Physical properties enhancement of porous silicon treated with In2O3 as a antireflective coating

Results in Physics 12 (2019) 1716–1724

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Physical properties enhancement of porous silicon treated with In2O3 as a antireflective coating

T

⁎

Afef Harizia,c, Fakher Laatarb,c, , Hatem Ezzaouiab a

Laboratoire de Photovoltaïque et Matériaux Semiconducteurs, ENIT-Université Tunis ElManar, BP 37, Le belvédère, 1002 Tunis, Tunisia Laboratory of Semiconductors, Nanostructures and Advanced Technology, Center for Research and Technology Energy, Tourist Route Soliman, BP 95, 2050 HammamLif, Tunisia c Physics Department, Faculty of Education of Afif, Shaqra University, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Porous silicon Indium oxide Reflectivity Photoluminescence Spectroscopic ellipsometry Lifetime

In this work, we investigate the effect of Indium Oxide (In2O3) on the microstructural, optical, optoelectrical, and electrical properties of Porous Silicon (PS) layer. PS film was prepared by electrochemical anodization technique. In2O3 thin film was coated onto PS layer by using simple chemical immersion method. The investigation showed a significant enhancement in the optoelectronic property of PS coated with In2O3. The In2O3/ PS layer was thermally annealed to improve the efficiency of the photovoltaic cells. Surface morphology and chemical composition modification of In2O3/PS were analyzed by atomic force microscopy (AFM) and Fourier transfer infrared (FTIR) spectroscopy. Optical reflectivity of sample In2O3/PS decreases significantly from 30% to 2.4% owing to an improvement in the light absorption. The optical parameters such as refractive index (n) and extinction coefficient (k), real and imaginary dielectric constants (ε1 & ε2), as well as the components percentage and thickness of samples were determined by analyzing the TanΨ and CosΔ ellipsometric parameters. Photoluminescence (PL) analysis of PS layer treated with In2O3 before and after annealing indicated a bleu emission shifts due to the quantum confinement effect of the oxidized silicon nanocrystals. Annealed PS coated In2O3 resulted a significant improvement in the minority carrier lifetime (τeff) from 2 to 16.4 μs. The optoelectronic and electronic quality of the treated PS layer with In2O3 was enhanced noticeably compared to the untreated porous layer, making it an ideal candidate for the solar cell applications.

Introduction Porous silicon (PS) has aroused a great interest due to its large advantages for many applications such as solar cells [1], opto-electronic [2–5], chemical [6–8], biological sensors [9–11], and biomedical devices [12]. However, the surface of PS layer can be oxidized in natural atmosphere which causes the change in chemical structure and optical properties [13,14]. In literature, several metals have been used for surface passivation to solve the mentioned problem above. These materials include transparent conducting metals and metal oxides such as Lithium [15], Iron Oxides [16,17], Ru-doped SnO2 and TiO2 [18], Aland Ga-doped ZnO [12,19], Indium Tin Oxide (ITO) [13,14] and semitransparent metals such as silver (Ag) and gold (Au) [20,21]. Several deposition methods have been used for deposing different metals on PS layers, including thermal evaporation, sputtering, electrochemical, electrolysis and immersion plating. Various chemical and physical

methods have been used to fabricate In2O3 thin films, such as laser deposition [20], atomic layer deposition [21], electrochemical deposition [22], sputtering [23], and chemical vapor deposition [24]. These different deposition methods have been widely used for thin film preparation due to accurate control of deposition parameters [25–29]. Howeover, only specified thin film area can be obtained due to the limitation of equipements of each deposition technique [14–21]. In contrast, thin film deposition via chemical routes such as chemical bath, spin-coating, and immersing methods are competitive alternatives to the conventional physical deposition techniques. It was reported that the immersion plating is a suitable process for large-area deposition of nanoparticles. This method is very easy and more practical than other methods. Indium oxide (In2O3) is an attractive semiconducting material due to its wide band-gap (3.7 eV) and their high transmittance in the visible range (> 90%) [28], which are used for many applications in nano-

