Le 3ème Séminaire International sur les Energies Nouvelles et
Renouvelables
The 3nd International Seminar on New and Renewable
Energies
Unité de Recherche Appliquée en Energies Renouvelables,
Ghardaïa – Algérie 13 et 14 Octobre 2014
Thickness and doping concentration optimization of
a-Si/c-Si layers by computer aided simulation for
development of performances solar cell
BOUZAKI Mohammed Moustafa*1 , BENYOUCEF Boumediene1, SOUFI Aicha1 , CHADEL Meriem1
1
URMER, Tlemcen University, 13000 Tlemcen Algeria
*Corresponding author. Tel: 00213771700915
*E-mail address: bouzaki_physique1@yahoo.fr
ABSTRACT-The doping concentration and the thickness of concentration, band gap, resistivity…) present in the
different layers in the Hetero-junction with Intrinsic Thin layers fabrication processing of HIT solar cells [7,8].
solar cells (HIT) strongly influence their performances. We In this paper, with the purpose to reduce the cost
simulated, using AFORS-HET simulation software, the following furthermore and promote the performance of hetero-junction
layers structure: ZnO/a-Si:H(n)/a-Si:H(i)/c-Si(p)/a-Si:H(p)/Ag. solar cells, we optimized the doping concentration and the
We optimized the thicknesses and doping concentration of the thickness of different layers of the solar cell.
emitter, buffer, absorber and the BSF layers.
II. SOLAR CELL STRUCTURE AND SIMULATION
Keywords- Simulation, AFORS-HET, performance, HIT silicon
DETAILS
solar cell. The simulated solar cell structure, as shown in Fig. 1, is
I. INTRODUCTION ZnO/a-Si:H(n)/a-Si:H(i)/c-Si(p)/a-Si:H(p)/Ag. The emitter,
Although the solar photovoltaic proportion in the global absorber, buffer and BSF layers are a-Si:H(n), c-Si(p), a-
energy market is currently insignificant, there are signs that Si:H(i) and a-Si:H(p), respectively. The ZnO layer is used as a
this is changing the demand is growing rapidly. This front contact and the Ag layer as a back contact.
technology is attracting a large academic and industrial
interest, and is considered by many to be the most promising
for energy generation. Among many available different
technologies for photovoltaic production, the HET silicon
solar cells has been developed by SANYO Ltd 1994 [1,2].
The hetero-junction a-Si:H/c-Si is formed by depositing
hydrogenated amorphous silicon on the crystalline silicon
substrate. The resulting cells can achieve high conversion
efficiencies while using thin film silicon processes to lower
the cost in comparison with c-Si solar cells [3]. SANYO
developed HIT with a very thin intrinsic hydrogenated
amorphous silicon (a-Si:H(i)) layer which is inserted between
two layers a-Si:H(p) and c-Si(n). However, most researchers
concentrate on the exploitation of HIT solar cells on c-Si(p)
substrates. This is motivated by the fact that the cost of c-Si(p) Figure I Schematic structure of the simulated solar cell
wafer is significantly lower than c-Si(n) on one hand and the
fact that the microelectronic industries widely use p-type Figure 2 shows the distributions of the gap state
wafer for device fabrication on the other hand [ 1,4,5]. densities of different layers in our solar cell. The defect
Numerical simulation is now almost indispensable for the density in crystalline silicon is chosen as single defect at
understanding and design of solar cells. AFORS-HET 0.56eV with a concentration of 1x1010 cm-3. For amorphous
(Automat FOR Simulation of HETero-structures) software has layers, the density of states has been assumed to be both
been developed by a group from the Hahn-Meitner Institute of acceptor like states (in the upper half of the gap) and donor
Berlin and is used for simulating hetero-junction in solar cells like states (in the lower half of the gap). Both of these
[6]. The software provides a convenient way to evaluate the acceptor and donor like states consist of exponential band tail
role of the various parameters (thickness, doping and Gaussian mid-gap states.
