P - and n - type microcrystalline silicon oxide (μc-SiOx: H) for applications in thin film silicon tandem solar cells

18 Pages·0.699 MB·English

by  Smirnov et al.

Page 1 of 18
d.
r
o
c
e
r
of P- and n- type microcrystalline silicon oxide (µc-SiOx:H) for applications in thin
n
o
si film silicon tandem solar cells
r
e
v
al
ci
fi V. Smirnov*, A. Lambertz, S. Tillmanns and F. Finger
f
o
al
4n
1fi
25/he IEK-5 Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
3/m t
0
n ro
of
y er * Corresponding author: Email: [email protected], Phone: [phone]
arff
brdi
Liy
s ma
nceIt
cieon.
y of Spositi Abstract
mm
eo
dc
Acage We report on the development and application of p- and n-type hydrogenated
a
an d p
ssian microcrystalline silicon oxide (µc-SiOx:H) alloys in tandem thin film silicon solar
Rung
by diti cells. Our results show that the optical, electrical and structural properties of
m e
oy
ess.co cop µc-SiOx:H can be conveniently tuned over a wide range to fulfil the requirements for
hpror t solar cell applications. We have shown that adding of PH gas during the deposition
arcpri 3
cresecript tends to increase crystallinity of µc-SiOx:H layers, while additional TMB tends to
nrus
w.n
a suppress crystalline growth. When applied in tandem solar cells, both p- and n-type
wm
wd
m e
frocept µc-SiOx:H lead to a remarkable increase in the top cell current. Taking advantage of
ded e ac low refractive index and high optical band gap of µc-SiO :H allows the achievement
ah x
Downloript is t of high efficiencies of 13.1% (initial) and 11.8% (stabilized).
c
s. us
yn
ha
Pm
J. N
n. -I
Caust
J
s
hi
T
y.
nl
o
e
s
u
al
n
o
rs
e
p
r
o
F
1
Page 2 of 18
d.
r
o
c 1. Introduction
e
r
f
o
n
o
si
r
e
al v As a wide optical gap material, microcrystalline silicon oxide (µc-SiOx:H) has
ci
fi
f been the subject of research as a material for photovoltaic applications [1-9].
o
al
4n
5/1e fi µc-SiOx:H, being a two phase material, is a mixture of microcrystalline silicon
2h
3/m t
n 0ro (µc-Si:H) and amorphous silicon oxide (a-SiOx:H) [5, 6]. The properties of µc-SiOx:H
of
y er
arff thin films are influenced by the doping, crystalline fraction and the oxygen content.
brdi
Liy
nces It ma Optical band gap (E04), refractive index (n), crystallinity (ICRS) and conductivity can
y of Scieposition. be modified over a wide range by varying gas flows during material growth.
mm Here we present the development and application of p- and n-type µc-SiO :H
x
eo
dc
Acage alloys, prepared over a wide range of compositions, for the applications in tandem
a
an d p
sin
sa junction thin film silicon solar cells. We show that p- type as well as n- type
Rung
m by editi µc-SiOx:H alloys with enhanced optical band gap energy, low refractive index and
oy
cp
ess.o co suitable electrical properties are beneficial for usage in tandem solar cells. In the case
hpror t
arcpri of tandem devices, the n/p contact between amorphous (a-Si:H) and microcrystalline
cresecript silicon (µc-Si:H) component cells can be substituted by µc-SiO :H doped layers [2, 5-
nrus x
w.n
a
wm 10] in order to enable reflection of short wavelength light back to the top (a-Si:H) cell
wd
m e
opt
frce due to a step in refractive index. This enhances the top cell current without an increase
ded e ac
Downloaript is th bine thape ptlhieicdk naess sa no f nt-hlea yae-rS io:Hf tahbes oµrcb-eSr i:lHay ebro. tAtodmd itcioelnla llleya dthineg n -ttoy pree dµucc-eSdi Opxa:Hra scitainc
c
s. us
yn
ha
J. PN m absorption. With the aid of µc-SiOx:H doped layers, high efficiencies of 13.1%
n. -I
Caust (initial) and 11.8% (stabilized) were achieved in the present work, for tandem solar
J
s
hi cells.
