Perc - The Technology Knocking at Your Doorstep
The history of solar cell dates as early as 1954 when Bell Laboratories demonstrated the first practical cell. After this, there had been a lot of development both in efficiency and novel technology capable of powering the future. However even after around 28 years the solar cell had few issues which still needed attention. The first among these issues was the reduced carrier lifetime i.e. the exited charges (from light) were unable to stay at the state for longer time which resulted in inefficient charge collection. The next issue was high surface recombination velocity meaning that the exited charges recombined at the surface of the solar cell with a greater speed. This along with the inefficient charge collection also resulted in an increase in the solar cell temperature (which was the third issue). All these issues were handled Passivated Emitter and Rear Contact (or PERC) solar cell.It was developed in 1982by scientist Martin Green and his team at University of New South Wales, Australia. During these times, researchers tried increasing the efficiency of solar cell by focusing on the front contact/ sunny side of the solar cell. PERC cell architecture had restructured both front and rear side of solar cell.This architecture (as shown in Figure 1) had the following differences while compared to other cells (available at that time):
- The contact at rear side of the solar cell had large number of contact holes via a passivated oxide layer. This enabled the light (photon) which just passed through the cell without exiting the electron (and hence generating current)to be reflected back into the cell.
- The rear side also had a localized BSF instead of a full BSF in the conventional cell. This enabled the un-collectedcharge carriers (electron) to be reflected back inside the cellallowing them to be collected back for current generation. Additionally this reflection also enabled in reduction in overall cell temperature which resulted in overall increase in efficiency of solar cell.
- The front side of the solar cell used the so called "inverted" pyramids. This pyramidsenables reducing the reflection from the cell while also ensuring efficient light trapping.
The output characteristics of the cell as independently measured by Sandia National Laboratories were open circuit voltage (Voc) 696 mV, short circuit current density (Jsc), fill factor (FF) of 81.4% and an efficiency of 22.8% which was the highest ever efficiency recorded during those times.
Figure 1: Schematic diagram of PERC cell (Source:Blakers A, Wang A, et. al. "22.8% efficient silicon solar cell", Appl. Phys. Lett. 55, 1363 (1989); https://doi.org/10.1063/1.101596)
With discovery of such highly efficient technology it was clear that the road ahead for these cell was promising. However field testing of this technology had presented few results which had restrained its use. The issues that affected either/ both mono and multi PERC cell were Light Induced Degradation (LID), Light and elevated temperature induced Degradation (LeTID) and Potential Induced Degradation (PID). Affecting primarily the mono PERC cell, LID affected the cell/module during their initial exposure to illumination resulting in degraded power output and hence its efficiency. LeTID as the name suggest affected the cell/module during their initial exposure to illumination at elevated temperature (from 50 to 95 °C). This degradation affected p type multi-crystalline PERC cell where (relative) efficiency loss of more than 10% was discovered. PID as we had explained earlier in our blog "What's and why's of PID" was significant in PERC cell which led to significant loss in power output and/or infant mortality of the module. These problems LID &LeTID (as claimed by the cell manufacturer) and PID (by ozonisation) have been resolved (to a certain extent) which has now again forced the world to shift its attention to this technology.It is noteworthy that LID and LeTID have significant effects only on p-type PERC cell and not on n-type cell. This has also led few manufacturers to focus more on n-type PERC cell.
The figure 2 and figure 3 below shows the market share of various cell technology and their stabilized efficiency value over a period of 10 years. It can be understood that PERC (and its cousin technologies) solar cell which has had mere 23% (appx) market share in 2017 would increase to around 60%. This shows that PERC technology would become the main stream technology in years to come. Additionally the efficiency of PERC solar cell is expected to rise to around 23.4% (both n & p type mono PERC cell) and 21.5% (p-type multi-crystalline cell) by around 2028 showing the true potential of this technology and self-explaining why this technology has a major market share by 2028.
Figure 2: World market share of different cell technology (Source: ITRPV)
Figure 3: Average stabilized efficiency of solar cell (Source: ITRPV)
The Figure 4 and Figure 5 shows the power output of (60 and 72 cell) solar module. It can be seen that the power output of both n & p type mono PERC solar module is expected to rise to around 350 Wp (60 cell) and around 415 Wp (72 cell) which is around 12% higher than the current value.
Figure 4: Module power for 60 cell module (Source: ITRPV)
Figure 5: Module power for 72 cell module (Source: ITRPV)
Waaree Energies had understood the market trend of PERC cell and has been offering its high efficiency and best quality PERC solar panels to optimize customer's space. The module has been certified by various national as well as international certifying agency ensuring that the module quantity is not compromised. We would continue this innovative drive to deliver the best of class products and service to our consumers.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION