Thin Film Si H Based Solarcells

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Vacuum 82 (2008) 11451150

Thin-film Si:H-based solar cells

C.R. Wronskia,1, B. Von Roedernb, A. Ko"odziejc,

aElectrical Engineering Department, Pennsylvania State University, University Park, PA 16802, USAbNational Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393, USA

cAGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland

Abstract

Recent developments in the photovoltaic (PV) industry, driven by a shortage of solar grade Si feedstock to grow Si wafers or ribbons,have stimulated a strong renewed interest in thin-film technologies and in particular in solar cells based on protocrystalline hydrogenated

amorphous silicon (a-Si:H) or nanocrystalline/microcrystalline (nc/mc)-Si:H. There are a number of institutions around the world

developing protocrystalline thin-film Si:H technologies as well as those based on tandem and triple junction cells consisting of a-Si:H,

a-Si:Ge:H and nc/mc-Si:H. There are also several large commercial companies actively marketing large production-scale plasma-

enhanced chemical vapor deposition (PECVD) deposition equipment for the production of such modules. Reduction in the cost of the

modules can be achieved by increasing their stabilized efficiencies and the deposition rates of the Si:H materials. In this paper, recent

results are presented which provide insights into the nature of protocrystalline Si:H materials, optimization of cell structures and their

light-induced degradation that are helpful in addressing these issues. The activities in these areas that are being carried out in the United

States are also briefly reviewed.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Thin silicon film; Phase transition; Solar cells; Stability

1. Introduction

1.1. Amorphous and nanocrystalline Si film-based

photovoltaic (PV) modules

Some 25 years ago, some researchers in the PV research

arena predicted a rapid transition from wafer or ribbon-

based Si modules to thin-film amorphous silicon (a-Si)-

based modules [1]. This switch did not occur because the

stabilized efficiency of a-Si-based modules did not reach

the expected 10% level. Also, the competing wafer (ribbon)Si-based technologies were able to implement, unforeseen

then, advances in both performance and manufacturing

cost reduction. However, a renewed interest in a-Si-based

modules appears to be driven by the following factors:

(a) Si feedstock shortages for wafer/ribbon Si; (b) the

advances in thin-film processing; (c) the quest for a low-

energy budget (low-Tprocess) solar module manufacturing

process relying on practical limitless silane feedstock;

(d) the advent of nanocrystalline/microcrystalline (nc/mc)

Si solar cells that triggered hope for greater stabilized

performance. Todays commercially available a-Si-based

modules range in efficiency (as rated by their manufac-

turers) 4.58.5% conversion efficiency. This performance is

high enough to compete with wafer/ribbon-based silicon

modules in a number of applications. There is also the hope

to further improvements in stabilized module performance.Research attempts to better the performance of a-Si:H

and/or nc-Si:H-based solar cells currently focus on two

primary factors that increase the cost of a-Si-based PV

modules: (a) low stabilized solar cell/module efficiency and

(b) low deposition rates requiring significant investment

into capital-intensive vacuum deposition equipment. A key

limitation on the stabilized efficiencies of the a-Si:H cells

are the light-induced defects associated with the Staeble-

rWronski effect (SWE) [2]. Recent progress in improving

the understanding of this effect is discussed below in terms

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0042-207X/$- see front matter r 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.vacuum.2008.01.043

Corresponding author. Fax: +4812 63323 98.

E-mail addresses: [email protected] (C.R. Wronski),

[email protected] (A. Kolodziej).1Also for correspondence.
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of the creation kinetics and nature of the light-induced

defect states as well as their dependence on microstructure

of the a-Si:H. Researchers pursued hydrogen dilution,

also discussed below, as a mitigation scheme that can

reduce the amount of StaeblerWronski degradation.

Unfortunately, hydrogen dilution also lowers the deposi-

tion rate. Today, champion solar cells are made usingdeposition times as low as 0.1 nm/s, and commercial

modules are produced at rates of 0.3 nm/s. This requires

production times on the order of 30min for the film

deposition; requiring large vacuum plasma-enhanced che-

mical vapor deposition (PECVD) coaters. Employing

even higher hydrogen dilution results in the deposition of

nc/mc films, which have also been used to fabricate low-T

deposited solar cells [3]. Some entities, especially in Europe

and Japan, expect that the incorporation of an nc/mc-Si cell

into a multijunction cell stack should improve its stabilized

performance.