T
y.
nl
o
e
s
u
al
n
o
s
r
e
p
r
o
F
2
Page 3 of 18
d.
r
o
c
e
r
f
o
n 2. Experimental
o
si
r
e
al v
ci
fi
f p- and n-type µc-SiO :H layers were deposited by RF (13.56 MHz) plasma
o x
al
4n
5/1e fi enhanced chemical vapour deposition (PECVD) technique at 185°C substrate
2h
3/m t
n 0ro temperature and a power of 50 W, using a mixture of silane (SiH4), hydrogen (H2) and
of
y er
arff carbon dioxide (CO ) gases. Trimethylboron (TMB) phosphine (PH ) gases were used
brdi 2 3
Liy
nces It ma for p- and n-type doping, respectively. The layers were deposited at varied CO2 flow
y of Scieposition. ratio rCO2 defined as the ratio between the CO2 flow and the SiH4 flow. Additional
mm details on the deposition conditions can be found elsewhere [7-10]. Optical and
eo
dc
Acage electrical properties of these layers were investigated by Photothermal Deflection
a
an d p
sin
sa Spectroscopy (PDS) and conductivity measurements, respectively. Conductivity
Rung
m by editi measurements were performed on films equipped with coplanar electrodes 5 mm in
oy
cp
ess.o co length separated by a 0.5 mm gap. The structural properties of µc-SiOx:H layers were
hpror t
arcpri probed using Raman spectroscopy. The ratio of integrated intensities attributed to
cresecript crystalline and amorphous regions, I = I / (I + I ) was used as semi-quantitative
nrus CRS c c a
w.n
a
wm value of the crystalline volume fraction [10].
wd
m e
opt
ded fre acce µc-SiOx:H layers were subsequently used as an intermediate reflector and as an
Downloaript is th nd-elpaoyseirt eodf btyh eP EµCcV-SDi: Hte cbhonttioqmue cuesliln gin e ittahnedr eRmF s(opl-a, ri -c, enl-ls l. aySeilrisc oonf tthhein t ofpil mcesl l,w aenrde
c
s. us
yn
ha
Pm n-layer of the bottom cell) or VHF (94.7 MHz, p- and i- layers of the bottom cell)
J. N
n. -I
Caust excitation. Solar cells were deposited on SnO2:F coated glass substrates from the
J
s
hi Asahi Glass Company (type VU). Additional details on the preparation of the solar
T
y.
nl
o cells can be found in refs [9, 11]. Solar cells were characterised by current-voltage
e
s
u
al (J-V) measurements under AM 1.5 illumination using a double source (Class A) sun
n
o
s
r
e
p
r
o
F
3
Page 4 of 18
d.
r
o
c simulator, and by Quantum Efficiency (QE) measurements, additional details are
e
r
f
o
n given in Ref. [9]. The light soaking of the solar cells was performed at temperature of
o
si
r
e
al v 55°C in the open circuit condition, using metal halide lamps with an intensity of 1000
ci
fi
f W/m² in the sample plane and a class B spectrum [12].
o
al
4n
5/1e fi
2h
3/m t
n 0ro 3. Results
of
y er
arff
brdi
Liy
es ma 3.1 Material properties
ncIt
y of Scieposition.
mm Fig. 1 presents the crystallinity (I ) of µc-SiO :H films, as a function of r ,
CRS x CO2
eo
dc
Acage depending on the PH (Fig. 1a) and TMB (Fig. 1b) flows. It is seen that I decreases
an d pa 3 CRS
sin
sa with an increase in r for all series of dopant gas flow. In the case of n-type
Rung CO2
m by editi µc-SiOx:H films, an increase in PH3 flow results in an increase in (ICRS) at a given
oy
cp
ess.o co rCO2. For example, in the case of the layers prepared at high rCO2 of 1, (ICRS) increases
hpror t
arcpri from around 25% up to around 70% with increase in the PH3 flow from 0 up to
cresecript 0.04 sccm. In contrast, an increase in TMB flow (Fig. 1b) results in the reduction of
nrus
w.n
a
wm crystallinity at a given r ratio.
