Evolution of c-Si PV Cell Technologies: Page 3 of 4
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Advances in c-Si PV
Cell architecture advancements in the 1980s and 1990s propelled c-Si PV technologies to conversion efficiencies that researchers had previously thought unimaginable. As early as the late 1970s, researchers were experimenting with a technique known as surface passivation, whereby they applied an oxide layer to the surface of the solar cell to reduce carrier recombination and improve open-circuit voltage levels. Green’s research group at the University of New South Wales (UNSW) achieved such success with surface passivation techniques that in 1985 it produced the first silicon cell to exceed 20% efficiency. This breakthrough was so extraordinary that Green has likened the accomplishment to breaking the 4-minute-mile mark in running.
While the theoretical maximum conversion efficiency for single-junction silicon solar cells is around 29%, industry experts such as Richard Swanson, SunPower’s retired founder, have concluded that the practical conversion efficiency limit in commercial mass production is likely in the 24%–25% range. (See the SunPower white paper in Resources.) To put the cell technology advances of the 1980s and ’90s into context, consider that researchers produced the first 24% efficient silicon cell in 1994 and extended this record to 24.5% by 1998. Fast-forward to January 2018, when Germany’s Institut für Solarenergieforschung announced that it had achieved a new international conversion efficiency record for a single-junction silicon cell of 26.1%. (See the PVTech article in Resources.)
The fact that it took two decades to improve single-junction cell efficiencies in the laboratory by 1.6% illustrates just how difficult it is to achieve additional incremental improvements in silicon cell performance. It also illustrates just how advanced silicon cell technologies were by the 1980s and ’90s. In fact, many of the high-efficiency cell technologies in mass production today date to this period of research and innovation. The most impactful advancements during this era were improvements in the manufacturing process, which drove PV module costs down from around $70 per watt in 1978, drawing a well-documented comparison to Moore’s Law.
Interdigitated back contact cells. Researchers at Stanford University developed the first high-effciency rear-contact silicon cell in the 1980s. Originally designed for concentrating PV applications, the Stanford cell was unusual in that it placed both the positive and negative contacts on the rear surface of the cell. On the face of it, this design is similar to the original Bell Solar Battery. However, the two cells operate very differently. By using extremely high-quality n-type silicon, Stanford’s rear-contact cell allowed light-generated carriers located near the top surface of the cell to travel to the rear contacts on the bottom surface of the cell.
To maximize the efficiency of a solar cell, the carriers must travel across the p-n junction as quickly and efficiently as possible. One simple but expensive way to improve carrier lifetime is to use ultra-purified silicon wafers, as this reduces the likelihood that a carrier will interact with impurities in the crystal structure. This is especially true for rear-contact solar cells, such as the Stanford cell, which need to transport carriers a further distance.
In 1988, the rear-contact cell achieved a conversion efficiency of 22%. Today, the industry refers to this silicon cell design as the interdigitated back contact (IBC) architecture due to the repeating pattern of positive and negative contacts on the backside of the cell. Incorporated in 1985 and publicly traded since 2005, SunPower is notable for having commercialized and improved upon this high-efficiency n-type silicon cell design, shown here in Figure 4. Trina Solar and Yingli Solar have both announced IBC cell efficiency records in recent years.
Passivated emitter and rear cell. For a period of time, it appeared as though rear-contact silicon cells had an insurmountable performance advantage over front-contact cell architectures. The latter, after all, must overcome the fact that metal contacts shade about 5% of the front cell surface. In the early 1990s, however, the research group at UNSW discovered a way to improve silicon cell efficiency. While experimenting with lasers to scribe and passivate the front cell surface, the Australian team discovered that cell performance improved when they applied higher concentrations of dopants precisely underneath the areas where they would later apply metal contacts.
The process of selectively doping cell areas effectively created a new solar cell category: selective emitter solar cells. The most common cell architecture in this category is the passivated emitter and rear cell (PERC) technology, which UNSW successfully demonstrated in 1989. The team started by combining, refining and building upon earlier technology advancements that improved the cell’s ability to capture light, such as front-surface texturing and using an AR coating. Its novel advancement was to use lasers to deposit much higher concentrations of phosphorous and boron selectively and accurately under the metal contact areas. The team’s 1989 PERC solar cell, shown in Figure 5, demonstrated a conversion efficiency of 22.8%.
The selective doping process improves performance by generating an additional electromagnetic pull in the crystalline structure that encourages carriers to travel to the exact locations where the circuit can use them. Today, the industry considers PERC architecture one of the most cost-effective ways to produce high-efficiency solar cells. Manufacturers with PERC modules in commercial mass production include Aleo, LONGi Solar, SolarWorld and Trina Solar.
Passivated emitter rear locally diffused cell. Not long after documenting the PERC architecture, the UNSW team announced that it had achieved a new world record for a single-junction silicon cell efficiency. The team had improved on its PERC design by laser-embedding tiny, highly doped dots on the rear surface of the cell rather than using a continuous metallic contact. In so doing, it was able to produce a 24% efficient solar cell in 1995 and extend the efficiency record for a single-junction c-Si solar cell to 24.5% by 1998— a record that stood until 2014.
The UNSW team dubbed its new architecture passivated emitter rear locally diffused (PERL), since it uses local diffusion at the rear-point contact to help collect light-induced carriers and reduce contact resistance. While the PERL architecture is extremely efficient, this cell technology and other specialized selective emitter techniques have yet to achieve commercialization due to the extremely high costs associated with the manufacturing process. However, the UNSW team made its mark on history, as PERC technology has become increasingly popular in both monocrystalline and multi-crystalline PV cell production.