Contributed paper
OPTO-ELECTRONICS REVIEW 12(1), 45–48 (2004)
Effect of macroporous silicon layer on opto-electrical parameters
of multicrystalline silicon solar cell
P. PANEK*
Institute of Metallurgy and Materials Science, Polish Academy of Sciences
25 Reymonta Str., 30-059 Cracow, Poland
The feasibility of improvement in multicrystalline silicon (mc-Si) solar cells is considered by applying porous silicon (PSi)
layers obtained and modified with chemical double-step method. PSi layers have a different diameter of pores which determines electrical and optical characteristics of solar cells. Structural properties of these layers were investigated by means of
scanning electron microscopy. The reflectance, internal quantum efficiency, and current-voltage characteristics of the silicon
solar cells with porous layers are reported. The PC1D computer program was applied to calculate the correlation between
texture slope angle and electrical parameters of the solar cell. The diffusion results are also demonstrated using POCl3 as a
donors source in an emitter with a sheet resistance in the range 30–80 9/) which depends on a structure of the porous layer
prepared before a diffusion process. Some technological problems using PSi in a solar cell structure and correlation between
porous layer morphology and opto-electrical parameters are discussed. As a result, the mc-Si solar cell with 12.74% efficiency was obtained. We think that macroporous silicon formation process can be applied in industry technology that allows
for exceeding efficiency limit of the mc-Si solar cells with TiOx, at the level of 13% at present.
Keywords: silicon solar cell, porous silicon.
1. Introduction
Crystalline silicon will remain an important and dominant
material in photovoltaics (PV) technology over the next
10–30 years, owing to its well-recognised desirable properties and also its established infrastructure for photovoltaic
manufacturing. Research on PV progress includes development of new separation processes for removing impurities
from silicon feedstock, better understanding of defect and
impurity interaction in multicrystalline silicon (mc-Si), the
development of novel, orientation-independent processes
for light-trapping, surface passivation at contacts and other
interfaces in thin Si structures and better understanding of
the role of hydrogen in crystalline silicon [1]. Porous silicon (PSi) plays significant role in photovoltaics [2] and has
a potential to be important material used for purification of
metallurgical-grade (MG) Si [3] and for enhancement of
light absorption [4]. This last problem is especially important for PV. The effective reflectance from planar as-cut
mc-Si surface is at the level of 35% and the surface texture
is necessary. Most chemical techniques using sodium or
potassium hydrooxide for chemical texturisation of
monocrystalline Si are ineffective when applying to mc-Si
due to a random distribution of the grain orientations. The
method which can cope with this problem is porous silicon
formation. It simultaneously fulfils the industry require*e-mail:
pan-kozy@wp.pl
Opto-Electron. Rev., 12, no. 1, 2004
ments, i.e., high output with low cost. In 2002, a
multicrystalline Si had about 55% market share relative to
other PV technologies when worldwide cell and module
production was 562 MW [5].
2. Experiment and device structure
The silicon samples used in this study were BAYSIX silicon produced by Bayer for PV industry and characterised
by the following technical data: p-type, thickness ~330 µm,
resistivity 1 Wcm, minority carriers diffusion length
~120 µm. The samples have been cut to the size of 25 cm2.
The main steps of solar cells manufacturing process are
presented in Table 1. The structure of solar cells obtained
due to the procedure, described in Table 1, is shown in
Fig. 1.
Fig. 1. Schematic structure of the silicon solar cell with porous Si
layer.
P. Panek
45
Effect of macroporous silicon layer on opto-electrical parameters of multicrystalline silicon solar cell
Table 1. Solar cells manufacturing process and chosen parameters
describing technological aspects influencing opto-electronics parameters of the solar cells.
No. Technology step – technical description and parameters
11
Chemical etching in 30% KOH – defected layer
removing ~15 µm
12
Texturization in solution 40% KOH-IPA-H2ODI
13
Porous Si layer formation in solution
C1= HF-HNO3-H2O-H2O2-C2H5OH in volume ratio
[2:1:2:2:1]
14
Porous Si layer modification in solution C2 = HNO3-HF
in volume ratio [98:2] in time range from 0 s to 300 s
15
Diffusion from POCl3 source at temperature 880°C.
Pre-diffusion time: 7 min.
Re-diffusion time: 4 min. Diffusion results n+ – donor
doping layer depending on modification time in C2
solution (see Table 2)
16
Parasitic junction removing
17
TiOx deposition at 280°C by spray method using
(C2H5O)4Ti as precursor
18
Back contact screen printing – aluminium paste
19
Front contact screen printing – Du Ponte 4943 paste
10
Metallization in 200 cm long infrared belt furnace at
peak temperature 880°C and belt speed 160 cm/min.
