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Improve efficiency of perovskite solar cells by using Magnesium doped ZnO and TiO2 compact layers
Accepted Manuscript Improve efficiency of Perovskite Solar Cells by using Magnesium Doped ZnO and TiO2 Compact Layers Ardeshir Baktash, Omid Amiri, Alireza Sasani PII:
S0749-6036(16)30027-1
DOI:
10.1016/j.spmi.2016.01.026
Reference:
YSPMI 4161
To appear in:
Superlattices and Microstructures
Received Date: 27 October 2015 Revised Date:
18 January 2016
Accepted Date: 20 January 2016
Please cite this article as: A. Baktash, O. Amiri, A. Sasani, Improve efficiency of Perovskite Solar Cells by using Magnesium Doped ZnO and TiO2 Compact Layers, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.01.026. 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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Improve efficiency of Perovskite Solar Cells by using Magnesium Doped ZnO and TiO2 Compact Layers
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
c
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Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA Department of Science, Karaj Islamic Azad University, Karaj, Alborz, P.O. Box 31485-313, Iran.
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Email: [email protected] and [email protected] *Corresponding author, Tel: +17343895728.
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Abstract
Here the effect of Magnesium doped TiO2 and ZnO as hole blocking layers (HBLs) are investigated by using solar cell capacitance simulator (SCAPS). The Impact of Magnesium concentration into the TiO2 and ZnO and effect of operating temperature on the performance of
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b
the perovskite solar cell are investigated. Best cell performance for both TiO2 and ZnO HBLs ( with cell efficiencies of 19.86% and 19.57% respectively) are concluded for the doping level of
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a
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Ardeshir Baktasha, Omid Amiri a,b*, Alireza Sasani c
10% of Mg into the structure of HBLs. Increase in operating temperature from 300 K to 400 K are decreased the performance of the perovskite solar cell with both pure and Mg-doped HBLs. However, the cells with pure ZnO layer and with Zn0.9 Mg0.1O layer show the highest (with a decline of 8.88% in efficiency) and the lowest stability (with a decline of 50.49% in efficiency) at higher temperatures respectively. Moreover, the cell with Ti0.9 Mg0.1O2 layer shows better
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stability (with 21.85% reduction in efficiency) than the cell with pure TiO2 compact layer (with 23.28% reduction in efficiency) at higher operating temperatures.
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Introduction Since five years ago thin film solar cells based on halide perovskites as light absorber have attracted considerable attention due to their low cost, high absorption coefficient, and suitable 1-6
. The improvement in cells efficiency improved rapidly and recently power
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band gap
conversion efficiency of close to 20% has been reported 7, 8.
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In perovskite solar cell, electron-hole pairs are produced in absorber layer and electrons move toward the Conduction Band Minimum (CBM) of the Electron Transfer Material (ETM) layer and the holes move toward the Valance Band Minimum (VBM) of the Hole Transfer Material (HTM) layer 9. For the ETM layer as a very important component in construction of perovskite
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solar cells, wide band gap materials such as TiO2 and ZnO have attracted scientists for their characteristics. For example TiO2 have properties such as wide range of applications, controllable framework compositions and tunable pore sizes
10, 11
. ZnO, on the other hand, have
price
12- 14
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characteristics such as superior physical and chemical stability, nontoxicity and low production . Because the ETM layer prevents recombination of available electrons of Fluorine-
doped Tin Oxide (FTO) layer and produced electrons and holes in absorber layer, this layer is
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1.
known as Hole Blocking Layer (HBL)
15, 16
. Therefore, it is very important to choose a suitable
HBL layer for perovskite solar cell.
One way to improve the performance of HBL is to dope a suitable material to improve the properties of the HBL. For this purpose, TiO2 and ZnO have been doped with a metal to improve photo anode properties of Dye Synthesized Solar Cell (DSSC) 17-20. One of the elements that are 2
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improved the performance of DSSC’s is Magnesium (Mg)
20
. The band gap of semiconductors
like TiO2 and ZnO can be tuned by the use of the Magnesium as a dopant 20, 21.
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In this study, our aim is to investigate the effect of Mg-doped TiO2 and Mg-doped ZnO as potential HBLs for the application in perovskite solar cell. Recently, Mg- doped TiO2 as a compact layer is used to improve the conversion efficiency of perovskite solar cell 22. However,
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to the best of our knowledge, the effect of Mg-doped ZnO compact layer is not investigated yet. In this work, the effect of doping concentration and effect of temperature on the performance of
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perovskite solar cell with pure and Mg-doped compact layers (TiO2 and ZnO) are investigated. For this purpose, we considered doping levels of 0.05 and 0.1 for TiO2 layer and 0.02, 0.05, 0.1 and 0.20 for ZnO compact layer. We used experimental data for the band gap of doped ZnO and TiO2 21, 22.
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1. Computational Method
In this work, SCAPS version 3.2.01(a Solar Cell Capacitance Simulator) software which is a one
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dimensional solar cell simulation program is used. This software is developed at Department of Electronics and Information Systems (ELIS) of the University of Gent, Belgium
23
. The
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simulated perovskite solar cell has layer configuration with transparent conductive oxide (TCO)/ hole blocking layer (HBL)/ absorber/ and hole transport material (HTM) layers. The considered materials for the mentioned layers are fluorine doped SnO2 (SnO2:F), pure and doped TiO2 and ZnO, CH3 NH3 PbI3-XCl3 and spiro-OMeTAD, respectively. For HBL, the effect of the Magnesium dopant on CBM of TiO2 and ZnO is considered. For HBL with TiO2, the concentration of 0.0, 0.05 and 0.1 Magnesium into the TiO2 layer are considered [22] and for HBL with ZnO layer, the concentration of Mg 0.0, 0.02, 0.05, 0.1 and 0.2 are considered 21 and 3
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accordingly the electron affinity of the layers is varied too. The descriptions of base parameters are available in Table 1. Table 2 and Table 3 show the base parameter set for different layers of the simulation that are used in this study. The thicknesses of layers are chosen based on 24, 25
. To consider interface recombination, the
interface layers INT1 and INT2 were defined from reference
26
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experimental works on perovskite solar cell
. In this study, to obtain carrier
diffusion lengths (Ln and Lp) of 1 µm that is similar to that of for experimental work 27, the value
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of defect parameters for all layers are considered identical and defect density for absorber is
2. Results and Discussion 2.1.
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assumed equal to Nt= 2.5 × 1013 cm3 26.
