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Solar cell for almost a decade had been stable with its size as M2 which was also the choice of the end customer. However, with the drive for enhanced power output alongside a reduction in solar PV’s Levelized cost of electricity (LCOE), the need for change was inevitable. The industry started taking cues from the semiconductor industry i.e. to increase the wafer size (refer to Figure 1) which would result in a direct increase in power output. The size of the wafer quickly jumped from 156.75 mm / 157 mm to an intermediate size of 161.75 mm / 166 mm and then to 182 mm / 210 mm within a span of around five years. With the current standard cell sizes, the PV module may have reached its size limits which currently span between 2.2 ~ 2.4 m in length to 1.1 ~ 1.3 m in width. With higher power output still being the demand from the end customer, any further increase in wafer size would lead to a proportional increase in module size. Such increases in module size have limitations like MMS cost, self-weight, handling limitation, etc. which would nullify the commercial gain from such enhancement. Further with PERC cell reaching its efficiency limits, the need to focus on alternative technologies is the need of the hour.

Figure 1: The evolution of the semiconductor market (on top) & PV market (on bottom) over the years
All the commercially available solar cells and the developments have only taken place on P-type solar cells (depends on the type of dopant used while manufacturing). This was contradictory to the fact that the first-ever developed solar cell was N-type but it couldn’t make it as the first used solar modules were in space. The initially developed N-type solar cell was obviously not stable under UV exposures and hence P-type became the only choice. However, with the current N-type solar cells having better reliability& higher efficiency, they are finding their way back from highly advanced solar labs to manufacturing facilities. The current offering in the N-type module is based on PERL, PERT, Topcon & heterojunction (HJT) solar cells. We would however focus more on HJT explaining to you its construction, features, its advantages and compare it to other traditional technologies at the later stage.

Construction of HJT cell

The HJT solar cell, as the name suggests is made up of different layers combined into one. The cell is made up of crystalline silicon cells sandwiched between (thin film) amorphous silicon on both sides. This means that the cell combines the advantage of better absorption of light (from the crystalline layer) alongside better passivation properties (from the amorphous layer) thus creating a superior cell. The current light conversion efficiency record for HJT cells stands at greater than 26.5% which clearly means that the cell has the potential to unlock the next generation of modules. However, efficiency improvement is only one of its many advantages. Before we jump into all its advantages, let us understand the construction of the HJT cell and then we would discuss its advantages in detail.
The HJT cells (refer to Figure2) contain an n-type crystalline silicon absorber at the center. It has both intrinsic (neutral) and doped layers of amorphous silicon on either side thus forming a p/i/n/i/n+ stacking. As we mentioned earlier, crystalline silicon has the property of better light absorption meaning that it can absorb almost all the light which falls on it thus generating more free carriers. Immediately around the n-type crystalline silicon are the intrinsic hydrogenated amorphous silicon (a-si: H(i)). Bareamorpho us silicon while is easy to deposit on the crystalline silicon but has a lot of surface defects, which means that there will be loss of carriers due to high resistance. Hydrogenating the bare amorphous silicon decreases the defect density drastically while also increasing their band-gap (when compared to the crystalline silicon).
The intrinsic (or un-doped) layer of a-si(H)(i) enables better surface passivation which means that the excited electrons and holes would not recombine before being collected. Moving on, further to the intrinsic layers are the p-type and n-type doped hydrogenated amorphous silicon layers which form the P-N junction in the solar cells. The top p-type layer collects part of the light falling onto it both directly and from inter-layer reflection. Similarly, the bottom layer captures the remaining amount of light that may have passed through the first two layers while also providing surface passivation. Generally, the conductivity of the a-Si: H layer is poor and may not be sufficient to provide a good carrier(charge) collection via the metal contacts. This is when the transparent conductive oxide (TCO) comes into play.
The TCO layer is deposited on both sides of the a-Si: H layer. They work by promoting a good ohmic contact, facilitating lateral carrier transportation, and also working as an anti-reflective coating (ARC)similar to the SiNx coatings in crystalline solar cells. There are many industry-standard TCO’s but indium tin oxide (ITO) is the most common of them. The thickness of the top and the bottom TCO may be different. The thickness of the top layer of the TCO and its oxygen content is optimized for a suitable sheet resistance for carrier transportation, good transparency to avoid unusual light absorption, and to further enhance light trapping. The rear TCO may be optimized for absorption in the infrared (IR) region.
Figure 2: A typical Heterojunction solar cell