3. Simulation by PC1D program
PC1D is a commercially attainable computer program from
the Sandia National Laboratory. PC1D solves the fully coupled nonlinear equations for the quasi-one-dimensional
transport of electrons and holes in crystalline semiconductor
devices, with emphasis on photovoltaic solar cells. This program includes arrangement for arbitrary doping profiles,
spatially varying material parameters, flexible boundary
conditions, and built-in models for calculating the
photogeneration profile resulting from a textured front surface [6]. The PC1D was applied to calculate the correlation
between texture slope angle and electrical parameters of the
solar cell taking, as a model, the surface texture for (100)
oriented Si wafer after texturisation in KOH or NaOH solution, when the texture slope angle 3 is constant and equal to
54.75 deg. The main parameters used for calculation of current-voltage characteristics were: mc-Si p-type with resistivity 1 W×cm, minority carriers diffusion length in the base
139 µm, cell surface 100 cm2, cell thickness 300 µm, donor
doping profile of erfc type with the sheet resistance
40.0 W/M and junction depth 0.30 µm, front surface recombination velocity for the electrons Sn and the holes Sp from
1.0 to 109 cm/s, back surface recombination velocity for
electrons and holes 105 cm/s, cell series resistance 0.015 W,
texture depth from 1 to 9 µm, photogeneration spectrum
AM1.5, and the texture slope angle 3 from 0.74 to
89.74 deg. In the case of PSi, 3 depends on pits shape. Figures 2 and 3 present the calculated results.
46
Fig. 2. Variation of the short circuit current Isc and the open circuit
voltage Voc as a function of the texture slope angle 3 for
Sn = Sp = 106 cm/s and for the texture depth 9 µm.
The variation of an open circuit voltage with a texture
slope angle for different sets of material parameters agrees
with those reported by the previous work [7]. Increase in
the surface area of the cell causes increase in the dark current density Jo by the factor (AT – A0)/A0 where AT, A0 are
the textured and planar total surface area, respectively.
Generally, this leads to decrease in the open-circuit voltage
since
,Voc =
k é (1 - RT ) (1 - R0 ) ù
ln
ú,
q êë
AT A 0
û
(1)
where RT, R0 is the reflection coefficient for the textured
and planar surface, respectively.
The area ratio AT/A0 is proportional to 1/cos3. On the
assumption for porous pit in Si that 3 is approximately going to 90 deg, the RT is going to 0. This combining with Eq.
(1) results at a room temperature in: ,Voc = –0.006 V for
3 = 60 deg and ,Voc = –0.106 V for 3 = 89 deg which confirms Voc decrease calculated by PC1D in Fig. 3.
For the texture slope angle 54.75 deg, the reflection coefficient is RT »0.1 but for 3 of above 80 deg the reflection
coefficient is reduced to zero which obviously causes the
Fig. 3. Calculated dependence of the texture slope angle on the
solar cell efficiency for solar cell with the surface texture depth of
9 µm.
Opto-Electron. Rev., 12, no. 1, 2004
© 2004 COSiW SEP, Warsaw
Contributed paper
Fig. 4. Surface SEM images of PSi layer prepared in C1 solution (a) and next modified in C2 solution in the time 300 s (b).
progress in Isc relation shown in Fig. 2 according to the role
played by the factor (1 – RT) in photocurrent generation in
the emitter, space charge region, and base. In order to
choose the most proper texture slope angle to achieve the
highest efficiency for the solar cell, the PC1D was used to
calculate the best achievable Eff versus 3.
According to the analysis, 3 has to be matched with the
surface recombination velocity which is closely related to
the material quality. If Si is of a poor quality (characterised
by Sn = 109 cm/s), then 3 should be around 64 deg but if Sn
reaches a typical value for the passivated surface
(Sn = 104 cm/s), the texture slope angle should be of
80.74 deg, which assures the best efficiency for the solar cell.
4. Structures of porous layers and their impact
on opto-electrical parameters
Fig. 5. Reflectance of mc-Si with phosphorous silica glass (PSG)
layer, mc-Si with PSi etched only in C1 solution and with PSG,
mc-Si with PSi obtained in C1 solution and subsequently modified
300 s in C2 solution, respectively.
Porous silicon layer formation mentioned in Table 1, in
steps No. 3 and 4, results in sponge-like structure. After
etching process in C1 solution, the pits have usually diameter about 100 nm [Fig. 4(a)] as observed by atomic force
microscopy and scanning electron microscope. The effective reflectance from such a prepared surface of mc-Si is
9.4%. Figure 4(b) shows the same surface after chemical
modification process in C2 solution for 300 s. The
sponge-like structure is observed and pores have their average diameters between 800 nm and 1000 nm what causes
increase in reflectance of above 20% in the 400–1100 nm
wavelength range.
