Influence of a Boron Precursor on the Growth and Optoelectronic

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Influence of a Boron Precursor on the Growth and Optoelectronic Properties of Electrodeposited Zinc Oxide Thin Film Fabien Tsin,†,§ Angélica Thomere,‡,§ Arthur Le Bris,‡,§ Stéphane Collin,∥,⊥ Daniel Lincot,‡,§,⊥ and Jean Rousset*,†,§,⊥ †

EDF R&D, 6 quai Watier, 78401 Chatou Cedex, France CNRS, 6 quai Watier, 78401 Chatou Cedex, France § IRDEP, Institute of Research and Development for Photovoltaic Energy, UMR 7174 CNRS-EDF-Chimie ParisTech, 78401 Chatou Cedex, France ∥ LPN, Laboratory for Photonics and Nanostructures, UPR 20 CNRS, Route de Nozay, 91460 Marcoussis, France ⊥ IPVF, 8 rue de la Renaissance, 92160 Antony, France ‡

ABSTRACT: Highly transparent and conductive materials are required for many industrial applications. One of the interesting features of ZnO is the possibility to dope it using different elements, hence improving its conductivity. Results concerning the zinc oxide thin films electrodeposited in a zinc perchlorate medium containing a boron precursor are presented in this study. The addition of boron to the electrolyte leads to significant effects on the morphology and crystalline structure as well as an evolution of the optical properties of the material. Varying the concentration of boric acid from 0 to 15 mM strongly improves the compactness of the deposit and increases the band gap from 3.33 to 3.45 eV. Investigations were also conducted to estimate and determine the influence of boric acid on the electrical properties of the ZnO layers. As a result, no doping effect effect by boron was demonstrated. However, the role of boric acid on the material quality has also been proven and discussed. Boric acid strongly contributes to the growth of high quality electrodeposited zinc oxide. The high doping level of the film can be attributed to the perchlorate ions introduced in the bath. Finally, a ZnO layer electrodeposited in a boron rich electrolyte was tested as front contact of a Cu(In, Ga)(S, Se)2 based solar cell. An efficiency of 12.5% was measured with a quite high fill factor (>70%) which confirms the high conductivity of the ZnO thin film. KEYWORDS: semiconductor, optical properties, X-ray diffraction, photoluminescence, boric acid, zinc oxide

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technology based on a Cu(In, Ga)(S, Se)2 (CIGS).14,15 Recently, high cell performances have been obtained with boron doped zinc oxide grown by metal−organic chemical vapor deposition (MOCVD).16 In both cases, the zinc oxide layer is produced by vacuum processes which require high cost equipment. One research direction aims to define low cost deposition techniques to produce high quality material. In this context, the electrodeposition process appears to be one of the most interesting approaches. Electrodeposition is an example of liquid and atmospheric based synthesis which involves low cost and abundant elements in a simple material configuration. This technique was first described in 1996 by Peulon17,18 and Izaki19,20 by two main synthesis routes: the use of dioxygen and nitrate as oxygen precursor, respectively. Inspired by these pioneering works, a lot of studies have been published to

INTRODUCTION Zinc oxide thin films and more generally transparent conductive oxides (TCO) have been widely used in several industrial fields such as photovoltaic devices, light emitting diodes, and touch screens.1,2 Additionally, this material presents a potential use in photocatalysis to degrade chemical agents.3,4 The lately renewed interest in zinc oxide semiconductor materials is due to its optoelectronic properties conferred by its direct wide band gap (Eg ∼ 3.3 eV at 300 K) adapted for recent technologies. The large variety of developed growth techniques allows researchers to adapt the properties of zinc oxide and to target the best balance between transparency and conductivity. In order to enhance the conductivity of the thin film layer, zinc oxide can be doped by cationic substitution with extrinsic elements such as aluminum, boron, indium, and gallium5−9 or by anionic substitution with halogen elements such as fluorine or chlorine.10−13 For example, aluminum doped zinc oxide (ZnO:Al) synthesized by sputtering technique is widely used as a transparent front contact for photovoltaic devices in thin film © XXXX American Chemical Society

Received: March 10, 2016 Accepted: April 25, 2016

A

DOI: 10.1021/acsami.6b02998 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Electrodeposition was performed in a transparent reactor with a three-electrode setup, using a SP-150 potentiostat from Bio-Logic Science Instruments and the EC-Lab software. The reference electrode was a saturated calomel electrode (SCE + 0.248 V vs normal hydrogen electrode (NHE)). A zinc foil served as the counter electrode while molybdenum coated glass substrate was used as the working electrode. Deposition experiments were performed under controlled potential (−1.1 V vs SCE) while the thicknesses of the films were controlled by the charge consumed during the growth and are equal to 1 μm. A thermal post-treatment is achieved by annealing at 150 °C in a stove for 30 min. The characterization of the electrical and optical properties of the films may be particularly challenging due to the presence of a conductive substrate required by the electrodeposition process. This one is generally a transparent SnO2 substrate doped by indium or fluorine (ITO and FTO) which may interact with the optoelectronic properties of ZnO and interfere with the measurements. In the present case, the ZnO films were synthesized on a Mo/glass substrate to study the influence of boric acid on the structural, optical, and electrical properties of the material. In order to acquire the TCO experimental optoelectronic properties we proposed an innovative approach which consists of a lift off method of the ZnO layer from the Mo/glass substrate to a transparent and nonconductive substrate such as glass lamella. Transfer is possible by applying an epoxy resin (Araldite 2020) between the glass and the ZnO layer which is then taken off by mechanical pressure. The stack of the samples is then ZnO/epoxy/ glass. Characterization Techniques. The morphology, composition, and thickness of the films were studied by scanning electron microscopy (SEM) using a Zeiss Merlin VP compact microscope coupled with an EDS analyzer. Chemical analysis of the material was performed by glow discharge optical emission spectrometry (GDOES) with a Horiba GD Profiler 2 spectrometer. The crystalline structure of the layers was examined by X-ray diffraction (XRD) with a Panalytical Empyrean X-ray diffractometer using Cu Kα1 radiation (1.5405 Å), in the classical Bragg−Brentano setup. Photoluminescence analysis was performed at room temperature with a UV-source of 325 nm by means of a Horiba-Jobin Yvon LabRAM HR system. The transmittance spectra were measured in the wavelength range 250−2000 nm by using a PerkinElmer Lambda 900 spectrophotometer in order to study the transparency of oxide layers and band gap evaluation. Reflectance spectra were measured with a Fourier-transformed infrared spectrometer (Bruker Vertex 70) and an MCT detector. A gold mirror was used as a reference. The deposition on CdS/CIGS substrate is performed without seed layer and under illumination. The ZnO growth is followed by an annealing at 150 °C during 30 min.

