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Comparison of theoretical and experimental results for band-gap-graded CZTSSe solar cell
Accepted Manuscript Comparison of theoretical and experimental results for band-gap-graded CZTSSe solar cell Mehran Minbashi, Mir Kazem Omrani, Nafiseh Memarian, Dae-Hwan Kim PII:
S1567-1739(17)30180-3
DOI:
10.1016/j.cap.2017.06.003
Reference:
CAP 4526
To appear in:
Current Applied Physics
Received Date: 9 February 2017 Revised Date:
27 May 2017
Accepted Date: 9 June 2017
Please cite this article as: M. Minbashi, M.K. Omrani, N. Memarian, D.-H. Kim, Comparison of theoretical and experimental results for band-gap-graded CZTSSe solar cell, Current Applied Physics (2017), doi: 10.1016/j.cap.2017.06.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparison of theoretical and experimental results for Band-Gap-Graded CZTSSe Solar Cell Mehran Minbashi1, Mir Kazem Omrani1, Nafiseh Memarian1*, Dae-Hwan Kim2
2
Faculty of Physics, Semnan University, Semnan, Iran, P.O. Box: 35131-19111
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1
Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
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Corresponding author email: [email protected]
Abstract:
The simulation of CZTSSe solar cells is presented in this paper. The simulation results are in reasonable agreement with the experimental data, indicating the reliability of simulation results. New structure is proposed to increase the functionality of the cell. Improved functional performances are achieved by inserting a P-Silicon (P-Si) layer as back surface
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field. Simulation results suggest that by inserting this P-Si layer, efficiency of the CZTSSe solar cell increases from 12.6 % to 16.59 %, which is a significant improvement. For the
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champion cell JSC=36.27 mA/cm2, VOC= 0.625 V and FF = 73.11 % has been achieved.
Keywords: CZTSSe solar cells; simulation; back surface field; P-type Silicon; S/(S+Se)
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weight ratio.
1. Introduction
Given the challenges related to climate and energy issues, there is a growing need for renewable energy sources [1]. Solar cells have the ability to convert large amounts of sunlight into electricity, and can be used in everyday life. High energy density with low cost is one of the advantages of solar cells. In this regard, various photovoltaic materials have been proposed and studied for use in PV devices [2, 3]. Thin-film solar cells have the potential for low-cost and large-scale photovoltaic applications. A number of semiconductor materials,
ACCEPTED MANUSCRIPT such as amorphous silicon (a-Si:H), polycrystalline cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and copper zinc tin sulfur selenide (CZTSSe) have been used for thin-film photovoltaic solar cells [4]. Thin-film solar cells based on CZTSSe (Cu2ZnSn(S,Se)4) absorber layer and related materials have attracted a lot of attention in the field of photovoltaic technology, since they have
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suitable and tunable band gap (1-1.5 eV) and absorption coefficient more than 104 cm-1 [5-7]. However, comparing with efficiency of CdTe and CIGS (21.5 % and 21.7 % respectively) CZTSSe efficiency is 12.6 % that should be further improved [8-10].
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To improve the efficiency of CZTSSe solar cells, it is important to minimize current and voltage losses. To do this, the growth mechanism of absorber layer must be improved [11, 12]. In addition the formation of the second phase should be eliminated [12-18]. Also, the
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presence of lattice defects must be eliminated [12, 17-21]. Furthermore, Sodium content [22, 23] and band gap of the absorber layer must be controlled [18, 24, 25]. If the band gap near the back contact rises, then more photo-generated carriers can transfer to the back contact which improves current density (JSC) by preventing recombination at back contact [18, 2628]. By these strategies despite the improvement of JSC, lower VOC is the key barrier to
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achieve higher PCE and CZTSSe solar cells [7, 29]. Band gap of CZTSSe compound is variable from 1eV for Cu2ZnSnSe4 to 1.5 eV for Cu2ZnSnS4 [24, 25, 30, 31]. Band gap tailoring can be achieved by adjusting the [S]/[S]+[Se] ratio in precursors or by two step annealing. However, few studies have been done and research in this area is ongoing.
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In this paper at first the experimental case [32] of CZTSSe solar cells with different SeS2/Se weight ratio (which is equivalent to [S]/([S]+[Se]) ratio) is studied and simulated by SCAPS -
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1D software. Comparison of simulation result with the experimental data validates the simulation method. After that a P-type Silicon BSF layer is proposed and the effect of addition of this layer on cell performance is studied.
2. Modeling
2.1 Cell structure The cell structure, which is illustrated in figure 1, is same as experimental work on CZTSSe solar cell with high efficiency [32]. It is Al/ ZnO:Al/ ZnO-i/ CdS/ CZTSSe /Mo,MoSe2/ Soda
ACCEPTED MANUSCRIPT Lime Glass. Zinc oxide (ZnO) thin films have emerged as an attractive option in the design of transparent electrodes in thin films solar cells due to the simultaneous occurrence of high transmittance in the visible region and a low resistivity [33]. Here, in our case, Aluminum doped zinc oxide (AZO) and intrinsic zinc oxide (ZnO-i) layers are used as transparent electrode and buffer layer, respectively. CZTSSe layer and Molybdenum are used as absorber
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layer and back contact, respectively. Since in experiment the formation of interfacial MoSe2 layer is inevitable [34], this interfacial layer is considered between CZTSSe and molybdenum back contact to have a more realistic simulation. Different cell codes denotes for different
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[S]/([S]+[Se]), same as experimental work [32].
