Issue 
Eur. Phys. J. Appl. Phys.
Volume 90, Number 1, April 2020



Article Number  10101  
Number of page(s)  9  
Section  Semiconductors and Devices  
DOI  https://doi.org/10.1051/epjap/2020190333  
Published online  13 May 2020 
https://doi.org/10.1051/epjap/2020190333
Regular Article
Modeling and optimization of nZnO/pSi heterojunction using 2dimensional numerical simulation
^{1}
Materials Physics Laboratory, Faculty of Sciences and Technologies, Sultan Moulay Slimane University, P.O. Box 523, 23000 Beni Mellal, Morocco
^{2}
Condensed Matters and Renewables Energies Laboratory, Faculty of Sciences and Technologies, Hassan II University,
P.O. Box 146, 20650 Mohammedia, Morocco
^{3}
Study Group of Optoelectronic Materials, Faculty of Sciences and Technologies, Cadi Ayyad University, P.O. Box 549,
40000 Marrakech, Morocco
^{4}
Laboratory of Physics Condensed Matter, Faculty of Sciences, University of Tunis EL Manar, 2092 Tunis, Tunisia
^{*} email: m.manoua@usms.ma
Received:
18
November
2019
Received in final form:
28
March
2020
Accepted:
3
April
2020
Published online: 13 May 2020
In this work, nZnO/pSi heterojunction was investigated using twodimensional numerical simulation. The effect of Zinc Oxide thickness, carrier concentration in Zinc Oxide layer, minority carrier lifetime of bulk Silicon and the interface states density on electrical properties were studied in dark and under illumination conditions. This study aimed to optimize these parameters in order to obtain nZnO/pSi solar cell with high conversion efficiency and low cost. The simulation was carried out by Atlas silvaco software. As results, a very low saturation current Is, low series resistance Rs, an ideality factor n between 1 and 1.5 were obtained for optimal charge carrier concentrations in the range [5 × 10^{19}–5 × 10^{21} cm^{−3}] and a thickness of Zinc Oxide between 0.6 and 2 µm. Moreover, a photovoltaic conversion efficiency of 24.75% was achieved without interfacial defect, which decreases to 5.49% for an interface defect density of 5 × 10^{14} cm^{−2}.
© EDP Sciences, 2020
1 Introduction
Front electrode heterojunctions made of Zinc oxide (ZnO) have been developed in order to produce solar cells with good electrical efficiency and less expensive [1,2]. These performances are due in large part to the very promising ZnO material that has both high transmittance in the visible range [3,4] and good electrical properties [5,6]. In addition, the cost of preparing this nontoxic material is relatively low compared to other materials used in the photovoltaic device industry [7,8]. Our study is aimed to integrate transparent conductive materials (TCO) such as zinc oxide (ZnO) into more controlled silicon technology, in order to obtain a new generation of solar cells with high photovoltaic performance. It is very interesting to report that ZnO can act as good emitter as well as an antireflective layer for silicon solar cells [9].
Several simulation researches have been reported on the purpose of optimizing the parameters and improving the performance of solar cells. The conversion efficiencies of 17.16, 19 and 21.88% were achieved by Chen et al. [10], Hussain et al. [11] and ElAmine [12], respectively, for nZnO/pSi solar cells.
In this work, the nZnO/pSi heterojunction was studied by examining the effect of charge carrier concentration in ZnO front electrode, the thickness of the ZnO layer, the lifetime of the minority carriers in pSi substrate and the interface states density on electrical properties in the dark and under illumination. For this purpose, a numerical simulation was carried out using TCAD AtlasSilvaco simulator.
2 Simulation details
ATLAS from silvaco international is physicallybased two and three dimensional simulator, which predicts the electrical characteristics and provides information on internal physical mechanisms associated with the device operation [13].
The simulated heterojunction consists of nZnO layer used as emitter and ptype Silicon as absorber. The schematic structure is shown in Figure 1. The physical parameters used in the modeling are summarized in Table 1.
Figure 2 shows the energy diagram of nZnO/pSi heterojunction extracted from AtlasSilvaco based on physical parameters illustrated in Table 1, with Zinc Oxide thickness of 600 nm and carrier concentration of 5 × 10^{17} cm^{−3}.
