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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

© 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 non-toxic 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 El-Amine [12], respectively, for n-ZnO/p-Si solar cells.

In this work, the n-ZnO/p-Si 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 p-Si 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 Atlas-Silvaco simulator.

2 Simulation details

ATLAS from silvaco international is physically-based 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 n-ZnO layer used as emitter and p-type 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 n-ZnO/p-Si heterojunction extracted from Atlas-Silvaco based on physical parameters illustrated in Table 1, with Zinc Oxide thickness of 600 nm and carrier concentration of 5 × 1017 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 drift-diffusion. The concentration dependent mobility model was used for carrier mobility. Shockley-Read-Hall (SRH) and Auger were included into the simulation for recombination models as a function of doping concentration. Thermionic emission across the n-ZnO/p-Si interface was also incorporated [13].

thumbnail Fig. 1

Schematic structure of ZnO/Si heterojunction.

Table 1

Physical parameters for ZnO/Si heterojunction.

thumbnail Fig. 2

Energy band diagram of ZnO/Si heterojunction.

3 Results and discussions

3.1 Dark parameters

The current-voltage characteristic in the dark condition is a valuable source of several parameters of the heterojunction such as series resistance Rs, ideality factor (n), barrier height φb and reverse saturation current IS.

The parameters were determined from the thermionic emission, where the current-voltage 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 Is 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 Is 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 φb is the barrier height.

thumbnail Fig. 3

for the ZnO/Si heterojunction.

thumbnail 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 × 1017 to 5 × 1021 cm−3, with Zinc Oxide thickness of 600 nm. The current-voltage (I-V) 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 turn-on voltage that decreases with the carrier concentration. This evolution is due to the decrease in diffusion voltage between n-ZnO and p-Si 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 n-ZnO 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 1020 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 n-ZnO zone, as mentioned by Belarbi [21]. The minimal value for series resistance is 213 Ω corresponding to carrier concentration of 5 × 1021 cm−3.

thumbnail Fig. 5

I-V characteristics of ZnO/Si heterojunction for different carrier concentrations.

thumbnail Fig. 6

Evolution of saturation current versus carrier concentration (a), reverse saturation current for limits carrier concentrations 5 × 1017 and 5 × 1021 cm−3 (b).

thumbnail Fig. 7

Evolution of ideality factor versus carrier concentration.

thumbnail 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 current-voltage characteristics for different n-ZnO thicknesses at a fixed charge carrier concentration equal to 5 × 1021 cm−3 (Fig. 9a). We observed a weak decrease in turn-on voltage with increasing thickness between 10 nm and 0.6 μm, but beyond 0.6 μm, we did not observe any real change in turn-on voltage.

The Figures 1012 show the evolution of the series resistance, ideality factor and saturation current as a function of n-ZnO 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 n-ZnO/p-Si heterojunction.

thumbnail Fig. 9

I-V characteristics of ZnO/Si heterojunction for different ZnO thickness.

thumbnail Fig. 10

Evolution of series resistance as a function of ZnO thickness.

thumbnail Fig. 11

Evolution of ideality factor as a function of ZnO thickness.

thumbnail 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 n-ZnO/Si heterounction, the Current density-Voltage characteristics has been simulated under standard light condition (AM1.5, 100 mW/cm2) with the same physical properties summarized in Table 1.

3.2.1 Effect of Zinc Oxide carrier concentration

From the simulated J-V curves, the short circuit current density Jsc, open circuit voltage Voc, 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 density-voltage characteristics for different n-ZnO carrier concentration, the rise of carrier density does not change the open circuit voltage significantly, however, we observes an increase of current density Jsc and then it stabilizes at 22.41 mA/cm2. This stabilization of the Jsc is probably due to the introduction of the defects accompanying the increase of the density of charge carriers in the n-ZnO film, as reported by Belarbi [11].

The variation of conversion efficiency and Fill Factor as a function of n-ZnO 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 × 1020 and 5 × 1021 cm−3.

Table 2

The variation of Jsc, Voc, FF and η with carrier concentration of ZnO/Si solar cell heterojunction.

thumbnail Fig. 13

J-V characteristics of ZnO/Si heterojunction for different carrier concentrations.

thumbnail Fig. 14

Variation of fill factor and efficiency as a function of carrier concentrations.

thumbnail 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 density-voltage 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 Jsc and a weak variation of Voc. The decrease of Jsc 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 Jsc 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 n-ZnO films become thicker.

Figure 18 shows the Fill Factor and the conversion efficiency versus n-ZnO 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 Jsc, FF and η as a function of n-ZnO thickness emitter are compatible with the results of Askari et al. [25] and Hussain et al. [11].

thumbnail Fig. 16

J-V characteristics of ZnO/Si heterojunction for different ZnO thicknesses.

