The rise of next-generation n-type solar PV cells

In 2022, the Renewable Energy Test Center (RETC) is closely monitoring a technology trend gaining market traction and acceptance: the rise of next-generation n-type PV cells with passivating contacts. These next-generation n-type PV cells are essential to the solar industry’s continued ability to drive down costs while improving performance. Here, we explore the promise of new n-type PV cell designs — and the potential challenges associated with scaling this promising technology.

Rise of TOPCon

Many industry analysts and material scientists believe emerging n-type PV cell designs are the next logical progression on the PV technology roadmap. In 2013, researchers at Germany’s Fraunhofer Institute for Solar Energy Systems presented a method of producing high-efficiency n-type silicon solar cells with a novel tunnel oxide passivated contact (TOPCon) structure. This novel cell design achieved high marks for open-circuit voltage (Voc), fill factor and efficiency thanks to excellent surface passivation and effective carrier transport.

Comparison of p-type PERC and n-type TOPCon silicon cell designs.

Less than a decade later, TOPCon is the buzziest word in solar. The largest module manufacturers in the world are beginning volume production of PV modules with TOPCon cells. While LONGi Solar is betting big on p-type TOPCon, many other leading module companies — such as JinkoSolar, Jollywood Solar Technology, JA Solar and Trina Solar — are making substantial investments in modules with n-type TOPCon cell designs.

This collective pivot in the market is primarily due to flattening efficiency curves for the p-type passivated emitter and rear-contact cell (PERC) modules. Although these have dominated the market in recent years, manufacturers are starting to reach the physical limits of p-type mono PERC cell designs. Transitioning to n-type TOPCon cells will allow module companies to boost cell efficiencies further in the laboratory and mass production.

“Everybody wants the highest possible module nameplate rating,” explained Kenneth Sauer, principal engineer at VDE Americas, a provider of technical due diligence and engineering services. “Via higher Voc values, you can achieve higher efficiencies and power ratings. In and of itself, that is likely going to move manufacturers to n-type TOPCon cell designs, as soon as they can get there.”

Benefits of n-type cells

Module characterization testing in progress at RETC’s accredited laboratory.

Solar manufacturers have long recognized the potential efficiency benefits of n-type PV cells. For example, Sanyo began developing n-type heterojunction technology (HJT) PV cells in the 1980s. In addition, SunPower has built its interdigitated back contact (IBC) PV cells upon a base of high-purity n-type silicon.

Due to the manufacturing complexities involved, high-efficiency PV modules based on n-type HJT and IBC cell designs are relatively expensive to produce and remain a niche part of the market. By comparison, n-type TOPCon cell manufacturing is similar to the PERC process. As a result, manufacturers can produce these next-generation high-efficiency TOPCon modules on upgraded PERC production lines.

Though today’s n-type TOPCon modules cost slightly more to produce on a per-watt basis than p-type mono PERC modules, the efficiency gains result in a lower levelized cost of energy (LCOE) in large-scale field deployments. Best of all, leading experts expect n-type TOPCon to benefit from an accelerated learning curve.

A primary material advantage of n-type TOPCon cells relative to p-type mono PERC cells is a lower degradation rate due to a decreased susceptibility to both light-induced degradation (LID) and light- and elevated temperature-induced degradation (LeTID). Additional advantages may include a higher bifaciality factor and improved performance under both low-light and high-temperature conditions.