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Silicon Heterojunction Solar Cell 

Characterization and 

Optimization Using In Situ and 

Ex Situ Spectroscopic 

Ellipsometry 

Preprint 

D. Levi, E. Iwaniczko, M. Page, Q. Wang, 

H. Branz and T. Wang 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39932 

May 2006 
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SILICON HETEROJUNCTION SOLAR CELL CHARACTERIZATION AND 

OPTIMIZATION USING IN SITU AND EX SITU SPECTROSCOPIC ELLIPSOMETRY* 

Dean Levi, Eugene Iwaniczko, Matthew Page, Qi Wang, Howard Branz, and Tihu Wang 

National Renewable Energy Laboratory, Golden, CO 80401 

ABSTRACT

We use in-situ and ex-situ spectroscopic ellipsometry 

to characterize the optical, electronic, and structural 

properties of individual layers and completed silicon 

heterojunction devices. The combination of in-situ

measurements during thin film deposition with ex-situ

measurements of completed devices allows us to 

understand both the growth dynamics of the materials and 

the effects of each processing step on material properties. 

In-situ ellipsometry measurements enable us to map out 

how the optical properties change with deposition 

conditions, pointing the way towards reducing the 

absorption loss and increasing device efficiency. We use 

the measured optical properties and thickness of the i-, n-, 

and p-layers in optical device modeling to determine how 

the material properties affect device performance. Our 

best solar energy conversion efficiencies are 16.9% for a 

non-textured, single-sided device with an aluminum back 

surface field contact on a p-type float zone silicon wafer, 

and 17.8% for a textured double-sided device on a p-type 

float zone silicon wafer. 

INTRODUCTION

Silicon heterojunction solar cells (SHJ), also known as 

HITTM cells (heterojunction with intrinsic thin layer), are a 

promising new hybrid amorphous silicon – crystal silicon 

technology that enables high efficiency with relatively 

simple, low temperature device processing [1]. Extremely 

thin layers of intrinsic and doped amorphous 

hydrogenated silicon (a-Si:H) are deposited onto a crystal 

silicon (c-Si) wafer to passivate the wafer surfaces while 

also creating an electric field for carrier collection. These 

devices present unique challenges for characterization 

and optimization because of the extremely thin layers 

used in their fabrication. The SHJ devices under 

development at NREL use 40Å intrinsic layers and 60Å 

doped layers deposited by hot wire chemical vapor 

deposition (HWCVD). Effective device optimization 

requires a method for accurately determining the layer 

thickness and material properties in a timely manner. 

Characterization is particularly challenging because it is 

difficult to measure the properties of such thin layers, and 

the material properties of a-Si:H change significantly with 

increasing film thickness and the type of substrate they 

are grown on. Hence it is not possible to accurately 

determine the properties of thin a-Si:H layers in the actual 

devices by growing thicker layers on transparent or other

test substrates. The only accurate method is to measure 

the material properties and film thickness in the actual 

device structures. 

Spectroscopic ellipsometry (SE) is a powerful method 

for characterizing the thickness and optical properties of 

thin films. Because it is an optical technique it can be 

applied in-situ to measure the film thickness and material 

properties of real SHJ devices during film deposition. 

Because it is a rapid, non-contact technique, it can be 

applied to characterize the device at each processing 

step, providing insight into how each step affects all of the 

layers in the device. We have made extensive use of both 

real-time in-situ and variable angle ex-situ SE (RTSE and 

VASE) for characterization and optimization of SHJ 

devices. Extensive details on film deposition conditions 

and ellipsometry data acquisition and analysis are 

available in the following reference [2]. 

SHJ DEVICE CHARACTERIZATION 

A prime example of the value of SE for device 

optimization is illustrated in figures 1 through 4. Figure 1 

shows a comparison of the internal quantum efficiency 

(IQE) for a standard, diffused junction c-Si solar cell with 

the IQE for a SHJ cell. Both cells were fabricated on 

equivalent p-type float zone c-Si wafers with an alloyed 

aluminum back surface field (BSF) back contact. The only 

difference between the two devices is the method used for 

Figure 1. Comparison of the internal quantum efficiency 

(IQE) for a standard, diffused junction crystal silicon solar 

cell with the IQE for a silicon heterojunction cell. The 

diamonds indicate the difference in IQE between the two 

devices. 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. 

