Amorphous and nanocrystalline silicon thin film photovoltaic technology on flexiblesubstratesBaojie Yan, Jeffrey Yang, and Subhendu Guha Citation: Journal of Vacuum Science & Technology A 30, 04D108 (2012); doi: 10.1116/1.4707154 View online: http://dx.doi.org/10.1116/1.4707154 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/30/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Quadruple-junction thin-film silicon-based solar cells with high open-circuit voltage Appl. Phys. Lett. 105, 063902 (2014); 10.1063/1.4892890 Microstructure factor and mechanical and electronic properties of hydrogenated amorphous and nanocrystallinesilicon thin-films for microelectromechanical systems applications J. Appl. Phys. 114, 184905 (2013); 10.1063/1.4829020 Influence of the absorber layer thickness and rod length on the performance of three-dimensional nanorods thinfilm hydrogenated amorphous silicon solar cells J. Appl. Phys. 113, 163106 (2013); 10.1063/1.4803045 Evolution of SiH x hydrides during the phase transition from amorphous to nanocrystalline silicon films J. Appl. Phys. 111, 043510 (2012); 10.1063/1.3686136 Light-induced metastability in hydrogenated nanocrystalline silicon solar cells Appl. Phys. Lett. 85, 1925 (2004); 10.1063/1.1790072

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Amorphous and nanocrystalline silicon thin film photovoltaic technologyon flexible substrates

Baojie Yan,a) Jeffrey Yang, and Subhendu GuhaUnited Solar Ovonic LLC, 1100 West Maple Road, Troy, Michigan 48084

(Received 13 January 2012; accepted 10 April 2012; published 26 April 2012)

This paper reviews our thin film silicon-based photovoltaic (PV) technology, including material

and device studies as well as roll-to-roll manufacturing on a flexible substrate. Our current thin

film silicon PV products are made with hydrogenated amorphous silicon (a-Si:H) and amorphous

silicon germanium (a-SiGe:H) alloys. The advantages of a-Si:H-based technology are low cost,

capability of large scale manufacturing, abundance of raw materials, and no environmental con-

cerns. One disadvantage of a-Si:H PV technology is lower energy conversion efficiency than solar

panels made of crystalline and polycrystalline silicon and compound crystal thin film semiconduc-

tors. Significant efforts have been made to improve efficiency. First, a-Si:H and a-SiGe:H material

quality has been improved by optimizing deposition conditions, especially using high hydrogen

dilution to deposit the amorphous materials close to the amorphous/nanocrystalline transition.

Second, cell efficiency has been improved by engineering the device structure, such as bandgap

profiling. In order to use the solar spectrum effectively, multijunction structures that incorporate

a-SiGe:H in the middle and bottom cells have been used. The authors achieved record high solar

cell efficiency with an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. Using the same

structure, we fabricated solar laminates on flexible stainless steel coils with roll-to-roll production

systems. Our current product has a stable aperture area efficiency of 8.2%. In recent years,

hydrogenated nanocrystalline silicon (nc-Si:H) has emerged as a potential replacement for the

a-SiGe:H bottom cell in multijunction structures. The authors have conducted a great deal of

research and development of a-Si:H- and nc-Si:H-based multijunction PV technology. The authors

have significantly improved the efficiency of a-Si:H and nc-Si:H multijunction solar cells and

modules by optimizing the nc-Si:H material quality and device structure. The authors achieved an

initial active-area (�0.25 cm2) solar cell efficiency of 16.3% using an a-Si:H/a-SiGe:H/nc-Si:H

triple-junction structure and an initial aperture-area module (�400 cm2) efficiency of 12.0% and a

stable aperture-area module (�800 cm2) efficiency of 11.3% using an a-Si:H/nc-Si:H/nc-Si:H

triple-junction structure. The authors expect to launch a new a-Si:H/nc-Si:H/nc-Si:H triple-

junction product in near future with much higher efficiency than the current product. VC 2012American Vacuum Society. [http://dx.doi.org/10.1116/1.4707154]