⁎ Corresponding author at: Laboratory of Semiconductors, Nanostructures and Advanced Technology, Center for Research and Technology Energy, Tourist Route Soliman, BP 95, 2050 Hammam-Lif, Tunisia. E-mail address: [email protected] (F. Laatar).

https://doi.org/10.1016/j.rinp.2019.01.076 Received 26 December 2018; Received in revised form 26 January 2019; Accepted 26 January 2019 Available online 01 February 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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devices, such as solar cells [29,30], optoelectronic [31], and gas sensors [32–34]. Several investigations of the origin and mechanism of light emission of In2O3 thin film in glass and silicon substrates have been carried out by PL studies. But, the influences of this metal oxide on the photoluminescence, optoelectrical, and electrical properties of PS layer are no studied yet. In this work, we used In2O3/PS as a new antireflection film structure and a front surface passivation onto c-Si based solar cells. We investigated the effects of the treated PS with In2O3 nanocrystals which leads to reduce of the reflectivity in the front surface and improve of the emission PL intensity, as well as the enhancement of the optoelectronic and electrical properties of the c-Si solar cells. In addition, we have determined some optical parameters such as the refractive index (n), the extinction coefficient (k), the real and imaginary parts of dielectric constants (ε1 and ε2) of the PS treated with indium oxide by using the spectroscopic ellipsometry (SE) technique. Characterizations Fig. 1. XRD patterns of PS substrate, as-deposited and annealed samples of In2O3/PS.

The crystallographic structure was carried out by XRD technique using a Bruker D 8 advance X-ray diffractometer with CuKα radiation (λCuKα = 1.5405 Å) for 2θ values in the range of 20–65°. The composition of PS and In2O3/PS samples was analyzed by Fourier transfer infrared (FTIR) spectroscopy in the 2200–400 cm−1 spectral range using a 560 Nicolet-Magna spectrometer. The surface morphology of the prepared samples were investigated by means of atomic force microscopy (AFM) in tapping mode configuration using a Nanoscope III microscope (Veeco AFM head RTESP silicon pur) to scan an area of 2μm × 2μm . The optical reflectivity spectra of samples was examined in the wavelength range 300–1200 nm using a UV/VIS/NIR Perkin-Elmer (Lambda 950) spectrophotometer. Photoluminescence analyses were carried out at room temperature and were registered on a PL-LS 55 Perkin-Elmer spectrometer using a Xe light source with an excitation of 448 nm. The films thickness and porosity or void% for samples were measured by a scientific Jobin Yvon Uvisel Ellipsometer by means of Bruggeman effective medium approximation (EMA) model in the 1.5–5 eV spectral range at an incidence angle of 70°. Furthermore, the effective minority carrier lifetime (τeff) was identified using the photoconductivity method by means of a Sinton WTC-120 set-up. The τeff values are obtained as a function of excess carrier density via the following steady-state condition:

τeff =

Δn G

typical experiment, two drops of 0.02 M indium (III) chloride butahydrate (InCl3·4H2O dissolved in 0.12 M hydrochloric acid solution) aqueous solution were added onto a PS layers at room temperature. Finally, the prepared sample of In2O3/PS was annealed at 200 °C in a closed furnace under air atmosphere for 90 min. Results and discussions X-ray diffraction study The crystallographic properties of PS and In2O3/PS layers before and after annealing has been measured by XRD technique in the range 20-65°. From Fig. 1, XRD spectra of the PS sample exhibited five diffraction peaks at 33.02°, 47.78°, 54.61°, 56.38°, and 61,73° corresponding to (2 0 0), (2 2 0), (1 2 0), (3 1 1), and (3 2 0) planes of c-Si substrate. From this figure, the XRD pattern of the prepared In2O3/PS presented three diffraction peaks at 30.30°, 35.43°, and 51.08° that corresponding to (2 2 2), (4 0 0), and (4 4 0) planes of In2O3 cubic structure (Ia3¯ space group) [JCPDS card No. 71-2194]. The position of these peaks indicates that the deposited In2O3 thin film on PS has a cubic structure with a lattice parameter a = 10.108 Å which is in good agreement with the value (a = 10.11 Å) as reported in JCPDS (712194). Furthermore, XRD patterns have been used to determine the average crystallites size (D), the micro-strain (ε), the dislocation density (δ), and stacking fault probabilities (α). The average nanocrystallites size of In2O3/PS layers before and after annealing were estimated using the Debye-Scherrer’s equation [35]:

(1)

where Δn and G are the injection minority carrier density and the generation rate of pairs electrons–holes in the samples, respectively. The I–V characteristic of prepared samples was measured by using twocontacts by means of printing silver as the front contact and silveraluminum as the back contact. I-V measurements were determined using source meter Keithley 2400 under illumination (100 mW/cm2).

D= Experimental details

kλ β cos θ

(2)

where D is the nanocrystallite size, λ is the wavelength of the X-ray radiations (1.5406 Å), β is the measured full-width at half maximum (FWHM) of the diffraction peak, θ is the Bragg's diffraction angle at the peak position, and k is a constant usually taken equal to 0.9. Table 1 presents the average crystallites size (D) of In2O3/PS before and after annealing. The obtained results indicate that the nanocrystallites size of In2O3 increased from 20 to about 26 nm after annealing, which confirms the obtained values by AFM measurements. The increase in the crystallite size is probably due to the decrease of void percent and the coalescence of small nanocrystals inside the pores during the annealing process, and this reason could be also enhance the crystallization in the film structure. The stresses have important effects on the structural properties of the In2O3/PS films, which results a geometric mismatch between crystalline lattices of In2O3 and PS thin

The study was carried out on p-type boron doped monocrystalline silicon substrate with a cristallographic orientation of (1 0 0) and a resistivity of 1–2 Ω.cm. PS layer was elaborated by a two-step anodizing process using electrochemical anodization in a solution of HF(20%)/ C2H5OH (1:1) under stirring at a constant current density of 5 mA/cm2 for an etching time of 3 min. The initial PS formed on the silicon substrate was removed by chemical etching using a NaOH solution and followed by rinsing in bi-distilled water and drying under nitrogen. The second electrochemical anodization step was performed in same condition as the first step in order to obtain a PS layer with an homogeneous pores structure. After anodization process, the samples were placed in ethanol solution for 10 min to remove the residual HF and followed by cleaning in bi-distilled water and drying under air. In a 1717

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Table 1 Microstructural parameters of In2O3/PS films before and after annealing. Sample treatment

2θ (degree)

FWHM β (°)

Crystallite size D (nm)

Dislocation density δ (1015 Lines/m2)

Microstrain ε (103)

Stacking fault probability α (10−2)

