3.3 Changes of the device structure
As can be extracted from the previous section, the coming future for the solar cells front face definition, to get an effective improvement on the energy conversion efficiency, is not only related to the use of an optimal metal grid definition, but is also related to the introduction of different device’s structures that will improve the performance of cells.
The Different working lines that can be followed by industry are related to the introduction in production lines of high efficiency concepts that are being successfully tested by research institutes and universities, among these lines it can be found several approach such as:
• To change the silicon surface topology introducing new more efficient texturization processing (Zhao et al., 1999), (Kumaravelu et al., 2002).
• And improving the optical performance of these with and optimised design of the antireflective coatings and rear reflector in order to get higher generated current (Nilsen et al., 2005); (Glunz et al., 2007).
• Improving the front, bulk and rear surface electrical passivation in order to enhance the final open circuit voltage of the devices, by means of better aluminium rear pastes, using gettering steps or introducing more radical changes in the cell design as a rear local contact with an improved electrical passivation. (Glunz et al., 2007).
Among all the high efficiency concepts, the idea of a ‘selective emitter’ in the devices is prone to be one of the first concepts introduced by the industry in its production lines without the need of radical changes in the production processes. This is why a special attention must be devoted to this concept.
The concept of ‘selective emitter’ consist in rising the surface doping level in the emitter area where metal grid will be deposited to improve later on its electrical contact (as it is plotted in Fig. 15), and keeping a low surface doping level in the rest of the front side of the cell (area that in that way will be later electrically better passivated) (Green, 1995). This concept not only improves the generated current due to the improvement of the low wavelength response of the resulting cell emitters, but also makes the open circuit voltage to rise due to the improvement in the front surface passivation.
But technical difficulty in obtaining this structure has made the selective emitter concept typical from fabrication processes of research labs, or implemented in solar cells with more complex structures and complicated fabrication processes due to the alignment requirements (as the LGBC or Laser Grove Buried Contact solar cell (Wenham & Green, 1993)), leaving this concept out of simple processing production due to its high implementation cost.
Apart from the typical microelectronic way to get a selective emitter with several diffusion steps and using diffusion barriers deposited on the (later on) non contacted areas (with an expensive photolithographic process due to the restrictive alignment between patterns and the high number of processing steps), new innovative techniques to get a selective emitter with screen-printing contacts are appearing recently due to the great industrial interest on improving photovoltaic device efficiencies (Raabe et al., 2007). All these new techniques can be classified in two big groups according to the way the selective emitter structure is obtained:
A. Selective emitter developed without masking processes, carrying out local phosphorus diffusion on the wafer surface.
Several alternatives for this kind of processes has been studied, among these, it could be emphasized:
1. The local deposition of phosphorus sources on the wafer surface by using a screenprinting process previous to the high temperature diffusion step, generating most highly doped areas in a self-aligned process that only requires the right location of the contact grid during the metallization step. Different examples of these processes can be found in (Horzel et al., A 1997); (Horzel et al., B 1997); (Salami et al., 2004).
2. The use of lasers to create, after a soft phosphorus diffusion step in all the cell area, a higher doped contact area (Besi-Vetrella et al., 1997).
3. Or the use of special metallization pastes that include doping material in its composition, thereby these pastes are used as a source of dopants for the contact area during the firing step of the metallization, creating a selective emitter structure when they are deposited on wafers with a low doped emitter in a self-aligned fabrication process. Examples of these processes can be found in (Rohatgi et al., 2001); (Porter et al., 2002); (Hilali et al., 2002).
B. Selective emitter developed using masking processes to protect, with barriers, part of the front area from the diffusion step (creating zones with a softer diffusion due to these barriers) (Bultman et al., 2000); (Bultman et al., 2001). Or masking to protect the surface from a selective etching. As it is refer in (Ruby et al., 1997); (Zerga, A. et al., 2006); (Haverkamp et al., 2008).
The alternatives exposed in the first group produce a correct selective emitter structure but have a drawback, after the phosphorus diffusion and the gettering step it introduces, when it is carried out in super-saturation conditions, the impurities concentrations in the silicon bulk keep constant, because impurities remain mainly trapped in the ‘dead layer’ that appears near the surface, without been effectively removed from the device, reducing the potential impact of the improvement this step could have.
Among the different alternatives of the second group, however, it exists processes that can carry out an effective reduction of impurities when these include a surface etching, what result in a better device performance; but also present some drawbacks related to the needed mask treatments and processing (such as deposition, curing and removing steps), giving slightly more complex fabrication process.
All the developed alternatives in both groups (with exception of the use of self-doped metallization pastes) present a fundamental problem for the selective emitter structure, that is the need of an alignment with the next processing steps for the contact definition, complicating the fabrication routes. This problem gets worse when it is taken into account the random deformation screen-printing technique presents for the transferred patterns with the increase in the number of prints (deformation that is associated to the relaxation of the fabrics, that compose the screens, and gets a maximum value in the mass production environment). But appearance of new alignment relaxed device structures concepts (Caballero, 2009) can help to develop new and easier industrial fabrication processes that could finally result in the implantation of the selective emitter as a common part of the typical industrial solar cell device structures. The introduction of the selective emitter structure would force the re-design of the front contact grid that could present a different optimal finger separation (with closer fingers) due to the increase in the emitter resistance of the new cells, but no other additional important changes.
