Cells used with concentrators have similar contact requirements to other cells, but operation at high intensity imposes more than the usual demands on the metallization. Overall contact requirements are listed and concentrator cell requirements are discussed

Iles, P. A.

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METALLIZATION PROBLEMS WITI-! COhCENTRATOR CELLS 
P.A. Iles N84 21990 
APPLIED SOLAR ENERGY CORPORATIOK 
City Of Industry, California 91749 
INTRODUCTION 
Cells used with concentrators have similar contact requirements to other cells, 
but operation at high intensity imposes more than the usual demands on the  
metallization. 
Table 1 lists the overall contact  requirements. W e  will discuss how concentrator 
cells can meet these requirements. 
REDUCING RESISTIVE LOSSES 
Since concentrator cells operate at high current density, any resistive losses a r e  
more severe. 
Resistive Loss Model . (See Reference I +  f o r  previous references.) 
The procedure used to design concentrator cells is as follows (see Figure I). 
Select the intrinsic cell design best suited to t h e  planned application -maximize 
collected current and voltage, minimize reflective and bulk resistance losses. 
These terms a r e  derived from previous experience gained in optimizing the  cell 
design. 
Select ca,tacts with low contact resistance, and design a grid pattern using 3-7 
below. 
The various resistive components (contact, front sheet, gridline, bulk) a r e  
analyzed as a network of resistors. This analysis generally uses parallel lines for 
rectangular or square cells, sometimes with the  cell divided into quandrants; for 
circular cells, radial lines are used, often with increasing line density towards 
the outer edge (bus contact)  of the cell. 
The resistive losses a r e  computed, and transformed into power losses e.g. by 
taking their ratio to the  internal impedance of the cell  at the  operating intensity 
X (this impedance = @.9 Vocx/Isc,). This impedance increases just less than 
linearly with X ( -  0.84X for Si, ), so that  the  relative 
percentage loss caused by resistive components increases at this same ratio. The 
shading loss does not vary with X, and is determined by the areal grid 
dimensions. 
The operating CFF of the cell under concentration is computed by subtracting 
the total resistive losses from an initial "ideal" CPF. 
The shading reduces the generated current density to the actual cell current 
den sit y . 
0.89X for CaAs 
*P.A. Iles "A Survey of Grid Technology" to be published in Confereace Rec. of 
16th Photovoltaic Specialists Conference, 1982. 
159 
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https://ntrs.nasa.gov/search.jsp?R=19840013922 2020-03-20T23:58:47+00:00Z
7 )  Criti line aimensions a r e  selected within the best available state-cf-the-art. 
The rationale for grid design is to minimize shading losses (at low concentrations 
up to 10-20X), and then to gradually reduce the  resistive losses more at higher 
concentrations (Reference 1). Figures 2 and 3 show the  good agreement obtained 
between measureti and calculateti values. The slight improvement in the  measure6 
CFF can be explained by slight differences in t h e  assumed and achieved gridline 
d i n  en sions. 
This agreement which has been repeatea for many different cell designs, shows 
that  the  loss model analysis is valid (especially at low concentrations), even using the 
simpler lumped resistance assumptions. This adds confidence to use of the  same 
model in designing grid (and husbar) configurations for large a rea  cells, or for cells 
where low costs limit the choice of grid formation methods. For concentrator cells 
practical grid formation and deposition techniques have been developed, and 
