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NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Distribution of Local Open-Circuit 

Voltage on Amorphous and 

Nanocrystalline Mixed-Phase Si:H 

and SiGe:H Solar Cells 

Preprint 

C.-S. Jiang, H.R. Moutinho, M.M. Al-Jassim, and 

L.L. Kazmerski 

National Renewable Energy Laboratory 

B. Yan, J.M. Owens, J. Yang, and S. Guha 

United Solar Ovonic Corporation

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39882 

May 2006 
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DISTRIBUTION OF LOCAL OPEN-CIRCUIT VOLTAGE ON AMORPHOUS AND 

NANOCRYSTALLINE MIXED-PHASE Si:H AND SiGe:H SOLAR CELLS*

C.-S. Jiang, H. R. Moutinho, M.M. Al-Jassim, L. L. Kazmerski 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 

B. Yan, J. M. Owens, J. Yang, and S. Guha 

United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084 

ABSTRACT 

Local open-circuit voltage (Voc) distributions on 

amorphous and nanocrystalline mixed-phase silicon solar 

cells were measured using a scanning Kelvin probe 

microscope (SKPM) on the p layer of an n-i-p structure 

without the top ITO contact. During the measurement, the 

sample was illuminated with a laser beam that was used 

for the atomic force microscopy (AFM). Therefore, the 

surface potential measured by SKPM is the sum of the 

local Voc and the difference in workfunction between the p

layer and the AFM tip. Comparing the SKPM and AFM 

images, we find that nanocrystallites aggregate in the 

amorphous matrix with an aggregation size of ~0.5 μm in 

diameter, where many nanometer-size grains are 

clustered. The Voc distribution shows valleys in the 

nanocrystalline aggregation area. The transition from low 

to high Voc regions is a gradual change within a distance of 

about 1 μm. The minimum Voc value in the nanocrystalline 

clusters in the mixed-phase region is larger than the Voc of 

a nc-Si:H single-phase solar cell. These results could be 

due to lateral photo-charge redistribution between the two 

phases. We have also carried out local Voc measurements 

on mixed-phase SiGe:H alloy solar cells. The magnitudes 

of Voc in the amorphous and nanocrystalline regions are 

consistent with the J-V measurements. 

INTRODUCTION 

It has been shown that the best hydrogenated 

amorphous silicon (a-Si:H) solar cells are deposited under 

the condition close to the amorphous/nanocrystalline 

transition, but still in the amorphous regime [1], whereas 

the best hydrogenated nanocrystalline silicon (nc-Si:H) 

solar cells are deposited under the condition close to the 

transition, but in the nanocrystalline regime [2]. Solar cells 

made under the transition condition are called mixedphase solar cells and show a distribution of open-circuit 

voltage (Voc) ranging from 0.5 to 1.0 V [3,4]. One 

interesting phenomenon in the mixed-phase solar cells is 

the light-soaking-induced Voc increase [3,4], which is 

opposite to the light-soaking-induced Voc decrease in 

conventional a-Si:H cells caused by the Staebler-Wronski 

effect [5]. Our original explanation for the light-induced Voc

increase was light-induced structural changes from the 

crystalline to amorphous phase [3,4]. Subsequently, we 

proposed a complementary model with two parallelconnected diodes (two-diode) for explaining the Voc

changes with crystalline volume fraction and light-induced 

Voc increase in the mixed-phase solar cells [6]. The key 

element in the two-diode model is that the amorphous 

phase and crystalline phase can be considered as two 

separate diodes with significantly different characteristics 

of a-Si:H and nc-Si:H solar cells. However, the challenge 

is that the nanocrystallites observed by X-ray diffraction 

and Raman are very small with a size of a few nm to 30 

nm. It is difficult to believe that such small grains can form 

complete diodes through the intrinsic layer. Recently, we 

used a conductive atomic force microscope (c-AFM) to 

measure the local current flow and found that the 

nanocrystallites aggregated and the nanocrystalline 

aggregation area showed a significantly higher forward 

current than the amorphous area [7]. 

In this paper, we report our new results of local Voc

distribution in the mixed-phase solar cells measured using 

a scanning Kelvin probe microscope (SKPM). We have 

established this nanometer-scale measurement technique 

[9,10] and applied this method for mapping the twodimensional electrical potential distributions in III-V, 

Cu(In,Ga)Se2, and a-Si:H solar cells [10-12]. The 

measurement results have improved our understanding of 

device physics in the solar cells and provided us a direct 

method to assess the quality of the p-n junctions. 