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Figure 1: Schematic of solar cell structure with CZTSSe absorber.
The main parameters of each layer used in the simulation are listed in Table 1. Different band
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gap values for CZTSSe absorber layer are given from experimental work. The effect of shunt resistance is neglected. In addition effect of dangling bands on the interface of materials ignored. The effect of light filtration by soda lime glass (SLG) is considered to have more realistic simulation. Work function of front contact is taken 4.3 eV [35]. For back contact flat band mode is used so the SCAPS calculates work function automatically to make an ohmic contact. Just for CZTSSe05 with [S]/([S]+[Se]) =0.6 work function of Mo (5 eV) is inserted manually [3, 36].
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Table 1: parameters set for the simulation of CZTSSe solar cell ZnO:Al
i-ZnO
CdS
CZTSSe
MoSe2
Thickness (nm)
300 [32]
50 [32]
50 [32]
1800 [32]
50 [32]
Electron affinity (eV)
4.600 [32]
4.600 [3]
4.450 [3]
Variable [37]
4.372 [38]
Band gap (eV)
3.400 [3, 39]
3.400 [3, 39]
2.400 [3, 39]
variable [32]
1.060 [40]
9.000 [3, 41]
9.000 [3, 36]
10.000 [36]
13.600 [42]
13.600 [38]
1×1018 [3]
1×1018 [3]
1×1018 [3]
2.2×1018 [42]
2.2×1018 [38]
1×1019 [3]
1×1019 [3]
1×1019 [3]
1.8×1019 [42]
1.8×1019 [38]
1×102 [3,
1×102 [3, 36] 1 2.500×10 [3, 36]
1×102 [42]
1×102 [38, 43]
2.500×10 [3, 41]
36] 1 2.500×10 [3, 36]
1×1018 [3]
1×1017 [3]
1×1018 [3]
0 [3]
0 [3]
0 [3]
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1
2.500×10 [42]
Data file
Data file
1
2.500×10 [38, 43] 0
15
[42]
1×1016 [43]
4
[32,
1×10
2×10
Data file
1
0 [3]
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1×102 [3, 41]
42]
Data file [44]
0 [42]
2.2 Numerical modeling
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dielectric permittivity (relative) CB effective density of states (cm-3) VB effective density of states (cm-3) electron mobility (cm²/Vs) hole mobility (cm²/Vs) shallow uniform donor density ND (cm-3) shallow uniform acceptor density NA (cm-3) absorption constant A (cm-1eV(½)) absorption constant B (eV(½)cm-1)
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Parameter and units
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SCAPS-1D (a Solar Cell Capacitance Simulator) is a one dimensional solar cell simulation program [45]. This package can be used for simulation of CIGS [3, 46, 47], CZTSSe [36, 37, 42] and other type of solar cells [48 – 50]. There is a good consistence between simulation
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results with SCAPS and experimental reported results, and so this package is superior to other solar cell simulation software [45]. The SCAPS program solves Poisson's equation coupled to electrons and holes continuity equations [51] at each position throughout the device by taking into account the boundary conditions. The program simulates device operation by taking into account the ShockleyRead-Hall (SRH) recombination statistics. In this work, an internal defect near the midgap states has been assumed for each layer. A 1.5 AM solar radiation with the power density of 100 mW/cm2 is used as the source of illumination and temperature is set as 300 K [52]. The absorption coefficient for the different layers, except CZTSSe layer, was already incorporated
ACCEPTED MANUSCRIPT in data files of SCAPS program. Since the absorption of CZTSSe layer is not constant and it is variable from one sample to another one, for absorption coefficient of that layer the following equation [45] is used:
B hυ − E g hυ
(1)
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α (λ ) = A +
Where A and B are constant and extracted from experimental data [32, 42].
In addition for electron affinity of absorber layer so called model-A has been used which shows the relation of electron affinity to [S] / ([S] + [Se]) ratio as the equation 2 [37].
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Model - A: χ CZTSSe = 4.505 - 0.35x
(2)
3. Results and Discussion
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Where χ denotes the electron affinity and x denotes the S/(S + Se) ratio.
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3.1 CZTSSe solar cell simulation and comparison with experimental cases Here the CZTSSe solar cell with defined structure is simulated. Figure 2 shows current voltage curves of the experimental and simulation results for 5 CZTSSe solar cells with different S/(S + Se) ratio. Furthermore, figure 3 presents the experimental data and simulation
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results of the quantum efficiency of five CZTSSe cells. The simulation results are in good agreement with experimental data.
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The best performance belongs to the CZTSSe02 solar cell. The main reason refers to good solfo-selenization process causing small band gap (1.097 eV) and low series resistance (0.96 Ω cm2). Moreover, the presence of interfacial MoSe2 layer should be considered as another important point for the reasonable performance of this case. As can be seen in figure 4, a
uniform, continuous and thin interfacial MoSe2 layer formed at the interface between CZTSSe absorber layer and Mo back contact. In CZTSSe cells with high S/(S+Se) ratio and
high amount of sulfur in absorber layer a MoS2 interface layer is created instead of MoSe2 [34]. This layer prevents ohmic contact at the junction of absorber layer and back contact of cell.
