Structural and Interfacial Properties of Large Area n-a-Si:H/i-a-Si:H/p-c-Si Heterojunction Solar Cells

Structural and Interfacial Properties of Large Area
n-a-Si:H/i-a-Si:H/p-c-Si Heterojunction Solar Cells
Özlem Pehlivana, Deneb Mendab, Okan Yılmaza, Alp Osman Kodolbaşa, Orhan Özdemirb*, Özgür
Duyguluc, Kubilay Kutlub and Mehmet Tomakd
a) TUBİTAK, National Metrology Institute (UME), TR-41470, Gebze, Kocaeli, Turkey
b) Yıldız Technical University, Department of Physics, Davutpasa Campus, TR-34210, Esenler,
Istanbul, Turkey
c) TUBITAK, Marmara Research Center (MAM), TR-41470, Gebze, Kocaeli, Turkey
d) Middle East Technical University, Department of Physics, TR-06800, Ankara
ABSTRACT
Large area (72 cm2) doping inversed HIT solar cells (n-a-Si:H/i-a-Si:H/p-c-Si) were investigated by High Resolution
Transmission Electron Microscopy (HR-TEM), Spectroscopic Ellipsometry (SE), Fourier Transform Infrared Attenuated
Total Reflection spectroscopy (FTIR-ATR) and current-voltage (I-V) measurement. Mixture of microcrystalline and
amorphous phase was identified via HR-TEM picture at the interface of i-a-Si:H/p-c-Si heterojunction. Using multilayer
and Effective Medium Approximation (EMA) to the SE data, excellent fit was obtained, describing the evolution of
microstructure of a-Si:H deposited at 225 °C on p-c-Si. Cody energy gap with combination of FTIR-ATR analyses were
consistent with HRTEM and SE results in terms of mixture of microcrystalline and amorphous phase. Presence of such
hetero-interface resulted poor open circuit voltage, Voc, of the fabricated solar cell devices, determined by I-V
measurement under 1 sun. Moreover, Voc was also estimated from dark I-V analysis, revealing consistent Voc values.
Efficiencies of fabricated cells over complete c-Si wafer (72 cm2) were calculated as 4.7 and 9.2 %. Improvement in
efficiency was interpreted due to the back surface cleaning and selecting aluminum/silver alloy as front contact.
Keywords: Doping inversed large area silicon heterojunction (SHJ), TEM, SE, epi-layer.
1. INTRODUCTION
The a-Si:H/c-Si interface is the key factor to obtain efficient heterojunction solar cell and the performance of the cell
depends on the recombination at the a-Si:H/c-Si interface (emitter-substrate) and substrate-rear contact. These losses can
be prevented by a strong band bending in the crystal wafer which leads to an inversion layer at the interface or by a low
density of interface states (Nss) at the junction. Not only Nss but also the optimum growth conditions of a-Si:H (optimum
emitter thickness, optimum doping) as well as band offsets in a-Si:H/c-Si junction impact the properties of
heterointerface, yielding better or worse cell properties. Nevertheless, the main challenge is on the suppression of
recombination of the a-Si:H/c-Si interface. To accomplish this goal, density of states (DOS) shall be modified by a pretreatment of the Si wafers that results in reducing Nss at the amorphous/crystalline interface or by depositing a thin
intrinsic a-Si:H buffer layer prior to the doped a-Si:H emitter layer, forming (p)a-Si:H/(i)a-si:H/(n)c-Si structure.
Efficiency of such kind of cells, known as Silicon Heterojunction (SHJ) or Heterojunction with intrinsic thin layer (HIT),
is reported over 23% by Sanyo group. Further improving of the efficiency of the solar cell structure is possible by
focusing on either preventing the optical losses that limit the short circuit current density, J sc, or the resistance losses
affecting the field factor, FF, or the recombination losses, impacting on the open-circuit voltage, Voc.
*ozdemir@yildiz.edu.tr; phone 0090 212 383-4279; fax 0090 212 383-3254
Providing efficient light trapping by texturizing the wafers reduces optical losses. By decreasing the series resistance of
the device with transparent conductive oxide (TCO) and good ohmic contacts leads to improve in the resistance losses.
