Review on screen printed metallization on p-type silicon

Transcription

1 Erschienen in: Energy Procedia ; 21 (2012). - S Available online at Energy Procedia 21 (2012 ) rd Workshop on Metallization for Crystalline Silicon Solar Cells, October 2011, Charleroi, Belgium Review on screen printed metallization on p-type silicon S. Riegel, F. Mutter, T. Lauermann, B. Terheiden, G. Hahn University of Konstanz, Department of Physics, P.O. Box X916, Konstanz, Germany Abstract Advanced solar cell contacts feature local contacts to p- and p + - Si. In is review existing models for contact formation to p and p + -Si are presented. The formation of e local Al BSF as applied in PERC like solar cell structures is inhibited by e formation of voids. It is shown at e formation of voids depends on process parameters and eir influence on e contact formation is reviewed. Using an analytical model it is possible to predict e dep of e resulting Al BSF in dependence of e contact geometry and e peak firing temperature. Contact formation to B doped Si wi pure Ag paste results in high contact resistances whilst contact formation to Al doped Si wi Ag paste is less difficult. The contact formation wi Ag paste to B doped Si is enhanced by e addition of Al to e Ag paste Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Guy Beaucarne, 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of Guy Beaucarne Gunnar Schubert and Jaap Hoornstra Open access under CC BY-NC-ND license. Keywords: screen printing; p-type; crystalline silicon; solar cell; local contact, contact formation Corresponding auor. Tel.: ; fax: address: stefanie.riegel@uni-konstanz.de Published by Elsevier Ltd. Selection and/or peer review under responsibility of Guy Beaucarne Open access under CC BY-NC-ND license. doi: /j.egypro Konstanzer Online-Publikations-System (KOPS) URL:

2 S. Riegel et al. / Energy Procedia 21 ( 2012 ) Introduction One approach to reduce costs per wattpeak is increasing solar cell efficiency while keeping solar cell production costs moderate. Following is approach, advanced solar cell concepts like Passivated Emitter and Rear solar Cells (PERC, first published by [1]) [2-8], bifacial solar cells [9-14] and n-type solar cells [9, 14-19] gain more and more attention. In e PERC concept e full area Al BSF (back surface field) is replaced by a dielectric passivation, which is locally opened for contact formation [1-8]. Contacts are formed by physical vapor deposition (CVD) of Al [1, 8] or screen printed Al [6-8, 20-30]. The dielectric passivation results in improved optical properties and less surface recombination [8]. But e local aluminum contact formation is challenging. The dielectric passivation layer must not be penetrated by Al [6]. Many auors report on e formation of voids undernea e Al contacts [7-8, 20-30] as well. If voids occur, only incomplete BSF formation takes place leading to enhanced surface recombination at e contacts and increased contact resistances. Bifacial solar cells commonly use a diffused B BSF passivated by a dielectric layer instead of a full area Al BSF [9, 10, 12-14]. For rear side metallization a finger grid is applied similar to at used for front side metallization. Therefore, bifacial solar cells which collect light from e rear side as well feature improved optical properties and less surface recombination [10, 12, 13]. Like front side metallization finger grids applied on e rear side must provide low contact resistances and good line conductivity. But using e same metallization scheme as for highly P doped Si on e front side of Si solar cells for contact formation to highly B doped Si results in increased contact resistances [14, 19, 31-35]. Silicon solar cells produced from n-type Si substrates allow for higher efficiencies an ose produced from p-type Si substrate as minority charge carrier lifetimes can be significantly larger [17, 36, 37]. Additionally, n-type Si is less sensitive to impurities [37]. Emitters for n-type solar cells may be fabricated by B diffusion or by screen print of Al paste [9, 14-16, 18, 19]. The latter is suitable for rear emitters only, if e Al paste is not removed. For contact formation to B diffused emitters e same difficulties as for B BSFs occur. An additional issue is e prevention of shunting of e emitter [14, 19, 31]. Besides eir advantages, ese novel solar cell concepts introduce new problems as described above. One problem crucial to all ese concepts is e contact formation to p- and p + -Si [7-8, 14, 19-35]. Various auors have investigated different aspects of contact formation techniques to p- and p + -Si. This contribution gives an overview of recent models and e state of e art. In e first part of e contribution current models on local Al contact formation are summarized. Results from oer auors concerning e influence of process parameters on e formation of local Al contacts are discussed. The second part deals wi e impact of e doping element and e paste composition on e contact formation of screen printed Ag based metallization paste to highly p-doped Si. 2. Formation of a local Al BSF In e PERC process rear contacts are formed locally after opening e dielectric layer. The opening of e dielectric may be done by laser, by using an etching paste or by masking of e dielectric and wet chemical etching. Afterwards e rear metallization usually using Al - is carried out. The Al may be screen printed, evaporated or sputtered on e full area of e rear side or locally as lines or dots. Finally, e contacts are fired. Detailed investigations on e contact formation between Al and Si, especially wi focus on e diffusion of Si in Al, have already been carried out in semiconductor research [38-47]. Early work on e