develop and optimize zinc oxide electrodeposition, understand its growth mechanism, and characterize its electrical and optical properties.21−25 However, few studies have taken an interest in the doping of zinc oxide by this deposition technique. Recently, it has been shown that chloride used as a support electrolyte has a doping effect on electrodeposited zinc oxide.26−28 Chlorine may represent an alternative to the aluminum classically considered in zinc oxide.29−31 Indeed, aluminum doping agent is quasi-insoluble at the pH (close to neutral) needed for the electrodeposition process. We have shown in a previous study that chlorine ions dissolved in the electrolyte are able to dope the electrodeposited ZnO layer up to 1020 cm−3. This doping level ensures a conductivity of about 5.2 × 104 S m−1 which has to be further increased in order to reach the performances required for transparent contact applications. Moreover, particular attention has to be paid to the material morphology for the applications that require layers with high lateral conductivity. In this case, very compact layers are needed. In this study we explore the influence of boric acid concentration on the structural and optoelectronic properties of electrodeposited zinc oxide layer grown in a perchlorate medium. The boric acid (H3BO3) has been selected as boron precursor as its use has been reported for growth of ZnO layers in liquid based techniques such as spray-pyrolysis,32−34 but organic boron compounds can also be used in chemical techniques.35−38 Finally, a ZnO layer was deposited by photoassisted electrodeposition from a boron rich electrolyte as front contact of a CIGS/CdS based front contact. The electrodeposition is a promising low cost technique that can be considered as a viable option to the widely used sputtering process. As a result, an encouraging efficiency of 12.5% has been obtained with an interesting 71% fill factor suggesting a high lateral conductivity of the electrodeposited ZnO film.

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EXPERIMENTAL SECTION

Electrochemical Synthesis. Depositions were carried out in solutions containing Zn2+ ions provided by Zn(ClO4)2·6H2O salt at a concentration of 5 × 10−3 M. To ensure good conductivity of the solution, KClO4 is added as a supporting electrolyte. Its concentration is adjusted in order to reach a total concentration of ClO4− equal to 0.1 M. Boric acid, H3BO3, is added to the solution as an assumed boron precursor at a concentration ranging from 10−4 to 2.5 × 10−2 M. In our very low concentration conditions and initial pH value close to neutral, boron is in the form B(OH)3, and it is very difficult to form a B3+ ion in solution. In solution, boron species exist at an equilibrium between B(OH)3 and the borate form. Oxygen precursor needed for the zinc oxide electrodeposition is introduced by bubbling O2 gas into the electrolytic solution which is maintained at 75 °C by means of a temperature regulation system. The mechanism envisaged by Peulon18 and considered here is

1/2O2 + H 2O + 2e− ⇌ 2OH−

(1)

Zn 2 + + 2OH− ⇌ Zn(OH)2

(2)

Zn(OH)2 ⇌ ZnO + H 2O

(3)

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RESULTS Morphology and Composition Aspects. The ZnO layers presented in this study were grown in potentiostatic conditions with an applied potential of −1.1 V versus SCE. For boron concentrations ranging from 0 mM to 15 mM, the chronoamperometric curve reaches a minimum value and suggests a three-dimensional electrocrystallization. After the nucleation step the current stabilizes around 0.75 mA cm−2 corresponding to a theoretical growth rate of 2 μm per hour. For higher concentrations the shape of the current density curve changes in particular during the first minutes of the deposition showing that the growth of the layer is dramatically affected. The SEM images of the ZnO thin films electrodeposited on molybdenum coated glass substrates in perchlorate electrolytes with different boric acid concentrations are shown in Figure 1. The growth of the zinc oxide layer in pure perchlorate medium leads to the formation of disjoined hexagonal columns (Figure 1a). The ZnO film morphology is significantly influenced by the addition of boric acid to the

The electrochemical formation of hydroxide ions at the surface vicinity of the working electrode by cathodic reduction of dissolved dioxygen in reaction 1 increases the local pH, and zinc ions precipitate to form zinc oxide by means of the dehydration of zinc hydroxide in reactions 2 and 3. During the increase of the pH to 11−12 due to the oxygen reduction at the vicinity of working electrode, boron adopts the solvated form [B(OH)4]− according to the fraction diagram of aqueous boron species.39 B