Efficient cleaning of the wafer surface before the a-Si:H deposition decreases the number of recombination centers and
interface trap density, Nss. As Nss decreases from 1012 to 1010 cm-2eV-1, Voc, increases from 600 to 680 mV; J sc, changes
from 38 to 38.5 mA/cm2; efficiency increases from 18% to 22% according to the simulation study [1]. As shown,
interface traps have dramatic influence on heterojunction, particularly on V oc. Moreover, deteriorating of Voc might also
be caused owing to the epitaxial (epi) growth formation at the a-Si:H/c-Si heterojunction [2,3], leading highly defective
region at the a-Si:H/c-Si interface. Epitaxial growth is often observed on <100> oriented p-type substrates, and at high
temperatures on n-type substrates. The formation can be avoided by tuning proper deposition conditions [4].
Nevertheless, if exist, presence of such layer modifies the transport of carrier paths, affecting the solar cell performance
under working condition.
Although, large area silicon heterojunction (SHJ) is fabricated on n-type c-Si wafer, studies on large area over p-type cSi are rather limited [5]. In the present work, improvement of the efficiency of the solar cells built on 72 cm 2 over p-type
c-Si wafer is presented. According to the recent studies, given in table 1, this is the largest active area SHJ solar cell
study so far. Cell and interface properties are investigated in detail using HR-TEM, SE, FTIR, and IV measurement and
we report improvement of the c-Si wafer surface passivation and of the electrodes on the cell performance.
Table 1 Literature survey on p- type crystalline silicon wafer

Voc
Jsc
FF
A
(%)
(mV)
(mA.cm-2)
(%)
(cm2)
EPFL,Switzerland
19.7
717
37.9
72.7
4, FZ
2011
NREL, USA
19.3
678
36.2
78.6
0.9, FZ
2010
Titech, Japan
19.1
680
36.6
76.9
0.8, FZ
2011
HZB, Germany
18.5
633
36.8
79.1
1
2009
Univ. Stuttgart,
Germany
18.1
670
35.7
75.6
2
2010
LPICM, France
17
662
33.0
77.6
25, CZ
2009
ENEA, Italy
17
601
37.1
76.3
2.25
2004
Univ. Hagen,
Germany
16.6
655
31.0
81.6
FZ
2009
NCHU, Taiwan
16.4
645
34.8
73.0
1
2008
IMEC, Belgium
16.4
644
1, FZ
2005
Univ. of Valencia,
Spain
15.2
591
33.8
77.6
1, CZ
2010
CAS, China
15.1
585
34.6
74.7
<1, CZ
2009
Ultrecht Univ., the
Netherlands
14.9
571
33.3
78
1, FZ
2005
UPC, Spain
14.5
613
30.3
77.9
FZ
2008
SUNY, USA
10.6
550
30
64
0.03
1997
Affillation
Year
2. EXPERIMENTAL
<100> oriented boron doped p-type crystalline silicon wafers were used. Two sets of solar cells were fabricated and
called as HIT 44 and 51. For both sets, prior to film deposition, substrates were cleaned through standard RCA cleaning
procedure in wet processing bench in ISO 6 cleanroom environment. Afterwards, wafers were immersed into the
H2SO4:H2O2 (1:1) solution for 10 minutes for chemical oxide growing and then after rinsing in de-ionized water, wafers
were dipped into 1% HF 60 seconds for the etching of the native silicon oxide. In order to prevent the surface oxidation,
cleaned substrates were introduced immediately into the chamber of plasma enhanced chemical vapor deposition
(PECVD) system. Note that before deposition, the chamber was cleaned with a NF 3 gas. Doped and undoped a-Si:H
films were grown in capacitive type 13.56 MHz, multi-chamber UHV-PECVD system at TUBITAK UME. Growth
conditions were as follows: substrate temperature was held constant at 225°C, deposition pressure was adjusted to 0.6
Torr, RF power density was set to 12mW/cm2. For intrinsic a-Si:H, SiH4 flow rate was adjusted to 40 sccm. For n type
doping, 20sccm PH3 was added to the silane. Aluminium (Al) busbars and fingers were deposited on the top of the cell
using thermal evaporation. Before full back contact Al deposition, rear side of the wafer was flown over 2% HF solution
for 1 minute and subsequently rinsed with DI water. For the second set (HIT 51), prior to back contact formation, the
wafer was immersed into 2% HF solution till hydrophobic surface was achieved and thickness of front contacts has been
increased as well as Al/Ag(silver) alloy was used instead of aluminium. In both sets, resume growth conditions were
used.
