ZnO:In Solar Cell Structure

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 84 (2015) 214 – 220

E-MRS Spring Meeting 2015 Symposium C - Advanced inorganic materials and structures for photovoltaics

Electrical study of Si/PS/ZnO:In solar cell structure H. Belaida, M. Nouiria, Z. Ben Ayadia, K. Djessasb, L. El Mira,c,* a

Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciences in Gabes, Gabes, Tunisia. b Laboratoire de Mathématiques et Physique des Systèmes (MEPS), Université de Perpignan, 52, avenue Paul Alduy, 66860 Perpignan Cedex, France. c Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Departement of Physics, Riyadh 11623, Saudi Arabia.

Abstract Indium doped zinc oxide (IZO) thin films have been grown on porous silicon, in order to develop solar cell structure, by rf-sputtering at room temperature using indium (4 at %) doped nanocrystalline powder previously synthesized by the sol-gel method. Such layers are polycrystalline with a hexagonal wurtzite structure with a thickness of about 400 nm and preferential orientation in (002) crystallographic direction. Electrical study under illumination reveals that this type of nanostructure is promising for solar cell application. By comparing Si/PS/IZO and Si/PS/ZnO/IZO structures, we illustrate a satisfactory photovoltaic conversion with these structures. Also we reveal that interposing an undoped ZnO layer leads to a degradation of PV parameters, specially the short-circuit current (Isc) was affected. In fact, Isc decreases from 3 mA to 0.8 mA by adding the undoped ZnO thin layer. The open circuit voltage is less affected. A decay of 30 mV was recorded. C-V analysis gave a carrier density of about 1016 cm-3 for both cells. © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The European Materials Research Society (E-MRS) Peer-review under responsibility of The European Materials Research Society (E-MRS) Keywords: Nanostructure, Zinc oxide, porous silicon, solar cells, sputtering.

* Corresponding author. Tel.: +21697408756; fax: +21675392421. E-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The European Materials Research Society (E-MRS) doi:10.1016/j.egypro.2015.12.316

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1. Introduction Transparent conductive oxide films (TCOs) have been extensively investigated due to their broad range of applications such as transparent electrodes in solar cells and in photovoltaic devices [1,2]. Most TCOs are based on SnO2, In2O3, ZnO and their mixed compounds. Among the available TCOs, ZnO reveals itself as a potential candidate for optoelectronic applications, and especially for solar cell, where it is used as a transparent and conducting top layer. Zinc oxide is II–VI compound semiconductor with wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV [3] at room temperature, with high transmittance in the visible, exceeding 90%, and a low resistivity, ranging from 103 to 104 Ω.cm for undoped ZnO films [4]. Undoped ZnO is usually an n-type semiconductor due to the presence of unintentionally introduced donor centres, usually identified as zinc interstitials or oxygen vacancies. However, experiments have been inconclusive as to which of these is the predominant defect [5]. Indium, as well as aluminum, is used for intentional n-type doping. Many different techniques have been used in the preparation of ZnO thin films, such as physical vapour deposition (PVD) [6], metal-organic chemical vapour deposition (MOCVD) [7], spray pyrolysis [8], sputtering [9], pulsed laser deposition (PLD) [10], ink-jet printing [11], electrochemical deposition [12], sol-gel technique [13]. Day by day this last technique is gaining territory in the synthesis of nanoparticules, due to several advantages: easily proportion and low cost. The essential novelty in our work is the coupling of the sol-gel and sputtering techniques. The sputtered target is a pellet of a ZnO sol-gel nanoparticules [14]. Earlier work resolves structural and electrical properties of thin film ZnO onto glass substrates [15]. The integration of silicon (Si) based structure allows for new opportunities. ZnO/Si heterojunction are of particular interest in optoelectronic devices. Much work has already been presented about ZnO thin layer elaboration onto Si substrates [16]. They reported several difficulties caused specially by the large lattice mismatch between Si and the ZnO layer and the formation of amorphous SiOx layers [17]. Ajimsha et al. [18] reported the electrical characteristics of n-ZnO/p-Si heterojunction diodes grown by PLD at different oxygen pressures. Kumar et al. [19] characterized sol-gel derived yttrium-doped n-ZnO/p-Si heterostructures. As we know, a very limited number of published studies of the properties of ZnO thin films on PS substrates [20]. The application of porous silicon PS in the field of optoelectronics attracted a great deal of attention. Heterostructures based on porous silicon PS/TCO junction have been attracted a high interest since they exhibit a wide range of properties, such as light emission from a PS/SnO2 junction [21]. It was reported in this work that the electrical contact take place in the volume of the PS film, which improve the electroluminescence efficiency [22]. In our previous work we have presented the PS/ZnO:Al3% junction as a promising solar cell [23]. This paper focuses on the electrical characterization and the photovoltaic parameters of solar cell based on n-IZO/p-PS heterostructure. We present two structures: IZO/PS and IZO/ZnO/PS. We reveal the effect of interposing of the undoped ZnO as a buffer layer between IZO and PS layers on the photovoltaic efficiency. 2. Experimental details 2. 1. Sample preparation The nanocristalline IZO powder were prepared by the sol-gel method using 20 g of ZAD [Zn(CH3COO)2 .2H2O] as precursor in a 140 ml of methanol [CH 3OH :99,8%]. After 10 min of magnetic stirring at room temperature, adequate quantity of [InCl3], corresponding to [In]/[Zn] ratio between 0.01 to 0.05, was added. After few min magnetic stirring, the solution was placed in an autoclave and dried under supercritical conditions of ethyl alcohol (EtOH). Further experimental details can be found elsewhere [24-28]. IZO thin films were deposited on p-type porous silicon by rf-magnetron sputtering (13.56 MHz). The sputtering chamber was evacuated to a base pressure of 2–3×10−4 Pa before admitting the sputtered high purity argon gas