EXPERIMENTAL 

Mixed-phase Si:H and SiGe:H n-i-p structures were 

deposited onto a 4×4 cm2

 stainless-steel substrate. The n

layer is an a-Si:H layer doped with phosphorus and the p

is a nc-Si:H layer doped with boron. The i layer was 

deposited under the condition of the 

amorphous/nanocrystalline transition regime. Due to the 

high sensitivity of the transition to the plasma properties, 

the mixed-phase solar cell shows different characteristics 

at different locations even on the same substrate. Figure 1 

(a) shows the solar cell distribution on the 4×4 cm2

stainless-steel substrate, where the large circles are 

indium tin oxide (ITO) dots with a area of 0.25 cm2

 and the 

small ones of 0.05 cm2

. Figure 1 (b) plots the Voc of the 16 

large solar cells. One can see that the center area shows 

an amorphous signature, the outer region a mixed-phase, 

and the corners substantially nanocrystalline 

characteristics. 

*This work at NREL has been authored by Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The United States Government 

retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to 

publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

1
The SKPM measurements were made at the three 

areas labeled with A, B, and C, as shown in Fig. 1(a), 

where no ITO was deposited. The SKPM technique is 

based on the non-contact mode of AFM [8-10]. Similar to 

the classical Kelvin probe, SKPM measures the contact 

potential difference (CPD) between the sample surface 

and the probe. However, unlike the classical Kelvin probe 

using a small ac current signal, SKPM measures the 

Coulomb force between the sample and the AFM tip. The 

extremely small distance between the tip and sample 

surface (~nm) and the extremely force-sensitive cantilever 

(~nN/m) ensure high resolutions in both spatial (~30 nm) 

and energy (~10 mV). Details of the technique are 

described elsewhere [9,10]. 

RESULTS AND DISCUSSIONS 

During the SKPM measurements, a built-in laser 

(used for the AFM operation) with 1.85-eV photon energy 

is used to illuminate the sample. In the thermal 

equilibrium state in the dark, the CPD measured by SKPM 

is the workfunction difference (WD) between the sample 

and the tip, as illustrated by Fig. 2(a). However, when the 

solar cell sample is illuminated, the CPD is the sum of WD 

and the local Voc, as shown in Fig. 2(b). To obtain the Voc

distribution, the WD of the p layer must be measured and 

subtracted from the measured CPD of the n-i-p structure. 

We deposited a p-type nc-Si:H layer directly on stainlesssteel substrates and measured the CPD. Because the p

layer is heavily doped and there is no p-n junction in the 

sample, the measured CPD should be close to the WD, 

even though the sample is illuminated by the laser light 

during the measurement. 

Figure 3 shows the SKPM CPD and the 

corresponding AFM topographic images taken on the 

amorphous, substantially nanocrystalline, and mixedphase regions, as well as on the p-type nc-Si:H single 

layer. The potential distributions are relatively uniform on 

the amorphous and nanocrystalline single-phase regions, 

as shown in Fig. 3 (a) and (c), as well as on the p layer in 

Fig. 3 (g). The microstructures on the morphology of the 

amorphous region [Fig. 3 (b)] show a small surface 

roughness in this region, while the cauliflower-like 

structures in the nanocrystalline region [Fig. 3 (d)] are 

aggregates of nanometer-size grains, which results in a 

large surface roughness. On the mixed-phase region, a 

few large cauliflower-like structures appear on the 

(a)

11 12 13 14

21 22 23 24

31 32 33 34

41 42 43 44

51 52 53 54 55

61 62 63 64 65

71 72 73 74 75

81 82 83 84 85

92 93 94

A

B

C

1 2 3 4

S1

S2

S3

S4

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Voc (V)

X

Y 

(b)

FIG. 1. (a) Solar cell distribution on a 4 cm × 4 cm 

substrate. The squares labeled A, B, and C are the 

three areas where SKPM electrical potential and AFM 

morphology images are taken. (b) Voc distribution of 

the sixteen 0.25-cm2

 cells on the same substrate. 

WD WD+Voc

KP=CPD=WD+V KP=CPD=WD oc

Voc

Evac

Ec

Ev

EF

EF

Evac

Ec

Ev

EFn EFp

EF

p

i

n n i p

(a) dark (b) illuminated

Fig. 2. Schematic band diagrams in the SKPM measurements. In (a), the SKPM measures the WD between the tip and 

the sample surface in the thermal equilibrium state; and in (b), it measures the sum of WD and Voc when the sample is 

illuminated. 

2
topographic image [Fig. 3 (f)], which correspond to the 

valleys in the Voc distribution image [Fig. 3 (e)]. Note that 

the sizes of the Voc valleys are larger than the 

corresponding nanocrystalline aggregates that appeared 

in the AFM image. 

Figure 3 (i) shows line profiles of the SKPM images of 

the amorphous, nanocrystalline, and mixed-phase regions 

along the lines indicated by the arrows in Fig. 3 (a), (c), 

and (e), respectively. The three dotted straight lines are 

the averaged values through several SKPM images of the 

amorphous and nanocrystalline regions, and the p layer. 

