CrystalClear is a large integrated project funded by the European Commission that aims to drastically reduce the cost of crystalline Si PV modules, down to 1 Euro/Wp. Among the different subprojects, the one dealing with the development of advanced solar cells is relatively large (with 11 partners out of the 15 Crystal Clear partners taking part) and has a crucial role. The goal of the subproject is to develop cell design concepts and manufacturing processes that would enable a reduction in the order of 40% of the cell processing costs per Wp. In this paper, we give an overview of all the development work that has taken place in the CrystalClear solar cells subproject so far. World class results have been achieved, particularly on high efficiency cells on Si ribbons, and on industrial-type solar cells on very thin (120 (j.m thick) substrates

Beaucarne, G.

Bitnar, B.

Cañizo Nadal, Carlos del

Choulat, P.

Evju, J.

Hahn, G.

Haverkamp, H.

Heemeier, M.

Hofmann, M.

Horzel, J.

John, Joachim

Junge, J.

Le Quang, N.

Ma, Y.

Nagel, H.

Nichiporuk, O.

Pan, Aline Cristiane

Preu, R.

Raabe, B.

Riegel, S.

Romijn, I.

Russel, R.

Seren, S.

Slaoui, A.

Solheim, H.

Strumpel, C.

Weber, T.

Weeber, A.

