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