The simulation of ZnO/Si heterojunction is based on the resolution of the fundamental equations: Poisson equation, electron and hole continuity equations [13,14]. Fermi statistic was used for carriers with driftdiffusion. The concentration dependent mobility model was used for carrier mobility. ShockleyReadHall (SRH) and Auger were included into the simulation for recombination models as a function of doping concentration. Thermionic emission across the nZnO/pSi interface was also incorporated [13].
Fig. 1 Schematic structure of ZnO/Si heterojunction. 
Physical parameters for ZnO/Si heterojunction.
Fig. 2 Energy band diagram of ZnO/Si heterojunction. 
3 Results and discussions
3.1 Dark parameters
The currentvoltage characteristic in the dark condition is a valuable source of several parameters of the heterojunction such as series resistance R_{s}, ideality factor (n), barrier height φ_{b} and reverse saturation current I_{S}.
The parameters were determined from the thermionic emission, where the currentvoltage relation of heterojunction is usually written as a function of the applied voltage (V) as described in the following equation [15]:(1) where q is the electronic charge, V is the applied voltage, n is the ideality factor, K is the Boltzmann's constant, T is the temperature and I_{s} is the reverse saturation current.
The series resistance, ideality factor and barrier height can be estimated by Cheung's model derived from I(V) forward characteristics [16]:(2) (3) (4) and H (I) curves presented in Figures 3 and 4 show the straight lines from which the ideality factor, series resistance and barrier height are estimated. The reverse current I_{s} is calculated by Bethe [17] as follows:(5)where A is the area of the diode, A ^{*} is the Richardson constant taken as 32 A cm^{−2} K^{−2} for ZnO [18,19] and φ_{b} is the barrier height.
Fig. 3 for the ZnO/Si heterojunction. 
Fig. 4 H(I) for the ZnO/Si heterojunction. 
3.1.1 Effect of Zinc Oxide carrier concentration
The carrier concentration effect is investigated by taking the carrier densities from 5 × 10^{17} to 5 × 10^{21} cm^{−3}, with Zinc Oxide thickness of 600 nm. The currentvoltage (IV) characteristics of ZnO/Si heterojunction are shown in Figure 5. It is clear that the heterojunction shows a good rectifying behavior under dark condition. In addition, the carrier concentration also affects the turnon voltage that decreases with the carrier concentration. This evolution is due to the decrease in diffusion voltage between nZnO and pSi regions, this result is consistent with the theory of heterojunctions mentioned by S.M. Sze [20].
Also, it can be observed that there is a decrease in the saturation current with the increasing of carrier density as shown in Figure 6. Indeed, the increase in carrier density reduces the minority hole concentration in the nZnO layer, which reduces the saturation current to its minimum value 2.32 × 10^{−12} mA.
Figure 7 illustrates the evolution of the ideality factor n as a function of the charge carrier concentration; the values taken by the factor n (from 1 to 1.8) indicate that both scattering mechanism and recombination mechanism of charge carriers must be taken into account in the electrical conduction of the ZnO/Si structure. In addition, this evolution shows that the scattering mechanism is predominant for the high concentration beyond 10^{20} cm^{−3}.
The series resistance decreases with increasing the carrier concentration as shown in Figure 8, this is probably due to an improvement in the conductivity of nZnO zone, as mentioned by Belarbi [21]. The minimal value for series resistance is 213 Ω corresponding to carrier concentration of 5 × 10^{21} cm^{−3}.
Fig. 5 IV characteristics of ZnO/Si heterojunction for different carrier concentrations. 
Fig. 6 Evolution of saturation current versus carrier concentration (a), reverse saturation current for limits carrier concentrations 5 × 10^{17} and 5 × 10^{21} cm^{−3} (b). 
Fig. 7 Evolution of ideality factor versus carrier concentration. 
Fig. 8 Evolution of series resistance versus carrier concentration. 
3.1.2 Effect of Zinc Oxide thickness
In this section, we started an optimization of ZnO thickness in order to obtain a thinner structure consuming less energy. For that, we are simulated the currentvoltage characteristics for different nZnO thicknesses at a fixed charge carrier concentration equal to 5 × 10^{21} cm^{−3} (Fig. 9a). We observed a weak decrease in turnon voltage with increasing thickness between 10 nm and 0.6 μm, but beyond 0.6 μm, we did not observe any real change in turnon voltage.