In2O3/PS Annealed In2O3/PS

30.59 30.32

0.41 0.32

20.08 25.72

2.48 1.51

6.54 5.15

92.6 24.3

layer and In2O3/PS before and after annealing. The vibration bonds at about 1068 cm−1 corresponds to stretch modes of SieOeSi. While the vibration bonds centered at 842 cm−1 and 977 cm−1 are attributed to scissors mode SieH2 and wagging mode O3SieH, respectively. All the peaks obtained in Fig. 2 are in compliance with the previous data mentioned in the literature [35–40]. Important changes are observed in the FTIR spectrum after treatment of PS with In2O3. These changes are pronounced particularly in SieH and SieO absorption peaks. We observe an increase in the peak intensity of the stretching mode SieOeSi located at 1068 cm−1. Moreover, the peaks intensity of SieHn (n = 1; 2; 3) in the range 2000–2300 cm−1 decreased in the case of as-deposited In2O3 on PS layer. After annealing In2O3/PS, the intensity of SieO bonds related asymmetric stretching modes and located in the range 1000–1300 cm−1 increased significantly, while the peak intensity of SieH2 located at 2194 cm−1 revealed a great decrease. The increase in the intensity and width of the absorption peak related SieOeSi and the decrease of the peak of SieH2 were presumably caused by the hydrogen atoms replacement with oxygen and/or indium atoms. As reported in the literature, the metal–oxygen–silicon bonding is located in the wavenumber range 300–700 cm−1 [41,42]. The new vibration mode that appeared at 790 cm−1 after treatment of PS with In2O3, corresponds to the OeSieO bending mode [43]. The vibration mode observed at 883 cm−1 after annealing can be attributed to the formation of IneO bond [44]. After the preparation and annealing samples In2O3/PS, the FTIR measurement shows the appearance of new peaks centered at about 459 cm−1 and 1625 cm−1, which can be assigned to the IneOeSi and IneSi bonds, respectively. These new peaks could be the result of an interaction between the In2O3 and PS interfaces. FTIR measurement indicates that SieO bonds take place instead of the unstable SieH bonds after treatment of PS with In2O3. Therefore, the increase in the absorption peak intensity of IneOeSi after annealing of the sample can be attributed to the increase in the concentration of oxygen atoms and this is confirms the formation of indium oxide on PS layer.

layers. The micro-strain and average dislocation density of In2O3/PS layers have been calculated using the following relations [36,37]:

ε=

β 4 tan θ

(3)

δ=

1 D2

(4)

As shown in the Table 1, the microstrain (ε) decreases from 6.54 × 103 to 5.15 × 103 after annealing temperature of In2O3/PS layers, leading to the decrease in the lattice imperfections. This could also demonstrate the increase in crystallites size. Furthermore, the dislocation density (δ) is decreased from 2.48 × 1015 to 1.51 × 1015 lines/m2 because stresses are released in the volume of sample during annealing process. This is an error in the wording of sentence. I want say “From these results and analyses, we noticed the improvement in the crystalline quality of In2O3/PS thin layers after annealing. The stacking fault probabilities (α) were estimated from the peak shifts of the X-ray lines of the In2O3/PS layers as referenced in the JCPDS cards No. 712194, using the following relation [38]:

2π 2 ⎤ Δ(2θ) α=⎡ ⎢ ⎣ 45 3 tan θ ⎥ ⎦

(5)

Table 1 shows that the stacking fault probability is reduced from 92.6 × 10−2 to 24.3 × 10−2 which indicate that the atoms of In2O3 nanocrystals become more able to diffuse to their correct positions inside the pores of PS layer. FTIR spectroscopic analysis In this work, FTIR spectroscopy was performed to reveal the chemical bond structure of samples. Fig. 2 shows the FTIR spectra of PS

AFM analysis Fig. 3 shows the AFM images of the surface morphology of untreated PS layer and In2O3/PS before and after annealing. From Fig. 3a, we observe a good arrangement of cylindrical micropores on the surface of PS layer with an average pores diameter of 60 nm. Fig. 3b shows AFM micrograph of the PS layer coated with In2O3 nanocrystals. This figure exhibits a homogeneous distribution of In2O3 nanocrystals that covered all the surface of PS layer. The AFM microgaph of the annealed In2O3/PS revealed the distribution of In2O3 clusters (agglomeration of small sized particles) with some voids percentage, such as seen from Fig. 3c. As known, the temperature in the annealing process enhances the mobility of molecules and/or ions in the layers, which may induce the crystallization and the change of surface morphology of the layer [45]. The instrumental AFM scan of the surface of sample before and after annealing indicates the increase of surface roughness from 5.3 to 7.8 nm and the mean crystallites size from 18 to about 25 nm. The increase of both surface roughness and crystallite size indicates the improvement of crystals quality of thin layers. Fig. 2. FTIR spectra of of PS, as-deposited and annealed In2O3/PS layers. 1718

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Fig. 3. 3-Dimonsionnal AFM micrographs of (a) PS layer, (b) as-prepared In2O3/PS, and (c) annealed In2O3/PS.

