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Practical guidelines for grid metallization in photovoltaic solar cell research
Solar Cells, 30 (1991) 459-472
459
Practical guidelines for grid metallization in photovoltaic solar cell research T. A. G e s s e r t , X. Li and T. J. C o u t t s Solar Energy Research Institute, Golden, CO 80401 (U.S.A.)
(Received October 25, 1990)
Abstract During the research stage, many photovoltaic solar cells suffer substantial power loss due to the use of non-optimum top contact grids. The very nature of solar cell research implies that many cell parameters will be in a continual state of change and thus the grid will seldom be truly optimized. However, several things can be done to ensure that a solar cell grid will perform well even ff the parameters vary. In this paper, critical parameters for solar cell grid modeling and design are identified and discussed. Particular attention is paid to the manner in which process aspects affect these parameters and the subsequent power loss of the grid. Finally, practical guidelines are presented, the use of which can minimize the effect of process variation.
1. I n t r o d u c t i o n A p h o t o v o l t a i c (PV) t e c h n o l o g y is only as v a l u a b l e as the p e r f o r m a n c e v s . c o s t c h a r a c t e r i s t i c s it c a n d e m o n s t r a t e . N e v e r t h e l e s s , during the r e s e a r c h
stage, t h e h i g h e s t p e r f o r m a n c e p o s s i b l e f r o m a g i v e n PV m a t e r i a l is o f t e n not demonstrated, leaving both researchers and other technology analysts with only s p e c u l a t i v e s t a t e m e n t s indicating h o w t h e p e r f o r m a n c e o f a g i v e n s o l a r cell will ( o r m i g h t ) b e i m p r o v e d if the " p r o c e s s i n g " a s p e c t s w e r e c o r r e c t l y p e r f o r m e d . T h e r e are m a n y r e a s o n s w h y this s i t u a t i o n exists; m o s t c e n t e r o n the f a c t t h a t a l t h o u g h the disciplines n e c e s s a r y to p r o c e s s s o l a r cells into h i g h - p e r f o r m a n c e d e v i c e s are c e r t a i n l y i n t e r r e l a t e d with t h o s e n e c e s s a r y to f a b r i c a t e h i g h quality s o l a r cell j u n c t i o n s , t h e y are still v e r y different. H o w e v e r , b e c a u s e j u n c t i o n s t u d i e s n o r m a l l y o c c u r b e f o r e p r o c e s s d e v e l o p m e n t , funding, too, usually flows f r o m j u n c t i o n to p r o c e s s d e v e l o p m e n t ; the latter o f t e n d e p e n d i n g on the f o r m e r f o r p e r s o n n e l a n d s u p p o r t . W h e n this s y m b i o t i c r e l a t i o n s h i p b e t w e e n j u n c t i o n f a b r i c a t i o n a n d p r o c e s s develo p m e n t is e s t a b l i s h e d , significant i n n o v a t i o n s o f t e n follow. H o w e v e r , f e w i n d e p e n d e n t r e s e a r c h g r o u p s h a v e t h e p e r s o n n e l a n d / o r e q u i p m e n t to d e v e l o p t h e s e s e p a r a t e t e c h n o l o g i e s fully. Additionally, w h e n a d v a n c e s in p r o c e s s t e c h n o l o g y do h e l p attain h i g h e r cell p e r f o r m a n c e , t h e s e details a n d t h e i r i m p o r t a n c e c a n o f t e n only b e p r e s e n t e d in a b b r e v i a t e d f o r m w h e n s u b s e q u e n t p u b l i c a t i o n s result. T h u s , a l t h o u g h p r o c e s s t e c h n o l o g y is likely to b e v e r y
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
460 difficult and time consuming to develop and test, the details necessary to transfer the process to other researchers are seldom discussed. It is believed that many different PV material technologies would realize substantial performance improvements through a better understanding and incorporation of more advanced metallization process technologies such as those currently used in the very large scale integration (VLSI) community. This paper presents some practical guidelines for effective modeling, design and fabrication of solar cell collector grids, focusing primarily on determination of the critical parameters necessary for grid modeling. The effect of variance in these critical parameters is also presented, illustrating how unexpected variations often lead to sources of resistive losses in an otherwise well designed solar cell grid. Although the discussion is fairly general, it will use, for example purposes, the particular material InP. This III-V material is currently of great importance for space applications because of its resistance to the damaging radiation experienced in space. The paper is organized into three sections. Each section first identifies the critical parameters being investigated and briefly discusses how even good laboratory conditions and practices can adversely affect the value of these parameters. Secondly, the possible effects of these parameters axe modeled, thereby enabling suggestions for possible design strategies which would limit the effect of critical parameter variation and/or instability.
2. M e t h o d s
Before a solar cell grid can be designed, the value of several critical parameters must be simultaneously determined or assigned [1 ]. However, accurate values of some of these basic parameters are often unknown, and measurement techniques for determining them must first be developed and tested. Since this is a long process, initially only a few of the parameters can be known with certainty and, although the subsequent design will only be as good as the parameters used to model it, estimates of the unknown parameters must initially be made. Nevertheless, the process of grid modeling should begin because the results of a well-considered modeling study will not only give insight into the probable areas of dominant cell power loss, but also indicate to the designer which of the critical parameter measurement techniques must be developed first. As mentioned above, the grid modeling, design and fabrication process must be highly interactive with the material/junction fabrication activities. This is because, in a laboratory environment, the material, electrical and process parameters are continuously changing. This, in turn, implies that, since a grid will seldom be perfectly optimum for the solar cell with which it is being used, the design and modeling should take into account reasonable variation in the critical parameters. This is normally done by considering a range of critical parameter values, thereby taking into account not only the uncertainty in an "estimated" parameter, but revealing what effect future
461 changes in the material/junction quality may have on the power losses of the grid. Essentially, six critical parameters must be known for solar cell grid modeling. They are: (1) the maximum power point voltage (Vmp); (2) the maximum power point, active area current density Jmp; (3) the emitter sheet resistance (R~); (4) the specific contact resistance between the contact metal and the semiconductor emitter (re); (5) the sheet resistance of the metal line at the assigned finger and bus bar width and metal thickness (RF and RB); and (6) the minimum metal fine width that can be processed with no electrical open circuits and with sufficient adhesion to withstand subsequent measurements and field testing (PVF). These parameters are presented in order of most difficult to easiest to modify, (1) and (2) being almost impossible to modify, (3) being possible but often difficult, and (4), (5) and (6) being relatively easy to modify and/or demonstrating the greatest degree of instability. For this reason, the grid modeling addressed here centers on variation in parameters (4), (5) and (6), with some mention of the benefits of varying parameter (3).
3. R e s u l t s 3.1. Contact r e s i s t a n c e The specific contact resistance rc of the metal to semiconductor interface is one of the most overlooked, yet very critical parameters of the grid design and modeling process. This is because accurate measurement of rc is difficult, often requiring highly specialized test structures and techniques. Additionally, even if a single value of re is determined, it is often not appreciated that it is strongly dependent on specific details of fabrication and processing and thus, power loss due to r~ variation is often not suspected or allowed for in the modeling. For these reasons, significant power loss due to re is often considered improbable until it can be proven to be a severe deterrent to the performance of a cell. Nevertheless, as will be shown, although the effect of variations in this parameter can be minimized, even relatively small variations can be detrimental to cell performance. Although it is generally believed that the re is primarily a function of the choice of metal and semiconductor (material and doping), experience will often show that the ideal metal-semiconductor values of re are seldom achieved with normal laboratory procedures. This is because, in addition to the specific metal and semiconductor chosen, many other process-related aspects are involved. These aspects include its purity, porosity, adhesion and interdiffusion of the metal layers and, relative to the semiconductor, the extent of surface cleaning prior to metal deposition, the type of deposition used (vacuum evaporation, sputtering, plating, etc.) and any intentional or unintentional thermal treatments experienced by the interface. Indeed, it has been observed that relatively slight changes in any one of these process parameters can cause a change in re of several orders of magnitude.
