Presently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- And y-axes of our graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are many different ways to achieve 22%. Our modelling predicts, for example, that 60 μm wide screen-printed metal fingers are sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90 μm. The rear may be contacted with 30 μm wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime of at least 1 ms at excess carrier densities near 10(14) cm(-3)

Altermatt, Pietro P.

McIntosh, Keith R.

AbstractPresently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- and y-axes of our graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are many different ways to achieve 22%. Our modelling predicts, for example, that 60μm wide screen-printed metal fingers are sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90μm. The rear may be contacted with 30μm wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime of at least 1ms at excess carrier densities near 1014 cm-3

Elsevier - Publisher Connector 

A Roadmap for PERC Cell Efficiency towards 22%, Focused on Technology-related Constraints 

Pietro P. Altermatt

Keith R. McIntosh

Crossref

Energy Procedia

A Roadmap for PERC Cell Efficiency towards 22%, Focused on Technology-related Constraints

 Energy Procedia  55 ( 2014 )  17 – 21 
Available online at www.sciencedirect.com
ScienceDirect
1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference
doi: 10.1016/j.egypro.2014.08.004 
4th International Conference on Silicon Photovoltaics, SiliconPV 2014 
A roadmap for PERC cell efficiency towards 22%, focused on 
technology-related constraints 
Pietro P. Altermatta, Keith R. McIntoshb 
a Dep. Solar Energy, Inst. Solid-State Physics, Leibniz University Hannover, Appelstr. 2, 30167 Hannover, Germany 
b PV Lighthouse,Coledale, 2512 NSW, Australia   
Abstract 
Presently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell 
efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. 
Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as 
possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, 
such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- and y-axes of our 
graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are 
many different ways to achieve 22%. Our modelling predicts, for example, that 60 m wide screen-printed metal fingers are 
sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90 m. The rear may be contacted with 30 m 
wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, 
they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime 
of at least 1 ms at excess carrier densities near 1014 cm-3. 
 
© 2014 The Authors. Published by Elsevier Ltd. 
Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference. 
Keywords: Device modeling; PERC cells; roadmap 
1. Evolutionary or revolutionary development? 
In the past, the c-Si PV industry went from 14% to 18% efficiency on an evolutionary path, where only single 
fabrication processes were optimised or altered. It is presently going from about 18% towards 20% by switching 
from standard cells to PERC cells. The switch cannot be achieved by an evolutionary approach, because both the 
front and the rear must be improved at the same time to make PERC design effective. Therefore, PERC cells need 
some new fabrication equipment. We show in this paper, however, that going beyond 20% can be achieved on an 
© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference
18   Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 
evolutionary path again. It is therefore the main purpose of this abstract to give quantitative foundations for choosing 
the new PERC production equipment so it stays useful in the near future for achieving 22% efficiency. For our 
predictions, we use state-of-the-art SENTAURUS [ 1 ] device modelling, combined with the EDNA and GRID 
calculators [2], and SPICE models. 
 
2. Reaching 22% efficiency on a standard path 
From the Shockley diode equation, it follows that to reach 22% efficiency, the total saturation current J0 must be 
no more than 200 fA/cm2. This conclusion arises by assuming Jsc = 40 mA/cm2, an ideality factor m = 1, and a total 
Rs = 0.4 ·cm2.  (A lower J0 is required if m > 1 or if Rs is larger.)  Since the Shockley equation serves only as a 
rough guide, we do not discuss its assumptions. 
Having set an upper limit to the J0 of a 22% solar cell, we next set a target J0 for the three major components of 
the solar cell:  the emitter J0e, the base J0,base, and the rear J0,rear.  We can then examine how those targets might be 
met by today’s equipment—or perhaps tomorrow’s equipment. 
In the following example, the target J0 of the three components is chosen to be J0e  80, J0,base  50, and J0,rear  70 
fA/cm2, but there is freedom in dividing the components. 
2.1. Constraints on the emitter 
The J0e of today's emitters is about 150-200 fA/cm2.  In our quest for J0e = 80 fA/cm2 and a 22% solar cell, it must 
be susbtationally improved. The J0e depends not only on the emitter’s dopant profile, but also on the amount of 
phosphorus precipitates [  3 ] introduced during fabrication, and on the quality of the surface passivation [4]. 
Because it is difficult to reach 22% cell efficiency with a homogeneous emitter and conventional screen-printing, 
we choose a selective emitters. J0e consists of three parts: the metal contacts J0,met , the passivated n++ part J0,n++ , and 
the passivated n+ part J0,n+, which sum to give J0e respective to their area fraction: 
 