Le 3ème Séminaire International sur les Energies Nouvelles et
Renouvelables
The 3nd International Seminar on New and Renewable
Energies
Unité de Recherche Appliquée en Energies Renouvelables,
Ghardaïa – Algérie 13 et 14 Octobre 2014
The front and the back contacts were assumed to be radiation was adopted as the illuminating source with the
flat band in order to neglect the contact potential influence. power density of 100mW/cm2.
The surface recombination velocities of electrons and holes
were both set to 107 cm/s (see Table 1). The solar AM1.5
Figure II The gap state distribution of different types of a-Si:H layers and c-Si in the simulations
Many other standard parameters are taken into consideration in the present simulation. Their values are reported in table 1.
Tableau 1 Parameter values adopted for the bifacial HIT solar cell in the simulation
Parameter a-Si :H (n) a-Si :H (i) c-Si (p) a-Si :H (p)
Thickness (nm) 10 7 300.000 10
Dielectric constant 11,9 11,9 11,9 11,9
Electron affinity (eV) 3,9 3,9 4,05 3,9
Band gap (eV) 1,74 1,72 1,12 1,74
Effective conduction band density (cm-3) 1020 1020 2,8x1019 1020
Effective valence band density (cm-3) 1020 1020 1,04x1019 1020
Electron mobility (cm2V-1s-1) 20 20 1040 20
Hole mobility (cm2V-1s-1) 5 5 412 5
Doping concentration of acceptors (cm-3) 0 0 1 x1016 1x1020
Doping concentration of donators (cm-3) 1 x1020 0 0 0
Thermal velocity of electrons (cm s-1) 107 107 107 107
Thermal velocity of holes (cm s-1) 107 107 107 107
Layer density (g cm-3) 2,328 2,328 2,328 2,328
Auger recombination coefficient for electron (cm6 s-1) 0 0 2,2x10-31 0
Auger recombination coefficient for hole (cm6 s-1) 0 0 9,9x10-32 0
Direct band-to-band recombination coefficient (cm3 s-1) 0 0 1,1x10-14 0
Le 3ème Séminaire International sur les Energies Nouvelles et
Renouvelables
The 3nd International Seminar on New and Renewable
Energies
Unité de Recherche Appliquée en Energies Renouvelables,
Ghardaïa – Algérie 13 et 14 Octobre 2014
III. RESULTS AND DISCUSION
III.1 Optimization the doping concentration of different
layers
The effects of the c-Si (p) doping concentration on the
performance of the solar cell are shown in Fig.3. The results
indicate that the doping concentration mainly influences VOC,
the fill factor and the efficiency. Preferably NA is required to
be higher than 8x1016cm-3 to obtain a good performance.
Hence, it can be considered that NA of 8x1016cm-3 can be
Figure IV Effects of the emitter doping concentration on the performance of
representative of the acceptable high doping concentration. the solar cell
Controlling the back surface field (BSF) is an
effective way to enhance the performance of HIT solar cells.
As seen in Fig.5, when the doping concentration of BSF layer
increases, the performance of the solar cell also increases
(VOC: 26,32 ~ 75,70mV; JSC: 4,8 ~ 40,8mA; FF: 36,65 ~
84,83%; η: 0,4 ~ 26,46%). But when the doping concentration
is higher than 8x1019 cm-3, the performances parameters of
the solar cell remain constant.
Figure III Effects of the c-Si (p) doping concentration on the performance of
the solar cell
Figure 4 demonstrates the effect of the a-Si:H(n)
layer doping concentration on the performance of the solar
cell. The requirement for such a high doping concentration is
due to the small conduction band offset between a-Si:H and c-
Si, as well as the distribution of the gap states in a-Si:H and
the interface states of a-Si:H/c-Si. We can see that when Ne
increases, FF and the efficiency also increase. However,
above a concentration of 3x1019 cm-3, the fill factor and the Figure V Effects of the a-Si:H (p) layer doping concentration on the
solar cell efficiency saturate. Therefore, an optimal performance of the solar cell
concentration could just be 3x1019 cm-3. Moreover, a larger
III.2 Optimization of thickness of different layers
acceptor concentration is difficult to obtain in the laboratory.