2. Hydrogen dilution and protocrystallinity

Continuous progress in the performance of a-Si:H-based

solar cells has been made by improving the quality of

the materials and cell structures. This, however, has been

achieved with approaches that were mainly empirical

due, primarily, to the absence of a clear understanding of

material growth and microstructure as well as the subtle

and not so subtle differences in their properties. As a

result even the improvements obtained by using a-Si:H

materials deposited with hydrogen dilution of silane [4,5]

were not understood. Thus it was difficult to control

them and carry out improvements in a systematic way.However, recently the growth and microstructure of these

hydrogen diluted protocrystalline Si:H materials has

been characterized [6,7] and new insights have been

obtained into both, their nature as well as their PV

properties. Under protocrystalline Si:H growth conditions

there is a unique evolutionary growth behavior, and the

protocrystalline material itself exhibits unique opto-

electronic properties. Using the real-time spectroscopic

ellipsometry (RTSE), first developed by Collins et al. [8,9],

to characterize thin-film growth and microstructure it

has been shown that the thin-film Si:H prepared under

moderate-to-high H2-dilution conditions evolves from the

amorphous phase to a mixed amorphous+nano/micro-

crystalline phase [(a+nc/mc)-Si:H] with the accumulated

thickness of the layer. The thin-film material in the

amorphous regime of growth has been called protocrystal-

line a-Si:H and exhibits a higher degree of ordering

than materials deposited under similar conditions without

H2-dilution [10]. Despite the evolutionary nature of the

Si:H materials prepared with moderate H2-dilution, they

exhibit relatively uniform bulk properties over certain

extended regions of thickness. Using phase diagrams

as a guide, it is possible to identify and control this

evolutionary nature in both films as well as solar cell

structures in which the effects of transitions between the

different phases in solar cell structures have been studied

and characterized.

A key capability of RTSE is the ability to generate

deposition phase diagrams for different deposition condi-

tions and substrate materials. These diagrams identify the

ranges of thickness over which it is possible to obtain the

pure protocrystalline phase and the ranges correspondingto the mixed-phase deposition as well as insights into the

control of the phase transitions for applications in device

fabrication. These uses of the phase diagram have allowed

characterization of the bulk properties of protocrystalline

Si:H films as well as the fabrication of highly controlled cell

structures.

In Fig. 1, it is shown a schematic of the structure of Si:H

films derived from phase diagrams prepared as a function

of hydrogen dilution R [H]/[SiH4] is shown for Si:H

deposited onto an undiluted R 0 substrate layer at a

substrate temperature of 2001C. It can be seen that

with decrease of R there is a continuous decrease of the

thickness at which the transition of protocrystalline a-Si:H

into the a+nc/mc phase occurs. This characteristic has very

important consequences in the design of protocrystalline

solar cells with different thickness, which until now have

not been recognized. Another feature is the increase with R

in the nucleation density of nc/mc-Si at the a-a+nc/mc

transition which are key to the subsequent growth of the

a+nc/mc mixed phase. A third point that can be made is

the decrease with R of the range of boundaries between

which the a+nc/mc-Si:H phase present before coalescing

into the mc-Si:H phase. Deposition phase diagrams have

also been developed to guide the increases in deposition

rates in which the effects of higher power densities on theevolution of protocrystalline Si:H have been characterized

[8]. Exemplary applications of phase diagrams have been

demonstrated for tandem and triple solar cell structures

with using a-Si:H, a-Si1xGex:H with low Ge content and

mc-Si:H received by hydrogen dilution and higher than

300 1C substrates temperature [11].

RTSE studies have also indicated that the way to

improve protocrystalline Si:H cells is to use what may be

called phase engineering of Si:H solar cells that exploits

microstructurally engineered structures for the improved

ARTICLE IN PRESS

Fig. 1. Schematic of the structure of Si:H films prepared as a function ofR.

