Antireflective downconversion ZnO:Er3+,Yb3+ thin film for Si solar cell ...

Antireflective downconversion ZnO:Er3+,Yb3+ thin film for Si solar cell applications R. Elleuch, R. Salhi, J.-L. Deschanvres, and R. Maalej Citation: Journal of Applied Physics 117, 055301 (2015); doi: 10.1063/1.4906976 View online: http://dx.doi.org/10.1063/1.4906976 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A study of the applicability of ZnO thin-films as anti-reflection coating on Cu2ZnSnS4 thin-films solar cell AIP Conf. Proc. 1451, 97 (2012); 10.1063/1.4732379 Effective light trapping in thin film silicon solar cells from textured Al doped ZnO substrates with broad surface feature distributions Appl. Phys. Lett. 100, 263508 (2012); 10.1063/1.4731775 Antireflection properties and solar cell application of silicon nanostructures J. Vac. Sci. Technol. B 29, 031208 (2011); 10.1116/1.3591344 Improvement in quantum efficiency of thin film Si solar cells due to the suppression of optical reflectance at transparent conducting oxide/Si interface by Ti O 2 ∕ Zn O antireflection coating Appl. Phys. Lett. 88, 183508 (2006); 10.1063/1.2200741 Application of plasma enhanced chemical vapor deposition silicon nitride as a double layer antireflection coating and passivation layer for polysilicon solar cells J. Vac. Sci. Technol. A 15, 1020 (1997); 10.1116/1.580509

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JOURNAL OF APPLIED PHYSICS 117, 055301 (2015)

Antireflective downconversion ZnO:Er31,Yb31 thin film for Si solar cell applications R. Elleuch,1,a) R. Salhi,2 J.-L. Deschanvres,3 and R. Maalej1 1

Laboratoire G eoressouces, Mat eriaux, Environnement et Changements Globaux, Facult e des Sciences de Sfax, Universit e de Sfax, 3018 Sfax, Tunisia 2 Laboratoire de Chimie Industrielle, Ecole Nationale d’ing enieurs de Sfax, Universit e de Sfax, 3018 Sfax, Tunisia 3 Laboratoire des Mat eriaux et du G enie Physique, 3 Parvis Louis N eel BP 257, 38016 Grenoble, France

(Received 15 July 2014; accepted 15 January 2015; published online 2 February 2015) Hexagonal wurtzite phased ZnO:Er3þ/Yb3þ thin films with various Yb concentrations were deposited on Si(111) substrate by Aerosol Assisted Chemical Vapor Deposition process. Post-annealed films at 1000 C in air atmosphere showed a crystallinity enhancement. Yb3þ (4F7/2 ! 4F5/2) 1000 nm emission increased with the increase of Yb3þ concentration emanating from an Er-Yb energy transfer. The reflectance percentage of 12% was achieved in the [250–1000 nm] range, and the refractive index of 1.97 was obtained for 632 nm wavelength. These results suggest that the (3 mol. % Er, 9 mol. % Yb) codoped film is a highly efficient antireflective C 2015 AIP Publishing LLC. downconversion layer for enhancing Si solar cell efficiency. V [http://dx.doi.org/10.1063/1.4906976]