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40
40
30 25 20 15
CZTSSe01 (exp) CZTSSe01 (simulation)
10 5 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
35 30 25 20 15 10 5 0 0.0
0.7
CZTSSe02 (exp) CZTSSe02 (simulation)
0.1
0.2
35
35
30 25 20 15
CZTSSe03 (exp) CZTSSe03 (simulation)
10 5
0.5
0.6
0.7
0.5
0.6
0.7
25 20 15
CZTSSe04 (exp) CZTSSe04 (simulation)
10 5
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
0.7
0 0.0
0.1
0.2
0.3 0.4 Voltage (V)
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0 0.0
30
0.3 0.4 Voltage (V)
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40
Current Density (mA/cm2)
40
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Current Density (mA/cm2)
Voltage (V)
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Current Density (mA/cm2)
Current Density (mA/cm2)
35
40
30 25
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Current Density (mA/cm2)
35
20 15
CZTSSe05 (exp) CZTSSe05 (simulation)
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10 5
0 0.0
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
0.7
Figure 2: Current density versus voltage curves for experimental and simulation results.
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100 80
60
EQE[%]
40
60 40
CZTSSe01(exp) CZTSSe01(simulation)
CZTSSe02(exp) CZTSSe02(simulation)
20
20
0 400
600
800 Wavelength [nm]
0
1000
400
600
100 100
EQE[%]
60 40
40 CZTSSe03(exp) CZTSSe03(simulation) 20
1000
CZTSSe04(exp) CZTSSe04(simulation)
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EQE[%]
60
800 Wavelength [nm]
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80
80
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EQE[%]
80
20
0
0 400
600
800 Wavelength [nm]
100
400
1000
600
800 Wavelength [nm]
1000
EQE[%]
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80 60 40
CZTSSe05(exp) CZTSSe05simulation)
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20
0
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400
600 Wavelength [nm]
800
Figure 3: Quantum efficiency of experimental and simulated CZTSSe solar cells.
ACCEPTED MANUSCRIPT MgF2 ZnO/AZO CdS
MoSe2 or Mo(Se,S)
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CZTSSe
2
Mo
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Figure 4: Cross-section SEM image of CZTSSe solar cell.
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3.2 Effect of P-Si BSF layer on CSTSSe solar cell performance In the following, CZTSSe02 which possess the highest efficiency among the set of cells has been selected for further study. A one micron-thick P type silicon (P-Si) layer is inserted at the absorber layer and back contact junction. This layer acts as a back surface field (BSF) layer. Recent theoretical research shows the beneficial effect of P type silicon layer on the
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performance of CIGS solar cells. The results suggest silicon layer works as an absorber layer so more photogenerated carriers improve the cell performance. In addition this layer can have good contact with molybdenum or molybdenum oxide [53]. So this could make back contact of solar cell more ohmic by decreasing the carrier recombination at back contact. Parameters
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of the P-Si layer are listed in table 2 [53].
Table2: parameter set for simulation of P-Si layer.
Parameter and units Thickness (nm) Electron affinity (eV) Band gap (eV) dielectric permittivity (relative) CB effective density of states (cm-3) VB effective density of states (cm-3) electron mobility (cm²/Vs) hole mobility (cm²/Vs) shallow uniform donor density ND (cm-3) shallow uniform acceptor density NA (cm-3) absorption coefficient
P-Si 1000nm 4.05 1.12 11.9 2.8×1019 2.65×1019 1450 500 0 1×1020 Data file
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A quantitative comparison of functional properties of experimental case, simulated cell and simulated cell with Silicon BSF layer is reported in table 3. Figure 5 shows current density versus voltage curves for three CZTSSe solar cells, (experimental case CZTSSe 02, simulation of CZTSSe 02 and simulation of CZTSSe 02 with BSF layer). From fig. 5 one can
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see the effect of adding P-Si layer to CZTSSe cell.
Table 3: functional parameters of experimental and simulated CZTSSe solar cell. J SC (mA/cm2)
VOC (v)
CZTSSe (experiment) CZTSSe (simulation) CZTSSe (simulation with p-Si layer)
34.98 34.73 36.27
0.521 0.514 0.625
67.2 69.53 73.11
η (%) 12.3 12.41 16.59
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30
20
CZTSSe02 (exp) CZTSSe02 (simulation) CZTSSe02 (simulation with p-si layer)
10
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Current Density (mA/cm2)
FF (%)
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Cell/ parameter
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0 0.0
0.1
0.2
0.3
0.4 0.5 0.6 Voltage (V)
0.7
0.8
0.9
Figure 5: Current density versus voltage curves for three CZTSSe solar cells, experimental, simulation and simulation with BSF layer.
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10
21
10
20
10
19
10
18
10
17
10
decreasing recombinition
16
10
15
10
0.0
0.5
1.0
1.5 2.0 x (µm)
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3
Total recombination (1/cm .s)
23
10
2.5
3.0
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Figure 6: Total recombination of photocarriers in the CZTSSe solar cell with and without BSF layer.
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For further study figure 6 shows total recombination of photocarriers in CZTSSe02 solar cell for two conditions, without and with silicon BSF layer. It should be noted that in figures 6 and 7, x axes is belong to the position in cell where 0 (zero) is the location of molybdenum back contact.
The band alignment is one of the most important parameters that influences the
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photogenerated carriers transport and the functionality of the solar cells. Figure 7 presents the band diagram of CZTSSe 02 solar cell under dark conditions, (a) without and (b) with psilicon BSF layer. It can be seen that with BSF layer the conduction and valance band has changed and this improves the carrier transfer from absorber layer to back contact and hence
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increase the VOC and cell efficiency.