HRTEM (High Resolution Transmission Electron Microscopy) techniques were applied to investigate the a-Si:H/c-Si
heterointerface formation on present sets with JEOL JEM 2100 at 200 kV. TEM samples were prepared with Gatan 691
Precision Ion Polishing System (PIPS). By Fourier Transform Infrared Attenuated Total Reflection spectroscopy (FTIRATR), the vibrational response of the Si-H bonds in the frequency range of 400-4000 cm-1 with a resolution of 4 cm-1
was studied.
Structural and interface properties were analyzed with a variable angle spectroscopic ellipsometer (SA Jobin YvonHoriba) of the rotating analyzer type. All measurements were performed in the wavelength range of 245 to 1200 nm.
Measurements were carried out at 70° angle of incidence. All data analyses were made using DeltaPsi II software. In the
analysis of SE data, multilayer model was used. The surface roughness layer was modeled as a mixture of the bulk
material and voids using the Bruggeman effective medium approximation.
For electrical analysis, the current voltage characteristics of solar cell devices were measured within dark and light
ambient by means of a computer controlled voltage-current Keithley 2600 source meter. Solar cell performance was
determined at Standart Test Conditions (STC) with the help of calibrated reference cell from Fraunhofer ISE.
3. RESULTS AND DISCUSSION
3.1 (i) a-Si:H/c-Si Heterointerface Properties by HR-TEM and SE Analysis
Figure 1-a depicted the cross-sectional TEM image of (i) a-Si:H film deposited onto c-Si substrate with the resumed
growth condition of that of the HIT solar cells (44 and 51). ZnO:Al was deposited on the top of a-Si:H film to improve
the quality of the TEM image. Obviously, co-existentence of epitaxial (epi) layer and the amorphous layer were shown
within the deposited film. Furthermore, a transition layer showed pyramidical epi growth followed by amorphous silicon
phase. Due to the changes in material properties like crystallinity and surface roughness, spectroscopic ellipsometry
analysis was used as one more confirmation. The dielectric function reflects the main absorption in the film due to band
to band transitions, while the evolution of the surface roughness yields information about nucleation process and surface
mechanisms [6]. For instance, as shown in Figure 1-b, two sharp peaks at 3.5 eV and 4.2 eV corresponded to the direct
optical transitions from the  (valance band) to  (conduction band) in the first Brillouin zone and transition in the
Brillouin zone along the X and R directions [7-8], respectively. A broad peak around 3.6 eV indicated amorphous
structure whilst narrower peak with a shoulder at smaller photon energies approved a mixture of crystallinity and
amorphous phase like nanocrystalline (nc) or microcrystalline structure [8, 9, 10]. Particularly, c phase has an
intermediate spectrum with a soft peak at 4.2 eV and a shoulder at low photon energies [10], whereas nc phase has a soft
peak at 3.6 eV and a shoulder at low photon energies.