Advantages of HJT

With the construction being clear, it’s time to understand what advantages HJT has to offer:

Manufacturing advantages: The first and foremost advantage of HJT is that it is a less energy-intensive process, thanks to the thin film depositions on either side. The HJT cells are processed at < 250 °C which saves a lot of energy during manufacturing cells. The number of steps required to manufacture these cells is halved compared to the industry standard PERC. Further at the module level, they are again processed at around the same temperature when stringing the HJT cells onto a module.

PID free technology: Potential Induced Degradation or better known as PID is known to affect almost all the type of solar modules. In a PID affected module, there is either a current leakage from the cell to the ground via the frame (better known as PID-polarization or PID-p) or there is a shift of positive ions (usually Na+) from the glass to the solar cell which leads to recombination loss (better-known as PID-shunting or PID-s). This PID mechanism usually attacks the insulating layer of the solar cell which if polar in nature accumulates electric charges under high potential difference. While this happens in the case of crystalline cells where the SiNx coating is polar in nature, the TCO used over the HJT cells is non-polar. This means that there would not be any charge built up in these cells and hence there would practically be no PID in them (refer to Figure 3).

Figure 3: Comparing crystalline & HJTcells for PID (Source: NREL)

No LID/ LeTID effect: Light Induced Degradation (LID) has been observed in nearly all silicon crystalline-based solar modules. In these modules, LID causes an initial power degradation of greater than or equal to 1%. LID is caused by either complexities in Boron Oxygen (B-O) or dissociation of Iron-Boron (Fe-B) pairs. During the wafer manufacturing process, oxygen reacts with the dopant (boron) to form a complex compound in B-O-based LID.

These complexes form intermediate energy levels where the exited electrons recombine, resulting in a loss of efficiency in solar cells. The iron present during manufacturing reacts with the boron in the dark due to Coulomb interaction in Fe-B-based LID. When the solar cell is illuminated, the iron ions separate and recombine with electrons, resulting in efficiency loss. LeTID (Light and Elevated Temperature Induced Degradation) is known to affect PERC cells, where both light and elevated temperatures (as high as 50 °C to 95 °C) affect cell efficiency. HJT cells are constructed without Boron, which eliminates the possibility of LID in them. Further, no such reported incidents at labs or at field confirm the effect of Le TID in these cells (refer to Figure 4)meaning that they are LID & LeTID free.

Figure 4: LeTIDresults of different modules (Source: Eternal Spire)

Lowest temperature coefficient: One of the most important parameters of a solar module is the temperature coefficient. It determines the amount of power loss that the cell or module will experience if the temperature rises. The temperature coefficient is affected by a variety of factors, including series and shunt resistance, surface passivation quality, the number of interstitial defects, and so on. With each interlayer of the HJT cell ensuring better light absorption, improved surface passivation at low series, and higher shunt resistances, the cell has the lowest thermal coefficients of any known solar PV technology.

Lowest degradation: With almost no known mechanism effectively affecting the HJT modules, they are known to have the lowest power degradation rates. Further, they are also known the have better reliability compared to their peer technologies.

Figure 5: An overview of all the advantages HJT has to offer

We will continue this article and explain the technical and commercial comparisons between HJT and other commercial technologies in the following article. We would also educate you on the global HJT technology roadmaps. Keep an eye on this space for a follow-up article.

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