In this study, we used the autocloning effect on pyramid structures to develop broad-bandwidth, omnidirectional antireflection structures for silicon solar cells. The angular dependence of reflectance on several pyramid structures was systematically investigated. The deposition of three-layer autocloned films reduced the refractive index gap between air and silicon, resulting in an increase in the amount of transmitted light and a decrease in the total light escaping. The average reflectance decreased dramatically to ca. 2–3% at incident angles from 0 to 60° for both sub-wavelength– and micrometer–scale pyramid structures. The measured reflectance of the autocloned structure was less than 4% in the wavelength range from 400 to 1000 nm for incident angles from 0 to 60°. Therefore, the autocloning technique, combined with optical thin films and optical gradient structures, is a practical and compatible method for the fabrication of broad-bandwidth, omnidirectional antireflection structures on silicon solar cells.
© 2010 OSA
Energy shortages and environmental issues are making solar cells more attractive as future energy devices. Because silicon (Si) has several practical advantages, including low cost, good thermal stability, mature fabrication technologies, and high efficiency, it is at present the major material used in the solar cell industry. The refractive index difference between air and Si, however, causes more than 40% of incident light to be reflected back through Fresnel reflection, thereby limiting the opto-electrical conversion efficiency of Si solar cells. Several methods have been developed to decrease the reflectance of Si surfaces, such as the use of antireflection (AR) coatings and textured structures. Although a single-layer of an AR coating, such as a Si3N4 film, can minimize the reflectance at a specific wavelength to almost zero, it does not provide broad-bandwidth AR properties over the visible and near-infrared (NIR) regions. Textured structures on Si surfaces decrease the reflectance over a wide wavelength range because they provide a graded effective refractive index profile. Several groups have developed broad-bandwidth AR textured structures on Si wafers. Anisotropic wet etching with alkaline solutions can form pyramidal structures that lower the reflectance to ca. 10% . Nanowire and nanocone arrays that provide a reflectance of less than 5% are also good candidates for AR structures [2–7]. Although these nanostructures exhibit good AR properties at normal incidence, the angle of the incident light varies during the day; therefore, it remains a challenge to develop broad-bandwidth, omnidirectional AR structures.
Chhajed et al.  used an oblique deposition method to develop three-layer, graded-index coatings with a ultralow refractive index (n) onto a SiO2 layer to reduce the average reflectance to less than 6% for wavelengths between 400 and 750 nm and for incident angles between 40 and 80°. Xi et al.  also used the oblique deposition process to deposit optical thin-film materials having a tunable refractive index to reduce the reflectance to 0.3% for angles of incidence from 0 to 55°, but only at a wavelength of 632.8 nm. Kuo et al.  used a similar method to fabricate a graded-index stack featuring six layers with quintic profiles  —which, theoretically, would have the best AR properties—to achieve a broad-bandwidth, omnidirectional AR surface exhibiting an average reflectance of 3.79% over the wavelength regime from 400 to 2000 nm and at angles of incidence from 8 to 60°. Although graded-index multilayer AR coatings can provide low reflectance, the applications of low-n thin films formed through oblique deposition are limited because of their porosity and difficulties in controlling the deposition process. Furthermore, the passivation properties of these porous films of AR coatings remain problematic.
Two groups have previously employed sub-wavelength structures to suppress the reflection of incident light. Huang et al.  reported that a non-periodic array of Si nanotips exhibited variable reflectance in the ultraviolet, visible, and terahertz regions when changing the height of the nanotips. Sai et al.  analyzed the wide-angle AR effect using solar cells possessing sub-wavelength structures. They found that although the average reflectance decreased to 3% over the incident angle range from 0 to 50°, the short-circuit current densities of solar cells incorporating the sub-wavelength structures did not increase, as expected, from the increased amount of incident light, presumably because of carrier recombination at the rough surface and poor contact between the electrodes and cells, due to the threadlike profile of these nanotip structures.