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for United States Government purposes.

1
Figure 2. Absorption and reflection components for a 

16.9% efficient p-i-n single-sided SHJ device. Values 

are calculated from optical constants and layer thickness 

determined from a VASE measurement on the device. 

front junction formation. As shown by the diamonds in the 

figure, the SHJ device suffers a loss in IQE between 400 - 

600 nm relative to the diffused junction cell. In order to 

maximize the current and efficiency in the SHJ device, we 

would like to determine the origin of this drop in IQE. To 

investigate this drop, we have measured ex situ variable 

angle spectroscopic ellipsometry, or VASE, on the 

completed SHJ device. The results of a VASE 

measurement consist of optical constants and layer 

thickness for each layer in the device. Using this accurate 

knowledge of the layer thickness and optical constants we 

are able to calculate the spectral dependence of the 

reflection, transmission, and absorption for the various 

layers that make up the SHJ device. Figure 2 shows the 

spectral dependence of absorption and reflection as 

calculated from the optical model of the SHJ device. The 

solid line is the percent of incident light that is transmitted 

through the indium-tin-oxide (ITO) and i- and n-layers and 

absorbed in the c-Si wafer. The dashed line is the total 

normal-incidence reflection from the device, the dotted line 

is the absorption in the ITO layer, the dot-dash line is the 

absorption in the n layer, and the short-dash line is the 

absorption in the i layer. We note that in comparison to 

the IQE data in figure 1, the c-Si absorption in figure 2 will 

correspond more closely to the external quantum 

efficiency rather than the IQE. 

Figure 3. Comparison of calculated normal-incidence 

reflection from device of figure 2 with measured normalincidence reflection. 

To verify the accuracy of the optical model we have 

measured normal incidence reflectivity for the complete 

SHJ device and compared this to the calculated reflectivity 

based on the VASE measurement. Note that although 

ellipsometry is a reflection measurement, it measures the 

phase and amplitude ratio for parallel and perpendicular 

polarization components at an oblique angle of incidence 

rather than the normal-incidence reflection amplitude. 

Hence, the information obtained in an ellipsometry 

measurement is quite distinct from that obtained in a 

normal-incidence measure of the reflectivity. Figure 3 

shows the normal-incidence reflection data as squares, 

and the calculated reflectivity as a solid line. Clearly the 

correspondence is excellent, except at wavelengths 

greater than 1000 nm. We believe this long-wavelength 

disparity is due to reflection from the BSF contact at 

wavelengths beyond the c-Si bandgap, which we have not 

included in our optical calculation. Based on the excellent 

correspondence between calculated and measured 

reflectivity above the c-Si bandgap we have confidence 

that the calculated values of absorption in the individual 

layers are also accurate.

This leads to a valuable insight into the nature of the 

drop in IQE for the SHJ device relative to the diffused 

junction device. Figure 4 compares the difference in IQE 

from figure 1 with the sum of the absorption in the i- and nlayers calculated from the ellipsometry results. Although 

the values diverge at the shortest wavelengths, the 

agreement is very good. We conclude that in our present 

device configuration there is almost no collection of 

carriers photoexcited within the a-Si:H emitter. We are 

currently working on strategies to reduce the absorption in 

these layers through increased bandgap or decreased 

thickness, or to find a way to collect carriers that are 

photoexcited within these layers. 

Figure 4. Difference in IQE between the SHJ device and 

an otherwise identical diffused junction device, compared 

with the calculated absorption in the i- and n-layers. 

IN SITU CHARACTERIZATION OF MATERIAL 

PROPERTIES 

In addition to VASE measurements, we have also 

used in-situ RTSE measurements to study how changes in 

deposition parameters affect material properties in SHJ 

devices. In situ measurements are particularly valuable 

for the very thin (40 - 60Å) layers used in SHJ devices 

because we are able to obtain a series of ellipsometry 

2
spectra that are constrained to have a linear increase in 

film thickness while maintaining the same optical 

properties. This is important because obtaining optical 

properties from layers less than 200Å thick is pushing the 

limits of ellipsometry. The additional information and 

thickness constraints of RTSE data enable accurate 

determination of the layer optical properties. 

