Broadband photocurrent enhancement and light-trapping in thin film Si solar cells with periodic Al nanoparticle arrays on the front C. Uhrenfeldt,1,2∗ T. F. Villesen,3 A. Tˆetu,4 B. Johansen,3 and A. Nylandsted Larsen2,3 1 Department
of Energy Technology, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark 2 Department of Physics and Astronomy, Aarhus University, Gustav Wieds vej 14, 8000 Aarhus C, Denmark 3 Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds vej 14, 8000 Aarhus C, Denmark 4 Department of Civil Engineering, Aalborg University, Sofiendalsvej 11, 9200 Aalborg, Denmark *[email protected]
Abstract: Plasmonic resonances in metal nanoparticles are considered candidates for improved thin film Si photovoltaics. In periodic arrays the influence of collective modes can enhance the resonant properties of such arrays. We have investigated the use of periodic arrays of Al nanoparticles placed on the front of a thin film Si test solar cell. It is demonstrated that the resonances from the Al nanoparticle array cause a broadband photocurrent enhancement ranging from the ultraviolet to the infrared with respect to a reference cell. From the experimental results as well as from numerical simulations it is shown that this broadband enhancement is due to single particle resonances that give rise to light-trapping in the infrared spectral range and to collective resonances that ensure an efficient in-coupling of light in the ultraviolet-blue spectral range. © 2015 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (240.6680) Surface plasmons; (050.1970) Diffractive optics.
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Palik, Handbook of Optical Constants of Solids (Academic, 1985). 32. D. R. Lide, Handbook of Chemistry and Physics, 84th ed. (CRC, 2003/2004). 33. M. A. Green and M. J. Keeves, “Optical properties of intrinsic silicon at 300K,” Prog. Photovoltaics: Res. and Appl. 3, 189–192 (1995). 34. H. Wang, K. Fu, R. A. Drezer, and N. J. Halas, “Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness,” Appl. Phys. B. 84, 191–195 (2006). 35. C. Battaglia, C.-M. Hsu, K. S¨oderstr¨om, J. Escarr, F.-J. Haug, M. Charrire, M. Boccard, M. Despeisse, D. T. L. Alexander, M. Cantoni, Y. Cui, and C. Ballif, “Light trapping in solar cells: Can peridodic beat random?,” ACS Nano 6, 2790–2797 (2012). 36. V. Ferry, M. A. Verschurren, H. B. T. Li, E. verhagen, R. J. Walters, R. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18, A237–A245 (2010). 37. E. Massa, V. Giannini, N. P. Hylton, N. J. Ekins-Daukes, S. Jain, O. E. Daif, and S. A. Maier, “Diffractive interference design using front and rear surface metal and dielectric nanoparticle arrays for photocurrent enhancement in thin crystalline silicon solar cells,” ACS Photonics 1, 871–877 (2014). 38. H. Ehrenreich, H. R. Philipp, and B. Segall, “Optical properties of aluminum,” Phys. Rev. 132, 1918–1928 (1963). 39. “Bonding silicon-on-insulator to glass wafers for integrated bio-electronic circuits,” Appl. Phys. Lett. 85, 2370–
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2372 (2004). 40. M. J. Madou, The MEMS Handbook (CRC, 2002).
1.
Introduction
Light collection in thin film Si solar cells is limited by silicon’s poor absorption efficiency towards longer wavelengths. Plasmonic solar cells, where plasmon resonances in metal nanoparticles are used to increase the absorption, have been proposed as a remedy to this deficiency [1]. Nanoparticles placed on the front of the solar cells can both increase the amount of light coupled into the solar cell, which appears as an antireflective effect, and also facilitate light-trapping by scattering light into guided modes in the solar cell [1,2]. Due to the large increase in the optical path in the solar cell the latter effect can lead to a strong relative increase in the absorption in the weakly absorbing spectral regions near the Si band gap [1, 3–5]. It has previously been shown that Al nanoparticles are good candidates for frontside plasmonic scatterers since they give a broadband increase in the incoupling of light into silicon [6–8]. For particles on the front of a silicon diode a photocurrent enhancement is generally observed at wavelengths larger than the plasmon resonance, whereas a reduction is observed at wavelengths below [6, 7, 9, 10]. This observation speaks in favor of tuning resonances to short wavelengths. On the other hand it has been reported that in order to ensure sufficient light-trapping in the spectral range of weak absorption in solar cells it is necessary to tune the resonances to the infrared (IR) part of the solar spectrum [4, 11, 12]. For particles which only support single strong resonances the latter point also implies a red-shift of the region of photocurrent enhancement, which can lead to detrimental losses in the blue region [6, 10]. Thus, it appears that using plasmon resonances in metal nanoparticles on the front involves a paradox: Tuning the nanoparticle resonance to ensure a broadband incoupling apparently comes at the expense of light-trapping which requires resonances tuned to the IR range where the absorption in Si is weak or vice versa. As will be shown in this paper this apparent paradox may be avoided if one considers particle arrays which support more than one strong resonance, and not only single particle radiative dipolar-like modes [2, 5]. It is known that larger particles near solar cell surfaces can support higher order modes related to the nanoparticle shape and the surroundings [4, 13, 14]. Moreover, it is possible to tailor additional resonances via interactions between particles. It was recently shown that placing large Al nanoparticles in a periodic array leads to diffractively coupled collective modes that induce additional resonances resulting in broadband photocurrent enhancement [15]. In the present work we demonstrate that it is indeed possible to obtain broadband enhanced incoupling of light into the solar cell front using such collective modes, while at the same time tuning the single particle dipolar-like modes towards the infrared to scatter light in the spectral range of weaker absorption, and thereby maintain strong light-trapping. 2.