generally require the  use of photolithographic methods. Of course to obtain high 
concentrator system efficiency, i t  is essential to have high cell  efficiency. 
INCREASED OPERATING STRESSES 
Items 5-7 in Table 1 a r e  especially important under the  increased operating 
stresses for concentrator cells. lr5 is important, especially for terrestrial  
concentrator systems where severe temperature cycling (many times per day) is 
possible. I t  also i n c e a s e s  the range of bonaing methods and conditions which can De 
used. lt6 is important because most concentrator cells operate hot (despite the use 
of cooling), and there may also be severe temperature gradients between the  front 
ana back surfaces of the cell. 
Also cells may have to operate a t  high temperatures for extended periods (say 20 
years), and i t  is considered that additional high temperature stability is required fc r  
concentrator cells. Some success has been achieved by using diftusion barriers tc 
minimize interlayer movement; silicon concentrator cells with TIN barriers have 
survived heating above 6OO0C for long periods (5-1 hour) with l i t t le degradation, and 
this work continues. 
The higher temperatures can also lead to accelerated corrosion rates  (d8)  when 
the cell is operating in the field; at present passivating layers (eg. Pd with Ti Ag) a r e  
used, but more testing is needed to show tha t  this is not a severe operating problem. 
The bonding of interconnects to concentrator cells ( # 7 )  involves the use of more 
massive interconnects (to reduce resistive losses), combined with larger bonding areas  
on the cells. The back surface must be well bonded to the substrate, to increase heat  
t r  arisf e r  . 
CONCLUSIONS 
Although there  a r e  some areas  not completely resolved, the  results obtained with 
concentrator cell contacts suggest tha t  the resistive loss models used a re  
satisfactory. The metallization behavior under the increased operating stresses 
supports the conclusion tha t  for cells operating around one-sun levels, that  some of 
the present metallization systems a r e  adequate. More work is required however, 
when compromises a r e  required between the metallization methods available, and the  
cost constraints of the cell or array processing methods. 
160 
Table 1 . Cell Metallization Requirements 
# REQU IREMENT 
1. 
2. 
3.  
4. 
5. 
6. 
7. 
8, 
9. 
10 I 
11. 
12. 
13. 
14. 
- 
- 
- 
LOW LUNTACT RESISTANCE TO SEMICONDUCTOR ( N  AND P I .  
HIGH CONDUCTIVITY TO DFCREASE RESISTIVE LOSSES. 
EASILY PATTERNED INTO GRIDS. 
MINIHIJM CHANCE OF GRID ACTING AS FUSE. 
GOOD ADHESION 
GOOIJ HIGH TEMPERATURE STABIL ITY  (UNDER SINTERING TO 
REDUCE CONTACT RESISTANCE, IWROVE AR COATING, OR 
UNDER OPERAT I NG C O N  I T  I ONS) , 
EASILY BONDABLE (BY SOLDERING OR WELDING) 
M l N l F l l t i  ‘CORROSION. 
SI W L E  METAL STACK. 
COWAT I BLE w I TH CELL FEATURES (TEXTURING, SHALLOW 
XJ).  
LOW COST (tl,’:,RIALS, DEPOSITION), EASILY APPLIED, 
LOW DENSITY TO REDUCE WEIGHT (SPACE-USE). 
THERML EXPANSION CLOSE TO SEMICONDUCTOR (ESPECIALLY 
ON BACK SURFACE). 
GOOD AT LOW TEMPERATURES (SPACE-CELLS) 
Figure 1. Resistive Loss Modeling 
SELECT Jsc, Voc, CELL DIMENSIONS, 
COMPUTE POWER LOSS COMPONENTS. 
MIN IMIZE SUM OF RESISTIVE AND SHADING LOSSES. 
EST I ElATE OPERAT I NG CFF, Jsc , Voc AT VAR I OUS I NTENS I T I ES. 
SELECT OPT I MUM GR I D  PATTERN, 
JET-UP RES 1 STOR NETWORK. 
U N C I  UCE GOOD AGREEMENT BETWEEN EXPER I MEN1 AND PRED I CT I ONS , 
INFER THAT BASlC MODEL IS VALID AND CAN APPLY TO ONE-SUN 
OPERATION, INCLUDING LARGE AREA CELLS, 
161 
ORIGINAL PAGE $9 
OF POOR QUALln 
Figure 2. Curve Fill Factor versus Solar Concentration 
162 