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diameter evolution is presented in Figure 1h and ranges from 180 nm without boric acid to 600 nm at [H3BO3] = 15 mM. Earlier studies have described the adsorption of boric acid H3BO3 or borate [B(OH)4]−, its solvated form, on different oxides.40,41 It has been observed that the maximal adsorption of boron is achieved in the pH range 7−9 which corresponds to the pH precipitation range (8−11) of zinc oxide in the electrode vicinity. A similar adsorption phenomenon can be expected on zinc oxide. Then, the growth mechanism of the ZnO columns could be similar to the one described in chloride electrolyte and generally attributed to the chloride adsorption at the top of the column which has a c-axis blocking behavior.28,42,43 Nevertheless, as the boric acid concentration exceeds 15 mM, the morphology is dramatically influenced. Larger columns are formed with almost 1 μm diameter leading to the growth of a noncompact layer. Figure 1g presents the consequence of an important increase of the boric acid concentration up to 50 mM: very large clusters replace the zinc oxide classical structure; the deposit is not fully covering, revealing large areas of molybdenum substrate. This structural evolution may be attributed to the acidic character of H3BO3. This weak acid shows a pKa of 9.3 and can act as a buffer limiting the pH increase at the electrode surface. This limitation does not entirely block the ZnO growth, but the pH stays too low to produce a large nuclei density on the substrate surface. Moreover, it has been established that boric acid forms polynuclear species,44,45when the total boron concentration is greater than 0.025 M, able to complex oxygen46 and limit its reduction. In these conditions, oxygen reduction into OH− ions is considerably inhibited, and the precipitation of ZnO is lowered. Glow discharge optical emission spectrometry is used in order to measure the composition profile along the cross section of the ZnO electrodeposited layers. Profiles are unsurprisingly substantially constituted of zinc and oxygen. Chlorine and boron are also detected. Different GDOES boron profiles are presented in the inset of Figure 2. The profile is roughly constant as a function of the sputtering time, and indicates a constant insertion of boron impurities during the deposition. Furthermore, a spike is obtained in the first seconds of the sputtering time and may confirm the adsorption of boric acid at the ZnO columns’ surface. In order to qualitatively estimate the boron incorporation evolution, the average ratio of boron and zinc intensities B/Zn is calculated for each sample and shown in Figure 2. This latter one gradually increases as a function of the boric acid concentration introduced in the bath. Chlorine is also detected in the layer and originates from the perchlorate support electrolyte used for the growth. The calculated Cl/Zn ratio follows a similar trend as B/Zn one while the perchlorate concentration remains unchanged for all experiments. An analysis by energy-dispersive X-ray spectroscopy (EDS) confirms this observation. Then, the chlorine concentration in the film seems to be linked to the boric acid concentration in the electrolyte. In small concentrations, the Cl/Zn ratio is relatively constant and increases slightly as the boric acid concentration becomes higher than 5 mM. Structural Study. XRD spectra of zinc oxide films grown with various concentrations of boric acid are presented in Figure 3a,b with a focus on the (002) peak position. This peak is systematically obtained and indicates a preferential orientation along the c-axis of the wurtzite lattice, perpendicular to the substrate, as suggested by the SEM observation. For a layer deposited in a pure perchlorate electrolyte, the position of

Figure 1. Morphology (surface and cross section) of ZnO layers deposited from perchlorate bath on glass/Mo substrate as a function of the boric acid concentration: (a) without boric acid, (b) 0.5 mM, (c) 1 mM, (d) 5 mM, (e) 10 mM, (f) 20 mM, and (g) 50 mM. (h) Evolution of column diameter as a function of the boric acid concentration.

electrolyte. From 0.1 (not shown) to 10 mM, corresponding to Figure 2b−e, the classical columnar hexagonal structure of electrodeposited ZnO is obtained. The increase of the boric acid concentration promotes the compactness of the zinc oxide layer due to a broadening of the ZnO columns. The column

Figure 2. B/Zn and Cl/Zn ratios from the composition profiles of ZnO layer measured by GDOES as a function of the boric acid concentration in the electrolyte, from 0 to 20 mM. Inset presents different GDOES boron profiles. C


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infrared regions is crucial for understanding the material properties. The electrical properties of ZnO are deduced from transmittance and reflectance spectra fitting the experimental curves by the Drude model applied to doped semiconductors, taking into account the reflection at each interface.50,51 The direct measurement of the electrical properties of an electrodeposited material is not straightforward due to the need for a conductive substrate for the growth. Thus, we developed a lift off process of the films from their growth substrate. If the appearance of microcracks in the layer during the lift off does not allow direct Hall measurement, the stack obtained (transparent epoxy glue/ZnO layer/glass) is optimal to carry out a complete optical characterization of the film. Our approach has been explained and described elsewhere. In this way, the carrier density and the intragrain mobility have been estimated. In general, the values calculated from the optical spectra (T and R) match quite well with those electrically measured7,52 with a slight overestimation of the charge mobility. These optical measurements probe the properties of the material at a local site and are not influenced by the grain boundaries unlike the long distance mobility measured by the Hall effect. Transmittance and Reflectance Studies. Figure 4a shows the transmittance spectra and the optical band gap evolution for 1 μm thick ZnO layers grown in electrolyte containing various boric acid concentrations and lifted off from their growth substrate. Their transmittance is compared to that of an intrinsic ZnO deposited by sputtering and an electrodeposited ZnO in a pure chloride electrolyte. The layers show a high transparency in the visible range that decreases in the nearinfrared region due to the absorption by the free carriers present in the material. Figure 4b shows the reflectance spectra of these layers and their fit by the Drude model for doped semiconductor. The reflectance of electrodeposited ZnO grown in boric acid/perchlorate electrolyte is lower than that of sputtered ZnO:Al but seems higher than that of electrodeposited ZnO:Cl. The influence of the boron concentration on the optoelectronic properties is presented in Figure 5. The optical carrier density and the intragrain mobility do not follow the expected trend. Without boric acid in a pure perchlorate electrolyte, a ZnO layer with a high carrier density is obtained with a value of about 1.6 × 1020 cm3. This result tends to demonstrate the potential of the perchlorate ions to dope ZnO. This effect is quite surprising, and further experiments are in progress to confirm it. On the contrary, the doping level remains roughly constant with the addition of boron to the bath. Moreover, the intragrain mobility is affected by the introduction of boron. The resistivity of the layer can be calculated from the equation:

Figure 3. Structural study: (a) typical X-ray diffractogram of ZnO thin film; (b) focus on the (002) peak position, for ZnO film electrochemically grown in perchlorate electrolyte containing various amounts of boric acid, from [H3BO3] = 0−20 mM. The dashed vertical line indicates the theoretical position of the (002) peak for an undoped zinc oxide while the dotted line shows the initial peak position for ZnO layer without boron inside. (c) Evolution of the a and c lattice parameters as a function of the boric acid concentration in electrolyte. (d) Boric acid concentration effect on crystallite size of zinc oxide.

the (002) peak is shifted to lower angles compared to the theoretical peak position (2θ = 34.43° from the PDF 00-0361451). This shift can be related to the insertion of a chlorinated species in the lattice in substitution for an oxygen atom or in an interstitial position and is the indication of the appearance of tensile stress in this crystallographic direction. The addition of acid boric to the solution slightly accentuates this shift; the c lattice parameter calculated from the XRD measurements increases from 5.212 Å (for a pure perchlorate bath) to 5.215 Å (for an electrolyte with [H3BO3] = 20 mM). The a lattice parameter follows the same trend and increases from 3.252 to 3.254 Å. More remarkably, the introduction of boric acid in the solution leads to a decrease of the crystallite size from 130 to 80 nm, estimated by the Scherrer formula47 and presented in Figure 3d. If all the B (under its B3+ form) atoms are substituted into Zn sites in the crystal, it should lead to a lattice parameter reduction, due to its largely lower ionic radius, as shown in previous studies.34,48,49 On the contrary, its occupancy of an interstitial site should lead to an increase of the parameter. As mentioned previously, it appears that the formation of B3+ in solution is very difficult. Then, boron impurity could be introduced into the ZnO matrix as borate which may contribute to this slight increase of the lattice constants. Optoelectronic Properties. To obtain films suitable for transparent conductive oxide application, the analysis of their optical behavior in the ultraviolet, visible, near-infrared, or

ρ=

1 Nopteμopt

Here Nopt and μopt are the carrier density and mobility, respectively, determined by the reflectance spectra fitting, and e is the electron charge. These results are detailed and compared with the properties of electrodeposited ZnO:Cl and sputtered ZnO:Al in Table 1. In a general manner the resistivity of the electrodeposited ZnO films remains constant, 4−7 times higher than that of the sputtered one for any boron concentrations tested. If the boron introduced in the film by an electrochemical does not improve D


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Table 1. Electrical Properties, Carrier Density, Mobility, and Resistivity of the ZnO Thin Films Calculated from the Reflectance Spectra as a Function of the Boric Acid Concentration [H3BO3] (mM) ZnO:Al ZnO:Cl [Cl−] ZnO:Cl [ClO4−] ZnO:Cl with B

doping level (1020 cm−3)

mobility (cm2 V−1 s−1)

resistivity (10−3 Ω cm)

4.7 1.5

32 17

0.5 2.2

0

1.6

16.7

2.3

0.1

1.7

16.8

2.2

0.5 1.0 5.0 10.0 15.0

1.7 1.9 1.6 1.3 1.6

15.4 15.7 11.7 12.3 17.4

2.3 2.1 3.2 3.7 2.2

the conductivity of the film at a local state, its influence on the morphology is expected to largely increase the lateral diffusion of the carrier over long distances. Photoluminescence. Figure 6 shows the photoluminescence (PL) spectra obtained for ZnO films annealed at different temperatures and grown in different bath compositions.

Figure 4. (a) Transmittance and (b) reflectance spectra for ZnO thin films after lift off step, for which the boric acid concentration in the bath was varied from 0 to 20 mM. The inset is the evolution of the energy band gap determined from the transmittance spectra. Representative spectra of sputtered i-ZnO, sputtered ZnO:Al, and electrodeposited ZnO:Cl are also shown for comparison.

Figure 5. Influence of the boric acid concentration on the optoelectronic properties of the electrodeposited ZnO. Optical mobility and carrier concentration extracted from fitted reflectance data. Properties of pure electrodeposited ZnO:Cl are also mentioned in the graphs for comparison and symbolized by the dashed lines.