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

SOLAR CELL PROCESS DEVELOPMENT IN THE EUROPEAN INTEGRATED PROJECT CRYSTALCLEAR 
G. Beaucarne1, J. John1, P. Choulat1, Y. Ma1, R. Russel2, I. Romijn3, A. Weeber3, M. Hofmann4, R. Preu4, A. Slaoui5, N. Le 
Quang6, O. Nichiporuk6, C. del Cañizo7, A. Pan7, H. Solheim8, J. Evju8, H. Nagel9, J. Horzel9, B. Bitnar10, M. Heemeier10, T. 
Weber10, B. Raabe11, H. Haverkamp11, C. Strümpel11, J. Junge11, S. Riegel11, S. Seren11, G. Hahn11'4 
'IMEC vzw, Kapeldreef75, B-3001 Leuven, Belgium 
2BP Solar 
3ECN 
4Fraunhofer Institute for Solar Energy, 
5InESS, 
6Photowatt 
7Polytechnical University of Madrid 
8REC Scancell 
9SCHOTT Solar 
10SolarWorld 
University of Konstanz 
ABSTRACT: CrystalClear is a large integrated project funded by the European Commission that aims to drastically 
reduce the cost of crystalline Si PV modules, down to 1 Euro/Wp. Among the different subprojects, the one dealing 
with the development of advanced solar cells is relatively large (with 11 partners out of the 15 Crystal Clear partners 
taking part) and has a crucial role. The goal of the subproject is to develop cell design concepts and manufacturing 
processes that would enable a reduction in the order of 40% of the cell processing costs per Wp. In this paper, we 
give an overview of all the development work that has taken place in the CrystalClear solar cells subproject so far. 
World class results have been achieved, particularly on high efficiency cells on Si ribbons, and on industrial-type 
solar cells on very thin (120 (j.m thick) substrates. 
Keywords: crystalline silicon solar cells ; processing 
1 INTRODUCTION 
CrystalClear (CC) is a large, 5-year joint effort of a 
large consortium of European companies, research 
institutes and university groups involved in crystalline 
silicon PV technology [1], see also 
www.ipcrystalclear.info. It is an Integrated Project 
carried out in the 6' Framework Program of the EU. The 
project covers the whole valué chain of Si PV up to 
module level. It therefore deals with feedstock 
crystallization, wafering, solar cell processing and 
module assembly. Apart from these main activities, there 
are two additional ones, one dealing with sustainability of 
the Si PV production, and another one dealing with the 
wafer equivalent concept, a thin-film alternative to the 
traditional bulk Si solar cells but which nevertheless 
show important similarities with conventional Si PV. 
Within CrystalClear, the subproject on solar cell 
processing is one of the largest, and involves 11 out of 
the 16 partners. The objectives of this subproject are to 
develop cell designs concepts and manufacturing 
processes that would enable a reduction in the order of 
40% of the cell processing costs per Wp. Activities are 
focused on high efficiency cell processing on low-cost 
materials and very thin wafers, and on innovative solar 
cell structures. 
2 OVERVIEW OF THE ACTIVITIES WITHIN THE 
CELL SUBPROJECT OF CRYSTALCLEAR 
The Subproject includes a variety of R&D activites 
with different implementation horizons (Figure 1). The 
activities range from quite fundamental research to large 
batch processing with part of the processes carried out in 
industrial production lines. The activities are determined 
to feed the roadmap technologies that have been 
determined in an integration effort within Crystal Clear 
[2,3] 
3 PHOTON CONVERSIÓN FOR SI CELLS 
ENHANCEMENT 
The most long-term activity investigates photon 
conversión applied to Si solar cells, evaluating the potential 
and the efficient integration in fabrication processes. The 
idea is to convert the incoming photons in a specific part of 
the spectrum into photons with different energies so that 
they can be absorbed or converted more efficiently by the 
Si solar cell. These photon converters can in principie be 
easily implemented on Si solar cells, by adding a layer on 
top (for conversión towards lower energies) or at the 
bottom of the cell (for up-conversion). This is vie wed as a 
big advantage as there is no need to modify the 
semiconductor; one can use the well-established crystalline 
Si technology as a base. 
To achieve photon shifting (shifting of very high 
energy photons towards lower energies for a better usage in 
the cell) silicon nanoparticles have been synthesized in a 
silicon nitride (SiN:H) host matrix and in a silicon 
dioxide/silicon nitride multilayer. The experiments have 
demonstrated that high temperature annealed samples with 
excess of 30% lead to the formation of percolated silicon 
particles of nanosize in width but very long chains. Bright 
photoluminescence has been observed on some samples, 
and these layers have been used in solar cell structures. 
c Closer to technology transfer I 
High efficiency 
manufacturing 
Novel processes 
forthin substrates 
High efficiency 
on low cost material 