 f met n++n++0 0,met 0,n++ 0,n+
f f f
– – e met
w p w wwJ J J J
p p p
     (1) 
 
where pf is the front finger pitch, wmet and wn++ are the width of the contact or n++ region, respectively. In the 
following, we assume that the emitter is fabricated with a POCl3 diffusion, and we use the experimental values from 
a recent study [5 ] shown in Fig. 1(a). Using these values, we optimise Eq. (1) and cross-check with SENTAURUS. 
 
Fig.1.  Experimentally achieved J0e by POCl3 diffusions [5] in dependence of em. 
 Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 19
 
For the simulation of selective emitters, we examine how the J0e depends on two technological constraints.  
Firsly, the alignment tolerance of the metal fingers wtol (in units of m) is varied; this tolerance implies that the n++ 
region must be wider than the metal contact by wtol. And secondly, the width of the metal contacts wf is varied. Fig. 2 
presents the results, giving a contour plot of J0e and n++ as as a function of wtol (x-axis) and wf (y-axis).  It was 
simulated for pf = 1.1 mm, J0,n+ = 60 fA/cm2 and ρn+ ≈ 120 Ω/sq. 
In Fig. 2, we see that as wtol decreases, the optimum n++ decreases and a lower J0e is attained.  We find that, for 
example, the total J0e = 80 fA/cm2 (or 90 fA/cm2) can indeed be achieved with 60 m wide metal contacts, if the 
alignement tolerance is maximally wtol  50 m (or 90 m). In this case, n++  40 /sq (or  50 /sq) is optimum. 
Note, how sensitively the wtol impacts on J0e. However, an alignment tolerance down to 30 m seems to be feasible 
[6]. 
 
 
Fig. 2. Simulation of a selective emitter with Jn+ = 60 fA/cm2. The minimally achievable total J0e (solid lines) is plotted with its corresponding 
optimum n++ (dashed lines), in dependence of the alignment tolerance of the front metal finger, and the width of the front contact (the 
technological restrictions). A front finger distance of pf = 1.1 mm is assumed (from Fig. 3). 
2.2. Optimization of the metal grid 
Optimizing the finger pitch is the next task. It affects the resistive losses in the emitter and in the front 
metallization, as well as Jsc, and is therefore a highly non-linear optimization task. The series resistance contribution 
(in units of Ωcm2) of the selective emitter can be calculated as 
 
 
2
2 2n+ n+
. n++ n++ n+ n+ n+ cell2 2
f f bb
1 1
3 (2 2 )s em
L LR L L L L A
Ll n n L L
 	

 
    
 
 
 
 
 
  (2) 
 
where lf is the finger length, nf (nbb) is the number of fingers (busbars) in the cell, L is half of the finger separation 
(which is the longest way electrons travel laterally in the emitter), Acell the cell area, and there is Ln+ = wn+/2, Ln++ = 
wn++/2, and L = pf – wmet. 
We now determine how the efficiency of the solar cell depends on technological constraints placed by the screen-
printing technology.  The results are plotted in Fig. 3(a), which plots efficiency contours against the finger width wf 
(x-axis) and the finger height (y-axis).  In these simulations, series resistance contribution of the metal grid is 
calculated with the freeware GRID [7] for a standard resistivity of screen-printed Ag fingers of 4.510-6 Ωcm; the 
shading is assumed to be 50% of the finger coverage and 90% of the busbar coverage, as was measured in a module 
[8]. For each simulation, the optimal finger pitch is selected, as plotted by dashed lines. 
Fig. 3(a) shows that with 60 μm wide and about 25 μm high fingers, 22% is possible if the front finger pitch pf 
20   Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 
is about 1.1 mm. Narrower fingers (and accordingly smaller pf) are only necessary if going above 22% cell 
efficiency, and instead, multi-wire busbars would be similarly effective.  
  