462
As m e n t i o n e d above, a c c u r a t e d e t e r m i n a t i o n o f the re is generally o n e o f the m o r e difficult m e a s u r e m e n t t e c h n i q u e s to d e v e l o p and qualify. Within the grid m o d e l i n g e x a m p l e s p r e s e n t e d here, several m e t h o d s h a v e b e e n u s e d , including t h e m e t h o d s o f K e r a m i n d a s [2] and C o x and Strack [3]. Often, h o w e v e r , o n e is m o r e interested in describing the nature o f t h e c o n t a c t resistivity as a f u n c t i o n o f s o m e o t h e r variable, s u c h a p r e d e p o s i t i o n c l e a n i n g p r o c e d u r e or p o s t d e p o s i t i o n h e a t treatments. In t h e s e c a s e s o f trend analysis, a simpler rc m e a s u r e m e n t t e c h n i q u e is o f t e n desirable, s u c h as the four-bar m e t h o d . A l t h o u g h this m e t h o d l a c k s the p r e c i s i o n o f t h e a b o v e m e n t i o n e d m e t h o d s , studies h a v e s h o w n that the four-bar m e t h o d will c o n s i s t e n t l y result in a v a l u e o f rc an order o f m a g n i t u d e higher than that o b t a i n e d u s i n g the C o x and Strack m e t h o d . The practical g u i d e l i n e s for m a x i m u m a c c e p t a b l e c o n t a c t r e s i s t a n c e for solar cells d e p e n d o n t w o (related) p r o c e s s aspects: (1) t h e m i n i m u m grid line width and (2) w h e t h e r the cell will be u s e d u n d e r c o n c e n t r a t i o n . Figure 1 illustrates h o w t w o different v a l u e s o f rc will affect t h e total p o w e r l o s s o f the m o d e l o n e - s u n solar cell. A l t h o u g h it is o f t e n b e l i e v e d that an re o f 1 × 10 -2 ~2 c m 2 is sufficient for o n e - s u n grids, the figure s h o w s , for the thinner line w i d t h s m o d e l e d here, this v a l u e o f rc will c a u s e t h e c o n t a c t r e s i s t a n c e to be the d o m i n a n t l o s s m e c h a n i s m ; substantial l o s s r e d u c t i o n w o u l d be a c h i e v e d if r~ c a n be r e d u c e d to 1 × 10 -4 ~ c m 2. A l s o s h o w n in Fig. 1 is the effect o f current c r o w d i n g o n the p o w e r l o s s and w h y , for n a r r o w lines, it is i m p o r t a n t to include this in the m a t h e m a t i c a l f o r m a l i s m . Figure 2 s h o w s similar calculations, but for a m o d e l c o n c e n t r a t o r cell o p e r a t e d at 1 0 0 0 suns. N o t e here that, unlike t h e c a s e o f t h e o n e - s u n design, an re o f e v e n 1 x 10 -4 ~ c m 2 will still h a v e a significant (although, n o t a d o m i n a n t ) effect o n t h e p o w e r loss. F o r this case, c o n t a c t r e s i s t a n c e v a l u e s typical o f t h o s e usually required in VLSI w o u l d be a d v a n t a g e o u s . Finally, Fig. 3 s h o w s t h e effect o f i n c r e a s i n g r~ f r o m 1 x 10 -4 ~ a m 2 to 1 x 10 -a ~ c m 2 as a 7.0 6.0
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Fig. 3. C o m p a r i s o n of re = 10 -3 v s . re = 10 -4 ~ c m 2 as a f u n c t i o n of c o n c e n t r a t i o n for a m o d e l c o n c e n t r a t o r cell. Note t h a t efficiency g a i n s w o u l d b e e x p e c t e d if re w e r e r e d u c e d b e l o w 10 -4 1"~ c m 2.
function of increasing concentration. Here, even a slight non-uniformity or instability in rc (as might be caused by progressive adhesion loss) would have a considerable effect on not only the efficiency but also the effective maximum concentration ratio. In developing methods to decrease re, one must note that rc will likely be interrelated with other process parameters that must be considered during a successful modeling study. For example, one must consider, because the emitter is often very thin, excessive heat treatment of the contacts will likely cause junction shorting and, thus, must be avoided. Similarly, plasma treatments, which have been shown to achieve notably low r~ [4] and improve adhesion through (predeposition) removal of surface contamination, may also adversely affect the junction through plasma damage to the junction. Additionally, annealing with a simple strip heater, as used in our work, is a sensitive process [5]. As a final example, Fig. 1 illustrates that the grid power loss due to rc is directly related to the line width used on the solar cell [1]. In light of these constraints, non-annealed, yet temperature stable contacts are often desirable. Examples include the use of plated, non-annealed gold onto bulk n+-InP on which measurements have demonstrated r~ values of about 10 -3 to about 10 -4 ~ cm 2. Recent work using similar InP material, involving a combination of low power argon plasma surface cleaning and Cr/Pd/Ag deposition indicates that specific contact resistance values of less than 10 -5 12 cm 2 may be routinely achievable for even narrow ( ~ 8 ]~m) line widths. Additionally, devices with thicker emitters, which have the advantage of allowing higher temperature operation, are currently being investigated. Finally, although it has not yet been attempted, rapid thermal annealing processing may be of considerable benefit [6]. 3.2. Gr id line r e s i s t a n c e The critical parameter of metal sheet resistance of the grid lines is another area of solar cell grid modeling and design technology where normal
464
process variations are often not fully appreciated and therefore, not account ed for in the grid design. Although the value of metal sheet resistance is associated primarily with the choice of metal, like contact resistance, it can vary greatly with the processes used for grid line fabrication. For example, the grid modeling program [1] assumes that the grid lines are perfectly rectangular and homogeneous, as shown in Fig. 4(a). However, experience reveals that this is seldom the case and the metal cross-section can vary greatly depending on process-related aspects as shown in Fig. 4. Because it is difficult to predict how these will combine to affect the sheet resistance, modeling efforts should include a measurement of the metal resistivity at typical line widths and thicknesses (i.e. sheet resistance values of a 1 mm wide metal strip may be different from those measured on an 8 /~m wide grid line). An example of a metallization pattern used for this purpose is shown in Fig. 5 where the measured resistivity is defined using the a v e r a g e m e t a l t h i c k n e s s and the a v e r a g e w i d t h o f line o b s c u r a t i o n . This definition allows the modeling program to incorporate accurately all of the above mentioned aspects of cross-section and homogeneity. Once this measured value of the effective metal resistivity is determined, it is often helpful (and insightful) to indicate the difference between the ideal resistance value (i.e. that determined from the pure, bulk value assuming perfectly rectangular grid lines) and the measured value (as defined above) as the resistance ratio RR, where R R = ( m e a s u r e d sheet reslstance)/(ideal sheet resistance). Even for a well established metallization process, there are many mechanisms that can greatly affect the actual sheet resistance. Consider, for example, the process of high purity plated gold contacts. In this case, it takes less than 1% of iron contamination to increase RR to 10 [7]. Increases in resistivity also occur for contamination of gold by other metals (such as cobalt, tin, nickel and indium) although to a lesser extent. Additionally, our own research has shown that the plating current has a substantial effect on Modeled Semiconductor (a) Plated Deposits
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465 I
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Fig. 6. (a, b) SEM micrographs of gold plated onto ITO: (a) 5 p~rn gold plated at high current density ( ~ 0 . 5 ~ m min-1); Co) 1 /~m gold plated at low current ( = 0 . 1 ~m min-~); (c) 120 nm evaporated gold. Deposit (a) had a resistivity ratio ~ 6 - 8 times that of pure gold, Co) had a resistivity ratio of about 2 to about 3.
both the porosity of the gold deposit and on the measured resistivity (Figs. 6 and 7). In particular, standard bus-bar patterns have been electroplated onto indium tin oxide (ITO) films (100 um thick) to a thickness of 1 - 4 /~m and current-voltage measurements performed. These measurements suggest that RR is from about 2 to about 8; the higher values associated with higher current densities and a darker deposit appearance. Furthermore, the greater porosity of the deposit in Fig. 6(a) could enable oxidizing substances to enter the metal, causing the grid resistivity (and perhaps re) to be unstable.
