Intrinsic Reliabilitty of Amorphous Silicon Thin Film Solar
S
Cells
M. A. Alam, S. Donngaonkar, Karthik Y., S. Mahapatra, D. Wang*, M. Frei*
Purdue Univversity, School of Electrical and Computer Engineeringg
465 Northw
western Avenue, West Lafayette, Indiana 47907, USA
* Applied Materials, Santa Clara, CA
Phoone: 765-494-5988 E-mail: [email protected]
1.
/
Introduction and Motivation
Although solar cells made of single crystalline
c
silicon
(c-Si) have long dominated the market for
fo their efficiency
and reliability, there has been a growing interest in exploring other (somewhat) less efficient materials (e.g.,
amorphous Si or α-Si:H, CIGs, CdTe) thhat offer improved
price/kWh, through significantly lower manufacturing
m
costs.
If the reliability challenges associated wiith these materials
could be understood and addressed, the economic
e
viability
of these PV options would improve dramaatically.
e
be extrinsic
The reliability issues of α-Si PV could either
(e.g., humidity, glass breakage, wiring faault)[1] or intrinsic
(e.g., light induced degradation). In this article, we will
discuss the physics and technological origgin of three intrinsic reliability concerns for α-Si solar cells: (i) shunt conduction, (ii) shadow degradation and hoot-spot generation,
and (iii) light induced degradation. Oveer the years, these
issues have been discussed in the literaturre by many groups,
although the context of the discussion was
w often isolated,
and the approach frequently empirical. Here we take a
comprehensive approach to show that thhese reliability issues are usually not independent, but can and should be
understood in a broader framework. Moreeover, many of the
reliability concerns of α-Si:H solar cells are
a shared by other
low-cost, thin-film PV materials (e.g. CIIGS, polymer) deposited by solution processing or chemicaal vapor deposition.
Therefore, a good understanding of the reeliability issues for
a-Si solar cells will also address broader questions regarding the reliability of other thin film technoologies.
2.
1
1 ;
is rate of generationn of electron-hole pairs (after
where
accounting for various opticall losses due to reflection and
free carrier absorption),
iss the series resistance, and n is
the ideality factor. (
1 for classical diode, while
2 for diodes dominateed by recombination in the
space charge region).
, the maximum
m
power delivered to
For a variable load
the load is given by (see Fig. 1b
1 for definitions)
;
where the open circuit voltagee
given by
2
(as shown in Fig. 1b) is
1
,
3
refllects the power lost to series
and the fill factor
resistance,
and other efffects. (see Fig. 1b, bottom).
Typically one would string 1000s of such diodes in series as
shown in Fig. 2 to increase thee output voltage.
Background Information
2. 1 Basics of PV Operation
The textbook description of the operationn of a solar cell is
o
as follows: The Sun radiates at 6000 C and
a the part of the
o
‘6000 C’ blackbody spectrum that is not absorbed by various molecules in the atmosphere and reaaches earth surface
can generate electron-hole pairs within thee p-n junction of a
solar cell (see Fig. 1a). If the charges cann be separated before they self-annihilate through recom
mbination, the collected photo-induced carriers can deliverr power to an external load. Assuming superposition is sattisfied [2],
978-1-4244-5431-0/10/$26.00©2010 IEEE
3E.2.1
(a)
(b)
Fig. 1: (a) Classical diode as solar cell, and equivalent circuit of actual solar cell. (b) Schematic diagram showing
light and dark IV characteristiics. Superposition is assumed.
The red-dot signifies maximum
m power point.
IRPS10-312
2.2 Basics of PV Reliability
Regardless of the material used for thee solar cells, i.e.,
crystalline or amorphous, the reliability issues
i
can be analyzed using Eq. (1). For a given value off
, Eq. (3) suggests that
(and therefore the outpuut power) decreases with increase in the ideality factor n, ‘dark current’
pre-factor . A time dependent shift in thhese parameters as
a consequence of operating conditions coonstitutes the reliability issues of a technology. Obviously,, the reliability issues that influence these parameters depend on the material
type and the configuration of the solar ceell. As a result, the
reliability concerns appear very differentt for various technologies; e.g., while light induced degraddation is an important reliability concern for a-Si:H solar cells, the issue is
less important for c-Si solar cells. The gooal of this paper is
to understand how various reliability issuees of a-Si:H affect
Eq. (2) and how they relate to the commoon reliability issues
for other solar cell technologies.