I. INTRODUCTION

Photovoltaic (PV) energy is expected to be the main candidate for sustainable energy production, which meets the worldwide energy demand. But presently, the contribution of photovoltaic energy based Si solar cell is limited due to its relatively low conversion efficiency of about 15%.1 The major energy loss in Si solar cells is the thermalization;2 when the absorbed photon energy is greater than Si band gap Eg ¼ 1.1 eV. Such a loss is expected to be considerably reduced if the absorbed photon (k < 550 nm) is cut into two other near-infrared (NIR) photons.3,4 This kind of twophoton emission by absorbing one high energy photon is called Downconversion (DC) through quantum cutting (QC).1 It has recently received considerable attention, thanks to its potential interest in enhancing the efficiency of Si solar cell.1,5,6 The assignment of rare earth (RE) doped material in front of a Si solar cell as a downconverter luminescent layer can boost its efficiency up to 40%.6 Moreover, when it is placed in front of the Si solar cell, DC layer should have a low reflectance by minimizing the scattering incoming light.5,7,8 Then, to further enhance the conversion efficiency of silicon solar cell, the advanced light trapping schemes are very sought after, because the optical path of the light inside the cells has to be increased in order to improve the photogenerated current.9 Generally, Si causes the reflection of more than 35% from ultraviolet to infrared light.10 It is well known that light manipulation by using an antireflective layer is one of the most important solutions for improving the PV solar cells performance.11–13 Antireflective film with an intermediate refractive index (2) between crystalline c-Si solar cell and the air increases light absorption and subsequent conversion efficiency.5,6,14,15 So far, the most a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ216 74 276 400. Fax: þ216 74 274 437.

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reported materials exhibiting DC emissions have been focused on powders, glass ceramics, or glasses.16–21 However, their poor compatibilities with c-Si solar cell technology limit their practical applications in c-Si solar cell. To overcome this problem, a transparent thin film deposited on c-Si solar cell presents an efficient way to boost their efficiencies.5–7 Rare earth doped semiconducting materials are very interesting for antireflective coating (ARC) and DC layers. The Er3þ ion is ideally suited for photon conversion since its emission at 980 nm originating from 4I11/2 ! 4I15/2 is well absorbed by Si solar cell.5,17,20 Furthermore, codoping with Yb3þ ions can improve this emission. Yb3þ ion has a relatively simple electronic structure of two energy-level manifolds: the 2F7/2 ground state and 2F5/2 excited state located around 1000 nm which is located just above the band gap of Si. It is recognized that the Yb3þ ion plays the role of a good emitter for DC process.20,22 Indeed, Yb ion has a large absorption cross section around 1000 nm wavelength and can efficiently transfer its energy to Er3þ ion.17,20 In this framework, we are interested in the deposition of Er/Yb codoped ZnO thin film via Aerosol Assisted Chemical Vapor Deposition (AACVD) process, since ZnO materials has a high transparency aspect with an ideal refractive index (1.9) intermediating the corresponding values of Si and air. An in-depth study, antireflection and DC emission properties of the films were investigated. Moreover, we investigate the emission and energy transfer between Yb3þ and Er3þ ions under 515 nm excitation. II. EXPERIMENTAL DETAILS

Zinc acetates dehydrate (C4H6O4Zn2H2O), Erbium (III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Er(TMHD)3)), and Ytterbium (III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Yb(TMHD)3)) purchased from the STREM society

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were dissolved in a mixing of methanol and butanol solvent with 0.05 M total concentration. Er(TMHD)3 molar percentage was fixed at 3 mol. %, and various Yb(TMHD)3 molar percentages were 6, 8, 9, and 10 mol. %. The ZnO:Er/Yb films were deposited on Si (1 1 1) substrate using AACVD atmospheric pressure technique which was described in a previous research work.23 The deposition temperature was fixed at 430 C and maintained for 30 min. Atmospheric air with 4 l/min flow rate was used as a carrier gas. The refractive index and thickness measurements were performed by an ellipsometer (Gaertner L116B) with exciting wavelength of 632 nm. Atomic force microscope (AFM) Veeco metrology group Di 3100 Nanoscope III instruments were performed to determine the roughness and morphological features of the films. Fourier Transformed IR spectra (FTIR) were recorded with a Bioard FTS65 spectrophotometer. Film structures were investigated by a Siemens D500 diffractometer using Cu Ka radiation in h/2h scan. Photoluminescence (PL) emissions in the range of [500–1600 nm] were examined by Coherent Innova Ar laser using green excitation (515 nm). NIR spectra were detected by Hammamatsu InGaAs diode in the range of [930–1600 nm]. Reflectance measurements were recorded from Perkin Elmer Lambda35 UV/Visible [250–1000 nm] spectrometer. III. RESULTS AND DISCUSSION