466 120 C"
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Fig. 7. Plot of plated gold resistivity as a function of plating current. As indicated, these results were obtained for grid lines plated both directly on an ITO film and onto Cr/Pd adhesion layers. Because the sheet resistance of the gold is low compared with that of the ITO and the Cr/Pd layers, a reasonable estimate of the resistivity may be made. On a c t u a l cells, m e a s u r e m e n t s indicate t h a t RR is f r o m a b o u t 2 to a b o u t 3. As will b e shown, this v a r i a t i o n is sufficient to n e c e s s i t a t e c o n s i d e r a t i o n in t h e grid design. In c o n t r a s t to p l a t e d metals, v a c u u m e v a p o r a t e d c o n t a c t s ( p h o t o l i t h o g r a p h i c lift-off p r o c e s s e d ) a p p e a r to suffer less f r o m s h e e t r e s i s t a n c e variations. Indeed, m e a s u r e m e n t s o f e l e c t r o n - b e a m - e v a p o r a t e d silver c o n t a c t s d e p o s i t e d o n t o g l a s s s u b s t r a t e s indicate that, w i t h o u t g r e a t effort or control, RRs o f 1 . 7 - 1 . 8 a r e r o u t i n e l y p o s s i b l e o v e r a fairly wide r a n g e o f d e p o s i t i o n r a t e s ( 1 0 - 5 0 ~ s - 1 ) . Similarly, e l e c t r o n - b e a m - e v a p o r a t e d a l u m i n u m d e m o n s t r a t e s e v e n a l o w e r RR o f 1.6 (20 /~ s - l ) . Thus, w h e n c h o o s i n g a m e t a l to u s e for a grid line, it is n o t o n l y i m p o r t a n t to c h o o s e o n e w i t h a relatively low ideal resistivity, b u t to k n o w h o w t h e resistivity is e x p e c t e d to c h a n g e as a f u n c t i o n of d e p o s i t i o n a n d e n v i r o n m e n t a l conditions. F i g u r e 8 s h o w s t h e p e r c e n t a g e p o w e r loss as a f u n c t i o n o f RR f o r a o n e - s u n cell. F o r e a c h o f t h e t h r e e d e p o s i t t h i c k n e s s e s shown, t w o different situations a r e c o n s i d e r e d : t h e u p p e r c u r v e o f e a c h p a i r a s s u m e s t h a t the d e s i g n e r is u n a w a r e o f t h e h i g h e r grid line resistivity; the l o w e r curves, a s s u m e s t h a t t h e d e s i g n e r h a d b e e n a w a r e t h a t the grid lines w e r e o f h i g h e r resistivity a n d h a d a c c o u n t e d f o r this. Obviously, t h e first s i t u a t i o n is m o r e likely. Realizing t h a t resistivities m a y b e u p to eight t i m e s t h e ideal value, m o d e l i n g s h o w s t h a t if h i g h e r resistivities are n o t a s s u m e d , t h e n e v e n at o n e sun, t h e s e l o s s e s will c e r t a i n l y d o m i n a t e . H o w e v e r , as also shown, a p r u d e n t d e s i g n i n c o r p o r a t i n g t h i c k e r a n d / o r w i d e r grid lines c a n b e u s e d to insure a g a i n s t t h e s e variations. T h e s a m e effects o f i n c r e a s i n g the grid line resistivity are m o d e l e d f o r the c o n c e n t r a t o r cell in Fig. 9. H e r e it is s h o w n t h a t if RE Can b e r e d u c e d f r o m 6 0 0 to 100 ~/E], a n d if n e a r ideal m e t a l resistivity c a n b e a c h i e v e d , t h e n the efficiency will i n c r e a s e to a c o n c e n t r a t i o n ratio of a b o u t 100 suns. A l t h o u g h c o n c e n t r a t o r s y s t e m s are o f t e n d e s i g n e d for o p e r a t i o n u p to 1 0 0 0 suns, w h i c h is still b e y o n d the efficiency m a x i m u m for t h e a b o v e design, this e x e r c i s e illustrates t h e a d v a n t a g e s of k n o w i n g w h i c h ceil p a r a m e t e r s to a d d r e s s first to a c h i e v e t h e g r e a t e s t benefit.
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Fig. 8. Percentage power loss as a function of grid Line resistance for three metal thickness for a one-sun cell. Fig. 9. Efficiency v s . concentration ratio for a concentrator cell with RE equal to 100 ~K:] in which the grid metal resistivity ratio is treated parametrically. Also shown is the modeled result fiRE had not b e e n decreased from 60012/[:], illustrating this requirement for a concentrator design.
3.3. M i n i m u m line w i d t h The critical parameter of minimum line width often represents not only the least optimized parameter of an existing cell design but also the best avenue for future cell improvements. In this section, two separate aspects of this critical parameter are addressed. The first describes the practical guideline that, for typical metallizations, it is generally better to design a grid using the minimum line width that can be fabricated. This guideline suggests that many existing solar cell grid designs are probably not optimum, especially if they have not incorporated state-of-the-art techniques currently used in the micro-electronics industry. The second aspect, on the other hand, cautions that, because fabricating optimized grids with thinner lines also implies a grid with more fingers and closer spacing, several new processrelated problems are likely to occur. Indeed, ff these problems are not properly addressed, designing for the minimum line width, in itself, will probably not yield a better-performing solar cell. As implied in Fig. 1, for one-sun cells, the total power loss will, to a point, decrease substantially as the line width decreases. For the cell modeled in this figure, the point of minimum power loss will depend on the process parameter leading to the limiting aspect ratio of the narrow grid line. For example, in this modeled grid, the metal thickness is fixed at 5 /~m and thus, at some line width less than 10 /~m (at 10 /zm, the aspect ratio of line w i d t h / h e i g h t = 2 ) , the metal thickness will likely have to be decreased (otherwise adhesion loss will result), thereby leading to a minimum in the power loss. Although the idea that there is a line width at which the power loss will reach a minimum, for one-sun ceils and typical metallizations, this width is often submicron, and therefore not practical for solar cells. The general guideline of using the minimum line width is also illustrated in Fig. 10, where a series of curves representing different metal thicknesses show not only how the total power loss decreases as the line width decreases,
468
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Fig. 11. Percentage power loss as a function of the grid line width (gold, RR= 1) and aspect ratio at 1000 suns. Note that there is a well-defined minimum position for each aspect ratio. Also shown is the effect of an Entech cover.
but also the p o w e r loss r e d u c t i o n as the metal thickness increases. Also s h o w n in this figure is the p o w e r loss which would b e e x p e c t e d if a grid with a fixed finger s p a c i n g (but varying finger width) w e r e used. Although the idea m e n t i o n e d a b o v e c o n c e r n i n g an o p t i m u m m i n i m u m line width is not usually c o n s i d e r e d for one sun grids, for c o n c e n t r a t o r cells o p e r a t i n g at high c u r r e n t density, it m u s t b e considered. This is illustrated in Fig. 11 (and Fig. 2); w h e n the a s p e c t ratio is t r e a t e d parametrically and the efficiency is c a l c u l a t e d as a f u n c t i o n o f the grid line width, t h e r e is an o p t i m u m line width for a given a s p e c t ratio which is in the 5 - I 0 ~ m range, and t h e r e f o r e c a n be fabricated. Also s h o w n in this figure is the m o d e l e d effect o f a t e c h n i q u e d e v e l o p e d b y E n t e c h Inc. in which the p o w e r loss due to s h a d o w i n g is r e d u c e d . This t e c h n i q u e is b a s e d o n the u s e o f a silicone plastic cover, the surface o f w h i c h is p a t t e r n e d to f o r m an a r r a y o f c o r r u g a t i o n s (prisms) which act to f o c u s the light away f r o m the grid lines [8]. As can be seen, w h e n this c o v e r is i n c o r p o r a t e d into cell c o n s t r u c t i o n , the influence
469
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10p, m= 1/2" Fig. 12. Photographs illustrating line width broadening from about 3 to about 8 /~m as solar
cell grid line goes through the processing stages of a photolithographically defined metal liftoff. Column 2 illustrates the improved edge uniformity that results when residual photoresist on the photomask (present in the column 1 sequence, photo lb) is removed.