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(99.9999%) without oxygen. The sputtering targets were prepared from the aerogel powder of ZnO:In as described in the first step. The silicon substrate was etched in a 10% HF for 5 min in order to remove the eventual oxide layer that may form during stirage, then rinsed in deionized water and dried in N 2 atmosphere. During the sputtering process, for each sample (Table 1) the substrate was set at room temperature, and the target to-substrate distance was 75mm. Table 1. Sample codes of the investigated cells. Code

Type of solar cell structure

SCT1

Si(p) / PS / ZnO:In(4%) / ITO

SCT2

Si(p) / PS / ZnO / ZnO:In(4%) / ITO

Porous silicon wafer was prepared with the vapour etching (VE) technique [29], using a phosphorus-doped (100) oriented Si substrate (ρ = 2 Ω.cm, ND = 5×1015 cm-3) with mirror-polished surface. The VE method involves exposing a Si substrate to acid vapours issued from a mixture of HNO3 and HF having a concentration of 65% and 40%, respectively. In order to obtain homogeneous PS layer, we need to control the HNO 3/HF volume ratio, the distance between the silicon substrate and the acid solution in the container, the temperature of the acid mixture and the exposure time of the Si substrate to the acid vapors. 2. 2. Characterization The crystalline structure and phase purity of the aerogel were investigated by XRD using the Cu K α radiation (λ = 1.5418 Ǻ) of a Bruker D5005 diffractometer. Typical θ-2θ spectra were collected between 2θ = 20° and 70° in 0.02° steps. The surface morphology of ZnO thin layer was investigated using atomic force microscopy (AFM, TopoMetrix). Current–voltage measurements were performed using a computer controlled set-up comprising a Keithley 220 current source and an Agilent 34401A multi-meter. To illustrate photovoltaic conversion of our proposed structures, we used simply an ordinary tungsten lamp placed at a fixed distance. Data was saved after 5 min to ensure an optimal efficiency of the investigated cells. C-V measurement was performed using an Agilent 4294A impedance analyzer. The aim of this work is to compare two proposed solar cell structures, specified in Table 1. We deduce from photoI(V) characteristic the effect of interposing a thin undoped ZnO layer between the PS and IZO layers. 3. Results and discussion Fig.1. shows the XRD spectrum of the nanoparticles of IZO as elaborated in the first step for the different In concentrations from 1 at% to 5 at% .Seven pronounced diffraction peaks appear in the 10–70° 2θ range, which can be attributed to the (100), (002), (101), (102), (110), (103) and (112) planes of ZnO [30]. This result indicates that ZnO:In nanopowder has a polycrystalline hexagonal wurtzite structure. The average grain size was calculated using Scherrer’s formula [31]:

G

0,9O B cosT B

(1)

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where λ is the X-ray wavelength (1.78901 Å), θB is the maximum of the Bragg diffraction peak and B is the linewidth at half maximum (in radians). The average grain size of the basal diameter of the cylinder-shape crystallites varies from 14 nm to 20 nm, whereas the height of the crystallites varies from 20 nm to 30 nm.