Fig. 3. (a) Predicted optimum front finger pitch pf (blue dashed lines) with the resulting maximally achievable cell efficiency (red solid lines) in 
dependence of the front finger width and front finger height (the technological restrictions). (b) Simulated demands of the rear metallisation, BSF 
and passivation: the optimum rear finger pitch (blue dashed lines) with the resulting maximally achievable cell efficiency (red lines). Note that 
these predictions depend on the emitter quality. 
2.3. Optimization of the rear 
To achieve 22% cell efficiency, the losses in the local BSFs and at the rear surface should combine to give a J0,rear 
of no more than 70 fA/cm2 (as stated in the beginning). Fig. 3(b) shows how this may be achieved assuming 30 m 
wide rear contact openings.  For example, a J0,rear of 70 fA/cm2 and therefore an efficiency of 22.0% arises when 
J0,BSF  800 fA/cm2 and Seff  10 cm/s, where the latter has been experimenally demonstrated for Al2O3/SiNx 
passivation.  In these simulations, the rear finger pitch that yields the maximised efficiency is selected—this is 
plotted as the dashed lines in Fig.3(b).  In the example case, the optimum rear finger distance then is 850 μm, but it 
may be chosen as wide as 1.2 mm without compromising on cell efficiency (not shown here). 
We note that with Seff =100 cm/s, only 21.5% efficiency is achievable, unless the emitter or the front 
metallization are made better than in the example of Fig. 2. 
2.4. Wafer quality requirements 
In all of our simulations, we assume a SRH lifetime  in the wafer of 1 – 2 ms. With lower ,  the equivalent J0,base 
exceeds 40 fA/cm2. This lifetime is presently achievable in boron-doped Cz materials by applying deactivation 
procedures [9]. It has been also achieved in Ga-doped or magnetic Cz materials. 
3. Conclusions 
To reach 22% cell efficiency, 60 μm wide and 25 μm high front metal fingers with a pitch of about 1.1 mm are 
sufficient, if an emitter saturation current density J0e = 80 fA/cm2 (including contacts) is realized, e.g. by a selective 
emitter with 60 μm wide n++ regions (with ρn++ ≈ 40 Ω/sq), and with an n+ region that has J0,n+ = 60 fA/cm2, and ρn+ 
≈ 120 Ω/sq. At the rear, Seff < 30 cm/s is favourable, as otherwise the front must be improved to compensate. Such a 
rear structure may be realised by a passivation with S = 10 cm/s, about 30 μm wide openings with a pitch near 1 mm, 
and local Al-BSFs with J0,BSF < 700 fA/cm2 . The main issue is  the bulk lifetime : it must reach about 2 ms at the 
 Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 21
prevailing excess carrier densities at MPP, which are near 11013 cm-3 (and at open-circuit conditions about 21015 
cm-3) in 1Ωcm material. 
References 
 
[1] Sentaurus User Manual, Synopsys Inc., Mountain View, CA, 2013. 
[2] McIntosh KR, Altermatt PP. A freeware 1D emitter model for silicon solar cells. 35th IEEE PVSC, Honolulu, 2010, pp. 2188–2193. Freely 
available online: www.pvlighthouse.com.au 
[3] Min B, Wagner W, Dastgheib-Shirazi A, Altermatt PP. Limitation of industrial phosphorus-diffused emitters by SRH recombination. This 
conference. 
[4] Altermatt PP, Schumacher JO, Cuevas A, Kerr MJ, Glunz SW, King RR, Heiser G, Schenk A. Numerical modelling of highly doped Si:P 
emitters based of Fermi-Dirac statistics and self-consistent material parameters. J Appl Phys 2002;92:3187–97. 
[5] Fischer B, Müller J, Altermatt PP. A simple emitter model for quantum efficiency curves and extracting the emitter saturation current. 28th 
EU Photovoltaic Solar Energy Conference, Paris, France; 2013. p. 840–45. 
[6] Voltan A, Galiazzo M, Tonini D, Casarin A, Cellere G and Baccini A. Advanced alignment technique for precise printing over selective 
emitter in c-Si solar cells. Prog PV 2012;20:670–80. 
[7] Grid calculator, freely available online: www.pvlighthouse.com.au 
[8] Woehl R, Hörteis M, Glunz SW. Determination of the effective optical width of screen-printed and aerosol-printed and plated fingers. 23rd 
EU Photovoltaic Solar Energy Conference, Valencia, Spain; 2008. p. 1377–80. 
[9] Walter DC, Lim B, Bothe K, Voronkov VV, Falster R, Schmidt J. Effect of rapid thermal annealing on recombination centres in boron-doped 
Czochralski-grown silicon. Appl Phys Lett 2014;104:042111. 