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(a)
7
CdS ZnO(i)
CZTSSe
Mo MoSe2
Al
ZnO:Al
6 Vacuum Level
4 3
Conduction Band
2
EFn
1
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Energy (ev)
5
0 -1 -2
Valence Band
-3 -4 0.5
1.0
1.5
Position (µm)
7
p-Si
Mo
6
Energy (ev)
5 3 2
0
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-1
-4
Conduction Band
EFn
1
-3
Al
Vacuum Level
4
-2
ZnO(i) CdS ZnO:Al
CZTSSe
2.5
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(b)
2.0
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0.0
Valence Band
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
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Position (µm)
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Figure 7: Band diagram of CZTSSe 02 solar cell (a) simulation without and (b) with Silicon BSF layer.
Finally the simulation results showed by inserting a p-Si layer as a BSF, the functionality of CZTSSe solar cell can improve by diminishing the photocurrent recombination at back contact and increasing the carrier transport and carrier collection efficiency.
Conclusion This work describes comparison of experimental and theoretical simulation of CZTSSe solar cells. Simulation results are in good agreement with experimental results. The results indicate the fact that CZTSSe solar cell behavior can be studied through SCAPS -1D simulation
ACCEPTED MANUSCRIPT software. MoSe2 interface layer, which its formation is inevitable in experimental work, is considered in simulation. For the first time we have reported the effect of addition a P-Si layer on Kesterite base inorganic solar cells. Silicon BSF layer prevents photogenerated carrier recombination at back contact. Hence back contact became more ohmic and the current density and open
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circuit voltage increase. Finally, this article states that by addition a p-type silicon layer the performance of CZTSSe solar cell can be improved and a cell with 16.59 % efficiency,
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JSC=36.27 mA/cm2, VOC= 0.625 V and FF = 73.11 % can be achieved.
Acknowledgments
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The authors thank Dr. Marc Burgelman from University of Gent for providing SCAPS simulation software. This work is supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning, KOREA (2016M1A2A2936781).
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[28] R. Ugarte, R. Schrebler, R. Cordova, E. A. Dalchiele, H. Gomez. Electrodeposition of CuInSe 2 thin films in a glycine acid medium. Thin Solid Films 340 (1999) 117-124. [29] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, W. Wang, High efficiency Cu2ZnSn (S, Se)4 solar cells by applying a double In2S3/CdS emitter. Advanced Materials 26 (2014) 7427-7431.
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[30] S. Chen, A. Walsh, J. H. Yang, X. G. Gong, L. Sun, P. X. Yang, J. H. Chu, S. H. Wei. Compositional dependence of structural and electronic properties of Cu2 ZnSn (S, Se)4 alloys for thin film solar cells. Physical Review B 83 (2011) 125201.
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[31] A. Walsh, S. Chen, S. H. Wei. X. G. Gong, Kesterite Thin‐Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4. Advanced Energy Materials 2 (2012) 400-409. [32] K.J. Yang, D.H. Son, S.J. Sung, J.H. Sim, Y.I. Kim, S.N. Park, D.H. Jeon, J. Kim, D.K. Hwang, C.W. Jeon, D. Nam, A band-gap-graded CZTSSe solar cell with 12.3% efficiency. Journal of Materials Chemistry A 4 (2016) 10151-10158. [33] S.M. Rozati, F. Zarenejad, N. Memarian, Study on physical properties of indium-doped zinc oxide deposited by spray pyrolysis technique. Thin Solid Films 520 (2011) 1259–1262. [34] D. K. Hwang, B. S. Ko, D. H. Jeon, J. K. Kang, S. J. Sung, K. J. Yang, D. Nam, S. Cho, H. Cheong, D. H. Kim, Single-step sulfo-selenization method for achieving low open circuit voltage deficit with band gap front-graded Cu2 ZnSn (S, Se)4 thin films. Solar Energy Materials and Solar Cells 161 (2017) 162-169.
ACCEPTED MANUSCRIPT [35] L. Lu, T. Xu, W. Chen, E.S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency. Nature Photonics 8 (2014) 716-722. [36] O. K. Simya, A. Mahaboobbatcha, K. Balachander. Compositional grading of CZTSSe alloy using exponential and uniform grading laws in SCAPS-ID simulation. Superlattices and Microstructures 92 (2016) 285-293.
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[37] D. Hironiwa, M. Murata, N. Ashida, Z. Tang, T. Minemoto, Simulation of optimum band-gap grading profile of Cu2ZnSn (S, Se)4 solar cells with different optical and defect properties. Japanese Journal of Applied Physics 53 (2014) 071201. [38] D. Cozza, C. M. Ruiz, D. Duché, J. J. Simon, L. Escoubas, Modeling the Back Contact of Cu2 ZnSnSe4 Solar Cells. IEEE Journal of Photovoltaics 6 (2016) 1292-1297.
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[39] T. Jäger, Y.E. Romanyuk, B. Bissig, F. Pianezzi, S. Nishiwaki, P. Reinhard, J. Steinhauser, J. Schwenk, A.N. Tiwari, Improved open-circuit voltage in Cu (In, Ga) Se2 solar cells with high work function transparent electrodes. Journal of Applied Physics 117 (2015) 225303.
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[40] S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, J. Wu, (2012). Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2. Nano letters, 12 (2012) 5576-5580. [41] G. Altamura, Development of CZTSSe thin films based solar cells. Diss. Université JosephFourier-Grenoble I, 2014.
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[42] O. K. Simya, A. Mahaboobbatcha, K. Balachander. A comparative study on the performance of Kesterite based thin film solar cells using SCAPS simulation program. Superlattices and Microstructures 82 (2015) 248-261. [43] A. Jäger-Waldau, M. Ch. Lax-Steiner, R. Jäger-Waldau, E. Bucher. Optical and Electronic Properties of MoSez thin Films Prepared by Soft Selenization. Springer proceedings in physics 45 (1991) 397-402.
