The rationale for pursuing high efficiency crystalline silicon technology research is discussed. Photovoltaic energy systems are reviewed as to their cost effectiveness and their competitiveness with other energy systems. The parameters of energy system life are listed and briefly reviewed

Prince, M. B.

English

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HIGH-EFFICIENCY CRYSTALLINE SILICON 
TXHNOLOGY DEVELOPMENT 
U.S. DEPARTMENT OF ENERGY 
Morton 6. Prince 
I have been asked t o  g ive  the  r a t i o n a l e  f o r  pursuing h igh-e f f i c i ency  
c r y s t a l l i n e  s i l i c o n  technology resea rch  and development a c t i v i t i e s .  
Let me s t a r t  by quot ing the  purposes of the  Nat ional  PV Program from t h e  
l a t e s t  Five-Year Research Plan. 
I t  In  accordance with l e g i s l a t i v e  mandates and recen t  po l i cy  guidance,  the  
Nat ional  Pho tovo l t a ics  Program sponsors high r i s k ,  p o t e n t i a l l y  high payoff 
r e sea rch  and developmerrt i n  pho tovo l t a ic  energy technology which w i l l  r e s u l t  
i n  a  technology base  from which p r i v a t e  enterprise can choose op t ions  f o r  
f u r t h e r  development and compet i t ive  a p p l i c a t i o n  i n  U.S. e l e c t r i c a l  energy 
markets." 
I n  o r d e r  f o r  ;he p r i v a t e  s e c t o r  t o  compete i n  t h e  U.S. e l e c t r i c a l  energy 
markets ,  pho tovo l t a ic  energy systems must be a b l e  t o  produce e l e c t r i c i t y  a t  o r  
about t h e  same c o s t  a s  o t h e r  competing energy systems such a s  o i l ,  g a s ,  c o a l ,  
nuc lea r  o r  any o t h e r  source  of  e l~e rgy .  PV may be a b l e  t o  command a  s l i g h t l y  
h igher  p r i c e  than some of t h e  o t h e r  sources  because of  i t s  s a f e t y ,  
non-pollution and o t h e r  b e n e f i t s .  However, t h i s  premium should not  b l ind  us 
t o  the  b r u t a l  f a c t  t h a t  c o s t  is  the  main d r i v i n g  f o r c e  i n  our  s o c i e t y  i n  
s e l e c t i n g  i t s  energy sources .  
Energy c o s t  a n a l y s t s  a t  JPL, Sandia,  E P R I  and t h e  Aerospace Corp. have 
come up wi th  e s s e n t i a l l y  t h e  same r e s u l t s  r ega rd ing  t h e  necessary  c o s t s  t h a t  
must be yeached. The 30-year l e v e l i z e d  c o s t  of  e l e c t r i c i t y  must g e t  down t o  
about ljd/kWh, o r  a t  today 's  c o s t  ebout 7d/kWh. 
The a n a l y s i s  has ended up wi th  s e v e r a l  g r a  h s  (which a r e  i n  the  5 Five-Year Plan) t h a t  r e l a t e  module c o s t s  ( i n  $/m and i n  $/wp), module 
e f f i c i e n c y  and l e v e l i z e d  e l e c t r i c i t y  c o s t s  (d/kWh). These graphs have been 
made w t h  many assurnp t ion~  t h a t  d i f f e r  i n  some parameters f o r  f l a t - p l a t e  
modules and concen t ra to r  modules 
These parameters inc lude  system l i f e ,  the  r a t i o  of  system e f f i c i e n c y  t o  
module e f f i c i e n c y ,  c a p a c i t y  f a c t o r ,  average peak i n s o l a t i o n ,  annual ObH c o s t s ,  
i n d i r e c t  c o s t  m u l t i p l i e r ,  p resen t  worth f a c t o r ,  f ixed  charge r a t e ,  c a p i t a l  
recovery and balance-of-system c o s t s ,  both a r e a - r e l a t e d  and power-related. I 
r e f e r  t o  t h e  Plan appendix which d e s c r i b e s  a l l  of t h e s e  parameters i n  d e t a i l  
and g i v e s  numerical  d a t a  f o r  them. I pe r sona l ly  have some q u e s t i o n s  about 