Figure 6. Top: Photoluminescence spectra of ZnO layers deposited in an electrolyte containing 10 mM of boric acid and annealed at different temperature. Bottom: Photoluminescence spectra of ZnO layers obtained for different boric acid concentration in perchlorate electrolyte and annealed at 150 °C. The dashed line represents the peak position of the reference sample grown without boric acid in the bath. E



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DISCUSSION The aim of this study was to determine the influence of the addition of boric acid to the electrolyte on the optoelectronic properties of zinc oxide electrochemically grown in a perchlorate medium. Besides the positive effect of the boron addition on the ZnO morphology, the association of perchlorate and boric acid was expected to lead to a codoping of the ZnO thin film. Indeed, boron belongs to column III on the Mendeleev table and should substitute for the zinc atoms in the lattice. The Burstein−Moss model56−59 permits linking the optical properties of a semiconductor to its electrical properties. According to this model, the widening of the optical band gap observed by transmittance spectroscopy would be due to the increase of the doping level in the semiconductor. However, it appears that the zinc oxide properties and especially the doping level and mobility measured by IR reflectance are not correlated with the band gap evolution and the increase of the boron ratio in the film. The doping level does not follow the expected trend and remains roughly constant around 1.5 × 1020 cm−3 for all the boron precursor concentrations tested. This carrier density is equal to that obtained in the case of the pure perchlorate electrolyte. That may indicate the nondoping behavior of the boron inserted in the layer during the electrodeposition and that the doping is fully ensured by a chlorinated species. Nevertheless, the effective widening of the optical band gap which evolves with the addition of boric acid remains unexplained. An answer can be found in the analysis of the crystallite size considering previously published studies. The XRD study shows that the coherent domain size decreases from 130 to 80 nm as a function of the boron concentration. On the contrary, we have shown in a previous study that the insertion of chloride in the layer does not influence the crystallite size.27 The evolution of the band gap has been displayed as a function of the crystallite size in Figure 8, showing that these two

Classically, the photoluminescence response of a zinc oxide thin film is composed of two major contributions: the first appears in the UV range and is associated with the free exciton while the second one is an emission in the visible range (known as a yellow band) related to the presence of defects such as oxygen and zinc interstitials, oxygen and zinc vacancies, and antisite oxygen.53,54 Remarkably, the PL spectrum carried out on a film that was deposited in an electrolyte containing a boron concentration of 10 mM and that did not undergo any posttreatment exhibits a dominant near-band-edge (NBE) emission in the UV range and a weak contribution in the visible range suggesting a low defect concentration and good quality of the material.55 As the sample is annealed, the free exciton peak is shifted to lower energies, and this shift monotonically increases with temperature. This effect which is not fully understood yet has been attributed to the reduction of defect concentration in the layer such as oxygen vacancies or the presence of zinc hydroxide. The high ratio between the intensity of the free exciton peak and that of the yellow band for all the conditions tested shows that the introduction of chlorine and boron in the films does not lower its optical properties. The chlorinated species introduced in the film and originating from the perchlorate support electrolyte seems to be mainly responsible for the high doping level of the film. Thus, this species (that remains to be clearly identified) is expected to create a very shallow defect and to not contribute to the PL emission in the visible range. The boron species could create defects leading to nonradiative recombinations; moreover, its concentration in the film can be too low to generate a detectable PL emission that originates from this impurity. The PL emission peak of the reference, grown without boron, is centered at 3.33 eV, corresponding to the optical band gap determined by transmission spectroscopy. Introduction of boric acid in the electrolyte induces a shift of the near band edge emission toward higher energies from 3.33 to 3.45 eV as the concentration varies from 0.0 to 20 mM. This evolution is presented in Figure 7 as a function of the boric acid concentration and compared with the optical band gap. Energy values determined in both cases are close, and follow a similar trend suggesting a widening of the band gap with an increase of boric acid concentration.

Figure 8. Correlation between energy band gap, determined by optical transmittance and photoluminescence, and the average crystallite size of the ZnO layer.

parameters seem to be linked. This result is coherent with previous studies which have reported the widening of the optical band gap60,61 as the crystallite size decreases. Particularly, Marotti et al. described this effect for an electrodeposited ZnO with a gap variation of 50 meV as the crystallite size ranges from 20 to 50 nm. Despite the nondoping

Figure 7. Comparison of the ZnO band gap determined by photoluminescence and optical transmittance measurements as a function of the boric acid concentration. F


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as front contact of a CIGS based solar cell with encouraging performances.

behavior of boron provided by the boric acid precursor, this species drastically enhances the compactness of the zinc oxide film. This property constitutes a very interesting advantage for the growth of a transparent and conductive window layer applied to photovoltaic devices. In order to estimate the performance of this electrodeposited ZnO thin film as TCO, it was deposited as the window layer of a CIGS based solar cell (with experimental conditions described previously and a concentration of boron precursor fixed at [H3BO3] = 10 mM). As shown in the SEM view in Figure 9, the electrodeposited thin film is compact and perfectly

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank S. Rives for the Mo/glass substrates supply, L. Michely for the GDOES measurements, M. Stanley for rereading the manuscript, and J. Vidal for fruitful discussions. This work was supported by the ANRT (CIFRE 2013/0386). The authors are grateful to the support from the SOLARERA.NET International program in framework of NovaZolar project (subproject n°1405C0005 funded by ADEME).