To obtain broad-bandwidth, omnidirectional AR structures without degrading the electrical properties of the solar cell, in this study we applied an autocloning technique to fabricate AR structures on Si solar cells. Autocloning is a deposition technique for the fabrication of three-dimensional (3D) photonic crystals [14–17]. A structure such as a two-dimensional (2D) pillar array is first prepared on the substrate using lithography and etching processes. Next, the stacking of alternating layers having different refractive indices onto the patterned substrate is performed through ion bombardment during sputtering or high density plasma chemical vapor deposition (HDP-CVD) processes . Although the deposition mechanisms of HDP-CVD and sputtering are different, both exhibit an angle-selective sputter etching effect, due to ion acceleration during the deposition process. The sputter etching effect can lead to good step coverage and conformality; therefore, it can be used for the deposition of multilayer thin films on patterned photonic crystals. Using the autocloning effect, the multilayer films can be deposited to follow the shape of the sub-wavelength structure on the substrate and, thereby, form a 3D photonic crystal structure. In this study, we developed a practical method, compatible with conventional processes for the fabrication of Si solar cells, by combining optical thin films with the autocloning technique. We analyzed the optical behavior of the autocloned AR structures through rigorous coupled wave analysis (RCWA) and the 3D finite-difference time domain (3D-FDTD) method to optimize their broad-bandwidth, omnidirectional AR properties for solar cell applications.
2. Simulation and experimental setup
The optical behavior of the autocloned AR structures was analyzed using the RCWA method, which directly solves Maxwell’s electromagnetic equations on periodic structures. Figure 1(a) presents the simulation model: a structure comprising close-packed pyramids, set in a periodic manner to form 2D structures having different periods and heights. In the simulation setup, pyramidal structures with or without autocloned multilayer films were constructed approximately in the form of a rectangular stack of gradually decreasing size. The inset to Fig. 1(a) reveals that if the three-layer optical thin film was composed of SiO2, Si3N4, and TiO2, the top view of each rectangular plate composed to the autocloned structure.
The degrees of reflection were calculated over the wavelength range between 400 and 1000 nm for both s- and p-polarized light; the reflectance of randomly polarized light was computed by averaging the reflectance of the s- and p-polarized light.
The averaged reflectance of the broad-bandwidth wavelength range was computed using the equation
The angular dependence of the reflectance was evaluated by varying the incident angle of the light from normal incidence to 60°. Randomly polarized incident light was assumed in the simulation. The total average reflectance was calculated using the equation
Pyramidal structures were fabricated on Si substrates on micrometer and sub-micrometer scales using conventional KOH wet etching and sequential electron-beam lithography/reactive ion etching, respectively. A transformer-coupled plasma reactive ion etcher (TCP-RIE) and the reactive gases Cl2, HBr, CF4, and SF6 were used to fabricate the pyramidal structures.
The mechanism of the autocloning effect is displayed in Fig. 1(b). The deposition and sputter etching processes were performed alternately. First, deposition of films was performed during the CVD or sputtering process. Next, sputter etching and corner chopping with an angular-dependent etching were performed. Sputter etching is a physical etching process, operating without chemical reaction, for thin-films; it usually operates under low pressure with an Ar ion plasma. The sputter-etching rate of the step corner will be faster than the removal of the thin films from the surface. Hence, the slope angle of the step corner will be retained. By balancing these two phenomena, the surface shapes remain the same before and after each processing cycle. Therefore, the corrugated shapes can be duplicated one after another. In this study, cross-section profiles of AR structures were observed using a JEOL JSM-6500F scanning electron microscope. The optical properties were measured using a Jasco V-570 spectrophotometer equipped with an integrating sphere.
3. Results and discussion
First, we used the RCWA method to simulate the relationship between the pyramidal structure size and the surface reflection. Figure 2(a) displays normal-incidence reflectance spectra of close-packed pyramidal Si structures having periods of 0.1 (black line), 0.5 (red line), and 5 (blue line) μm. We fixed the ratio of the structure height to the period at 0.7. Figure 2(a) reveals that the reflectance of the structure having a period of 0.1 μm was high for the small structure height. We also find that the reflectance was less than 5% in the visible and NIR regimes for the structure having a period of 0.5 μm. Increasing the period to 5 μm caused the reflectance to increase, because structures having large periods possess poor gradient indices. Figure 2(b) displays the average reflectance at normal incidence for the various pyramidal structures. Here, we fixed the aspect ratios of the pyramidal structures at 0.7, close to that of the real structures etched in base. The average reflectance decreased upon decreasing the period because the sub-wavelength structures exhibited superior gradient index behavior. If the structure height is small, however, the average reflectance will be increased.
