Amorphous silicon/crystalline silicon heterojunctions:
The future of high-efficiency silicon solar cells
Zachary Holman
Arizona State University
February 28, 2014
The importance of efficiency
Goodrich et al., NREL/PR-6A20-50955 (2011).
2
Record efficiencies
3
Record efficiencies
4
Record efficiencies
5
Diffused-junction solar cells
Diffused-junction solar cell
No passivation
Chemical passivation
 Front passivation is insulating;
contact is made to the emitter
by “spiking” the Ag through the
passivation layer
Field-effect
passivation
6
 Poor passivation at front and
rear contacts limits Voc to ~650
mV
Silicon heterojunction solar cells
Heterojunction solar cell
Chemical
passivation
Chemical
passivation
7
 a-Si:H provides excellent
passivation of c-Si surface
 Charge can trickle through aSi:H layers; recombinationactive contacts are displaced
from c-Si surface
Voc and silicon heterojunction solar cells
 Lifetime > 1 ms; Voc > 730 mV
 Excellent Voc is trademark of
heterojunctions
Descoeudres et al., IEEE JPV 3, 83 (2013).
8
Getting to S-Q: Voc
 For 100-µm-thick c-Si wafer, S-Q
predicts Voc, max = 760-770 mV
 Commercially available c-Si cells:
Voc ≈ 650 mV
 Record-efficiency PERL c-Si cell:
Voc = 706 mV
 Sanyo Si heterojunction cell:
Voc = 750 mV!
S. De Wolf, et al., Green 2, 7 (2012).
9
Voc: What next?
 Increase Voc by squeezing more charge into a
smaller volume
 Can’t reduce recombination further, but can
thin wafer, which helps because generation is
mostly at the front
Generation
Generation
Generation
Recombination
Recombination
Recombination
S.C Baker-Finch, et al., PIP 20, 51 (2011).
10
Voc: What next?
τbulk=0.1 ms
 Voc improves with
decreasing wafer
thickness for surface
recombination
velocities < 100 cm/s
 Silicon heterojunctions
best technology for
thin wafers
Voc (V)
 Surface recombination
gains importance as
wafer is thinned
0.74
0.72
0.70
0.68
0.66
0.64
0.62
0.60
0.58
0.56
τbulk=1 ms
τbulk=10 ms
sf=sb=1 cm/s
10 cm/s
100 cm/s
1000 cm/s
50 100150200 50 100150200 50 100150200
Wafer thickness (um)
11
Record-high Vocs, but… low Jsc and FF
Current density (mA/cm2)
40
IMT heterojunction
Voc = 727 mV
2
Jsc = 38.9 mA/cm
FF = 78.4%
Eff. = 22.1%
30
20
UNSW PERL
Voc = 706 mV
2
Jsc = 42.7 mA/cm
FF = 82.8%
Eff. = 25.0%
10
0
0
200
400
600
Voltage (mV)
A. Descoeudres et al., IEEE J. Photovoltaics 3, 83 (2013).
12
800
Where does the light go?
100-µm-thick wafer; 20.8%
Holman et al., IEEE JPV (submitted).
13
Where does the light go?
 Reflection from Ag grid and TCO anti-reflection coating
 UV and blue parasitic absorption in front a-Si:H layers
 UV and IR parasitic absorption in front TCO; IR parasitic
absorption in rear TCO
 Incomplete trapping of IR light
14
UV and blue parasitic absorption
Holman et al., IEEE JPV 2, 7 (2012).
15
15
Parasitic absorption in the front a-Si:H layers
3
 All light absorbed in a-Si:H p-layer is lost
 70% of light absorbed in a-Si:H i-layer is lost
2
 UV loss in TCO is small (0.2 mA/cm2)
1
i-la 12
ye 9
r th 6
ick
ne 3
ss 0
(nm
)
16
Jsc loss (mA/cm2
)
4
0
12 )
9
m
6 ess (n
3
ckn
i
h
t
0 yer
p-la
Thinner layers?