Total reflectivity analysis The reflectivity spectra of untreated PS layer, and In2O3/PS layers before and after annealing, are enregistred in the wavelength region 300–1200 nm, as shown in Fig. 4. Annealing temperature has a significant effect on the surface morpholgy and optical property of the In2O3/PS films. Therefore, the annealing process activates the diffusion of In2O3 inside the PS layer [15,46,47], which may lead to enhancing the surface passivation quality of the PS layer. The porosity of the PS layer reduces the reflection loss, allowing the light trapping within the cell by internal multi-reflection. Furthermore, the treated PS layer with Indium oxide shows a significant reflection loss in the visible light range. Annealing sample results the reduction of surface reflectivity from 30% to 2.4% in the wavelength range 300–500 nm. The significant reduction in the reflectivity of annealed In2O3/PS compared to the untreated PS may be due to the textured surface enhancement. The important decrease of the reflectivity is related to the surface morphology and roughness of prepared samples, as discussed in the AFM

Fig. 5. Ellipsometric optical models of the multilayer structure of (a) In2O3/PS and (b) PS.

study. Therefore, this antireflection coating can be used as a light trapping to improve the efficiency of the photovoltaic cells.

Spectroscopic ellipsometry SE measurements To study the influence of the treatment of PS on their optical properties, we concentrated on the spectroscopic ellipsometry measurements. A physical model is necessary to determine the optical parameters such as refractive index “n”, the extinction coefficient “k”, and the dielectric constant “ε1” and “ε2”, as well as the chemical composition of each sub-layer by analyzing the TanΨ and CosΔ ellipsometric parameters. To determine the required parameters, the upper porous samples were considered as a mixture of Si/void (for PS) and In2O3/void (for In2O3/PS) according to the Bruggeman effective medium approximation (EMA) model [48,49], such as seen from Fig. 5. The analysis of the ellipsometric measurements needs a suitable optical model to describe precisely the samples structures. In our case, the constructed model is based partially on the foregoing works [50–53]. Fig. 5 ullistrates the ellipsometric optical models of PS before and after deposition of In2O3. These optical models are constituted of four layers, and each sublayer is formed by a thickness and a volume fractions of components. The optical model of PS reveals that the four layer are formed by Si and void components (see Fig. 5b). As shown in Fig. 5a, the optical model of In2O3/PS indicates that the first thin layer is composed by Si surrounded by void. The second and third layers consist of Si, In2O3 and void. The last layer is formed by the In2O3 particles separated by micro-voids. The fitted TanΨ and CosΔ of the multilayer structure using the optical models were shown in Fig. 6. This figure exhibits the measured theoretical and experimental ellipsometric parameters of samples PS and In2O3/PS before and after annealing. The showed oscillations as function of photon energy are due to the interference phenomena resulted by the multireflection in the samples. In

Fig. 4. Reflectivity spectra of PS, as-deposited and annealed In2O3/PS layer structure. 1719

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Fig. 6. SE spectra of the ellipsometric angles tanΨ and cosΔ and their mathematical fittings calculated using optical model of the PS and In2O3/PS layers.

datas of the samples, such as shown in the Fig. 6. The spectra exhibit a close experimental measurements to the theoretical ones which allowing to determine the exact thickness of the layers suggested by the model. The layer thickness, volume fraction of components and the RMSE values are illustrated in Table 2. SE analysis allows to deduce the evolution of optical parameters n and k with the treatments of PS layer (see Fig. 7). As shown in the Fig. 7a and b, the refractive index n of all samples increase as function of photon energy upto 2.8 eV. Afterthat, we can observed the increase of the refractive index of annealed In2O3/ PS compared to the untreated PS layer from 3.74 to 4.48 at the photon energy range 2.8–3.9 eV. Finally, the refractive index of each sample decrease continuously up to low values at the higher photon energies. Fig. 7b shows the increase of extinction coefficient k for each samples as function of photon energy. From this figure, we can also observe the increase of extinction coefficient k for the treated samples comparing to the untreated one. It is well known that the refractive index n and