I. INTRODUCTION

Thin film silicon solar cell technology represents one of

the photovoltaic (PV) solar energy technologies capable of

providing low production costs, environmentally friendly,

terawatt solar panel production. Hydrogenated amorphous

silicon (a-Si:H), silicon germanium (a-SiGe:H), and nano-

crystalline silicon (nc-Si:H) are the three major intrinsic

materials used in multijunction thin film silicon-based solar

cells.1 The drawbacks of a-Si:H-based solar cells are rela-

tively low efficiency and light-induced degradation caused

by the Staebler-Wronski effect.2 Nonetheless, continued

research and development has improved (i) material quality,

(ii) multijunction solar cell structures, and (iii) effective light

trapping so as to make high stable efficiency thin film silicon

solar cells. We have studied various cell structures, including

a-Si:H single-junction, a-Si:H/a-Si:H double-junction,

a-Si:H/a-SiGe:H double-junction, a-Si:H/nc-Si:H double-

junction, a-Si:H/a-SiGe:H/a-SiGe:H triple-junction, a-Si:H/a-

SiGe:H/nc-Si:H triple-junction and a-Si:H/nc-Si:H/nc-Si:H

triple-junction structures.3–10 Each cell structure has its own

unique advantage in terms of manufacturing cost and product

application. For example, the manufacturing cost of a-Si:H

single-junction solar cells is lower than that of multijunction

solar cells, but it has lower conversion efficiency and poorer

light-induced stability. On the other hand, multijunction solar

cells perform better than single-junction cells, but manufac-

turing becomes more complex; thus increasing manufacturing

cost. We have developed and launched products based on a

spectrum splitting, a-Si:H/a-SiGe:H/a-SiGe:H triple-junction

structure. Such triple-junction solar cell structures provide

high efficiencies and the product shows reduced light-induced

degradation after prolonged light soaking. World-record initial

and stable cell efficiencies of 14.6% and 13.0% were achieved

in 1997, respectively.3,4 In the meantime, our manufacturing

technology employing a roll-to-roll deposition technique on

flexible stainless steel substrates has continuously improved.5

Since 2001, we have been working on nc-Si:H (also

referred to as lc-Si:H in the literature) as an intrinsic

absorber layer.6 As initially reported by the group at Neucha-

tel University, nc-Si:H solar cells provide higher photocur-

rent and lower light-induced degradation.11,12 Over the

years, significant progress has been made by many groups

a)Author to whom correspondence should be addressed; electronic mail:

byan@uni-solar.com

04D108-1 J. Vac. Sci. Technol. A 30(4), Jul/Aug 2012 0734-2101/2012/30(4)/04D108/10/$30.00 VC 2012 American Vacuum Society 04D108-1

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around the world.13–16 To date, both the cell and module effi-

ciencies for multijunction structures incorporating a-Si:H

and nc-Si:H have exceeded the records achieved using

a-Si:H and a-SiGe:H materials in triple-junction structures.

There are two major advantages to use nc-Si:H solar cells:

superior high long wavelength response and low light-

induced degradation. However, because of indirect transition

in the crystalline phase, nc-Si:H intrinsic layers are usually

made much thicker than corresponding a-Si:H and a-SiGe:H

intrinsic layers. For example, the total thickness of a typical

a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells is

about half a micrometer, while the intrinsic layer in a single-

junction nc-Si:H solar cell is at least one micrometer and a

few micrometers of total cell thickness are needed in multi-

junction cell structures. Therefore, high rate deposition is a

necessary requirement for manufacturing a-Si:H and nc-Si:H

multijunction solar cells. Although several high rate deposi-

tion methods have been proposed, including very high fre-

quency (VHF) glow discharge,11,12 microwave glow

discharge,5 thermal expansion plasma,17 and hot-wire chemi-

cal vapor deposition,18 VHF glow discharge is the most

promising technique for the production of nc-Si:H material

at high rates. It was found that the ion energy is lower and

ion flux is higher in VHF than in rf glow discharges, which

provides proper bombardment for high quality nc-Si:H

growth. When using very high frequency (40–130 MHz),

achieving large area uniformity is a major challenge for

VHF nc-Si:H deposition.

We have been investigating a-Si:H and nc-Si:H multi-

junction solar cells since 2001.6 Significant progress has

been made in material characterization and optimization,

solar cell design, and device fabrication.6–11 In this paper,

we briefly review the current a-Si:H/a-SiGe:H/a-SiGe:H

triple-junction PV technology and product in Sec. II; we

discuss the progress of improving material and device

structures of a-Si:H- and nc-Si:H-based PV technologies in

Sec. III; and we present a summary in Sec. IV.

II. a-Si:H- AND a-SiGe:H-BASED TRIPLE-JUNCTIONPV TECHNOLOGY

We have studied a-Si:H and a-SiGe:H materials and solar

cells for more than 30 years. Significant efforts have been

made in the optimization of deposition processes and in solar

cell design. In the following we summarize several major tech-

niques for obtaining high stable cell and module efficiencies.

A. High hydrogen dilution

In plasma-enhanced chemical vapor deposition (PECVD),

hydrogen dilution is critical for improving thin film silicon

material quality and device performance. Normally, the

materials made at no or low hydrogen dilution show

amorphous structures, in which long range order disappears,

but short range order remains. With increasing hydrogen

dilution, isolated nanometer-sized crystallites or linear

structures are included in the amorphous materials.19,20

From optical and electrical points of view, this material still

has an amorphous signature, but has significant improved

quality for device application. Because this type of material

is made under conditions in which the hydrogen dilution is

close to the transition from the amorphous phase to the nano-

crystalline phase, it is often called “edge” material or “proto-

crystalline” material. The a-Si:H films made with high

hydrogen dilution show a lower defect density, narrower

bandtails, and lower light-induced defect density than those

with no or low hydrogen diluted a-Si:H films.21,22 The high

hydrogen diluted a-Si:H solar cells show higher open circuit

voltage and better stability after light soaking. Increasing the

hydrogen dilution further promotes more nanocrystalline

formation. When the crystalline phase forms percolation

paths in the vertical direction, the material shows crystal-like

properties with high conductivity. This type of material is

normally called nanocrystalline or microcrystalline silicon.

For a-Si:H solar cell optimization, one can use open cir-

cuit voltage (Voc) as a measure of the amorphous/nanocrys-

talline transition. An example is given in Fig. 1, where the

Voc values of three sets of a-Si:H solar cells with different

thicknesses are plotted as a function of normalized hydrogen

dilution ration (R). R¼ 1 corresponds to the optimized

hydrogen dilution for a-Si:H top cell in a-Si:H/a-SiGe:H/a-

SiGe:H triple-junction solar cells. One can see that in the

amorphous region, Voc increases slightly with hydrogen dilu-

tion; it reaches the highest value just before a sharp decrease.