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[44] https://refractiveindex.info/
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[45] A. Niemegeers, S. Gillis, M. Burgelman. "A user program for realistic simulation of polycrystalline heterojunction solar cells: SCAPS-1D." Proceedings of the 2nd World Conference on Photovoltaic Energy Conversion, JRC, European Commission, July. 1998. [46] M. Burgelman, K. Decock, S. Khelifi, A. Abass, Advanced electrical simulation of thin film solar cells.Thin Solid Films 535 (2013) 296–301. [47] K. Decock, P. Zabierowski, M. Burgelman, Modeling metastabilities in chalcopyrite-based thin film solar cells. Journal of Applied Physics 111 (2012) 043703. [48] L. Boudaoud, S. Khelifi, M. Mostefaoui, A. K. Rouabhia, N. Sahouane, Numerical study of InGaN based photovoltaic by SCAPs simulation. Energy Procedia 74 (2015) 745 – 751. [49] M. Burgelman, P. Nollet, S. Degrave, Modelling polycrystalline semiconductor solar cells. Thin Solid Films 361-362 (2000) 527-532.
ACCEPTED MANUSCRIPT [50] K. Decock, S. Khelifi, M. Burgelman, Modelling multivalent defects in thin film solar cells. Thin Solid Films 519 (2011) 7481–7484. [51] S. M. Sze, K. Ng. Kwok Physics of semiconductor devices. John wiley & sons, 2006. [52] S. Fonash, Solar cell device physics. Elsevier, 2012.
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[53] H. Heriche, Z. Rouabah, N. Bouarissa, New ultra-thin CIGS structure solar cells using SCAPS simulation program, International Journal of Hydrogen Energy 42 (2017) 9524–9532.
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Highlights: 1- Simulation of the champion CZTSSe solar cell 2- Comparison and validation of simulation with the experimental case.
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4- Highest achieved efficiency for CZTSSe solar cell.
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3- Proposing a simple and structurally matched BSF to increase the efficiency.
S1567-1739(17)30180-3
DOI:
10.1016/j.cap.2017.06.003
Reference:
CAP 4526
To appear in:
Current Applied Physics
Received Date: 9 February 2017 Revised Date:
27 May 2017
Accepted Date: 9 June 2017
Please cite this article as: M. Minbashi, M.K. Omrani, N. Memarian, D.-H. Kim, Comparison of theoretical and experimental results for band-gap-graded CZTSSe solar cell, Current Applied Physics (2017), doi: 10.1016/j.cap.2017.06.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparison of theoretical and experimental results for Band-Gap-Graded CZTSSe Solar Cell Mehran Minbashi1, Mir Kazem Omrani1, Nafiseh Memarian1*, Dae-Hwan Kim2
2
Faculty of Physics, Semnan University, Semnan, Iran, P.O. Box: 35131-19111
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1
Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
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Corresponding author email: [email protected]
Abstract:
The simulation of CZTSSe solar cells is presented in this paper. The simulation results are in reasonable agreement with the experimental data, indicating the reliability of simulation results. New structure is proposed to increase the functionality of the cell. Improved functional performances are achieved by inserting a P-Silicon (P-Si) layer as back surface
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field. Simulation results suggest that by inserting this P-Si layer, efficiency of the CZTSSe solar cell increases from 12.6 % to 16.59 %, which is a significant improvement. For the
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champion cell JSC=36.27 mA/cm2, VOC= 0.625 V and FF = 73.11 % has been achieved.
Keywords: CZTSSe solar cells; simulation; back surface field; P-type Silicon; S/(S+Se)
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weight ratio.
1. Introduction
Given the challenges related to climate and energy issues, there is a growing need for renewable energy sources [1]. Solar cells have the ability to convert large amounts of sunlight into electricity, and can be used in everyday life. High energy density with low cost is one of the advantages of solar cells. In this regard, various photovoltaic materials have been proposed and studied for use in PV devices [2, 3]. Thin-film solar cells have the potential for low-cost and large-scale photovoltaic applications. A number of semiconductor materials,
ACCEPTED MANUSCRIPT such as amorphous silicon (a-Si:H), polycrystalline cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and copper zinc tin sulfur selenide (CZTSSe) have been used for thin-film photovoltaic solar cells [4]. Thin-film solar cells based on CZTSSe (Cu2ZnSn(S,Se)4) absorber layer and related materials have attracted a lot of attention in the field of photovoltaic technology, since they have
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suitable and tunable band gap (1-1.5 eV) and absorption coefficient more than 104 cm-1 [5-7]. However, comparing with efficiency of CdTe and CIGS (21.5 % and 21.7 % respectively) CZTSSe efficiency is 12.6 % that should be further improved [8-10].
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To improve the efficiency of CZTSSe solar cells, it is important to minimize current and voltage losses. To do this, the growth mechanism of absorber layer must be improved [11, 12]. In addition the formation of the second phase should be eliminated [12-18]. Also, the
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presence of lattice defects must be eliminated [12, 17-21]. Furthermore, Sodium content [22, 23] and band gap of the absorber layer must be controlled [18, 24, 25]. If the band gap near the back contact rises, then more photo-generated carriers can transfer to the back contact which improves current density (JSC) by preventing recombination at back contact [18, 2628]. By these strategies despite the improvement of JSC, lower VOC is the key barrier to
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achieve higher PCE and CZTSSe solar cells [7, 29]. Band gap of CZTSSe compound is variable from 1eV for Cu2ZnSnSe4 to 1.5 eV for Cu2ZnSnS4 [24, 25, 30, 31]. Band gap tailoring can be achieved by adjusting the [S]/[S]+[Se] ratio in precursors or by two step annealing. However, few studies have been done and research in this area is ongoing.
