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A single-step, low-cost fabrication method to generate resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. A guided-mode resonant structure is obtained by subsequent deposition of thin films of transparent conductive oxide and amorphous silicon on the imprinted area. Referenced to equivalent planar structures, around 25% and 45% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. The fabricated elements provided have 300-nm periods. Thermally imprinted thermoplastic substrates hold potential for low-cost fabrication of nano-patterned thin-film solar cells for efficient light management.
©2013 Optical Society of America
1. Introduction
In order to achieve efficient light absorption in thin-film solar cells, photonic nano-patterns are of great interest. Two basic approaches are considered to increase light interaction in solar media through nano- and micro-structures. One approach is to use an antireflective layer with a gradually changing refractive index profile to enhance optical transmission [1]. The other approach is to trap light inside the cell. Common light trapping schemes include use of front and/or back random texturing to scatter light as well as deployment of metallic or dielectric gratings to induce plasmon or quasi-guided modes, respectively, to confine light in the waveguide region [2, 3]. Along with one-dimensional (1D) and two-dimensional (2D) patterns, three-dimensional (3D) dome-, cone-, or rod-shaped structures are reported with enhanced conversion efficiencies [4–6]. The resonant photonic patterns are applicable to silicon solar cells and are promising for third-generation organic photovoltaics. However, additional fabrication steps of nano- or micro-domain features using conventional photolithography and subsequent etching of respective materials adds to production cost. Thus, the main challenge remains to improve low-cost and large-area fabrication techniques of such patterns. To overcome this issue, nano-imprint lithography (NIL) is a promising alternative.
The basic concept of NIL is to transfer a pattern from a master (or a mold) to various substrates under pressure [7–9]. The desired nano-patterns are normally fabricated first as master templates using photolithography or e-beam lithography and wet or dry etching. Various materials such as silicon, quartz, or metals can be used as masters. The substrate may have a spin-coated polymer resist layer that is subsequently cured by ultra-violet (UV) or infrared (IR) light, or it can be a thermoplastic substrate softened by a thermal source. Thermally activated NIL process is also known as hot embossing lithography. The master may or may not need a release agent depending on the type of substrate material used. Laser-assisted direct imprinting on inorganic films using excimer lasers has also been reported [10].
NIL is applied in vast topical areas including organic and inorganic thin-film transistors, light emitting diodes (LEDs) and organic LEDs, and organic and thin-film solar cells [7]. For thin-film solar cell applications in particular, NIL technique is mostly applied to fabricate antireflection layers. One- or double-sided imprinted films are fabricated separately and later attached on top of the solar cells [11–14]. For light trapping, imprinted randomly textured substrates are fabricated to scatter light in thin films of solar cells [15–17]. In contrast to scattering, light trapping through coupling quasi-guided or leaky modes into the waveguide region of solar cells requires resonant structures with precise shape and periodicity on the order of the operating wavelength. Few studies have been done on periodic nano-pattern fabrication using soft-NIL and nanomoulding [18, 19], and simple and easy fabrication techniques based on these concepts still needs to be conquered.
We have reported on enhanced solar uptake in thin-film hydrogenated amorphous silicon solar cells through guided-mode resonance (GMR) effects [20]. Here we complement our research by presenting a simple, easy, and faster fabrication approach of the nano-patterns with periods smaller than the operating wavelength using thermal nano-imprinting lithography. The technique involves fewer processing steps than soft-NIL and nanomoulding, circumvents solution process spin coating or UV-source curing, ensures high repeatability and a large printing area, and uses low-cost materials. These characteristics make the method suitable as an integrated step in production of thin-film photovoltaic devices.
2. Experiments
2.1 Master fabrication
The master grating is made of quartz, and Fig. 1 schematically shows the fabrication steps. A 1x1-inch2 quartz substrate is cleaned with acetone, isopropanol, and deionized water; the substrate is then dried with blown nitrogen. An 80-nm-thick bottom antireflection coating (BARC) is spin-coated at 1200 rpm and baked for 60 seconds on a heating plate adjusted to 205°C. A 300-nm-thick photoresist (PR) layer is subsequently spin-coated at 1100 rpm and baked for 90 seconds on a heating plate at 110°C. Both 1D and 2D grating patterns with a period of 300 nm are transferred to the PR using UV laser interferometric lithography. 2D grating patterns require double exposure with 90° sample rotation between exposures. After developing, the BARC layer in the PR windows is removed by oxygen plasma. Quartz is etched down to 66 nm for the 1D and 75 nm for the 2D grating using argon (Ar) and trifluromethane (CHF3) gas mixture in a reactive-ion etch (RIE) chamber. The remaining PR and BARC are removed using oxygen plasma.