We have obtained a Voc¼ 1.05 V, which is the record for

a-Si:H solar cells. The materials of the solar cells in the

region showing the sharp Voc decrease with the increase of

hydrogen dilution are called “mixed-phase.” Solar cells de-

posited in the mixed-phase regime show several unique

properties, including a large spread of Voc when measuring

performance across the same substrate and an increase of

light-induced Voc.23,24 The mixed-phase solar cells have very

complex structures and provide a good platform for

fundamental studies, but they are not used to achieve high

efficiency solar cells because of poor material quality.

Therefore, we will not discuss the mixed-phase cells in

detail. Because the transition from amorphous phase to nano-

crystalline phase not only depends on hydrogen dilution, but

FIG. 1. Open circuit voltage (Voc) of a-Si:H solar cells as a function of nor-

malized hydrogen dilution ratio (R). Three sets of a-Si:H cells with different

intrinsic layer thicknesses are plotted.

04D108-2 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-2

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also on other parameters such as substrate temperature, pres-

sure, and excitation power, the absolute hydrogen dilution at

the phase transition can be very different from one set of

samples to another or from one reactor to another. Therefore,

we use normalized hydrogen dilution (R), in which R¼ 1 is

the value corresponding to best cell performance. The gen-

eral guideline for a-Si:H solar cell optimization is to find the

phase transition and then reduce the hydrogen dilution

slightly to obtain the best a-Si:H solar cells.

B. a-SiGe:H solar cells as low bandgap componentcells

In order to effectively use the solar spectrum to achieve

high efficiency solar cells, multijunction solar structures with

component cells having different bandgaps tailored to absorb

different wavelengths of the solar spectrum are normally

used. In the a-Si:H-based thin film solar cells, a-SiGe:H has

been widely used as the narrow bandgap material. The

bandgap of a-SiGe:H can be changed by the Ge/Si ratio in the

material. Compared to a-Si:H, a-SiGe:H materials usually

have a high defect density because the H–Ge bond is much

weaker than H–Si bond; therefore, the Ge dangling bond

density is much higher than the Si dangling bond density.22

Further, the defect density increases with the Ge/Si ratio.

We developed several techniques to improve a-SiGe:H

material quality and solar cell performance. Among them,

hydrogen dilution is also an important method for improving

a-SiGe:H quality.5 In addition, proper bandgap profiling and

optimized buffer layers at the p/i and i/n interface are critical

components for high efficiency a-SiGe:H solar cells. Figure 2

shows an illustration of a bandgap profile in optimized

a-SiGe:H solar cells.25 The p/i and i/n buffer layers are thin

high quality a-Si:H layers that have two functions: reducing

the interface defect density and forming a field to reduce the

photocarrier backdiffusion. A properly designed buffer layer

can significantly improve the fill factor (FF) and Voc.

Design of the bandgap profiling is based on the fact that

electron mobility is much higher than hole mobility in

a-Si:H and a-SiGe:H materials; therefore cell performance is

mainly dominated by the hole transport.26 When an

a-SiGe:H solar cell is designed to have a narrow bandgap in

the region adjacent the p/i interface and a wider bandgap to-

ward the i/n interface, a high density of electron–hole pairs

is generated in the region close to the p/i interface, thereby

reducing the distance that photogenerated holes must travel

to the p layer. In this case, the FF is improved. In addition,

the increased bandgap adjacent to the i/n interface keeps the

designed average Ge/Si ratio, which improves the Voc. In

order to reduce the defect density at the p/i interface, a thin

layer of a-SiGe:H with a ramping up the bandgap toward the

p/i interface is also used. Therefore, the optimized bandgap

is normally called “V-shaped” bandgap profiling as shown in

Fig. 2. Experimentally, we used dynamically controlled SiH4

and GeH4 flow rates to get a desired Ge/Si ratio. Because the

bandgap of a-SiGe:H depends on the Ge/Si ratio in the de-

posited materials, one can easily control the bandgap profile

in a-SiGe:H solar cells. Table I compares a pair of a-SiGe:H

solar cells made with flat and profiled bandgaps in the intrin-

sic layers. One can see demonstrably better performance for

the cell with the profiled bandgap. The solar cell with proper

bandgap profiling shows a significant improvement in the

FF, especially the FF measured under a 530 nm long-pass fil-

ter and under a weak red light (FFr), indicating improved

collection for holes generated by long wavelength light. The

slightly reduced blue light FF (FFb) in the a-SiGe:H solar

cell with bandgap profiling implies that the high Ge content

in the region close to the p/i interface could cause additional

defects in the p/i region. However, the overall performance

is improved by the bandgap profiling.

C. a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells

In general, multijunction solar cells have an advantage

over single-junction solar cells because they use the solar

FIG. 2. Schematic illustration of bandgap profiling in a-SiGe:H solar cells.

TABLE I. J–V characteristics of a-SiGe:H solar cellsa made with a flat bandgap (21089) and a profiled bandgap (21029).b

Run Source Voc (V) FF FFb FFr Jsc (mA/cm2) Pmax (mW/cm2)c Comment

21029 Color 0.731 0.702 Profiled bandgap

>530 0.774 0.694 9.85 5.29

AM1.5 0.791 0.699 14.84 8.21

21089 Color 0.75 0.552 Flat bandgap

>530 0.809 0.603 9.57 4.67

AM1.5 0.829 0.641 14.82 7.88

aSolar cells were made on flat SS without BRs.bMeasurements were made under an AM1.5 solar simulator, AM1.5 solar simulator with a long pass filter (>530 nm), and with blue and red color light under

low light intensity.cMaximum power.