Experimental methods
The samples consist of a layered structure with Al nanoparticles placed on top of a ∼40 nm SiO2 spacer layer on a Si thin-film test solar cell as well as a reference test solar cell with the same layer structure but no nanoparticles. The dielectric spacer layer thickness of 40 nm SiO2 serves to ensure a larger driving field of the particle resonances [3, 12]. The thin film Si test solar cell consists of a 1160 nm c-Si diode stack which has been bonded onto a glass (pyrex) substrate. It involves a highly B doped ∼60 nm p++ Si top layer followed by a 500 nm intrinsic Si layer and an Sb doped 600 nm n+ base layer. Aluminum electrodes were formed by photolithography and followed by Al evaporation. Further details of the steps involved in the synthesis of the test solar cell can be found in the appendix. A schematic of the test solar
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cell is shown in Fig. 1(a). The SiO2 spacer layer was deposited by rf-magnetron sputtering using a commercial sputtering system [16], and the thickness was determined from reflectance measurements. The aluminum nanoparticles were defined in periodic arrays with a particle
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Fig. 1. (a) Schematic of the solar cell structure. (b) Picture of the thin film test solar cell (diameter ca. 4 mm) where the nanoparticle array can be seen as a very dark area. The AFM profile and SEM image of the nanoparticle array can be seen in (c) and (d) respectively. The pitch of the square array is 380 nm.
pitch of 380 nm using electron beam lithography (EBL). The nanoparticle arrays were made in areas of 2 × 2 mm2 consisting of multiple write-fields with dimensions of 100 × 100 μ m2 . The aluminum nanoparticles were formed by thermal evaporation of aluminum and subsequent lift-off in acetone. The shapes and particle profiles were investigated using scanning electron microscopy (SEM). A SEM image of the nanoparticles is shown in Fig. 1(d). The diameters of the particles were determined from the SEM images to be ∼190 nm, and the height determined from AFM measurements to be ∼ 70 nm. The nanoparticle diameter decreases slightly towards the nanoparticle top (see the AFM profile in Fig. 1(c)) which is ascribed to a slight narrowing of the resist holes during Al evaporation. The total reflectance (specular and diffuse) and the transmittance were measured in both sand p-polarization using a Perkin Elmer Lambda 1050 double beam spectrophotometer fitted with an 150 mm integrating sphere and a wire grid polarizer. The polarization dependent total reflectance was measured at an 8◦ angle of incidence in the spectral range of 300-1200 nm relative to a Spectralon SRS-99 reflectance standard [17]. The 8◦ angle of incidence is used in most integrating sphere reflectance accessories, and serves to ensure that the specularly reflected light is captured within the integrating sphere in the reflection measurements. Polarization dependent measurements were made since optical measurements on periodic structures like those investigated here are sensitive to polarization at off-normal angles of incidence just as in conventional diffraction gratings [15]. The numerical aperture was ∼0.2 in both transmittance and reflectance measurements. The transmittance measurements were also made at an 8◦ angle of incidence by rotating the sample stage with respect to the incident beam around #233121 - $15.00 USD (C) 2015 OSA
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the y-axis (see Fig. 1(d) for the orientation) in order to match the reflectance measurements. Polarization resolved photocurrent measurements were made at normal incidence and at an 8◦ angle of incidence in a custom built setup equipped with a Keithley 6517B electrometer, an Oriel MS257 monochromator, and a wire grid polarizer. During the measurements the numerical aperture was ∼ 0.24. The photon flux was measured with a Newport 818-UV photodiode with calibrated quantum efficiency in the 350-1100 nm spectral range, from which the external quantum efficiencies (EQE) of the test-solar cells were obtained. 3.
Results and discussions
In Fig. 1(b) the area covered by nanoparticle arrays can be seen as a dark region in the small circular test solar cell, indicating a strongly enhanced absorption by the arrays. 3.1.
Optical reflectance and transmittance measurements
Reflectance (%)
The results from polarization-resolved reflectance and transmittance measurements are shown in Figs. 2(a) and 2(b) respectively. It may be noted that, while the measurements of the refer(a) 80 60 40 20
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Fig. 2. Measured reflectance (a), transmittance (b) and absorptance (c) for the reference sample (grey lines) and the NP array sample in s-polarization (blue lines) and in ppolarization (red lines).
ence sample (grey lines) were independent of the polarization as expected, the s-polarization measurement of the nanoparticle array sample (blue lines) differs from that measured with ppolarization (red lines), the difference being more easily observed in the reflectance data. This difference arises due to the 8◦ angle of incidence used in the optical measurements which has a strong influence on the response of the periodic nanoparticle-array-structure [15]. Absorptance (A) spectra were derived from the reflectance (R) and the transmittance (T) measurements by the relation A=100-R-T. and the result is shown in Fig. 2(c) [19].
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For the reference sample the IR part of the reflectance spectra (grey line in Fig. 2(a)) is dominated by interference in the solar cell layer and the thin dielectric spacer layer. At visible wavelengths the interference is damped due to the increased absorption efficiency in silicon at smaller wavelengths. At wavelengths below ∼450 nm the solar cell layer becomes nontransparent (as evidenced in the transmittance measurement in Fig. 2(b)) and the reflectance resembles that of bulk Si with a dielectric spacer layer on top, as expected. For the solar cell with the nanoparticle array it is clear that the nanoparticles on the front of the solar cell have a profound influence on the reflectance with a broadband reduction in the reflectance throughout the solar spectrum compared to that of the reference sample. In p-polarization two distinct dips are observed near 350 nm and 450 nm while a single strong drop in reflectance is observed in s-polarization between ∼360 nm and ∼420 nm. In addition it can be seen that the interference fringes in the optical spectra for the nanoparticle solar cell have additional polarizationdependent fine-structure. In the transmittance measurements the nanoparticles also cause a substantially reduced transmittance through almost the entire measured range, and a polarization dependent fine-structure. The use of front-side nanoparticles for solar cells is often compared to well-established and simpler-to-make antireflective layers. However, if the nanoparticle array only serves as an antireflective front side layer, the transparency of the solar cell material in the IR spectral range would imply that a corresponding larger fraction of “non-reflected” light would be effectively transmitted by the solar cells leading to an increased transmittance. The fact that a strong reduction is observed in both the reflectance and the transmittance, with the large enhancement in the absorptance shown in Fig. 2(c) as a result, shows that the nanoparticle array is effective in enhancing the light absorbing properties of the composite structure beyond that of a pure antireflective effect. 3.2.
