incident light of rear side can enter cell from area that is not covered by Al layer, so that bifacial photoelectric conversion function can be realized.
PERC Cell and Bifacial PERC Cell Structure
It can be seen that, bifacial PERC cell comes up with some special process requirements to rear side Al paste and metallization: 1) Rear side printing precision requirement is slightly higher than mono-facial PERC cell; 2) Rear side needs screen printing Al grid, and comes up with higher requirement to Al paste.
Bifacial PERC cell rear side uses Al grid design, which can effectively reduce local contact cavity, and it is helpful to improve part Al-BSF uniformity, so that cell open circuit voltage can be increased. Key technologies of bifacial PERC Al paste are as below: 1) Al grid after sintering needs to have a certain of height weight ratio, which can increase illumination area, benefit for collecting more photo-generated currents, increase PERC rear side transfer efficiency; 2) Has good contact place local filling effect and suitable thickness Al-BSF, to reduce volume resistivity.
Bifacial PERC can realize bifacial power generation in case of almost not increase costs, which remarkably reduce the cost per kWh of PV system, and greatly strengthen the competitiveness of PERC technology and future development potential. Trina Solar, Longi Solar, JA Solar and Aiko Solar etc. manufacturers all launch bifacial PERC products. It is estimated that bifacial PERC will become important route of follow-up development of PERC cell.
PERC technology is becoming new generation conventional technology of solar cell
Matured manufacturing technique and quite low capital input make PERC cell easily to expand capacity, adding with downstream market requirement to high power module, PV industry is actively expanding PERC cell capacity. In 2017, global solar cell capacity was about 125GW, among, PERC cell capacity reached 35GW. In 2018, new built or upgraded P type c-Si cell production lines basically all use PERC technology. It can be seen that PERC technology is becoming new generation conversional technology of solar cell.
Currently, mono-Si PERC cell mass production efficiency can reach 21.5%, multi-Si PERC cell that is overlapped black silicon mass production efficiency can reach 20.5%, and each major manufacturer is continuing devoting to increase PERC cell mass production efficiency. Through overlapped with MBB, selective emitter and TOPCon etc. technologies, and continue optimizing of paste and metallization etc. technical methods that can be mass production, mass production efficiency of PERC mono-Si and multi-Si will exceed 22% and 21% recently, and competitiveness will be further increased, and will bring huge market opportunities to front side Ag paste, rear side Al paste and rear side Ag paste that suit for PERC technology.