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Discussion About N-Type And P-type Crystalline Silicon Solar Cell
Mar 14, 2019

N type HJT or HIT solar cell


Discussion in the solar PV industry about which crystalline silicon (c-Si) technology is dominant has been a long time:  Monocrystalline, grown through the Czochralski method, or Multicrystalline, manufactured through directional solidification. Recently, traditionally higher cost mono is becoming comparable on a $/W basis installed to multi, leading to significant growth in mono market share in 2016.  Now it begins to become interesting to examine the different merits and shortcomings of different types of mono c-Si technology.


Mono c-Si cells can be broadly divided into two categories; p-type and n-type.  P-type cells are doped with atoms that have one less electron that silicon, such as boron, resulting in a positive (p) charge.  N-type cells, on the other hand, are doped with atoms that have one more electron than silicon, making them negative (n).  While n-type cells are offer higher efficiency potential than p-type cells, they are more costly (Lai, Lee, Lin, Chuang, Li, & Wang, 2016).  

The main issue faced by cell manufacturers when trying to sell p-type c-Si cells is light induced degradation (LID).  LID is a phenomenon that leads to the degradation of the carrier lifetime of p-type monocrystalline silicon cells during exposure to light; the minority carrier lifetime is impacted by the light as excess carriers are injected into the cell (Walter, Pernau, & Schmidt, 2016).  The minority carrier lifetime of a cell, which is defined as the average time a carrier can spend in an excited state after electron-hole generation before combination, determines the efficiency of the cell.  Cells with shorter minority carrier lifetimes will usually be less efficient than cells with long lifetimes .


The n-type materials for the solar cell fabrication process demands some additional step compared to solar cells fabricated on p-type substrates. In fact, the p-type substrates have some advantages in terms of the processing of solar cells, such as the convenience of phosphorus gettering, which assists improvement in cell efficiency, specifically for mc-Si wafers. The emitter formation in the case of n-type substrates has to be done via the boron diffusion process, which requires higher temperatures compared to the phosphorus diffusion for p-type cells, which makes the cell fabrication process more complex. Moreover, the process for two separate diffusion steps (emitter and BSF) renders it even more complicated and costly. During the boron diffusion process, another important issue is the formation of born rich layer (BRL) which is good for the gettering purpose but degrades the carrier lifetime in bulk. Recently, a particularly effective method of removing BRL without the injection of gettering impurities has been developed.


There are a number of solar cell structures with higher efficiencies that have already been implemented successfully using n-type substrates. Figure 1 illustrates these solar cell structures on n-type substrates briefly. The cell structures designed on n-type substrates will be discussed briefly in the preceding sections. These cell structures can be categorized according to the techniques used for cell processing and are described as follows: (1) front surface field (FSF) Al rear-emitter cells (n+np+ cells) can either have the contacts at the front or at the rear and normally has phosphorus diffused FSF; (2) back surface field (BSF) front-emitter cells (p+nn+ cells) can also have the contacts either on front or rear and are commonly boron-doped emitters with phosphorus-doped BSF; (3) ion implanted emitters cells have the emitter formed by ion implantation process and can be realized for both front and rear contact schemes on n+np+ and p+nn+ structures; (4) heterojunction with intrinsic thin-layer (HIT) cell structure.


N type substrate solar cell structure chart

Figure 1: N type substrate solar cell structures

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