on t h e m o d e l e d p e r f o r m a n c e is quite s p e c t a c u l a r . H o w e v e r , o n e s h o u l d n o t e t h a t in this m o d e l i n g , a d d i t i o n a l reflection effects h a v e n o t b e e n c o n s i d e r e d . From the above mentioned arguments, one may be convinced that narrower grid lines a r e v e r y o f t e n a s i m p l e p a t h to a b e t t e r p e r f o r m i n g s o l a r cell grid. A l t h o u g h this is t r u e f r o m t h e m o d e l i n g s t a n d p o i n t , in p r a c t i c e , m a n y p r o c e s s r e l a t e d p r o b l e m s c a n a r i s e w i t h t h e s e thin-line grid d e s i g n s w h i c h c a n m o r e t h a n offset t h e m o d e l e d gains. T h e p r i m a r y r e a s o n f o r this is t h a t the relative size ( a n d t h u s t h e effect) o f a v a r i a t i o n in line w i d t h i n c r e a s e s d r a m a t i c a l l y as t h e line w i d t h n a r r o w s . In o t h e r w o r d s , b e c a u s e a thin-line d e s i g n will h a v e m a n y m o r e , v e r y closely s p a c e d grid fingers, o b s c u r a t i o n l o s s e s c a u s e d b y p r o c e s s r e l a t e d line b r o a d e n i n g will t e n d to d o m i n a t e a n y r e l a t e d gains due to d e c r e a s e s in line r e s i s t a n c e . As a n e x a m p l e , c o n s i d e r a grid line 50 ~ m wide, h a v i n g a 2 ~ m r e g i o n o n e a c h e d g e o f t h e line w h e r e irregularities o f s o m e s o r t are o b s e r v e d . A l t h o u g h it is b e l i e v e d t h a t this 2 ~ m r e g i o n h a s s u s p e c t r e s i s t a n c e a n d / o r a d h e s i o n quality, b e c a u s e it r e p r e s e n t s only
470 O.t 0.09 0.08 Plated Au, ~ / / RR=5
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Fig. 13. Diagram combining the effects of ideal resistance of the grid metal, m e a s u r e d resistance ratio and minimum line width. Because many process-related aspects are already incorporated, illustrations such as this can be used with the modeling program to determine process-specific grid loss.
8% of the grid line, these uncertainties can often be incorporated into the model through appropriate assignment of the related critical parameters. However, if the line width is decreased to 6 ~m, and the metallization process is not improved, the region of suspect quality suddenly has increased to 67% of the metallized area and the line resistance a n d / o r adhesion will likely suffer (e.g. Fig. 12(d)). Another problem in fabricating thin-line grid designs is that when a new (thin) line width is incorporated into a design, there is often great uncertainty concerning how the imtial line width, assigned at the photomask development stage, will c om par e with the postprocessing line width a n d / o r what degree of variation can be e x p e c t e d during normal processing. This point, which is especially true in contact print lithography, is illustrated in Fig. 12 where two columns of photographs are c o m p a r e d showing how line width broadening propagates through a photolithographic lift-off process. Note that although the initial line width specified during phot om ask manufacture is 2 ~m, the final metallized line width is closer to 8 /~m. Additionally, not only does the line width expand (causing increased obscuration), but unless the photolithographic process is pe r f or m e d very carefully, the edge definition is often poor, causing the metal sheet resistance to increase more than would be
471 assumed. These line width constraints, t o g e t h e r with the line resistance constraints mentioned in the preceding section, are illustrated in Fig. 13. Figures such as this not only indicate how different metallization/process technologies compare, but, when considered with a p r o p e r modeling study, can also illustrate how solar cell grids fabricated with typical research-based technologies (evaporation, photolithography) com pare with grids fabricated with typical production technologies (plating, screen printing).
4. C o n c l u s i o n This study has established several guidelines for designing front contacts to solar cells that are not intuitively apparent. For one-sun applications incorporating relatively narrow grid lines, unless it is possible to establish a contact resistance of less than 10 -2 ~ am 2, this p a r a m e t e r will dominate the resistance losses. For concentration applications however, a minimum value of 10 -4 ~ cm 2 must be achieved, while even lower values, comparable to those of VLSI, would be preferred. Although, in practice, there may not be complete f r eedom t o lower the contact resistance without junction damage or adversely affecting cell performance, substantial reduction in re may be achieved by using techniques such as plasma cleaning and rapid thermal annealing. The resistivity of the grid line metal, its thickness and aspect ratio are important parameters in the grid design process. It has b e e n indicated that for evaporated metal deposits, variations in resistivity of twice the ideal value are usual. Similarly, for plated deposits, resistivities of up to eight times the ideal value are m or e usual than may be expected. If not account ed for, these variations in resistivity can dominate the solar cell power loss. Modeling reveals that, for solar cells operat ed at one sun using c o m m o n metallizations, it is always better to design the device with the narrowest grid line that can be fabricated. However, one must be cautious of several new problems that are often e n c o u n t e r e d when thin-line designs are fabricated. The most c o m m o n of these problems is that, as the line width narrows, the effect of line broadening b e c o m e s m uc h more severe. Also, although narrow grid lines are normally desirable, it is shown that at a concentration of 1000 suns, there is an optimum grid line width which depends on the metal resistivity and the aspect ratio. Finally, although the basic p r o c e d u r e s for studying grid losses were established by previous researchers, the practical application of these techniques is often neglected resulting in an unfiecessarily pessimistic view of the potential of many types of PV solar cells.
Acknowledgments The authors wish to thank A. Mason for SEM photographs. This work was s u p p o r t e d by the U.S. Department of Energy under Contract No. DE-
472
AC02-83CH10093 and by NASA Lewis Research Center under Interagency Order No. C-3000-K.
References 1 T. A. Gessert and T. J. Coutts, in Mat. Res. Soc. Symp. Proc., Vol. 181, Advanced MetaUizations in Microelectronics, MRS, Pittsburgh, PA, 1990, p. 301. 2 V.G. Karamidas, inProc. SeventhInt. Symp. on GaUiumArsenide andRelated Compounds, St. Louis, September 1978, Conf. Series Number 45, Institute of Physics, Bristol and London, p. 396. 3 R. H. Cox and H. Strack, Solid State Electron., 10 (1967) 1213. 4 W. C. Dautremont-Smith, P. A. Barnes and J. W. Staylt, Jr., J. Vac. Svi. TechnoL B, 2 (1984) 620. 5 T. A. Gessert, X. Li, T. J. Courts, M. W. Wan]ass and A. B. Franz, in Proc. First Int. Conf. on I n d i u m Phosphide and Related Materials f o r Advanced Electronic and Optical Devices, SPIE Proc., Vol. 1144, SPIE, Bellingham, WA, 1989, p. 476. 6 A. Katz, B. E. Weir, D. M. Maher, P. M. Thomas, M. Soler, W. C. Dautermont-Smith, R. F. Karlicek, Jr., J. D. Wynn and L. C. Kimerling, Appl. Phys. Left., 55 (1989) 2220. 7 F. H. Reid and W. Goldie (cds.), Gold Plating Technology, Electrochemical Publications, Ayr, 1974, p. 14. 8 M.J. O'Neill and M. G. Piszczor, in Proc. 20th IEEE Photovoltaics Specialists Conf, IEEE, New York, 1988, p. 1007.
459
Practical guidelines for grid metallization in photovoltaic solar cell research T. A. G e s s e r t , X. Li and T. J. C o u t t s Solar Energy Research Institute, Golden, CO 80401 (U.S.A.)
(Received October 25, 1990)
Abstract During the research stage, many photovoltaic solar cells suffer substantial power loss due to the use of non-optimum top contact grids. The very nature of solar cell research implies that many cell parameters will be in a continual state of change and thus the grid will seldom be truly optimized. However, several things can be done to ensure that a solar cell grid will perform well even ff the parameters vary. In this paper, critical parameters for solar cell grid modeling and design are identified and discussed. Particular attention is paid to the manner in which process aspects affect these parameters and the subsequent power loss of the grid. Finally, practical guidelines are presented, the use of which can minimize the effect of process variation.