3 16 S. Riegel et al. / Energy Procedia 21 ( 2012 ) Al-Si contact formation targeting e application in solar cells was done by Finetti et al. [48]. A model of e Al-Si alloying process wi regard to screen printed Al is given by Huster [49] and Popovich et al. [50], based on e equilibrium phase-diagram of Al and Si of Murray and McAllister [51]. A detailed analysis of e formation of e Al-Si eutectic is published by Bock et al. [52], which gives an understanding of e high acceptor concentration normally found at e surface of aluminum-doped regions. Thickness homogeneity of full area Al BSFs is achieved by fast temperature ramp rates and a proper adjustment of e peak firing temperature and e amount of Al deposited [53-55]. The first solar cells to use a local Al BSF were published by von Finckenstein et al. [56] leading to efficiencies of up to 14.1% [57]. A first analysis of local Al contact formation is given by Agostinelli et al. [6] and Meemongkolkiat et al. [20]. To our knowledge e latter are e first to observe e formation of voids in local Al contacts and showed at ey can be avoided by using a suitable Al paste. However, ey do not comment on e proper paste composition. This is done by Rauer et al. [21] who proved at adding Si powder to e Al paste prevents e formation of voids and increases e BSF ickness. Furer work targeting e influence of process steps and parameters on e contact formation is done by several auors. These investigations reveal at e meod applied for opening e dielectric layer [22, 23], e contact geometry [7, 21, 23-29], e amount of Al [22, 24-29] and e firing profile [7, 23, 24, 27] influence e contact formation significantly. An extended analysis of local Al contact formation is done for example by Uruena et al. [8] wi e result, at after opening of e dielectric barrier by laser, screen printing and firing of Al paste up to 60 μm deep Al-Si pyramids are seen. Uruena et al. [8] and Grasso et al. [30] propose simple models for e local Al alloying process which were extended by Lauermann et al. [7] and Urrejola et al. [25-29]. An analytical model allowing to calculate e Al BSF ickness is introduced by Müller et al [58] Model for local alloying In a PERC like solar cell structure e rear side is locally contacted (first published by Blakers et al. [1]). The local contact formation is often realized by screen printing Al paste on a locally opened dielectric layer [6-8, 20-30]. The Al pastes used for is purpose do not fire rough e dielectric [6-8, 20-30]. During e firing process e Al particles melt and start to alloy locally wi e Si surface [49]. Due to e high solubility of Si in Al at temperatures above e eutectic point (e.g. [51]), Si from e Si substrate diffuses into e Al paste [7, 22, 26, 29] (see Fig. 1). For a full area contact e Si concentration in e liquid Al depends on e firing temperature and reaches its maximum at e peak firing temperature [49]. In case of a local Al-Si contact a concentration gradient between e high Si concentration above e contact openings and e Si-free Al between e openings. Hence, e Si diffuses laterally into e liquid Al. At peak temperature a lateral Gaussian distribution of e Si in e Al-Si alloy can be assumed [26]. The analytical model by Müller et al. [58] includes e influence of e peak firing temperature, e area over which e dielectric layer is opened and e shape of e openings (points or lines), but e shape of e firing profile is not taken into account. While e calculated dep of e Al BSF fits well wi experimental data, e formation of voids cannot be explained wiin is model C and R C Urrejola et al. [25 C shows little variation wi peak firing temperature. But e specific contact resistivity C increases wi e dielectric barrier opening wid d 1 while e total contact resistance R C does not vary wi e dielectric barrier opening wid d 1 (see Figure 1). This may be explained by e fact, at for larger opening wid d 1 e planar surface between

