Journal of Non-Crystalline Solids 299–302 (2002) 663–667
www.elsevier.com/locate/jnoncrysol
Interface recombination in heterojunctions of amorphous
and crystalline silicon
A. Froitzheim *, K. Brendel, L. Elstner, W. Fuhs, K. Kliefoth, M. Schmidt
Hahn-Meitner-Institut Berlin, Abt. Silizium-Photovoltaik, Kekul
estr. 5, D-12489 Berlin, Germany
Abstract
Heterojunction solar cells consisting of an n-type a-Si:H(n) emitter and a p-type monocrystalline silicon wafer have
been studied with particular emphasis on the role of interface recombination. It is shown that the form of the I–V
characteristics and the effective interface recombination velocity depend on the treatment of the Si-wafer prior to the
deposition of the amorphous emitter. Numerical simulation suggests that the non-exponential (S-shape) dependence of
the I–V curves under illumination arises from a high density of interface states which results in enhanced recombination
via interface states. Ó 2002 Elsevier Science B.V. All rights reserved.
PACS: 85.30.De; 73.70.Lq
1. Introduction
Heterojunctions of amorphous and crystalline
silicon are promising candidates for highly efficient
solar cells processed at low temperatures. High
efficiencies have been obtained in the laboratory
and industrialization has already been announced
[1,2]. Inspite of this technological progress surprisingly little is known about the properties of the
heterojunction interface. Particularly interesting
problems are the passivation and role of interface
states. Optimization of the device performance
requires proper pretreatment procedures of the Siwafers and optimized deposition conditions of the
amorphous emitter. In a recent paper we reported
that the best result was obtained if the deposition
occurs on flattened and hydrogen-terminated surfaces [3]. Another important question concerns the
magnitude and influence of the bandoffsets DEC
and DEV in the conduction and valence band. The
values quoted in the literature differ appreciably
[3–5]. In this study we investigate the system aSi:H(n)/c-Si(p) consisting of an n-type amorphous
silicon emitter deposited by PECVD onto floatzone silicon wafers. We discuss the influence of the
wafer pretreatment and show by simulations of the
device that the interface recombination has a
pronounced influence on the I–V characteristics
once the density of interface states exceeds a value
of 1012 cm2 .
2. Experimental procedure
*
Corresponding author. Tel.: +49-30 67053 305; fax: +49-30
67053 333.
E-mail address: froitzheim@hmi.de (A. Froitzheim).
We prepared heterojunction solar cells by depositing n-type a-Si:H onto p-type c-Si substrates
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 0 2 9 - 8
664
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
(FZ, 0.5–2 X cm, (1 1 1), 330 lm) with back surface field. The nþ -a-Si:H(n) films were deposited
by plasma enhanced chemical vapour deposition
(PECVD) at a substrate temperature of 250 °C
using standard conditions for high quality films
(p ¼ 400 mTorr, 0.6% gas phase concentration
phosphine/silane, P ¼ 15 W). The thickness of the
nþ -a-Si:H(n) emitter was kept at 30–40 nm. Prior
to the deposition, the p-type c-Si wafers were
treated in three different ways: (a) HF-dip which
causes roughening, (b) wet chemical oxidation and
etching in buffered NH4 F which is considered to
smoothen the surface and to saturate dangling
bonds (H-termination), (c) growth of an ultrathin
oxide layer for passivation. Contact to the a-Si:H
emitter was made by a 80 nm thick ZnO(Al)
ðR ¼ 50 XÞ deposited by sputtering. Evaporated
aluminium was used for the grid and rear contacts.
I–V characteristics were measured at 300 K
with under AM1.5 conditions. The measurements
of the temperature dependence between 100 and
300 K were performed in a liquid nitrogen cryostat
using a tungsten halogen lamp for illumination.
Numerical simulations were performed which
consisted of the simultaneous solution of the
Poisson equation and the continuity equations for
electrons and holes with a quasi-continuum of gap
states in the amorphous layer and a single gap
state in the crystalline wafer.
3. Results
Table 1 summarizes previously published results of a measurement of the effective interface
recombination velocity S for the various wafer
pretreatment procedures (see [3] for details). We
conclude that the interface recombination is
strongly influenced by the density of interface
states Dit which is determined by the pretreatment
Table 1
Effective recombination velocity [3]
Wafer pretreatment
Seff (cm/s)
Tunnel-oxide
HF-Dip
H-termination
6000
1000
600
Fig. 1. I–V characteristics at 300 K under AM 1.5 – illumination.
procedure [6]. S is smallest for the sample where
the deposition of the amorphous emitter was performed after careful H-termination of the Siwafer. This result is consistent with our experience
that this treatment leads to highest cell efficiencies
of above 13%. The I–V characteristics, too, exhibit
a characteristic dependence on the wafer pretreatment (Fig. 1). Whereas the behaviour of the
H-terminated sample is close to normal, the other
pretreatments led to S-shaped I–V characteristics.