Effect of doping concentration of Mg into TiO2 layer
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Magnesium as a dopant into TiO2 could widen the band gap by moving the CBM toward the higher energy levels. So, to simulate the effect of different doping concentration of Mg into the TiO2 compact layer, the data for band gap of the layer with different doping levels are chosen
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from experimental work 22. The information for Band gap changing are shown in Table 4. As it can be found from table 4, Mg as an additive into TiO2 structure is widening the bang gap. The
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reason for this band gap widening is the differences in band gap energy of TiO2 (~3.2 ev) 38 and MgO ( ~7.8 ev) 39.
The efficiencies of the cell with pure and Mg-doped TiO2 compact layers are shown in Figure 1. As seen from figure 1, use of Mg as an additive (up to 10%) into the HBL leads to higher efficiency in perovskite solar cell. Previously, Mg-doped TiO2 layer is used to improve the 4
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efficiency of perovskite solar cell
22
. For our simulation, the cell’s efficiency (η) of pure TiO2
layer, 0.05 and 0.10 Mg-doped TiO2 layer are 17.91%, 18.53% and 19.86% respectively. Mgdoped TiO2 acts as the HBL in the structure and reduces recombination rate of electrons and
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holes. As Figure 2 shows, by increasing the amount of Mg into the HBL, short circuit current density (Jsc) and open circuit voltage (Voc) of the cell increases and this means that the recombination rate is decreased and consequently more electrons are injected from the absorber
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layer into the HBL. Therefore, upward move of CBM toward the LUMO level of the absorber, facilitates the electron injection from the absorber into the compact layer and leads to better
2.2.
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performance of the cell.
Effect of doping concentration of Mg into ZnO layer
In this Section, the effect of Mg-doepd ZnO compact layer on the performance of perovskite
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solar cell is investigated. In our simulation, the used information for the band gap energies of compact layer with different dopant concentrations is gathered from experimental work
21
. The
band gap energies for ZnO layer with different doping levels of Mg are tabled in Table 5.
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The efficiency (η) and fill factor (FF) of the simulated cells are shown in Figure 3. As shown in figure 3, the η of the cell with lightly Mg doped (up to 10% Mg) is improved. However, by using
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Zn0.8Mg0.2O compact layer, the efficiency is reduced. Efficiency of the cell with pure ZnO compact layer is 16.78% and the efficiency for 2, 5, 10 and 20% Mg-doped ZnO are 17.19%, 18.21%, 19.57% and 18.17% respectively. The FF of the cell with pure and 0.2 Mg doped ZnO layer is increased from 68.24% to 84.16%. The FF of 69.93%, 74.11%, and 79.85% are calculated for the cells with 2, 5 and 10% Mg-doped ZnO compact layers. Introduction of Magnesium into the structure of ZnO, shifts the CBM toward the LUMO level of the absorber
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and facilitates the injection of electrons from the absorber toward the compact layer. Previously the impact of higher concentration of Mg into the TiO2 structure for the application in perovskite solar cell was investigated and it was found that the higher density of Mg into the TiO2 structure 22
. Based on the results of our
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would deteriorate the performance of the perovskite solar cell
simulation, existence of 20% Mg into the ZnO structure has negative impact on the cell’s efficiency and the optimum level of Mg dopant based on our calculations is 10%. The reason for
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reduction in efficiency of cell with doping level of 20% of Mg is because this doping level moves the CBM of the HBL by ~0.41ev upward. Closer CBM of HBL and LUMO of absorber
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may facilitate the electron injection from the absorber toward the HBL. On the other hand, when CBM moves upward, in fact it is moving far from CBM of FTO layer. So, it would increases the recombination rate inside the HBL and reduces the electron injection from CBM of HBL to CBM of FTO layer.
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According to the figure 4, when the doping level is increased from 10% to 20%, the quantum efficiency declines at shorter wavelengths, which can be a reason for the reduction in efficiency of the cell with 20% Mg-doped ZnO HBL. The band gap of ZnO reaches to 3.7ev when dopant
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concentration increased to 20%. These Mg doped ZnO absorb light with 300=350 nm. Therefore,
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the quantum efficiency is decreased in 300=350 nm. Figure 5 shows the variation of open circuit voltage (Voc) and short circuit current density (Jsc) of the cell with pure and Mg doped ZnO HBLs. As illustrated in the figure, when the amount of Mg in the compact layer is increased, the Jsc of the cell moves toward higher Jsc. In fact existence of Mg impurity into ZnO structure leads to widening the band gap energy and consequently moves the CBM closer to the LUMO level. Therefore, electrons would move faster from the LUMO level of the absorber to the conduction band of the compact layer. On the other hand, when the 6
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level of Mg is increased, the Voc of the cell stay constant. The Voc for the cell with pure ZnO and Mg-doped ZnO simulations is ~1.08 V.
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2.3. Comparing pure and Mg doped TiO2 and ZnO compact layers Previously researchers were shown that the best device performance was achieved by using TiO2 blocking layer
40, 41
. ZnO blocking layer, however, attracted a lot of attention due to its 42
.
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abundance in nature, easy preparation and inexpensive price and high electron mobility
Therefore, higher cell efficiency for the cell with TiO2 HBL and higher Voc for the cell with ZnO
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HBL is anticipated. As it can be found from former sections, results from our simulation proves that TiO2 blocking layer resulted in better cell performance (17.91% compared to 16.78%), however, the use of ZnO blocking layer improves electron transfer (Voc= 1.08 V compared to Voc= 0.95 V). The rate of improvement in efficiency of perovsike solar cell with different
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blocking layers is shown in figure 6. As it can be seen from the figure, the rate of improvement for the cell with up to 10% Mg-doped ZnO compact layer (16.63% improvement in efficiency) compared to the cell with up tp 10% Mg-doped TiO2 layer (10.89% improvement in efficiency)
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is higher. The results show that, using certain amount of Mg impurity is more effective to improve the efficiency of the cell with ZnO HBL rather than the one with TiO2 HBL. The Effect of Operating Temperature
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2.4.
