Commercialized c-Si solar cell structures

Cell structures targeting at high efficiency (e.g. PERL [1] or PERT [2]) were designed and made in lab with 24% of cell efficiency by Prof. Martin Green's group at UNSW in early 1995.  However, in reality, it is the cost that has limited the full replication of such structures in industry. For example, SunTech's Pluto line which is said associated with such structures still has not been on production mode yet mainly due to cost issue, I think. Thus, current majority cells only partly possess above structures without full optimization.

The conventional commercialized cell structure is mainly based on p-wafer, front contact, heavily-doped emitter, front-side SiNx passivation/ARC, and Al BSF. The efficiency is obviously compromised by cost reduction and general ball park cell eff of monocrystal Si is 15-18%. But, this structure is still commonly used as a base and entry point of many manufacturers.

Fig1: SunPower's IBC vs. Conventional Front-Contact Cell (Courtesy of Doug Rose)

As an improved cell architecture, Interdigitated Back Contact (IBC) structure has also been commercialized but with relatively lower production quantity. This type of cell was invented and still its progress is led by SunPower. It is n-wafer based due to high lifetime requirement (>1ms). Also it is more difficult for p-type Si to meet this spect due to oxygen sensitivity of B caused lifetime stability issue.[3] It is both side passivated (e.g. SiNx front and SiO2 back) and interdigitated with all metal contacts on the back. Although the efficiency is very appealing (24.2% [4] cell eff and >20% module eff in Maylasia factory from 2011 press release), it still not cost effective. Fig.1 is a comparison of SunPower's IBC and conventional cell.

To improve cell efficiency/cost value, there's other structures and processes. But, most of them are not fully ready for commercialization yet. Selective-emitter (SE) based cell has started in production but of course in very small quantity, for examples, Schmid's etch-back SE and Centrotherm's laser selective doping. The detailed SE will be discussed later.

In summary, current cell structures need to be further improved based on cost reduction and/or efficiency enhancement to meet the requirement of competitive dollar-per-watt value of final panels. Cell structure and process methods to make it definitely are some of the keys to play.

References

[1] Jianhua Zhao et. al. "Twenty-four percent efficient Si solar cells with double layer antireflection coatings and reduced resistance loss", Appl. Phys. Lett., 66, 1995, pp.3636-3638.

[2] Jianhua Zhao et. al. "High efficiency PERT cells on N-type Si substrates", 29th IEEE PVSC, 2002, pp.218-221.

[3] D. Macdonald et. al. "Boron-oxygen defects in compensated P-type of cz Si", 24th EPSEC, 2009, pp.877-882.

[4] Peter J. Cousins et. al. "Generation 3: Improved performance at lower cost", 35th IEEE PVSC, 2010, pp000275-000278.