2.3 Thin Film Technology
l
area macroeThe fabrication and installation costs of large
lectronic devices like solar cells can be minimized
m
only if
they are processed with low energy input in high-efficiency
equipment using inexpensive and durablle substrate to reduce the use of raw materials to the minimum. While crystalline silicon requires 100s of μm thicck active layer[3],
thin film solar cells use only 100s of nm of
o intrinsic material sandwiched between 10-20nm thick, heeavily doped p and
n layers. Fluorinated Tin Oxide (SnO:F) or Indium Tin
e
while Al
Oxide (ITO) is used as transparent front electrode,
acts as back electrode. The cells are coonnected in series
using laser scribes on TCO and a-Si:H laayers (see Fig. 2).
This long string of series-connected cellss produces a module output of ~100V at ~1A. The qualityy and thickness of
the ITO and Al determine the optical prooperties and series
resistance of the system.
Fig. 2: Schematic diagram showing thee series connected
solar cells supplying a load in an equivallent circuit picture
(top) and the actual device layout schemaatic (bottom), with
the arrows indicating the current paths.
The economical use of materials in thin film technologies,
IRPS10-313
3E.2.2
like a-Si:H or CIGS, is not duue to any fundamental innovation, but rather related to the generic
g
wavelength dependent
absorption in semiconductors. Typically, a-Si:H has larger
bandgap (~1.72 eV) comparedd to c-Si (~1.1 eV); given that
absorption in silicon scales with
w
density of states and is
higher at shorter wavelengths, only a thin film of crystalline
or amorphous silicon is neccessary to completely absorb
(E > 1.72 eV) part of the solarr spectrum (discussed in more
detail in Fig. 5). While the manufacturing
m
cost is greatly
reduced by low-temperature vapor phase deposition of
thin-films on inexpensive glaass substrate, there are some
unique reliability concerns of thin film solar cells like light
induced degradation, shunt leaakage, etc.
3.
Intrinsic Reliability Issu
ues of a-Si:H Solar Cells
3.1 Shunt Leakage
Since the features of the darkk current (I-V characteristics
without sunlight shining on) dictate
(Eq. (3)), measurement of this characteristiccs offers insight into ultimate
device efficiency. While darkk I-V of thin film PV at sufficiently large forward biases appears
a
purely classical (with
1
2 ), one of the most
m
interesting and universal
feature is an anomalous leakage component at low forward
A
this feature has long
and reverse bias conditions. Although
been accounted for in equivallent circuit models with a parallel ohmic shunt resistance,
(see Fig. 1(a), bottom),
and has often been eliminatedd/reduced by so called ‘shunt
busting’, the physical origin of
o the phenomena and the universality of leakage across various
v
thin-film technologies
have remained unclear [4].
c
is defined by four chaWe find that this anomalous current
racteristics;
namely,
c
current-voltage
symmetry
(
); voltage nonlinearity
(
~
);
temperature
insensitivity
~
), annd large statistical fluctuation
(
in the leakage current magnittude from sample to sample.
These observations cannot be explained by intrinsic device
properties (e.g., defect distribbutions), but rather lead us to
attribute this anomalous leakage to generalized
space-charge limited currennt through localized metal-semiconductor-metal strucctures, and described by of
⁄
~
, where ~1 2 (to be published).
These localized structures shhould not be confused with
pin-holes related to inadequaate deposition of Si, but are
likely to arise from surface noon uniformity of TCO coated
glass substrates. Recent mapping
m
of the localized
light-spots by lock-in thermometry appears to support the
conclusion[5]. Other expperiments involving metal-a-Si-metal resistive memoories are consistent with the
hypothesis[6]. Indeed, this leeakage must be reduced to a
level so that it does not degraade
and becomes a reliability concern for thin film
m technologies. Fortunately,
however, shunt conductance may
m be reduced by improved
surface planarization[7], blocking layerrs[8], shunt busting[4], etc. The choice of a specific appproach depends on
application.
Initial experiments with “shadow stress” demonstrate that
degradation may be describbed by a power law, i.e.,
Δ
~
with ~1/4 (see
(
Fig. 4(b)), and the voltage acceleration is exponenttial (to be published). These
two equations are sufficient too predict the effect of shadow
degradation in large scale solaar cell installations.
The problem of shadow deggradation might be addressed
both at device as well as circuuit levels. At device level, the
reduction of Zener voltage reduces
r
reverse stress on the
affected cell and reduces defeect generation for a given duration of shadow. If cell redesign is difficult or expensive,
circuit/system solutions are apppropriate.
Fig. 3: Dark IV showing anomalous leakaage currents in (a)
a-Si:H p-i-n solar cells; (b) Organic BHJ cells
c
from [7].