4 S. Riegel et al. / Energy Procedia 21 ( 2012 ) e Al-Si alloy half circles shows a relatively high contact resistance while e Al-Si alloy at e edges provides a low contact resistance [25] Influence of e firing parameters After firing e rear side of a solar cell wi locally alloyed Al contacts shows dark-grey lines [6, 8, 26]. This is an Al-Si alloy indicating e approximate limit of e spread d 2 (see Figure 1) of Si in Al [6, 8, 26]. Urrejola et al. show at e line spreading depends linearly on e firing temperature [27]. Fig. 1. : Model for e formation of a homogenous Al BSF (left) and local Al line contacts (right) as proposed by Lauermann et al. [24]. Process time proceeds from top to bottom of e chart. The blue solid line represents e Si content as measured by [27]. The dashed line represents e Si solubility in Al at a given temperature. The dielectric barrier opening wid is named d 1. The spread of Si in e Al matrix is referred to as line spreading d 2. Besides e peak temperature e ramp-up and ramp-down rates, adjusted by e belt speed, influence e contact formation [22, 23]. The impact of e temperature ramp rates is investigated for constant peak temperature [23] as well as for constant ermal budget [22]. It depends on e shape of e dielectric barrier openings [23]. For line openings e resulting BSF becomes inner when smaller ramp rates are applied and less voids are formed [22, 23]. In contrast, for point openings e BSF becomes icker and more voids occur for smaller ramp rates [23].

5 18 S. Riegel et al. / Energy Procedia 21 ( 2012 ) Influence of e contact spacing and Al paste composition on e BSF ickness If e contact spacing is too large, voids are formed [28]. Additionally, Rauer et al. show at e BSF ickness depends on e contact spacing [21]. For contact spacing below 500 μm, e BSF ickness increases significantly from zero up to 4 μm [21]. Meemongkolkiat et al. already showed in 2006, at a suitable additive can prevent e formation of voids and increase e ickness of e local BSF [20]. But it took until 2011 at Rauer et al. published at Si powder is a suitable additive for Al paste to increase e ickness of a local Al BSF and to reduce e dep of e local contacts [21]. Additionally ey demonstrate a reduction in j 0e by increasing e Si content in e paste [21] Influence of e meod used for opening of e dielectric barrier The formation of e BSF is influenced by e meod used for opening of e dielectric barrier. Fang et al. [22] and Bähr et al. [23] observed icker and more homogenous BSFs and less voids when an etching paste was applied to open e dielectric barrier instead of laser ablation using ns ( = 1064 nm [23], 532 nm [22]) and fs ( = 1025 nm [23]) lasers. The suboptimal BSF formation after laser ablation is traced back to an increased surface damage compared to dielectric barrier opening by an etching paste. 3. Contact formation to p + Si Using n-type Si substrates e emitter formation may be done by B diffusion. In case of p-type Si substrate B diffusion is often applied for BSF formation in bifacial solar cell devices. As a finger grid + provides little shading losses it is often applied as contact formation scheme to B doped p -Si. Therefore, it is essential at e contact formation scheme allows for small contact and line resistances. Additionally, e diffusion profile must remain unchanged by contact formation. Contacting p + -Si surfaces wi Ag ick film paste results in a high C of ~ 100 m 2 [14, 19, 31, 32]. The barrier height of e Ag - p-si Schottky contact of 0.43 ev is near e critical value of 0.45 ev needed for a reasonable contact resistance [33, 59]. The barrier height can be reduced by increasing e acceptor impurity concentration under e contact [33, 59]. This can be achieved by adding Al to e Ag ick film paste and is first shown by Kopecek et al. [19]. Al contained in e metallization paste melts during e firing process. Where e liquid Al gets in contact wi e Si wafer surface it forms Al doped regions on e Si wafer surface [32]. On Al doped Si surfaces e grow of Ag crystallites and erefore contact spots is enhanced [34]. The more Al is added to e Ag ick film paste, e more contact points are detected in e SEM surface analysis and smaller C are reached [14, 31, 35]. The contact points contain a relatively large concentration of Al [32, 34, 35]. An enhanced amount of Al is found at e wafer-contact point interface [32]. If e amount of Al added to e Ag ick film paste is too high, large Al spikes are identified in e SEM surface analysis [14, 31] leading to increased leakage currents or shunting of e solar cell [31]. This is due to e fact, at Al in e Ag/Al ick film paste is strongly attracted into e Si wafer and alloys deeply wi e Si as observed for local Al contacts [7, 20-29]. The strong attraction of Al wiin e Ag/Al ick film paste by Si of e wafer can be lessened by adding some Si to e Ag/Al ick film paste as demonstrated by [14]. Thereby, e shunt conductance GSH and e current density j 01 are reduced [14], but e line resistance is increased.