To analyze the angular-dependence Ravg (θ) of the different light trapping structures, we computed the average reflectance with respect to the incident angle. Figure 3(a) presents the values of Ravg (θ) of the pyramidal structures having periods of 0.5 and 5 μm. Although the structure having a period of 0.5 μm exhibited a value of Ravg (θ) of ca. 3.5% at normal incidence, this value increased to more than 15% upon increasing the incident angle to 60°. In contrast, the structure having a period of 5 μm exhibited higher average reflectance for light at normal incidence, but its reflectance remained at ca. 7.5% for incident angles ranging from 0 to 60°. Overall, the pyramid structures having periods of 0.5 and 5 μm had a similar average reflectance (Rtotal-avg) of ca. 7.5% over the range of incident angles from 0 to 60°. Figure 3(b) displays the reflectance spectra of a conventional single-layer film of Si3N4, having an optical thickness of one quarter wavelength, at incident angles of 0 (blue) and 60° (red). A single-layer AR coating reduces the reflectance through destructive interference of reflected light; only light of a specific wavelength and incidence angle can satisfy the specific conditions. When the incident angle increases from 0 to 60°, the wavelength of lowest reflectance is blue-shifted and the value of Ravg (θ) increases from 9.2 to 16.8%. Therefore, a single-layer AR coating cannot be applied as a broad-bandwidth, omnidirectional structure in a working solar cell.
Next, we designed a three-layer AR coating having a gradient refractive index profile for application in broad-bandwidth, omnidirectional solar cells; the three layers were, from bottom to top, TiO2 (126 nm), Si3N4 (69 nm), and SiO2 (102 nm). Figure 3(c) displays calculated reflectance spectra for the three-layer AR coating on a Si substrate for incident angles of 0 and 60°. At normal incidence, the value of Ravg of the three-layer AR coating was 4.7%; it increased to 9.8% at an incident angle of 60°. In previous studies, graded-index AR coatings combined with ultralow-refractive-index films prepared using oblique deposition have exhibited very good AR properties [8–11], although the applications of ultralow-refractive-index films are limited, as we mentioned earlier. In our three-layer structure, a refractive index difference existed between the top SiO2 layer and air; therefore, the reduction in its value of Rtotal-avg is not comparable with that of the three-layer AR coatings with ultralow refractive index designed by Chhajed et al. .
We also used RCWA and 3D-FDTD methods to study the AR properties of the structures reported by Huang et al.  From the RCWA simulation, Fig. 3(d) displays the reflectance spectrum of the sub-wavelength nanotip structure having a period of 0.2 μm and a height of 1.6 μm. The reflectance of this nanotip structure was less than 0.9% over the entire wavelength range from 400 to 1000 nm at normal incidence; the average reflectance was only 0.33%. The reflectance at an incident angle of 60° was at most 2.1%, with an average reflectance of only 0.78%. Although the high aspect ratio of this nanotip structure provided superior AR behavior relative to that of other AR structures, using this kind of structure introduces several problems, including surface defects and poor diffusion and optical properties. We discuss the optical behavior of the nanotip structure in further detail in our description below of the near-field simulations.
In this study, instead of using oblique deposition to form ultralow-refractive-index AR coatings or threadlike nanotip structures, we combined multilayer optical thin films [Fig. 4(a) ] with a textured Si surface to form an autocloned multilayer AR structure [Fig. 4(b)]. Figure 4(c) displays the periodically autocloned structure that we prepared on a Si substrate using the autocloning technique. The SEM image reveals that the SiNx/SiO2 films, which had a total thickness greater than 2 μm, each followed the shape of the textured Si surface; thus, the autocloning technique can be used to generate multilayer optical thin films.