04D108-3 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-3

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spectrum more effectively. Normally, the top cell intrinsic

layer has a wider bandgap to absorb the short wavelength

light and reduce the energy loss during the thermalization

that occurs in narrow bandgap semiconductors, where the

extra photo-energy above the bandgap is lost during thermal-

ization. The middle and bottom cells have narrower bandg-

aps to absorb the longer wavelength light. In a-Si:H-based

thin film solar cells, the second advantage of multijunction

cell structures is their use of relatively thin intrinsic layers in

each component cell, which minimizes light-induced degra-

dation. Combining these two advantages normally results in

high stable efficiencies in multijunction solar cells.

We have studied several kinds of a-Si:H and a-SiGe:H

multijunction solar cells, including a-Si:H/a-Si:H double-

junction, a-Si:H/a-SiGe:H double-junction, and a-Si:H/a-

SiGe:H/a-SiGe:H triple-junction structures.3–5 We found

that the a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure

provides the best performance. Figure 3 shows an illustration

of the triple-junction structure, where the substrate is flexible

thin stainless steel (SS) and the backreflector (BR) is con-

structed with a thin layer of metal (Ag or Al) for reflection

and a dielectric layer (such as ZnO) to reduce the metal/

semiconductor interface loss and enhance the scattering by

additional texture. The a-SiGe:H middle and bottom cells

normally have bandgaps around 1.6–1.7 and 1.5–1.6 eV,

respectively, achieved by changing the Ge/Si ratio. The a-

Si:H intrinsic layer in the top cell has a bandgap �1.85 eV.

The top indium–tin–oxide (ITO) contact serves two func-

tions: it serves as a transparent top electrical contact and as

an anti-reflection coating to reduce reflection loss. In order

to reduce series resistance loss, a metal grid or wire is placed

on top of the ITO. Using the a-Si:H/a-SiGe:H/a-SiGe:H

triple-junction structure, we achieved record initial and

stable active-area cell efficacies of 14.6% and 13.0%, respec-

tively.3 Figure 4 shows the current density versus voltage

(J–V) characteristics of the record efficiency solar cell.

D. Solar laminate production using a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell structure

Based on a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell

structures, we built three generations of manufacturing

lines. Current production lines were designed and built in

2002–2003. It simultaneously and continuously processes

six rolls of 2.5 km long and 0.36 m wide SS substrates. It

takes �62 h to finish the deposition of a nine-layered triple-

junction structure on the six long rolls. After that, the coils

are processed in another roll-to-roll machine for ITO deposi-

tion. The solar cell coils are then processed to make modules

using steps that include cutting into slabs, putting grid wires,

interconnecting, encapsulating, and final testing.5

Our current product contains 22 solar cells connected in

series. The power rating is 144 W, corresponding to a stable

aperture-area efficiency of 8.2%. Compared to the cell effi-

ciency of 13.0% achieved in the research and development

laboratory, the product efficiency is lower. Several loss

mechanisms have been identified, including the deterioration

of material quality at increased deposition rate, nonuniform-

ity over large area, poorer reflection from Al/ZnO compared

to Ag/ZnO, and encapsulation loss. For building-integrated

photovoltaic applications, our a-Si:H/a-SiGe:H/a-SiGe:H

triple-junction have significant advantages as compared to

crystalline silicon solar panels: our flexible laminates

provide ease of installation and architectural compatibility

with roofs of commercial and residential buildings.

FIG. 3. (Color online) Schematic illustration of a-Si:H/a-SiGe:H/a-SiGe:H

triple-junction solar cell structure.

FIG. 4. (Color online) Initial J–V characteristics and QE curves of an

a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cell with initial and stable

active-area efficiencies of 14.6% and 13.0%, respectively.

04D108-4 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-4

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III. a-Si:H- AND nc-Si:H-BASED MULTIJUNCTIONPV TECHNOLOGY

The efficiency of a-Si:H- and a-SiGe:H-based multijunc-

tion solar cells appear to have reached a plateau, indicating

that the efficiency has been limited by the intrinsic layer

material quality. Unless one can reduce the defect density

significantly, it may not be possible to further improve

a-Si:H- and a-SiGe:H-based solar cells. The main limitation

is the poorer quality of a-SiGe:H than a-Si:H. In order to

improve the efficiency, new narrow bandgap materials are

needed. The group at Neuchatel University conducted early

research and development of nc-Si:H solar cells,11,12 a better

narrow bandgap material with higher carrier mobility and

better stability than a-Si:H and a-SiGe:H. We have worked

on nc-Si:H solar cells since 2001 and have made significant

progress in solar cell efficiency.6 We have developed

advanced nc-Si:H deposition processing parameters and so-

lar cell structures, which led to the records in cell and mod-

ule efficiencies.10,13 In the following, we summarize the key

issues in nc-Si:H solar cells and the techniques used to

resolve those issues to improve nc-Si:H solar cell efficiency.

A. High hydrogen dilution and hydrogen dilutionprofiling

As discussed in the previous section, hydrogen dilution is

a key parameter employed to change thin film silicon mate-

rial from the amorphous phase to the nanocrystalline phase.

Therefore, much higher hydrogen dilution is needed in nc-

Si:H deposition than in a-Si:H deposition. Proper hydrogen

dilution is critical for high efficiency nc-Si:H solar cells.