The results of the variation of VOC, JSC, FF and η as a
function of the absorber layer thickness are shown in Fig.6. It
is clear that increasing the thickness of the absorber gives a
reduction in VOC (from 792mV to 758mV), but it also
produces an important increase in JSC (from 33mA to 40mA)
and for η (from 23% up to 26%). In contrast, FF is almost
constant (85%). the best performance is obtained with a
thickness of c-Si equalizes to 300 μm, but we still can find
good performances with a thickness inferior. for a thickness of
150, there is 772,6mV; 37,27mA; 85.46% and 24.61% for
VOC, JSC, FF and η, respectively.
3
Le 3ème Séminaire International sur les Energies Nouvelles et
Renouvelables
The 3nd International Seminar on New and Renewable
Energies
Unité de Recherche Appliquée en Energies Renouvelables,
Ghardaïa – Algérie 13 et 14 Octobre 2014
Figure VI Effects of the c-Si (p) thickness on the performance of the solar cell
Figure VIII Effects of the a-Si:H(p) thickness on the
With the purpose of lowering the cost of solar cells, performance of the solar cell
we have chosen an absorber layer thickness of 150μm. The
optimization of the thickness of the emitter layer a-Si:H(n) is The dependence of the parameters VOC, JSC, FF and η on the
performed by keeping the thicknesses of a-Si:H(i), c-Si(p) and thickness of the a-Si:H(i) layer is shown on Fig.9. it is seen
a-Si:H(p) constant at 7nm, 150 μm and 10nm, respectively. that VOC and FF show no dependency and remain constant.
JSC, on the other side, is found to drop from 42,40mA/cm2 to
41,29mA/cm2 when the thickness changes from 3nm to 10nm.
Similarly η decreases from 28,1% to 27,3%.
Figure VII Effects of the a-Si:H(n) thickness on the performance of the solar
cell
Figure IX Effects of the a-Si:H(i) thickness on the performance of the solar
cell
The dependence of the cell performance on the
thickness of the emitter layer is presented in Fig.7. The IV. CONCLUSION
reduction of the considered thickness results in an increase of The effects of the doping concentration and the thickness
the JSC and η of the solar cell, although the VOC and the FF of different layers on the performance of hetero-junction with
remain nearly constant. The simulation shows that a thickness intrinsic thin layer (HIT) solar cell have been studied using
of 1nm is optimal. This, however, is difficult to achieve in the AFORS-HET simulation software.
laboratory, and a value of 5nm is therefore adopted. It is shown that, after parameter optimization, a record
According to figure 8, the thickness of a-Si:H(p) is efficiency of 28,1% could be obtained. Relevant parameters
chosen as 5nm. values are VOC(774,22mV), JSC (42,40mA/cm2) and FF of
85,57%. these results are obtained from the thickness of 5nm,
3nm, 150μm and 5nm of -Si:H(n) (emitter layer), a-Si:H(i)
(buffer layer), c-Si(p) (absorber layer) and the a-Si:H(p) (BSF
layer) , respectively. Concerning the concentration of doping,
we have 3x1019cm-3, 8x1016cm-3 and 8x1019cm-3 at
emitter, absorber and BSF layers, respectively.
4
Le 3ème Séminaire International sur les Energies Nouvelles et
Renouvelables
The 3nd International Seminar on New and Renewable
Energies
Unité de Recherche Appliquée en Energies Renouvelables,
Ghardaïa – Algérie 13 et 14 Octobre 2014
[4] L. Zhao, C.L.Li, Designe optimization of bifacial HIT
V. ACKNOWLEDGMENTS solar cells on p-type silicon substrates by simulation, Solar
The authors wish to thank Helmoltz-Zentrum Berlin for Energy Materials & Solar Cells 92 (2008) 673-681.
providing the AFORS-HET simulation software. A free copy
can be downloaded from http://www.helmoltzberlin.de/fors [5] Dwivedi, Sunhil Kumar, Atul Bist, Kamlesh Patel, S.
chung/enma/sipv/projekte/asicsi/afors-het/index_en.html. Sudhakar, Simulation approach for optimization of device
structure and thickness of HIT solar cells to achieve 27%
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