This effect is most pronounced in case where a thin
tunnel oxide has been formed which is known to
lead to high Dit . Such effects have been observed
only under illumination, there was no anomaly in
the dark characteristics. Similar S-shaped characteristics of a-Si:H(n)/c-Si(p) junctions have been
reported before by Unold et al. [4] who related this
effect to a large bandoffset in the conduction band
of DEC ¼ 0:3 eV. However, we found much smaller values of less than 0.1 eV [3]. In situ constantfinal-state photoemission led to DEC ¼ 0:16 eV
which might be considered to be the most reliable
value at present [5]. It is interesting to notice that
generally at low temperature the S-shape form of
the I–V characteristics becomes much more pronounced. Fig. 2 gives an example for the case of
the HF-dipped specimen.
The results presented so far show that the value
of S as well as the form of the I–V curves can
considerably be influenced by the pretreatment of
the surface of the Si-wafer. Since the pretreatment
procedure is known to vary the density of interface
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
Fig. 2. Temperature dependence of the I–V characteristics for
samples when illuminated (tungsten halogen lamp). The sample
that is shown has an HF-dip as wafer pretreatment.
states [6] we argue that interface recombination is
responsible for both features. To further investigate these effects we have performed numerical
simulations using a program that allows to model
the device with special regard to the interface. The
Poisson equation and the transport equations for
electrons and holes have been solved with the
following boundary conditions for the front (0)
and back side ðwÞ of the cell:
uð0Þ ¼ 0;
uðwÞ ¼ Vbi V ;
ð1Þ
jn ð0Þ ¼ Snf Dnð0Þ;
jn ðwÞ ¼ Snr DnðwÞ;
ð2Þ
jp ð0Þ ¼ Spf Dpð0Þ;
jp ðwÞ ¼ Spr DpðwÞ:
ð3Þ
665
In these expressions u denotes the electrostatic
potential, jn and jp are the electron and hole current densities, Dn and Dp are the excess conf=r
centrations under illumination, Sn=p are the
recombination velocities of the electrons (n) and
holes (p) at the front (f) and rear (r) contact. All
f=r
Sn=p were chosen to 107 cm=s. Vbi is the built in
voltage and V is the applied voltage. We assume
flat band conditions on both contacts. For the aSi:H(n) the position of the Fermi level at the surface was kept at EC EF ¼ 0:25 eV which is the
activation energy measured for thick a-Si:H(n)
samples. The interface is assumed to be abrupt
with bandoffsets DEC and DEV . The boundary
conditions for the potential at the interface are
uðx1 Þ uðx2 Þ ¼ 0;
ð4Þ
e0 ec-Si Eðx2 Þ e0 ea-Si Eðx1 Þ Qit ¼ 0:
ð5Þ
Physically this means that the potential is continuous across the interface, where x1 is at the interface on the amorphous side and x2 is at the
interface on the crystalline side, such that dipoles
are excluded (Eq. (4)). The Gauss equation for
the dielectric displacement (Eq. (5)) has to be fulfilled including Qit which is the charge localized at
the interface. e0 ; ea-Si and ec-Si are the dielectric
constants and E is the electric field. Transport
across the interface is simulated assuming thermionic emission of holes and electrons. The interface
states have been positioned at midgap of the c-Si.
Table 2
Parameters used for simulation (data for a-Si:H(n) similar to [8])
Material parameters
a-Si:H(n)/c-Si(p)
Thickness (cm)
EG (eV)
ln and lp ðcm2 =V sÞ
NC and NV ðcm3 Þ
Doping [P], [B] ðcm3 Þ
3 105 =3:3 102
1.7/1.12
5/1400 and 1/480
1 1020 =2:86 1019 and 1 1020 =3:19 1019
1 1019 =1 1016
Tail states parameters a-Si:H(n)
DOS at CB/VB edge ðcm3 =eVÞ
CB/VB Urbach energy (eV)
cn;p charged/neutral states ðcm3 =sÞ
1 1021 =1 1021
0.080/0.170
1 108 =1 1010
Dangling bond states in a-Si:H(n)
ND ðcm3 Þ
Eþ=0 =E0= above EV (eV)
Gaussian distribution
cn;p charged/neutral states ðcm3 =sÞ
1:5 1019
0.4/0.6
r ¼ 0:15
1 108 =1 1010
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A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
The defect structure of the a-Si:H(n) is modelled
similar to [8] by two Gaussian distributions of the
dangling bond states and exponential tail states at
both bandedges. For c-Si a single trap at midgap is
assumed. The most important parameters used are
listed in Table 2.