2.2 Imprinted plexiglas fabrication
Plexiglas (polymethyl methacrylate, PMMA) is a thermoplastic material transparent to visible light. It has a luminous transmittance of 92%. Figure 2 shows the measured transmittance over the 400-nm to 700-nm wavelength range. The refractive index is 1.49 at the 589.3-nm wavelength [21], and the glass transition temperature (Tg) is 105°C. Plexiglas with higher Tg, up to 165°C, is also available. A ~1.5-mm thick plexiglas sheet is used for the experiments.
A schematic view of the pattern transfer setup is shown in Fig. 3. A quartz master is placed on a heating plate, and the plexiglas substrate sits on top of it. A load in the form of a metal chunk is applied from the top side to create a pressure of 1.57 bar (22.8 psi). The pressure is applied before the heating plate is turned on. When the temperature rises above Tg, the plexiglas behaves as a viscous liquid and under pressure allows the quartz grating to penetrate the substrate. The temperature of the heating is adjusted so that the temperature of the master, as measured by a thermocouple, is maintained at 160°C. After 60 minutes at 160°C, the heating plate is turned off and cooled down to 60°C after which the pressure is released. The imprinted plexiglas is ejected without any adhesion or damage to the master. We notice that the fabricated grating area on the master and the imprinted area on plexiglas is the same, 5x5mm2. With a larger fabricated master grating area and uniform application of pressure over plexiglas, larger imprinted grating areas can be obtained.
The grating geometry of a GMR element is defined by the period (Λ), fill factor (F), and grating depth (dg); Fig. 4 shows a schematic picture. Atomic force microscope (AFM) images and scanning electron microscope (SEM) images are taken to characterize the profile dimensions of the fabricated nano-patterns. The AFM images of 1D and 2D master grating profiles are shown in Figs. 5 and 6, respectively. Figures 7 and 8 present the AFM images of respective imprinted plexiglas. Table 1 gives the profile dimensions obtained from the AFM/SEM images. The period of both master and imprinted gratings is 300 nm as measured with AFM. The 1D master grating depth is 66 nm whereas the imprinted grating depth is 65 nm as measured with AFM. The 2D master grating and the imprinted grating depths are 75 nm as measured with AFM; the fill factors are 0.45 and 0.55, respectively. The dimensions obtained from the AFM images closely match the SEM images shown in Fig. 9. Figure 9(a) and 9(d) show the SEM images of quartz masters with a 1D and 2D grating, respectively, and Figs. 9(b) and 9(e) show the corresponding imprinted plexiglas. These topological SEM images show that the fill factor is approximately the same in the copy as in the master. Small cracks are observed on the imprinted plexiglas, which are due to the gold films deposited for improved visibility of the SEM images.
Fig. 4 Schematic view of grating geometry showing period (Λ), fill factor (F), and grating depth (dg).
Table 1. Characteristic numbers of grating profiles obtained from AFM and SEM images
Fig. 9 SEM images of the (a) 1D quartz master grating, (b) 1D imprinted plexiglas grating, (c) ITO-coated 1D imprinted plexiglas grating, (d) 2D quartz master grating, (e) 2D imprinted plexiglas grating, and the (f) ITO-coated 2D imprinted plexiglas grating.
2.3 ITO and a-Si film deposition on plexiglas grating
To observe the optical absorbance enhancements enabled via the imprinted grating structures, 80-nm-thick indium-tin-oxide (ITO) layers and 200-nm-thick amorphous silicon (a-Si) layers are deposited via sputtering over the imprinted plexiglas. As a reference sample, identical films are deposited on top of a planar plexiglas substrate. In principle, the presence of the ITO film has little effect on the optical enhancement but is applied to mimic a thin-film solar cell in order to demonstrate the potential of the imprint method for solar cell fabrication. Light is shone from the plexiglas side to realize a superstrate configuration of a solar cell with ITO serving as the front transparent conducting oxide. Figure 10 gives a schematic view of the structure with incident (I), reflected (R), and transmitted (T) beams. Figures 11 and 12 visualize the AFM surface image of ITO-coated patterned plexiglas. The grating depths after ITO coating remain similar, 67 nm for the 1D grating and 76 nm for the 2D grating. The SEM images of the ITO-coated nano-patterns are included in Fig. 9. Shallow grating patterns with grating depths of approximately 22 nm for the 1D grating and 35 nm for the 2D grating are transferred to the top surface of the a-Si layer after a 200-nm-thick a-Si deposition over the ITO pattern as shown in Fig. 13.
Fig. 10 Schematic view of fabricated nano-patterned a-Si film; arrows indicate the directions of the incident, reflected, and transmitted beams.
3. Results and discussion
A fiber optic spectrometer is used to measure the transmitted and reflected light from the fabricated devices shown in Fig. 13. Light is shone normally from the plexiglas side as Fig. 10 depicts. Taking the refractive index of plexiglas as 1.49 and the period of the grating as 300 nm, the Rayleigh wavelength of the patterned structure is λR = nΛ ≈450nm. Hence, only zero-order diffraction prevails beyond 450 nm. Reflectance (R0) and transmittance (T0) spectra normalized to the source spectrum are measured, and the absorbance is obtained as A = 1-R0-T0. Figure 14 shows the comparison of unpolarized absorbance between patterned and unpatterned reference samples at normal incidence. Integrated optical absorbance enhancement of about 25% for 1D and 45% for 2D nano-patterns is observed over the 450-nm to 900-nm wavelength range. Figure 15 gives the polarization dependent absorbance for a 1D grating pattern.
