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V(m
V )
DISTRIBUTION OF LOCAL OPEN-CIRCUIT VOLTAGE ON AMORPHOUS AND NANOCRYSTALLINE MIXED-PHASE Si:H AND SiGe:H SOLAR CELLS
National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, USA
United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI, USA
m
Evac
Ec
Ev
EFn EFp
ni
when it is illuminated.
Mixed phase
a-phase 1200
V oc
(mV )
(e)
(f)
(i): Example potential line profiles in the SKPM images. (j): Line profile of Voc deduced from the potential profiles.
•Nanocrystallites aggregate in the amorphous matrix with an aggregation size of
distribution shows valleys in the nanocrystalline aggregation area. regions is a gradual change within a distance
oc of a nc-Si:H single phase solar cell.
oc
Potential measurement on SiGe:H alloy cells
Line profile.
on the aggregation is more smooth-out than the case of Si:H
The information contained in this poster is subject to a government license. 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion,
C.-S. Jiang, H. R. Moutinho, M. M. Al-Jassim, L. L Kazmerski
B. Yan, J. Yang, and S. Guha
BACKGROUND RESULTS
• The best a-Si:H solar cells are deposited under the condition close to the (a) dark (b) illuminated
amorphous/nanocrystalline transition but still in the amorphous regime. p i WD+Voc• The best nanocrystalline silicon (nc-Si:H) solar cells are deposited under the WD pn
condition close to the transition but in the nanocrystalline regime Evac• The devices made on the transition condition show characteristics of a-Si:H/nc-Si:H
mixed-phase, having an V between 1.0 and 0.5 V. Ec EFEFoc
• The mixed-phase solar cells show light-soaking-induced Voc increase, which is EF Vocopposite to the light-soaking-induced Voc decrease in conventional a-Si:H cells.
• An original explanation for the light-induced V increase was light-induced Ev oc
structural changes from the crystalline to amorphous phase. • A complementary model is two parallel-connected diodes, one with a-Si:H diode
KP=CPD=WD KP=CPD=WD+Voccharacteristics and the other with nc-Si:H diode characteristics. • We have recently found an aggregation of nc-Si:H phase in the a-Si:H phase matrix, •KP measures the workfunction difference (WD) when it is in the thermoequilibrium
by imaging the local current flow using conductive AFM. state in the dark. •KP measures the sum of WD and local Voc• In this presentation, we report on local Voc distribution on the mixed-
phase solar cells, by using Scanning Kelvin Probe Microscopy (SKPM). Potential measurement on Si:H cells(c) (g)(a)
1200EXPERIMENTAL 1250 850
1100Scanning Kelvin Probe Microscopy (SKPM) 1200
8002 μmSemiconductor sample Metal probe
EVac mVmV mV 1000mV
PSPD 2MHz (b) (d) (h)Laser 150CPDHF 200Lock-in-amp(ωsf )
Filter EVacφLF 50Bimorph 150 100φ
Negative EC
s
feedback EF
100Frequency IG PG -+Cantilever 50Oscillate counterfeedback
Subs
trat
e 50+ Vdc Kelvin probe 0 nm 0 nm0 nm 0 nmEV+ Signal
Z Feedback ~ Vacsin(ωsf t) (j)(i)Scanner controller 1400 a-phase- 1100
1000
AFM Image 1200 Mixed phase
E = 12 C(Vt +VacSin(ωt) −Vs)
2 If Vt = Vs 1000
F1 = 0,1
Pot
entia
l (m
V)2 F F0 + F1 + F2E C (Vt −Vs) = 800 900= nc-phase2 1 ∂C ⎡(Vt −Vs)2 + 1 Vac
2 ⎤ Vt =
=
Wt +Vb 800F0 600∂E 1 ∂C
= = ∂z 2 ∂z
= ∂z ⎢⎣ ⎥⎦ Vs Ws2 2 2F (Vt −Vs)
nc-phase700400F1 =
∂C (Vt −Vs)VacSin(ωt) Vb = Ws −Wt
p-nc layer∂ Wt − − − Constantz 600200F 2 = −
∂∂ Cz Vac 2 COS (2ωt) Vb maps Ws 0 5000 1 2 3 40 1 2 3 4
Distance (μm)Distance (μm)
•SKPM is based on the non-contact mode of atomic force microscopy (NC-AFM) (a) and (b): SKPM potential and AFM topographic images on a-Si:H region. •SKPM measures contact potential difference (CPD) between the tip and sample (c) and (d): on nc-Si:H region.surface (e) and (f): on mixed-phase region. •SKPM detects the Coulomb force between the tip and sample using second resonant (g) and (h): on p-layer directly deposited on stainless steel substrate.frequency of the cantilever•Spatial resolution of SKPM: ~30 nm; Energy resolution: ~10 meV
Sample Preparation
(a) (b) ~0.5 μm. 51 C 52 53 54 55 1.2 •The VocB
•The transition from low to high Voc11 12 13 14
1.0
61 62 63 64 65 of around 1 μm.0.8 value in the nanocrystalline clusters in the mixed-phaseoc0.6 region is larger than the V
•The minimum VA 21 22 23 24
V oc (
V)
71 72 73 74 75 0.4
31 32 33 34 S40.2
S381 82 83 84 85 S2 Y0.0
S141 42 43 44 1 2 (c)(a) (b)3 4 110092 93 94 X
1000900
100•i-layer was deposited in a-Si:H/nc-Si:H transition condition. 8001000•Characteristics of i-layer are sensitive to location on the 700
substrate, center: a-Si:H, corner: nc-Si:H; edge: mixed-phase. 50 6002 μm 500950
mV 4000SUMMARY 0 nm 1 2 3 4 Distance (μm)
By combining SKPM and AFM, we have developed a method to measure the local V (a): SKPM potential image; (b) AFM; (c): Vocdistribution in mixed-phase solar cells. The results clearly show the nanocrystalline oc
aggregation. The Voc is smaller in the nanocrystalline aggregates than in the •nc-SiGe:H aggregation is smaller than the case of Si:Hsurrounding amorphous matrix, and the transition from the low to high Voc is a gradual •Vchange. Although there are some lateral charge redistributions, a clear distinction oc
between the amorphous and nanocrystalline regions has been observed. The current SKPM results and previous C-AFM results provide extra support for the two-diode model Waikoloa, Hawaii; May 7-12, 2006; NREL/PO-520-39865.
for explaining the carrier transport in the mixed-phase solar cells
150
200
20
40
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