Corresponding author : XU Ying E2mail : yingxu @red. semi . ac. cn
mc2Si :HPc2Si solar cell prepared by PECVD
XU Ying1 ,2) , LIAO Xianbo1) , DIAO Hongwei1) , Li Xudong2) , ZENG Xiangbo1) , LIU Xiaoping2) ,
WANG Minhua2) , and WANG Wenjing2)
1) State Laboratory for Surface Physics , Institute of Semiconductors and Center for Condensed Matter Physics , Chinese
Acad2bmy of Sciences , Beijing 100083 , China
2) Beijing Solar Energy Research Institute , Beijing 100083 , China
(Received 2006206228)
Abstract : Hetero2junction solar cells with an mc2Si :H window layer were achieved. The open voltage is in2
creased while short current is decreased with increasing the mc2Si :H layer′s thickness of emitter layer. The
highest of Voc of 597 mV has obtained. When fixed the thickness of 30 nm, changing the N type from amor2
phous silicon layer to micro2crystalline layer , the efficiency of the hetero2junction solar cells is increased. Al2
though the hydrogen etching before deposition enables the c2Si substrates to become rough by AFMimages , it
enhances the formation of epitaxial2like micro2crystalline silicon and better parameters of solar cell can be ob2
tained by implying this process. The best result of efficiency is 13. 86 % with the Voc of 549. 8 mV , J sc of
32. 19 mA·cm- 2 and the cell′s area of 1 cm2 .
Key words : solar cell ; hetero2junction ; amorphous silicon ; plasma enhanced CVD
[ This project was financially supported by the National Science Foundation of Beijing , China (No. 04D063) ]
1. Introduction
So2call hetero2junction silicon solar cells
has got more attentions by a lot of researchers in
recent years for its advantages such as higher
performance , lower fabrication temperature and
related lower quality of the substrates [ 124 ] .
The performance of the hetero2junction solar cell
is dramatically influenced by the thin2filmPab2
sorber interface [ 5 ] . Actually , an intermediate
buffer layer is always needed to passivate the
surface. Some group used intrinsic2amorphous 2
silicon thin film to be the buffer layer , others
prefer the hybrid layer[627 ] . Lots of researches
on different substrates etching or treatments
method , and variety instinct layer depth and in2
terface parameters have been studied[8 ] .
In this paper , we achieved an epitaxial2like
micro2silicon emitter or nano2crystalline window
layer to realize the hetero2junction solar cells ,
which means our hetero2junction solar cells with2
out any intrinsic amorphous buffer layer. The
hetero2junction solar cell was prepared by plasma
enhanced CVD on P type crystalline silicon sub2
strates without any texture. The structure was
AgPITOPn2mc2Si :HPp2c2SiPAl . ITO film is sput2
tered after the PECVD process , and the front
grid was formed by a mask2evaporation technolo2
gy. Rear contact was formed before PECVD pro2
cess by thermal evaporation and fired at 450 ℃
to form the ohm2contact .
2. Experimental
Phosphorous doped mc2Si :H materials was
made by PECVD in a three2chamber deposition
system , excited by RF frequency (13. 56 MHz)
with a power density of 1 W·cm- 2 . A mixture
of SiH4 , PH3 and H2 was used with a high
hydrogen dilution ratio of H2
PSiH4 = 100. For
optical and electrical characterization , films
were deposited on Coming 7059 glass substrates
with a thickness about 500 nm and examined by
RARE METALS
Vol . 25 , Spec. Issue , Oct 2006 , p. 176
Raman scattering to determine the grain size and
crystallinity. Raman spectra of the films are
taken in backscattering geometry with the 532
nm line of an Ar laser. The temperature depen2
dence of dark conductivity (σD ) was measured
using a pair of Al coplanar gap2cell electrodes.
The activation energy ( Ea ) of the electric con2
ductivity was deduced.
Commercial Czochralski c2Si (100) wafers
of 340μm thick and 1. 3Ω·cm were used. Sil2
icon wafers were cleaned by RCA process. Be2
fore deposition of the emitter , an Al film was
thermally evaporated on the rear side of the wa2
fers and followed by an annealing at 450 ℃to
form the ohm2contact .
The hetero2junctions were then fabricated
by depositing of nc2Si : H to mc2Si : H n2layer
emitter on the polished side of the silicon wa2
fer. Before deposition , a hydrogen plasma treat2
ment is applied to clean the c2Si surface in a
low gas pressure of 70 Pa , and a power density
of 0. 5 W·cm- 2 . Atom Force Microscopy
(AFM) and High2Resolution electron microsco2
py (HRTEM) were used to investigate the effect
of the hydrogen plasma treatment .