1. I n t r o d u c t i o n A p h o t o v o l t a i c (PV) t e c h n o l o g y is only as v a l u a b l e as the p e r f o r m a n c e v s . c o s t c h a r a c t e r i s t i c s it c a n d e m o n s t r a t e . N e v e r t h e l e s s , during the r e s e a r c h
stage, t h e h i g h e s t p e r f o r m a n c e p o s s i b l e f r o m a g i v e n PV m a t e r i a l is o f t e n not demonstrated, leaving both researchers and other technology analysts with only s p e c u l a t i v e s t a t e m e n t s indicating h o w t h e p e r f o r m a n c e o f a g i v e n s o l a r cell will ( o r m i g h t ) b e i m p r o v e d if the " p r o c e s s i n g " a s p e c t s w e r e c o r r e c t l y p e r f o r m e d . T h e r e are m a n y r e a s o n s w h y this s i t u a t i o n exists; m o s t c e n t e r o n the f a c t t h a t a l t h o u g h the disciplines n e c e s s a r y to p r o c e s s s o l a r cells into h i g h - p e r f o r m a n c e d e v i c e s are c e r t a i n l y i n t e r r e l a t e d with t h o s e n e c e s s a r y to f a b r i c a t e h i g h quality s o l a r cell j u n c t i o n s , t h e y are still v e r y different. H o w e v e r , b e c a u s e j u n c t i o n s t u d i e s n o r m a l l y o c c u r b e f o r e p r o c e s s d e v e l o p m e n t , funding, too, usually flows f r o m j u n c t i o n to p r o c e s s d e v e l o p m e n t ; the latter o f t e n d e p e n d i n g on the f o r m e r f o r p e r s o n n e l a n d s u p p o r t . W h e n this s y m b i o t i c r e l a t i o n s h i p b e t w e e n j u n c t i o n f a b r i c a t i o n a n d p r o c e s s develo p m e n t is e s t a b l i s h e d , significant i n n o v a t i o n s o f t e n follow. H o w e v e r , f e w i n d e p e n d e n t r e s e a r c h g r o u p s h a v e t h e p e r s o n n e l a n d / o r e q u i p m e n t to d e v e l o p t h e s e s e p a r a t e t e c h n o l o g i e s fully. Additionally, w h e n a d v a n c e s in p r o c e s s t e c h n o l o g y do h e l p attain h i g h e r cell p e r f o r m a n c e , t h e s e details a n d t h e i r i m p o r t a n c e c a n o f t e n only b e p r e s e n t e d in a b b r e v i a t e d f o r m w h e n s u b s e q u e n t p u b l i c a t i o n s result. T h u s , a l t h o u g h p r o c e s s t e c h n o l o g y is likely to b e v e r y
0379-6779/91/$3.50
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460 difficult and time consuming to develop and test, the details necessary to transfer the process to other researchers are seldom discussed. It is believed that many different PV material technologies would realize substantial performance improvements through a better understanding and incorporation of more advanced metallization process technologies such as those currently used in the very large scale integration (VLSI) community. This paper presents some practical guidelines for effective modeling, design and fabrication of solar cell collector grids, focusing primarily on determination of the critical parameters necessary for grid modeling. The effect of variance in these critical parameters is also presented, illustrating how unexpected variations often lead to sources of resistive losses in an otherwise well designed solar cell grid. Although the discussion is fairly general, it will use, for example purposes, the particular material InP. This III-V material is currently of great importance for space applications because of its resistance to the damaging radiation experienced in space. The paper is organized into three sections. Each section first identifies the critical parameters being investigated and briefly discusses how even good laboratory conditions and practices can adversely affect the value of these parameters. Secondly, the possible effects of these parameters axe modeled, thereby enabling suggestions for possible design strategies which would limit the effect of critical parameter variation and/or instability.
2. M e t h o d s
Before a solar cell grid can be designed, the value of several critical parameters must be simultaneously determined or assigned [1 ]. However, accurate values of some of these basic parameters are often unknown, and measurement techniques for determining them must first be developed and tested. Since this is a long process, initially only a few of the parameters can be known with certainty and, although the subsequent design will only be as good as the parameters used to model it, estimates of the unknown parameters must initially be made. Nevertheless, the process of grid modeling should begin because the results of a well-considered modeling study will not only give insight into the probable areas of dominant cell power loss, but also indicate to the designer which of the critical parameter measurement techniques must be developed first. As mentioned above, the grid modeling, design and fabrication process must be highly interactive with the material/junction fabrication activities. This is because, in a laboratory environment, the material, electrical and process parameters are continuously changing. This, in turn, implies that, since a grid will seldom be perfectly optimum for the solar cell with which it is being used, the design and modeling should take into account reasonable variation in the critical parameters. This is normally done by considering a range of critical parameter values, thereby taking into account not only the uncertainty in an "estimated" parameter, but revealing what effect future
461 changes in the material/junction quality may have on the power losses of the grid. Essentially, six critical parameters must be known for solar cell grid modeling. They are: (1) the maximum power point voltage (Vmp); (2) the maximum power point, active area current density Jmp; (3) the emitter sheet resistance (R~); (4) the specific contact resistance between the contact metal and the semiconductor emitter (re); (5) the sheet resistance of the metal line at the assigned finger and bus bar width and metal thickness (RF and RB); and (6) the minimum metal fine width that can be processed with no electrical open circuits and with sufficient adhesion to withstand subsequent measurements and field testing (PVF). These parameters are presented in order of most difficult to easiest to modify, (1) and (2) being almost impossible to modify, (3) being possible but often difficult, and (4), (5) and (6) being relatively easy to modify and/or demonstrating the greatest degree of instability. For this reason, the grid modeling addressed here centers on variation in parameters (4), (5) and (6), with some mention of the benefits of varying parameter (3).
3. R e s u l t s 3.1. Contact r e s i s t a n c e The specific contact resistance rc of the metal to semiconductor interface is one of the most overlooked, yet very critical parameters of the grid design and modeling process. This is because accurate measurement of rc is difficult, often requiring highly specialized test structures and techniques. Additionally, even if a single value of re is determined, it is often not appreciated that it is strongly dependent on specific details of fabrication and processing and thus, power loss due to r~ variation is often not suspected or allowed for in the modeling. For these reasons, significant power loss due to re is often considered improbable until it can be proven to be a severe deterrent to the performance of a cell. Nevertheless, as will be shown, although the effect of variations in this parameter can be minimized, even relatively small variations can be detrimental to cell performance. Although it is generally believed that the re is primarily a function of the choice of metal and semiconductor (material and doping), experience will often show that the ideal metal-semiconductor values of re are seldom achieved with normal laboratory procedures. This is because, in addition to the specific metal and semiconductor chosen, many other process-related aspects are involved. These aspects include its purity, porosity, adhesion and interdiffusion of the metal layers and, relative to the semiconductor, the extent of surface cleaning prior to metal deposition, the type of deposition used (vacuum evaporation, sputtering, plating, etc.) and any intentional or unintentional thermal treatments experienced by the interface. Indeed, it has been observed that relatively slight changes in any one of these process parameters can cause a change in re of several orders of magnitude.