Figure 4. Increased Operating Stresses 
o ADHESION - FOR TEHPERATURE CYCLING, BONDING. 
o HIGH TEMPERATURE STAB1 L I TY - PROCESS SI NTEB I NG 
- OPERAT I NG CONDITIONS 
- DECREASED CORROSION 
TESTING DIFFUSION BARRIERS, Sl f lPLE STACKS AND 
PASS I VAT I NG LAYERS, 
o BONDING - THICKER INTERCONNECTS 
- LARGER BONDliJG AREA 
- THEtKAL CONTACT 
Figure 5. Conclusions 
o RESISTIVE LOSS ANALYSIS IS VALID, SINCE CONCENTRATOR 
CELLS PROVIDE TEST UNDER VERY SEVERE CONDITIONS, 
o BEHAV!OR OF CONCENTRATOR CELLS UNDER ENHANCED OPERATING 
STRESSES SUGGESTS V A T  HIGH COST CONTACi SYSTEMS HAVE 
PROMI SEI 
o flORE WORK I: REQUIRED FOR LOU COST CONTACT SYSTEMS TO 
MEET MOST REQUIREMENTS EVEN AT ONE-SUN LEVELS, 
164 
DISCUSS ION 
WONG: Do you have any grid line patterns available here? 
ILES: I don't have any here, you have to know what the concentrator 
intensity distribution is and what the size is; that's fairly obvious. 
They are all custom made, there i: no such thing as a standard 
concentrator, there are several liaear focus systems in darious sizes 
and several point-focus, so that if you have an intensity and you have 
some idea of what the distribution is then we can find a pattern close 
t o  it. If you are just talking hypothetically we can send you a picture 
of a thing that looks .ery pretty with lots of lines on it, but if ycu 
have a Fresnel with a lOOX lens, which may have a half-inch focused 
image, that's how we can design a cell with fairly precise dimensions. 
BICKLER: Pete, is it true that the concentrator cells are shallow diffusion, 
slanted toward tile blue response? 
ILES: Yes, when you do t.,is analysis, when you put the grids close together, 
the sheet resistance then becomes negligibly small. 
BICKLER: Oh, I was going to say, it souids like a paradox in that there is an 
advantage to over-shadowing, as you mentioned, in the metallization 
system to reduce the series resistance. 
parallel with the diffusion depth and that's a lack of transmission or 
analogous to a shadow, and well worth the -- 
I would assume you drad a 
ILES: That's true, but the scale of the two is so different -- it is typic !1 
to say 50X -- you are getiing 3% or 4% shadowing and maybe 5% or 6% 
total resistive lcss. If you go to 400X then you slide thc shadow mask 
up t J  about 10% by putting more lines closer together. Putting the 
lines closer together is not to reduce the sheet resistance, it is to 
reduce the power through the zrid lines. The grid lines are the more 
import ant res i s tor block. 
CAMPBELL: Am I correct that the concentrator cells have a generally lower 
resistivity base than -- 
ILES: They generally are, particularly for high concentration, ab---? about 
50X or lOOX, but they have been good; typically the ones that are made 
in the range of 0.1 to 0.3 or 0.4 ohm-cm, and have 1017 carriers per 
cubic meter, but there have been some quite gooi cells made with 1 
ohm-cm, with high-quality material, and there have even been some made 
with io  ohm-.cm but they were thin, they were 2 mils thick, so that the 
bulk loss was reduced. 
CAMPBELL: Other than putting this number into these equations, would t3is L - v !  
any effect on the contact pattern? 
ILES: No, there is a discrete bulk loss in the equation, *<Yen I can calcalate 
that one, but apart from that, no problem, if yotr to put it with a 
web you could easily calcJlate thc pattern. 
165 
CAUPBELL: Are not a number of concentrator cells p on n? 
ILES: Yes, the ones that Sandia made with great success are p on n; the ones 
that we make are n on p and p on n. 
on n m’sht be a little better as you go to higher intensities because 
the Dember effect is assisting yo-1. No one has seen that >et; the 
resolution of all the other variables hasn‘t been good enough, but we 
also haven’t seen conductivity modulation in either of them up to lOOOX, 
which is also predicted by theory. 
There is a theoretical reason why p 
lo6  


Metallization problems with concentrator cells

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