Fig. 1(a) displays the 3D-AFM image of the as-deposited ZnO:3 mol. % Er, 9 mol. % Yb thin film. The as-deposited nanostructure films are the aggregation of nanograins with the average diameter about 120 nm and the tip size is 10 nm. The other films showed identical surface morphology with fairly homogeneous nanograins distribution. The image shown in Fig. 1(a) suggest a Volmer-Weber type film growth for ZnO:Er/Yb films when deposited at 430 C.23 The nanograins were assumed as 3D island growth mode resulting from the nucleation of ZnO:Er/Yb onto the Si substrate surface.24 The ZnO:Er/Yb films grown at 430 C reveal a growth rates of 620–640 nm/h with film thicknesses of 310, 314, 316, and 320 nm corresponding to 6, 8, 9, and 10 mol. % Yb, respectively. These results are is in good agreement with those reported for the ZnO:Er films presented in our previous work.23 Regarding the CVD conditions, the as-deposited films are adherent. In the first step, droplets were formed ultrasonically and conveyed by air as a carrier gas to the Si substrate,

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which is maintained at 430 C deposition temperature. Then, the precursor droplets are quickly dissociated due to the low dissociation enthalpy of the zinc, erbium, and ytterbium precursors.23,25,26 Finally, droplets are dissociated before their complete spreading on the substrate surface due to the large surface tension of the precursors. The 3D-AFM images of as-deposited films with well-defined nanograins are grown perpendicular to substrate surfaces. The root mean square (RMS) roughness of the as-deposited films was measured and found to increase with the increase Yb content in the films, with values of 92 nm, 98 nm, 100 nm, and 101 nm for the Yb concentrations of 6 mol. %, 8 mol. %, 9 mol. %, and 10 mol. %, respectively. It is well known that the organic and hydroxyl OH contaminations are inevitable when using b-dictonate precursors.23,27 These contaminations are still remained on the top of the film surface after film deposition.23,27 Moreover, their presence eradicates the RE luminescence. To check their presence, the FTIR measurements of the as-deposited films was recorded and presented in Fig. 2(a). The obtained data reveal the presence of the Zn-O vibration band at 410 cm1, which is characteristic of ZnO in hexagonal structure. It is observed also the presence of the O-H vibration bands at 3500 cm1 and the [C ¼ C, C ¼ O] and c CH3 vibration bands at 1300–1700 cm1. To overcome this problem, postannealing at 1000 C is an efficient treatment which eliminates these contaminations and enhances the crystallinity of the film.23,27 Thus, all the films have undergone postannealing treatments in air atmosphere at 1000 C for 1 h. It is also shown in Fig. 2(a) that the Zn-O vibration band intensity is increased with the annealing at 1000 C occurring from the enhancement of the crystallinity of the films, and also the disappearance of O-H and the contaminations groups. Indeed, We observed also two bands at 912 cm1 and 1077 cm1, which are attributed to the Si-O vibrations of SiO2 thin layer originating from the reaction between the oxygen atoms and Si substrate during the thermal annealing.23,27 Fig. 2(b) shows the XRD patterns of the as-deposited ZnO thin films codoped with 3 mol. % Er and 9 mol. % Yb ions. What is worthy to note is that all the samples exhibit the same XRD patterns. ZnO:Er/Yb thin films consist of diffraction peaks similar to those of the undoped ZnO with the hexagonal wurtzite structure (JCPDS Card No. 36–1451). The results indicate that the ZnO structure is not altered by the presence of Yb3þ and Er3þ dopant ions, likely obtained

FIG. 1. 3D-AFM images of (a): asdeposited and (b) annealed film of ZnO: 3 mol. % Er, 9 mol. % Yb.