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REFERENCES

(1) Ö zgür, Ü .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doğan, S.; Avrutin, V.; Cho, S.-J.; Morkoç, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (2) Klingshirn, C. ZnO: Material, Physics and Applications. ChemPhysChem 2007, 8, 782−803. (3) Mills, A.; Le Hunte, S. An Overview of Semiconductor Photocatalysis. J. Photochem. Photobiol., A 1997, 108, 1−35. (4) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G.-C.; Choi, W. Photocatalysis Using ZnO Thin Films and Nanoneedles Grown by Metal-Organic Chemical Vapor Deposition. Adv. Mater. 2004, 16, 1661−1664. (5) Sernelius, B. E.; Berggren, K.-F.; Jin, Z.-C.; Hamberg, I.; Granqvist, C. G. Band-Gap Tailoring of ZnO by Means of Heavy Al Doping. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 10244− 10248. (6) Wenas, W. W.; Yamada, A.; Takahashi, K.; Yoshino, M.; Konagai, M. Electrical and Optical Properties of Boron-Doped ZnO Thin Films for Solar Cells Grown by Metalorganic Chemical Vapor Deposition. J. Appl. Phys. 1991, 70, 7119−7123. (7) Steinhauser, J.; Fay, S.; Oliveira, N.; Vallat-Sauvain, E.; Ballif, C. Transition between Grain Boundary and Intragrain Scattering Transport Mechanisms in Boron-Doped Zinc Oxide Thin Films. Appl. Phys. Lett. 2007, 90, 142107. (8) Lucio-López, M.; Luna-Arias, M.; Maldonado, A.; Olvera, M. de la L.; Acosta, D. R. Preparation of Conducting and Transparent Indium-Doped ZnO Thin Films by Chemical Spray. Sol. Energy Mater. Sol. Cells 2006, 90, 733−741. (9) Hirata, G. A.; McKittrick, J.; Cheeks, T.; Siqueiros, J. M.; Diaz, J. A.; Contreras, O.; Lopez, O. A. Synthesis and Optelectronic Characterization of Gallium Doped Zinc Oxide Transparent Electrodes. Thin Solid Films 1996, 288, 29−31. (10) Olvera, M. de la L.; Maldonado, A.; Asomoza, R.; MeléndezLira, M. Effect of the Substrate Temperature and Acidity of the Spray Solution on the Physical Properties of F-Doped ZnO Thin Films Deposited by Chemical Spray. Sol. Energy Mater. Sol. Cells 2002, 71, 61−71. (11) Olvera, M. de la L.; Maldonado, A.; Asomoza, R. ZnO: F Thin Films Deposited by Chemical Spray: Effect of the Fluorine Concentration in the Starting Solution. Sol. Energy Mater. Sol. Cells 2002, 73, 425−433. (12) Tchelidze, T.; Chikoidze, E.; Gorochov, O.; Galtier, P. Perspectives of Chlorine Doping of ZnO. Thin Solid Films 2007, 515, 8744−8747. (13) Chikoidze, E.; Modreanu, M.; Sallet, V.; Gorochov, O.; Galtier, P. Electrical Properties of Chlorine-Doped ZnO Thin Films Grown by MOCVD. Phys. Status Solidi A 2008, 205, 1575−1579.

Figure 9. Morphology and performances of the electrodeposited ZnO as front contact of a CIGS based solar cell. The inset represents an SEM image of the solar cell cross section of an electrodeposited ZnO on a CIGS (electrodeposited)/CdS substrate as well as the spectral response corresponding to the current−voltage curve.

covers the substrate. The external quantum efficiency (EQE) of the cell presented in the inset confirms the high transparency of the material in the visible range, and an effective solar cell has been obtained with an encouraging efficiency of 12.5%. This cell performance is lower than that we previously reported28 due to lower Voc and current density. These electrical parameters are not directly related to the quality of the ZnO front contact but mainly originate from the CIGS absorber which shows a different composition as compared to the one used in the previous paper. On the contrary, the fill factor is largely influenced by the conductivity of the ZnO front contact. In the example presented in the paper, this parameter is high (71.5%), even higher than that previously reported (68%).

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CONCLUSION Boric acid addition in the perchlorate electrolyte had a major effect on the morphology of the electrodeposited zinc oxide layer. The compactness of the film was strongly improved by the adsorption of this species at the top of the grains up to a critical concentration (between 15 mM and 20 mM). Despite the evidence of boron incorporation in the material, electrical properties of the film, the doping level in particular, remained unaffected. Nevertheless, boric acid had an influence on the optical band gap and contributed to its widening. This observation is contrary to the Burstein−Moss model which links an increase of the doping level to the band gap widening. This effect had been attributed to the decrease of the crystallite size observed by X-ray diffraction. Finally, a compact transparent and conductive ZnO layer was electrodeposited G