some of t h e  numbers, but  the  d i f f e r e n c e s  do not s i g n i f i c a n t l y  change t h e  
r e s u l t s .  Let us  look a t  two of  the  f i g u r e s  from t h e  Plan. 
The f i r s t  f i g u r e  is f o r  f l a t - p l a t e  PV systems wi th  $75/m2 f o r  area-  
r e l a t e d  BOS c o s t s  (Figure  l ) .  A s  you can s e e ,  i n  o r d e r  f o r  the  PV system t o  
meet t h e  15//kWh l i n e ,  the  module e f f i c i e n c y  must be g r e a t e r  than IS%, and 
https://ntrs.nasa.gov/search.jsp?R=19850024082 2020-03-22T20:05:41+00:00Z
PLENARY SESSIONS 
even then the module cost must he less than $35/m2 (or approximately 
20Clwatt). If the module efficiency can te increased to 20%, the cost of the 
module in the system can be up to $75/m2 (or about 37d/watt). Thus, one can 
start to see the pressure on obtaining high efficiency. 
Figure 2 shows similar information for a concentrator system with 
$125/m2 area-related BOS cost. In this case, one can see that the minifilm 
meaningful cell efficiency is 15%, which will allow a mcdule cost of $50/m2 
of aperture. The $50/m2 level will be extremely difficult to meet. A more 
realistic set of numbers is efficiency of 35% and module cost of $135/m2 of 
aperture. 
Thus, it is necessary to get to flat-plate module efficiencies of 
approximately 20% (and thus cell efficiencies of 22%) and concentrator cell 
efficiencies of about 35%. Also, you now have a better idea of what same of 
our problems are in designing the PV program within the limited budgets that 
we have been working with during the past few years. Tt is possible to trade 
off lower B X  costs to allow lower cell and module efficiencies, but unless 
some breakthrough is made in the BOS area of work, T believe the BOS costs 
assumed above will be difficult to beat. Besides the PV Five-Year Plan, T 
recommend that you read the EPRI Report AP-3351 titled "PV Power Systems 
Research Evaluation," especially Section 2, if you are interested in more 
detailed information on the economic requiremants for PV systems. 
The Program Office has, with the help of the three lead lab -tories, 
come up with a set of activities that, we believe, will help industry reach 
these high efficiencies. This set includes work on silicon devices, 
single-junction multielement devices, multijunction thin-film devices (for 
flat-plate modules) and multijunction structures for concentrator modules. 
Data on single-junction cells are shown in Figure 3. These data are for 
laboratory devices. The expected near-term efficiencies are given in the 
right hand columns. As you can see, none of these single-junction cells will 
ever meet the 35% efficiency needed for the concentrator module. Only a 
couple of materials have the potential far meeting the flat-plate cell 
efficiency of 22% or more. In the near term, tying costs with efficiency 
results in the fact that single-crystal silicon is the strongest contender for 
a single-junction device that may meet the objectives discussed above. 
As most of you know, it is possible to obtain higher efficiencies by 
stacking more than one cell on top of another to make better use of the solar 
spectrum. Several analysee have been nade that indicate that the efficiency 
can be increased to over 55% with about 10 to 20 junctions of different 
materials stacked upon one another. However, the difficulties of developing 
such a structure are so horrendous that we can only think in terms of two-cell 
and three-cell stacks at this time. These smaller stacks obtain the major 
increase of efficiency over the single-junction device efficiencies in any 