So far, the operating temperature for all the simulations is considered 300K. Figure 7, Figure 8 and Figure 9 declare that raising the operating temperature leads to lower cell efficiency, fill factor and Voc. However, the rate of reduction in efficiency of the cell with different HBLs is different. In figure 7, when the operating temperature is increased from 300 K to 400 K, the efficiency of the cell with pure TiO2 reduces from 17.91% to 13.74% and the FF declines from 7
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84.03% to 79.14%. While the reduction for the cell with 0.1Mg doped-TiO2 compact layer is from 19.86% to 15.52%. Figure 8 shows the variation of efficiency and FF for the cell with Mgdoped TiO2 compact layer. The FF of the cell with doped HBL reduces from 84.65% to 80.64%.
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According to the figure 9 when the operating temperature is increased, the Voc of the cell moves toward the lower Voc values (reduces from 1.04 V at 300K to 0.85V at 400 K). The Jsc, however, has not changed considerably at higher temperatures. In fact, Physical parameters such as the
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electron and hole mobility as well as carrier concentration and band gap of the layers could be affected by changing in operating temperature 44. Higher operating temperature could leads to the
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higher short circuit current density (Jsc) due to the production of more electrons into the conduction band at higher operating temperatures. On the other hand, the band gap energy was reduced at higher temperatures, so this would lead to more recombination of electrons and holes and finally leads to the reduction in Jsc 45.
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Figure 10 and Figure 11 show the effect of operating temperature on the efficiency of perovskite cell with pure ZnO and Mg-doped ZnO HBLs, respectively. As in can be seen from figure 10, for the cell with pure ZnO compact layer, cell’s efficiency has been declined from 16.78% to
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15.29%. In figure 11, on the other hand, the reduction in efficiency for the cell with Mg-doped
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ZnO compact layer is increased from 19.57% to 9.69%. The calculated efficiencies at temperatures 300K, 325K, 350 K, 375 K and 400K are 19.57%, 18.62%, 16.92%, 14.1% and 9.69%, respectively. As it can be seen from figure 12, when the temperature increases from 300K to 400K, the Voc of the cell moves toward lower voltages (from 1.08V to 0.86V). From figure 13 it can be found that the cell with pure ZnO compact layer with 8.88% reduction rate would have the highest stability at higher temperatures. The cell with Mg-doped ZnO with 50.49% reduction rate, however, would have the lowest stability at higher operating 8
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temperatures. The percentage of reduction in efficiency of the cell with pure TiO2 HBL is 23.28%, while the percentage is 21.85% for the cell with Mg-doped TiO2 HBL. Therefore, Mg as a dopant into the TiO2 compact layer not only improves cell efficiency at room temperature, but
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also improves stability at higher operating temperatures. So, based on our simulation, use of both Mg-doped TiO2 and Mg-doped ZnO HBLs at low temperatures would result in better cell performance. While, at higher operating temperatures, use of pure ZnO or Mg-doped TiO2 are
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favorable.
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Conclusion
The effect of Magnesium doped ZnO and TiO2 HBLs on the performance of perovskite solar cell at different operating temperatures are studied. When the doping level is increased up to 10%, the cell’s efficiency is improved from 17.91% to 19.86% and from 16.78% to 19.57% for the cell
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with TiO2 and ZnO compact layers, respectively. However, Increase in operating temperature in general, deteriorated the performance of the cell with pure and Mg-doped HBLs. When the temperature is increased from 300K to 400K the reduction in efficiency of the cell with pure
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TiO2 HBL is from 17.91% to 13.74% and for the Mg-doped TiO2 HBL is from 19.86% to 15.52%. At the same range of temperature the reduction in efficiency for the cell with pure and
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Mg-doped ZnO HBLs are from 16.78% to 15.29% and from 19.57% to 9.69% respectively. Therefore, the cell with pure ZnO compact layer and Mg-doped ZnO compact layer are the most stable and the most unstable samples at higher operating temperatures. References 1
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Figure Caption Figure 1. Cell’s efficiency for different doping levels of Mg into the TiO2 HBL
Figure 2. Open circuit voltage (Voc) and short circuit current density (Jsc) for different doping
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levels into TiO2.
Figure 3. Cell’s efficiency and fill factor for different doping levels of Mg in ZnO HBL.
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Figure 4. Quantum efficiency of the perovskite solar cell with different doping levels of Mg in ZnO HBL.
Figure 5. Open circuit voltage (Voc) and short circuit current density (Jsc) for different doping
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levels of Mg in ZnO.
Figure 6. Efficiency improvement rate for TiO2 and ZnO HBLs.
Figure 7. Effect of temperature on FF and η of perovskite cell with pure TiO2 compact layer.
layer.
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Figure 8. Effect of temperature on FF and η of perovskite cell with Mg-doped TiO2 compact
Figure 9. Effect of temperature on Voc and Jsc of perovskite cell with Mg-doped TiO2 compact
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layer.
layer.
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Figure 10. Effect of operating temperature on the η of perovskite cell with pure ZnO compact
Figure 11. Effect of temperature on the FF and η of perovskite cell with Mg-doped ZnO compact layer.
Figure 12. Effect of temperature on Voc and Jsc of perovskite cell with Mg-doped ZnO compact layer.
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Figure 13. Comparison between the stability of pure and Mg-doped ZnO HBLs and Pure and Mg-doped TiO2 compact layers at different operating temperatures (from 300K- 400K).
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Table Caption: Table 1. Definition of electronic properties
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Table 2. Base Parameters for simulated perovskite solar cell with TiO2 compact layer. Table 3. Base Parameters for simulated perovskite solar cell with ZnO compact layer.
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Table 4. The energy band gap for different concentrations of Mg into TiO2 film
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.