First, a high crystalline volume fraction is required to form

carrier transport paths and absorb long wavelength light.

However, a too high crystalline volume fraction normally is

associated with large grains and poor grain boundary passi-

vation. It is often found that material with a high crystalline

volume fraction has a high density of microvoids and micro-

cracks, which causes ambient degradation of cell efficiency

without intentional light soaking.6,7 We identified that ambi-

ent degradation in nc-Si:H solar cells is caused by impurity

diffusion into the intrinsic layer causing shallow donor state

formation. We found that optimized nc-Si:H solar cells

should have a crystalline volume fraction of about 50%

estimated by Raman spectroscopy.

The structure of nc-Si:H material also changes with film

thickness under constant deposition conditions. Raman spec-

tra and cross-sectional TEM images demonstrate that an

amorphous incubation layer is formed in the early stage of

film growth, nanocrystallites form at certain thickness, and

the crystallites grow as the film gets thicker.1,12 Such a pro-

cess is called nanocrystalline evolution with low crystallinity

in the bottom region of the film and high crystallinity in the

top region. This nonuniform crystallinity in nc-Si:H solar

cells has a negative impact on solar cell performance. First,

if the amorphous incubation layer is too thick, the photocar-

riers cannot readily transport through the incubation layer,

which causes poor FF and high series resistance (Rs).

Second, the increased crystallinity at the top region often

causes a high density of microvoids and microcracks as

observed in nc-Si:H materials made with very high hydrogen

dilution. The porosity in the top region can result in poor

solar cell performance and ambient degradation in solar cell

efficiency. To overcome the crystalline evolution issue, we

invented a hydrogen dilution profiling technique that con-

trols crystallinity along the growth direction.27,28 Because

crystallinity strongly depends on the hydrogen dilution ratio,

we use very high hydrogen dilution at initial deposition to

reduce or remove the amorphous incubation layer and form

nc-Si:H within a very thin layer that is of the desired volume

fraction. During deposition, the hydrogen dilution is reduced

continually to ensure crystallinity is controlled properly.

Table II lists two pairs of nc-Si:H solar cells made using con-

stant hydrogen dilution and profiled hydrogen dilution. The

first pair of early samples (L14559 and L14560) were made

several years ago and showed a large difference in cell per-

formance,27 especially in the Jsc. At the early stages of our

nc-Si:H study (an example of samples L14559 and L1466),

we used rf glow discharge to deposit the nc-Si:H intrinsic

layers under relatively low pressure of �1–2 Torr, under

which significant ion bombardments lead to the nc-Si:H

materials having a high defect density and (111) preferential

orientation. The material quality was not as good in these so-

lar cells; therefore, the hydrogen dilution profiling had a

large effect on the cell performance. The second pair

(B15422 and B15423) was made recently with improved nc-

Si:H material quality, and therefore the improvement from

hydrogen dilution profiling was not as large as before, but it

still resulted in a 7.8% increase in efficiency. Compared to

the previous nc-Si:H solar cells, the recent improvement of

nc-Si:H solar cell performance comes from two areas:

change from rf to VHF deposition and optimized deposition

parameters. Over the years, we have found that VHF glow

discharge at higher pressure is characterized by low ion

energy and high ion flux intensity which is beneficial for nc-

Si:H depositions. In addition, the other deposition parame-

ters, such as substrate temperature, VHF power, and the

degree of active gas depletion, have been systematically

optimized to achieve high nc-Si:H solar cell efficiency.

Similar to any other process parameter, hydrogen dilution

profiling has to be optimized as a function of time. An insuf-

ficient ramping of hydrogen dilution may not show the effect

on cell performance; but too steep of hydrogen dilution

ramping also results in a poor cell performance caused by

TABLE II. J–V characteristics of two pairs of nc-Si:H single-junction solar

cells made using constant and profiled hydrogen dilution during the intrinsic

layer depositions.

Sample

number

Jsc

(mA/cm2)

Voc

(V) FF

Efficiency

(%) Comment

L14559a 21.48 0.482 0.632 6.54 Constant H2 dilution

L14660a 25.15 0.502 0.663 8.37 Profiled H2 dilution

B15423b 26.61 0.504 0.665 8.92 Constant H2 dilution

B15422b 26.73 0.502 0.685 9.12 Profiled H2 dilution

aL14559/L14660 is a pair made a few years ago and reported in Ref. 28.bB15422/B15423 is a pair made recently.

04D108-5 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-5

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too large an amorphous component in the region close to the

p/i interface. Raman measurements demonstrated that the

best nc-Si:H solar cells have a slightly inverted crystallinity

distribution,29 which means that the crystalline volume frac-

tion in the p/i region should be slightly lower than in the

bulk or the i/n region. The inverted crystallinity distribution

helps reduce porosity in the film and improve carrier trans-

port. Hydrogen dilution profiling has been adopted by many

groups.30–32

The parameters for depositing thin film silicon is a multi-

variable system that includes controlling substrate tempera-

ture, pressure, gas flow rate, excitation power, and chamber

geometry. In addition to hydrogen dilution and hydrogen

dilution profiling, other parameters also affect the amor-

phous/nanocrystalline transition, and therefore, can be used

to control the crystalline volume fraction during the deposi-

tion of nc-Si:H solar cells. Han et al. demonstrated a power

profiling method to control the crystalline volume fraction.33

B. nc-Si:H single-junction design and high efficiencync-Si:H cells

Compared to a-Si:H solar cells, high efficiency nc-Si:H

solar cells have a complicated structure as shown in Fig. 5,

where an n-i-p structure on flexible SS substrate is illus-

trated. Similar to high efficiency a-Si:H and a-SiGe:H solar

cells, a Ag/ZnO BR is used to enhance the light trapping, nand p layers are used to form the junction, and ITO and

metal grids form the top electrode. The differences reside in

the i/n buffer layer, seed layer, and p/i buffer layer.