wd CO2
m e
opt
frce The corresponding dark conductivity values are presented in Fig. 2 versus the
ded e ac
Downloaript is th fooprt itchael bmaantde rgiaalps (fEo0r4 )t.h Se uacphp plirceasteionntast iionn tsaenrdveems asso ala fri gcuerlels .o fF more rbiot tfho rn t-h aen sdu ipta-b tiyliptye
c
s. us
yn
ha
J. PN m µc-SiOx:H films (Fig. 2a and 2b respectively) the general trend indicates a reduction
n. -I
Caust in dark conductivity with increasing optical band gap (E04): σdark reduces from around
J
s
hi 10 S/cm down to below 10-12 S/cm with increase in the E from 1.9 eV up to 2.9 eV
T 04
y.
nl
o (2.6 eV) in the case of n-type (p- type) µc-SiO :H films.
e x
s
u
al
n
o
s
r
e
p
r
o
F
4
Page 5 of 18
d.
r
o
c 3.2 Tandem a-Si:H / µc-Si:H solar cells
e
r
f
o
n
o
si
r
e
al v A schematic structure of the a-Si:H/µc-Si:H tandem solar cells with integrated p- and
ci
fi
f n-type doped µc-SiO :H material is shown in Fig. 3. Selected n- and p- type
o x
al
4n
5/1e fi µc-SiOx:H layers were subsequently used as an intermediate reflector between a-Si:H
2h
3/m t
n 0ro and µc-Si:H component cells and additionally as an n-type contact layer (n2) in
of
y er
arff µc-Si:H bottom cell. The properties of these layers are listed in Table 1.
brdi
Liy
es ma
ncIt
y of Scieposition. The resulting current densities calculated from the external quantum efficiencies QE
mm of the a-Si:H top cell (J ) and of the µc-Si:H bottom cell (J ) and also the
QE,Top QE,Bot
eo
dc
Acage sum of both cells (J ) are shown in Fig. 4 for various cell structures, where the
an d pa QE,Sum
sin
sa legend on the x-axis denotes the parts of the tandem solar cell where the µc-SiO :H
Rung x
m by editi was utilized.
oy
cp
ess.o co Fig. 4 shows in the case of the standard tandem cell, where no µc-SiOx:H
hpror t
arcpri layers is used (“no SiOx”, cell 1), the current densities of the top and bottom cells
cresecript (J and J ) are 13 mA/cm² and 14.2 mA/cm², respectively. For this cell the
nrus QE,Top QE,Bot
w.n
a
wm current matching requirements are not fulfilled since the J is 1.2 mA/cm² lower.
wd QE,Top
m e
opt
frce The next solar cell, which has “µc-SiOx:H as n1” (cell 2) with a refractive index of
ded e ac
Downloaript is th 2d.e3c reaas sethde t oi n1t2e.r1m medAia/ctem ²r.e flector, JQE,Top is increased to 14.2 mA/cm² and JQE,Bot
c
s. us
yn
ha
Pm In a tandem solar cell with a p-type intermediate reflector (marked as “p2” in
J. N
n. -I
Caust Fig. 4), the JQE,Top decreases to 13.7 mA/cm² and the JQE,Bot increases to 13 mA/cm²
J
s
hi with respect to the previously mentioned cell. In the next solar cell (cell 4), n-type and
T
y.
nl
o p-type µc-SiOx:H are combined together as an intermediate reflector in the n/p tunnel
e
s
u
al recombination junction (n1+p2) of the tandem cell. Cell 5 has an additional µc-
n
o
s
r
e
p
r
o
F
5
Page 6 of 18
d.