Figures 4(d) and (e) display the reflectance spectra of the autocloned pyramid structures having periods of 0.5 and 5 μm, respectively. When the three-layer autocloned AR coatings described in Fig. 3(c) were deposited on the pyramidal structures of these textured Si surfaces, the reflectance from both structures decreased dramatically over the entire range of working wavelengths of a Si solar cell. The values of Ravg (θ) at normal incidence decreased from 3.76 to 0.21% for the autocloned structure having a period of 0.5 μm, and from 7.32 to 2.99% for that having a period of 5 μm.
To study the omnidirectional properties of our autocloned AR structures, Figs. 5(a) and (b) displays the simulated angle-dependent values of Ravg (θ) of structures having periods of (a) 0.5 and (b) 5 μm in the presence and absence of autocloned multilayer thin films. Upon increasing the incident angle from 0 to 60°, the average reflectance of the 0.5-μm pyramidal structure without autocloning increased from ca. 4 to 16%. After adding the three-layer autocloned film, the average reflectance decreased dramatically to the range from ca. 0.2 to 2.5% for incident angles from 0 to 60°. The average reflectance of the 5-μm pyramidal structure without autocloning was ca. 7.5% for incident angles from 0 to 60°. After adding the three-layer autocloned film, the average reflectance decreased significantly to ca. 3% for incident angles from 0 to 60°. These simulations suggested that the average reflectance of the 0.5-μm pyramidal structure had greater angular dependence than that of the 5-μm structure. The autocloned films effectively reduced the omnidirectional reflectance of both the 0.5-and 5-μm pyramidal structures, with an especially great effect for the sub-wavelength structure (0.5 μm) in the large-incidence-angle regime.
To analyze the optical behavior of autocloned structures in the near-field regime, we used the 3D-FDTD method to simulate the behavior of light incident to the textured structures in the presence and absence of autocloned multilayer films. Figures 6(a) and (b) display the electric field intensity distributions of plane waves propagating into a pyramidal structure having a period of 0.5 μm at normal incidence and at an incident angle of 45°, respectively. The plane waves had a wavelength of 520 nm and were applied incident from 1.3 μm above the substrate; therefore, any electric field distribution above this position would have arisen from reflected waves. The absorption of Si at this wavelength led to a gradual decay in the amount of transmitted light. Because the sub-wavelength pyramidal structure possessed good gradient index properties at normal incidence, Fig. 6(a) reveals the presence of a weak reflection wave on the pyramid surface. In Fig. 6(b), the reflection of the oblique incidence wave increased because of poorer optical gradient behavior when the plane wave propagated toward the textured Si surface at an incident angle of 45°. Because its reflection would be higher when light propagates into the structure at larger incident angles, this kind of pyramidal structure would not be suitable as an omnidirectional AR structure for Si solar cells.
Figures 6(c) and (d) display the electric field distributions of plane waves propagating toward a pyramidal structure possessing a three-layer autocloned film at normal incidence and at an incident angle of 45°, respectively. The presence of the autocloned films on the pyramidal structures provided superior AR properties relative to the unmodified Si pyramidal structures. The three-layer autocloned films reduced the refractive index gap between air and Si, thereby increasing the amount of transmitted light and decreasing the amount of total escaped light. Comparing Figs. 6(b) and (d) reveals that the autocloned films greatly reduced the reflection of light at large incidence angles.
Next, we analyzed the transmitted waves of the light propagating through the textured Si surfaces. Because the absorption of light in the depletion regions near p–n junctions in solar cells is more effective than at other positions, the electric field induced by the built-in voltage can separate electron/hole pairs, resulting in electric current. Figures 7(a–c) display the electric field distributions resulting from plane waves propagating into (a, b) autocloned pyramid structures [having periods of (a) 5 and (b) 0.5 μm] and (c) high-aspect-ratio nanotips (having a period of 200 nm and a height of 1.6 μm); the depletion regions in these structures are highlighted by dotted lines. Conventionally, doping processes are performed through thermal diffusion of dopants to fabricate p–n junctions in solar cells. Diffusion is an isotropic process that generally forms highly doped regions away from the Si surface. In Fig. 7(a), we observe that the junction formed along the autocloned surface of the pyramidal structure having a period of 5 μm. Because of the limitations of the thermal diffusion process, it is generally difficult to form a continuous doped region near the surface of a rough structure having a scale of less than 500 nm. Therefore, to fabricate a continuous doping region that would retain the conduction of the photocurrent, the p–n junction should be formed below the bottom of the sub-wavelength pyramid and nanotip structures, as displayed in Figs. 7(b) and (c), respectively.