6 S. Riegel et al. / Energy Procedia 21 ( 2012 ) Contact formation Contact formation wi Ag ick film paste to p + -Si (B, Al) The specific contact resistance C, Ag, p+ of pure Ag ick film paste to highly boron doped Si (N surface =9E18cm -3 ) is reported to be between 50 and 130 m 2 [14, 19, 31, 32]. After removing e Ag ick film paste in HF aq (~5%) e Ag paste Si interface contains very small Ag crystallites and inverted pyramids. Wi increasing firing temperature e number and size of e Ag crystals and inverted pyramids does not vary [32]. The pyramids are observed preferably at e <111> edges formed during sample processing by etching in hot NaOH aq [34]. Enhanced grow of Ag crystallites along <111> oriented planes is observed for e Ag ick film firing process on n +- Si as well [60, 61]. Butler et al. demonstrated by atomistic modeling at is enhanced Ag crystal grow can be explained by a reduction of e Schottky barrier of e Ag Si contact down to 0.3 ev due to different charge densities at <111> and <110> interfaces [62]. For aluminum doped surfaces (Nsurface =4E18cm -3 ) e Si wafer Ag paste interface after removing e Ag ick film paste in HF aq (~5%) looks different: The number and size of Ag crystallites is significantly increased and depends on e surface conditions of e wafers before metallization wi Ag paste [32]. If e Ag paste is screen printed on e residuals of e Al-Si eutectic resulting from Al doping, more and larger Ag crystallites are detected. The Ag crystallites cluster along e lamellas of e Al-Si eutectic. Additionally, e Ag paste penetrates into e wafer at ese structures. If e Al-doped surface is treated wi NaOH aq (N surface =3E18cm -3 ), e Ag crystallites are smaller and cover e whole wafer surface regardless of e crystal orientation of e wafer surface [34]. Fig. 2. : SEM micrographs of p + Si layers metalized wi Ag ick film paste. Ag ick film paste removed in HF aq (5%). Left: highly B doped substrate: very few inverted pyramids are visible. The pyramids are mainly located at e edges between <100> and <111> surfaces formed during saw damage removal in NaOH aq (e.g. blue oval). Middle: Al doped wafer surface, not wet chemically treated: Ag crystallites (e.g. yellow arrows) are preferably found along e lamellas (red ovals) of e Al-Si eutectic resulting from e Al alloying. Right: Al doped wafer surface etched in NaOH aq before metallization. Very small Ag crystallites cover e whole wafer surface regardless of e crystal orientation of e surface [34] Contact formation wi Ag/Al ick film paste to p -Si (B) The addition of Al to Ag ick film paste results in a reduction of e specific contact resistance C, Ag/Al, p+ down to 0.7 to 8 m 2 at optimum firing temperature [14, 19, 32, 35]. Additionally, e addition of Al to e Ag ick film paste leads to a more pronounced temperature dependency of e contact formation compared to pure Ag ick film paste. Already after drying e Ag/Al ick film paste at 290 C first contact spots are observed, even ough e PbO is not molten yet [32]. After firing at 575 C most of e PbO is molten and contact spots containing Ag and Al are visible [32]. Furermore, Al doped rectangles are found at e Si wafer surface [32]. For firing at 625 C all Al rectangles observed +