Institutionelles Repositorium der Leibniz Universität Hannover

 Energy Procedia  55 ( 2014 )  17 – 21 Available online at www.sciencedirect.comScienceDirect1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conferencedoi: 10.1016/j.egypro.2014.08.004 4th International Conference on Silicon Photovoltaics, SiliconPV 2014 A roadmap for PERC cell efficiency towards 22%, focused on technology-related constraints Pietro P. Altermatta, Keith R. McIntoshb a Dep. Solar Energy, Inst. Solid-State Physics, Leibniz University Hannover, Appelstr. 2, 30167 Hannover, Germany b PV Lighthouse,Coledale, 2512 NSW, Australia   Abstract Presently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- and y-axes of our graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are many different ways to achieve 22%. Our modelling predicts, for example, that 60 m wide screen-printed metal fingers are sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90 m. The rear may be contacted with 30 m wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime of at least 1 ms at excess carrier densities near 1014 cm-3.  © 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference. Keywords: Device modeling; PERC cells; roadmap 1. Evolutionary or revolutionary development? In the past, the c-Si PV industry went from 14% to 18% efficiency on an evolutionary path, where only single fabrication processes were optimised or altered. It is presently going from about 18% towards 20% by switching from standard cells to PERC cells. The switch cannot be achieved by an evolutionary approach, because both the front and the rear must be improved at the same time to make PERC design effective. Therefore, PERC cells need some new fabrication equipment. We show in this paper, however, that going beyond 20% can be achieved on an © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference18   Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 evolutionary path again. It is therefore the main purpose of this abstract to give quantitative foundations for choosing the new PERC production equipment so it stays useful in the near future for achieving 22% efficiency. For our predictions, we use state-of-the-art SENTAURUS [ 1 ] device modelling, combined with the EDNA and GRID calculators [2], and SPICE models.  2. Reaching 22% efficiency on a standard path From the Shockley diode equation, it follows that to reach 22% efficiency, the total saturation current J0 must be no more than 200 fA/cm2. This conclusion arises by assuming Jsc = 40 mA/cm2, an ideality factor m = 1, and a total Rs = 0.4 ·cm2.  (A lower J0 is required if m > 1 or if Rs is larger.)  Since the Shockley equation serves only as a rough guide, we do not discuss its assumptions. Having set an upper limit to the J0 of a 22% solar cell, we next set a target J0 for the three major components of the solar cell:  the emitter J0e, the base J0,base, and the rear J0,rear.  We can then examine how those targets might be met by today’s equipment—or perhaps tomorrow’s equipment. In the following example, the target J0 of the three components is chosen to be J0e  80, J0,base  50, and J0,rear  70 fA/cm2, but there is freedom in dividing the components. 2.1. Constraints on the emitter The J0e of today's emitters is about 150-200 fA/cm2.  In our quest for J0e = 80 fA/cm2 and a 22% solar cell, it must be susbtationally improved. The J0e depends not only on the emitter’s dopant profile, but also on the amount of phosphorus precipitates [  3 ] introduced during fabrication, and on the quality of the surface passivation [4]. Because it is difficult to reach 22% cell efficiency with a homogeneous emitter and conventional screen-printing, we choose a selective emitters. J0e consists of three parts: the metal contacts J0,met , the passivated n++ part J0,n++ , and the passivated n+ part J0,n+, which sum to give J0e respective to their area fraction:   f met n++n++0 0,met 0,n++ 0,n+f f f– – e metw p w wwJ J J Jp p p     (1)  where pf is the front finger pitch, wmet and wn++ are the width of the contact or n++ region, respectively. In the following, we assume that the emitter is fabricated with a POCl3 diffusion, and we use the experimental values from a recent study [5 ] shown in Fig. 1(a). Using these values, we optimise Eq. (1) and cross-check with SENTAURUS.  