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Table 5. The energy band gap for different concentrations of Mg into ZnO film [21].
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η (%)
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19
18
0.00
0.02
0.04
0.06
0.08
0.10
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Doping (%)
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η (%)
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Figure 1
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22.60
Voc
1.04
Jsc
22.56
0.98
22.54
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Voc (V)
1.00
0.96
22.52
0.00
0.02
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0.94 0.04
0.06
Doping (%)
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Figure 2
17
0.08
0.10
22.50
Jsc (mA/cm2)
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22.58
1.02
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20.0 19.5
η (%)
84
FF (%)
82 80
19.0
η (%)
76 74
18.0
72
17.5
FF (%)
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78 18.5
70 68
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17.0
66
16.5 0.0
0.1
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Doping (%)
0.2
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Figure 3
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Pure ZnO 0.02MgZnO 0.05MgZnO 0.1MgZnO 0.2MgZnO
100
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QE (%)
60
40
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400
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600
λ (nm)
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Figure 4
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1.10
Voc Jsc
1.08
22.64
22.62
1.02
22.60
1.00 0.98
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Voc (V)
1.04
22.58
0.96 0.94 0.0
0.1
0.2
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Doping (%)
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Figure 5
20
Jsc (mA/cm2)
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1.06
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118
Mg-doped TiO2
116
Mg-doped ZnO
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112 110 108 106 104 102 100 98 0.00
0.02
0.04
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0.08
0.10
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Doping (%)
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Efficiency Variation (%)
114
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Figure 6
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η (%)
18
84
FF (%)
η (%)
16
80
FF (%)
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82
78
14
300
320
340
360
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380
400
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Operating temperature (K)
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Figure 7
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η FF
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η (%)
18
82
FF (%)
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80
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Operating Temperature (K)
400
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Figure 8
23
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22.60
Voc
Jsc 22.55
22.50
0.9
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22.45
22.40
300
320
340
360
380
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Operating Temperature (K)
400
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Figure 9
24
Jsc (mA/cm2)
Voc (V)
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1.0
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16.8
η (%)
16.6
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16.4
η (%)
16.2 16.0 15.8
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15.6 15.4
300
320
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15.2 340
360
380
Operating Temperature (K)
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Figure 10
25
400
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80
η
20
FF
75
18
16
η (%)
65
14
60
12
10
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55 50
8
45
320
340
360
380
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300
Operating Temperature (K)
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Figure 11
26
400
FF (%)
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22.70
Voc
1.1
Jsc
Voc (V)
1.0
22.60
22.55
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0.9
22.50
320
340
360
380
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300
Operating Temperature (K)
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Figure 12
27
400
Jsc (mA/cm2)
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22.65
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1.0
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0.8
0.7
Mg-doped TiO2 Pure ZnO Mg-doped ZnO
0.5 300
320
340
360
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Pure TiO2
0.6
380
400
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Normalized Efficiency
0.9
Operating Temperature (K)
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Figure 13
28
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Table 1
Definition
Ӽ
Electron Affinity
Ԑr
Relative permittivity
µN(cm2/Vs)
Electron band mobility
µP (cm2/Vs)
Hole band mobility
NA(cm-3)
Acceptor concentration
ND(cm-3)
Donor concentration
Nt (cm-2)
Defect Density
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Parameter
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Table 2
Parameters
TCO
INT1
Absorber
INT2
HTM
100
10
330
10
350
3.5
Varied[22]
1.55
1.55 [29]
1.55
3 [28]
Ӽ (ev)
4
4.1[34]
3.9
3.9[30]
3.90
2.45 [28]
Ԑr
10
10
6.5
6.5 [32]
6.5
3 [31]
µN (cm2/Vs)
15
15 [35]
2.0
2.0 [33]
2.0
2×10-4
2.0
2.0 [33]
2.0
2×10-4 [28]
1013
1013
1013
-
-
-
-
2×1018
10
10 [35]
ND(cm-3)
2×1019
1018 [35]
NA(cm-3)
-
-
1015
1016
1017
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Nt (cm-2)
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µP (cm2/Vs)
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Eg(ev)
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Thickness(nm) 500
TiO2
30
2.5×1013
[28]
[27] 1017
1015
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Table 3
Parameters
TCO
INT1
Absorber
INT2
HTM
100
10
330
10
350
3.5
Varied[21]
1.55
1.55 [29]
1.55
3 [28]
Ӽ (ev)
4
4.5 [36]
3.9
3.9[30]
3.90
2.45 [28]
Ԑr
10
10
6.5
6.5 [32]
6.5
3 [31]
µN (cm2/Vs)
15
100 [37]
2.0
2.0 [33]
2.0
2×10-4
2.0
2.0 [33]
2.0
2×10-4 [28]
1013
1013
1013
-
-
-
-
2×1018
10
25 [37]
ND(cm-3)
2×1019
1018 [37]
NA(cm-3)
-
-
1015
1016
1017
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Nt (cm-2)
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µP (cm2/Vs)
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Eg(ev)
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Thickness(nm) 500
ZnO
31
2.5×1013
[28]
[27] 1017
1015
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Table 4
Bang Gap Energy (ev)
0.0 (Pure)
3.33
0.05
3.36
0.10
3.44
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Amount of Magnesium
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Table 5
Bang Gap Energy (ev)
0.0 (Pure)
3.30
0.02
3.32
0.05
3.37
0.10
3.45
0.20
3.71
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Amount of Magnesium
33
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1. We investigate the possibility of using Mg-doped TiO2 as ETL in perovskite solar cell for the first time. 2. The effects of Mg concentration into the buffer layer were investigated.
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3. Buffer film thickness and operating temperature have significant influence on the performance of the perovskite solar cell.
