Heterojunction - The technological way ahead for solar PV! – Part 2

24 August 2021 at Waaree

The first part of the article "Heterojunction - The technological way ahead for solar PV!" introduced the heterojunction (HJT) technology and also explained its advantages. We would now discuss about the comparison between HJT & other cell technologies both technically and commercially. Further we would also discuss on how the world is going ahead on HJT.

Technical comparison between HJT & other cell technology

As we discussed earlier, the HJT technology is by default bifacial in nature. And with the technology having symmetric structure and alike-manufacturing methodology, the technology has one of the best bifaciality. Theoretically, its bifaciality is above 90% and the recorded bifaciality of the technology on field goes around 93% which is highest amongst all the competing technology. Most prevalent commercial cells are crystalline in nature which means they may not be able to resist micro crack effectively. Comparing HJT to it, the technology has amorphous layers on both the sides which gives it much needed flexibility which improves its resistance to micro cracks. Further, as we described earlier, the technology has no initial LID which makes it the leading contender for lowest initial degradation. Additionally with no PID/ LeTID/ other degradation mechanism known affecting the HJT module in general, they also have one of the lowest year-on-year degradation rates. Finally comparing the temperature coefficients of all available technologies, the Δ is 0.10%abs between HJT & mono PERC which means that in a country like India, implementing HJT module would lead to reduced energy loss due to higher temperature.

Figure 1: Technical comparision of HJT vs other technologies

Commercial comparison between HJT & PERC

We understood that HJT is technically superior amongst all the available technologies. However it is also important to compare HJT to the on-going passivated emitter and rear contact (PERC) technology. We simulated a 1 MW power plant in the state of Gujarat (refer Figure 2). We kept all the other factors like tilt angle, module arrangement, inverter loading ratio, etc. constant in order to have a comparative analysis. We know from the graph above (Figure 1) that the Δ of temperature coefficient between HJT & mono PERC is 0.10%abs. While this number seem small, when we look at the power plant output the temperature losses in a mono PERC module based plant are around 50% higher. The quantum of this energy loss is huge when we consider (ultra) mega power plants and with further in temperate zones like India where ambient temperatures always stays above 35 degrees. Next, with HJT having better surface passivation & low light performance along with lowest initial degradation, we find that the specific energy output of HJT based power plant stands at 1922 kWh/kWp/year which is around 6% higher than PERC based power plants. Finally, talking about performance ratio (PR) which indicates the how good or bad a plant is performing. The PR of HJT based plant is 4.7%abs greater than the PERC based plant which clearly shows the advantages of utilizing HJT technology. With such advantages, HJT stands out tall when compared to its competitors in almost all the fields.

Figure 2: Comparison between HJT & PERC

Things to take care while making HJT

HJT cells have higher charge density in the quasi neutral region (q-n-r) and higher effective carrier minority lifetime than other typical c-Si cells. This causes slower response to changes in voltage and current therefore these cells will have larger electrical capacitance. Typically, module manufacturers use Flash Test to measure and characterize the modules. In a flasher, I-V curve is obtained by changing from open-circuit to short-circuit and plotting the current vs voltage. The current PV modules in production demands fairly lower sweep times (ranging from 10 millisecond (ms) to 100 ms) for determining the electrical characteristics accurately. However with the HJT cells having higher electrical capacitance, flashing them at the current sweep times would result in significant measurement error. This error can range anywhere from 2% to as high as 8% (refer Figure 3). Thus in order to limit this error to less than 1%, it is suggested that the sweep time should be more than 170 ms.

Figure 3: Capacitance effect of HJT at forward and reverse sweeps (Source: Eternal Spire)

Where is the world going ahead with HJT?

With the advantages being clear on the technology, we know that HJT is definitely the thing to look forward in years to come. The solar PV community has been voicing the same idea with majority manufacturers having already announced plans of the shift. Let us understand first what the future holds for HJT. Three technologies i.e. PERx, TopCon & HJT are compared in terms of their power output & Bifaciality (refer Figure 4). For simplification, we have normalized the power output to M6 144 cell based solar module. The current power output of all the three technologies are 450 Wp+ with the delta being 10 Watts between PERx & TopCon and TopCon & HJT respectively. However, with the PERx technology reaching efficiency limits, the maximum enhancement in power output would only be around 20 Wp per module. Comparing this to TopCon & HJT, the maximum enhancement currently stands at 25 Wp per module with this number going up to as much as ~40 Wp for HJT based module. Further, even in terms of Bifaciality, the PERx technology has almost reached upper limit at around 75%. Compare that to HJT, where maximum Bifaciality of around 95% is expected making it a strong contender for all the future modules.

Figure 4: Normalized power output & bifaciality for M6 module from different technologies (Source: ITRPV)

The current nameplate capacity vs actual production of HJT modules is expected to stay somewhere at 50% in 2021. However with many manufacturers jumping in, the total announced HJT capacity cumulates to 50+ GW. Further with the capacity expansion spread out almost equally around the world, we expect that HJT technology would pick up pace by the mid of this decade with more than 70% of the world's PV capacity turning to HJT.

Figure 5: Capacity vs actual production & country wise production capacity announced for HJT (Source: Taiyang news)

With all the given technical and commercial advantages, HJT definitely seems to be the technology of the future. And with further new cell architecture almost utilizing HJT in them, we strongly believe that the future of solar PV is HJT.