r
o
c SiO :H layer as the n-layer of the bottom cell (n2) demonstrating a significant
e x
r
f
o
n improvement in the J and J . The quantum efficiency curves of this solar
o QE,Bot QE,Sum
si
r
e
al v cell are compared with a reference tandem solar cell (without µc-SiOx:H) in Fig. 5. It
ci
fi
f can be seen that in the case of the solar cell with µc-SiO :H layers, QE of the top cell
o x
al
4n
5/1e fi is enhanced in the wavelength region between 500 nm and 750 nm and QE of the
2h
3/m t
n 0ro bottom cell is enhanced in the wavelength region above 900 nm.
of
y er
arff
brdi
Liy
es ma
ncIt
y of Scieposition. 4. Discussions
mm
eo
dc
Acage The link between dark conductivity (σ ), crystallinity (I ) and optical gap
an d pa dark CRS
sin
sa (E ) is evident from Figs 1 and 2. The results indicate that when r increases, I
Rung 04 CO2 CRS
m by editi decreases while E04 increases, leading to a reduction of σdark, in agreement with
oy
cp
ess.o co previous reports [7-9]. In the case of p-type µc-SiOx:H films, a strong reduction in
hpror t
arcpri ICRS with increasing TMB flow is observed (Fig. 1a), indicating that the
cresecript microcrystalline growth is suppressed by boron incorporation. Such behaviour is also
nrus
w.n
a
wm known for the p-type µc-Si:H [13]. In contrast, in the case of phosphorous doping, an
wd
m e
opt
ded fre acce improvement of crystallinity at a given rCO2 is observed. An increase of the µc-Si:H
Downloaript is th fµrca-cStiio:Hn . with increasing the phosphine flow can also be found in literature [14] for
c
s. us
yn
ha
J. PN m For the applications of doped µc-SiOx:H in thin film solar cells, a sufficient
Can. ust-I dark conductivity above 10-5 S/cm is required [9] as indicated by the dashed area in
J
s
hi Fig. 3. The trade-off between electrical and optical properties is also evident in Fig. 3.
T
y.
nl
o While the material with high E would be optically favourable to be used as doped
e 04
s
u
al layers in solar cells due to reduced parasitic absorption, the electrical properties of
n
o
s
r
e
p
r
o
F
6
Page 7 of 18
d.
r
o
c such µc-SiO :H layers are rather poor. That is why the layers with intermediate values
e x
r
f
o
n of E (see Table 1) were selected for incorporation in tandem solar cells.
o 04
si
r
e
al v The favourable effect of the optical properties of µc-SiOx:H is evident in Fig.
ci
fi
f 4. It is seen that both p- and n-type µc-SiO :H, when used as an intermediate reflector
o x
al
4n
5/1e fi between the sub-cells, result in an increase in the top cell current. This can be related
2h
3/m t
n 0ro to a low refractive index n of µc-SiOx:H (see Table 1), enhancing the reflection of
of
y er
arff short wavelength light into the top cell [2-5]. This is also accompanied by a reduction
brdi
Liy
nces It ma in JQE,Bot of the bottom cells, indicating a so-called “current transfer” from bottom to
y of Scieposition. top component cells (enhanced absorption of light in the top cell and corresponding
mm reduced absorption of light in the bottom cell, resulting in an increase in J and a
QE,Top
eo
dc
Acage corresponding reduction in J ). The lower J of cell 3 (where a p-type
an d pa QE,Bot QE,Top
sin
sa µc-SiO :H layer is used) with respect to a solar cell 2 (where an n-type µc-SiO :H
Rung x x
m by editi layer is employed) is most likely related to the slightly higher refractive index n of 2.5
oy
cp
ess.o co (see Table 1). In turn, an increase in JQE,Bot of cell 3 can be attributed to the reduced
hpror t
arcpri parasitic absorption, visible in the cell reflection curves as discussed in Ref. [9]. In the
cresecript case of cell 4, J is slightly increased relative to cell 3, while J of cell 3 and
nrus QE,Bot QE,Sum
w.n
a
wm cell 4 remains at a similar level due slightly reduced top cell current J of cell 4.