7 20 S. Riegel et al. / Energy Procedia 21 ( 2012 ) include a contact spot [32]. Increasing e firing temperature furer on does not change e contact structure, but leads to more and larger contact spots [32]. The contact spot size increases up to over 25 μm [40]. After firing at temperatures between 710 and 880 C Al precipitates at e Si wafer Ag/Al ick film paste interface [32]. Fig. 3. : Ag/Al contact after etching in HF (5%), T peak=880 C. The area marked wi e red box was measured wi an EDX scan. Red: Si, green: Ag, blue: Al. One can see at e pellets in e right part of e contact consist of silver while e walls or lamellas in e middle of e red box consist of Al [32]. Fig. 4. : SEM micrograph and EDX scan of e contact area after etching in HF (5%), T peak =800 C. Red: Si, green: Ag, blue: Al. Most part of e contact area is filled wi Ag and Al. In e lower right part of e scan area only Ag is detected. A part of e paste filling e upper contact is lifted. Therefore, e interface between Ag/Al ick film paste and e Si wafer is visible. At e Si-paste interface Al is detected (white arrow) [32] Prevention of spiking Contacting boron emitters wi Ag/Al ick film pastes gives low R S as mentioned above. But e inclusion of Al into e paste often leads to shunting of e cell or an increased j 0e as Al in e Ag/Al ick film paste is strongly attracted into e Si wafer. This problem can be overcome by adding of Si powder to e Ag/Al paste [14]. Lago et al. investigate a set of eight Ag based pastes for contacting boron

8 S. Riegel et al. / Energy Procedia 21 ( 2012 ) emitters containing different amounts of Al and Si. Large spikes (up to 5x5 μm 2 and 5 μm deep) are observed if e Al is not compensated by Si [14]. If Si is included in e paste no spikes are detected. On cell level RS is reduced from ~2.5 2 down to wi increasing Al content [14]. Regarding e influence of Si, R S is lowest for low Si content [14], but e higher e concentration of silicon in e metallization paste, e lower G SH is observed [14]. The higher e Al content in e paste e more pronounced is e dependence of G SH on e Si content in e paste [14]. The saturation current density j 01 also shows a clear dependence on e Si content in e paste. Including more Si in e paste reduces e measured j 01 [14]. 4. Summary and conclusion In is paper e existing phenomenological model for local Al contact formation and an analytical model to predict e BSF ickness depending on e peak firing temperature and e contact shape and size have been reviewed, and investigations concerning e influence of e process parameters on e contact formation have been summarized. While C does not vary wi peak firing temperature, e Al BSF ickness and e formation of voids change wi increasing ramp rates. Weaer e Al BSF formation and e formation of voids increase or decrease depends on e contact shape (lines or points). For a contact spacing smaller an 500 μm a significantly icker BSF is observed, whereby e exact value may vary wi firing parameters and Al metallization paste properties. Contact areas opened by etching wi an etching paste reveal inner and more homogenous BSFs and less voids an ose opened by laser ablation. Knowing e influence of process parameters in e contact formation allows for a phenomenological understanding of e contact formation and e definition of stable solar cell processes wi homogenous BSF formation and e avoidance of voids. Though e influence of contact shape and peak firing temperature is understood in a more quantitative way, e quantitative understanding of e influence of e temperature ramp rates and e surface conditions is still lacking. When contacting B doped Si wi Ag metallization paste, e density of Ag crystallites or inverted pyramids does not increase wi increasing peak firing temperature. Pyramids are preferably observed at e edges between <100> and <111> oriented surfaces. For Al doped Si significantly more Ag crystallites are observed an for B doped Si. The location of ese crystallites does not depend on e Si crystal orientation. Adding Al to e Ag metallization paste reduces C by two orders of magnitude. Al in e metallization paste locally dopes e Si. Wiin ese Al doped areas contact formation is enhanced and can lead to large (up to 5 μm) inverted pyramids ( spikes ). Adding Si powder to e paste prevents e formation of spikes. Gsh is increased and C and spiking are reduced by adding Al and Si powder to e Ag metallization paste. On cell level R S is increased for increasing Si content in e Ag/Al metallization paste. To e knowledge of e auors studies regarding e line conductivity of Ag metallization pastes containing Al and Si powder have not been published yet. Additionally, furer work regarding e influence of e doping element and e surface conditions on e contact formation to B doped Si is necessary. Acknowledgements The financial support from e BMU projects FKZ and FKZ is gratefully acknowledged in particular for e sample characterization. The content of is publication is e responsibility of e auors.

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