UV range, the spectra of samples In2O3/PS show an abrupt attenuation of absorption at ∼3.7 and ∼3.9 eV, which corresponds to the optical gaps of In2O3. The fit quality of the optical model is extremely good, which indicating that the root means square error (RMSE) is very weak. The RMSE is given by the following relation [50,51]:

RMSE =

1 2N − P − 1

N

∑ [(tan ψjm − tan ψjs )2 + (cos Δmj − cos Δsj )2] j=1

(6) where N is the number of measured ψ and Δ pairs spectrum, P is the number of fitted parameters in the optical model, and m and s refer to measured and simulated spectra, respectively. The RMSE value of the treated PS sample was found between 5% and 7%, indicating a best fit [54,55] (see Table 2). Therefore, the ellipsometric theoretical fits are in good agreement with the experimental 1720

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Table 2 Minority carrier lifetime τeff and minority surface recombination Seff values of untreated PS, and In2O3/PS before and after annealing. Volume fraction of composition

Layer 1

Layer 2

Layer 3

Layer 4

Untreated PS Thickness (520 nm) RMSE (0.054)

Si Void

94% 6%

82% 18%

48% 52%

30% 70%

In2O3/PS Thickness (638 nm) RMSE (0.062)

In2O3 Si Void

0% 94% 6%

4% 82% 14%

38% 16% 46%

66% 0% 34%

Annealed In2O3/PS Thickness (626 nm) RMSE (0.072)

In2O3 Si Void

0% 96% 4%

6% 76% 8%

40% 20% 40%

78% 0% 12%

Fig. 8. Imaginary and real parts (ε1 and ε2) of the dielectric functions for PS, unannealed and annealed In2O3/PS.

Fig. 7. Refractive index and extinction coefficient measurements of the PS layer and In2O3/PS structure before and after annealing.

∼4.2 eV, which corresponds to the conversion area from anomalous to normal dispersion. Whereas, the bands positions were shifted to a lower wavelength arround 275 nm (∼4.5 eV), as the imaginary part ε2 is largely follow the evolution of the curve of extinction coefficient (see Fig. 8b). In addition, the amplitude of the real dielectric function of annealed In2O3/PS around 3.5 eV indicates an increase of both crystallite size and crystallinity of In2O3 layer. The good quality of structural and optical properties of the sample In2O3/PS making it a potential substrate for solar cells and optoelectronic applications.

extinction coefficient k depend heavily on the layer porosity. As given in Table 2, the void percentages in the layers decreases after treatments of the PS sample with indium oxide and also after annealing temperature. The decrease of void percentage and the increase of volume fraction of In2O3, as well as the decrease of layer thickness after annealing sample induces the improvement of the sample crystallization. Fig. 8a and b exhibits the photon energy dependency of the real and imaginary parts (ε1 and ε2) of dielectric constant. It is noticed that the values of ε1 and ε2 of the treated PS layer become higher than that of the freshly porous layer. Also, obvious bands have been showed in both graphs. The bands position in the real dielectric ε1 constant is located at 1721

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Table 3 Parameters extracted from the SE spectral-fitting for the PS and In2O3/PS layers. RMSE are reported in the same table. Sample treatment

Effective minoritary carrier lifetime τeff (μs)

Effective surface recombination Seff (cm·s−1)

PS In2O3/PS Annealed In2O3/ PS

1.85 5.65 16.4

5405 336 245

layer. The annealed sample In2O3/PS shows also an improvement in the τeff from 1.85 μs to 16.4 μs, due to the surface passivation and the minimization of the dangling bonds density at the c-Si surface. Table 3 indicates that the superior limit of the surface recombination velocity has been diminished noticeably. The Seff in the PS sample have been decreased from 5405 cm·s−1 to 245 cm·s−1 in the annealed sample of In2O3/PS. The values of Seff indicate an enhancement in the surface quality of PS sample, which can be due to the saturation of dangling bonds and the surface passivation of the PS layer. Mainly, these enhancement are due to the coordination of In atoms to the Si atoms through the oxygen (IneOeSi bonds) as indicated in the FTIR analysis.