462
As m e n t i o n e d above, a c c u r a t e d e t e r m i n a t i o n o f the re is generally o n e o f the m o r e difficult m e a s u r e m e n t t e c h n i q u e s to d e v e l o p and qualify. Within the grid m o d e l i n g e x a m p l e s p r e s e n t e d here, several m e t h o d s h a v e b e e n u s e d , including t h e m e t h o d s o f K e r a m i n d a s [2] and C o x and Strack [3]. Often, h o w e v e r , o n e is m o r e interested in describing the nature o f t h e c o n t a c t resistivity as a f u n c t i o n o f s o m e o t h e r variable, s u c h a p r e d e p o s i t i o n c l e a n i n g p r o c e d u r e or p o s t d e p o s i t i o n h e a t treatments. In t h e s e c a s e s o f trend analysis, a simpler rc m e a s u r e m e n t t e c h n i q u e is o f t e n desirable, s u c h as the four-bar m e t h o d . A l t h o u g h this m e t h o d l a c k s the p r e c i s i o n o f t h e a b o v e m e n t i o n e d m e t h o d s , studies h a v e s h o w n that the four-bar m e t h o d will c o n s i s t e n t l y result in a v a l u e o f rc an order o f m a g n i t u d e higher than that o b t a i n e d u s i n g the C o x and Strack m e t h o d . The practical g u i d e l i n e s for m a x i m u m a c c e p t a b l e c o n t a c t r e s i s t a n c e for solar cells d e p e n d o n t w o (related) p r o c e s s aspects: (1) t h e m i n i m u m grid line width and (2) w h e t h e r the cell will be u s e d u n d e r c o n c e n t r a t i o n . Figure 1 illustrates h o w t w o different v a l u e s o f rc will affect t h e total p o w e r l o s s o f the m o d e l o n e - s u n solar cell. A l t h o u g h it is o f t e n b e l i e v e d that an re o f 1 × 10 -2 ~2 c m 2 is sufficient for o n e - s u n grids, the figure s h o w s , for the thinner line w i d t h s m o d e l e d here, this v a l u e o f rc will c a u s e t h e c o n t a c t r e s i s t a n c e to be the d o m i n a n t l o s s m e c h a n i s m ; substantial l o s s r e d u c t i o n w o u l d be a c h i e v e d if r~ c a n be r e d u c e d to 1 × 10 -4 ~ c m 2. A l s o s h o w n in Fig. 1 is the effect o f current c r o w d i n g o n the p o w e r l o s s and w h y , for n a r r o w lines, it is i m p o r t a n t to include this in the m a t h e m a t i c a l f o r m a l i s m . Figure 2 s h o w s similar calculations, but for a m o d e l c o n c e n t r a t o r cell o p e r a t e d at 1 0 0 0 suns. N o t e here that, unlike t h e c a s e o f t h e o n e - s u n design, an re o f e v e n 1 x 10 -4 ~ c m 2 will still h a v e a significant (although, n o t a d o m i n a n t ) effect o n t h e p o w e r loss. F o r this case, c o n t a c t r e s i s t a n c e v a l u e s typical o f t h o s e usually required in VLSI w o u l d be a d v a n t a g e o u s . Finally, Fig. 3 s h o w s t h e effect o f i n c r e a s i n g r~ f r o m 1 x 10 -4 ~ a m 2 to 1 x 10 -a ~ c m 2 as a 7.0 6.0
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function of increasing concentration. Here, even a slight non-uniformity or instability in rc (as might be caused by progressive adhesion loss) would have a considerable effect on not only the efficiency but also the effective maximum concentration ratio. In developing methods to decrease re, one must note that rc will likely be interrelated with other process parameters that must be considered during a successful modeling study. For example, one must consider, because the emitter is often very thin, excessive heat treatment of the contacts will likely cause junction shorting and, thus, must be avoided. Similarly, plasma treatments, which have been shown to achieve notably low r~ [4] and improve adhesion through (predeposition) removal of surface contamination, may also adversely affect the junction through plasma damage to the junction. Additionally, annealing with a simple strip heater, as used in our work, is a sensitive process [5]. As a final example, Fig. 1 illustrates that the grid power loss due to rc is directly related to the line width used on the solar cell [1]. In light of these constraints, non-annealed, yet temperature stable contacts are often desirable. Examples include the use of plated, non-annealed gold onto bulk n+-InP on which measurements have demonstrated r~ values of about 10 -3 to about 10 -4 ~ cm 2. Recent work using similar InP material, involving a combination of low power argon plasma surface cleaning and Cr/Pd/Ag deposition indicates that specific contact resistance values of less than 10 -5 12 cm 2 may be routinely achievable for even narrow ( ~ 8 ]~m) line widths. Additionally, devices with thicker emitters, which have the advantage of allowing higher temperature operation, are currently being investigated. Finally, although it has not yet been attempted, rapid thermal annealing processing may be of considerable benefit [6]. 3.2. Gr id line r e s i s t a n c e The critical parameter of metal sheet resistance of the grid lines is another area of solar cell grid modeling and design technology where normal
464
process variations are often not fully appreciated and therefore, not account ed for in the grid design. Although the value of metal sheet resistance is associated primarily with the choice of metal, like contact resistance, it can vary greatly with the processes used for grid line fabrication. For example, the grid modeling program [1] assumes that the grid lines are perfectly rectangular and homogeneous, as shown in Fig. 4(a). However, experience reveals that this is seldom the case and the metal cross-section can vary greatly depending on process-related aspects as shown in Fig. 4. Because it is difficult to predict how these will combine to affect the sheet resistance, modeling efforts should include a measurement of the metal resistivity at typical line widths and thicknesses (i.e. sheet resistance values of a 1 mm wide metal strip may be different from those measured on an 8 /~m wide grid line). An example of a metallization pattern used for this purpose is shown in Fig. 5 where the measured resistivity is defined using the a v e r a g e m e t a l t h i c k n e s s and the a v e r a g e w i d t h o f line o b s c u r a t i o n . This definition allows the modeling program to incorporate accurately all of the above mentioned aspects of cross-section and homogeneity. Once this measured value of the effective metal resistivity is determined, it is often helpful (and insightful) to indicate the difference between the ideal resistance value (i.e. that determined from the pure, bulk value assuming perfectly rectangular grid lines) and the measured value (as defined above) as the resistance ratio RR, where R R = ( m e a s u r e d sheet reslstance)/(ideal sheet resistance). Even for a well established metallization process, there are many mechanisms that can greatly affect the actual sheet resistance. Consider, for example, the process of high purity plated gold contacts. In this case, it takes less than 1% of iron contamination to increase RR to 10 [7]. Increases in resistivity also occur for contamination of gold by other metals (such as cobalt, tin, nickel and indium) although to a lesser extent. Additionally, our own research has shown that the plating current has a substantial effect on Modeled Semiconductor (a) Plated Deposits
Ev(b)orated Depos.ts(C)
(d)
(f) (h) Metal to Semiconductor Connection
(i)
?
(k)
Fig. 4. Diagram illustrating possible metal cross-sections for different deposition techniques.
465 I
I
T
probe Pads
.I l
Fig. 5. Pattern used to measure postprocess metal resistivity and line width.
a
b
c
lgm
1
Fig. 6. (a, b) SEM micrographs of gold plated onto ITO: (a) 5 p~rn gold plated at high current density ( ~ 0 . 5 ~ m min-1); Co) 1 /~m gold plated at low current ( = 0 . 1 ~m min-~); (c) 120 nm evaporated gold. Deposit (a) had a resistivity ratio ~ 6 - 8 times that of pure gold, Co) had a resistivity ratio of about 2 to about 3.
both the porosity of the gold deposit and on the measured resistivity (Figs. 6 and 7). In particular, standard bus-bar patterns have been electroplated onto indium tin oxide (ITO) films (100 um thick) to a thickness of 1 - 4 /~m and current-voltage measurements performed. These measurements suggest that RR is from about 2 to about 8; the higher values associated with higher current densities and a darker deposit appearance. Furthermore, the greater porosity of the deposit in Fig. 6(a) could enable oxidizing substances to enter the metal, causing the grid resistivity (and perhaps re) to be unstable.