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In this paper at first the experimental case [32] of CZTSSe solar cells with different SeS2/Se weight ratio (which is equivalent to [S]/([S]+[Se]) ratio) is studied and simulated by SCAPS -
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1D software. Comparison of simulation result with the experimental data validates the simulation method. After that a P-type Silicon BSF layer is proposed and the effect of addition of this layer on cell performance is studied.
2. Modeling
2.1 Cell structure The cell structure, which is illustrated in figure 1, is same as experimental work on CZTSSe solar cell with high efficiency [32]. It is Al/ ZnO:Al/ ZnO-i/ CdS/ CZTSSe /Mo,MoSe2/ Soda
ACCEPTED MANUSCRIPT Lime Glass. Zinc oxide (ZnO) thin films have emerged as an attractive option in the design of transparent electrodes in thin films solar cells due to the simultaneous occurrence of high transmittance in the visible region and a low resistivity [33]. Here, in our case, Aluminum doped zinc oxide (AZO) and intrinsic zinc oxide (ZnO-i) layers are used as transparent electrode and buffer layer, respectively. CZTSSe layer and Molybdenum are used as absorber
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layer and back contact, respectively. Since in experiment the formation of interfacial MoSe2 layer is inevitable [34], this interfacial layer is considered between CZTSSe and molybdenum back contact to have a more realistic simulation. Different cell codes denotes for different
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[S]/([S]+[Se]), same as experimental work [32].
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Figure 1: Schematic of solar cell structure with CZTSSe absorber.
The main parameters of each layer used in the simulation are listed in Table 1. Different band
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gap values for CZTSSe absorber layer are given from experimental work. The effect of shunt resistance is neglected. In addition effect of dangling bands on the interface of materials ignored. The effect of light filtration by soda lime glass (SLG) is considered to have more realistic simulation. Work function of front contact is taken 4.3 eV [35]. For back contact flat band mode is used so the SCAPS calculates work function automatically to make an ohmic contact. Just for CZTSSe05 with [S]/([S]+[Se]) =0.6 work function of Mo (5 eV) is inserted manually [3, 36].
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Table 1: parameters set for the simulation of CZTSSe solar cell ZnO:Al
i-ZnO
CdS
CZTSSe
MoSe2
Thickness (nm)
300 [32]
50 [32]
50 [32]
1800 [32]
50 [32]
Electron affinity (eV)
4.600 [32]
4.600 [3]
4.450 [3]
Variable [37]
4.372 [38]
Band gap (eV)
3.400 [3, 39]
3.400 [3, 39]
2.400 [3, 39]
variable [32]
1.060 [40]
9.000 [3, 41]
9.000 [3, 36]
10.000 [36]
13.600 [42]
13.600 [38]
1×1018 [3]
1×1018 [3]
1×1018 [3]
2.2×1018 [42]
2.2×1018 [38]
1×1019 [3]
1×1019 [3]
1×1019 [3]
1.8×1019 [42]
1.8×1019 [38]
1×102 [3,
1×102 [3, 36] 1 2.500×10 [3, 36]
1×102 [42]
1×102 [38, 43]
2.500×10 [3, 41]
36] 1 2.500×10 [3, 36]
1×1018 [3]
1×1017 [3]
1×1018 [3]
0 [3]
0 [3]
0 [3]
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1
2.500×10 [42]
Data file
Data file
1
2.500×10 [38, 43] 0
15
[42]
1×1016 [43]
4
[32,
1×10
2×10
Data file
1
0 [3]
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1×102 [3, 41]
42]
Data file [44]
0 [42]
2.2 Numerical modeling
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dielectric permittivity (relative) CB effective density of states (cm-3) VB effective density of states (cm-3) electron mobility (cm²/Vs) hole mobility (cm²/Vs) shallow uniform donor density ND (cm-3) shallow uniform acceptor density NA (cm-3) absorption constant A (cm-1eV(½)) absorption constant B (eV(½)cm-1)
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Parameter and units
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SCAPS-1D (a Solar Cell Capacitance Simulator) is a one dimensional solar cell simulation program [45]. This package can be used for simulation of CIGS [3, 46, 47], CZTSSe [36, 37, 42] and other type of solar cells [48 – 50]. There is a good consistence between simulation
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results with SCAPS and experimental reported results, and so this package is superior to other solar cell simulation software [45]. The SCAPS program solves Poisson's equation coupled to electrons and holes continuity equations [51] at each position throughout the device by taking into account the boundary conditions. The program simulates device operation by taking into account the ShockleyRead-Hall (SRH) recombination statistics. In this work, an internal defect near the midgap states has been assumed for each layer. A 1.5 AM solar radiation with the power density of 100 mW/cm2 is used as the source of illumination and temperature is set as 300 K [52]. The absorption coefficient for the different layers, except CZTSSe layer, was already incorporated
ACCEPTED MANUSCRIPT in data files of SCAPS program. Since the absorption of CZTSSe layer is not constant and it is variable from one sample to another one, for absorption coefficient of that layer the following equation [45] is used:
B hυ − E g hυ
(1)
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α (λ ) = A +
Where A and B are constant and extracted from experimental data [32, 42].