3.4 The future of the metallic contact definition
Apart from the improvement of the general parameters of the device (Voc and Jsc), as previous sections has shown, improving the front contact of solar cells is possible and it can produce an increase in the final efficiency of the industrial solar cells. In this section it will be reviewed the strategies that research centres and industry are following for a future improvement of the front grid.
The approach for improving the front grid is based on increasing the aspect ratio of the cross-section of fingers as it is shown in Fig. 16, reducing the seepages that increase the shadowing factor of the grid without reducing the grid resistance, increasing the finger height, and reducing its width in order to produce a lower grid shadowing.
Fig. 16. Future improvement of the metal finger cross-section
Several strategies can be followed or will be followed by the industry with this purpose; from the introduction of slight modifications or changes of the nowadays production technology, such as:
1. Optimising the paste composition with different combinations of the silver particles with different shapes and sizes, in order to maximise the finger heights after printing.
2. Optimising the printing process, modifying the fabrics and emulsions characteristics in the screens, and the processing parameters to reduce the paste seepages during printing.
3. Introducing a heated chuck inside the standard printer units in order to produce an increase in resolution by heating the wafer substrates during printing process. Reaching finger width as lower as 50 microns (Erath et al., 2009).
4. Substituting the standard pastes by the Hotmelt technology pastes (Williams et al., 2002) that produce an improved aspect ratio of the final metal fingers. This technology changes the traditional solvent of the paste by a long Chain alcohol with a melting point in the range of 40 to 90°C, so final paste needs a heated screen to be disposed.
To the introduction of additional processing steps, that needs the addition of new machines in the production lines, such as:
5. Growing pure silver over the screen-printed contact in an electrolytic bath, reducing the resistance of the fingers and improving the contact resistance in its edges, due to the silver filling of the empty space of the fingers volume.
Several approaches can be found in the market, based on the classical electrolytic growing, that needs a current contact with the front grid, or based on the LIP technique (Pysch et al., 2008); (Glunz et al., 2008) that doesn’t need any contact. In both cases grid fingers width increases after processing.
Or the complete change of the technology for the contact definition using new techniques nowadays under development, in research projects of several companies and institutions, such as:
6. Growing the complete finger in an electrolytic processing, but defining previously a grid which is used as a seed for the electrolytic grown with narrow lines to avoid an excessive increase of the final finger width.
This seed for growing could be produced with an electroless nickel plating over the bare emitter silicon (Glunz et al., 2008) (processing that would introduce previously, a masking of the front silicon nitride layer using an inkjet printing system to deposit the mask with the needed definition of lines; and an etching of the masked structure to open the nitride layer), or defined with new promising techniques such as inkjet printing (Mills & Branning, 2009), aerosol jet printing (technique able to reach finger definition of 40 microns) (Horteis et al., 2008), or laser direct-printing (able to reach a finger definition below 20 microns) (Shin et al., 2008); (Arnold et al., 2004) over the silicon nitride layer (with its later firing through previous to the electrolytic growing) or also over the bare silicon emitter (processing that would need also a previously masked etchings of the nitride).
The introduction of the plating of new materials to grow the contact grid can reduce additionally the final cost of the metallization step. Thus, copper with a good conductive properties (97.61% of the silver conductivity (Brady et al., 2002)), is stirring up the industrial interest on the creation of new multiple metal layer contact such as:
Seed of Nickel/ Copper/ Tin
Seed of Silver/ Nickel/ Cooper/ Tin or
Directly Copper without seed/ Tin
On the other hand, situation for the back aluminium contact of cells is different because it is not related with the improvement of resolution, but with the need of an improvement in Voc to increase the final cell efficiencies. In this case the possible paths for industry to follow are:
• Improving the characteristics of the Aluminium pastes for the total bsf device structure, designing new formulations of the used pastes and removing the limitation that nowadays the aluminium contact has, related with the bowing of devices during the firing process, by means of, for example, adding special thermal treatments processing (Huster, 2005).
• Changing the total rear contact by a local rear contact where non contacted areas must be passivated with a new deposited layer with improved properties of passivation for p type silicon. Laser techniques for the creation of the local contacts (Schneiderlochner et al., 2002); (Tucci et al., 2008) are taking a good position to be industrialised for the creation of this kind of local contacted structures.
This chapter has presented the technique used by the mass production industry to define the contacts of the silicon solar cell, its basic principles and factors that have an influence in its result with the main aim of giving an introductory view of a technique responsible for the development and expansion of the nowadays photovoltaic terrestrial market.
It has been shown how through a simple analytical modelling the performance of different designs for the front contact in the commercial solar cells can be optimised and compared. And a quick review of the coming changes in the device structure design, and the future techniques that are under research to substitute the screen-printing technology have been done in order to give an idea of how the industry can evolve in the coming years.