External quantum efficiency determinations
In the case of the reference sample all of the light absorption occurs in the Si layer of the solar cell, while in the nanoparticle solar cell part of the light absorption may occur as parasitic absorption in the nanoparticles. Thus, a key merit is whether this increased absorption can be converted to photovoltaic power or whether it is lost as parasitic absorption. To address this point the key is to measure the external quantum efficiency, since this is related to the light absorbed only in the photoactive layers of the solar cell. Figures 3(a) and 3(b) show the external quantum efficiencies measured at normal incidence in s-polarization and p-polarization, respectively, for the nanoparticle array sample (blue lines) and for the reference sample (grey lines). Measurements at an 8◦ angle of incidence for the nanoparticle sample are also included (red lines) in order to provide EQE data that mimic the measurement geometry of the reflectance and transmittance measurements. In order to allow a more easy identification of regions of photocurrent enhancements the relative photocurrent or gain was derived by normalizing the measured photocurrent for the nanoparticle sample to the photocurrent measured for the reference sample. The results are shown in Fig. 3(c) for spolarization and in Fig. 3(d) for p-polarization. In the infrared range the EQE of the reference solar cell is declining due to the decreasing absorption efficiency in Si towards longer wavelengths while it may be noted that the external quantum efficiency in both samples displays a distinct drop in the 350-400 nm range. The latter is ascribed to front surface recombination at the interface between the Si surface and the sputtered SiO2 layer. For real devices the SiO2 spacer layer should be optimized with respect to both optical properties and to interface quality but for the purpose of this study the optically optimized sputtered SiO2 suffice. In the normal-incidence measurements the EQE for the nanoparticle sample is similar for the two polarizations as expected, and the EQE is increased with respect to the reference solar
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Fig. 3. External quantum efficiency (EQE) measured in s-polarization (a) and in ppolarization (b). The grey lines refer to the reference solar cell while the blue lines and red lines correspond to measurements on the nanoparticle array made at an angle of incidence of 0◦ and 8◦ respectively. The relative EQE with respect to the reference sample is shown in (c) and (d) for s- and p-polarization respectively.
cell from the UV-visible spectral region to the infrared. The photocurrent enhancement is more easily identified from the gain profiles shown in Figs. 3(c) and 3(d), where it is seen that the cross-over point from photocurrent reduction to photocurrent enhancement occurs near 400 nm and the region of enhancement is maintained throughout the measurement range towards longer wavelengths where the gain is increasing to very large values near the Si band gap at 1100 nm. By inspection of the EQE data in Figs. 3(a) and 3(b), and the absorptance data in Fig. 2(c) it is observed that the enhancement appears to be stronger in two regions: In the region from the crossover point to ∼500 nm and between ∼550 nm and ∼1000 nm, where a broad hump can be observed. In the first region the gain curve has a distinct Fano-like profile just below 400 nm [20]. Fano-like profiles in the photocurrent gain curves for solar cells with frontside particles, like that observed below 400 nm, have been shown in the literature to correlate with resonances in the metal nanoparticles. [6, 10, 13]. Moreover, in [15] Uhrenfeldt et al. recently showed that diffractively coupled collective modes in periodic nanoparticle arrays may introduce additional Fano-like profiles in the photocurrent gain. The diffractive coupling of these collective modes is mediated by Rayleigh-Wood anomalies (RA), which is an optical phenomenon related to diffracted light propagating parallel to the array plane [21–23]; this diffracted light leads to a strong coupling of the particles which induces collective resonant modes in the array near the spectral positions of the RA [21–29]. An important hallmark of these collective array modes is a strong dependence on the angle of incidence and the polarization of the incident light [15],
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which enables an easier identification of such modes. For the EQE measurements made at an incidence angle of 8◦ [Fig. 3] it is seen that the gain for s-polarization is almost the same as that for the normal incidence EQE measurements, whereas the EQE has changed in the ultravioletvisual spectral range for p-polarization. Thus it is evident that the Fano-like profile near 400 nm has a clear dependence on the angle of incidence and on the polarization. For later use it is noted that since the EQE measurements in s-polarization at normal incidence and at an 8◦ angle of incidence are almost the same, the transmittance and reflectance measurements made in s-polarization may be considered as approximative normal incidence measurements. For normal incidence light, the spectral position of the Rayleigh Wood anomalies can be calculated from ni p λ0 = √ (1) n2 + m2 where λ0 is the RA free-space wavelength, p is the pitch of the array, (n, m) are integers for the diffraction order, and ni is the refractive index of the material into which the light is scattered. Thus ni = 1 (air) for light reflected from the array into the air superstratum and ni = nSiO2 ≈ 1.46 (at 600 nm) for light scattered from the array into the SiO2 dielectric layer below. The position of the reflected light Rayleigh-Woods anomalies with (n, m) = (±1, 0) and (0, ±1) becomes λ0 =380 nm, which is just the array pitch value for the array studied here. This value may be compared to the s-polarized reflectance measurements shown in Fig. 2(a), since these resemble normal incidence measurements as noted previously. In the figure the λ0 =380 nm position has been marked as a vertical dashed line which can be seen to coincide with the strong drop in reflectance observed between ∼360 nm and ∼420 nm in s-polarization. The correlation of the spectral position of the reflectance drop with the particle pitch combined with the strong dependence of the Fano-profile on the angle of incidence and the polarization strongly supports that the Fano-like profile below 400 nm and the photocurrent enhancement in the ∼400-500 nm spectral range is indeed associated with diffractively coupled collective modes in the Al nanoparticle array in line with the findings in [15]. Turning to the second region of EQE enhancement between ∼550 nm and ∼1000 nm, the “hump” observed in Fig. 2(c) and Figs. 3(a) and 3(b) arises from the convolution of the strongly decreasing absorption efficiency of Si towards the IR, as seen in the reference sample, and the strong increase in the relative gain observed above ∼550 nm in Figs. 3(c) and 3(d). The fact that the gain increases to very large values above 550 nm indicates that the nanoparticle array causes light-trapping beyond this point. It should be noted that the EQE gain is larger than unity for most of the part of the solar spectrum that can be absorbed by Si. This implies that the Al nanoparticle array leads to both a broadband increased in-coupling of light into the solar cell and also facilitates light-trapping in the solar cell. 3.3.