wd QE top
m e
opt
ded fre acce The JQE,Bot of the bottom cell can be improved when an n2 µc-SiOx:H layer is used
Downloaript is th inn2s tµeacd-S oiOf a: sHta nladyaerrds aa-rSei :Hus end-l,a ytheer . tIont athl ec ucrarseen ot fo tfh eth tea nsdoelmar cceellll 5i,s w ihmeprero nv1e,d p 2u pa ntod
c x
s. us
yn
Phma 27.8 mA/cm2, as evident in Fig. 5. It shows that implementation of n1 and p2
J. N
Can. ust-I µc-SiOx:H layers results in the increase in the top cell current (JQE,Top) by 0.8 mA/cm2,
J
s
hi while n2 µc-SiO :H layer is responsible for the improvement in (J ) at the
T x QE,Bot
y.
nl
o wavelength range above 820 nm, consistent with reduced cell reflection in this
e
s
u
al wavelength range. We note that in the case of cell 5, a slight increase in cell reflection
n
o
s
r
e
p
r
o
F
7
Page 8 of 18
d.
r
o
c R (see Fig. 5) is observed at the wavelengths between 450 to 620 nm and between 700
e
r
f
o
n to 750 nm. A similar effect on reflection has been reported in [10]. Fig. 5 indicates a
o
si
r
e
al v gain in the total current of the tandem cell (JQE,Sum) by 0.55 mA/cm2 relative to the
ci
fi
f reference solar cell without µc-SiO :H layers. A part of the reduction in (J ) could
o x QE,Bot
al
4n
5/1e fi be assigned to the increased cell reflection as noted above. This solar cell, prepared
2h
3/m t
n 0ro with n1, p2 and n2 µc-SiOx:H layers shows a high initial efficiency of 13.1%, and
of
y er
arff 11.8% after 1000 hours of light soaking. The J-V characteristics of this solar cell,
brdi
Liy
es ma measured with anti-reflection coating, in the initial and stabilized performance are
ncIt
y of Scieposition. summarized in Table 2.
mm
eo
dc
Acaage 5. Conclusions
an d p
sin
sa
Rung
m by editi Electrical, optical and structural properties of p- and n-type µc-SiOx:H were
oy
cp
ess.o co investigated over a wide range of properties with respect to photovoltaic applications.
hpror t
arcpri We have found that at a given rCO2, adding of PH3 gas tends to increase crystallinity
cresecript of the µc-SiO :H layers, while TMB tends to suppress crystalline growth. Our results
nrus x
w.n
a
wm demonstrate that the properties of doped µc-SiO :H can be conveniently adjusted to
wd x
m e
opt
frce fulfil various requirements for applications in tandem solar cells. A remarkable
ded e ac
Downloaript is th inn-ctyrepaes µe ci-nS tioOpx c:Hel ll acyuerrres ndte amnodn osvtreartael lt heeff sicuiietanbciyl idtyu ea ntod ihnicgohr ppoorteantitoianl ooff bµoct-hS piO- a:nHd
c x
s. us
yn
ha
Pm as a functional layer in tandem devices. High efficiencies of 13.1% (initial) and
J. N
n. -I
Caust 11.8% (stabilized) were achieved for tandem solar cells with doped µc-SiOx:H layers.
