PERC Technology

PERC stands for Passivated Emitter Rear Cell or Passivated Emitter Rear Contact or Passivated Emitter and Rear Cell. The industry is moving towards PERC and most industry experts are these contemplating a complete move toward PERC from the current mono and polycrystalline technologies. For those who are not aware of this technology, I have tried to present an overview and, advantages and challenges of using PERC.

PERC is not at all a new concept and was initially developed by Prof. Martin Green around 1982 at the University of South Wales (UNSW).

In PERC technology, an additional dielectric layer is added on the backside of the conventional solar cell process flow which acts as passivation and reflection layer. It reduces carrier recombination significantly and also reflects light which otherwise would get transmitted from the backside. Al_BSF contacts are made by opening windows in the dielectric layer using laser or chemical technology.

Advantages

The PERC technology can be applied to either polycrystalline or monocrystalline Si. The efficiencies increases around 21-23% and even PERC cells with >23% efficiencies are recorded. The conventional Screenprint Al BSF solar cells will have efficiencies in the range of 18-19%. The increased efficiency is due to higher short circuit current (Isc) and thus higher open-circuit voltage (Voc). Thus the significant gain in efficiency is the single largest advantage of PREC technology and it can be achieved by not modifying the conventional screen print manufacturing set-up, significantly. The photon absorption is higher in the longer wavelength region for PERC cells which enables them to harvest more energy during low light conditions, especially in the morning and afternoons. 

 Challenges

The biggest challenge with PERC technology is the LeTID effect. LeTID stands for Light and elevated Temperature Induced Degradation. It can be seen in the field and is accelerated by high irradiance at the higher temperatures (>50℃). The effect from LeTID, which is known since 2012, is more pronounced in PERC modules than LID in the screen print AI BSF technology and can take 1-2 years to appear. The magnitude of this degradation can reach up to 10% for both mono-crystalline and multi-crystalline PERC modules. Higher temperatures and more intense light levels can greatly accelerate LeTID. The speed of degradation is therefore location-dependent. 

A study conducted by PI Berlin shows that it took almost 10 years for LeTID to appear in Germany compared to only 4 years in Cyprus. Indian climatic conditions vary significantly from region to region and therefore a detailed study needs to be done before the installation of PERC technology in India to evaluate the impact of LeTID and consequences on the sustainability of the project given the highly price-sensitive market here. 

It is desirable to have a pilot plant constructed in different climatic zones of the Indian sub-continent by agencies like MNRE, NISE, NCPRE (IIT Bombay) and study the LeTID effects for different regions. This will help ascertain the degradation, incremental gains and financial feasibility of PERC technology in different regions before the solar plant developers blindly adopt this technology to improve yields.