Finite difference time domain simulations
To strengthen this point further, numerical finite difference time domain (FDTD) simulations of the nanoparticle and solar cell structure were performed. The calculations were made using the commercial three dimensional FDTD solver provided by Lumerical [30] using a unit cell with periodic boundary conditions on the sides, and perfectly matched layer boundary conditions on the top and bottom. In all the calculations a normal plane wave is incident from the top onto a periodic array of Al nanoparticles placed on a 40 nm thick SiO2 film on top of a slab of 1160 nm Si, which in turn was placed on a SiO2 substrate. The incident wave was polarized along the y-axis [see Fig. 1(d)]. This corresponds to the experimental s-polarization configuration but due to the symmetry of the array at normal incidence the results are identical to polarization along the x-axis. In order to limit the simulation volume the extent of the 300 μ m SiO2 (Pyrex in the experiments) was omitted from the simulation volume. This omission implies that the
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Reflectance (%)
relatively weak reflectance at the pyrex-air backside is not accounted for, which may give slight deviations from the experiments. The optical constants for Al, Si, and SiO2 were taken from Refs. [31–33], respectively, and fitted using the routines provided by Lumerical. In order to include the slight narrowing effect which occurred in the metal evaporation, the particle shape was modeled as truncated cones. As in the experiments, the height and base diameter of the nanodiscs were chosen to be 70 nm and 190 nm, respectively. The top diameter was set to the base diameter minus ∼ 40% of the height to match the experimental conditions. With these parti-
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Fig. 4. Simulated normal incidence reflectance (a), transmittance (b), and absorptance (c) spectra for the solar cell with nanoparticles for different array pitch values.
cle dimensions the reflectance, transmittance and absorptance for the composite structure were calculated for four different array pitch values of p=360 nm, p=380 nm, p=400 nm, and p=460 nm, and the results are shown in Fig. 4. By comparison with Fig. 2 it is seen that the calculated reflectance and transmittance for p=380 (green lines in Fig. 4) are in very good agreement with the experimental curves, although the pronounced increase in the transmittance below 550 nm is not observed in the experiments. To highlight the effect of the array pitch on the optical properties of the nanoparticle solar cell, the positions of selected Rayleigh-Woods anomalies have been included in Figs. 4(a) and 4(b). As mentioned previously, the spectral positions of the (±1, 0) and (0, ±1) Rayleigh-Woods anomalies for reflected light is just the array pitch values, which are shown in Fig. 4(a) as vertical lines. In the case of the transmittance measurements the RA spectral positions for transmitted light were calculated using Eq. 1, which for the (±1, 0) and (0, ±1) modes reduces to λ0 = nSiO2 p ≈ 1.46p. The spectral positions of these modes are shown as vertical lines in Fig. 4(b). It is clear that the strong reflectance dip observed in the short wavelength region in the simulations correlates with the (±1, 0) and (0, ±1) RA positions for reflected light, and the corresponding RA values for transmitted light are seen to correlate with the pronounced drops in the simulated transmittance in the 500-700 nm spectral range.
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400 500 600 700 800 900 1000 Wavelength (nm)
Gain
8 6 4 2
Fig. 5. (a) Comparison of the measured s-polarized absorptance (blue lines) and simulated normal incidence absorptance (red lines) for the nanoparticle sample as well as the experimental (black line) and simulated (grey line) absorptance for the reference solar cell. (b) Relative gain in the measured (blue line) and simulated (red line) absorptance shown in (a). (c) Comparison of the s-polarized normal incidence external quantum efficiency measurements with the simulated normal incidence absorptance in the 1160 nm Si layer for the nanoparticle sample (blue and red lines) and the reference sample (black and grey lines). (d) Comparison of the relative gain in the measured EQE and the simulated Si layer absorptance shown in (c).
3.4.
Comparison between experiment and simulation
To allow a more detailed comparison of the simulations with the experiments, the experimental absorptance measured in s-polarization for the nanoparticle (blue line) and reference samples (black line) are shown in Fig 5(a) together with the simulated values for p=380 nm. Furthermore, the gain in absorptance of the nanoparticle sample relative to that of the reference sample were also derived for both the experiment and the simulation and the results are shown in Fig. 5(b). As can be seen from Figs. 5(a) and 5(b) the simulations are in excellent agreement with the experimental data except from a few minor deviations: Some of the peaks in the simulations are less distinct in the measurements and the nanoparticle induced modulation of the absorptance in the ∼350 - 500 nm range is less pronounced in the experiments. This may be due to experimental non-idealities, such as corner rounding, surface roughness, and grains in the Al nanoparticles, which are not included in the simulation and which may affect the optical properties [34]. To be able to compare the simulations with the experimental EQE, the amount of light absorbed solely in the Si layer was extracted from the simulations since this quantity can be considered an ideal EQE. The simulated Si layer absorptance for the nanoparticle sample with
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p=380 nm and the reference sample are shown in Fig. 5(c) as red and grey lines, respectively. By comparing Figs. 5(a) and 5(c) it is seen that the Si absorptance is less than the total absorptance in the nanoparticle sample which is due to parasitic absorption in the nanoparticles. The experimental EQE measured in s-polarized at normal-incidence for the nanoparticle and reference sample are also shown in Fig. 5(c) for comparison. It can be seen that the measured EQE data of both the nanoparticle cell and the reference cell are markedly reduced relative to the simulated ideal EQE. Since the absorptance data in Figs. 5(a) and 5(c) show that the simulations are fairly accurate with respect to the optical absorption, this difference with respect to the ideal EQE may be ascribed to a reduced electron-hole collection efficiency in the experimental samples, caused by interface recombination at the front and back interfaces of the Si absorption layer. This effect of recombination is largely eliminated when comparing the relative gain in EQE from the simulations with the experiments. This is seen in Fig. 5(d) where the gain in experimental EQE and simulated Si absorptance derived from the data in Fig. 5(b) are compared. As can be seen the experimental and simulated EQE gain curves are in very good agreement apart from the same minor deviations commented on previously. An ideal shortcircuit current density Jsc may be obtained from the simulations by convoluting the simulated ideal EQE shown in Fig. 5(c), with the solar spectrum. For the reference sample with a 40 nm SiO2 layer used here this ideal Jsc yields ∼9.8 mA/cm2 while a more conventional