Haze ratio enhancement using a closely packed ... - OSA Publishing

Haze ratio enhancement using a closely packed ZnO monolayer structure Shih-Shou Lo1*, Dison Haung1, and Der-Jun Jan2 1 Department of photonics, Feng-Chia University, No.400, Whenwha Rd. Seatwen area, Taichung, Taiwan Physics Division, Institute of Nuclear Energy Research, No. 1000, Wenhua Rd., Jiaan Village, Longtan Township, Taoyaun County, Taiwan 32546, Republic of China *[email protected]

2

Abstract: A ZnO thin-film that comprised a ZnO monolayer close-packed structure embedded in ZnO sol-gel was fabricated. The structure contained ZnO spheres of various sizes. At normal incidence, the mean optical transmittance of the thin-film was 60% in the visible region when the structure contained spheres with a diameter of 300 nm. An average haze ratio of 80% was obtained from the thin-film when the structure was formed using 500 nm-diameter spheres. The thin-film exhibits broad-band transparency and a high haze ratio. It can be used as a light-trapping structure in an ultrathin solar cell. ©2010 Optical Society of America OCIS codes: (310.0310) Thin-Film; (310.6628) Subwavelength Structures

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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1. Introduction The surface roughness of a thin-film dominates the scattering of light from it. Such films are critical in thin-film solar cells as they are used to optimize the surface roughness of the front or back transparent conductive oxide (TCO). The film layer is that through which the light

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Received 18 Nov 2009; revised 13 Dec 2009; accepted 16 Dec 2009; published 4 Jan 2010

18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 662

enters or reflects back to the solar cell. It scatters all incoming light, and its surface roughness must be especially suited to diffusing long-wavelength light. The main required properties of TCO layers in solar cells are high transparency and conductivity. The most extensively adopted transparent semiconductors are made of InsO3, SnO2, ITO or ZnO. However, in thin-film solar cells, the TCO layer must also act as a light scatterer, to increase the overall absorption, and thus allow the thickness of the cell to be reduced, for a given value of the absorption. For amorphous silicon solar cells, a drop in the thickness reduces the light-induce degradation of cell efficiency. For microcrystalline solar cells, whose deposition rates are typically low, reducing the cell thickness enables the deposition time to be reduced lowering fabrication costs. Most approaches depend on a particular structure of the top surfaces of the soar cell for trapping the light inside the cell [1]. Their ultimate goal is to trap the light perfectly in the absorbed layer. These processes do not have to be efficient in all spectral domains, only in the spectral at the long wavelength, where the absorption by the solar cell material is low but not negligible, and where absorbed photons contribute to the short-circuit current. The structure of the back-side of solar cell has attracted relatively little research interest. Thin-film solar cells benefits form the trapping of light by a back reflector with a suitable structure. Since the back reflector is not at all (exposed to the short-wavelength region of the solar spectrum (UV and blue light); this part of the spectrum is absorbed by the solar cell before it reaches the back reflector. Since the back reflector only reflects the light and not to transmit it, the proposed configuration provides more design flexibility in the implementation of improved light-scattering schemes. The transmission of light is more easily achieved using a conductive electrode in a solar cell. When a material with low absorption is used to solar cell, a highly reflective material can be adopted as an efficient reflector at the rear of the cell to double the light path. Again, for an a-Si thin film, this high back reflection is required only for light in the red and near-infrared spectral domains, since light of shorter wavelengths is sufficiently efficiently absorbed in a primary passage through the cell. In 1995, Claus et al. introduced blazed gratings with periods of less than a micrometer as back-reflectors for solar energy applications [2], increasing the optically effective thickness of the cell by a factor of five. In 1998, Zettner et al. proposed another approach for fabricating a back reflector. They adopted a Bragg-type multi-layer structure that comprised porous Si as a highly efficient back reflector in a spectrally narrow domain [3]. Recently, L. Zeng et al. demonstrated the enhancement of the efficiency of an Si solar cell using a textured photonic crystal back reflector: external quantum efficiency was significantly improved between the wavelengths of 1000 nm and 1200 nm [4–7]. Such a structure in a solar cell does have the disadvantage of involving an expensive high-precision photolithograph process and layerdeposition process. To eliminate the need for an expensive high-precision and complex photolithographic process, randomly textured back reflectors with various nano-structural dimensions have been fabricated using plasmas, acid-base solvents or self-assembled photonic crystals structure [8–11]. A current gain of 20% over or absorption enhancement that of an identical cell that is deposited on a flat, mirror-like back reflector was achieved using these structures. In this work, a ZnO thin-film with a ZnO monolayer close-packed structure is fabricated and embedded in a ZnO sol-gel thin-film. The optical properties of thin-film were studied. 2. Experimental details Synthesizing ZnO spheres requiring two steps. In the primary reaction, an appropriate amount of zinc acetate dehydrate was added into solution. The reaction solution was heated to 170 °C. ZnO spheres precipitated out when the working was reaching, the solvent was placed in a centrifuge. After centrifuging for several minutes, the supernatant was used in the second step and the sediment was discarded. A secondary reaction was then performed to yield monodispersed ZnO spheres. It began in a manner similar to the primary reaction. ZnAc was added into the reaction solution was heated under reflux. Before the working temperature (140 #120170 - $15.00 USD