Here we present results for which the only
varied parameters were the density Dit of acceptorlike states, their electron and hole capture rate
coefficients cn=p and the temperature. Fig. 3 shows
the effect of increasing Dit . The cn;p were assumed
to be strongly asymmetric, cn ¼ 108 cm3 =s and
cp ¼ 1012 cm3 =s, in accordance with literature
data for the SiO2 =Si interface [7]. The bandoffset is
Fig. 3. Simulation of I–V characteristics at 300 K. Parameter is
the density of interface states Dit : (a) 0 1012 cm2 , (b) 1 1012 cm2 , (c) 3 1012 cm2 (d) 4 1012 cm2 , (e) 5 1012
cm2 , (f) 6 1012 cm2 .
Fig. 4. Simulation of the I–V characteristics at various temperatures ðDit ¼ 4 1012 cm2 Þ.
kept constant at DEC ¼ 150 meV. As a result, the
fill factor of the I–V curve is reduced and the Sshape evolves when the Dit exceeds a value of
about 1012 cm2 . Fig. 4 displays the simulation of
the temperature dependence using a value for Dit
of 4 1012 cm2 . While at 300 K the I–V curve is
almost normal, the S-shape develops rapidly with
decreasing temperature. These results of the numerical simulation thus show that the S-shape of
the I–V characteristics can arise from enhanced
interface recombination.
4. Discussion
For a qualitative understanding of the physical
reasons for the occurrence of an S-shaped I–V
curve (double exponential) we show in Fig. 5 the
band diagram under short circuit conditions with
the Dit as parameter. If Dit is smaller than
1012 cm2 the band bending is not affected by the
Dit because the Qit in these states is small compared
to the space charge QS . If Qit becomes comparable
with QS , the bands are rearranged to keep charge
balance. As a result, the band bending in the
crystalline layer is reduced, while the band bending
in the amorphous layer is enhanced, such that the
built-in voltage Vbi remains constant. The reduction of the band bending results in an enhancement of the concentration of holes at the interface
and a reduction of the electron concentration in
Fig. 5. Band bending diagrams of the heterojunction under illumination for short circuit conditions (V ¼ 0 V). Note that the
x-axis has logarithmical scaling. Details see text.
A. Froitzheim et al. / Journal of Non-Crystalline Solids 299–302 (2002) 663–667
the amorphous layer. This allows the interface
states to be more efficient for recombination of the
minorities which reduces the electron current in
the amorphous layer. With a high Dit it is possible
to reach an accumulation layer in the crystalline
wafer if a forward bias V is applied to the junction
for V close to VOC . This means that the recombination at the interface changes drastically and a
significant loss current flows across the interface
thus reducing the photocurrent. For V > VOC the
current flow occurs in the opposite direction. The
recombination current at the interface then does
not appear as a loss current such that an exponential increase of the I–V characteristic is obtained. It should be noted that such a behaviour is
not obtained for defect states of donor type, because occupied donor states are neutral and do not
result in a modified band bending.
In this simple picture the temperature dependence, too, can qualitatively be explained. As the
temperature is reduced Vbi increases. This gain in
Vbi drops off almost entirely in the crystalline wafer
due to the high density of states in the amorphous
silicon. This results in an enhanced electron concentration at the interface and, again, if forward
bias is applied, these electrons recombine with the
increasing number of holes at the interface, resulting in a loss of the photocurrent. Since more
electrons are available for recombination, the recombination current is enhanced and the S-shape
becomes more pronounced.
5. Conclusion
Modifications of the wafer surface which enhance interface recombination may have consid-
667
erable influence on the properties of a-Si/c-Si
heterojunctions. S-shaped I–V characteristics can
result if the charge in interface states becomes
comparable with the space charge in the Si-wafer.
In the present case this critical value of the density
of interface states is at about Dit ¼ 1012 cm2 .
Acknowledgements
We thank H. Angermann, G. Keiler, F. Fenske
for preparation and the Bundesministerium f€
ur
Bildung und Forschung (BMBF) for partial financial support (contract number 01SF0012).
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