Ultra-thin wafers 
Photon conversión 
forSi cells 
enhancement 
Inline characterization 
Advanced cell designs 
Ribbonchamp 
Superslice 
Multistar SupersIicE 
1 > Roadmap timeline 
Figure 1: Overview of the activities within the subproject and positioning of these activities with respect to 
CrystalClear roadmap technologies 
the 
Various Er-based upconverting materials have been 
tested at the rear of high efficiency bifacial cells. The most 
efficient up-converter was found to be NaYF4:Er +, with an 
EQE of up to 0.3% under excitation at 1522 nm with about 
3 mW power. A new up-convertor, Er-doped barium 
chloride was developed and prepared in the project [4]. 
4 HIGH EFFICIENCY ON LOW COST MATERIAL 
An important task in the subproject is to evalúate the 
potential of low-cost materials like RGS, EFG and block 
cast me material from alternative feedstock sources. 
For RGS material a record efficiency for 2x2 cm2 
solar cells of 14.4% could be confirmed at FhG-ISE 
CalLab. To check the influence of grain size on 
efficiency potential, a special reference material was used 
for cell processing. Multicrystalline wafers grown with 
the float zone (FZ) were used and compared with RGS 
wafers. It was concluded that that grain size in current 
RGS material does not limit efficiency in principie to 
valúes below 15.5%. Intragrain defeets (extended defeets 
like dislocations, impurities) are expected to be the 
limiting factor [5]. 
On EFG, a high efficiency process has been 
developed and further improved. A record efficiency of 
18.2% (confirmed at FhG-ISE CalLab) was achieved 
(Figure 2) [6]. This cell featured a thin Si02 layer 
underneath the SiN at the front, and an Al BSF. The 
LFC process is being tested on such cells, promising even 
better performance [7]. A screenprinting process has also 
been implemented on EFG substrates, leading to an 
efficiency of 16 % (100 cm2) on a 170 (j,m thick ribbon 
[8]. 
J _ 
a— lo .s 
RTR 
L Ú E 1.21 [W @ 9BG nm 
26. 
63^32a_25n25.lbs 
2007 04:50:10 
0 BAO nrr. i¡$ títp\ i 
1 = 20.375 x 20.4 ™ 
Figure 2: Internal Quantum Efficiency (IQE) at 980 nm 
of the best EFG solar cell (efficiency=18.2%). The cell 
shows good material quality, including some grain 
boundaries. 
5 NOVEL PROCESSES FOR THIN SUBSTRATES 
A major effort in the subproject is the development of 
new processes and solar cell structures. In the earlier part 
of CrystalClear, some very high efficiencies on relatively 
thick samples were demonstrated, including a 18.1 % 
large-area multicrystalline Si cell [9,10]. This cell 
featured a texture based on mechanical grooving, láser 
grooved buried contacts, and an Al BSF. However, for a 
few years now, the focus has been on industrially 
applicable processes adapted for very thin substrates, and 
the consensus has grown that another type of surface 
passivation than the conventional full Al BSF needs to be 
used. Over the course of CrystalClear, three solar cell 
concepts (j-PERC concept, bifacial structure with fire-
through contacts, and Láser Fired Contacts based solar 
cells) have emerged as very promising for the next 
generation of solar cells (Figure 3). 
Bifacial 
LFC 
. . . . . . . . . . . . . . . ..ft 
/-PERC 
S i i i i i i i i i i i i i i 
Figure 3 : The passivated rear surface solar cells on low 
cost substrates can be classified in three groups: bifacial 
solar cells with fire-through contacts, láser fired contact 
solar cells, and selective alloying, thermally fired LBSF 
solar cells (i-PERCs) 
These concepts have the potential for high efficiency. 
This was illustrated by a laboratory cell of 21.7 %, 
obtained with a passivating stack consisting of an a-Si 
layer and a PECVD Si02 layer, and LFC firing [11]. 
Excellent results have been achieved on very thin 
large-area multicrystalline and monocrystalline Si 
substrates (Table 1) [12,13,14]. 
Table 1 : Solar cell results for very thin 120 - 130 (xm 
thick solar cells on multi- and monocrystalline Si 
(screenprinted cells) 
Material/process 
mc-Si(156cm2) 
/ i-PERC 
Cz-Si(100cm2) 
/ i-PERC 
Cz-Si(12.5PS) 
/ bifacial, B-BSF 
[mA/cm2] 
34.7 
35.1 
36.2 
[mV 
1 
627 
633 
635 
FF 
[%] 
77.3 
79.1 
75.2 
Eff. 
[%] 
16.8 
17.6 
17.2 
Various specific process steps have been developed 
that are especially adapted to very thin substrates. As an 
example, we show in Figure 4 a study of in-line diffusion 
based on spray-on sources. These experiments revealed 
that the homogeneity of the obtained emitters was similar 
to those achieved with POCl3 diffusion. 
b) 
.0 0.5 1.0 1.5 2.0 
distance from S surface (j_im) 
.0 0.4 0.8 1.2 1.6 2.0 2.4 
distance from S surface d_im) 
Figure 4 : Emitter profile measured by secondary electrón 
SEM. Darker regions correspond to n-type silicon. a) 
spray-on in-line diffusion, b) POCl3 sample. The profiles 
are in both case about 0.5-1 um at the pyramid flank, and 