The Figures 10–12 show the evolution of the series resistance, ideality factor and saturation current as a function of nZnO thickness. It can be noted that these three parameters decrease thus improve with thickness and saturate in the range [0.6–2 µm]. As a result, beyond this limit thickness of 0.6 µm, the thickness is no longer considered as a parameter limiting the electrical conduction in the nZnO/pSi heterojunction.
Fig. 9 IV characteristics of ZnO/Si heterojunction for different ZnO thickness. 
Fig. 10 Evolution of series resistance as a function of ZnO thickness. 
Fig. 11 Evolution of ideality factor as a function of ZnO thickness. 
Fig. 12 Evolution of saturation current as a function of thickness. 
3.2 Photovoltaic response of ZnO/Si heterojunction
in order to evaluate the evolution of the solar cell performances of nZnO/Si heterounction, the Current densityVoltage characteristics has been simulated under standard light condition (AM1.5, 100 mW/cm^{2}) with the same physical properties summarized in Table 1.
3.2.1 Effect of Zinc Oxide carrier concentration
From the simulated JV curves, the short circuit current density J_{sc}, open circuit voltage V_{oc}, Fill Factor FF and efficiency η has been extracted. The values of these four essential solar cell parameters as a function of the carrier concentration are illustrated in Table 2.
Figure 13 displays current densityvoltage characteristics for different nZnO carrier concentration, the rise of carrier density does not change the open circuit voltage significantly, however, we observes an increase of current density J_{sc} and then it stabilizes at 22.41 mA/cm^{2}. This stabilization of the J_{sc} is probably due to the introduction of the defects accompanying the increase of the density of charge carriers in the nZnO film, as reported by Belarbi [11].
The variation of conversion efficiency and Fill Factor as a function of nZnO carrier concentration are displayed in Figure 14, the increase of Fill Factor and conversion efficiency are observed, the maximal values reached of FF and η are 80.93 and 10.61%, respectively.
The External Quantum Efficiency (EQE) is an important property that qualifies a solar cell; it is defined by the ratio of the number of carriers collected to the number of incident photons [22,23]:(6)
Figure 15 shows the external quantum efficiency for different carrier concentrations. The external quantum efficiency improved with the increase of the carrier concentration, which confirms the results obtained in Figure 13. Also we are observed good quantum efficiency in the visible range, which reaches 70% for high carrier concentrations such as 5 × 10^{20} and 5 × 10^{21} cm^{−3}.
The variation of J_{sc}, V_{oc}, FF and η with carrier concentration of ZnO/Si solar cell heterojunction.
Fig. 13 JV characteristics of ZnO/Si heterojunction for different carrier concentrations. 
Fig. 14 Variation of fill factor and efficiency as a function of carrier concentrations. 
Fig. 15 External Quantum Efficiency for different carrier concentrations. 
3.2.2 Effect of Zinc Oxide thickness
The impact of Zinc Oxide thickness on photovoltaic parameters is investigated in this part. We simulated the current densityvoltage characteristics for different thicknesses from 10 to 2000 nm (Fig. 16). Table 3 summarizes the values of the solar cell parameters obtained. We note a decrease of J_{sc} and a weak variation of V_{oc}. The decrease of J_{sc} is explained by an increase of the recombination rate of carriers in the thicker ZnO layer as mentioned by Dwivedi et al. [24]. Also, this decrease of J_{sc} may be due to the decrease of photons number that reaches the space charge zone. These results are confirmed by the diminution of external quantum efficiency for different Zinc Oxide thicknesses, as seen in Figure 17. Also, we notice a strong absorption in the UV range gradually as the nZnO films become thicker.
Figure 18 shows the Fill Factor and the conversion efficiency versus nZnO thickness. We remark that the optimal thickness of Zinc Oxide domain is [50–100 nm], giving the good values for FF and η, the maximal values of conversion efficiency and the Fill Factor are 11.51 and 80.67%, respectively.
These observed evolutions of J_{sc}, FF and η as a function of nZnO thickness emitter are compatible with the results of Askari et al. [25] and Hussain et al. [11].
Fig. 16 JV characteristics of ZnO/Si heterojunction for different ZnO thicknesses. 
Photovoltaic cell parameters for different ZnO thickness.
Fig. 17 External Quantum Efficiency for different ZnO thicknesses. 
Figure 18 Variation of fill factor and efficiency as a function of ZnO thicknesses. 
3.2.3 Effect of Silicon minority carrier lifetime