The i/n buffer layer is an a-Si:H layer. The function of

the i/n buffer layer is to prevent P contamination in the nc-

Si:H intrinsic material. As nc-Si:H is sensitive to doping

impurities, a very small amount of P could cause a reduction

in the quantum efficiency (QE) long wavelength spectrum.

Figure 6 shows nc-Si:H solar cells made with and without

the i/n buffer layer. The solar cell made without the i/nbuffer layer (b) shows a large difference between the QE

curves measured under a 3 V reverse bias and under short

circuit conditions, whereas the cell made with the i/n buffer

(a) shows no difference between the two QE curves. We

believe the difference is caused by the shifting of the Fermi

level toward the conduction band edge that reduces the

electric field in the region close to the i/n interface. The P

contamination of the nc-Si:H intrinsic layer could come

from two processes. First, P could be sputtered from the nlayer at the beginning of nc-Si:H deposition because of the

high hydrogen dilution and high excitation power in the seed

layer deposition; second, P could diffuse into the nc-Si:H

intrinsic layer during the nc-Si:H deposition through micro-

voids or grain boundaries at elevated temperature. Currently,

we do not have conclusive evidence of which process is the

major source of P contamination. However, the QE loss as

shown in Fig. 6(b) is sensitive to the subsequent seed layer

deposition, indicating the gas phase process could cause the

major P contamination. Nevertheless, a proper a-Si:H buffer

layer effectively reduces P contamination from the n layer.

The a-Si:H n/i buffer layer thickness is in the range of

10–30 nm, depending on the hydrogen dilution of the a-Si:H

layer and the growth condition of the seed layer and nc-Si:H

i layer. In addition, the a-Si:H n/i buffer layer reduces the

negative effect of substrate texture on nc-Si:H solar cell per-

formance and improves the performance of nc-Si:H solar

cells on very textured substrates.34

After the i/n buffer layer, a seed layer is deposited, which

is normally made with very high hydrogen dilution to create

crystallite seeds for further nc-Si:H growth. The seed layer

could be combined with hydrogen dilution profiling during

FIG. 5. (Color online) Schematic illustration of a nc-Si:H single-junction

solar cell.

FIG. 6. (Color online) QE curves of nc-Si:H solar cells made without the i/nbuffer layer and with the i/n buffer layer. The measurements are made under

short circuit condition and under �3.0 V electrical bias.

04D108-6 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-6

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intrinsic layer deposition. The main goal is to reduce the

thickness of the amorphous incubation layer and grow an

nc-Si:H intrinsic layer with a desired crystalline structure

along the growth direction.

The p/i buffer layer is also used to improve nc-Si:H cell

performance. It mainly improves FF and Voc. For p-i-n struc-

tured nc-Si:H solar cells, the p/i buffer layer is mainly used

to make a proper transition from the a-SiC:H or nc-SiC:H player to the nc-Si:H i layer and to reduce the amorphous

incubation layer.35 For n-i-p structured nc-Si:H solar cells,

because of the growth sequence, nc-Si:H solar cells without

the p/i buffer show normally large shunt current, which

reduces the FF and Voc.36 Although it is not clear how the

shunt paths are formed in nc-Si:H, a proper a-Si:H buffer

layer reduces shunt current and improves FF and Voc as

shown in Fig. 7. The high current from the cell without the

p/i buffer in the low voltage region is an indication of shunt

current. Increasing the p/i buffer layer thickness reduces the

shunt current and makes the dark J–V approach a standard

diode characteristic. Further the increasing p/i buffer layer

thickness can cause a high Rs and poor FF. Therefore, the p/ibuffer layer must be optimized properly. We experimentally

found that the optimized a-Si:H p/i buffer layer is

�20–30 nm. In some case, it can be as thick as 42 nm as

previously reported.37

C. Light trapping

Because of low absorption of long wavelength light, nc-

Si:H solar cell efficiency strongly depends on light trapping.

In other words, effective light trapping can significantly

improve nc-Si:H cell efficiency. Compared to solar cells de-

posited on flat SS, the gain in Jsc of a-SiGe:H solar cells is

�20%–30% when the cells are deposited on Ag/ZnO BRs,

whereas it can be �50%–60% for nc-Si:H solar cells. Figure

8 shows a comparison of QE curves from a pair of (a)

a-SiGe:H and a pair of (b) nc-Si:H solar cells deposited on

different substrates. Because of the strong dependence of

nc-Si:H cell performance on light trapping, there have been

significant efforts to improve light trapping, especially by

advanced nanotechnologies, such as metal nanoparticles for

plasmonic scattering and periodic structures for photonic

scattering.37,38 Although theoretical calculations show that

light trapping using advanced nanostructures have advan-

tages over conventional randomly textured BRs,39 in prac-

tice no solar cell with such light trapping structures has

FIG. 7. Dark J–V characteristics of three nc-Si:H solar cells with different

p/i buffer layer thicknesses as given in the inset.