The dashed and dotted lines identify the a-(a+nc/mc), and (a+nc/mc)-mc

transitions, respectively. Also indicated are the corresponding densities Nd

of nc-Si:H nuclei present at the a-(a+nc/mc) transition.

C.R. Wronski et al. / Vacuum 82 (2008) 114511501146


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performance [12]. Two examples are presented here to

illustrate how the ability to control the microstructure of

the materials allows phase engineering to be carried out in

a systematic way. The first shows how a protocrystalline

a-Si:H silicon carbide pin cell structure has been

optimized with a microstructurally engineered p/i interface

region obtained by a two-step variation of the i-layerH2-dilution ratio [13,14]. This is illustrated with results on

customized cells in which different thickness R=40 Si:H

layers were inserted into cells having 4000 A , R=10

i-layers. The light IVs of cells having 100, 200, 300, and

400A thick R=40 layers in the p/i interface region are

shown in Fig. 2. The corresponding AFM images of the

R 40 i-layers are shown in Fig. 3. The introduction of the

R 40 p/i interface regions of 100 and 200 A thickness

increases the Voc from the 0.88 V baseline value obtained

without an interface region to 0.92 and 0.93 V, respectively,

without altering the fill factors (FF). Subsequent increase

in the thickness of the R 40 layer to 300 and 400A ,

however, significantly reduces both the Voc and the FF.

These changes can be related to the transition into the

mixed phase identified with SRTE and consistent with the

AFM results in Fig. 3.

The other example is for p-Si:H contacts on a-Si:H nip

solar cells where the results of these layers deposited on

a-Si:H clearly show a very strong thickness dependence

of the electronic properties of the film, as it evolves from

protocrystalline Si:H to a (a+nc/mc)-Si:H mixed phase.

The evolution of the microstructure with thickness in the

p-Si:H layers deposited with constant doping ratio, but

different R values, is illustrated in Fig. 4. In Fig. 4, the

regions of protocrystalline Si:H, mixed (a+nc/mc)-Si:H,and purely microcrystalline Si:H phases can be clearly

identified.

Also shown in Fig. 4 are the 1sun Voc values obtained

from corresponding nip solar cell structures fabricated

ARTICLE IN PRESS

Fig. 2. Light IV characteristics of the customized pin solar

cells fabricated with the two-step i-layer having different thickness

p/i interface layers.

Fig. 3. 22mm2 AFM images of the R 40 Si:H films at the same

thickness used in the customized cell structures.

Fig. 4. Extended phase diagram for p-Si:H deposited on protocrystalline

a-Si:H as function ofR and thickness. Also shown is the 1 sun Voc for cell

structures with corresponding $200A p-layers.

C.R. Wronski et al. / Vacuum 82 (2008) 11451150 1147


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with $200A of these p-layers. The highest Voc is achieved

by the cell having a R 150 p-layer which is in the

protocrystalline a-Si:H phase and where its growth is

terminated at, or close to the transition into the mixed

(a+nc/mc)-Si:H phase [15,16]. In the case of R=100,

the p-layer is amorphous throughout its thickness and the

200A thickness is nowhere close to the transition into amixed phase.

On the other hand, for the R=200 cell, the p-layer is in

the form of a mixed (a+nc/mc)-Si:H phase right from the

beginning and evolves into a purely microcrystalline phase

at the thickness of$200A . These results clearly point to

the highest VOC being obtained with a protocrystalline

a-Si:H p-layer which is deposited at a maximum R value

that allows the desired thickness to be obtained without

crossing the transition into the mixed-phase growth regime.

It is interesting to point out here that until these deposition

phase diagrams were obtained it was claimed that the

optimum p contact layers were microcrystalline. This is the

p-layers that were characterized were much thicker than

the 100200 A in the solar cells so that transitions to the

purely mc phase were present in those films [17].

3. Light-induced changes in thin films and solar cells

There is a serious challenge that has still to be overcome

for further advances to be made in a-Si:H-based solar cell

technology.