Fig. 14 Unpolarized absorbance spectra of planar reference and imprinted patterned samples at normal incidence of light.
Fig. 15 TE (electric field vector normal to the plane of incidence) and TM (electric field vector parallel to the plane of incidence) polarized components of absorbance of the 1D grating patterned sample at normal incidence of light.
We recall that even in these thin patterned films, a large number of resonant leaky modes fundamentally exists across the spectral region of interest in this work. The modes can be visualized by artificially setting the imaginary part of the complex refractive index to zero as exemplified by Wu et al. in [22]. In a real material, the modes at shorter wavelengths experience large absorption and the spectrum appears smooth without discernible resonance peaks. As the absorption falls with increasing wavelength as in normal material dispersion, the modal signatures become more distinct. Thus in our experiments, besides broadband monotonic absorption at shorter wavelengths, distinguishable resonance-based absorption peaks are observed at longer wavelengths as particularly evident at 762 nm for transverse electric (TE) and at 792 nm for transverse magnetic (TM) polarizations. At first, this result may appear curious to an experimentalist assuming that these are the signatures of the fundamental TE and TM modes as the TE modal resonance should appear at the longer wavelength. Here, the TE mode is in fact not the fundamental mode but rather the TE1 mode. To illustrate this, we compute the total electric and magnetic field distributions at resonance using rigorous coupled-wave analysis [23]. Figure 16(a) shows that a TE1-like mode profile belongs to the resonant peak at 762 nm. Figure 16(b) demonstrates the TM0 mode profile associated with the peak at 792 nm. As an approximation, we model the GMR structure with a rectangular grating profile in Fig. 16. Changing the grating shape to a rounded profile, as shown in Fig. 13, has a relatively small effect on the electromagnetic field patterns. In general, a resonance wavelength shift occurs on the profile variation as discussed by Shin et al. in [24]. In the present context, this wavelength shift has a minor effect on the resulting absorption enhancement.
Fig. 16 Total (a) electric field distribution for TE1 mode excitation at the 762-nm wavelength (b) magnetic field distribution for TM0 mode excitation at the 792-nm wavelength, observed in the absorbance spectra of the 1D grating sample shown in Fig. 15. The fill factors of plexiglas, ITO, and a-Si grating layers are 0.5, 0.66, and 0.66, respectively.
Conversely, the 2D grating pattern exhibits a polarization-independent response as shown in Fig. 17. In general, for both 1D and 2D patterned devices, the absorbance response and enhancement will vary widely for different photovoltaic absorbing materials and embodiments. Indeed, the occurrence and density of GMRs depends on the complex dielectric constant and its dispersion properties as well as on the thickness, fill factor, and grating modulation strength of the absorbing waveguide-grating layer system.
Fig. 17 TE and TM polarized components of absorbance of the 2D grating patterned sample at normal incidence of light.
As visualized by the surface images in Fig. 13, the sharpness of the 1D and 2D patterns decreases on account of the ITO deposition and still further upon a-Si film deposition. This does not substantially affect the optical enhancement obtained since the pattern period and dimensional regularity persist.
4. Conclusion
A low-cost approach to nano-pattern fabrication of resonant thin-film solar cells for efficient light trapping is proposed and experimentally demonstrated. The process includes a single step to fabricate the pattern on a substrate, which eliminates the need for photolithography and RIE steps. Imprinted 1D and 2D elements containing nano-patterns with a 300-nm period on thermoplastic substrates are designed and fabricated. The spectral variation of optical absorbance in thin a-Si films is measured, and it shows integrated absorption enhancement of ~25% for 1D and ~45% for 2D grating patterns across the 450-nm to 900-nm wavelength range, referenced to planar samples, for the particular device embodiments studied. Future research work may extend this methodology to real thin-film p-i-n solar cell fabrication over patterned imprinted substrates. Single-step fabrication methods facilitate production by enabling roll-to-roll printing of substrates for thin-film solar cells including organic photovoltaics. The thermal nano-imprint fabrication technique described in this study is not limited in shape or dimension of the pattern profile. Once a master grating is fabricated with a desired photonic structure and reasonable aspect ratios, concomitant patterns can be replicated with high repeatability and a long master lifetime.
Acknowledgments
This research was supported in part by the Energy Research Fund of the National Power Company of Iceland and by the UT System Texas Nanoelectronics Research Superiority Award funded by the State of Texas Emerging Technology Fund. Additional support was provided by the Texas Instruments Distinguished University Chair in Nanoelectronics endowment.
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