Table 3

Photovoltaic cell parameters for different ZnO thickness.

thumbnail Fig. 17

External Quantum Efficiency for different ZnO thicknesses.

thumbnail 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 n-ZnO/p-Si 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 n-ZnO fixed at 5 ×1021 cm−3 and 50 nm, respectively. We note a clear improvement of solar cell parameters: Jsc, Voc, 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 Jsc. 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 Jsc.

Our optimized n-ZnO/p-Si structure had good photoelectric properties, compared to other recently published results referenced in Table 5.

Table 4

Effect of minority carrier lifetime of bulk Si on solar cell parameters.

thumbnail Fig. 19

J-V characteristics of ZnO/Si heterojunction for different minority carrier lifetimes of bulk Si.

thumbnail Fig. 20

Variation of fill factor and efficiency as a function of minority carrier lifetimes of bulk Si.

thumbnail Fig. 21

External Quantum Efficiency for different minority carrier lifetimes of bulk Si.

Table 5

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 n-ZnO/p-Si structure, it is essential to take into account the density of interface states at the junction of n-ZnO and p-Si 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 n-ZnO and p-Si. The defect distribution in this interface layer was assumed to be acceptor-like states [13,26].(7)where NTA,NGA are the density of states of tail-acceptor and deep-acceptor, respectively. EC is the minimal conduction band energy. WTA and WGA are the characteristic decay energy, taken as 0.07 and 0.2 eV, respectively. EGA 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 Jsc current and a clear deterioration of Voc 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].

Table 6

ZnO/Si performances for different interface state densities.

4 Conclusions

In this paper, n-ZnO/p-Si 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 n-ZnO/p-Si structure with low series resistance about 200 Ω, low saturation current density about 10−12 mA/cm2 and an ideality factor ranging from 1 to 1.5. Secondly, the effect of carrier concentration and thickness of n-ZnO 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 n-ZnO/p-Si heterojunction using 2-dimensional numerical simulation, Eur. Phys. J. Appl. Phys. 90, 10101 (2020)

All Tables

Table 1

Physical parameters for ZnO/Si heterojunction.

Table 2

The variation of Jsc, Voc, FF and η with carrier concentration of ZnO/Si solar cell heterojunction.

Table 3

Photovoltaic cell parameters for different ZnO thickness.

Table 4

Effect of minority carrier lifetime of bulk Si on solar cell parameters.

Table 5

Comparison of our results with recent works from literature.

Table 6

ZnO/Si performances for different interface state densities.

All Figures

thumbnail Fig. 1

Schematic structure of ZnO/Si heterojunction.

In the text
thumbnail Fig. 2

Energy band diagram of ZnO/Si heterojunction.

In the text
thumbnail Fig. 3

for the ZnO/Si heterojunction.

In the text
thumbnail Fig. 4

H(I) for the ZnO/Si heterojunction.

In the text
thumbnail Fig. 5

I-V characteristics of ZnO/Si heterojunction for different carrier concentrations.

In the text
thumbnail Fig. 6

Evolution of saturation current versus carrier concentration (a), reverse saturation current for limits carrier concentrations 5 × 1017 and 5 × 1021 cm−3 (b).

In the text
thumbnail Fig. 7

Evolution of ideality factor versus carrier concentration.

In the text
thumbnail Fig. 8

Evolution of series resistance versus carrier concentration.

In the text
thumbnail Fig. 9

I-V characteristics of ZnO/Si heterojunction for different ZnO thickness.

In the text
thumbnail Fig. 10

Evolution of series resistance as a function of ZnO thickness.

In the text
thumbnail Fig. 11

Evolution of ideality factor as a function of ZnO thickness.

In the text
thumbnail Fig. 12

Evolution of saturation current as a function of thickness.

In the text
thumbnail Fig. 13

J-V characteristics of ZnO/Si heterojunction for different carrier concentrations.

In the text
thumbnail Fig. 14

Variation of fill factor and efficiency as a function of carrier concentrations.

In the text
thumbnail Fig. 15

External Quantum Efficiency for different carrier concentrations.

In the text
thumbnail Fig. 16

J-V characteristics of ZnO/Si heterojunction for different ZnO thicknesses.

In the text
thumbnail Fig. 17

External Quantum Efficiency for different ZnO thicknesses.

In the text
thumbnail Figure 18

Variation of fill factor and efficiency as a function of ZnO thicknesses.

In the text
thumbnail Fig. 19

J-V characteristics of ZnO/Si heterojunction for different minority carrier lifetimes of bulk Si.

In the text
thumbnail Fig. 20

Variation of fill factor and efficiency as a function of minority carrier lifetimes of bulk Si.

In the text
thumbnail Fig. 21

External Quantum Efficiency for different minority carrier lifetimes of bulk Si.

In the text

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