(a)
(b)
Figure 1. (a) Cross-Sectional TEM image of (n)a-Si:H/(i)a-Si:H/)p)c-Si Heterostructure, (b) Photon Energy versus 2 graph for c-Si
and 60s deposited intrinsic a-Si:H film
For the surface roughness, top layer was defined as the composite structure including void and the bulk by using
Bruggeman Effective Medium Approximation (EMA) method in which each component material was identified by its
percentage of the overall volume of the relevant layer [8-10]. Effective medium approximation method based on the
addition of the polarization effects, and given as:
 eff   h
  h
  fi i
 eff  2 h
 i  2 h
i
(1)
Where
, , and fi were the effective medium dielectric function, the host dielectric function, and the dielectric
function and volume fraction of the ith component, respectively. In the Bruggeman approximation (EMA),
relation was hold and Eqn. 1 turned into [11-13]:
f
i
i
i  h
0
 i  2 h
(2)
Generally Tauc-Lorentz dispersion law was used to describe the a-Si:H thin film [14] and hence, imaginary part of the
dielectric function was defined as:
 AE o C(E  E g ) 2

ε 2 (E)   E (E 2  E o2 ) 2  C 2 E 2
0



if E  E g
(3)
if E  E g
where, Eg = optical band gap energy, Eo = energy of maximum absorption (peak transition energy), A = the amplitude
factor proportional to the density of the material, C = the broadening parameter that is inversely related with the short
range order of the material [15-18]. Keeping those in mind, we have successfully applied a multilayer model to fit the
measured SE data and depicted in inset of Figure 2-a. In this model, a crystalline silicon wafer was a substrate, followed
by two linearly graded breakdown layers (void on bottom and c-Si on top and a transition from crystal silicon at the
bottom and amorphous silicon at the top). Subsequently, an amorphous layer itself and a layer constituted by amorphous
silicon and void, which yielded a full history of the deposition, including the epitaxial layer thickness and the evolution
of the breakdown into a-Si:H. Low 2, 0.066, indicated a good matching of model with the experimental data as shown
by solid lines in Figure 3-a. Consequently, retrieved parameters as a result of fit were compared with that of the HRTEM
results (given in Table 2), presenting a very good agreement in both film thickness and proposed composition for a-Si:H
film. As partial conclusion, the model successfully described the evolution of microstructure of a-Si:H deposited at 225
°C on p-c-Si substrate. For the bandgap estimation, another clue for the existence of microcrystalline phase of present
thin films, we determined Cody gap from the energy position of
in the plot of
versus En and
obtained as 1.59 eV (see figure 2-b) [19].
Surface roughness analysis of the films, deposited at the same deposition conditions with different deposition times
consisted also a mixed a-Si:H and c-Si:H phases together within the. Surface roughness a-Si:H layer was calculated to
be 5.7 A.
(a)
(b)
Fgiıre 2. (a) Matching of SE measurements (symbols) with model fitting (solid lines), for 120 s deposited sample. The inset shows
multi layer model for 120 s deposited sample, representing the structure of the material as accurately as possible (b) Variation of
dielectric function versus photon energy. Note that energy gap of the film was obtained as 1.59 eV from Cody analysis.
3.2
FTIR-ATR of (n)a-Si:H/(i)a-Si:H(p)c-Si Heterostructure
Fourier Transform Infrared Attenuated Total Reflection spectroscopy (FTIR-ATR) technique was applied to measure the
vibrational response of the Si-H bonds in the frequency range of 400-4000 cm-1 with a resolution of 4 cm-1. Figure 3
showed a typical IR spectrum in the domains of absorption coefficient with a spectral range of 1800-2300 cm-1 (denoted
as symbol).
Figure 3. Raw (symbol) and deconvoluted (solid line) FTIR-ATR spectrum of (n)a-Si:H/(i)a-Si:H/(p)c-Si heterostructure
At priori, wavenumbers of Si-Hx stretching modes (SM) and their corresponding bond types on device grade a-Si:H films
without microcrystalline phase were given in Table 3 for comparison purpose.
