Amorphous Silicon-Based Solar Cells

TLDR: In this paper, the authors describe the diamond lattice of crystal silicon as a regular array or lattices, which must be consistent with the underlying chemical bonding properties of the atoms, such as the four covalent bonds of a silicon atom.

Abstract: Crystalline semiconductors are very well known, including silicon (the basis of the integrated circuits used in modern electronics), Ge (the material of the first transistor), GaAs and the other III-V compounds (the basis for many light emitters), and CdS (often used as a light sensor). In crystals, the atoms are arranged in near-perfect, regular arrays or lattices. Of course, the lattice must be consistent with the underlying chemical bonding properties of the atoms. For example, a silicon atom forms four covalent bonds to neighboring atoms arranged symmetrically about it. This “tetrahedral” configuration is perfectly maintained in the “diamond” lattice of crystal silicon.

Figures

Figure 12.3 In a pin photodiode, excess electrons are donated from the n-type to the p-type layers, leaving the charges and electric fields illustrated. Each photon absorbed in the undoped, intrinsic layer generates an electron and a hole photocarrier. The electric field causes these carriers to drift in the directions shown. pin diodes are incorporated into solar cells in either the superstrate or substrate designs. For amorphous silicon–based cells, photons invariably enter through the p-type window layer as shown here

Figure 12.7 (a) Computer model of the chemical bonding of hydrogenated amorphous silicon. The larger, gray spheres indicate Si atoms; the smaller, white spheres indicate hydrogen atoms, which are found in clustered and relatively isolated, dilute-phase configurations as indicated. (b) Correlation of the defect (dangling bond) density in a-Si:H with the density of hydrogen removed from the material by heating (the hydrogen deficit). The data points are derived from deuterium and defect profiles by Jackson et al. [31] (350◦C deuteration). The curve is a fit to a model proposed by Zafar and Schiff [32]

Figure 12.20 Model calculations of the short-circuit current JSC (mA/cm2), open-circuit Voltage VOC, (V), fill factor FF, and power P under AM1.5 illumination for a-Si:H-based pin solar cells with varying intrinsic layer band gaps and thicknesses. No back reflector or texturing effects are included

Figure 12.14 Bandedge and Fermi-level profiles in a pin solar cell under open-circuit conditions. The open-circuit voltage is precisely the value of EFh at the left interface (x = 0). The built-in potential VBI is illustrated. Note that the p-layer has a slightly (0.2 eV) larger band gap than the i-layer; the calculation assumes symmetrical offsets of the valence and conduction bands at the p/i interface

Figure 12.28 Cell interconnection of superstrate-type solar cells, used at BP Solar

Figure 12.12 Deposition rate for a-Si:H films as a function of RF excitation frequency (at constant power); (After Shah A et al., Mater. Res. Soc. Symp. Proc. 258, 15 (1992) [83])

Full text

Syracuse University Syracuse University
SURFACE SURFACE
Physics College of Arts and Sciences
2003
Amorphous Silicon Based Solar Cells Amorphous Silicon Based Solar Cells
Xunming Deng
University of Toledo
Eric A. Schiff
Syracuse University
Follow this and additional works at: https://surface.syr.edu/phy
Part of the Physics Commons
Recommended Citation Recommended Citation
"Amorphous Silicon Based Solar Cells," Xunming Deng and Eric A. Schiff, in Handbook of Photovoltaic
Science and Engineering, Antonio Luque and Steven Hegedus, editors (John Wiley & Sons, Chichester,
2003), pp. 505 - 565.
This Book Chapter is brought to you for free and open access by the College of Arts and Sciences at SURFACE. It
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12
Amorphous Silicon–based Solar Cells
Xunming Deng
1
and Eric A. Schiff
2
1
University of To ledo, Toledo, OH, USA,
2
Syracuse University,
Syracuse, NY, USA
12.1 OVERVIEW
12.1.1 Amorphous Silicon: The First Bipolar Amorphous
Semiconductor
Crystalline semiconductors are very well known, including silicon (the basis of the inte-
grated circuits used in modern electronics), Ge (the material of the first transistor), GaAs
and the other III-V compounds (the basis for many light emitters), and CdS (often used
as a light sensor). In crystals, the atoms are arranged in near-perfect, regular arrays or
lattices. Of course, the lattice must be consistent with the underlying chemical bonding
properties of the atoms. For example, a silicon atom forms four covalent bonds to neigh-
boring atoms arranged symmetrically about it. This “tetrahedral” configuration is perfectly
maintained in the “diamond” lattice of crystal silicon.
There are also many noncrystalline semiconductors. In these materials the chemical
bonding of atoms is nearly unchanged from that of crystals. Nonetheless, a fairly small,
disorderly variation in the angles between bonds eliminates the regular lattice structure.
Such noncrystalline semiconductors can have fairly good electronic properties – sufficient
for many applications. The first commercially important example was xerography [1, 2],
which exploited the photoconductivity of noncrystalline selenium. As do all semicon-
ductors, selenium absorbs those photons from an incident light beam that have photon
energies exceeding some threshold e nergy. The photon that is absorbed generates a posi-
tively charged “hole” and a negatively charged electron that are separated and swept away
by the large electric fields used in xerography.
However, solar cells require that photogenerated electrons and holes be separated
by relatively modest electric fields that are “built-in” to the device, and selenium and
many other noncrystalline semiconductors proved unsuitable for making efficient cells.
Handbook of Photovoltaic Science and Engineering. Edited by A. Luque and S. Hegedus
 2003 John Wiley & Sons, Ltd ISBN: 0-471-49196-9