 Current decreases with p-layer
thickness, but FF improves
 Current decreases with i-layer
thickness, but Voc improves
 Overall trend is in favor of Jsc
 i-layer should be thick enough to
provide passivation, but no thicker
0
5 10 15 20
p-layer thickness (nm)
Efficiency (%)
74
73
0
5 10 15 20
p-layer thickness (nm)
FF (%)
680
20.5
76
20.0
75
19.5
19.0
Jsc (mA/cm2)
690
Jsc = 38.0–0.16 tp-layer
0
5 10 15 20
p-layer thickness (nm)
77
75
700
36
35
76
710
17
37.0
0
5
10
15
i-layer thickness (nm)
0
5
10
15
i-layer thickness (nm)
20.5
74
73
0
5 10 15 20
p-layer thickness (nm)
37.5
36.5 Jsc = 37.9–0.10 ti-layer
Efficiency (%)
715
37
FF (%)
Voc (mV)
720
720
Voc (mV)
Jsc (mA/cm2)
38
710
38.0
730
725
0
5
10
15
i-layer thickness (nm)
20.0
19.5
19.0
18.5
0
5
10
15
i-layer thickness (nm)
More transparent materials?
 n and k both decrease as CO2 is
added to plasma: a-Si:H → a-SiOx
 µc-Si has lower k than a-Si:H
 Growth is substrate dependent: hard to
grow thin, highly crystalline layers on,
e.g., a-Si:H
 FF falls very rapidly
18
18
IR parasitic absorption
Holman et al., JAP. 113, 013107 (2013).
Barraud et al. SolMat, 115, 151 (2013).
Holman et al., SolMat. (in press).
Holman et al., Light (submitted).
Holman et al., IEEE JPV (submitted).
19
19
IR EQE and reflectance: From ideal to real
100
EQE
EQE and
and 1-R
1-R (%)
(%)
80
60
40
20
0
900
Increasing
parasitic absorption
1000
1100
Wavelength (nm)
1200
 Parasitic absorption reduces both EQE and R
 Rsub (i.e., R @ λ=1200 nm) is metric of IR parasitic absorption that can be
measured on insulating test structures
20
20
IR parasitic absorption
21
High-mobility hydrogen-doped indium oxide (IO:H)
 Replace front ITO with highmobility IO:H (µ > 100 cm2/Vs)
 Less (parasitic) free-carrier
absorption for equivalent sheet
resistance
 An ultrathin ITO layer is still
required for good contact to
screen-printed Ag fingers
Koida et al., several papers between 2007-2013.
22
Optimized rear TCO layers
100
90
80
70
60
t
50 Rear ITO thickness (nm)
40
147
65
30
33
20
8
10
0
0
400600800 1000
1100
Wavelength (nm)
EQE and 1-reflectance (%)
EQE and 1-reflectance (%)
100
90
N
80
N
70
60
-3
)
(cm
50 Rear ITO carrier density
19
2.6 x 10
40
6.5 x 1019
30
1.0 x 1020
20
2.0 x 1020
10
3.0 x 1020
0
400600800 1000
1100
1200
Wavelength (nm)
 Ideal rear TCO layer is very transparent (to avoid FCA in the TCO), and …
 >150 nm thick (to supress plasmon excitation in the metal reflector)
23
1200
Record cells
24
Adding a low-refractive-index rear dielectric layer
SiNx, 70 nm
Si
dielectric, variable t, nd
Ag
For a 300-nm-thick layer of air (nd = 1), only 4% of
incident light is absorbed in the Ag layer (rr = 99.8%)
25
Really Rockin’ Rear Reflector (RRRR)
26
RRRR performance
It’s really rockin’…but the old reflector was
already very good. In industry, probably not
worth the cost of the extra processing step.
27
RRRR and CheapRRRR performance
Best rear reflector ever?
28
Summary
24
Acknowledgments
 A. Descoeudres, J. Wu, D. Rowe, D. Alexander, J. Seif, S.
De Wolf, U. Kortshagen, C. Ballif
 European Union 7th Framework Program (FP7/2007-2013),
Project '20plµs‘, #256695
 Axpo Naturstrom Fonds, Switzerland
29
Sanyo
22
<100 µm
20
18
16
14
>200 µm
IMT
2x2
10x10
12.5x12.5
200
0
200
2
200
4
200
6
200
8
201
0
201
2
Efficiency (%)
 a-Si:H provides excellent surface passivation and
charge extraction; recombination-active metal
contacts are removed from the wafer surface
 Heterojunction design enables Voc > 730 mV;
further improvements only if wafer is thinned
 But…heterojunction design compromises FF, Jsc
 Reduce blue parasitic absorption in a-Si:H layers
with thin layers, transparent materials, backcontacted design, or downshifters
 Reduce IR parasitic absorption with high-mobility
TCOs, and “detached” rear metal reflector