S0749-6036(16)30027-1
DOI:
10.1016/j.spmi.2016.01.026
Reference:
YSPMI 4161
To appear in:
Superlattices and Microstructures
Received Date: 27 October 2015 Revised Date:
18 January 2016
Accepted Date: 20 January 2016
Please cite this article as: A. Baktash, O. Amiri, A. Sasani, Improve efficiency of Perovskite Solar Cells by using Magnesium Doped ZnO and TiO2 Compact Layers, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.01.026. 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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Improve efficiency of Perovskite Solar Cells by using Magnesium Doped ZnO and TiO2 Compact Layers
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
c
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Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA Department of Science, Karaj Islamic Azad University, Karaj, Alborz, P.O. Box 31485-313, Iran.
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Email: [email protected] and [email protected] *Corresponding author, Tel: +17343895728.
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Abstract
Here the effect of Magnesium doped TiO2 and ZnO as hole blocking layers (HBLs) are investigated by using solar cell capacitance simulator (SCAPS). The Impact of Magnesium concentration into the TiO2 and ZnO and effect of operating temperature on the performance of
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b
the perovskite solar cell are investigated. Best cell performance for both TiO2 and ZnO HBLs ( with cell efficiencies of 19.86% and 19.57% respectively) are concluded for the doping level of
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a
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Ardeshir Baktasha, Omid Amiri a,b*, Alireza Sasani c
10% of Mg into the structure of HBLs. Increase in operating temperature from 300 K to 400 K are decreased the performance of the perovskite solar cell with both pure and Mg-doped HBLs. However, the cells with pure ZnO layer and with Zn0.9 Mg0.1O layer show the highest (with a decline of 8.88% in efficiency) and the lowest stability (with a decline of 50.49% in efficiency) at higher temperatures respectively. Moreover, the cell with Ti0.9 Mg0.1O2 layer shows better
1
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stability (with 21.85% reduction in efficiency) than the cell with pure TiO2 compact layer (with 23.28% reduction in efficiency) at higher operating temperatures.
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Introduction Since five years ago thin film solar cells based on halide perovskites as light absorber have attracted considerable attention due to their low cost, high absorption coefficient, and suitable 1-6
. The improvement in cells efficiency improved rapidly and recently power
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band gap
conversion efficiency of close to 20% has been reported 7, 8.
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In perovskite solar cell, electron-hole pairs are produced in absorber layer and electrons move toward the Conduction Band Minimum (CBM) of the Electron Transfer Material (ETM) layer and the holes move toward the Valance Band Minimum (VBM) of the Hole Transfer Material (HTM) layer 9. For the ETM layer as a very important component in construction of perovskite
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solar cells, wide band gap materials such as TiO2 and ZnO have attracted scientists for their characteristics. For example TiO2 have properties such as wide range of applications, controllable framework compositions and tunable pore sizes
10, 11
. ZnO, on the other hand, have
price
12- 14
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characteristics such as superior physical and chemical stability, nontoxicity and low production . Because the ETM layer prevents recombination of available electrons of Fluorine-
doped Tin Oxide (FTO) layer and produced electrons and holes in absorber layer, this layer is
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1.
known as Hole Blocking Layer (HBL)
15, 16
. Therefore, it is very important to choose a suitable
HBL layer for perovskite solar cell.
One way to improve the performance of HBL is to dope a suitable material to improve the properties of the HBL. For this purpose, TiO2 and ZnO have been doped with a metal to improve photo anode properties of Dye Synthesized Solar Cell (DSSC) 17-20. One of the elements that are 2
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improved the performance of DSSC’s is Magnesium (Mg)
20
. The band gap of semiconductors
like TiO2 and ZnO can be tuned by the use of the Magnesium as a dopant 20, 21.
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In this study, our aim is to investigate the effect of Mg-doped TiO2 and Mg-doped ZnO as potential HBLs for the application in perovskite solar cell. Recently, Mg- doped TiO2 as a compact layer is used to improve the conversion efficiency of perovskite solar cell 22. However,
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to the best of our knowledge, the effect of Mg-doped ZnO compact layer is not investigated yet. In this work, the effect of doping concentration and effect of temperature on the performance of
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perovskite solar cell with pure and Mg-doped compact layers (TiO2 and ZnO) are investigated. For this purpose, we considered doping levels of 0.05 and 0.1 for TiO2 layer and 0.02, 0.05, 0.1 and 0.20 for ZnO compact layer. We used experimental data for the band gap of doped ZnO and TiO2 21, 22.
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1. Computational Method
In this work, SCAPS version 3.2.01(a Solar Cell Capacitance Simulator) software which is a one
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dimensional solar cell simulation program is used. This software is developed at Department of Electronics and Information Systems (ELIS) of the University of Gent, Belgium
23
. The
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simulated perovskite solar cell has layer configuration with transparent conductive oxide (TCO)/ hole blocking layer (HBL)/ absorber/ and hole transport material (HTM) layers. The considered materials for the mentioned layers are fluorine doped SnO2 (SnO2:F), pure and doped TiO2 and ZnO, CH3 NH3 PbI3-XCl3 and spiro-OMeTAD, respectively. For HBL, the effect of the Magnesium dopant on CBM of TiO2 and ZnO is considered. For HBL with TiO2, the concentration of 0.0, 0.05 and 0.1 Magnesium into the TiO2 layer are considered [22] and for HBL with ZnO layer, the concentration of Mg 0.0, 0.02, 0.05, 0.1 and 0.2 are considered 21 and 3
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accordingly the electron affinity of the layers is varied too. The descriptions of base parameters are available in Table 1. Table 2 and Table 3 show the base parameter set for different layers of the simulation that are used in this study. The thicknesses of layers are chosen based on 24, 25
. To consider interface recombination, the
interface layers INT1 and INT2 were defined from reference
26
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experimental works on perovskite solar cell
. In this study, to obtain carrier
diffusion lengths (Ln and Lp) of 1 µm that is similar to that of for experimental work 27, the value
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of defect parameters for all layers are considered identical and defect density for absorber is
2. Results and Discussion 2.1.
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assumed equal to Nt= 2.5 × 1013 cm3 26.