Fig. 1. X-ray diffraction of IZO aerogel nanoparticles.

Fig. 2. shows typical XRD spectra of doped ZnO thin films on PS substrate deposited by rf-magnetron sputtering. Thin film exhibit an intensive (002) XRD peak, indicating that they have c-axis-preferred orientation due to the self-texturing mechanism as discussed by Deng et al. [32]. They concluded that film crystal orientation is caused by minimization of the crystal surface free energy as well as by the interaction between the deposited material and the substrate surface. The 2T value of the diffraction peak (002) is located at 33.46°, which is very close to standard ZnO crystal. A small displacement of the (002) peak position was observed compared to the same peak in the powder, indicating that some residual stress inside the film may exist [33,34]. 100

(400)

(002)

80

Intensity (a. u.)

T ZnO U Si

T

U

60 40 20 0 20

30

40

50

60

70

2TCu (degree) Fig. 2. Typical X-ray diffraction of doped ZnO thin film deposited on porous silicon substrate by rf-magnetron sputtering.

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CV measurement at room temperature confirms the establishment of a pn junction through the structure. The voltage dependence of the capacitance is displayed as Mott-Schottky plot in Fig. 3. We deduce a carrier density about 1016 cm-3. This value leads us to confirm that the depletion region is established, especially through the porous silicon layer.

7,0

-2

C (10 nF cm )

6,5

-2

-3

6,0 5,5 5,0 4,5

-2,0

-1,5

-1,0

-0,5

0,0

Bias (V)

Fig. 3. Mott-Schottky plot of the capacitance of SCT2 solar cell structure.

Fig. 4. Illustrates the dark and the light J–V characteristics of the investigated structures. Corresponding curves show similar shapes to each other. Dark measurements demonstrate a good rectifying behaviour, characterized by a rectifying ratio RR about 32 and 8, with SCT1 and SCT2 structures respectively.

2

2

0 -200

Current density (mA/cm )

(b)

Obscurity Illumination

4

2

Current density (mA/cm )

(a)

0

200

-2

400

1.5

Obscurity Illumination

1.0 0.5 -100

0.0 0

100

200

300

400

-0.5 -1.0 -1.5

-4

-2.0

Polarisation (mV)

Polarisation (mV)

Fig. 4. J-V characteristics for (a): SCT1; (b): SCT2 structures.

Photo-J(V) characteristics show an obvious photovoltaic effect with the two proposed structures. The open circuit voltage is about 150 mV for SCT1 structure. By interposing an un-doped ZnO layer a decay of 30 mV was recorded.