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The observed luminescence is attributed to the intra 4f–4f transitions of Er3þ incorporated in ZnO matrix,23 since Yb3þ ions do not have any emissions in visible wavelength range. Three emission bands are observed in the range of [510–680 nm]. The observed green luminescence occurring from 515 nm to 535 nm is attributed to 2H11/2 ! 4I15/2 transitions and the second ranging from 540 to 565 nm is ascribed to 4S3/2 ! 4I15/2 transitions. The third emission localized at 660 nm is attributed to 4F9/2 ! 4I15/2 transitions. To understand the DC mechanism of our annealed ZnO:Er/Yb thin film, the photoluminescence spectrum of ZnO film doped by 3 mol. % Er3þ only was measured and presented in Fig. 3(b). This spectrum indicates the Er3þ downconversion emission under 515 nm via the 4I11/2 ! 4I15/2 transition at 980 nm and the 4I13/2 ! 4I15/2 transition at 1540 nm. Indeed, this spectrum shows two strong and resolved DC emissions peaks, which is similar to that obtained in our previous work.5 The possible emission process can be explained by the energy transfer mechanism according to the following equation: Er3þ ð4 S3=2 –2 H11=2 Þ þ Er3þ ð4 I15=2 Þ ! Er3þ ð4 I9=2 Þ þ Er3þ ð4 I13=2 Þ:

FIG. 2. (a) FTIR and (b) XRD patterns of as-deposited and annealed ZnO: 3 mol. % Er, 9 mol. % Yb film.

by Chawla et al. for Li and Na doped ZnO matrix.28 It is clearly observed that no other phase of Yb2O3 and/or Er2O3 appeared in XRD patterns, indicating a well incorporation of Er3þ and Yb3þ ions into ZnO matrix during deposition process. Actually, the Er and Yb ions were incorporated in the matrix by the substitution of Zn2þ ions.23 It is found that the (101) peak shifts to the lower diffraction angle compared to the undoped ZnO film. This is due to the radii of both Er3þ (0.088 nm) and Yb3þ (0.086 nm) which are higher than that of the Zn2þ (0.074 nm) ions. This radius mismatching increases the lattice constants “a” and “c,” implying a unit cell expansion in ZnO:Er/Yb films.22,28 Besides, it can be seen that the sample’s mean grain size D determined by the Debye-Scherrer method from the (002) diffraction peak is slightly affected by the Yb content from 6 mol. % to 10 mol. % with 12 6 0.4 and 14 6 0.2 nm, respectively. To study the effect of the annealing on the crystallinity of all annealed films at 1000 C, XRD measurements were recorded. Besides, the patterns shown in Fig. 2(b) confirm the hexagonal wurtzite structure of ZnO:Er/Yb films. The annealing treatment in air atmosphere was proven to enhance the crystallinity of the films.23,27 Fig. 3(a) shows the room temperature (RT) PL emission spectra of the annealed films of ZnO doped Er3þ 3 mol. % only and Er codoped 9 mol. % Yb3þ under 515 nm excitation. Since all the spectra were measured under identical conditions, the intensities of the emissions may be compared.

(1)

Fig. 3(c) shows the DC spectrum of ZnO: 3 mol. % Er, 9 mol. % Yb annealed films excited with 515 nm in the 4F7/2 level of Er3þ, which then results in the emission of Er3þ itself at about 1540 nm and that of Yb3þ at about 1000 nm. Upon increasing Yb3þ concentrations, the intensity of the Er3þ 980 nm emission decreases, leading to the increase of the 1000 nm Yb3þ emission. With the addition of Yb3þ ions, the DC emission indicates that an energy transfer from Er3þ to Yb3þ ions was taking place.17 This is due to the substantially higher emission cross-section and concentration of the Yb3þ dopants. This can be understood because the Yb3þ–Er3þ inter dopant distances decrease due to the incorporation of these dopants in the ZnO matrix. The decrease in the Yb3þ-Er3þ inter-dopant distances results in the increase of the rate of the energy transfer Er3þ-Yb3þ.29 However, the transitions from Er3þ 4I13/2 are reduced by increasing the population of 4I11/2 multiplets. Nevertheless, the effective 4 I11/2 level multiplets decrease with the addition of Yb ions, which have only one emission at 1000 nm (2F7/2 ! 2F5/2). The considered energy transfer mechanism for the Er3þYb3þ ions is given by the following equation: Er3þ ð4 F7=2 Þ þ Yb3þ ð2 F7=2 Þ ! Er3þ ð4 I11=2 Þ þ Yb3þ ð2 F5=2 Þ: (2) Therefore, the significant increase of the relative emission intensity at 1000 nm of Yb3þ with the increase of the concentration ratio of Yb ions from 6 to 9 mol. % was observed (seen in the inset of Fig. 3(c)). The resolved and strongest emissions were detected for the film having 9 mol. % Yb3þ concentration, indicating a maximum doping of the Yb incorporated in the matrix. Over this Yb3þ concentration, the PL intensity of DC emission decreased, when the Yb3þ emission is largely quenched through concentration quenching.20