Effect of doping concentration of Mg into TiO2 layer
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Magnesium as a dopant into TiO2 could widen the band gap by moving the CBM toward the higher energy levels. So, to simulate the effect of different doping concentration of Mg into the TiO2 compact layer, the data for band gap of the layer with different doping levels are chosen
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from experimental work 22. The information for Band gap changing are shown in Table 4. As it can be found from table 4, Mg as an additive into TiO2 structure is widening the bang gap. The
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reason for this band gap widening is the differences in band gap energy of TiO2 (~3.2 ev) 38 and MgO ( ~7.8 ev) 39.
The efficiencies of the cell with pure and Mg-doped TiO2 compact layers are shown in Figure 1. As seen from figure 1, use of Mg as an additive (up to 10%) into the HBL leads to higher efficiency in perovskite solar cell. Previously, Mg-doped TiO2 layer is used to improve the 4
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efficiency of perovskite solar cell
22
. For our simulation, the cell’s efficiency (η) of pure TiO2
layer, 0.05 and 0.10 Mg-doped TiO2 layer are 17.91%, 18.53% and 19.86% respectively. Mgdoped TiO2 acts as the HBL in the structure and reduces recombination rate of electrons and
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holes. As Figure 2 shows, by increasing the amount of Mg into the HBL, short circuit current density (Jsc) and open circuit voltage (Voc) of the cell increases and this means that the recombination rate is decreased and consequently more electrons are injected from the absorber
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layer into the HBL. Therefore, upward move of CBM toward the LUMO level of the absorber, facilitates the electron injection from the absorber into the compact layer and leads to better
2.2.
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performance of the cell.
Effect of doping concentration of Mg into ZnO layer
In this Section, the effect of Mg-doepd ZnO compact layer on the performance of perovskite
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solar cell is investigated. In our simulation, the used information for the band gap energies of compact layer with different dopant concentrations is gathered from experimental work
21
. The
band gap energies for ZnO layer with different doping levels of Mg are tabled in Table 5.
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The efficiency (η) and fill factor (FF) of the simulated cells are shown in Figure 3. As shown in figure 3, the η of the cell with lightly Mg doped (up to 10% Mg) is improved. However, by using
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Zn0.8Mg0.2O compact layer, the efficiency is reduced. Efficiency of the cell with pure ZnO compact layer is 16.78% and the efficiency for 2, 5, 10 and 20% Mg-doped ZnO are 17.19%, 18.21%, 19.57% and 18.17% respectively. The FF of the cell with pure and 0.2 Mg doped ZnO layer is increased from 68.24% to 84.16%. The FF of 69.93%, 74.11%, and 79.85% are calculated for the cells with 2, 5 and 10% Mg-doped ZnO compact layers. Introduction of Magnesium into the structure of ZnO, shifts the CBM toward the LUMO level of the absorber
5
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and facilitates the injection of electrons from the absorber toward the compact layer. Previously the impact of higher concentration of Mg into the TiO2 structure for the application in perovskite solar cell was investigated and it was found that the higher density of Mg into the TiO2 structure 22
. Based on the results of our
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would deteriorate the performance of the perovskite solar cell
simulation, existence of 20% Mg into the ZnO structure has negative impact on the cell’s efficiency and the optimum level of Mg dopant based on our calculations is 10%. The reason for
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reduction in efficiency of cell with doping level of 20% of Mg is because this doping level moves the CBM of the HBL by ~0.41ev upward. Closer CBM of HBL and LUMO of absorber
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may facilitate the electron injection from the absorber toward the HBL. On the other hand, when CBM moves upward, in fact it is moving far from CBM of FTO layer. So, it would increases the recombination rate inside the HBL and reduces the electron injection from CBM of HBL to CBM of FTO layer.
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According to the figure 4, when the doping level is increased from 10% to 20%, the quantum efficiency declines at shorter wavelengths, which can be a reason for the reduction in efficiency of the cell with 20% Mg-doped ZnO HBL. The band gap of ZnO reaches to 3.7ev when dopant
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concentration increased to 20%. These Mg doped ZnO absorb light with 300=350 nm. Therefore,
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the quantum efficiency is decreased in 300=350 nm. Figure 5 shows the variation of open circuit voltage (Voc) and short circuit current density (Jsc) of the cell with pure and Mg doped ZnO HBLs. As illustrated in the figure, when the amount of Mg in the compact layer is increased, the Jsc of the cell moves toward higher Jsc. In fact existence of Mg impurity into ZnO structure leads to widening the band gap energy and consequently moves the CBM closer to the LUMO level. Therefore, electrons would move faster from the LUMO level of the absorber to the conduction band of the compact layer. On the other hand, when the 6
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level of Mg is increased, the Voc of the cell stay constant. The Voc for the cell with pure ZnO and Mg-doped ZnO simulations is ~1.08 V.
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2.3. Comparing pure and Mg doped TiO2 and ZnO compact layers Previously researchers were shown that the best device performance was achieved by using TiO2 blocking layer
40, 41
. ZnO blocking layer, however, attracted a lot of attention due to its 42
.
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abundance in nature, easy preparation and inexpensive price and high electron mobility
Therefore, higher cell efficiency for the cell with TiO2 HBL and higher Voc for the cell with ZnO
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HBL is anticipated. As it can be found from former sections, results from our simulation proves that TiO2 blocking layer resulted in better cell performance (17.91% compared to 16.78%), however, the use of ZnO blocking layer improves electron transfer (Voc= 1.08 V compared to Voc= 0.95 V). The rate of improvement in efficiency of perovsike solar cell with different
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blocking layers is shown in figure 6. As it can be seen from the figure, the rate of improvement for the cell with up to 10% Mg-doped ZnO compact layer (16.63% improvement in efficiency) compared to the cell with up tp 10% Mg-doped TiO2 layer (10.89% improvement in efficiency)
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is higher. The results show that, using certain amount of Mg impurity is more effective to improve the efficiency of the cell with ZnO HBL rather than the one with TiO2 HBL. The Effect of Operating Temperature
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2.4.
