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 

n

i 

when it is illuminated. 

Mixed phase 

a-phase 

1200

V

o

c

m

(

V

)

(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.

o

c

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+V • The best nanocrystalline silicon (nc-Si:H) solar cells are deposited under the WD p oc n

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 E oc EF F

• The mixed-phase solar cells show light-soaking-induced Voc increase, which is EF Voc opposite 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+V characteristics and the other with nc-Si:H diode characteristics. oc

• 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 mixedphase solar cells, by using Scanning Kelvin Probe Microscopy (SKPM). Potential measurement on Si:H cells

(a) (c) (g) 1200 EXPERIMENTAL 1250 850 

1100 Scanning Kelvin Probe Microscopy (SKPM) 1200 

2 μm 800

Semiconductor sample Metal probe 

EVac 

mV mV mV 1000mV

PSPD 2MHz Laser CPD (b) (d) 150 (h) HF Lock-in-amp 200

(ωsf ) Filter EVac φ
 Bimorph LF φ 50 150 100

Negative EC 

s 

feedback 

EF

Frequency 100 Cantilever IG PG + - Oscillate counter 50

feedback

Substrate

+ V 50 dc 

Kelvin probe 

+ EV 0 nm 0 nm 0 nm 0 nm Signal

Z Feedback ~ Vacsin(ωsf Scanner t) (i) (j) controller 1400 a-phase - 1100

1000

AFM Image 1200 Mixed phase E = 1

2

C(Vt +VacSin(ωt) −Vs)

2 If Vt =Vs

1000 

F1 1 = 0,

Potential (mV)

2 F F0 + F1 E C(Vt −Vs) + F2 = = 800 900 2 nc-phase 1 ∂C ⎡

(Vt −Vs)

2 +

1Vac

2 ⎤ Vt =

=

Wt +Vb

∂E 1 ∂C F0 600 800

= =

∂z 2 ∂z

= ∂z ⎢

⎣ ⎥⎦ 2 Vs Ws F (Vt −Vs) 2 2

400 700 nc-phase

F1 = ∂C (Vt −Vs)VacSin(ωt) Vb = Ws −Wt

∂z Wt − − − Constant p-nc layer 200 600

F2 = − ∂

∂

C

z

Vac

2

COS(2ωt) Vb maps Ws 0 500 0 1 2 3 4 0 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

B

•The Voc

•The transition from low to high Voc

11 12 13 14 1.0

61 62 63 64 65 of around 1 μm. 0.8

value in the nanocrystalline clusters in the mixed-phase oc 0.6 region is larger than the V

•The minimum V A 

21 22 23 24 

Voc (V) 

71 72 73 74 75 

0.4

31 32 33 34 

0.2 S4

S3 81 82 83 84 85 

0.0 S2 Y

S1 41 42 43 44 1 2 3 (a) (b) (c) 4 1100 92 93 94 X

1000

900

100

•i-layer was deposited in a-Si:H/nc-Si:H transition condition. 1000 800

•Characteristics of i-layer are sensitive to location on the 700

substrate, center: a-Si:H, corner: nc-Si:H; edge: mixed-phase. 50 600

2 μm 950 500

mV 400 SUMMARY 0 nm 0 1 2 3 4 

Distance (μm) 

By combining SKPM and AFM, we have developed a method to measure the local Voc (a): SKPM potential image; (b) AFM; (c): V distribution 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:H surrounding amorphous matrix, and the transition from the low to high Voc is a gradual •V change. 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