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ACS Applied Materials & Interfaces (14) Niki, S.; Contreras, M.; Repins, I.; Powalla, M.; Kushiya, K.; Ishizuka, S.; Matsubara, K. CIGS Absorbers and Processes. Prog. Photovoltaics 2010, 18, 453−466. (15) Jackson, P.; Hariskos, D.; Wuerz, R.; Wischmann, W.; Powalla, M. Compositional Investigation of Potassium Doped Cu(In,Ga)Se2 Solar Cells with Efficiencies up to 20.8%. Phys. Status Solidi RRL 2014, 8, 219−222. (16) Nakamura, M.; Yoneyama, N.; Horiguchi, K.; Iwata, Y.; Yamaguchi, K.; Sugimoto, H.; Kato, T. Recent R&D Progress in Solar Frontier’s Small-Sized Cu(InGa)(SeS)2 Solar Cells. In Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th; IEEE, 2014; pp 0107−0110.10.1109/PVSC.2014.6925346 (17) Peulon, S.; Lincot, D. Cathodic Electrodeposition from Aqueous Solution of Dense or Open-Structured Zinc Oxide Films. Adv. Mater. 1996, 8, 166−170. (18) Peulon, S.; Lincot, D. Mechanistic Study of Cathodic Electrodeposition of Zinc Oxide and Zinc Hydroxychloride Films from Oxygenated Aqueous Zinc Chloride Solutions. J. Electrochem. Soc. 1998, 145, 864−874. (19) Izaki, M.; Omi, T. Transparent Zinc Oxide Films Prepared by Electrochemical Reaction. Appl. Phys. Lett. 1996, 68, 2439−2440. (20) Izaki, M.; Omi, T. Electrolyte Optimization for Cathodic Growth of Zinc Oxide Films. J. Electrochem. Soc. 1996, 143, L53. (21) Canava, B.; Lincot, D. Nucleation Effects on Structural and Optical Properties of Electrodeposited Zinc Oxide on Tin Oxide. J. Appl. Electrochem. 2000, 30, 711−716. (22) Goux, A.; Pauporté, T.; Chivot, J.; Lincot, D. Temperature Effects on ZnO Electrodeposition. Electrochim. Acta 2005, 50, 2239− 2248. (23) Laurent, K.; Wang, B. Q.; Yu, D. P.; Leprince-Wang, Y. Structural and Optical Properties of Electrodeposited ZnO Thin Films. Thin Solid Films 2008, 517, 617−621. (24) Shinagawa, T.; Chigane, M.; Murase, K.; Izaki, M. Drastic Change in Electrical Properties of Electrodeposited ZnO: Systematic Study by Hall Effect Measurements. J. Phys. Chem. C 2012, 116, 15925−15931. (25) Manzano, C. V.; Caballero-Calero, O.; Hormeño, S.; Penedo, M.; Luna, M.; Martín-González, M. S. ZnO Morphology Control by Pulsed Electrodeposition. J. Phys. Chem. C 2013, 117, 1502−1508. (26) Cui, J. B.; Soo, Y. C.; Chen, T. P.; Gibson, U. J. LowTemperature Growth and Characterization of Cl-Doped ZnO Nanowire Arrays. J. Phys. Chem. C 2008, 112 (12), 4475−4479. (27) Rousset, J.; Saucedo, E.; Lincot, D. Extrinsic Doping of Electrodeposited Zinc Oxide Films by Chlorine for Transparent Conductive Oxide Applications. Chem. Mater. 2009, 21, 534−540. (28) Tsin, F.; Venerosy, A.; Vidal, J.; Collin, S.; Clatot, J.; Lombez, L.; Paire, M.; Borensztajn, S.; Broussillou, C.; Grand, P. P.; Jaime, S.; Lincot, D.; Rousset, J. Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar Cell. Sci. Rep. 2015, 5, 8961. (29) Baka, O.; Azizi, A.; Velumani, S.; Schmerber, G.; Dinia, A. Effect of Al Concentrations on the Electrodeposition and Properties of Transparent Al-Doped ZnO Thin Films. J. Mater. Sci.: Mater. Electron. 2014, 25, 1761−1769. (30) El Hichou, A.; Diliberto, S.; Stein, N. Influence of the Aluminum Incorporation on the Properties of Electrodeposited ZnO Thin Films. Surf. Coat. Technol. 2015, 270, 236−242. (31) Calnan, S.; Riedel, W.; Gledhill, S.; Stannowski, B.; Lux-Steiner, M. C.; Schlatmann, R. Comparison of the Influence of Boron and Aluminium Doping on the Material Properties of Electrochemically Deposited ZnO Films. Thin Solid Films 2015, 594, 215−224. (32) Lokhande, B. J.; Patil, P. S.; Uplane, M. D. Studies on Structural, Optical and Electrical Properties of Boron Doped Zinc Oxide Films Prepared by Spray Pyrolysis Technique. Phys. B 2001, 302, 59−63. (33) Pawar, B. N.; Jadkar, S. R.; Takwale, M. G. Deposition and Characterization of Transparent and Conductive Sprayed ZnO:B Thin Films. J. Phys. Chem. Solids 2005, 66, 1779−1782. (34) Gaikwad, R. S.; Bhande, S. S.; Mane, R. S.; Pawar, B. N.; Gaikwad, S. L.; Han, S.-H.; Joo, O.-S. Roughness-Based Monitoring of