FIG. 8. (Color online) QE curves of (a) a-SiGe:H solar cells and (b) nc-Si:H

solar cells made on SS, Al/ZnO BRs and Ag/ZnO BRs.

FIG. 9. (Color online) QE curves of nc-Si:H solar cells on (A) SS and on (B)

Ag/ZnO BR along with the calculation using classical limit of 4n2 without

(C) and with (D) the consideration of losses in the BR, doped layer, and

reflection by ITO.

04D108-7 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-7

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shown a Jsc higher than what we achieved in nc-Si:H solar

cells using our optimized Ag/ZnO BRs.40 We have estimated

gains and losses in our nc-Si:H solar cells and found the gain

in Jsc from an optimized BR is close to the classical limit of

4n2, where n is the refractive index of silicon.41 Figure 9 shows

QE plots of nc-Si:H on (a) flat SS and (b) textured Ag/ZnO

BR, and the calculations based on the classical theory without

(c) consideration of losses and with (d) consideration of losses

in the BR, doped layers, and reflection by ITO. It is clear that

our optimized BR results in light scattering close to the classi-

cal limit; however further improvement of photocurrent density

could come from the reduction of losses in various layers.

D. High efficiency a-Si:H- and nc-Si:H-basedmultijunction solar cells

We have investigated several multijunction solar cell struc-

tures, including a-Si:H/nc-Si:H double-junction, a-Si:H/a-

SiGe:H/nc-Si:H triple-junction, and a-Si:H/nc-Si:H/nc-Si:H

triple-junction solar cell structures. Each structure has advan-

tages and disadvantages. The a-Si:H/nc-Si:H double-junction

structure is simple and can be easily manufactured at

relatively low cost. However, it has lower efficiency than

triple-junction structures and it suffers large light-induced

degradation because of the relatively thick a-Si:H top cell.

Although advanced cell structures, such as those incorporat-

ing an inter-reflection layer between the a-Si:H top cell and

nc-Si:H bottom cells to reduce the a-Si:H top cell thickness,

have been developed, the light-induced degradation in a-Si:H/

nc-Si:H double-junction remains higher than 10%.

a-Si:H/a-SiGe:H/nc-Si:H triple-junction is a promising cell

structure capable of attaining high initial cell efficiency. We

have achieved an initial active-area efficiency of 16.3% using

this cell structure with a nc-SiOx:H dual-function layer in the

a-SiGe:H middle cell, where the P-doped nc-SiOx:H layer

serves as both the n layer of the a-SiGe:H middle cell and an

inter-reflection layer to increase the middle cell current.42

Figure 10 shows the J–V characteristics and QE curves of the

triple-junction solar cell with the highest initial efficiency. The

disadvantage of this cell structure is that the a-Si:H top cell

and a-SiGe:H middle cell both suffer light-induced degrada-

tion. Therefore, the light-induced degradation is normally

higher than a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells.

a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells have

the lowest light-induced degradation, normally, �3%–6% in

optimized cells. Therefore, this cell structure can give the

highest stable efficiency. We achieved an initial active area

efficiency of 14.5% using this cell structure, which is similar

to the record efficiency achieved with a-Si:H/a-SiGe:H/a-

SiGe:H triple-junction solar cell.3 Because the light-induced

degradation is lower, the stable total area efficiency of

12.5% as measured at the National Renewable Energy Labo-

ratory (NREL) was attained, which is the highest cell effi-

ciency for thin film silicon solar cells.

E. High rate nc-Si:H deposition

The second critical issue for nc-Si:H solar cells is deposi-

tion rate. Because nc-Si:H is a mixture of nanometer-sized

crystallites embedded in amorphous tissues, the absorption

of long wavelength light mainly occurs in the crystalline

phase where momentum conservation may still hold even if

the crystallites are small. Therefore, the indirect transition

results in low absorption coefficients in the long wavelength

region and in order to obtain high photocurrent density, a

thick intrinsic layer is normally needed. For example, the

intrinsic layer thickness of a-Si:H or a-SiGe:H solar cells is

�0.2�0.3 lm, but the intrinsic layer of nc-Si:H cells should

be �2–3 lm depending on the multijunction structure. In

order for nc-Si:H to become a viable technology for PV,

high rate deposition manufacturing technology is necessary.

We estimate that the deposition rate should be �1–2 nm/s,

which is much higher than the 0.3 nm/s rate of current

a-Si:H and a-SiGe:H solar cells. We investigated several

methods for high rate deposition, including radio frequency

PECVD, VHF PECVD, and microwave PECVD.8 We found

the most promising method to be VHF PECVD with high

pressure and high power. The high frequency in VHF plasma

increases plasma density and reduces ion energy. We found

ion energy is much lower and ion density is much higher in

VHF plasmas as compared to rf plasmas at the same process

conditions. High VHF power ensures a high deposition rate

and the high pressure reduces the ion bombardment while

keeping good material quality in nc-Si:H films. In addition,

high VHF power produces more atomic hydrogen in the

plasma, which can reduce the required hydrogen dilution

necessary for keeping deposition in the nc-Si:H region.

Figure 11 shows nc-Si:H solar cell efficiency plotted as a

FIG. 10. (Color online) (a) J–V characteristics and (b) QE curves of an

a-Si:H/a-SiGe:H/nc-Si:H triple-junction cell with an initial active-area

efficiency of 16.3%.