It is the SWE, which was discovered in 1977 [2] but still

imposes serious limitations on the performance of a-Si:H

cells and panels. It is in the form of metastable defects that

are created under illumination, which can be annealed outat temperatures above $150 1C.

The reversible changes that occur between the annealed,

initial, and the light soaked, degraded, states have

become one of the most often investigated phenomena in

a-Si:H-based material and solar cells.

However, progress has been relatively slow in obtaining

a definitive understanding and systematic control of the

SWE. This is in large part due to the absence of a unique

a-Si:H material so that the creation of the light-induced

defects depends on the deposition conditions and resultant

microstructure of the materials. It is also due to the studies

focusing on the prevailing theories that attempted to

explain all metastable phenomena solely in terms of a

metastable light-induced neutral dangling bond defect [D0].

Nevertheless, an empirical approach has led to some

progress being made over the years not only in improving

the initial properties of a-Si:H-based material but also in

reducing their SWE by optimizing the growth conditions to

improve their microstructure. The development of proto-

crystalline a-Si:H materials has resulted in solar cells that

not only have higher initial efficiencies, but even more

importantly better end of life performance [18]. The

significant improvements obtained with protocrystalline

a-Si:H are illustrated in Fig. 5. Fig. 5 shows the changes of

the FF under 1 sun illumination at 25 1C for a 4000 A pin

solar cell with a protocrystalline (R 10) a-Si:H layer and

an undiluted (R 0) intrinsic layer deposited under the

same conditions. It can be pointed out here that in the

corresponding films after such degradation the densities of

the neutral dangling bonds, D0, were the same.

These light-induced defects affect both the hole and

electron transport that results in the significant reduction

of the FF and performance in cells seen here and for

example photoconductivity in corresponding thin-film

materials. For a long time there has been a striking

absence of direct correlations between the results of such

measurements on films and cells, which has seriouslylimited the ability to systematically improve the stability of

a-Si:H solar cells based on the optimization studies on thin-

film materials. Recently however, results have been

obtained which not only clearly establish direct and

quantitative correlations between the light-induced defects

created in a-Si:H films and their corresponding cells but

which have also shed new light on the nature of the light-

induced defects as well as the possible mechanisms for

SWE. An example of such correlations is illustrated with

results that are shown in Fig. 5. In Fig. 5, there are

shown the excellent overlaps in the changes of the gap

states under 1 sun illumination as reflected in the solar cell

by the recombination of injected carriers at 0.4 V forward

bias [19] and in the corresponding film by the reciprocal

of the photocurrent for a carrier generation rate of

2 1016 cm2 s1 [20]. It is interesting to note that in the

past, when the characterization of cells relied on effici-

encies or FF, such direct comparison of recombination

through the midgap states as illustrated here could not be

carried out.

Similar direct correlations have also been now obtained

between the results of the recombination currents in cells

and the densities of midgap states in films as measured with

the Dual Beam Photoconductivity technique [20]. Results

from such studies that are being carried out on both solar

ARTICLE IN PRESS

Fig. 5. Fill factor under 1 sun illumination at 25 1C for pin solar cells

with 400 nm i-layers deposited with R 0 (K) and R 10 (W) at 200 1Cas well as changes in the forward bias current in an a-Si:H cell (J) and the

reciprocal of the current in the corresponding film (.) for a carrier

generation rate of 21016 cm2 s1.



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cells and their corresponding intrinsic thin films have not

only further substantiated the inadequacy of the extensive

past characterization of light-induced changes in terms of

just neutral dangling bond, D0, states [21], but also yielded

important new information about the nature of the other

states associated with SWE. These recent studies, which

have also included the kinetics of the light-induced changesin the gap states of a-Si:H, have confirmed regimes with a

t1/3 dependence such as it has been extensively reported

for results on thin films. However, they have also clearly

established that the evolution of just the light-induced

midgap defect states follow a t1/2 dependence, which is

inconsistent with the rate equation for the creation of light-

induced defects that is generally used in modeling of SWE.