SiO2 antireflective layer thickness of 100 nm gives 11.6 mA/cm2 . In the case of the Al nanoparticle sample cell investigated here the idealized Jsc yields ∼ 14.6 mA/cm2 which constitutes an enhancement of 49% relative to reference sample and 26% relative to the antireflective layer configuration. This is contrary to the findings of Massa et al. in [37] where Al nanoparticle arrays on the front were also studied numerically but where collective resonances were not utilized. From Fig. 4 it is clear that the Rayleigh-Woods anomalies have a strong influence on the absorptance profile of the solar cell as it also appears from Fig. 4(c). A full analysis of the modes and coupling efficiencies of the compound structure is beyond the scope of the present experimental work, since these are likely to depend on the array pitch as well as on the solar cell layer configuration in a complex manner [23,35,36]. The latter may be hinted by observing that the detailed spike structure in the simulated absorptance depends on the particle pitch in a nontrivial manner [see Fig. 4(c)]. Nevertheless some important implications of the array pitch influence can still be given based on a more qualitative account as follows: At wavelengths below the (±1, 0) and (0, ±1) RA positions for the reflected light, the light scattered by the nanoparticle array back into the air can couple to radiative diffractive modes [21]. On the high wavelength side above the (±1, 0) and (0, ±1) RA positions these modes are evanescent on the air side and as a consequence they are confined to the vicinity of the array. This can account for the abrupt drop in the reflectance spectra observed at the RA positions in Fig. 4(a). The amplitude of this reflectance reduction is most pronounced near the RA which indicate the resonant nature of the diffractive coupling. The light scattered by the array into the SiO2 layer can couple to radiative diffractive modes in the SiO2 if the wavelength is below the (±1, 0) and (0, ±1) RA positions for transmitted light at ∼ 555 nm. These radiative modes in the SiO2 layer can in turn couple to radiative modes in the Si slab which are partially transmitted into the pyrex substrate employed here, and hence this light is not trapped in the Si. However, at wavelengths larger than 555 nm the transmitted RA related modes in the glass layer are evanescent in nature. Since the SiO2 is very thin these evanescent modes may couple via mode overlap to internal guided modes in the nearby Si slab and this can lead to light trapping inside the Si layer. This is in good agreement with the strong increase in EQE gain above ∼ 550 nm observed in Figs. 3(c) and 3(d) and is further evidenced by the strong and narrow peaks that starts to dominate the simulated Si absorptance in Fig. 5(b) at wavelengths
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above 555 nm. A nanoparticle array may support collective resonances in one spectral range and still display single-particle like resonances in other parts of the spectrum if these are detuned from the RA anomalies that mediate the diffractive coupling [22]. In order to elucidate the role of such single-particle-like resonances in the EQE enhancement the scattering cross sections and absorption cross sections were calculated for a single Al nanoparticle placed on a Si surface with a 40 nm SiO2 spacer layer using FDTD. The calculations are made for normal incidence using the ”Total-Field Scattered-Field” source provided by Lumerical [30], where the scattering and absorption cross sections are calculated by placing frequency domain power monitors outside and inside the region of the source respectively and by using perfectly matched layers on all boundaries. The scattering and absorption efficiencies Q, which are obtained by normalizing the respective cross sections to the geometric cross section, are shown in Fig. 6(a) as full and dashed lines, respectively, for three different particle geometries; for a base width of 190 nm and a height of 70 nm as in the experiments (blue lines), for an increased base width of 220 nm (red lines) as well as for an increased height of 110 nm (green lines). The Si absorptance was also calculated for a p=380 nm nanoparticle array with these particle geometries using the simulation settings as those used in Figs. 4 and 5, and the results are shown in Fig. 6(b).
5
(a)
4 Q
3 2 1
Absorptance (%)
0
(b)
Wavelength (nm)
80 60
Reference Exp. geometry NP D190 h110 NP D220 h70
40 20 0 300
400
500
600
700
800
900
1000
1100
Fig. 6. (a) Calculated normal incidence scattering efficiency (full lines) and absorption efficiencies (dashed lines) for the experimental nanoparticle geometry (blue), for a nanoparticle with a larger basediameter of 220 nm (red) and for a nanoparticle with a height of 110 nm (green), all other structural dimensions being similar. (b) The simulated normal incidence absorptance for nanoparticle solar cells using the nanoparticle geometries in (a) (blue,red,green) as well as for the reference cell (grey).
It can be seen that the Al nanoparticles with the experimental geometry have a strong scattering efficiency in most of the investigated spectral range with a broad dipole-like resonance centered around 650 nm and an additional resonance-like peak structure near 300 nm, which may be a quadropole-like resonance [13]. The corresponding absorption efficiency is seen to be substantially smaller with a faint peak structure centered near 830 nm, which coincides with the narrow interband transition in Al at 1.5 eV [38]. For fairly large nanoparticles radiative damping of the single particle resonances becomes pronounced and leads to a broadening of dipolar modes and increases the scattering-to-absorption ratio, which is in agreement with the #233121 - $15.00 USD (C) 2015 OSA
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trends observed. It is also noted that the RA spectral positions near 400 nm correspond to regions where the single particle scattering is still pronounced, which is a necessary condition for an efficient coupling. By comparing the single particle resonances with the corresponding Si absorptance in Fig. 6(b), it can be seen that the profile of the dipole-like mode coincides with the light-trapping hump in the Si absorptance enhancement, although this is truncated at wavelengths below 555 nm due to the influence of the array structure as discussed above. Thus, while the diffractive coupling accounts for the steep increases in absorptance near 400 nm and 555 nm, the broad absorption enhancement region in the IR is due to light-trapping caused by scattering from the single particle dipole-like resonances. 4.
Conclusion
Broadband photocurrent enhancement and light trapping in the IR spectral range have been demonstrated using periodic arrays of large Al nanoparticles on the front side of a test solar cell. It is demonstrated that the steep increase observed in the EQE enhancement in the ultravioletblue region is caused by diffractively coupled collective modes in the array, while the broad absorption enhancement region in the IR is due to light-trapping caused by scattering from the single particle dipole-like resonances. The results demonstrate that an apparent paradox between broadband photocurrent enhancement and light-trapping for nanoparticle arrays on the front side of a solar cell may be avoided if the properties of the particle arrays are tailored to support multiple resonances. A.