466 120 C"
10.0
• Cr/Pd Auon[/[~
80 ~
60
~ 4o 2.0 01
012
013
014
015 010 07
Platingrate(#m/minl
Fig. 7. Plot of plated gold resistivity as a function of plating current. As indicated, these results were obtained for grid lines plated both directly on an ITO film and onto Cr/Pd adhesion layers. Because the sheet resistance of the gold is low compared with that of the ITO and the Cr/Pd layers, a reasonable estimate of the resistivity may be made. On a c t u a l cells, m e a s u r e m e n t s indicate t h a t RR is f r o m a b o u t 2 to a b o u t 3. As will b e shown, this v a r i a t i o n is sufficient to n e c e s s i t a t e c o n s i d e r a t i o n in t h e grid design. In c o n t r a s t to p l a t e d metals, v a c u u m e v a p o r a t e d c o n t a c t s ( p h o t o l i t h o g r a p h i c lift-off p r o c e s s e d ) a p p e a r to suffer less f r o m s h e e t r e s i s t a n c e variations. Indeed, m e a s u r e m e n t s o f e l e c t r o n - b e a m - e v a p o r a t e d silver c o n t a c t s d e p o s i t e d o n t o g l a s s s u b s t r a t e s indicate that, w i t h o u t g r e a t effort or control, RRs o f 1 . 7 - 1 . 8 a r e r o u t i n e l y p o s s i b l e o v e r a fairly wide r a n g e o f d e p o s i t i o n r a t e s ( 1 0 - 5 0 ~ s - 1 ) . Similarly, e l e c t r o n - b e a m - e v a p o r a t e d a l u m i n u m d e m o n s t r a t e s e v e n a l o w e r RR o f 1.6 (20 /~ s - l ) . Thus, w h e n c h o o s i n g a m e t a l to u s e for a grid line, it is n o t o n l y i m p o r t a n t to c h o o s e o n e w i t h a relatively low ideal resistivity, b u t to k n o w h o w t h e resistivity is e x p e c t e d to c h a n g e as a f u n c t i o n of d e p o s i t i o n a n d e n v i r o n m e n t a l conditions. F i g u r e 8 s h o w s t h e p e r c e n t a g e p o w e r loss as a f u n c t i o n o f RR f o r a o n e - s u n cell. F o r e a c h o f t h e t h r e e d e p o s i t t h i c k n e s s e s shown, t w o different situations a r e c o n s i d e r e d : t h e u p p e r c u r v e o f e a c h p a i r a s s u m e s t h a t the d e s i g n e r is u n a w a r e o f t h e h i g h e r grid line resistivity; the l o w e r curves, a s s u m e s t h a t t h e d e s i g n e r h a d b e e n a w a r e t h a t the grid lines w e r e o f h i g h e r resistivity a n d h a d a c c o u n t e d f o r this. Obviously, t h e first s i t u a t i o n is m o r e likely. Realizing t h a t resistivities m a y b e u p to eight t i m e s t h e ideal value, m o d e l i n g s h o w s t h a t if h i g h e r resistivities are n o t a s s u m e d , t h e n e v e n at o n e sun, t h e s e l o s s e s will c e r t a i n l y d o m i n a t e . H o w e v e r , as also shown, a p r u d e n t d e s i g n i n c o r p o r a t i n g t h i c k e r a n d / o r w i d e r grid lines c a n b e u s e d to insure a g a i n s t t h e s e variations. T h e s a m e effects o f i n c r e a s i n g the grid line resistivity are m o d e l e d f o r the c o n c e n t r a t o r cell in Fig. 9. H e r e it is s h o w n t h a t if RE Can b e r e d u c e d f r o m 6 0 0 to 100 ~/E], a n d if n e a r ideal m e t a l resistivity c a n b e a c h i e v e d , t h e n the efficiency will i n c r e a s e to a c o n c e n t r a t i o n ratio of a b o u t 100 suns. A l t h o u g h c o n c e n t r a t o r s y s t e m s are o f t e n d e s i g n e d for o p e r a t i o n u p to 1 0 0 0 suns, w h i c h is still b e y o n d the efficiency m a x i m u m for t h e a b o v e design, this e x e r c i s e illustrates t h e a d v a n t a g e s of k n o w i n g w h i c h ceil p a r a m e t e r s to a d d r e s s first to a c h i e v e t h e g r e a t e s t benefit.
467 80
i
i
i
3C
i
2E 6O
2E 24
O,o ~ 2O
22 2O
I
.
I
,
I 5.0 u,m thick grid, lines
2
3
4
5
6
7
Ratio of grid metal resistivity/ pure metal resistivity
8
9
18 10
16
i
10 100 Concentration ratio (x one sun)
1000
Fig. 8. Percentage power loss as a function of grid Line resistance for three metal thickness for a one-sun cell. Fig. 9. Efficiency v s . concentration ratio for a concentrator cell with RE equal to 100 ~K:] in which the grid metal resistivity ratio is treated parametrically. Also shown is the modeled result fiRE had not b e e n decreased from 60012/[:], illustrating this requirement for a concentrator design.
3.3. M i n i m u m line w i d t h The critical parameter of minimum line width often represents not only the least optimized parameter of an existing cell design but also the best avenue for future cell improvements. In this section, two separate aspects of this critical parameter are addressed. The first describes the practical guideline that, for typical metallizations, it is generally better to design a grid using the minimum line width that can be fabricated. This guideline suggests that many existing solar cell grid designs are probably not optimum, especially if they have not incorporated state-of-the-art techniques currently used in the micro-electronics industry. The second aspect, on the other hand, cautions that, because fabricating optimized grids with thinner lines also implies a grid with more fingers and closer spacing, several new processrelated problems are likely to occur. Indeed, ff these problems are not properly addressed, designing for the minimum line width, in itself, will probably not yield a better-performing solar cell. As implied in Fig. 1, for one-sun cells, the total power loss will, to a point, decrease substantially as the line width decreases. For the cell modeled in this figure, the point of minimum power loss will depend on the process parameter leading to the limiting aspect ratio of the narrow grid line. For example, in this modeled grid, the metal thickness is fixed at 5 /~m and thus, at some line width less than 10 /~m (at 10 /zm, the aspect ratio of line w i d t h / h e i g h t = 2 ) , the metal thickness will likely have to be decreased (otherwise adhesion loss will result), thereby leading to a minimum in the power loss. Although the idea that there is a line width at which the power loss will reach a minimum, for one-sun ceils and typical metallizations, this width is often submicron, and therefore not practical for solar cells. The general guideline of using the minimum line width is also illustrated in Fig. 10, where a series of curves representing different metal thicknesses show not only how the total power loss decreases as the line width decreases,
468
15'~ 14~ 13 ~-",
",
'
'--Constan't finge'r spac'ing --Variable finger spacing
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lo
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-
-
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~
~
-I
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1'4 1~
1'8 2o
Finger width (p.m) Fig. I0. Diagram illustrating the percentage power loss for a 4 am 2 grid as a function of line width. The grid metal thickness (silver, RR--1) has been treated parametrically. 40
i
35 \ ~ 3O
r
,
~
,
With Entech cover
20 15 ~ o- 10 5 ~
2 "WIH=t
W/H=2 - ~ - ~
/With H=IO Nm
Entech cover ,
I
0
10
20 30 40 Width of grid lines (Hm)
50
60
Fig. 11. Percentage power loss as a function of the grid line width (gold, RR= 1) and aspect ratio at 1000 suns. Note that there is a well-defined minimum position for each aspect ratio. Also shown is the effect of an Entech cover.
but also the p o w e r loss r e d u c t i o n as the metal thickness increases. Also s h o w n in this figure is the p o w e r loss which would b e e x p e c t e d if a grid with a fixed finger s p a c i n g (but varying finger width) w e r e used. Although the idea m e n t i o n e d a b o v e c o n c e r n i n g an o p t i m u m m i n i m u m line width is not usually c o n s i d e r e d for one sun grids, for c o n c e n t r a t o r cells o p e r a t i n g at high c u r r e n t density, it m u s t b e considered. This is illustrated in Fig. 11 (and Fig. 2); w h e n the a s p e c t ratio is t r e a t e d parametrically and the efficiency is c a l c u l a t e d as a f u n c t i o n o f the grid line width, t h e r e is an o p t i m u m line width for a given a s p e c t ratio which is in the 5 - I 0 ~ m range, and t h e r e f o r e c a n be fabricated. Also s h o w n in this figure is the m o d e l e d effect o f a t e c h n i q u e d e v e l o p e d b y E n t e c h Inc. in which the p o w e r loss due to s h a d o w i n g is r e d u c e d . This t e c h n i q u e is b a s e d o n the u s e o f a silicone plastic cover, the surface o f w h i c h is p a t t e r n e d to f o r m an a r r a y o f c o r r u g a t i o n s (prisms) which act to f o c u s the light away f r o m the grid lines [8]. As can be seen, w h e n this c o v e r is i n c o r p o r a t e d into cell c o n s t r u c t i o n , the influence
469
1a
2a
_L 3.2 pm
2.8p,m ~ T
T lb ..i_
2b ~4~m
_L ~4pm T
lc 3_ ~7.urn T
2c _L ~6p,m _
T
r
ld _.L
2d ..L ~8pm
m m
~7pm
T
T l
I
10p, m= 1/2" Fig. 12. Photographs illustrating line width broadening from about 3 to about 8 /~m as solar
cell grid line goes through the processing stages of a photolithographically defined metal liftoff. Column 2 illustrates the improved edge uniformity that results when residual photoresist on the photomask (present in the column 1 sequence, photo lb) is removed.