Fig. 9. PL spectra of PS, as-deposited and annealed In2O3/PS.

PL spectroscopic analysis The PL spectra of PS and In2O3/PS layers before and after annealing are presented in Fig. 9. As shown in this figure, the PL measurement of the untreated PS layer exhibits a PL band located at 1.85 eV. In Fig. 9, the PL analysis reveals an increase in PL-peak intensity and a bleu shift from 1.88 eV (for In2O3/PS) to 1.9 eV (for annealed In2O3/PS). The origin of these evolutions can be attributed to the increase in the concentration of oxygen atoms incorporated into In2O3/PS layer structure and the quantum confinement effect of the oxidized silicon nanocrystals. The enhancement of the PL spectra of sample In2O3/PS is due to the surface passivation improvement and the decrease of recombination activities. The thermal treatment promote the decrease in the void concentration of assembly In2O3/PS, which resulting an enhancement of the crystals quality of layers [58]. We can also attribute the shift to higher energy by the trapped electrons at the located states owing to Si]O bond of PS layer [36,56,57] and the substitution of hydrogen atoms by the oxygen atoms [58,59] and the oxidized indium forming IneOeSi according to the FTIR analysis.

I-V characteristic measurement The current-voltage characteristics (I-V) of the treated PS layer based c-Si solar cell has been measured under illumination such as seen from Fig. 10. The different electrical parameters of silicon solar cell devices such as open circuit voltage (VOC), short circuit current (ISC), fill factor (FF) and the conversion efficiency (η) were determined from the I-V curves and presented in the Table 4. This table shows an increase in the values of VOC which indicating the enhancement of the treated PS sample with indium oxide. As consequently, due to the filling of PS pores by In atoms, both short circuit current density (JSC) and open circuit voltage (VOC) were increased from 74 to 80 mA/cm2 and from 420 to 450 mV, respectively. The increase of the JSC and VOC after treatement of PS layer are due to the recombination surface reduction. The conversion efficiency (η) of the treated PS based solar cell was enhanced from 6.7% to 9.3%, as presented in the Table 4. In this work, the enhancement of the I–V characteristics is due to both effects: the antireflection coating and the surface passivation by the indium oxide treatment.

Minority-carrier lifetime measurements To study the effect of the treatment of PS on the electronic quality of the Si wafer, we measured the effective carrier lifetime (τeff) of PS layer and indium oxide treated PS sample before and after annealing. The τeff is measured using the WCT-120 photoconductance tester in the quasisteady mode which is expressed as following [60]:

2Seff 1 1 = + τeff τbulk W

(7)

where τbulk is the effective carrier lifetime of c-Si bulk, Seff is the effective surface recombination velocities, and W is the thickness of the samples. Generally, the Seff is closed to about 0 cm/s when a good surface passivation has performed. Therefore, the effective lifetime become dominated by intrinsic recombination states, which reducing the value of Seff that could be expressed as following:

Seff ⩽

W 2τeff

(8)

Table 3 shows the enhancement of the τeff and Seff after annealing the treated PS with In2O3. For the treated sample of PS layer with In2O3, the τeff is increased significantly compared to the PS substrate, which could be attributed to the increase of the defects onto the porous silicon

Fig. 10. I-V characterisctic of PS, In2O3/PS before and after annealing. 1722

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Table 4 Electrical parameters of obtained solar cell. Sample treatment

JSC (mA/cm2)

VOC (mV)

FF

η (%)