3.2 Shadow Degradation
Shadow degradation is related to the reequirement that N
solar cells are series connected in a module to achieve large
. If the various structuraal elements of the
assembly (e.g., antennas, solar paddles, booms,
b
etc. in satellite applications) or other natural elem
ments (e.g. clouds,
leaves, trees, dirt, etc. for terrestrial appplications) cast a
shadow on one or several elements of thee solar module, the
current in the affected cells is reduced. Current
C
continuity
dictates the voltage be redistributed in suuch a way among
the N-cells so that
, where
is the voltage across the load,
is the voltage generated
illuminated cells of the array
across each of the
and
is the voltage developed across the shadowed part
(
cells). The negative sign of
reflects the fact that
the affected cells are reversed biased in thhe Zener tunneling
or Avalanche breakdown mode (see Fig 4(a)).
4
Indeed, shadowing not only eliminates the cells from
m power generation,
but dramatically degrades the power output from the rest of
the system by reducing the output voltagee.
One may wonder if shadowing, like radiation induced soft
errors in CMOS circuits, is a transient effect, and if the
system might be restored to its pristine performance once
the shadow is lifted (e.g., with change in orientation of the
satellite with respect to the Sun, or cleaaning of the solar
panels). Indeed, most papers in the literatture treat shadowing as a power management problem[9], with
w no discussion
of long term consequences. The large revverse bias endured
by the shadowed α-Si:H cells however wiill depassivate SiH
and/or weak Si-Si bonds, create mid-gaap defects, reduce
recombination lifetime , and increase
. The corresponding shift in
reflects the integratted duration of the
shadowing (and the corresponding stresss) endured by the
cells. The change in the maximum poweer point would reduce power output of the system. Details of the system level effects will be discussed elsewhere.
3E.2.3
(aa)
(bb)
Fig. 4: (a) Schematics showinng the operating points before
and after shading; identifyinng voltages across individual
cells (V1), reverse voltage devveloped across shaded device
(VS), and the corresponding load
l
voltage (VL). (b) Power
law time dependent degradatioon of solar cells under different reverse bias stresses that has been scaled by constant
factors to highlight the power--exponent of degradation.
Recall that for SRAM memorry, use of redundant arrays in
post-Si phase dramatically im
mproves yield by allowing remapping of defective cells (due to process related
f
leading to READ or
fluctuation or fluctuation in for
WRITE failures)[10]. Similar approaches may be appropriate for PV modules as well[11]. For example, a significant
IRPS10-314
reduction in
(see Eq. (3)) would indicate onset of
shadowing; a sweep across the elements of the module
would determine the affected row; and a standby redundant
array would then be switched in to bypass the affected cell.
Obviously, once the shadow is lifted, the same redundant
cell can be released and can be reconfigured such that they
can be used by other shadowed cells. Simulations show that
small degree of redundancy and management overhead
result in large improvements [11].
In some cases, rapid defect generation in cells affected by
intense shadows may lead to catastrophic failure of solar
cells by a process called the “hot-spot” formation[12]. Hot
spots may delaminate the mirrors and destroy the cells.
Like ‘hard breakdown’ in thick gate dielectrics, hot-spot
formation and propagation is likely to involve a positive
feedback and interplay between temperature and current[13,
14]. Like the early literature on gate dielectric breakdown,
there is considerable debate regarding whether the hot-spot
is related to pre-existing defects like etch pits as discussed
in Section 3.1 (thermal images seems to indicate such possibility) or intrinsic defect generation terminated by a runaway process. Further work is needed to identify the mechanism of hot-spot generation.
Si/SiO2 interface of c-Silicon solar cells (See Fig. 5). These
low energy photons
~1
that reach the
back-interface cannot dissociate SiH bonds. In contrast, the
distance over which the high energy photons are absorbed
in a-Si solar cells are replete with SiH bonds, and therefore
it is hardly surprising that Si-H bonds dissociation is a
concern. In short, film thickness of solar cells, bandgap of
the PV material and the light-induced degradation are related by fundamental consideration and must be understood
within this comprehensive framework. It is important to
note that LID is not absent in c-Si, but involves dissociation
processes (e.g. of B-H complexes) that can be initiated by
lower energy photons.