In addition for electron affinity of absorber layer so called model-A has been used which shows the relation of electron affinity to [S] / ([S] + [Se]) ratio as the equation 2 [37].
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Model - A: χ CZTSSe = 4.505 - 0.35x
(2)
3. Results and Discussion
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Where χ denotes the electron affinity and x denotes the S/(S + Se) ratio.
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3.1 CZTSSe solar cell simulation and comparison with experimental cases Here the CZTSSe solar cell with defined structure is simulated. Figure 2 shows current voltage curves of the experimental and simulation results for 5 CZTSSe solar cells with different S/(S + Se) ratio. Furthermore, figure 3 presents the experimental data and simulation
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results of the quantum efficiency of five CZTSSe cells. The simulation results are in good agreement with experimental data.
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The best performance belongs to the CZTSSe02 solar cell. The main reason refers to good solfo-selenization process causing small band gap (1.097 eV) and low series resistance (0.96 Ω cm2). Moreover, the presence of interfacial MoSe2 layer should be considered as another important point for the reasonable performance of this case. As can be seen in figure 4, a
uniform, continuous and thin interfacial MoSe2 layer formed at the interface between CZTSSe absorber layer and Mo back contact. In CZTSSe cells with high S/(S+Se) ratio and
high amount of sulfur in absorber layer a MoS2 interface layer is created instead of MoSe2 [34]. This layer prevents ohmic contact at the junction of absorber layer and back contact of cell.
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40
40
30 25 20 15
CZTSSe01 (exp) CZTSSe01 (simulation)
10 5 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
35 30 25 20 15 10 5 0 0.0
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CZTSSe02 (exp) CZTSSe02 (simulation)
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CZTSSe03 (exp) CZTSSe03 (simulation)
10 5
0.5
0.6
0.7
0.5
0.6
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25 20 15
CZTSSe04 (exp) CZTSSe04 (simulation)
10 5
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
0.7
0 0.0
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0.3 0.4 Voltage (V)
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0.3 0.4 Voltage (V)
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Current Density (mA/cm2)
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Current Density (mA/cm2)
Current Density (mA/cm2)
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CZTSSe05 (exp) CZTSSe05 (simulation)
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10 5
0 0.0
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
0.7
Figure 2: Current density versus voltage curves for experimental and simulation results.
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100 80
60
EQE[%]
40
60 40
CZTSSe01(exp) CZTSSe01(simulation)
CZTSSe02(exp) CZTSSe02(simulation)
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600
800 Wavelength [nm]
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1000
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EQE[%]
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CZTSSe05(exp) CZTSSe05simulation)
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600 Wavelength [nm]
800
Figure 3: Quantum efficiency of experimental and simulated CZTSSe solar cells.
ACCEPTED MANUSCRIPT MgF2 ZnO/AZO CdS
MoSe2 or Mo(Se,S)
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CZTSSe
2
Mo
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Figure 4: Cross-section SEM image of CZTSSe solar cell.
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3.2 Effect of P-Si BSF layer on CSTSSe solar cell performance In the following, CZTSSe02 which possess the highest efficiency among the set of cells has been selected for further study. A one micron-thick P type silicon (P-Si) layer is inserted at the absorber layer and back contact junction. This layer acts as a back surface field (BSF) layer. Recent theoretical research shows the beneficial effect of P type silicon layer on the
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performance of CIGS solar cells. The results suggest silicon layer works as an absorber layer so more photogenerated carriers improve the cell performance. In addition this layer can have good contact with molybdenum or molybdenum oxide [53]. So this could make back contact of solar cell more ohmic by decreasing the carrier recombination at back contact. Parameters
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of the P-Si layer are listed in table 2 [53].
Table2: parameter set for simulation of P-Si layer.
Parameter and units Thickness (nm) Electron affinity (eV) Band gap (eV) dielectric permittivity (relative) CB effective density of states (cm-3) VB effective density of states (cm-3) electron mobility (cm²/Vs) hole mobility (cm²/Vs) shallow uniform donor density ND (cm-3) shallow uniform acceptor density NA (cm-3) absorption coefficient
P-Si 1000nm 4.05 1.12 11.9 2.8×1019 2.65×1019 1450 500 0 1×1020 Data file
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A quantitative comparison of functional properties of experimental case, simulated cell and simulated cell with Silicon BSF layer is reported in table 3. Figure 5 shows current density versus voltage curves for three CZTSSe solar cells, (experimental case CZTSSe 02, simulation of CZTSSe 02 and simulation of CZTSSe 02 with BSF layer). From fig. 5 one can
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see the effect of adding P-Si layer to CZTSSe cell.
Table 3: functional parameters of experimental and simulated CZTSSe solar cell. J SC (mA/cm2)
VOC (v)
CZTSSe (experiment) CZTSSe (simulation) CZTSSe (simulation with p-Si layer)
34.98 34.73 36.27
0.521 0.514 0.625
67.2 69.53 73.11
η (%) 12.3 12.41 16.59
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CZTSSe02 (exp) CZTSSe02 (simulation) CZTSSe02 (simulation with p-si layer)
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Current Density (mA/cm2)
FF (%)
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Cell/ parameter
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0 0.0
0.1
0.2
0.3
0.4 0.5 0.6 Voltage (V)
0.7
0.8
0.9
Figure 5: Current density versus voltage curves for three CZTSSe solar cells, experimental, simulation and simulation with BSF layer.