So far, the operating temperature for all the simulations is considered 300K. Figure 7, Figure 8 and Figure 9 declare that raising the operating temperature leads to lower cell efficiency, fill factor and Voc. However, the rate of reduction in efficiency of the cell with different HBLs is different. In figure 7, when the operating temperature is increased from 300 K to 400 K, the efficiency of the cell with pure TiO2 reduces from 17.91% to 13.74% and the FF declines from 7
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84.03% to 79.14%. While the reduction for the cell with 0.1Mg doped-TiO2 compact layer is from 19.86% to 15.52%. Figure 8 shows the variation of efficiency and FF for the cell with Mgdoped TiO2 compact layer. The FF of the cell with doped HBL reduces from 84.65% to 80.64%.
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According to the figure 9 when the operating temperature is increased, the Voc of the cell moves toward the lower Voc values (reduces from 1.04 V at 300K to 0.85V at 400 K). The Jsc, however, has not changed considerably at higher temperatures. In fact, Physical parameters such as the
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electron and hole mobility as well as carrier concentration and band gap of the layers could be affected by changing in operating temperature 44. Higher operating temperature could leads to the
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higher short circuit current density (Jsc) due to the production of more electrons into the conduction band at higher operating temperatures. On the other hand, the band gap energy was reduced at higher temperatures, so this would lead to more recombination of electrons and holes and finally leads to the reduction in Jsc 45.
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Figure 10 and Figure 11 show the effect of operating temperature on the efficiency of perovskite cell with pure ZnO and Mg-doped ZnO HBLs, respectively. As in can be seen from figure 10, for the cell with pure ZnO compact layer, cell’s efficiency has been declined from 16.78% to
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15.29%. In figure 11, on the other hand, the reduction in efficiency for the cell with Mg-doped
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ZnO compact layer is increased from 19.57% to 9.69%. The calculated efficiencies at temperatures 300K, 325K, 350 K, 375 K and 400K are 19.57%, 18.62%, 16.92%, 14.1% and 9.69%, respectively. As it can be seen from figure 12, when the temperature increases from 300K to 400K, the Voc of the cell moves toward lower voltages (from 1.08V to 0.86V). From figure 13 it can be found that the cell with pure ZnO compact layer with 8.88% reduction rate would have the highest stability at higher temperatures. The cell with Mg-doped ZnO with 50.49% reduction rate, however, would have the lowest stability at higher operating 8
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temperatures. The percentage of reduction in efficiency of the cell with pure TiO2 HBL is 23.28%, while the percentage is 21.85% for the cell with Mg-doped TiO2 HBL. Therefore, Mg as a dopant into the TiO2 compact layer not only improves cell efficiency at room temperature, but
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also improves stability at higher operating temperatures. So, based on our simulation, use of both Mg-doped TiO2 and Mg-doped ZnO HBLs at low temperatures would result in better cell performance. While, at higher operating temperatures, use of pure ZnO or Mg-doped TiO2 are
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favorable.
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Conclusion
The effect of Magnesium doped ZnO and TiO2 HBLs on the performance of perovskite solar cell at different operating temperatures are studied. When the doping level is increased up to 10%, the cell’s efficiency is improved from 17.91% to 19.86% and from 16.78% to 19.57% for the cell
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with TiO2 and ZnO compact layers, respectively. However, Increase in operating temperature in general, deteriorated the performance of the cell with pure and Mg-doped HBLs. When the temperature is increased from 300K to 400K the reduction in efficiency of the cell with pure
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TiO2 HBL is from 17.91% to 13.74% and for the Mg-doped TiO2 HBL is from 19.86% to 15.52%. At the same range of temperature the reduction in efficiency for the cell with pure and
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Mg-doped ZnO HBLs are from 16.78% to 15.29% and from 19.57% to 9.69% respectively. Therefore, the cell with pure ZnO compact layer and Mg-doped ZnO compact layer are the most stable and the most unstable samples at higher operating temperatures. References 1
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195450 (2007). 40
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J. Jung, and K. Kern, Nano Lett., 14, 6533 (2014).
H. Zhou, Q. Chen, G. Li, S. Luo, T. -B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y.
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Yang, Science, 345, 542 (2014).
A. Yella, L.-P. Heiniger, P. Gao, M. K. Nazeeruddin and M. Grätzel, Nano Lett., 14, 2591
(2014). 42
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W.A. Laban, and L. Etgar, Energy Environ. Sci. 6, 3249 (2013). D. Bi, G. Boschloo, S. Schwarzmuller, L Yang,. E. M. J. Johanssona, and A.Hagfeldt,
Nanoscale 5, 11686 (2013). 12
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T. Nakada, and M. Mizutani, Jpn. J. Appl. Phys. 41, 165 (2002).
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P. Singh, and N.M. Ravindra, Solar Energy Materials & Solar Cells 101, 36 (2012).
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Figure Caption Figure 1. Cell’s efficiency for different doping levels of Mg into the TiO2 HBL
Figure 2. Open circuit voltage (Voc) and short circuit current density (Jsc) for different doping
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levels into TiO2.
Figure 3. Cell’s efficiency and fill factor for different doping levels of Mg in ZnO HBL.
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Figure 4. Quantum efficiency of the perovskite solar cell with different doping levels of Mg in ZnO HBL.
Figure 5. Open circuit voltage (Voc) and short circuit current density (Jsc) for different doping
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levels of Mg in ZnO.
Figure 6. Efficiency improvement rate for TiO2 and ZnO HBLs.
Figure 7. Effect of temperature on FF and η of perovskite cell with pure TiO2 compact layer.
layer.
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Figure 8. Effect of temperature on FF and η of perovskite cell with Mg-doped TiO2 compact
Figure 9. Effect of temperature on Voc and Jsc of perovskite cell with Mg-doped TiO2 compact
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layer.
layer.
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Figure 10. Effect of operating temperature on the η of perovskite cell with pure ZnO compact
Figure 11. Effect of temperature on the FF and η of perovskite cell with Mg-doped ZnO compact layer.
Figure 12. Effect of temperature on Voc and Jsc of perovskite cell with Mg-doped ZnO compact layer.
14
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Figure 13. Comparison between the stability of pure and Mg-doped ZnO HBLs and Pure and Mg-doped TiO2 compact layers at different operating temperatures (from 300K- 400K).