Appendix
The procedure to fabricate the thin crystalline Si solar cell is illustrated in Fig. 7. A commercial silicon-on-insulator (SOI) wafer (SOITEC) is used as a starting substrate presenting a 290 nm top silicon layer and a 400 nm buried oxide layer [Fig. 7(a)]. The diode stack (DS) is grown by molecular beam epitaxy (MBE) on the top Si layer of the SOI wafer [Fig. 7(b)]. The DS which is also shown in Fig. 1(a) consists of a highly B doped ∼60 nm p++ Si layer (1 × 1020 B/cm3 ), a 500 nm intrinsic Si layer and a Sb doped 600 nm n+ Si layer (5 × 1018 Sb/cm3 ). The highly B doped layer is utilised as an etch stop in one of the fabrication steps described below. The pressure under growth is 1.5 × 10−10 mbar. After careful cleaning, the structure is anodically bonded in vacuum to a borosilicate glass substrate [Fig. 7(c)] at a temperature of 400◦ C, with an applied voltage of 700 V modulated at 1 Hz [39]. The structure is then flipped up-side down [Fig. 7(d)], and the SOI wafer is then removed in a series of chemical wet-etching steps. The silicon substrate of the SOI wafer is removed by a 30% KOH wet-etch at 85◦ C where the buried oxide layer acts as an etch stop [Fig. 7(e)]. The second etch step uses a 30% HF solution to remove the 400 nm oxide layer [Fig.7(f)]. The remaining inactive Si layer from the SOI wafer is then etched away in a solution of ethylene diamine pyrocatechol (EDP) at 100◦ C utilizing the highly B doped p++ Si layer of the DS as a very efficient etch stop [Fig. 7(g)] [40]. The EDP etchant combined with the B etch stop leaves a very uniform surface confirmed by atomic force microscopy (roughness of ∼1.2 nm RMS). The DS has hereby been transferred onto a glass substrate. The native oxide has been removed using a 30% HF solution prior to defining the contacts. Ring shaped top electrodes were made by photolithography masking followed by evaporation of a ∼300 nm thick Al layer. Subsequently the diode and the top electrodes were masked with black wax [Fig. 7(i)] in order to define the mesa diode using a 1 : 9 : 1 HNA wetetching solution [Fig. 7(j)]. The wax is stripped off [Fig. 7(k)] and backside Al contacts were made by a second photolithography process. The resulting Si test solar cell with the electrode pattern can be seen in Fig. 1(b).
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Fig. 7. (a) Schematic illustrating the different fabrication steps to obtain a thin mono crystalline Si solar cell on a glass substrate starting with (a) a SOI substrate on which (b) a DS is grown by MBE. (c) This structure is bonded to a borosilicate glass substrate and (d) the whole structure is flipped up-side down. (e)(f)(g) Chemical wet-etching is used to remove the SOI wafer: KOH is used to remove the thick Si substrate, HF is used to remove the SiO2 layer, and ethylene diamine pyrocathecol is used to remove the top Si inactive layer. (h) The thermally evaporated top Al electrodes are (i) masked with wax to isolate the diode with (j) a HNA wet-etch. (k) The wax is removed and (l) the back Al electrodes are thermally evaporated.
Acknowledgments The authors greatly acknowledge the financial support from the project Localized surface plasmons and silicon thin-film solar cells-PLATOS financed by the Villum Foundation.
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of Energy Technology, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg, Denmark 2 Department of Physics and Astronomy, Aarhus University, Gustav Wieds vej 14, 8000 Aarhus C, Denmark 3 Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds vej 14, 8000 Aarhus C, Denmark 4 Department of Civil Engineering, Aalborg University, Sofiendalsvej 11, 9200 Aalborg, Denmark *[email protected]
Abstract: Plasmonic resonances in metal nanoparticles are considered candidates for improved thin film Si photovoltaics. In periodic arrays the influence of collective modes can enhance the resonant properties of such arrays. We have investigated the use of periodic arrays of Al nanoparticles placed on the front of a thin film Si test solar cell. It is demonstrated that the resonances from the Al nanoparticle array cause a broadband photocurrent enhancement ranging from the ultraviolet to the infrared with respect to a reference cell. From the experimental results as well as from numerical simulations it is shown that this broadband enhancement is due to single particle resonances that give rise to light-trapping in the infrared spectral range and to collective resonances that ensure an efficient in-coupling of light in the ultraviolet-blue spectral range. © 2015 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (240.6680) Surface plasmons; (050.1970) Diffractive optics.
References and links 1. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9, 205–213 (2010). 2. P. Spinelli, M. Hebbink, R. de Waele, L. Black, and A. Polman, “Optical impedance matching using coupled plasmonic particle arrays,” Nano Lett. 11, 1760–1765 (2011). 3. K. R. Catchpole and A. Polman, “Design principles for particle plasmon enhanced solar cells,” Appl. Phys. Lett. 93, 191113 (2008). 4. F. J. Beck, S. Mokkapati, and K. R. Catchpole, “Light trapping with plasmonic particles: beyond the dipole model,” Opt. Express 19, 25230–25241 (2011). 5. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109, 073105 (2011). 6. C. Uhrenfeldt, T. F. Villesen, B. Johansen, T. G. Pedersen, and A. N. Larsen, “Tuning plasmon resonances for light coupling into silicon: a “rule of thumb” for experimental design,” Plasmonics 8, 79–84 (2013). 7. T. F. Villesen, C. Uhrenfeldt, B. Johansen, J. L. Hansen, H. U. Ulriksen, and A. N. Larsen, “Aluminum nanoparticles for plasmon-improved coupling of light into silicon,” Nanotechnology, 23, 085202 (2012).