The values of nZnO/pSi solar cell parameters as a function of minority carrier lifetime in Si bulk are enlisted in Table 4. These results are obtained for the carrier concentration and thickness of nZnO fixed at 5 ×10^{21} cm^{−3} and 50 nm, respectively. We note a clear improvement of solar cell parameters: J_{sc}, V_{oc}, FF and η with increasing of minority carrier lifetime as shown in Figures 19 and 20. Indeed, when the carrier lifetime increases, the electrons generated in Si bulk have more chance to cross the front heterojunction region, giving rise to an increase of J_{sc}. These results are also confirmed by Diouf [14]. The maximum values of FF and η are 83.17 and 24.75% respectively. The external quantum efficiency for different minority carrier lifetime is shown in Figure 21. The EQE is improved with the increasing of minority carrier lifetime, which confirms the rise of J_{sc}.
Our optimized nZnO/pSi structure had good photoelectric properties, compared to other recently published results referenced in Table 5.
Effect of minority carrier lifetime of bulk Si on solar cell parameters.
Fig. 19 JV characteristics of ZnO/Si heterojunction for different minority carrier lifetimes of bulk Si. 
Fig. 20 Variation of fill factor and efficiency as a function of minority carrier lifetimes of bulk Si. 
Fig. 21 External Quantum Efficiency for different minority carrier lifetimes of bulk Si. 
Comparison of our results with recent works from literature.
3.2.4 Effect of ZnO/Si interface defects
For a more realistic modelling of the nZnO/pSi structure, it is essential to take into account the density of interface states at the junction of nZnO and pSi materials. In this part we have proceeded to study the effect of interface defect, for that an interfacial layer of 1 nm has been taken between nZnO and pSi. The defect distribution in this interface layer was assumed to be acceptorlike states [13,26].(7)where N_{TA},N_{GA} are the density of states of tailacceptor and deepacceptor, respectively. E_{C} is the minimal conduction band energy. W_{TA} and W_{GA} are the characteristic decay energy, taken as 0.07 and 0.2 eV, respectively. E_{GA} is the peak energy of the Gaussian distribution taken as 0.5 eV.
The Table 6 summarizes the photovoltaic parameter values obtained. As can be seen, there is a slight decrease in J_{sc} current and a clear deterioration of V_{oc} with increasing interface states density. Indeed, This strong deterioration is due to the trapping effect of charge carriers by interface states, resulting a decrease of fill factor and conversion efficiency. The same degradation of photoelectrical properties with the interface states was observed by Askari et al. [25] and Diouf et al. [14].
ZnO/Si performances for different interface state densities.
4 Conclusions
In this paper, nZnO/pSi heterojunction is simulated under dark and illumination conditions using Atlas Silvaco software. Firstly, the effect of carrier concentration and thickness of Zinc Oxide are investigated in the dark condition. The good rectifying behavior is obtained for nZnO/pSi structure with low series resistance about 200 Ω, low saturation current density about 10^{−12} mA/cm^{2} and an ideality factor ranging from 1 to 1.5. Secondly, the effect of carrier concentration and thickness of nZnO emitter, minority carrier lifetime in the bulk silicon and interfacial defects density are studied under illumination condition. The good fill factor and conversion efficiency: FF = 83.17% and η = 24.75% are obtained without interfacial defects. These two photoelectric parameters are strongly deteriorated and become FF = 62.56%, η = 5.49%, if we take into account the interfaces states density.
Author contribution statement
M. Manoua carried out the numerical simulation and wrote the manuscript. T. Jannane finalized the graphs and the tables of values. N. Fazouan, A. Almaggoussi and N. Kamoun: Discussion and participation in the results interpretation. A. Liba and O. Abouelala: Orientation, planning and validation of results. M. Manoua and A. Liba: Response to questions and reviewers' comments.
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Cite this article as: Mohamed Manoua, Tariq Jannane, Otmane Abouelala, Nejma Fazouan, Abdelmajid Almaggoussi, Najoua Kamoun, Ahmed Liba, Modeling and optimization of nZnO/pSi heterojunction using 2dimensional numerical simulation, Eur. Phys. J. Appl. Phys. 90, 10101 (2020)
All Tables
The variation of J_{sc}, V_{oc}, FF and η with carrier concentration of ZnO/Si solar cell heterojunction.
All Figures
Fig. 1 Schematic structure of ZnO/Si heterojunction. 