506 AMORPHOUS SILICON–BASED SOLAR CELLS
In Dundee, Scotland, Walter Spear and Peter LeComber discovered around 1973 that
amorphous silicon prepared using a “glow discharge” in silane (SiH
4
) gas had unusually
good electronic properties; they were building on earlier work by Chittick, Sterling, and
Alexander [3]. Glow discharges are the basis for the familiar “neon” light; under certain
conditions, an electric voltage applied across a gas can induce a significant electrical
current through the gas, and the molecules of the gas often emit light when excited by the
current. Amorphous silicon was deposited as a thin film on substrates inserted into the
silane gas discharge.
1
Spear and LeComber reported in 1975 [4] that amorphous silicon’s
conductivity could be increased enormously either by mixing some phosphine (PH
3
)gasor
some diborane (B
2
H
6
) gas with the silane. Just as for crystal silicon, the phosphorus doping
of the amorphous silicon had induced a conductivity associated with mobile electrons (the
material was “n-type”), and the boron doping had induced a conductivity associated with
mobile holes (the material was “p-type”).
In 1974, at the Radio Corporation of America (RCA) Research Laboratory in
Princeton, David Carlson discovered that he could make fairly efficient solar cells using
a silane glow discharge to deposit films. In 1976, he and Christopher Wronski reported
a solar cell based on amorphous silicon [5] with a solar conversion efficiency of about
2.4% (for historical discussion see Reference [6, 7]).
Carlson and Wronski’s report of the current density versus output voltage is pre-
sented in Figure 12.1 (along with the curve from a far more efficient cell r eported in
1997 [8]). As these scientists had discovered, the optoelectronic properties of amorphous
silicon made by glow discharge (or “plasma deposition”) are very much superior to
the amorphous silicon thin films prepared, for example, by simply evaporating silicon.
−10
−8
−6
−4
−2
0
012
1997 − 14.6%
1976 − 2.4%
Voltage
[V]
Current density
[mA/cm
2
]
Figure 12.1 Current density versus voltage under solar illumination for a very early single-junction
amorphous silicon solar cell (Carlson and Wronski [5]) and from a recent “triple-junction” cell
(Yang, Banerjee, and Guha [8]). The stabilized efficiency of the triple-junction cell i s 13.0%; the
active area is 0.25 cm
2
1
The term amorphous is commonly applied to noncrystalline materials prepared by deposition from gases.