~ 1-1.5 um atthe peaks. 
The core research in solar cell processing is 
complemented with a study of the mechanical stability of 
very thin wafers (impact of different types of cracks on 
the breakage of wafers, before and after etching) [15,16]. 
6 ADVANCED BACK-CONTACT CELL DESIGNS 
While the developed processes are first implemented 
in structures with conventional front and rear contacting, 
a strong activity within the subproject concerns the 
development of solar cells with innovative structures, 
featuring all main contacts on the rear surface and having 
the potential advantages of low-cost module 
manufacturing and higher efíiciency thanks to higher 
packing density. A solar cell concept based on a bifacial 
structure with fire-through contacts combined with a 
Metallization Wrap-Through structure (ASPIRe cell) was 
successfully demonstrated, with an efíiciency of 16.4 % 
(180 \im thick, 243 cm2) [17]. On very thin (130 (j,m 
thick) large área (15.6 x 15.6 cm2) substrates efíiciencies 
up to 15.5 % were reached. 
Figure 5: Photo of the front and the rear of an ASPIRe 
cell 
An EWT cell process featuring only industrially 
applicable process steps was also developed and 
implemented on Cz, resulting in an efficiency of 17.3 % 
sofar[18]. 
3 - lOQcm, Cz. tcxtured, elcaned 
5 N deposition on rear {serves as diffusion 
Scrccn Printing and drying / firing of Ctch F 
Patlcming of SiN in HF 
Romoval of Ctch Rosist, Clcaning of Wafer 
POCIj, 50í2/sq Standard Emiltcr Diffusion 
| Phosphorus Glas and roma ¡ni ng SiN Removí 
| Rear Sidc PECVO SiN - |FSF Etch Back) - Fro 
L _ ^ B _ _ ^ _ _ J Screcn Printing + Firing of N-Contacts 
L _ _ _ _ _ J Al evaporation and LF of Base Contacts 
Figure 6 : Processing scheme of EWT cells 
7 ULTRA-THIN WAFERS 
The CrystalClear roadmap has focused on substrates 
with thickness of 120 |i.m, and defined the roadmap 
technologies for those substrates. Therefore, most of the 
R&D cell activities have been geared towards this 
thickness. Nevertheless, it is interesting to investígate 
whether the developed technologies can be pushed 
further to ultra-thin substrates, 100 (j,m and below. This 
somewhat longer term activity may give a hint as to how 
long crystalline Si will be the dominant PV technology. 
Interestingly, it appears that some of the concepts for 
rear passivated cells can be implemented on such 
extremely thin wafers without major process issues. On 
a 4 inch round Cz wafer (p-type) with a thickness of only 
80 (xm, an j-PERC solar cell was made with an efíiciency 
of 16.6 % (35.6 mA/cm2, 632 mV, FF 75.8 %) [19]. 
Figure 7 : Extremely thin (80 (j,m) screenprinted solar cell 
with an efíiciency of 16.6 % 
8 CHARACTERIZATION 
The development of new characterization techniques 
has been an integral part of the subproject. An important 
focus has been the development of techniques that can be 
implemented in-line. For instance, the illuminated Lock-
In thermography technique was successfully adapted to 
make it suitable for in-line characterization [20]. 
3s 2s 
I i 
Figure 8 : iLIT measurements at 1 Hz, 1 sun and Voc for 
varying measurement times 
Several round robins on characterization and 
processing have also been organized, which give a very 
good view on the stand of each tested technique, as the 
consortium is a quite large and relevant sample of the PV 
'population'. A comparison of the illuminated IV results 
for instance revealed a significant spread in the results 
obtained, though a check with a calibration laboratory 
revealed that the CrystalClear partners tended to 
underestimate the quoted cell efficiencies. 
9 HIGH EFFICIENCY MANUFACTURING 
The three different solar cell concepts for industrial 
type rear passivated solar cells are presently being 
implemented in joint experiments between institutes and 
companies. This involves large batches of wafers which 
are partly processed in production lines and partly in the 
R&D facilities in the institutes. The purpose is to start 
assessing the issue for industrial application, as well as 
producing a sufíicient number of cells for module 
assembly research. 
10 CONCLUSIÓN 
CrystalClear involves a large volume of R&D on solar 
cell processing. World class results and world records 
have been achieved, particularly with industrial cells on 
very thin wafers and with ribbons. The project has been 
effective in enhancing collaboration and communication 
between some major PV partners in the EU. The fruits 
of the research are expected to be reaped in the next few 
years, as the developed technologies will come on-line in 
industrial production lines. 
11 ACKNOWLEDGEMENTS 
CrystalClear (contract SES6-CT-2003-502583) is an FP6 
Integrated Project partly funded by the European 
Commission. 
We thank the CrystalClear coordinator, Wim Sinke, as 
well as the whole of the Project Management office for 
the excellent work and trust. 