3.3 Light Induced Degradation
Light induced degradation (LID) involves time dependent
reduction in output current of a solar cell, under solar illumination. Since the early definitive studies of LID in 1977s
by Staebler and Wronski[15], this notorious reliability issue
has been studied in depth; and several features are known
thereof; (i) the degradation follows a power law of the form
with ~1/3 as the power exponent [16]
∆
(see Fig. 6a), (ii) there is evidence of some recovery once
light is removed, (iii) the degradation increases with light
intensity, (iv) many studies suggest dissociation of Si-H
complexes during light soaking, (v) degradation is reduced
if H is replaced by isotope Deuterium, and finally, (vi) and
most intriguingly, thin-films are susceptible to it, while
the effect, although present, is considerably suppressed in
crystalline silicon solar cells[17].
The last three observations appear to be contradictory; as
the backplane of most c-Si solar cells involve large Si-SiO2
interface passivated by SiH bonds, otherwise minority carand decrease
and
rier recombination will increase
hence the efficiency. If light induced dissociation of SiH
bonds creates defects in thin -Si:H solar cells, why isn’t
LID degradation a similar concern for c-Si solar cells? After all, in c-Si cells, light bounces many times between the
electrodes for full absorption[3].
To resolve this puzzle, recall that absorption length of high
energy photons are relatively small (~10s of nm) and it is
low-energy, near gap, radiation that reaches the bottom
IRPS10-315
3E.2.4
Fig. 5: Surface plot showing absorbance of crystalline silicon vs. thickness ( ) and photon energy (eV).
There have been several models proposed to explain the
time dependent kinetics of LID issue. Some involve time
dependent recombination of electron-hole pairs breaking
weak Si-Si bonds[16], while others have explicitly attributed the defect generation to light induced dissociation of
SiH bonds[18, 19]. LID continues to be an active topic of
research, therefore more work is needed before a robust
theory is developed. If the time-exponent of LID is robust,
as several group have reported, a reaction-diffusion based
model might be appropriate. Below we offer a simple derivation and numerical solution of the concept. Detailed prediction of the model needs to be explored by experiments.
Let us assume that the small wavelength photons of flux
have sufficient energy to dissociate the weak Si-H
) at the rate of
. The
bonds (initial concentration
remain fixed in space to act as
broken dangling bonds
recombination centers, while the freshly released atomic H
diffuse laterally through the film. These two populations of
mobile H and static dangling bonds interact throughout the
volume of the thin film, with opportunities for repassiva). In sum, therefore, we can write the
tion (rate constant
rate of defect creation as .
4
In addition to repassivation, H can react with each other
and be lost from the kinetics of the problem by forming
. Therefore the evolution of H is given by
molecular
.
5
The coupled equations can be solved numerically (Fig. 6),
although the following analytical solution provides additional insight. After an initial transient, the rate of change
of the system can be presumed to be slow, compared to the
fluxes sustaining them. Therefore, the forward dissociation
and reverse repassivation of the Si-H bonds are evenly balanced as
~
;
6
⁄ ~
and
together, we find
/
so that
3
/ ~
. Taken
.
7
Reassuringly, this theoretical result compares well with the
experimental results from literature, as shown in Fig. 6. The
scaling of degradation rate (observation 2 above) as a function of flux G (for concentrator PV applications, for
and has been noted by many groups. This should motivate a
NBTI like thorough study of solar cell degradation, with
the full understanding that in addition to SiH
bond-dissociation, other bond dissociation processes (e.g.
weak Si-Si bonds, B-O complexes) may also contribute to
LID.
4.
Conclusion
In this paper, we have discussed three intrinsic reliability
issues of thin-film -Si:H solar cells; space charge limited
shunt
conduction
through
localized
metal-semiconductor-metal structures; shadow degradation in
series connected cells in a module, and light induced degradation. Despite their distinct external manifestation,
these intrinsic reliability issues appear to share common
physical phenomena. For example, the light induced and
the shadow degradation may be related because they are
described by very similar time-exponents (see Fig. 4c and
6a). While the physics of G are different (e.g. photon induced dissociation for LID and (possibly) electron-hole
recombination induced dissociation for shadow degradation), it is likely that they both break SiH bonds and are
subsequently follow similar diffusive kinetics. Finally,
analogies to CMOS reliability; e.g., shunt conduction related to non uniform conduction through oxides, shadow
degradation to bulk defect generation and TDDB in gate
dielectric, and light induced degradation to NBTI in PMOS
transistors; may help illuminate many aspects of the degradation processes.
Acknowledgements
Fig. 5 is taken from unpublished work by Mohammad
Ryyan Khan, a graduate student in Prof. Alam’s group. We
gratefully acknowledge Applied Materials for samples, the
Birck Nanotechnology Center for characterization facilities,
and the Network of Computational Nanotechnology for
computational resources. The work is supported by grants
from Applied Materials and Columbia EFRC.
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3E.2.5
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