The reflection measurements of the samples were carried out with a Perkin-Elmer Lambda-19 spectrophoto-
Table 2. The main electrical parameters I-V light characteristics under AM1.5 solar photon flux for solar cells with porous silicon layer.
The parameters for the solar cell without porous Si obtained with the same technology process are given for comparison.
Modification time in C2 solution
(s)
RH
(W/M)
Isc
(A)
Voc
(V)
Rsh
(kW)
Rs
(mW)
Pm
(W)
FF
Eff
(%)
Mc-Si without PS
36.9
0.663
0.572
0.0744
70.9
0.283
0.747
11.34
0
85.0
0.650
0.491
0.0018
153.5
0.139
0.438
5.59
40
40.0
0.735
0.582
0.0052
90.8
0.258
0.603
10.32
90
35.8
0.713
0.579
0.0191
68.6
0.302
0.732
12.10
300
32.1
0.722
0.582
0.0212
54.6
0.318
0.757
12.74
where: RH is the sheet resistance, Isc is the short circuit current, Voc is the open circuit voltage, Rsh is the shunt resistance, Rs is the series
resistance, Pm is the optimum output power, FF is the fill factor, Eff is the efficiency.
Opto-Electron. Rev., 12, no. 1, 2004
P. Panek
47
Effect of macroporous silicon layer on opto-electrical parameters of multicrystalline silicon solar cell
Fig. 6. Internal quantum efficiency for samples which reflectance is
plotted in Fig. 5.
meter with an integral sphere and the obtained characteristics are plotted in Fig. 5.
It can be seen from Fig. 5 that PSi layer without modification and with phosphorosilica glass (PSG) left after diffusion gives the excellent reflectivity below 5% between
400–1100 nm wavelength and can act as perfect antireflection coating, neglecting other effects.
The internal quantum efficiency (IQE) was calculated
after the spectral response measurements using a
Jobin-Yvon H20 monochromator with a 75 W tungsten-iodine lamp and calibrated Hamamatsu Si photodiode.
In the full wavelength range, the IQE of a cell with unmodified PSi layer is low because the sheet resistance of
85.0 W/M is too high for a screen-printing technology. It
gives shunt resistance at an unacceptable value of 1.8 W
(see Table 2). For a solar cell with the modified PSi structures, IQE increases but it is still lower than for mc-Si solar
cell in the short wavelength range. This is caused by the
surface recombination. The IQE gain for the wavelengths
over 600 nm is lost below 550 nm, which indicates that
PSG is not suitable for a passivation layer.
The Eff of the mc-Si solar cell with PSi modified for 300 s
is 12.74% what is better than the reported in the previous
work concerning similar PSi technology [8]. As it can be seen
from Table 2, the sheet resistance depends on porous layer
morphology. RH has the highest value for PSi layer after etching only in C1 solution, for which the size of pits is the smallest one. In this case, PSi acts as a diffusion barrier for phosphorous atoms implanted into a bulk region [9]. The Rs decreases with increasing modification time, exhibits dependence on a diameter of the porous pits and affects FF.
5. Conclusions
Comparison of the experimental results and those simulated by PC1D gives the following conclusions:
• efficiency of Si solar cell depends on the texture slope
angle 3 which should be matched with the surface recombination velocity. This can assure the best effi-
48
ciency if the 3 reaches 80 deg for Si characterised by a
properly passivated surface with a surface recombination velocity at the level of 104 cm/s.
• macroporous silicon layer, prepared before donor doping process, results in surface texturisation of mc-Si
with the larger 3 angle than that obtained using classical texturisation based on KOH. The experimentally
measured surface reflectance dropped below 10%.
The photovoltaic parameters of the solar cells with
macroporous silicon layers represent possibility to reach
conversion efficiency over the standard multicrystalline solar cells but further work should be performed and focused
on:
• passivation of macroporous Si layers to improve a spectral response, especially in a short wavelength range,
• diffusion process to obtain proper sheet resistance (below 40 W/M) on mc-Si with a porous layer prepared in
C1 solution and characterised by the lowest value of
reflectance.
A macroporous silicon formation process is alternative
for obtaining low reflectance multicrystalline Si surface
and can be used in the photovoltaic industry as a standard
procedure.
Acknowledgements
This work was supported by the Polish Ministry of Science
and Information Technology in the frame of the Project No.
4 T08A 04623.
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Opto-Electron. Rev., 12, no. 1, 2004
© 2004 COSiW SEP, Warsaw