The nc2Si :H to mc2Si :H n2layer was depos2
ited on dilution ratio (H2
PSiH4 ) of 100 , a high
gas pressure of 650 Pa , under the condition of a
high power density of 1 W·cm- 2 and a substrate
temperature of 350 to 430 ℃. About 80 nm thick
ITO coating was formed at 200 ℃ by using an
RF2magnetron sputtering system from a sintered
ceramic target (3 inch in diameter) of In2O3 ∶
SnO2 (10 wt . %) after the PECVD process. The
Ag front grids were formed by a mask2evapora2
tion technique. Solar cells were made in config2
uration of Ag (200 nm)PITO (80 nm)Pn2mc2Si :
H (30 nm)Pp2c2Si (340μm)PAl (200 nm) .
Fig. 1. Structure of heterojunction solar cell.
Fig. 1 shows the structure of the solar cell . Illu2
minated I2V characteristics of solar cells were
measured under a sun simulator of AM1. 5 and
100 mW·cm- 2 power density at room tempera2
ture , using a 42probes system.
3. Results and discussion
Fig. 2 (a) gives the AFM mapping of the c2
Si substrates of the RCA cleaning. The average
roughness ( Ra ) of c2Si substrate is 0. 047392.
After a low gas pressure of 70 Pa , a power
density of 0. 5 W·cm- 2 hydrogen plasma treat2
ment , the c2Si surface was etched as shown in
Fig. 2 (b) . The Ra is increased to 0. 1581 , i .
e. , hydrogen plasma treatment of the c2Si sub2
strate can enhance the roughness of the c2Si
surface. The HRTEM mapping also shows the
same results mentioned later.
Fig. 2. AFM image of initial c2Si substrate ( a) ;
AFM image of c2Si substrate with hydrogen plasma
treatment ( b) .
Fig. 3 shows the Raman spectrumof the nc2
Si :H to mc2Si :H n2layer which are deposited on
Xu Y. et al . , mc2Si :HPc2Si solar cell prepared by PECVD 177
dilution ratio (H2
PSiH4 ) of 100 , under the con2
dition of a high gas pressure of 650 Pa , a highly
power density of 1 W·cm- 2 and substrate tem2
perature of 350 to 430 ℃. It can be seen that
the films become micro2amorphous silicon at the
temperature of 430 ℃. The mc2Si :H film has a
1. 78 eV band gap and Ea of 0. 2 eV.
Fig. 3. Raman spectrum of n2layer on different dep2
osition temperature.
The grain size of mc2Si : H layer can be
calculated as follows [ 9 ] : by analysis Raman
spectrum through Gaussian distribution fitting.
The grain size of nc2Si :H (350 ℃) is 7 nm ,
and mc2Si :H is 4. 8 nm.
Δω( D) = - A ( a
D
) r (1)
The cross section of the interface between
the c2Si substrates and the mc2Si :H film is in2
vestigated by HRTEM as shown in Fig. 4. The
mc2Si : H is like cone growth when applying
highly power density of hydrogen treatment as
shown in Fig. 4 (b) .
Hetero2junction solar cells with different
mc2Si : H thickness are summarized in Fig. 5.
The open voltage is increased while short current
is decreased with increasing the micro2crystalline
silicon layer′s thickness of emitter layer , when
the thickness increases to 170 nm , the highest
Voc of 597 mV is obtained with a related lower
J sc of 20. 676 mA·cm- 2 and high FF of 0. 79.
All the samples are obtained with hydrogen plas2
ma treatment , while others without hydrogen
plasma treatment has bad parameter. The best
result of efficiency is 1 3 . 8 6 %with the Voc is
Fig. 4. HRTEM image of mc2Si :Hand c2Si inter2
face with hydrogen plasma treatment of 0. 05
W·cm- 2 , 10 min ( a) ; HRTEM image of mc2Si :H
and c2Si interface with hydrogen plasma treatment of
015 W·cm- 2 , 10 min ( b) .
Fig. 5. Parameters solar cell vs mc2Si :Hthickness.
549. 8 mV , J sc of 32. 19 mA·cm- 2 , FF of
01783 and the cell′s area of 1 cm2 , and the
mc2Si :H thickness of 20 - 30 nm.
4. Conclusions
Phosphorous doped mc2Si :H n2layers were
realized and characterized for hetero2junction
solar cell application. The open voltage is in2
178 RARE METALS , Vol . 25 , Spec. Issue , Oct 2006
creased while short current is decreased with in2
creasing the micro2crystalline silicon layer′s
thickness of emitter layer , while decreasing the
fill factor. The best thickness may be 20 to 30
nm. Although H plasma treatment enhances the
roughness of the silicon wafer , the higher effi2
ciency can be obtained by applying this pro2
cess . The best result of efficiency is 13. 86 %
with the Voc of 549. 8 mV , J sc of 32. 19
mA·cm- 2 and the cell′s area of 1 cm2 , by us2
ing H treatment of 50 W, 10 min.
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Xu Y. et al . , mc2Si :HPc2Si solar cell prepared by PECVD 179