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FIG. 3. (a) RT photoluminescence of 3 mol. % Er ions doped ZnO and 3 mol. % Er, 9 mol. % Yb codoped ZnO annealed films at 1000 C. (b) DC emission of 3 mol. % Er ions doped ZnO annealed film. (c) DC emission of 3 mol. % Er, 9 mol. % Yb codoped ZnO annealed film. Inset shows the Yb3þ 1000 nm emission intensity versus Yb concentrations. (d) Energy diagram of optical transitions and Er-Yb energy transfer related to ZnO:Er3þ/Yb3þ thin films under 515 nm excitation.

The increase of the Yb3þ doping in the films results in the shifting of the equilibrium between the population of the 4 I11/2 (Er3þ) and 2F5/2 (Yb3þ) levels, favoring the emission at about 1000 nm of Yb3þ compared to the 1540 nm emission of Er3þ.29,30 Generally, annealing in air rearrange the oxygen environment of the ions and lead to an enhancement of the optical activity of the Er and Yb levels transitions.23,31 For the as-deposited films, Er3þ and Yb3þ ions are surrounded by five O atoms and second nearest neighbors of eight O atoms, leading to a suppressing of the intra-4f transitions, giving a low PL intensity. Therefore, the annealing in air could reorganize the oxygen atoms around the Er ions, leading to a much stronger Er3þ emissions.23,31 Indeed, the optical active center of Er3þ in the ZnO lattice has a local structure similar to that of the ErO6 unit.31 After annealing, the local structure around Er probably forms a similar pseudo-octahedron with C4v point structure by the diffusion of Oxygen O excess, leading to higher emission intensity.23,31 The DC emission of Er3þ ions via the undesired 4 I13/2 state was confirmed to be suppressed by the addition of Yb ions. When the Er ions are surrounded by Yb, the 980

emission would be favored and weak emission at 1540 nm was recorded. In Fig. 3(d), we presented a simplified energy diagram of optical transitions and Er-Yb energy transfer related to ZnO:Er3þ/Yb3þ thin films under 515 nm excitation. Generally, the antireflection films exhibit a refractive index, which is commonly around 2 at 632 nm.32 The refractive indices of all annealed films are summarized in Table I. As regards the as-deposited films, the refractive index (n) values were in the 1.86–1.89 range and were noted to increase with annealing at 1000 C. At this annealing temperature, the refractive indices were in the range of 1.95 and TABLE I. Refractive index (n), RMS roughness and reflectance percentages of the Er,Yb:ZnO films annealed at 1000 C as a function of Yb concentration. Yb concentration (mol. %) Refractive index (n) RMS roughness (nm) Reflectance (%)