on t h e m o d e l e d p e r f o r m a n c e is quite s p e c t a c u l a r . H o w e v e r , o n e s h o u l d n o t e t h a t in this m o d e l i n g , a d d i t i o n a l reflection effects h a v e n o t b e e n c o n s i d e r e d . From the above mentioned arguments, one may be convinced that narrower grid lines a r e v e r y o f t e n a s i m p l e p a t h to a b e t t e r p e r f o r m i n g s o l a r cell grid. A l t h o u g h this is t r u e f r o m t h e m o d e l i n g s t a n d p o i n t , in p r a c t i c e , m a n y p r o c e s s r e l a t e d p r o b l e m s c a n a r i s e w i t h t h e s e thin-line grid d e s i g n s w h i c h c a n m o r e t h a n offset t h e m o d e l e d gains. T h e p r i m a r y r e a s o n f o r this is t h a t the relative size ( a n d t h u s t h e effect) o f a v a r i a t i o n in line w i d t h i n c r e a s e s d r a m a t i c a l l y as t h e line w i d t h n a r r o w s . In o t h e r w o r d s , b e c a u s e a thin-line d e s i g n will h a v e m a n y m o r e , v e r y closely s p a c e d grid fingers, o b s c u r a t i o n l o s s e s c a u s e d b y p r o c e s s r e l a t e d line b r o a d e n i n g will t e n d to d o m i n a t e a n y r e l a t e d gains due to d e c r e a s e s in line r e s i s t a n c e . As a n e x a m p l e , c o n s i d e r a grid line 50 ~ m wide, h a v i n g a 2 ~ m r e g i o n o n e a c h e d g e o f t h e line w h e r e irregularities o f s o m e s o r t are o b s e r v e d . A l t h o u g h it is b e l i e v e d t h a t this 2 ~ m r e g i o n h a s s u s p e c t r e s i s t a n c e a n d / o r a d h e s i o n quality, b e c a u s e it r e p r e s e n t s only
470 O.t 0.09 0.08 Plated Au, ~ / / RR=5
~[] 0.07 0.06
Evap.Ag / RR=2
~ 0.05
\
0.04 0.03
*~,
0.02
"*~"-
•~
-
x%%% 0.01
~-o 0 '
' 8 ' 12 1'6 ' 2'0 ' 2'4 ' 218 ' 3~2 ' 36 Minimum grid line width (p.m)
Fig. 13. Diagram combining the effects of ideal resistance of the grid metal, m e a s u r e d resistance ratio and minimum line width. Because many process-related aspects are already incorporated, illustrations such as this can be used with the modeling program to determine process-specific grid loss.
8% of the grid line, these uncertainties can often be incorporated into the model through appropriate assignment of the related critical parameters. However, if the line width is decreased to 6 ~m, and the metallization process is not improved, the region of suspect quality suddenly has increased to 67% of the metallized area and the line resistance a n d / o r adhesion will likely suffer (e.g. Fig. 12(d)). Another problem in fabricating thin-line grid designs is that when a new (thin) line width is incorporated into a design, there is often great uncertainty concerning how the imtial line width, assigned at the photomask development stage, will c om par e with the postprocessing line width a n d / o r what degree of variation can be e x p e c t e d during normal processing. This point, which is especially true in contact print lithography, is illustrated in Fig. 12 where two columns of photographs are c o m p a r e d showing how line width broadening propagates through a photolithographic lift-off process. Note that although the initial line width specified during phot om ask manufacture is 2 ~m, the final metallized line width is closer to 8 /~m. Additionally, not only does the line width expand (causing increased obscuration), but unless the photolithographic process is pe r f or m e d very carefully, the edge definition is often poor, causing the metal sheet resistance to increase more than would be
471 assumed. These line width constraints, t o g e t h e r with the line resistance constraints mentioned in the preceding section, are illustrated in Fig. 13. Figures such as this not only indicate how different metallization/process technologies compare, but, when considered with a p r o p e r modeling study, can also illustrate how solar cell grids fabricated with typical research-based technologies (evaporation, photolithography) com pare with grids fabricated with typical production technologies (plating, screen printing).
4. C o n c l u s i o n This study has established several guidelines for designing front contacts to solar cells that are not intuitively apparent. For one-sun applications incorporating relatively narrow grid lines, unless it is possible to establish a contact resistance of less than 10 -2 ~ am 2, this p a r a m e t e r will dominate the resistance losses. For concentration applications however, a minimum value of 10 -4 ~ cm 2 must be achieved, while even lower values, comparable to those of VLSI, would be preferred. Although, in practice, there may not be complete f r eedom t o lower the contact resistance without junction damage or adversely affecting cell performance, substantial reduction in re may be achieved by using techniques such as plasma cleaning and rapid thermal annealing. The resistivity of the grid line metal, its thickness and aspect ratio are important parameters in the grid design process. It has b e e n indicated that for evaporated metal deposits, variations in resistivity of twice the ideal value are usual. Similarly, for plated deposits, resistivities of up to eight times the ideal value are m or e usual than may be expected. If not account ed for, these variations in resistivity can dominate the solar cell power loss. Modeling reveals that, for solar cells operat ed at one sun using c o m m o n metallizations, it is always better to design the device with the narrowest grid line that can be fabricated. However, one must be cautious of several new problems that are often e n c o u n t e r e d when thin-line designs are fabricated. The most c o m m o n of these problems is that, as the line width narrows, the effect of line broadening b e c o m e s m uc h more severe. Also, although narrow grid lines are normally desirable, it is shown that at a concentration of 1000 suns, there is an optimum grid line width which depends on the metal resistivity and the aspect ratio. Finally, although the basic p r o c e d u r e s for studying grid losses were established by previous researchers, the practical application of these techniques is often neglected resulting in an unfiecessarily pessimistic view of the potential of many types of PV solar cells.
Acknowledgments The authors wish to thank A. Mason for SEM photographs. This work was s u p p o r t e d by the U.S. Department of Energy under Contract No. DE-
472
AC02-83CH10093 and by NASA Lewis Research Center under Interagency Order No. C-3000-K.
References 1 T. A. Gessert and T. J. Coutts, in Mat. Res. Soc. Symp. Proc., Vol. 181, Advanced MetaUizations in Microelectronics, MRS, Pittsburgh, PA, 1990, p. 301. 2 V.G. Karamidas, inProc. SeventhInt. Symp. on GaUiumArsenide andRelated Compounds, St. Louis, September 1978, Conf. Series Number 45, Institute of Physics, Bristol and London, p. 396. 3 R. H. Cox and H. Strack, Solid State Electron., 10 (1967) 1213. 4 W. C. Dautremont-Smith, P. A. Barnes and J. W. Staylt, Jr., J. Vac. Svi. TechnoL B, 2 (1984) 620. 5 T. A. Gessert, X. Li, T. J. Courts, M. W. Wan]ass and A. B. Franz, in Proc. First Int. Conf. on I n d i u m Phosphide and Related Materials f o r Advanced Electronic and Optical Devices, SPIE Proc., Vol. 1144, SPIE, Bellingham, WA, 1989, p. 476. 6 A. Katz, B. E. Weir, D. M. Maher, P. M. Thomas, M. Soler, W. C. Dautermont-Smith, R. F. Karlicek, Jr., J. D. Wynn and L. C. Kimerling, Appl. Phys. Left., 55 (1989) 2220. 7 F. H. Reid and W. Goldie (cds.), Gold Plating Technology, Electrochemical Publications, Ayr, 1974, p. 14. 8 M.J. O'Neill and M. G. Piszczor, in Proc. 20th IEEE Photovoltaics Specialists Conf, IEEE, New York, 1988, p. 1007.