Fig.1.  Experimentally achieved J0e by POCl3 diffusions [5] in dependence of em.  Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 19 For the simulation of selective emitters, we examine how the J0e depends on two technological constraints.  Firsly, the alignment tolerance of the metal fingers wtol (in units of m) is varied; this tolerance implies that the n++ region must be wider than the metal contact by wtol. And secondly, the width of the metal contacts wf is varied. Fig. 2 presents the results, giving a contour plot of J0e and n++ as as a function of wtol (x-axis) and wf (y-axis).  It was simulated for pf = 1.1 mm, J0,n+ = 60 fA/cm2 and ρn+ ≈ 120 Ω/sq. In Fig. 2, we see that as wtol decreases, the optimum n++ decreases and a lower J0e is attained.  We find that, for example, the total J0e = 80 fA/cm2 (or 90 fA/cm2) can indeed be achieved with 60 m wide metal contacts, if the alignement tolerance is maximally wtol  50 m (or 90 m). In this case, n++  40 /sq (or  50 /sq) is optimum. Note, how sensitively the wtol impacts on J0e. However, an alignment tolerance down to 30 m seems to be feasible [6].   Fig. 2. Simulation of a selective emitter with Jn+ = 60 fA/cm2. The minimally achievable total J0e (solid lines) is plotted with its corresponding optimum n++ (dashed lines), in dependence of the alignment tolerance of the front metal finger, and the width of the front contact (the technological restrictions). A front finger distance of pf = 1.1 mm is assumed (from Fig. 3). 2.2. Optimization of the metal grid Optimizing the finger pitch is the next task. It affects the resistive losses in the emitter and in the front metallization, as well as Jsc, and is therefore a highly non-linear optimization task. The series resistance contribution (in units of Ωcm2) of the selective emitter can be calculated as   22 2n+ n+. n++ n++ n+ n+ n+ cell2 2f f bb1 13 (2 2 )s emL LR L L L L ALl n n L L 	            (2)  where lf is the finger length, nf (nbb) is the number of fingers (busbars) in the cell, L is half of the finger separation (which is the longest way electrons travel laterally in the emitter), Acell the cell area, and there is Ln+ = wn+/2, Ln++ = wn++/2, and L = pf – wmet. We now determine how the efficiency of the solar cell depends on technological constraints placed by the screen-printing technology.  The results are plotted in Fig. 3(a), which plots efficiency contours against the finger width wf (x-axis) and the finger height (y-axis).  In these simulations, series resistance contribution of the metal grid is calculated with the freeware GRID [7] for a standard resistivity of screen-printed Ag fingers of 4.510-6 Ωcm; the shading is assumed to be 50% of the finger coverage and 90% of the busbar coverage, as was measured in a module [8]. For each simulation, the optimal finger pitch is selected, as plotted by dashed lines. Fig. 3(a) shows that with 60 μm wide and about 25 μm high fingers, 22% is possible if the front finger pitch pf 20   Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 is about 1.1 mm. Narrower fingers (and accordingly smaller pf) are only necessary if going above 22% cell efficiency, and instead, multi-wire busbars would be similarly effective.    Fig. 3. (a) Predicted optimum front finger pitch pf (blue dashed lines) with the resulting maximally achievable cell efficiency (red solid lines) in dependence of the front finger width and front finger height (the technological restrictions). (b) Simulated demands of the rear metallisation, BSF and passivation: the optimum rear finger pitch (blue dashed lines) with the resulting maximally achievable cell efficiency (red lines). Note that these predictions depend on the emitter quality. 