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The same trend was found with the short-circuit current, but it is more pronounced. J SC is about 5.5 mA/cm2 for SCT1 and 1.75 mA/cm2 for SCT2, corresponding to a decrease of about 68%. It is noteworthy, with SCT2, that the light-curve intersects the dark-one, known as the crossover behaviour. This trend was reported by several authors. This can be related to defect centres at the ZnO/PS interface caused by the dangling bonds, since the large lattice mismatch between ZnO and Si. Thereby, recombination at the interface takes place and then limits the photo-current through the cell structure. Additionally, we remark, with SCT2, that the photo-characteristic seems to bend with a current saturation for large forward bias. This behaviour, known as rollover effect, was reported by Shen et al. with ZnO/p-Si junction [35] and others works [36,37]. This current-saturation was ascribed to an additional barrier at the back contact. 4. Conclusion We have fabricated ITO/IZO/ZnO/PS/Si(p) and ITO/IZO/PS/Si(p) for photovoltaic conversion. Despite the used light source was an ordinary tangsten lamp, our structures exhibit a limited photovoltaic effect as solar cell, leading us to consider them as promising solar cell structures. The structure with an undoped ZnO layer between IZO and PS layer shows a crossover behaviour of the dark and light J-V curves, which indicates the existence of recombination at the ZnO/PS interface. Thus, the photo-current is affected. A decay of about 68 % is detected. The bending and the saturation of the photo-current at high forward bias were attributed to the existence of an additional barrier at the back contact. References [1] Tonooka K, Bando H, Aiura Y. Photovoltaic effect observed in transparent p-n heterojunctions based on oxide semiconductors. Thin Solid Films 2003;445:327-331. [2] Senadeera GKR, Nakamura K, Kitamura T, Wada Y, and Yanagida S. Fabrication of highl efficient polythiophene-sensitized metal oxide photovoltaic cells. App. Phys. Lett. 2003;83:5470-5472. [3] Özgür Ü, Alivoy YI, Liu C, Teke A, Reshchikoy MA, Dogan S, Avrutin V, Cho SJ, Morkoc H. A comprehensive review of ZnO materials and devices. Journal of Applied Physics 2005;98:41301-41404. [4] Chopra KL, Major S, Pandya DK. Indium-doped zinc oxide thin films deposited by chemical spray starting from zinc acetylacetonate: effect of the alcohol and substrate temperature. Thin Solid Films 1983;102:1-46. [5] Look DC, Claflin B. P-type doping and devices based on ZnO. Phys. stat. sol. 2004;241:624-630. [6] Kim YS, Tai WP, Shu SJ. Effect of preheating temperature on structural and optical properties of ZnO thin films by sol–gel process. Thin Solid Films 2005;491:153-160. [7] Zhao JL, Sun XW, Ryu H, Moon YB. Thermally stable transparent conducting and highly infracted relective Ga-doped ZnO thin films by metal organic chemical vapor deposition. Opt. Mater. 2011;33:768-772. [8] Siva Kumar VV, Singh F, Kumar A, Avasthi DK. Growth of ZnO nanocrystals in silica by rf co-sputter deposition and post-annealing. Nucl. Instr.Method. Phys. Res. Sect. 2006;244:91-94. [9] Tao YM, Ma SY, Chen HX, Meng JX, Hou LL, Jia YF, Shang XR. Effect of the oxygen partial pressure on the microstructure and optical properties of ZnO:Cu films. Vacuum 2011;85 :744-748. [10] Zhaoyang W, Liyuan S, Lizhong H. Effect of laser repetition frequency on the structural and optical properties of ZnO thin films by PLD. Vacuum 2010;85:397-399. [11] Shena W, Zhao Y, Zhang T. The preparation of ZnO based gas-sensing thin films by ink-jet printing method. Thin Solid Films 2005;483: 382-387. [12] Lupan O, Pauporté T, Ursaki VV, Tiginyanu IM. Highly luminescent columnar ZnO films grown directly on n-Si and p-Si substrates by low-Temperature electrochemical deposition .Opt. Mater. 2011;33:914-919. [13] Maiti UN, Ghosh PK, Nandy S, Chattopadhyay KK. Effect of Mn doping on the optical and structural properties of ZnO nano/micro-fibrous thin film synthesized by sol–gel technique. Physica B 2007;387:103-108. [14] Ben Ayadi Z, El Mir L, Djessas K, Alaya S. Effect of substrate temperature on the properties of Al-doped ZnO films sputtered From aerogel nanopowders for solar cells applications. Thin Solid Films 2011;519:7572-7574. [15] Ben Ayadi Z, El Mir L, Djessas K, Alaya S. Effect of the annealing temperature on transparency and conductivity of ZnO:Al thin films. Thin Solid Films 2009;517:6305- 6309. [16] Kim MS, Kim GS, Cho MY, Kim DY, Choi HY, Jeon SM, Yim KG, Lee DY, Kim JS, Kim JS, Son JS, Lee JI, JY. Leem. Nitrogen passivation effects of Si Substrates of the properties ZnO epitaxy. J. Korean Phys. Soc. 2010;56:827-831. [17] Xiu F, Yang Z, Zhao D, Liu J, Alim KA, Balandin AA, Itkis ME, Kaddon RC. ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE. J. Cryst. Growth 2006;286:61-65.

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