In the text 
Fig. 2 Energy band diagram of ZnO/Si heterojunction. 

In the text 
Fig. 3 for the ZnO/Si heterojunction. 

In the text 
Fig. 4 H(I) for the ZnO/Si heterojunction. 

In the text 
Fig. 5 IV characteristics of ZnO/Si heterojunction for different carrier concentrations. 

In the text 
Fig. 6 Evolution of saturation current versus carrier concentration (a), reverse saturation current for limits carrier concentrations 5 × 10^{17} and 5 × 10^{21} cm^{−3} (b). 

In the text 
Fig. 7 Evolution of ideality factor versus carrier concentration. 

In the text 
Fig. 8 Evolution of series resistance versus carrier concentration. 

In the text 
Fig. 9 IV characteristics of ZnO/Si heterojunction for different ZnO thickness. 

In the text 
Fig. 10 Evolution of series resistance as a function of ZnO thickness. 

In the text 
Fig. 11 Evolution of ideality factor as a function of ZnO thickness. 

In the text 
Fig. 12 Evolution of saturation current as a function of thickness. 

In the text 
Fig. 13 JV characteristics of ZnO/Si heterojunction for different carrier concentrations. 

In the text 
Fig. 14 Variation of fill factor and efficiency as a function of carrier concentrations. 

In the text 
Fig. 15 External Quantum Efficiency for different carrier concentrations. 

In the text 
Fig. 16 JV characteristics of ZnO/Si heterojunction for different ZnO thicknesses. 

In the text 
Fig. 17 External Quantum Efficiency for different ZnO thicknesses. 

In the text 
Figure 18 Variation of fill factor and efficiency as a function of ZnO thicknesses. 

In the text 
Fig. 19 JV characteristics of ZnO/Si heterojunction for different minority carrier lifetimes of bulk Si. 

In the text 
Fig. 20 Variation of fill factor and efficiency as a function of minority carrier lifetimes of bulk Si. 

In the text 
Fig. 21 External Quantum Efficiency for different minority carrier lifetimes of bulk Si. 

In the text 
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