case. Figure 4 shows what is possible theoretically for single-junction, 
two-cell stack, and three-cell stack material systems for flat-plate modules 
and 500X concentrator modules. This chart also shows the optimum band gaps 
that should be used., Note the bottom line combination which uses silicon as 
the lower cell. Flgure 5 (from John Fan) shows that the band-gap selection is 
not very critical. Thus many combinations of materials are po~sible that will 
meet both the flat-plate module and concentrator-module requirements. 
PLENARY SESSIONS 
To b u i l d  s u c c e s s f u l  s t a c k s  wi th  t h e s e  e f f i c i e n c i e s ,  i t  i s  necessary  f o r  
each c e l l  i n  the  s t a c k  t o  be e f f i c i e n t  i n  i t s  own r i g h t .  Thus, a g a i n ,  s i l i c o n  
appears  t o  be a  r e l a t i v e l y  near-term component f o r  a  u s e f u l  system, and i t  
. must be h igh ly  e f f i c i e n t .  
Now t h a t  I have developed the  r a t i o n a l e  f o r  the  need t o  maximize the  
e f f i c i e n c y  of c r y s t a l l i n e  s i l i c o n  PV d e v i c e s ,  l e t  me very b r i e f l y  s e t  t h e  
background f o r  the  o t h e r  speakers  of t h i s  morning's program. 
Figure  6 l i s ts  many of the  f a c t o r s  t h a t  must be considered i n  the  des ign 
of  a  h igh-eff ic iency device.  Many of these  f a c t o r s  can only  be optimized a t  
t h e  expenec of o t h e r  f a c t o r s ,  and i t  is  t h i s  complex o v e r a l l  opt imizat ior i  t h a t  
has given us d i f f i c u l t i e s  over  the  pas t  many years .  However, we a r e  g radua l ly  
c l o s i n g  i n  on the  opt imal  t r ade -of f s ,  and a s  you know c e l l s  with over  19% have 
been obta ined f o r  s i l i c o n .  
The l a s t  f i g u r e  (Figure  7 )  shows how many of  these  f a c t o r s  f i t  i n  the  
conceptual  pho tovo l t a ic  c e l l .  
PLENARY SESSIONS 
Figure 1 .  Module Costs and Efficiencies vs 36-Year 
Levelized Electricity Costs for Flat-Plate 
Photovoltaic Systems 11 751m2 Area-Related BOS) 
hl 
E 100- 
% h
w 
F1 
Q 
z' Module Efficiency 
Y 
- 
Y) 
0 
0 
0, 
- 
2 5 0 -  
r" 
5 10 15 20 2 5 
Levelired Electricity Cost ( t l kWh in current $1 
PLENARY SESSIONS 
Figure 2. Module Costs and Cell Efficiencies vs 30-Year 
Levelized Electricity Costs for Concentrator 
Photovoltaic Systems ( 5  1 251m2 Area-Related BOS) 
Cell Efficiency 
Levohzed Electricity Cost (;/kwh h current $1 
PLENARY SESSIONS 
Figure 3. Singie-Junction Photovoltaic Cells 
CELL MATERIALS 
SINGLE-CRYSTAL 
SILICON 
SILICON (SOOX) 
GaAa THlN FILM (THICK) 
GaAs (580X) 
AMORPHOUS SILICON 
~ u l n S c ~ / ~ d ~  ** 
C ~ T ~ I C ~ S  ** 
- 
LABORATOR CELL EFFICIENCY (%)* 
rt (THEORY) 
*CONCENTRATION RATIO = I SUN (1x1 UNLESS DESIGNATED OTHERWISE 
(500 SE'Y CONCENTRATION RATIO = SOOX) 
**POLYCRYSTALLINE THlN FILM CELLS 
Figure 4. Theoretical Efficiencies 
- 
OPTIMAL BANDGAP(S) ft (THEORY) 
(TOP TO BOTTOM) 2 8 ' ~  
SINGLE-JUNCTION CELL ( I  SUN) 28% 
1.45 EV 
SINGLE-JUNCTION CELL (SOOX) 32% 
TWO-CELL STACK ( I  SUN) 
TWO-CELL STACK (SOOX) 
THREE-CELL STACK ( I  SUN) 
THREE-CELL STACK (SOOX) 
C 
TWO-CELL STACK (I SUN) 1.8 - 1.1 EV 34% 
(LOWER CELL-SILICON) 
PLENARY SESSIONS 
Figure 5, Two-Junction Photovoltaic Converter 
Iso-Eff iciency Lines 
Independent calls 
T = 3 0 0 ~ ~  
C = 1000 AM2 suns 
Top Cell Bandgap (eV) 
Figure 6. Factors Affecting Efficiency 
MECHANICAL FACTORS DEVICE FACTORS 
PHYSICAL FACTORS 
PLENARY SESSIONS 
Figure 7 


High-efficiency crystalline silicon technology development

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