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Table Caption: Table 1. Definition of electronic properties
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Table 2. Base Parameters for simulated perovskite solar cell with TiO2 compact layer. Table 3. Base Parameters for simulated perovskite solar cell with ZnO compact layer.
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Table 4. The energy band gap for different concentrations of Mg into TiO2 film
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.
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Table 5. The energy band gap for different concentrations of Mg into ZnO film [21].
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η (%)
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19
18
0.00
0.02
0.04
0.06
0.08
0.10
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Doping (%)
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η (%)
20
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Figure 1
16
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22.60
Voc
1.04
Jsc
22.56
0.98
22.54
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Voc (V)
1.00
0.96
22.52
0.00
0.02
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0.94 0.04
0.06
Doping (%)
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Figure 2
17
0.08
0.10
22.50
Jsc (mA/cm2)
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22.58
1.02
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20.0 19.5
η (%)
84
FF (%)
82 80
19.0
η (%)
76 74
18.0
72
17.5
FF (%)
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78 18.5
70 68
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17.0
66
16.5 0.0
0.1
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Doping (%)
0.2
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Figure 3
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Pure ZnO 0.02MgZnO 0.05MgZnO 0.1MgZnO 0.2MgZnO
100
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80
QE (%)
60
40
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20
300
400
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0 500
600
λ (nm)
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Figure 4
19
700
800
900
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1.10
Voc Jsc
1.08
22.64
22.62
1.02
22.60
1.00 0.98
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Voc (V)
1.04
22.58
0.96 0.94 0.0
0.1
0.2
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Doping (%)
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Figure 5
20
Jsc (mA/cm2)
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1.06
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118
Mg-doped TiO2
116
Mg-doped ZnO
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112 110 108 106 104 102 100 98 0.00
0.02
0.04
0.06
0.08
0.10
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Doping (%)
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Efficiency Variation (%)
114
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Figure 6
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η (%)
18
84
FF (%)
η (%)
16
80
FF (%)
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82
78
14
300
320
340
360
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76
380
400
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Operating temperature (K)
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Figure 7
22
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86
η FF
20
η (%)
18
82
FF (%)
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84
80
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16
78
300
320
340
360
380
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Operating Temperature (K)
400
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Figure 8
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22.60
Voc
Jsc 22.55
22.50
0.9
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22.45
22.40
300
320
340
360
380
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Operating Temperature (K)
400
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Figure 9
24
Jsc (mA/cm2)
Voc (V)
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1.0
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16.8
η (%)
16.6
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16.4
η (%)
16.2 16.0 15.8
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15.6 15.4
300
320
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15.2 340
360
380
Operating Temperature (K)
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Figure 10
25
400
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80
η
20
FF
75
18
16
η (%)
65
14
60
12
10
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55 50
8
45
320
340
360
380
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300
Operating Temperature (K)
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Figure 11
26
400
FF (%)
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22.70
Voc
1.1
Jsc
Voc (V)
1.0
22.60
22.55
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0.9
22.50
320
340
360
380
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300
Operating Temperature (K)
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Figure 12
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400
Jsc (mA/cm2)
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22.65
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1.0
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0.8
0.7
Mg-doped TiO2 Pure ZnO Mg-doped ZnO
0.5 300
320
340
360
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Pure TiO2
0.6
380
400
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Normalized Efficiency
0.9
Operating Temperature (K)
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Figure 13
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Table 1
Definition
Ӽ
Electron Affinity
Ԑr
Relative permittivity
µN(cm2/Vs)
Electron band mobility
µP (cm2/Vs)
Hole band mobility
NA(cm-3)
Acceptor concentration
ND(cm-3)
Donor concentration
Nt (cm-2)
Defect Density
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Parameter
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Table 2
Parameters
TCO
INT1
Absorber
INT2
HTM
100
10
330
10
350
3.5
Varied[22]
1.55
1.55 [29]
1.55
3 [28]
Ӽ (ev)
4
4.1[34]
3.9
3.9[30]
3.90
2.45 [28]
Ԑr
10
10
6.5
6.5 [32]
6.5
3 [31]
µN (cm2/Vs)
15
15 [35]
2.0
2.0 [33]
2.0
2×10-4
2.0
2.0 [33]
2.0
2×10-4 [28]
1013
1013
1013
-
-
-
-
2×1018
10
10 [35]
ND(cm-3)
2×1019
1018 [35]
NA(cm-3)
-
-
1015
1016
1017
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Nt (cm-2)
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µP (cm2/Vs)
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Eg(ev)
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Thickness(nm) 500
TiO2
30
2.5×1013
[28]
[27] 1017
1015
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Table 3
Parameters
TCO
INT1
Absorber
INT2
HTM
100
10
330
10
350
3.5
Varied[21]
1.55
1.55 [29]
1.55
3 [28]
Ӽ (ev)
4
4.5 [36]
3.9
3.9[30]
3.90
2.45 [28]
Ԑr
10
10
6.5
6.5 [32]
6.5
3 [31]
µN (cm2/Vs)
15
100 [37]
2.0
2.0 [33]
2.0
2×10-4
2.0
2.0 [33]
2.0
2×10-4 [28]
1013
1013
1013
-
-
-
-
2×1018
10
25 [37]
ND(cm-3)
2×1019
1018 [37]
NA(cm-3)
-
-
1015
1016
1017
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Nt (cm-2)
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µP (cm2/Vs)
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Eg(ev)
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Thickness(nm) 500
ZnO
31
2.5×1013
[28]
[27] 1017
1015
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Table 4
Bang Gap Energy (ev)
0.0 (Pure)
3.33
0.05
3.36
0.10
3.44
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Amount of Magnesium
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Table 5
Bang Gap Energy (ev)
0.0 (Pure)
3.30
0.02
3.32
0.05
3.37
0.10
3.45
0.20
3.71
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Amount of Magnesium
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1. We investigate the possibility of using Mg-doped TiO2 as ETL in perovskite solar cell for the first time. 2. The effects of Mg concentration into the buffer layer were investigated.
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3. Buffer film thickness and operating temperature have significant influence on the performance of the perovskite solar cell.