During the HWCVD deposition process the substrate 

temperature Ts can have a significant and systematic 

impact on the properties of the deposited film. Optical 

properties are affected by the structural and electronic 

properties of the material; hence the optical properties 

provide a sensitive measure of how the electronic and 

structural properties are changing. The optical properties 

of a material can be expressed in terms of the dielectric 

function ε, where ε has both real and imaginary parts, ε1

and ε2, respectively. ε1 and ε2 are related by the KramersKronig relation, hence knowing the ε2 spectrum provides 

ε1 as well [3]. ε2 is proportional to the joint density of 

states for optical transitions between occupied states in 

the valence band and unoccupied states in the conduction 

band. As such it is directly related to both the electronic 

structure and optical properties of the material and we will 

focus on ε2 for our discussion of the properties of the thin 

a-Si:H layers used in SHJ devices. 

Figure 5 shows ε2 for a series of i-layers grown on 

HF-etched <100> c-Si substrates over a range of Ts from 

90o

C to 160oC. The four lowest-temperature films exhibit 

the broad featureless ε2 spectrum characteristic of a-Si:H. 

There is a monotonic increase in amplitude and decrease 

in center energy with increasing Ts. These changes in ε2

indicate a decrease in hydrogen content in the film as Ts

increases [4]. The films deposited at 145o

C and 160oC 

exhibit qualitative differences in their ε2 spectra relative to 

the lower-temperature films. The 145o

C film appears to be 

nanocrystalline silicon (nc-Si), while the 160o

C film 

exhibits the dielectric spectrum of crystal silicon, indicating 

epitaxial deposition. As we have shown previously, high 

efficiency SHJ PV devices depend on the highly effective 

passivation of the c-Si surface provided by a-Si:H 

deposited directly on a clean c-Si surface [5]. Hence it is 

imperative that i-layer deposition temperatures be kept 

below 145o

C. 

Quantitative analysis of the optical properties provides 

further insight into how the i-layers change with Ts. Figure 

Figure 5. ε2 spectra for a series of i-layers grown on 

HF-etched <100> c-Si substrates over a range of Ts

from 75o

C to 160oC. 

Figure 6. Dependence of i-layer ε2 peak energy and 

Tauc’s bandgap on substrate temperature for i-layers 

grown on HF-etched <100> c-Si substrates. 

6 shows the Ts dependence of the ε2 peak energy and the 

Tauc’s bandgap for the i-layers. Tauc’s bandgap is a 

standard means for expressing the band gap of a-Si:H and 

is determined by calculating the zero intercept of the 

tangent to the linear portion of sqrt(αE), where α is the 

absorption coefficient and E is the photon energy [6]. The 

range of Ts in the figure is limited to the range where the ilayers are amorphous. The linear fit to the ε2 peak 

indicates a drop of 210 meV for each 100oC increase in 

Ts. From reference [4] we know that the peak in ε2 drops 

17 meV for each 1% decrease in the hydrogen 

concentration of the film, hence the hydrogen content in 

the i-layers likely decreases by 6% over the 50o

C range in 

Ts depicted in figure 6. The linear fit to the bandgap 

indicates a decrease of 30 meV over the 50oC range in Ts. 

Figure 7 shows how the ε2 spectra evolve with Ts for 

n-layers deposited on 40Å thick i-layers. Although the 

graph covers a much larger temperature range of 90-

270o

C, the ε2 spectra change much less than the i-layer 

spectra do over a much smaller range. This is because 

the n-layers are deposited on an a-Si:H i-layer, hence the 

onset of nc-Si deposition occurs at much higher 

temperatures for the n-layer and n-layer optical properties 

are much less sensitive to substrate temperature. 

Figure 8 presents the quantitative analysis of the nlayer dielectric function. Here the linear fit to the ε2 peak 

vs. Ts has a slope of –120 meV / 100oC and the fit to Eg

Figure 7. ε2 spectra for a series of n-layers grown on 40Å 

thick amorphous i-layers over a range of Ts from 90o

C to 

270o

C. 

3
Figure 8. Dependence of n-layer ε2 peak energy and 

Tauc’s bandgap on substrate temperature for n-layers 

grown on 40Å thick amorphous i-layers over a range of 

Ts from 90o

C to 270oC. 

vs. Ts has a slope of +20 meV / 100o

C. It is notable that 

the rate of change of the ε2 peak is nearly half that of the ilayers, while the rate of change of Eg has the opposite 

sign of that for the i-layers. This is likely due to two 

competing factors in the n-layer properties. Because the 

n-layers are deposited on amorphous silicon layers, the 

transition to nc-Si growth occurs at a much higher 

temperature and short-range order can increase without 

initiating nc-Si growth. Increasing short-range order tends 

to increase the bandgap and the ε2 peak energy. Hence 

the effects of reduced hydrogen composition and 

increased short-range order tend to offset each other and 

reduce the overall changes in the optical properties with 

substrate temperature. 

