Realizing increases in solar cell efficiency or production yield today requires the use of high-performance chemicals capable of optimizing multiple process steps. Mallinckrodt Baker’s Johan Hoogboom, et al., discuss where such chemistry can help realize efficiency increases in the key steps in cell manufacturing, focusing mainly on texturing and emitter optimization.
by Johan Hoogboom, Suzanne Kuiper, Paul Thomassen, Mallinckrodt Baker BV
Whether it’s monocrystalline, multicrystalline, batch, or inline, every solar cell manufacturing process contains several key steps that rely on wet chemical modification. Until a few years ago, the chemistry involved was no more than simple acidic or alkaline dips, using commodity chemicals. With cell efficiencies continuously on the rise, and new cell concepts slowly being introduced into high-volume manufacturing, attaining even an incremental increase in conversion efficiency or process yield can make a huge difference in company revenue. Instead of using commodity chemicals, realizing increases in cell efficiency or production yield today requires the use of high-performance chemicals that are capable of optimizing multiple process steps, such as tuning emitters. As a result, cell manufacturers increasingly turn to performance chemicals to adjust their standard processes.
In standard cell manufacturing, most wet chemistry is used in wafer sawing/cleaning, texturing, emitter deposition, phosphosilicate glass (PSG) removal, emitter optimization, electrode formation, and edge isolation. For emerging higher-efficiency concepts, selective etching and increasing surface passivation are key. This review shows how chemistry goes beyond performance in realizing efficiency increases in the key steps in cell manufacturing, focusing mainly on texturing and emitter optimization (Figure 1).
|Figure 1. Process flow diagram for high-volume manufacturing of cr-Si cells.|
Lowering the surface reflection of silicon wafers by texturization is one of the main processes to improve solar cell efficiency. Increased light absorption, as schematically depicted in Figure 2(a) will result in higher currents from the cell, thus resulting in up to several tenths of a percent higher efficiency. Therefore, texturing remains one of the key issues in the industrial fabrication of crystalline silicon solar cells, and suitable texturing methods that result in optimal cell performance, are still being developed.
|Figure 2. a) schematic representation of increase in light absorption on a textured silicon surface; b) Silicon wafer after treatment with an alkaline texturing solution; c) Silicon wafer after treatment with an acidic texturing solution.|
There are two main approaches used in the industry for texturing of mono- and multicrystalline silicon wafers. One approach is anisotropic alkaline texturing, mainly used for monocrystalline wafers. The difference in etch rates between the different crystal planes of silicon results in the formation of pyramidal structures on the wafer’s surface, consisting of the Si(111) planes, which are the most stable planes towards alkaline etchants (Fig. 2b). A much-used solution for alkaline texturing is based on a potassium hydroxide (KOH)/isopropanol (IPA) mixture, which requires processing at elevated temperatures (80°-90°C) for extended periods (roughly 30 minutes). However, at these higher temperatures, evaporation of IPA is difficult to control and hence it is difficult to retain bath stability, making this a batch-type process. To solve this problem, extensive research is being done to decrease the usage of IPA, or fully replace IPA in alkaline texturing solutions, in principle prolonging the current bath life of six to eight hours. Replacement of IPA by high-boiling alcohols was introduced by Fraunhofer ISE . Several other solutions have also been published that are specifically designed for alkaline monocrystalline texturing, either decreasing or replacing IPA [2,3].
The second approach is aimed at the texturing of multicrystalline silicon wafers, using an isotropic acidic etch. Acidic textured surfaces have a less ordered structure (as can be seen in Fig. 2c) than alkaline textured (monocrystalline) wafers, which can be partly attributed to the formation of gas bubbles during reaction, and the isotropic nature of the acidic etch. This method relies on the use of a hydrofluoric acid (HF)/ nitric acid (HNO 3) mixture, in which HNO 3 is added to oxidize the silicon surface and HF to strip the oxide. One of the main employed methods for acidic texturing is the UKON etch, developed by the University of Konstanz, which consists of an HF/HNO 3 mixture . This texturing solution, albeit very inexpensive, results in significant defect etching and has high usage of the two acids. When there are many defects on the wafer surface, this can result in undesired charge recombination and the wafer becoming more brittle, decreasing the average yield. Recently, several alternatives were reported that aim to reduce defect etching and increase production yield [5,6].
There is a great amount of effort within the industry, as well as at research institutes, directed towards gaining more control over multicrystalline texturing. A prime example of obtaining highly ordered structures on a mc-Si wafer was presented recently by Fraunhofer . By treating the silicon wafer with UV nanoimprint lithography (NIL) and subsequent plasma etching, they successfully applied a honeycomb structure on the surface. The results of these experiments are very promising, however there is still much room for improvement before industrial-scale implementation can be achieved.
An approach to overcome the high costs that are inherently coupled with lithography and plasma etching was recently presented by the University of New South Wales (UNSW) [8,9]. UNSW presented a way to make use of inkjet printing to locally etch the surface of a multicrystalline silicon wafer, and thereby also creating either honeycomb or U-groove textures on the wafer surface. Furthermore, this approach has the added advantage of using less HF than conventional texturing solutions.
In a p-type silicon solar cell, charge separation is enabled by the emitter, which is formed by the thermal diffusion of phosphorous into a boron-doped silicon wafer. This leaves a highly doped top layer, which is a major source of charge recombination. This top layer is comprised of two zones. The top zone consists of PSG leftover from the emitter source material (phosphoric acid or POCl 3), which is partially dissolved in a standard manufacturing process by a simple HF dip. The second zone is the so-called dead layer, which consists of non-electrically active phosphorous. This layer cannot be substantially removed by a simple HF dip. Removing the PSG layer and etching part of the dead layer reduces the amount of surface recombination of charge carriers, substantially increasing short-circuit current and thereby the efficiency of the cell.