The difference between the amorphous and 

nanocrystalline regions is 0.45 V, which is consistent with 

the Voc difference between a-Si:H and nc-Si:H solar cells 

measured by J-V characteristics. Figure 3 (j) show the Voc

line profile of the mixed-phase region obtained by 

subtracting the p-layer potential from the potential profile 

in Fig. 3 (i). Two important features are observed from 

this plot. First, the minimum Voc value in the 

nanocrystalline valleys is much larger than the average 

value in the fully nanocrystalline region, as indicated by 

the bottom straight line. Second, the transition from the 

low to high Voc regions is a gradual change within a 

distance of about 1 μm. We think these two phenomena 

are related and caused by the lateral charge redistribution. 

The electrical interaction between the two phases by the 

relatively conductive p layer and the charge redistribution 

near the amorphous/nanocrystalline boundary in the bulk 

of the i layer could be a mechanism. 

1200

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(b)

(c)

(d)

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nc-phase

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(j)

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(f)

(g)

(h)

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(j)

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mV

0 nm

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100

150

150

200

mV

0 nm

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40

(e)

(f)

(g)

(h)

2 μm

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Fig. 3. SKPM potential and the corresponding AFM topographic images taken on (a) and (b) the amorphous 

phase, (c) and (d) the nanocrystalline phase, (e) and (f) the mixed-phase regions of an n-i-p solar cell. (g) and (h) 

were taken on a p-type nc-Si:H layer directly deposited on the stainless-steel substrate. Solid-line profiles in (i) 

show the potentials along arrows in (a), (c), and (e). Dotted straight lines in (i) show averaged potential values 

over several SKPM images. (j) shows the Voc line profile in the mixed-phase deduced from the potentials in (i). 

Although there is a lateral interaction between the 

amorphous phase and nanocrystalline aggregation areas, 

the Voc characteristic shows a clear distinction between 

the two phases. This is consistent with our recent C-AFM 

measurement on the mixed-phase cells [7]. The twodimensional electrical current images show much larger 

local current flow through the nanocrystalline aggregates 

than through the surrounding a-Si:H matrix. However, 

unlike the gradual potential transition that is related to the 

charge redistribution around the 

amorphous/nanocrystalline boundary, the transition of 

local current flow between the two phases is sharp at the 

boundary, because of the much smaller bandgap and 

higher conductivity in the nanocrystalline aggregates than 

in the surrounding amorphous regions. 

We also measured the local Voc distributions in mixedphase SiGe:H alloy solar cells (Fig. 4). In general, the 

3
results are similar to the mixed-phase Si:H cells. 

However, some differences are noticed. First, the Voc 

values are lower than the corresponding values in the 

mixed-phase Si:H cells. Second, the size of the 

nanocrystalline aggregates is also smaller than that in the 

mixed-phase Si:H cells. Third, the Voc of the 

nanocrystalline aggregate is more heavily influenced by 

the surrounding amorphous matrix, showing smaller depth 

in the valley than the mixed-phase Si:H cells. Fourth, the 

transition from the low to high Voc regions becomes even 

broader. 

The phenomena above may result from the small size 

of nanocrystalline aggregates and/or heavy electrical 

interaction between the amorphous phase and 

nanocrystalline aggregates. The electrical interaction may 

relate to the electronic structure in the i layer, such as 

band alignment at the amorphous/nanocrystalline 

boundary. In addition, the conductivity of the SiGe:H alloy 

materials is higher than that of the Si:H materials. The 

heavier Voc smooth-out in the mixed-phase SiGe:H cell 

could be partially caused by the high conductivity in the 

SiGe:H materials. 

SUMMARY 

By combining SKPM and AFM, we have developed 

a method to measure the local Voc distribution in mixedphase solar cells. The results clearly show the 

nanocrystalline aggregation. The Voc is smaller in the 

nanocrystalline aggregates than in the surrounding 

amorphous matrix, and the transition from the low to high 

Voc is a gradual change. Although there are some lateral 

charge redistributions, a clear distinction 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 for 

explaining the carrier transport in mixed-phase solar cells. 

ACKNOWLEDGEMENT 

This work at NREL is supported or funded under DOE 

Contract No. DE-AC36-99GO10337. The work at United 

Solar was supported by NREL under the Thin Film 

Partnership Program Subcontract No. ZXL-6-44205-14. 

500

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Voc m( V)

Distance (μm) 

01234

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mV

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0 nm

50

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(a) (b) (c)

2 μm

Fig. 4. (a) A SKPM electrical potential and (b) the corresponding topographic images taken on a mixed-phase a-SiGe:H 

solar cell. The solid curve in (c) shows a potential line profile along the arrows in (a), and the two dotted straight lines 

show averaged potential values take on the amorphous and nanocrystalline regions. 

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