The improved insights that are being developed into the

nature of metastable defects point to new approaches for

obtaining a better understanding of the role that micro-

structure plays in determining the stability of a-Si:H

and the mechanisms responsible for SWE. This offers the

real possibility of further improving the performance of

a-Si:H-based solar panels.

4. Nanocrystalline/microcrystalline solar cells and

micromorph multijunction cells

In the mid-1990s, the University of Neuchatel

(Switzerland) first produced nc-Si solar cells and micro-

morph multijunction cells. The nc/mc-Si layers were

obtained by high hydrogen dilution of the silane feed gas

(490% of the gas being H2) as well as by changes in the

frequency of the PECVD deposition (very high frequency,

50100MHz, instead of the typical radio frequency13.56 MHz). This invention spurred the hope of achieving

greater stabilized efficiency for a-Si film-based solar cells

and modules [3]. Hence, several research groups around

the world prepared nc/mc-Si and micromorph multi-

junction [a-Si:H/nc/mc-Si:H] solar cells. The first commer-

cial offering has been recently introduced by the Sharp

Corporation, followed by a recent announcement of very

high volume manufacturing capacity [100 MWp annual

capacity after 2010].

nc/mc-Si is a potential replacement for a-SiGe:H-based

sub-cells in multijunction solar cells, because the nc-Si has

absorption coefficients greater than a-Si [3]. Hence, an

expectation exists of beating the performance of a-SiGe:H-

based cells and simultaneously eliminating the use of

expensive GeH4 gas for manufacturing. Drawback to these

schemes is that nc-Si solar cells have the absorber

thicknesses about 10 times thicker than a-SiGe:H cells

(1.52mm) and that for these cells too, best results are

obtained using slow deposition rates.

Work by the Ju lich group suggested in 1999 that the best

cells are obtained by growing the mixed-phase nc/mc layers

[22] near the nanocrystalline/amorphous growth transition.

These results were somewhat unexpected, but have been

ubiquitously confirmed by other groups researching these

materials. Since the microcrystalline deposition regime has

to be avoided when fabricating high-efficiency solar cells,

such cells can be referred to as nc-Si, although many

groups also use the term microcrystalline for such cells.

Based on the knowledge of a nanomicrocrystalline

structure evolving with film thickness (Fig. 1), Uni-Solar

has implemented the graded hydrogen dilution (reducing

dilution as the films grow) to keep thicker films near thenc/mc-Si to a-Si transition [23]. Only time will tell if nc-Si

solar cells, when incorporated into multijunction struc-

tures, can enhance stabilized cell performance. The Uni-

Solars current position is that nc/mc-Si cells are equivalent,

but not yet better than a-SiGe:H-based solar cells.

A concern with these rather thick cells is the long

deposition time required. The performance of such cells is

also extremely sensitive to processing detail, which may

make scale up of the process (from R&D areas to

commercial module areas) more challenging than PECVD

deposition of a-Si:H-based cells. The United States PV

program has supported a research effort on nc-Si solar cells

at United Solar, and if better results can be obtained, they

stand ready to implement manufacturing of nc-Si contain-

ing micromorph modules.

5. Conclusion and outlook

In order to achieve further cost reduction for a-Si:H-

based PV modules, it is highly desirable to increase their

stabilized performance and simultaneously increase their

deposition rates. A study has been conducted on estimating

the potential module cost for various PV technologies. This

study compares commercial module performance of PV

modules using different cell technology fabricated withcomparable maturity in module manufacturing. It is

argued that all successful PV technologies will eventually

reach a similar degree of manufacturing maturity. It is then

further assumed that a thin-film module will eliminate the

costs fraction attributable to the Si wafer in a crystalline Si

PV module. Based on this analysis, a-Si-based thin-film

modules appear competitive, but in the near and midterm

they may not provide the lowest-cost module option

because of their lower performance [24]. However, the

advantages of a low-temperature growth process, a

virtually unlimited feedstock supply for manufacturing

them, and the renewed interest from display manufacturingequipment manufactures is inherent to this PV technology.

It is therefore imperative to continue work on overcoming

the current stabilized performance shortfalls.

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Thin Film Si H Based Solarcells