Figures 3(d) and 7(c) reveal that the reflection loss from the nanotip structure was extremely low because of the good gradient refractive index profile of this structure and because the incident waves bounced off the sidewalls of the nanotips. The electric field intensity of the transmitted intensity near the junction region beneath the nanotips was, however, lower than those of the autocloned pyramid structures having periods of 0.5 and 5 μm. Figure 7(d) displays the power fluxes of transmitted waves passing through the autocloned pyramid structure and the nanotip structure. The transmitted power flux of the nanotip structure was only one-third of that of the autocloned structure, presumably because a plane wave transmitting into the nanotips would have to propagate across the absorbed tip to reach the junction region. Moreover, the low degree of carrier recombination and the poor contact of electrodes on nanotip structures are two key disadvantages hindering their practical application. Autocloned structures do not suffer from these problems because their p–n junction regions are located close to the Si surface.
In an experimental study, we fabricated three-layer autocloned films on pyramidal structures to demonstrate their broad-bandwidth, omnidirectional AR effects. Figure 8 displays the measured values of Ravg of a conventional randomly arranged pyramid structure (black line) and a pyramid structure featuring a three-layer autocloned film (red line) at incident angles from normal to 60°. The value of Ravg of the conventional pyramid structure was ca. 12.5% at normal incidence; it increased dramatically upon increasing the incident angle, reaching greater than 30% at an incident angle of 60°. This behavior resulted from the random arrangement of the pyramid structure and from the size distribution of the pyramids (a range of several micrometers). After combining the autocloned multilayer thin film with the textured Si surface, the measured reflectance was suppressed to ca. 3% at normal incidence. Moreover, the values of Ravg remained less than 4% for incidence angles from normal to 60°. Thus, the autocloned sample displayed lower reflectance and superior omnidirectional properties relative to those of the conventional pyramid structure.
In this study, we used the autocloning effect to combine multilayer optical thin films with pyramidal structures to develop broad-bandwidth, omnidirectional AR structures for Si solar cells. We systematically investigated the effects of several pyramid structures on the angular dependence of their reflectance. Sub-micrometer–scale pyramid structures displayed low reflectance at normal incidence, but it increased rapidly upon increasing the incident angle; micrometer-scale pyramid structures exhibited higher reflectance at normal incidence, but with no obvious increase upon increasing the incident angle. After depositing the autocloned multilayer films, the various pyramidal structures—on both sub-wavelength and micrometer scales—exhibited obvious improvements in their AR properties. The presence of a three-layer autocloned film reduced the refractive index gap between air and Si, thereby increasing the amount of transmitted light while reducing the amount of total escaped light. The autocloned films provided a particularly dramatic reduction in the amount of reflected light at large incidence angles for the sub-wavelength–scale pyramid structures. For both the sub-wavelength– and micrometer–scale pyramid structures, the addition of three-layer autocloned films caused the average reflectance to decrease dramatically to ca. 2–3% at incident angles from 0 to 60°. Although a related nanotip structure displayed excellent broad-bandwidth, omnidirectional AR properties, it possessed several disadvantageous features, including poor contact with electrodes and large amounts of light absorbed by the tips before it could reach the junction. In an experimental study, we found that the measured reflectance of an autocloned structure was less than 4% for wavelengths in the range from 400 to 1000 nm and incident angles from 0 to 60°. Therefore, the autocloning technique combined with optical thin films is a practical and compatible approach for the fabrication of broad-bandwidth, omnidirectional AR structures for Si solar cells. This autocloning method has great potential for practical applications, because this method does not need to largely change the manufacture process of a silicon solar cell.
We thank the National Science Council, Taiwan, for supporting this study under contracts NSC-97-2221-E-002-046-MY3 and NSC-97-2623-7-002-008-ET.
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