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Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9, 707–715 (2010). 21. B. Lamprecht, G. Schider, R. T. Lechner, H. Ditlbacher, J. R. Krenn, A. Leitner, and F. R. Aussenegg, “Metal nanoparticle gratings: influence of dipolar particle interaction on the plasmon resonance,” Phys. Rev. Lett. 84, 4721–4724 (2000). 22. S. R. K. Rodriguez, A. Abass, B. Maes, O. T. A. Janssen, G. Vecchi, and J. G´omez Rivas, “Coupling bright and dark plasmonic lattice resonances,” Phys. Rev. X 1, 021019 (2011). 23. S. Murai, M. A. Verschuuren, G. Lozano, G. Pirrucio, S. R. K. Rodriguez, and J. G´omez Rivas, “Hybrid plasmonic-photonic modes in diffractive arrays of nanoparticles coupled to light-emitting waveguides,” Opt. Express 21, 4250–4262 (2013). 24. S. Zou and G. C. Schatz, “Narrow plasmonic/photonic extinction and scattering line shapes for one and twodimensional silver nanoparticle arrays,” J. Chem. Phys. 121, 12606–12612 (2004). 25. N. 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Palik, Handbook of Optical Constants of Solids (Academic, 1985). 32. D. R. Lide, Handbook of Chemistry and Physics, 84th ed. (CRC, 2003/2004). 33. M. A. Green and M. J. Keeves, “Optical properties of intrinsic silicon at 300K,” Prog. Photovoltaics: Res. and Appl. 3, 189–192 (1995). 34. H. Wang, K. Fu, R. A. Drezer, and N. J. Halas, “Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness,” Appl. Phys. B. 84, 191–195 (2006). 35. C. Battaglia, C.-M. Hsu, K. S¨oderstr¨om, J. Escarr, F.-J. Haug, M. Charrire, M. Boccard, M. Despeisse, D. T. L. Alexander, M. Cantoni, Y. Cui, and C. Ballif, “Light trapping in solar cells: Can peridodic beat random?,” ACS Nano 6, 2790–2797 (2012). 36. V. Ferry, M. A. Verschurren, H. B. T. Li, E. verhagen, R. J. Walters, R. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18, A237–A245 (2010). 37. E. Massa, V. Giannini, N. P. Hylton, N. J. Ekins-Daukes, S. Jain, O. E. Daif, and S. A. Maier, “Diffractive interference design using front and rear surface metal and dielectric nanoparticle arrays for photocurrent enhancement in thin crystalline silicon solar cells,” ACS Photonics 1, 871–877 (2014). 38. H. Ehrenreich, H. R. Philipp, and B. Segall, “Optical properties of aluminum,” Phys. Rev. 132, 1918–1928 (1963). 39. “Bonding silicon-on-insulator to glass wafers for integrated bio-electronic circuits,” Appl. Phys. Lett. 85, 2370–
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2372 (2004). 40. M. J. Madou, The MEMS Handbook (CRC, 2002).
1.
Introduction
Light collection in thin film Si solar cells is limited by silicon’s poor absorption efficiency towards longer wavelengths. Plasmonic solar cells, where plasmon resonances in metal nanoparticles are used to increase the absorption, have been proposed as a remedy to this deficiency [1]. Nanoparticles placed on the front of the solar cells can both increase the amount of light coupled into the solar cell, which appears as an antireflective effect, and also facilitate light-trapping by scattering light into guided modes in the solar cell [1,2]. Due to the large increase in the optical path in the solar cell the latter effect can lead to a strong relative increase in the absorption in the weakly absorbing spectral regions near the Si band gap [1, 3–5]. It has previously been shown that Al nanoparticles are good candidates for frontside plasmonic scatterers since they give a broadband increase in the incoupling of light into silicon [6–8]. For particles on the front of a silicon diode a photocurrent enhancement is generally observed at wavelengths larger than the plasmon resonance, whereas a reduction is observed at wavelengths below [6, 7, 9, 10]. This observation speaks in favor of tuning resonances to short wavelengths. On the other hand it has been reported that in order to ensure sufficient light-trapping in the spectral range of weak absorption in solar cells it is necessary to tune the resonances to the infrared (IR) part of the solar spectrum [4, 11, 12]. For particles which only support single strong resonances the latter point also implies a red-shift of the region of photocurrent enhancement, which can lead to detrimental losses in the blue region [6, 10]. Thus, it appears that using plasmon resonances in metal nanoparticles on the front involves a paradox: Tuning the nanoparticle resonance to ensure a broadband incoupling apparently comes at the expense of light-trapping which requires resonances tuned to the IR range where the absorption in Si is weak or vice versa. As will be shown in this paper this apparent paradox may be avoided if one considers particle arrays which support more than one strong resonance, and not only single particle radiative dipolar-like modes [2, 5]. It is known that larger particles near solar cell surfaces can support higher order modes related to the nanoparticle shape and the surroundings [4, 13, 14]. Moreover, it is possible to tailor additional resonances via interactions between particles. It was recently shown that placing large Al nanoparticles in a periodic array leads to diffractively coupled collective modes that induce additional resonances resulting in broadband photocurrent enhancement [15]. In the present work we demonstrate that it is indeed possible to obtain broadband enhanced incoupling of light into the solar cell front using such collective modes, while at the same time tuning the single particle dipolar-like modes towards the infrared to scatter light in the spectral range of weaker absorption, and thereby maintain strong light-trapping. 2.
Experimental methods
The samples consist of a layered structure with Al nanoparticles placed on top of a ∼40 nm SiO2 spacer layer on a Si thin-film test solar cell as well as a reference test solar cell with the same layer structure but no nanoparticles. The dielectric spacer layer thickness of 40 nm SiO2 serves to ensure a larger driving field of the particle resonances [3, 12]. The thin film Si test solar cell consists of a 1160 nm c-Si diode stack which has been bonded onto a glass (pyrex) substrate. It involves a highly B doped ∼60 nm p++ Si top layer followed by a 500 nm intrinsic Si layer and an Sb doped 600 nm n+ base layer. Aluminum electrodes were formed by photolithography and followed by Al evaporation. Further details of the steps involved in the synthesis of the test solar cell can be found in the appendix. A schematic of the test solar
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cell is shown in Fig. 1(a). The SiO2 spacer layer was deposited by rf-magnetron sputtering using a commercial sputtering system [16], and the thickness was determined from reflectance measurements. The aluminum nanoparticles were defined in periodic arrays with a particle
(a)
(b)
40 nm SiO2
Al nanoparticles
++
60 nm p trinsic 500 nm in +
600 nm n
(d)
strate Pyrex sub
(c)
y
x 500 nm
Fig. 1. (a) Schematic of the solar cell structure. (b) Picture of the thin film test solar cell (diameter ca. 4 mm) where the nanoparticle array can be seen as a very dark area. The AFM profile and SEM image of the nanoparticle array can be seen in (c) and (d) respectively. The pitch of the square array is 380 nm.