The discussion up to this point has focused on i- and 

n-layers used in n-i-p SHJ devices deposited on p-type FZ 

c-Si wafers. Our research team at NREL has recently 

transitioned to fabrication of p-i-n SHJ devices deposited 

on n-type FZ c-Si wafers. This necessitates developing an 

understanding of the growth dynamics and properties of players as a function of deposition conditions. 

The dependence of the ε2 on substrate temperature 

for the p-layers is quite similar to the n-layer dependence, 

although the bandgap tends to slightly decrease with 

increasing Ts, as opposed to the n-layers, where it slightly 

increases. For the p-layers the linear fit to the ε2 peak vs. 

Ts has a slope of –135 meV / 100oC and the fit to Eg vs. Ts

has a slope of -10 meV / 100o

C. The difference in trends 

is likely due to differences in doping with di-borane as 

opposed to phosphene used for n-layers. 

As efforts proceed to optimize the p-i-n SHJ devices, 

our research team has initiated a study of the effect of 

doping in the p-layers through variation of the 

concentration of B2H6 during p-layer deposition. This is 

accomplished by varying the flow rate of B2H6 gas into the 

HWCVD deposition chamber. Figure 9 shows how the player Tauc’s bandgap changes with B2H6 flow. For flow 

rates between 5 and 18 sccm, we find that the bandgap 

decreases slightly, at a rate of 8.5 meV per 1 sccm of 

B2H6 flow. At the flow rate of 2 sccm we find that the ε2

spectrum is beginning to show signs of a transition to ncSi growth. Although the Tauc’s gap bandgap analysis is 

no longer rigorously accurate for this material that is 

Figure 9. Tauc’s bandgap vs. B2H6 flow for p-layers 

deposited on 40Å thick amorphous i-layers at Ts of 200o

C. 

beginning to transition to an indirect gap, we include a 

point on the plot to emphasize the qualitative change that 

occurs at 2 sscm. 

CONCLUSIONS 

The combination of in situ and ex situ spectroscopic 

ellipsometry provides unique capabilities in characterizing 

the properties of very thin layers used in SHJ PV devices. 

Due to the thinness of these layers and the template 

effects associated with growing test films on non-device 

substrates, ellipsometry provides one of the only 

techniques capable of obtaining information on the 

material properties of these layers in actual device 

structures. This knowledge of the i-, n-, and p-layer 

properties has proven useful in delineating the effects that 

changes in these layers have on the device function and 

efficiency, and also provide guidance in optimizing SHJ 

device function and efficiency. 

ACKNOWLEGEMENTS 

This work was supported by the US DOE under 

Contract No. DE-AC36-99GO10337. The authors thank 

Russell Bauer, Lorenzo Royball, Helio Moutinho, Bobby 

To, Yanfa Yan, and Yueqin Xu of NREL, and Ajeet 

Rohatgi and Vijay Yelundur of Georgia Technical Institute. 

 

[1] M. Tanaka, et al. Jpn. J. Appl. Phys. 31, p. 3518. 

[2] D.H. Levi, et al.,”Real-time spectroscopic ellipsometry 

studies of the growth of amorphous and epitaxial silicon 

for photovoltaic applications”, J. Vac Sci. Technol. A 24(4), 

Jul/Aug 2006. 

[3] R.M.A. Azam and N.M. Bashara, Ellipsometry and 

Polarized Light, 1st ed. (North-Holland, Amsterdam, 1977). 

[4] G.F. Feng, at al., “Dielectric functions and electronic 

band states of a-Si and a-Si:H”, Phys. Rev. B 45, 1992, 

pp. 9103-9107. 

[5] D.H. Levi, et al., MRS Symposia Proceedings No. 808

(Materials Research Society, Pittsburg, 2004) p.239. 

[6] J. Tauc, Optical Properties of Solids, ed. F. Abeles 

(North-Holland, Amsterdam, 1969). 

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