04D108-8 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-8

J. Vac. Sci. Technol. A, Vol. 30, No. 4, Jul/Aug 2012

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function of nc-Si:H intrinsic layer time, where the intrinsic

layer thicknesses were kept �2.4 lm. In order to keep the

same thickness with various deposition times, the deposition

parameters had to change to increase the deposition rate.

The simple way to increase the deposition rate is to increase

the VHF power density. However, increasing the power den-

sity normally is associated with a deterioration of material

quality. In addition, an increased VHF power causes an

increase of crystalline volume fraction. Therefore, in opti-

mizing the nc-Si:H solar cells at high rates, we normally

increase VHF power and at the same time reduce hydrogen

dilution and increase gas pressure. At each point of Fig. 11,

a significant optimization has been made to reach the highest

efficiency at a given deposition rate. One can see that by

doing the optimization we achieved a constant efficiency at a

wide range of deposition times from 25 to 40 min; reducing

the deposition time further causes the cell efficiency to

decrease. Currently, we are optimizing the deposition condi-

tions to improve high rate nc-Si:H cell efficiency further.

F. Large area a-Si:H/nc-Si:H/nc-Si:H triple-junctionsolar modules

We have conducted research and development of large-

area a-Si:H/nc-Si:H/nc-Si:H triple-junction solar modules

using batch large-area deposition systems and VHF PECVD.

Batch machines allow us to explore and resolve issues that

can then be transferred to large volume production; these

issues include large area uniformity, module fabrication, and

demonstration of high module efficiency. We designed large

area VHF deposition chambers that can deposit nc-Si:H uni-

formly. We made a-Si:H/nc-Si:H/nc-Si:H triple-junction so-

lar modules with an initial aperture area efficiency �12%

and stable efficiency over 11%. Table III lists a few exam-

ples of J–V characteristics in the initial state measured by

United Solar and by NREL. The consistency of the measure-

ments is noteworthy. Recently, we obtained a stable aperture

area efficiency of 11.3% from an 800 cm2 module made at

high deposition rates, which represents the highest thin film

silicon solar module measured by NREL.

Based on solar cell and module results, United Solar has

conducted extensive research and development of next

generation roll-to-roll manufacturing systems to fabricate

a-Si:H/nc-Si:H/nc-Si:H triple-junction product. The target

for aperture-area stable module efficiency is 12.0%.

IV. SUMMARY

In this paper, we reviewed our current a-Si:H/a-SiGe:H/

a-SiGe:H triple-junction spectrum splitting PV technology

using roll-to-roll deposition systems on flexible substrates.

This technology is based on high efficiency solar cell tech-

nology developed in our research and development labora-

tory, which holds the record efficiency of 13.0% measured

by NREL. Key techniques of the high efficiency a-Si:H and

a-SiGe:H solar cells have been discussed, including hydro-

gen dilution, bandgap profiling in a-SiGe:H component cells,

and multijunction cell design. The production process, prod-

uct performance, and installation examples have also been

presented.

We also summarized the progress made in research and

development of a-Si:H- and nc-Si:H-based solar cells and

modules for future high efficiency products. The nc-Si:H

material properties, cell design, optimization of cell fabrica-

tion, high rate deposition, and large area solar modules have

been discussed. The key techniques for providing high effi-

ciency nc-Si:H solar cells have been identified as VHF

PECVD deposition at high pressure and high power regime,

proper hydrogen dilution profiling, and interface layers. By

combining all efforts, we have demonstrated record high sta-

ble cell and module efficiencies measured by NREL, includ-

ing 12.5% stable total area (0.268 cm2) cell efficiency and

11.3% stable aperture area (800 cm2) module efficiency. We

are now focused on the development of next generation roll-

to-roll manufacturing systems to fabricate high efficiency

triple-junction products incorporating nc-Si:H.

ACKNOWLEDGMENTS

We sincerely acknowledge the dedicated work of the

research and development department at United Solar

Ovonic LLC. A persistent team effort has led us to make the

record solar cell and module efficiencies. The work has been

supported by the U.S. Department of Energy through the So-

lar America Initiative Program under Sub-Contract No.

FIG. 11. nc-Si:H single-junction solar cell efficiency as a function of

nc-Si:H deposition time, where the intrinsic layer thickness was kept constant.

TABLE III. Initial J–V characteristics of a-Si:H/nc-Si:H/nc-Si:H modules

measured by United Solar Ovonic (USO) and by NREL.

Module

number Measurement

Area

(cm2)

Tempa

(�C)

Voc

(V)

Iscb

(A)

FF

(%)

Effc

(%)

3D10527 USO 400 24.2 1.87 3.43 71.7 11.50

NREL 399.8 25.2 1.882 3.525 67.1 11.10

3D10529 USO 400 25.4 1.912 3.36 72.5 11.67

NREL 399.8 25.1 1.925 3.5 68.9 11.60

3D10554 USO 400 24.5 1.936 3.41 73.2 12.09

NREL 399.8 25.1 1.948 3.57 69.2 12.00

2D10555 USO 400 25.3 1.906 3.53 71.3 12.02

NREL 399.8 25.1 1.92 3.676 68 12.00

aMeasurement temperature.bShort circuit current.cEfficiency.

04D108-9 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-9

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FC36-07 GO 17053, and by the U.S. Department of Defense

through US Army under Sub-Contract No. W9132 T-11-C-

0007.

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04D108-10 Yan, Yang, and Guha: Amorphous and nanocrystalline silicon thin film photovoltaic technology 04D108-10

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