J
s
hi
T
y.
nl
o
e
s
u
al
n
o
s
r
e
p
r
o
F
8
Page 9 of 18
d.
r
o
c Acknowledgment
e
r
f
o
n
o
si
r
e
al v The authors thank A. Bauer, R. Carius, M. Hülsbeck, J. Klomfaß, G. Schöpeand C.
ci
fi
f Zahren for their contributions to this work. This work was partially supported by the
o
al
4n
5/1e fi German Federal Ministry for the Environment, Nature Conservation and Nuclear
2h
3/m t
n 0ro Safety (BMU) under contract number 0325442D and by the EC under grant
of
y er
arff agreement no 283501 (Fast Track).
brdi
Liy
es ma
ncIt
y of Scieposition. References:
mm
eo
dc
Acage [1] D. Das, M. Jana, A. K. Barua, Sol. Energy Mat. Sol. Cells 63, 285 (2000).
a
an d p
sin
sa [2] D. Domine, P. Buehlmann, J. Bailat, A. Billet, A. Feltrin, C. Ballif, Phys. Stat.
Rung
m by editi Sol. (RRL) 2, 4, 163 (2008).
oy
cp
ess.o co [3] V. Smirnov, W. Böttler, A. Lambertz, H. Wang, R. Carius, F. Finger, Phys. Status
hpror t
arcpri Solidi C7, 1053 (2010).
cresecript [4] P. Delli Veneri, L.V. Marcaldo, I. Usatii, Appl. Phys. Lett. 97 (2), 023512 (2010).
nrus
w.n
a
wm [5] A. Lambertz, T. Grundler, F. Finger, J. Appl. Phys. 109 (2011) 113109.
wd
m e
opt
frce [6] B.Yan, G. Yue, L. Sivec, J. Yang, S. Guha and C.-S. Jiang, Appl. Phys. Lett. 99,
ded e ac
Downloaript is th 1[71]3 V51. 2S m(2i0r1n1o)v., A. Lambertz, B. Grootoonk, R. Carius, and F. Finger, J. Non-Cryst.
c
s. us
yn
ha
Pm Solids 358, 1954 (2012).
J. N
n. -I
Caust [8] A. Lambertz, F. Finger, B. Holländer, J.K. Rath, R.E.I. Schropp, J. Non-Cryst.
J
s
hi Solids 358, 1962 (2012).
T
y.
nl
o [9] A. Lambertz, V. Smirnov, T. Merdzhanova, K. Ding, S. Haas, G. Jost, R.E.I.
e
s
u
al Schropp, F. Finger, and U. Rau, Sol. Energy Mat. Sol. Cells 119, 134 (2013).
n
o
s
r
e
p
r
o
F
9
Page 10 of 18
d.
r
o
c [10] S. Kirner, O. Gabriel, B. Stannowski, B. Rech, R. Schlatmann, Appl. Phys. Lett.
e
r
f
o
n 102, 051906 (2013).
o
si
r
e
al v [11] V. Smirnov, C. Das, T. Melle, A. Lambertz, M. Hülsbeck, R. Carius, F. Finger,
ci
fi
f Mater. Sci. Eng. B 159, 44 (2009).
o
al
4n
5/1e fi [12] IEC 60904-9 Photovolataic devices– Part 9: Solar simulator performance
2h
3/m t
n 0ro requirements, (2007).
of
y er
arff [13] A. Dasgupta, A. Lambertz, O. Vetterl, F. Finger, R. Carius, U. Zastrow, and H.
brdi
Liy
es ma Wagner, in Proceedings of the 16th European Photovoltaic Solar Energy Conference,
ncIt
y of Scieposition. Glasgow, UK, May 1 - 5, 2000 (2000), p. 557.
mm [14] A. Matsuda, S. Yamasaki, K. Nakagawa, H. Okushi, K. Tanaka, S. Iizoma, et al.,
eo
dc
Acage Jpn. J. Appl. Phys. 19 (1980) L305–L308.
a
an d p
sin
sa
Rung
m by editi
oy
cp
ess.o co
hpror t
arcpri
cresecript
nrus
w.n
a
wm
wd
m e
opt
frce
ded e ac
Downloaript is th
c
s. us
yn
ha
Pm
n. J. -IN
Caust
s J
hi
T
y.
nl
o
e
s
u
al
n
o
s
r
e
p
r
o
F
10