2.3. Optimization of the rear To achieve 22% cell efficiency, the losses in the local BSFs and at the rear surface should combine to give a J0,rear of no more than 70 fA/cm2 (as stated in the beginning). Fig. 3(b) shows how this may be achieved assuming 30 m wide rear contact openings.  For example, a J0,rear of 70 fA/cm2 and therefore an efficiency of 22.0% arises when J0,BSF  800 fA/cm2 and Seff  10 cm/s, where the latter has been experimenally demonstrated for Al2O3/SiNx passivation.  In these simulations, the rear finger pitch that yields the maximised efficiency is selected—this is plotted as the dashed lines in Fig.3(b).  In the example case, the optimum rear finger distance then is 850 μm, but it may be chosen as wide as 1.2 mm without compromising on cell efficiency (not shown here). We note that with Seff =100 cm/s, only 21.5% efficiency is achievable, unless the emitter or the front metallization are made better than in the example of Fig. 2. 2.4. Wafer quality requirements In all of our simulations, we assume a SRH lifetime  in the wafer of 1 – 2 ms. With lower ,  the equivalent J0,base exceeds 40 fA/cm2. This lifetime is presently achievable in boron-doped Cz materials by applying deactivation procedures [9]. It has been also achieved in Ga-doped or magnetic Cz materials. 3. Conclusions To reach 22% cell efficiency, 60 μm wide and 25 μm high front metal fingers with a pitch of about 1.1 mm are sufficient, if an emitter saturation current density J0e = 80 fA/cm2 (including contacts) is realized, e.g. by a selective emitter with 60 μm wide n++ regions (with ρn++ ≈ 40 Ω/sq), and with an n+ region that has J0,n+ = 60 fA/cm2, and ρn+ ≈ 120 Ω/sq. At the rear, Seff < 30 cm/s is favourable, as otherwise the front must be improved to compensate. Such a rear structure may be realised by a passivation with S = 10 cm/s, about 30 μm wide openings with a pitch near 1 mm, and local Al-BSFs with J0,BSF < 700 fA/cm2 . The main issue is  the bulk lifetime : it must reach about 2 ms at the  Pietro P. Altermatt and Keith R. McIntosh /  Energy Procedia  55 ( 2014 )  17 – 21 21prevailing excess carrier densities at MPP, which are near 11013 cm-3 (and at open-circuit conditions about 21015 cm-3) in 1Ωcm material. References  [1] Sentaurus User Manual, Synopsys Inc., Mountain View, CA, 2013. [2] McIntosh KR, Altermatt PP. A freeware 1D emitter model for silicon solar cells. 35th IEEE PVSC, Honolulu, 2010, pp. 2188–2193. Freely available online: www.pvlighthouse.com.au [3] Min B, Wagner W, Dastgheib-Shirazi A, Altermatt PP. Limitation of industrial phosphorus-diffused emitters by SRH recombination. This conference. [4] Altermatt PP, Schumacher JO, Cuevas A, Kerr MJ, Glunz SW, King RR, Heiser G, Schenk A. Numerical modelling of highly doped Si:P emitters based of Fermi-Dirac statistics and self-consistent material parameters. J Appl Phys 2002;92:3187–97. [5] Fischer B, Müller J, Altermatt PP. A simple emitter model for quantum efficiency curves and extracting the emitter saturation current. 28th EU Photovoltaic Solar Energy Conference, Paris, France; 2013. p. 840–45. [6] Voltan A, Galiazzo M, Tonini D, Casarin A, Cellere G and Baccini A. Advanced alignment technique for precise printing over selective emitter in c-Si solar cells. Prog PV 2012;20:670–80. [7] Grid calculator, freely available online: www.pvlighthouse.com.au [8] Woehl R, Hörteis M, Glunz SW. Determination of the effective optical width of screen-printed and aerosol-printed and plated fingers. 23rd EU Photovoltaic Solar Energy Conference, Valencia, Spain; 2008. p. 1377–80. [9] Walter DC, Lim B, Bothe K, Voronkov VV, Falster R, Schmidt J. Effect of rapid thermal annealing on recombination centres in boron-doped Czochralski-grown silicon. Appl Phys Lett 2014;104:042111. 

A roadmap for PERC cell efficiency towards 22%, focused on technology-related constraints

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A roadmap for PERC cell efficiency towards 22%, focused on technology-related constraints

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Institutionelles Repositorium der Leibniz Universität Hannover