OVERVIEW 507
After several years of uncertainty, it emerged that plasma-deposited amorphous silicon
contained a significant percentage of hydrogen atoms bonded into the amorphous silicon
structure and that these hydrogen atoms were essential to the improvement of the elec-
tronic properties of the plasma-deposited material [9]. As a consequence, the improved
form of amorphous silicon has generally been known as hydrogenated amorphous silicon
(or, more briefly, a-Si:H). In recent years, many authors have used the term amorphous
silicon to refer to the hydrogenated form, which acknowledges that the unhydrogenated
forms of amorphous silicon are only infrequently studied today.
Why was there so much excitement about the amorphous silicon solar cells fab-
ricated by Carlson and Wronski? First, the technology involved is relatively simple and
inexpensive compared to the technologies for growing crystals. Additionally, the opti-
cal properties of amorphous silicon are very promising for collecting solar energy, as
we now explain. In Figure 12.2, the upper panel shows the spectrum for the optical
absorption coefficients α(hν) for amorphous silicon and for crystalline silicon [10].
2
In
the lower panel of the figure, we show the spectrum of the “integrated solar irradiance;”
this is the intensity (in W/m
2
) of the solar energy carried by photons above an energy
threshold hν [11].
0
0123
200
400
600
800
Absorbed
Photon energy hn
[eV]
a-Si:H (500 nm)
Transmitted
10
0
10
1
10
2
10
3
10
4
10
5
Absorption a
[cm
−1
]
Solar irradiance above hn
[W/m
2
]
a-Si:H
c-Si
Figure 12.2 (Upper panel) Spectra of the optical absorption coefficient α(hν) as a function of
photon energy hν for crystalline silicon (c-Si) and for hydrogenated amorphous silicon (a-Si:H).
(Lower panel) The solid curve indicates the irradiance of photons in the solar spectrum with energies
hν or larger. An a-Si:H film that is 500 nm thick mostly absorbs photons above 1.9 eV; as indicated
by the shaded areas, this corresponds to an absorbed irradiance of about 390 W/m
2
.AfterVan
ˇ
e
ˇ
cek
M et al., J. Non-Cryst. Solids 227–230, 967 (1998) [10]
2
We assume familiarity with the concept of a photon energy hν and of an optical absorption coefficient α;see
Chapter 3.

508 AMORPHOUS SILICON–BASED SOLAR CELLS
We use these spectra to find out how much solar energy is absorbed by layers
of varying thickness. The example used in the fi gure is an a-Si:H layer with a thickness
d = 500 nm. Such a layer absorbs essentially all photons with energies greater than 1.9 eV
(the energy at which α = 1/d). We then look up how much solar irradiance lies above
1.9 eV. Assuming that the reflection of sunlight has been minimized, we find that about
420 W/m
2
is absorbed by the layer (the gray area labeled “absorbed”). Through such a
layer 580 W/m
2
of energy is transmitted. These energies may be compared to the results
for c-Si, for which a 500-nm-thick layer absorbs less than 200 W/m
2
.
To absorb the same e nergy as the 500-nm a-Si:H layer, a c-Si layer needs to be
much thicker. The implication is that much less material is required to make a solar
cell from a-Si than from c-Si.
3
In the remainder of this section, we first describe how
amorphous silicon solar cells are realized in practice, and we then briefly summarize some
important aspects of their electrical characteristics.
12.1.2 Designs for Amorphous Silicon Solar Cells: A Guided Tour
Figure 12.1 illustrates the tremendous progress over the last 25 years in improving the
efficiency of amorphous silicon–based solar cells. In this section we briefly introduce three
basic ideas involved in contemporary, high-efficiency devices: (1) the pin photodiode
structure, (2) the distinction between “substrate” and “ superstrate” optical designs, and
(3) multijunction photodiode structures. A good deal of this chapter is devoted to more
detailed reviews of the implementation and importance of these concepts.
12.1.2.1 pin photodiodes
The fundamental photodiode inside an amorphous silicon–based solar cell has three layers
deposited in either the p-i-n or the n-i-p sequence. The three layers are a very thin
(typically 20 nm) p-type layer, a much thicker (typically a few hundred nanometer),
undoped intrinsic (i) layer, and a very thin n-type layer. As illustrated in Figure 12.3, in
this structure excess electrons are actually donated from the n-type layer to the p-type
layer, leaving the layers positively and negatively charged (respectively), and creating a
sizable “built-in” electric fi eld (typically more than 10
4
V/cm).
Sunlight enters the photodiode as a stream of photons that pass through the p-type
layer, which is a nearly transparent “window” layer. The solar photons are mostly absorbed
in the much thicker intrinsic layer; each photon that is absorbed will generate one
electron and one hole photocarrier [12, 13]. The photocarriers are swept away by the
built-in electric field to the n-type and p-type layers, respectively – thus generating solar
electricity!
The use of a pin structure for a-Si:H-based solar cells is something of a departure
from solar cell designs for other materials, which are often based on simpler p-n structures.
3
The very different optical properties of c-Si and a-Si reflect the completely different nature of their elec-
tronic states. In solid-state physics textbooks, one learns about the “selection rules” that greatly reduce optical
absorption in c-Si, which is an “indirect band gap” semiconductor. Such selections rules do not apply to a-Si.
Additionally, the “band gap” of a-Si is considerably larger than that for c-Si.