Transparency and Conductivity in Boron-Doped ZnO Thin Films Prepared by Spray Pyrolysis. Mater. Res. Bull. 2012, 47, 4257−4262. (35) Izaki, M.; Katayama, J. Characterization of Boron-Incorporated Zinc Oxide Film Chemically Prepared from an Aqueous Solution. J. Electrochem. Soc. 2000, 147, 210. (36) Kumar, V.; Singh, R. G.; Purohit, L. P.; Mehra, R. M. Structural, Transport and Optical Properties of Boron-Doped Zinc Oxide Nanocrystalline. J. Mater. Sci. Technol. 2011, 27, 481−488. (37) Tsay, C.-Y.; Hsu, W.-T. Sol−gel Derived Undoped and BoronDoped ZnO Semiconductor Thin Films: Preparation and Characterization. Ceram. Int. 2013, 39, 7425−7432. (38) Ishizaki, H.; Imaizumi, M.; Matsuda, S.; Izaki, M.; Ito, T. Incorporation of Boron in ZnO Film from an Aqueous Solution Containing Zinc Nitrate and Dimethylamine-Borane by Electrochemical Reaction. Thin Solid Films 2002, 411, 65−68. (39) Hinz, K.; Altmaier, M.; Gaona, X.; Rabung, T.; Schild, D.; Richmann, M.; Reed, D. T.; Alekseev, E. V.; Geckeis, H. Interaction of Nd(III) and Cm(III) with Borate in Dilute to Concentrated Alkaline NaCl, MgCl2 and CaCl2 Solutions: Solubility and TRLFS Studies. New J. Chem. 2015, 39 (2), 849−859. (40) Blesa, M. A.; Maroto, A. J.; Regazzoni, A. E. Boric Acid Adsorption on Magnetite and Zirconium Dioxide. J. Colloid Interface Sci. 1984, 99, 32−40. (41) Goldberg, S.; Glaubig, R. A. Boron Adsorption on Aluminum and Iron Oxide Minerals. Soil Sci. Soc. Am. J. 1985, 49, 1374−1379. (42) Tena-Zaera, R.; Elias, J.; Wang, G.; Levy-Clement, C. Role of Chloride Ions on Electrochemical Deposition of ZnO Nanowire Arrays from O2 Reduction. J. Phys. Chem. C 2007, 111, 16706−16711. (43) Tena-Zaera, R.; Elias, J.; Lévy-Clément, C.; Bekeny, C.; Voss, T.; Mora-Seró, I.; Bisquert, J. Influence of the Potassium Chloride Concentration on the Physical Properties of Electrodeposited ZnO Nanowire Arrays. J. Phys. Chem. C 2008, 112, 16318−16323. (44) Ingri, N.; Lagerström, G.; Frydman, M.; Sillén, L. G. Equilibrium Studies of Polyanions. II. Polyborates in NaClO4 Medium. Acta Chem. Scand. 1957, 11, 1034−1058. (45) Ingri, N.; et al. Equilibrium Studies of Polyanions. 8. On the First Equilibrium Steps in the Hydrolysis of Boric Acid, a Comparison between Equilibria in 0.1 and 3.0 M NaClO4. Acta Chem. Scand. 1962, 16, 439−448. (46) Jovancicevic, V.; Zelenay, P.; Scharifker, B. R. The Transport Properties of Oxygen in Aqueous Borate Solutions. Electrochim. Acta 1987, 32, 1553−1555. (47) Jenkins, R.; de Vries, J. L. Worked Examples in X-Ray Analysis; Springer: New York, 1970. (48) Wang, X. B.; Song, C.; Geng, K. W.; Zeng, F.; Pan, F. Luminescence and Raman Scattering Properties of Ag-Doped ZnO Films. J. Phys. D: Appl. Phys. 2006, 39, 4992−4996. (49) Kim, K. H.; Park, K. C.; Ma, D. Y. Structural, Electrical and Optical Properties of Aluminum Doped Zinc Oxide Films Prepared by Radio Frequency Magnetron Sputtering. J. Appl. Phys. 1997, 81, 7764. (50) Fox, M. Optical Properties of Solids, 2nd ed., reprinted; Oxford Master Series in Condensed Matter Physics; Oxford University Press: Oxford, 2012. (51) Jin, Z.-C.; Hamberg, I.; Granqvist, C. G. Optical Properties of Sputter-Deposited ZnO:Al Thin Films. J. Appl. Phys. 1988, 64, 5117. (52) Steinhauser, J.; Faÿ, S.; Oliveira, N.; Vallat-Sauvain, E.; Zimin, D.; Kroll, U.; Ballif, C. Electrical Transport in Boron-Doped Polycrystalline Zinc Oxide Thin Films. Phys. Status Solidi A 2008, 205, 1983−1987. (53) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. Mechanisms behind Green Photoluminescence in ZnO Phosphor Powders. J. Appl. Phys. 1996, 79, 7983. (54) Rodnyi, P. A.; Khodyuk, I. V. Optical and Luminescence Properties of Zinc Oxide (Review). Opt. Spectrosc. 2011, 111, 776− 785. (55) Manzano, C. V.; Alegre, D.; Caballero-Calero, O.; Alén, B.; Martín-González, M. S. Synthesis and Luminescence Properties of Electrodeposited ZnO Films. J. Appl. Phys. 2011, 110, 043538. H


Research Article

ACS Applied Materials & Interfaces (56) Burstein, E. Anomalous Optical Absorption Limit in InSb. Phys. Rev. 1954, 93, 632−633. (57) Moss, T. S. The Interpretation of the Properties of Indium Antimonide. Proc. Phys. Soc., London, Sect. B 1954, 67, 775. (58) Roth, A. P.; Webb, J. B.; Williams, D. F. Absorption Edge Shift in ZnO Thin Films at High Carrier Densities. Solid State Commun. 1981, 39, 1269−1271. (59) Roth, A. P.; Webb, J. B.; Williams, D. F. Band-Gap Narrowing in Heavily Defect-Doped ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 1982, 25, 7836. (60) Marotti, R. E.; Giorgi, P.; Machado, G.; Dalchiele, E. A. Crystallite Size Dependence of Band Gap Energy for Electrodeposited ZnO Grown at Different Temperatures. Sol. Energy Mater. Sol. Cells 2006, 90, 2356−2361. (61) Tan, S. T.; Chen, B. J.; Sun, X. W.; Fan, W. J.; Kwok, H. S.; Zhang, X. H.; Chua, S. J. Blueshift of Optical Band Gap in ZnO Thin Films Grown by Metal-Organic Chemical-Vapor Deposition. J. Appl. Phys. 2005, 98, 013505.

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