After PSG removal by an HF dip, substantial amounts of PSG remain on the surface. Traditionally, cleans like RCA clean are used for additional surface cleaning. Unfortunately, these cleans involve a two-step process (excluding rinsing) at very high temperatures, which makes this industrially unattractive. In 2006, Mallinckrodt Baker and the Energy research Centre of the Netherlands (ECN) developed a one-step alternative, which was presented as the ECN-Clean process, containing Mallinckrodt Baker’s J.T.Baker PV-160 solar cell surface modifier (Fig. 1) [10-12].
Laboratory experiments have shown that improvements in efficiency resulting from the ECN-Clean step were due to an increase in voltage and current (both by almost 1%), while the loss in fill factor was small. This increase was found to be irrespective of wafer position in the ingot [10,11]. In contrast, applying a standard RCA clean resulted in a dramatic loss of fill factor of several percent, drastically reducing final cell efficiency [13-14].
|Figure 3. SEM/EDX micrographs after application of a standard HF dip, PV-160 and PV-200, respectively. White shows the presence of phosphorous. Images are 50 × 30mm.|
The application of PV-160 resulted in an increased blue response from the cell, as shown by IQE measurements [10-12]. It was concluded that phosphorous was removed from the top of the wafer, which was corroborated by SEM/EDX measurements ( Figure 3).
The ECN-Clean process has been implemented on various industrial production lines of multi-crystalline wafers using H 3PO 4 as the phosphorous source. Using tailor made start-up and spiking protocols, the chemical bath can be kept stable over several days, resulting in stable cell parameters and an increased cell efficiency of up to 0.3% absolute [12,15].
In 2009, Mallinckrodt Baker launched its Advanced Emitter Optimization platform, adding two patented, highly engineered second generation products, J.T.Baker PV-162 and J.T.Baker PV-200 [15-16]. PV-162 is a product with much of the same characteristics as PV-160, but has been tailored to overcome the small drop in fill factor often seen upon the implementation of PV-160, leading to increases in the cell efficiency in inline solar cell manufacturing processes of up to 0.5% absolute. It has been shown in laboratory experiments that this product will also increase the cell efficiency in lines that employ POCl 3 as the phosphorous source by up to 0.3%.
|Figure 4. Effect of temperature on the increase in sheet resistance for treatment with PV-200. Delta Rsheet is in the range of 2-20 W/Ω¯.|
In a totally different approach, the etching mechanism of PV-200 is radically different than that of its counterparts in the product line. This adjustable product allows for a specific and controlled etching rate that can be tailored to the customer’s needs by varying the ratio of the chemicals in the etching bath and/or by adjusting its temperature. This tailoring allows for the achievement of specific sheet resistance values of the wafers with low standard deviations across and between wafers ( Figure 4), something which can be hard to achieve by controlling the phosphorous diffusion oven alone. In laboratory and small-scale industrial tests, it has been shown that the implementation of PV-200 in an inline solar cell production process can lead to an increase of the cell efficiency of up to 0.7% absolute and 0.5% for POCl 3 lines ( see table).
|Process||Multi inline H3PO4||Mono inline H3PO4||Multi batch POCl3||Mono batch POCl3||Multibatch POCl3 metallurgical grade Si|
|Efficiency gain (abs. %)||0,3-0,7||0,1-0,2||0,2-0,3||0,1-0,3||0,4-0,7|
Texturing and emitter optimization show that the use of performance chemistry can increase both cell efficiency and production yield, while simultaneously lowering cost of ownership. There are, however, many more opportunities for the application of performance chemistry in solar cell manufacturing. In standard cell production, wafer sawing and cleaning will be focal points for wet performance chemical applications going forward. Removal of organics and silicon carbide from the sawing slurry, as well as dissolution of ingot glue is currently done in multi-step processes that have reached the limits of their performance. Consolidation into fewer steps, and improving on performance will ensure increased ease-of-use for wafer producers and cell manufacturers alike . Additionally, the introduction of new pastes, capable of contacting ever higher ohmic emitters , and the use of new printing techniques such as stencil printing  will also continue to increase cell efficiencies.
Reducing surface recombination will become crucial, as normal production processes use ever thinner wafers, and with high-efficiency cell concepts slowly coming to market. Increasing surface passivation with wet chemistry, or ensuring that the surface is clean and homogeneous prior to deposition of passivation layers, will be crucial for attaining ever higher cell efficiencies .
As interfaces become more and more important in increasing cell efficiency, solutions that can control surface properties and homogeneity will surely be incorporated into industrial production schemes. This entails that the development and application of cutting-edge performance chemistries will surely be a focal point for technologically innovative chemical companies for some time to come.
J.T.Baker is a registered trademark of Mallinckrodt Baker.
Johan Hoogboom received his PhD from the Radboud U. Nijmegen, The Netherlands, and performed postdoctoral studies at the Massachusetts Institute of Technology. He is group leader, energy conversion at Mallinckrodt Baker BV, Teugseweg 20, NL-7418AM Deventer, The Netherlands, e-mail Johan.Hoogboom@covidien.com.
Suzanne Kuiper received her PhD from the Radboud U. Nijmegen, The Netherlands and is scientist, photovoltaic R&D at Mallinckrodt Baker BV.
Paul Thomassen received his PhD from the Radboud U. Nijmegen and performed postdoctoral studies at the U. of Sydney. He is application project manager photovoltaics at Mallinckrodt Baker BV.
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