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°C) was reached, a suitable amount of the primary reaction supernatant was added to the solution. At 170 °C, the solution was stirred for one hour. The as-synthesized sample was washed six-times in de-ionized water. Finally, the solution was dropped on glass and heated to 120 °C to dry in air. The schematic diagram of ZnO nanparticles synthesis process was displayed in the Fig. 1.

Fig. 1. Schematic diagram of fabrication process of proposed ZnO thin-film.

The as-synthesized ZnO spheres with diameter of 300 nm were dissolved in a mixed solution. The ZnO nanoparticles were deposited on the surface of B270 glass in a dip-coating at a rate 0.13 mm/s of dip-coating process. After the ZnO spheres were uniformly deposited on the substrate, a suitable amount of ZnO sol-gel solution infiltrated the close-packed monolayer structure in a spin-coating process at 1500 rpm. Finally, the ZnO thin-film was annealed at 550 °C for one hr. 3. Results and discussions Morphological and structural analyses of ZnO spheres were performed by SEM and HRTEM. Figure 2(a) presents SEM and HRTEM images of ZnO spheres. The inset in Fig. 2 presents a detailed image, from which shows nano-entities on the surface of each sphere. The sizes of the ZnO spheres and the nano-entities are 300 nm and 15 nm, respectively. Figure 2(b) shows a representative HRTEM image of ZnO spheres. The image indicates that the ZnO spheres have an inseparable structure. Figure 3 displays XRD patterns of the ZnO spheres. The strong and clear peaks in this figure show the high purity and crystallinity of the as-obtained product. All of the peaks in the figure are associated with the wurtzite also ZnO structure, according to JCPDS card file No.36-1451. Five reflections along , , , and are associated with common pure ZnO.

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Fig. 2. (a) SEM image of as-synthesized ZnO spheres. (b) HRTEM image of ZnO spheres.

Fig. 3. XRD pattern of as-synthesized ZnO spheres.

Figure 4 presents the room-temperature PL spectrum of ZnO spheres, obtained using a 193 nm KrF pulsed laser at room temperature. The spectrum of typical ZnO spheres comprises weak UV band emission and green emission. The green emission corresponds to a single ionized oxygen vacancy in the ZnO, which is formed by the recombination of a photogenerated hole with the single ionized charge state of this defect. The weak green emission also reveals few surface defects in the ZnO spheres. Strong PL peaks are observed at 383 nm. The sharp peaks were presumably produced by excitons. The full width at half maximum (FWHM) of the peak associated with ZnO spheres was 16 nm.

Fig. 4. Room-temperature photoluminescence spectrum of as-synthesized ZnO spheres.

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Figure 5(a) shows the morphology of the ZnO monolayer structure, which is deposited at a concentration of ZnO spheres solution of 10 wt. %, in a dip-coating process at a rate of 0.13 mm/s. A monolayer loosely packed structure is formed on B270 glass. The pattern obtained at a concentration of ZnO spheres solution of 20 wt.% reveals, a close-packed structure, as presented in Fig. 5(b). Few defects are observed in the structure because the synthesized ZnO spheres are asymmetrical. Therefore, the self-assemble monolayer structure has some defects. Notably, a uniform monolayer with a large area can be easily fabricated following the process that was adopted in the experiment. After the sol-gel has infiltrated the interstitial space of the prepared monolayer structure, the ZnO spheres were successfully embedded in the ZnO solgel thin-film.

Fig. 5. SEM image of monolayer structure (a) unclose-packed pattern (b) close-packed pattern.

Various ZnO thin-films were fabricated for comparison. Sample A is a typical ZnO sol-gel thin-film that was fabricated using the process of Ohya et al. [12]. Sample B is a proposed ZnO thin-film. Figure 6(a) presents the schematic diagram of proposed ZnO thin-film. Figure 6(b) shows a photographic image of samples A and B. The back-ground of sample A is clear and visible due to ZnO is transparent material and less scattering was caused by sol-gel thinfilm.

Fig. 6. (a) Schematic diagram of as-fabricated ZnO thin-film. (b)Photographic image of ZnO thin-film. Sample A is sol-gel thin-film without ZnO spheres. Sample B is proposed ZnO thinfilm.