Crystal Clear is a long project. As a result, the people 
involved did not remain the same throughout the course 
of the project, as the researchers sometimes moved on to 
new jobs. We therefore thank all the CrystalClear 
alumni, who are not in the authors' list but did bring 
important contributions to the subproject. Particular 
thanks to the previous Subproject Leaders : Guido 
Agostinelli, Kristian Peter, Peter Fath and Eric Rüland. 
12. REFERENCES 
[1] W. C. Sinke, "The CRYSTALCLEAR Integrated 
Project: next generation crystalline silicon technology 
from lab to production", Proceedings 20 European 
Photovoltaic Solar Energy Conference (2005). 
[2] W. C. Sinke, Proceedings 22n European 
Photovoltaic Solar Energy Conference (2007). 
[3] W. Sinke, G. del Coso, C. del Cañizo, "1 € per Watt-
peak advanced cristalline silicon modules The 
CrystalClear integrated project", these proceedings 
[4] C. Strümpel, G. Hahn, M. J. McCann, "Influence of 
hygroscopy on the optical properties of the up-converter 
BaCl2:Er3+", these proceedings 
[5] U. Hess, T. Lauermann, S. Seren, G. Hahn, A. 
Schónecker, "Impact of the crystal structure on solar cell 
parameters of Ribbon Growth on Substrate (RGS) solar 
cells", these proceedings 
[6] M. Kaes, G. Hahn, A. Metz, G. Agostinelli, Y. Ma, J. 
Junge, A. Zuschlag, D. Groetschel, "Progress in high 
efficiency processing of EFG silicon solar cells," 
Proceedings 22nd European Photovoltaic Solar Energy 
Conference (2007) 897-902. 
[7] J. Junge, A. Zuschlag, S. Seren, G. Hahn, M. Kaes, A. 
Metz, „Laser fired contacts for high efficiency solar cells 
based on EFG material", these proceedings 
[8] P. Choulat, Y. Ma, J. John, B. T. Chan, G. 
Agostinelli, H. Nagel, J. Horzel, S. Forment, G. 
Beaucarne, "High efficiency industrial type PERC Solar 
Cell on very thin EFG Substrates", these proceedings 
[9] M. McCann, B. Raabe, W. Jooss, R. Kopecek, P. 
Fath, "18.1% efficiency for a large área, multi-crystalline 
silicon solar cell," Proceedings 4th World Conference on 
Photovoltaic Energy Conversión (2006) 894-899. 
[10] A. W. Weeber, C. J. J. Tool, P. Manshanden, H. 
Tathgar, O. Gjerstad, M. McCann, B. Raabe, F. Huster, 
P. Fath, S. Ponce-Alcantara, J. Coello, C. del Cañizo, S. 
Roberts, T. M. Bruton, K. C. Heasman, S. Devenport, H. 
Nagel, B. Lenkeit, W. Schmidt, R. Russel, "Record cell 
efficiencies on mc-Si and a roadmap towards 20 %, the 
EC project TOPSICLE," Proceedings 21st European 
Photovoltaic Solar Energy Conference and Exhibition 
(2006). 
[11] M. Hofmann, S. Glunz, R. Preu and G. Willeke, 
"21 %-Efficient Silicon Solar Cells Using Amorphous 
Silicon Rear Side Passivation, Proceedings of the 21st 
European Photovoltaic Energy Conference", Proceedings 
21 st European Photovoltaic Solar Energy Conference and 
Exhibition (2006) p. 609 
[12] M. Hofmann, D. Erath, B. Bitnar, L. Gautero, J. 
Nekarda, A. Grohe, D. Biro, J. Rentsch, R. Preu, 
"Industrial type Cz solar cells with screen-printed fine 
line front contacts and passivatied rear contacted by láser 
firing", these proceedings 
[13] G. Agostinelli, P. Choulat, H.F.W. Dekkers, Y. Ma, 
G. Beaucarne, "Silicon solar cells on ultra-thin substrates 
for large scale production", Proceedings 21s t European 
Photovoltaic Solar Energy Conference (2006) 601-604. 
[14] P. Choulat, G. Agostinelli, Y. Ma, F. Duerinckx, G. 
Beaucarne, "Above 17 % industrial type PERC Solar Cell 
on thin multi-crystalline silicon substrate," Proceedings 
22n European Photovoltaic Solar Energy Conference 
(2007). 
[15] J. Gustafsson, H. Larsson, T. Bostróm, H. J. 
Solheim, "Mechanical stress tests on mc-Si wafers with 
microcracks", these proceedings 
[16] H. Larsson, J. Gustafsson, T. Bostróm, H. J. 
Solheim, "The impact of saw damage etching on 
microcracks in solar cell production", these proceedings 
[17] I.G. Romijn, A.A. Mewe, M.W.P.E. Lamers, A.F. 
Stassen, E. Kossen, I.J. Bennett, E.E. Bende, A.W. 
Weeber, "An Overview of MWT Cells and Evolution to 
the ASPIRe Concept: A New Integrated mc-Si Cell and 
Module Design for High-Efficiencies", these proceedings 
[18] H. Haverkamp, G. Hahn, "Development of a high 
throughput masking scheme for emitter wrap through 
cells", these proceedings 
[19] Y. Ma, P. Choulat, G. Agostinelli, J. John, X. 
Loozen, G. Beaucarne, "Industrial-type PERC solar cell: 
towards 80 micron thickness", these proceedings 
[20] S. Seren, G. Hahn, M. Kas, H. Nagel, "Shunt 
detection with illuminated lock-in thermography on 
inline relevant time scales", these proceedings 


Solar cell process development in the european integrated project crystalclear

Archivo Digital UPM

http://oa.upm.es/3588/2/INVE_MEM_2008_55839.pdf

Solar cell process development in the european integrated project crystalclear

Abstract

Similar works

Full text

Available Versions

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

Archivo Digital UPM