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10
21
10
20
10
19
10
18
10
17
10
decreasing recombinition
16
10
15
10
0.0
0.5
1.0
1.5 2.0 x (µm)
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Total recombination (1/cm .s)
23
10
2.5
3.0
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Figure 6: Total recombination of photocarriers in the CZTSSe solar cell with and without BSF layer.
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For further study figure 6 shows total recombination of photocarriers in CZTSSe02 solar cell for two conditions, without and with silicon BSF layer. It should be noted that in figures 6 and 7, x axes is belong to the position in cell where 0 (zero) is the location of molybdenum back contact.
The band alignment is one of the most important parameters that influences the
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photogenerated carriers transport and the functionality of the solar cells. Figure 7 presents the band diagram of CZTSSe 02 solar cell under dark conditions, (a) without and (b) with psilicon BSF layer. It can be seen that with BSF layer the conduction and valance band has changed and this improves the carrier transfer from absorber layer to back contact and hence
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increase the VOC and cell efficiency.
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(a)
7
CdS ZnO(i)
CZTSSe
Mo MoSe2
Al
ZnO:Al
6 Vacuum Level
4 3
Conduction Band
2
EFn
1
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Energy (ev)
5
0 -1 -2
Valence Band
-3 -4 0.5
1.0
1.5
Position (µm)
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Energy (ev)
5 3 2
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EFn
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Al
Vacuum Level
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ZnO(i) CdS ZnO:Al
CZTSSe
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(b)
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0.0
Valence Band
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
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Position (µm)
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Figure 7: Band diagram of CZTSSe 02 solar cell (a) simulation without and (b) with Silicon BSF layer.
Finally the simulation results showed by inserting a p-Si layer as a BSF, the functionality of CZTSSe solar cell can improve by diminishing the photocurrent recombination at back contact and increasing the carrier transport and carrier collection efficiency.
Conclusion This work describes comparison of experimental and theoretical simulation of CZTSSe solar cells. Simulation results are in good agreement with experimental results. The results indicate the fact that CZTSSe solar cell behavior can be studied through SCAPS -1D simulation
ACCEPTED MANUSCRIPT software. MoSe2 interface layer, which its formation is inevitable in experimental work, is considered in simulation. For the first time we have reported the effect of addition a P-Si layer on Kesterite base inorganic solar cells. Silicon BSF layer prevents photogenerated carrier recombination at back contact. Hence back contact became more ohmic and the current density and open
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circuit voltage increase. Finally, this article states that by addition a p-type silicon layer the performance of CZTSSe solar cell can be improved and a cell with 16.59 % efficiency,
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JSC=36.27 mA/cm2, VOC= 0.625 V and FF = 73.11 % can be achieved.
Acknowledgments
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The authors thank Dr. Marc Burgelman from University of Gent for providing SCAPS simulation software. This work is supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning, KOREA (2016M1A2A2936781).
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References:
[1] S. Hallegatte, M. Fay, M. Bangalore, T. Kane, L. Bonzanigo, Shock waves: managing the impacts of climate change on poverty. World Bank Publications, 2015.
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[3] A. Chirilă, S. Buecheler, F. Pianezzi, P. Bloesch, C. Gretener, A.R. Uhl, C. Fella, L. Kranz, J. Perrenoud, S. Seyrling, R.Verma, Highly efficient Cu (In, Ga) Se2 solar cells grown on flexible polymer films. Nature materials 10 (2011) 857-861. [4] M. Elbar, S. Tobbeche, A. Merazg, Effect of top-cell CGS thickness on the performance of CGS/CIGS tandem solar cell, Solar Energy 122 (2015) 104–112. [5] K. Ramasamy, M. A. Malik, P. O'Brien. Routes to copper zinc tin sulfide Cu2 ZnSnS 4 a potential material for solar cells. Chemical Communications 48 (2012) 5703-5714. [6] M.T Winkler, W. Wang, O. Gunawan, H.J. Hovel, T.K. Todorov, D.B. Mitzi,Optical designs that improve the efficiency of Cu2 ZnSn (S, Se)4 solar cells. Energy & Environmental Science 7 (2014) 1029-1036. [7] J. Jean, P. R. Brown, R. L. Jaffe, T. Buonassisi, V. Bulović, Pathways for solar photovoltaics. Energy & Environmental Science 8 (2015) 1200-1219.
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[12] K.J. Yang, J.H. Sim, D.H. Son, D.H. Kim, G.Y. Kim, W. Jo, S. Song, J. Kim, D. Nam, H. Cheong, J.K. Kang, Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu2ZnSnS4 solar cells. Progress in Photovoltaics: Research and Applications 23 (2015) 1771-1784. [13] A. Fairbrother, X. Fontané, V. Izquierdo‐Roca, M. Placidi, D. Sylla, M. Espindola‐Rodriguez, S. López‐Mariño, F.A. Pulgarín, O. Vigil‐Galán, A. Pérez‐Rodríguez, E. Saucedo, Secondary phase formation in Zn‐rich Cu2ZnSnSe4‐based solar cells annealed in low pressure and temperature conditions. Progress in Photovoltaics: Research and Applications 22 (2014) 479-487.
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[53] H. Heriche, Z. Rouabah, N. Bouarissa, New ultra-thin CIGS structure solar cells using SCAPS simulation program, International Journal of Hydrogen Energy 42 (2017) 9524–9532.
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Highlights: 1- Simulation of the champion CZTSSe solar cell 2- Comparison and validation of simulation with the experimental case.
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4- Highest achieved efficiency for CZTSSe solar cell.
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3- Proposing a simple and structurally matched BSF to increase the efficiency.