Figure 7 shows the transmission spectra of samples B with ZnO nanoparticles of various sizes (300, 500 and 650 nm) obtained at normal incidence to the thin-film. For comparison, Fig. 7 also presents the transmission spectrum of sample A. The transmittance of sample A in the wavelength range 380–800 nm is 90%. This result is in good agreement with the transmittance spectra of bulk ZnO. When spheres with a diameter of 650 nm were used to form a monolayer structure in sample B, the transmittance is 50%. Moreover, the maximal transmittance exceeded 60% at a wavelength of 800 nm.

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Fig. 7. Transmittance spectra of ZnO thin-film with different structure.

A mean optical transmittance of 60% was obtained when spheres with a diameter of 300 nm were used to form the monolayer structure. Consider a monolayer of dielectric spheres arrayed periodically in the xy-plane. The dielectric constants of the sphere and the outer region are denoted εsol-gel and εsphere, respectively. Reducing the scattering effect of the thinfilm increase transmittance. The transmission curve exhibits a broad-dip for sphere diameter of 500 nm and 600 nm. Each broad dip is attributable to intrasphere Mie scattering, or more precisely, the geometric sum of Mie scattering by the spheres [13]:

  1 E ( r ) = ∫ 1 + 2 ∇ 2  G ( r , r ′ ) × V ( r ′ ) E ( r ′ ) dr ′  ksol − gel   

(1)

where

2 V ( r ) = (ω / c ) ε sol − gel − ε ( r )  (1.a) G ( r , r ′ ) = − exp ( iksol − gel r − r ′ / 4π r − r ′ ) (1.b) Equation (1.a) indicates that E(r) = 0 at εsol-gel = ε(r). In this experiment, the scattering was huge. Therefore, the ZnO monolayer close-packed structure that is well embedded in ZnO solgel can scatter effectively in optoelectronic devices. Since light scattering by a thin-film depends mainly on its surface roughness. The surface roughness of front or back TCO must be optimized for light trapping ability of a thin ZnO thin-film is given by its haze ratio. A preliminary index of light trapping ability of the ZnO thin film is its haze ratio. To measure the degree of elastic light scattering, the haze-ratio H, given by

H=

Td

Tt

(2)

is used, where Td is the diffusivity and Tt is total transmittance, respectively. Equation (2) indicates the high H value can be achieved using high scattering effect or less transmittance. However, the less transmittance of thin-film can be used in top TCO of solar cell. Figure 8 presents the haze ratio of sample B with various sizes of as a function of operating wavelength in the range of 380~800 nm. For comparison, Fig. 8 also presents the haze ratio of sample A. The haze ratio of sample B exceeds 60% and markedly exceeds that of sample A (15%). Notably, the largest haze ratio of any thin-film is 80%, exhibited by the film of 500 nmdiameter spheres. This result motivates further work to evaluate the light trapping ability of the proposed thin-film when used in ultrathin solar cell.

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Fig. 8. Haze ratio of ZnO thin-film on B270 with different structure.

To demonstrate the light trapping ability of proposed ZnO thin-film as a proof of concept, an ultrathin a-Si layer with the thickness of 0.26 µm was deposited on the proposed thin-film. Figure 9 shows a photographic image of an ultrathin a-Si layer deposited on samples A and B. The sample B is less transparent than the sample A. It should be noted that an uniform a-Si layer can be obtained on sample B due to the ZnO nanoparticles were embedded in ZnO solgel thin-film. Figure 10 presents the absorption spectra of sample B with nanoparticles of various sizes as a function of operating wavelength in the range of 350~800 nm. For comparison, Fig. 10 also presents the absorption of sample A. Since the absorption coefficient of a-Si at short wavelength (λ600 nm), the optical absorption of sample A and B is identical due to the thickness of a-Si is enough to absorb the incident light. At the long wavelength (λ>600 nm), the optical absorption of sample B with ZnO nanoparticles of various sizes (300, 500 and 650 nm) is higher than that of sample A due to the light path in sample B become longer caused by multiple scattering and reflection of proposed thin-film.

Fig. 9. Photographic image of 0.26 µm a-Si deposited on sample A and sample B.

4. Conclusions

The work successfully demonstrated a ZnO thin-film that comprised a ZnO monolayer closepacked structure that was embedded in ZnO sol-gel. A monolayer ZnO close-packed structure was developed. The transmittance and haze ratio of proposed thin-film were measured. A broad-band transparency and high haze ratio of ZnO thin-film was obtained. The absorption enhancement of ultrathin a-Si layer deposited on proposed thin-film has been demonstrated. If the proposed ZnO thin-film exhibits excellent conductivity characteristics, it may be used as a back TCO layer of ultrathin solar cell in future.

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Fig. 10. Absorption spectra of 0.26 µm a-Si deposited on ZnO thin-film with different structure.

Acknowledgements

The author would like to thank the Institute of Nuclear Energy Research of Republic of China, Taiwan, for financially supporting research under contract No.(982001INER027).

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