6 1.95 104 15

8 1.96 107 14

9 1.97 114 12

10 1.97 115 12


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1.97, corresponding to 6 and 10 mol. % of Yb doping, respectively. The findings, therefore, suggest that n increases with the increase of both annealing temperature and Yb concentration. In fact, the film density increases with the increase of substrate temperature.33 Indeed, annealing treatments enhance the crystallinity and the density of the films, since the grain boundaries are disappeared and the nanograins are more agglomerated and enlarged, showed in Fig. 1(b). These results are explained also by the increase of the RMS roughness values of the annealed films, which are 104, 107, 114, and 115 nm corresponding to 6, 8, 9, and 10 mol. % Yb doping films annealed at 1000 C, respectively. In fact, the refractive indices of our ZnO:Er,Yb annealed films are high when compared to the refractive index of the ZnO bulk (n ¼ 1.9), and the more the refractive index is increased, the more the crystallinity is enhanced and the grain size is enlarged. The same results were obtained for our previous work reported on the optical study of Er:ZnO film.5 These results demonstrate that under these annealing conditions, the quality of the films is improved and, thanks to the better crystallinity and enhanced optical properties, the annealing process could lead to higher transmission. It is well known that over 35% of the incoming light power is reflected over the complete air-mass AMG1.5 spectrum by air–Si solar cell interface. To minimize the light reflection, the surface roughness scale of the film should be much smaller than the wavelength range of the incident light.34 For all our ZnO:Er,Yb films annealed at 1000 C, the RMS roughness increased compared to the as-deposited ones. As shown in Fig. 1(b), the more the crystallinity is improved by annealing, the more the grain size is enlarged. The morphology surfaces of the annealed films at 1000 C are similar to those of the as-deposited films. The RMS roughness values of the annealed films were summarized in Table I. This increase of the RMS roughness of the annealed films may be explained by the increase of the density of the films and consequently by the decrease of the grain boundaries, which are the dissipation sources of the light generated inside the film.35

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Moreover, surface shape grating structures made on ARC films have also been used to increase the efficiency of solar cell.7 But, the fabrication of such subwavelength grating structure still requires complex processes such as nanolithography, multilayer porous films, self-masking plasma etching, and polymer replication and so on.9,10,36 Fig. 4 shows the reflectance spectra of all the films annealed at 1000 C in the wavelength range of [250–1000 nm]. The observed oscillations result from the constructive or destructive interference caused by the multiple reflections of the light at the interfaces of the air/ZnO:Er,Yb film and ZnO:Er,Yb/Si substrate.5,7 We noted that the ZnO:Er,Yb films annealed at 1000 C decrease the reflectance response of Si substrate by light trapping, and the reflectance spectra detailed in Table I were found between 12% and 15% for all the wavelengths. For the film possessing 9 mol. % Yb doping, the reflectance percentage was 12% in [250–1000 nm], referring to 114 nm RMS roughness value. This reflectance percentage is considered as significant compared to the ZnO:Er films produced by the same technique and reported in our previous work.5 Also, our films are considered as efficient light trapping layer than Y2O3:Bi,Yb film, which possessed a reflectance percentage of 15.8%.7 It can be observed also that all our annealed films have a minimum reflection at 610 nm which is useful for Si solar cells applications.6 These results revealed that the reflectance percentages were decreased when the light reflection is suppressed by ZnO:Er,Yb/Si interface ensured by nanograins structure film. IV. CONCLUSION

In summary, Er3þ,Yb3þ:ZnO DC thin films have been successfully deposited using AACVD process. The morphology and XRD spectra show the preferential orientations of the hexagonal phased Er3þ,Yb3þ:ZnO which are grown perpendicularly to the Si substrate. The refractive indexes of the annealed films in air atmosphere at 1000 C were in the range of 1.95–1.97. This result shows that these annealed films are very transparent with a higher crystallinity than the as-deposited ones. The ZnO:Er,Yb thin films reduces the average reflectance of Si substrate from 35% to 12% for the film having 3 mol. % Er, 9 mol. % Yb concentration. This film also shows intense NIR luminescence Yb3þ emission at 1000 nm originating from an energy transfer between Er-Yb ions. These results prove that the ZnO:Er,Yb thin films have combined both light trapping and spectrum shifting properties for enhancing the efficiency of c-Si solar cell. ACKNOWLEDGMENTS

We thank Professor R. Chtourou for his kind help in AFM measurements and discussions. 1

FIG. 4. Reflectance percentages versus wavelength of all ZnO:Er,Yb thin films annealed at 1000 C.

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