Table 2, Silicon-hydrogen stretching modes for FTIR measurement,
Surface Hydrogen
Bulk Hydrogen
Peak position
(cm-1)
Hydride
Type
References
Peak position
(cm-1)
Hydride Type
References
2070-2080
SiH
[22, 23, 24]
1980-2010
(LSM)
SiH
vacancies
[25, 26, 27, 23]
2095-2105
SiH
[22, 23, 24]
2070-2100
(HSM)
SiH clustered
[27,28]
2110-2120
SiH2
[22, 23, 24]
2190-2210
SiHx(Oy)
[23]
2135-2145
SiH3
[22, 23, 24]
2240-2260
SiH(O3)
[23,26]
2255-2265
SiH(O2)
[22]
in
As clear from TEM and SE analyses, microcrystalline phase existed at the interface. Solid line in figure 4 designated
deconvolution process of the spectrum with Gaussian modes. The extreme low SM (ELSM: 1862, 1884, 1917, 1937,
1955 and 1964), the middle SM (MSM: 2024, 2041, 2055), the high SM (HSM: 2122, 2138, 2150, 2169) were attributed
to microcrystalline phase. ELSM reflected thin hydride dense a-Si:H tissue, ascribed either passivates the crystalline
grain boundaries or fills the small pores whereas MSM were associated with SiH x groups. HSM showed the contribution
of mono-, di-, and trihydrides on crystalline surfaces, assigned to crystalline grain boundaries in the bulk. The
wavenumber at 2000 cm−1 was attributed to the stretching mode (SM) of the Si-H bond whereas the wavenumber at 2090
cm-1 peak revealed the stretching mode of the Si-H2 bond, respectively. Presence of the Si-H2 stretching mode proved
increase of a dangling bond in the film due to the microcrystallization. The intensity of the Si-H stretching mode on the
other hand, indicated good quality a-Si:H thin films [20,21]. At last, wavenumber range of 2180-2250 is associated to
oxygen related bonds: SiH2(O2) at 2185-2210 and SiH(O3) at 2250 cm-1 [1].
3.3
Electrical Properties of of (n)a-Si:H/(i)a-Si:H(p)c-Si Heterostructure
(a)
(b)
Figure 4. (a) Isc-Voc variation of fabricated solar cells under 1 sun illumination. (b) Dark I-V characteristics for the present cells.
Figure 4 showed Isc-Voc curve under 1-sun illumination of solar cells (HIT 44 and 51), built over 72 cm2 on p-c-Si. The
only difference between HIT44 and 51 were the back surface cleaning of the c-Si wafer and usage of Al/Ag as top
electrode instead of sole Al. As a consequence of that V oc was changed from 410 mV to 430 mV while I sc was altered
from 0.9 to 2.6 Amperes. In light of structural and heterointerface analyses carried out via HR-TEM, SE and FTIR-ATR
measurements, such poor Voc was expected due to the microcrystalline phase formation at the interface of i-a-Si:H/p-c-Si
heterojunction. Since Voc was directly related with surface passivation of p-c-Si wafer.
On the other side, dark current-voltage curves were used to investigate the fundamental characteristics of solar cell
device, particularly to extract Voc without the need of a solar simulator. For the two-diode equivalent circuit models,
forward current would be [1]
(4)
In there,
was the photo-induced current, V was the applied bias voltage, I0,1(I0,2) represented the
saturation current, n1 was the diode ideality factor for the high-forward bias region, A was the temperature (in)dependent
factor for the low-forward bias region, Rs was the series resistance, Rp was the parallel or shunt resistance, k was
Boltzman constant and T was the temperature. Avoiding the series resistance region simplified the relation 4 into an
analytical expression as
(5)
where the first terms corresponded to high forward bias region and the second term denoted to low bias voltage region.
Under high forward bias within dark condition, setting I F=0 and solving the equation leaded an oppurtunity to estimate
the Voc from dark I-V characteristic as,
(6)
The estimated Voc values (538 mV For HIT 44 and 458 for HIT 51) were in agreement with the actual V oc (410 mV For
HIT 44 and 430 for HIT 51). On the other side, short circuit current increased due to improvement of the front contacts.
As a result of better back surface cleaning of the c-Si wafer and improvement of top electrodes, solar cell efficiency has
been increased from 4.7% to 9.2%.
4. CONCLUSION
Two sets of doping inversed HIT solar cells were fabricated over the largest area so far on p-c-Si wafers (72 cm2). Owing
to the resumed growth conditions, similar structural and heterointerface formation was observed for the two sets. Drastic
difference in between fabricated solar cells in terms of efficiency was find out the largest studied area (4.7% and 9.2%)
on p type c-Si wafer. The reasons behind were due to better back surface cleaning of the c-Si wafer and choosing alloys
as top electrodes rather than aluminium.
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