PS In2O3/PS Annealed In2O3/PS

74 77 80

420 428 450

73 75 77

6.7 7.8 9.3

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Conclusion In this work, we have investigated the influence of treatment of PS layer with indium oxide on the optical optoelectronic and electrical properties of c-Si based solar cells. Annealing In2O3/PS sample reduces the reflectance from 30% to 2.4%. FTIR spectroscopy analysis have shown the increase SieOeIn bond intensity after annealing In2O3/PS film structure, and found to be a replacement of SieH bonds. A modification in the surface morphology of PS layer after treatment with In2O3 is observed. Annealing temperature of In2O3/PS layers leads to a noticeable improvement in carrier lifetime from 1.85 μs to 16.4 μs. We have also demonstrated that the annealed PS layer coated In2O3 film enhances the PL intensity due to the decrease of the voids density and the improvement of crystals quality of films. The decrease of voids percentage of the film structure enhances the crystalline quality of layers, and this is due to the decrease in the microstrain and the dislocation density. Optical reflectivity of In2O3/PS was decreased significantly from 30% to 2.4% owing to the improvement in the light absorption, which resulting the increase of the refractive index and extinction coefficient. We found that the treated PS layer with indium oxide acts as an efficient antireflection coating. The annealing sample In2O3/PS layer structure leads to dangling bonds passivation at the surface. The conversion efficiency of the PS coated In2O3 was enhanced from 6.7% to 9.3%. The optoelectronic and electronic quality of the treated PS layer with indium oxide film was enhanced noticeably, making it an ideal candidate for the solar cell applications. Acknowledgements This work was funded by Ministry of Higher Education and Scientific Research of Tunisia. References [1] Hang BR, Yang YK, Lin TC, Yang WL. A simple and low-cost technique for silicon nanowire arrays based solar cells. Sol Energy Mater Sol Cells 2012;98:357. [2] Ahmadi MT, Lau HH, Ismail R, Arora VK. Current–voltage characteristics of a silicon nanowire transistor. Microelectron J 2009;40:547. [3] Collins RT, Fauchet PM, Tischler MA. Porous silicon: from luminescence to LEDs. Phys Today 1997;50:24. [4] Lazarouk S, Jaguiro P, Katsouba S. Stable electroluminescence from reverse biased n-type porous silicon–aluminum Schottky junction device. Appl Phys Lett 1996;68:2108. [5] Steiner P, Kozlowski F, Lang W. Light-emitting porous silicon diode with an increased electroluminescence quantum efficiency. Appl Phys Lett 1993;62:2700. [6] Zhang GJ, Ning Y. Silicon nanowire biosensor and its applications in disease diagnostics: a review. Anal Chim Acta 2012;749:1. [7] Foucaran A, Pascal-Delannoy F, Giani A, Sackda A, Combette P, Boyer A. Porous silicon layers used for gas sensor applications. Thin Solid Films 1997;297:317. [8] Sailor MJ, Link JR. “Smart dust”: nanostructured devices in a grain of sand. Chem Commun 2005;94:1375. [9] Chan S, Fauchet PM, Li Y, Rothberg LJ, Miller BL. Porous silicon microcavities for biosensing applications. Phys Status Solidi A 2000;182:541. [10] Lin VSY, Motesharei K, Dancil KS, Sailor MJ, Ghadiri MR. A porous silicon-based optical interferometric biosensor. Science 1997;278:840. [11] Thust M, Schoning MJ, Frohnhoff S, Arens-Fischer R. Porous silicon as a substrate material for potentiometric biosensors. Meas Sci Technol 1996;7:26. [12] Coffer JL, et al. Porous silicon-based scaffolds for tissue engineering and other biomedical applications. Phys Status Solidi A 2005;202:1451. [13] Kim HS, Kim MG, Ha YG, Kanatzidis MG, Marks TJ, Facchetti A. Low-temperature solution-processed amorphous indium tin oxide field-effect transistors. J Am Chem Soc 2009;131:10826–7. [14] Park JS, et al. Effects of Zn content on structural and transparent conducting properties of indium–zinc oxide films grown by rf magnetron sputtering. J Vac Sci

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