pitch of 380 nm using electron beam lithography (EBL). The nanoparticle arrays were made in areas of 2 × 2 mm2 consisting of multiple write-fields with dimensions of 100 × 100 μ m2 . The aluminum nanoparticles were formed by thermal evaporation of aluminum and subsequent lift-off in acetone. The shapes and particle profiles were investigated using scanning electron microscopy (SEM). A SEM image of the nanoparticles is shown in Fig. 1(d). The diameters of the particles were determined from the SEM images to be ∼190 nm, and the height determined from AFM measurements to be ∼ 70 nm. The nanoparticle diameter decreases slightly towards the nanoparticle top (see the AFM profile in Fig. 1(c)) which is ascribed to a slight narrowing of the resist holes during Al evaporation. The total reflectance (specular and diffuse) and the transmittance were measured in both sand p-polarization using a Perkin Elmer Lambda 1050 double beam spectrophotometer fitted with an 150 mm integrating sphere and a wire grid polarizer. The polarization dependent total reflectance was measured at an 8◦ angle of incidence in the spectral range of 300-1200 nm relative to a Spectralon SRS-99 reflectance standard [17]. The 8◦ angle of incidence is used in most integrating sphere reflectance accessories, and serves to ensure that the specularly reflected light is captured within the integrating sphere in the reflection measurements. Polarization dependent measurements were made since optical measurements on periodic structures like those investigated here are sensitive to polarization at off-normal angles of incidence just as in conventional diffraction gratings [15]. The numerical aperture was ∼0.2 in both transmittance and reflectance measurements. The transmittance measurements were also made at an 8◦ angle of incidence by rotating the sample stage with respect to the incident beam around #233121 - $15.00 USD (C) 2015 OSA
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the y-axis (see Fig. 1(d) for the orientation) in order to match the reflectance measurements. Polarization resolved photocurrent measurements were made at normal incidence and at an 8◦ angle of incidence in a custom built setup equipped with a Keithley 6517B electrometer, an Oriel MS257 monochromator, and a wire grid polarizer. During the measurements the numerical aperture was ∼ 0.24. The photon flux was measured with a Newport 818-UV photodiode with calibrated quantum efficiency in the 350-1100 nm spectral range, from which the external quantum efficiencies (EQE) of the test-solar cells were obtained. 3.
Results and discussions
In Fig. 1(b) the area covered by nanoparticle arrays can be seen as a dark region in the small circular test solar cell, indicating a strongly enhanced absorption by the arrays. 3.1.
Optical reflectance and transmittance measurements
Reflectance (%)
The results from polarization-resolved reflectance and transmittance measurements are shown in Figs. 2(a) and 2(b) respectively. It may be noted that, while the measurements of the refer(a) 80 60 40 20
Transmittance (%)
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Ref. NP p−pol. NP s−pol.
60 40 20 0
Absorptance (%)
(b)
80
(c)
80 60 40 20 0 300
400
500
600 700 800 Wavelength (nm)
900
1000
1100
Fig. 2. Measured reflectance (a), transmittance (b) and absorptance (c) for the reference sample (grey lines) and the NP array sample in s-polarization (blue lines) and in ppolarization (red lines).
ence sample (grey lines) were independent of the polarization as expected, the s-polarization measurement of the nanoparticle array sample (blue lines) differs from that measured with ppolarization (red lines), the difference being more easily observed in the reflectance data. This difference arises due to the 8◦ angle of incidence used in the optical measurements which has a strong influence on the response of the periodic nanoparticle-array-structure [15]. Absorptance (A) spectra were derived from the reflectance (R) and the transmittance (T) measurements by the relation A=100-R-T. and the result is shown in Fig. 2(c) [19].
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For the reference sample the IR part of the reflectance spectra (grey line in Fig. 2(a)) is dominated by interference in the solar cell layer and the thin dielectric spacer layer. At visible wavelengths the interference is damped due to the increased absorption efficiency in silicon at smaller wavelengths. At wavelengths below ∼450 nm the solar cell layer becomes nontransparent (as evidenced in the transmittance measurement in Fig. 2(b)) and the reflectance resembles that of bulk Si with a dielectric spacer layer on top, as expected. For the solar cell with the nanoparticle array it is clear that the nanoparticles on the front of the solar cell have a profound influence on the reflectance with a broadband reduction in the reflectance throughout the solar spectrum compared to that of the reference sample. In p-polarization two distinct dips are observed near 350 nm and 450 nm while a single strong drop in reflectance is observed in s-polarization between ∼360 nm and ∼420 nm. In addition it can be seen that the interference fringes in the optical spectra for the nanoparticle solar cell have additional polarizationdependent fine-structure. In the transmittance measurements the nanoparticles also cause a substantially reduced transmittance through almost the entire measured range, and a polarization dependent fine-structure. The use of front-side nanoparticles for solar cells is often compared to well-established and simpler-to-make antireflective layers. However, if the nanoparticle array only serves as an antireflective front side layer, the transparency of the solar cell material in the IR spectral range would imply that a corresponding larger fraction of “non-reflected” light would be effectively transmitted by the solar cells leading to an increased transmittance. The fact that a strong reduction is observed in both the reflectance and the transmittance, with the large enhancement in the absorptance shown in Fig. 2(c) as a result, shows that the nanoparticle array is effective in enhancing the light absorbing properties of the composite structure beyond that of a pure antireflective effect. 3.2.
External quantum efficiency determinations
In the case of the reference sample all of the light absorption occurs in the Si layer of the solar cell, while in the nanoparticle solar cell part of the light absorption may occur as parasitic absorption in the nanoparticles. Thus, a key merit is whether this increased absorption can be converted to photovoltaic power or whether it is lost as parasitic absorption. To address this point the key is to measure the external quantum efficiency, since this is related to the light absorbed only in the photoactive layers of the solar cell. Figures 3(a) and 3(b) show the external quantum efficiencies measured at normal incidence in s-polarization and p-polarization, respectively, for the nanoparticle array sample (blue lines) and for the reference sample (grey lines). Measurements at an 8◦ angle of incidence for the nanoparticle sample are also included (red lines) in order to provide EQE data that mimic the measurement geometry of the reflectance and transmittance measurements. In order to allow a more easy identification of regions of photocurrent enhancements the relative photocurrent or gain was derived by normalizing the measured photocurrent for the nanoparticle sample to the photocurrent measured for the reference sample. The results are shown in Fig. 3(c) for spolarization and in Fig. 3(d) for p-polarization. In the infrared range the EQE of the reference solar cell is declining due to the decreasing absorption efficiency in Si towards longer wavelengths while it may be noted that the external quantum efficiency in both samples displays a distinct drop in the 350-400 nm range. The latter is ascribed to front surface recombination at the interface between the Si surface and the sputtered SiO2 layer. For real devices the SiO2 spacer layer should be optimized with respect to both optical properties and to interface quality but for the purpose of this study the optically optimized sputtered SiO2 suffice. In the normal-incidence measurements the EQE for the nanoparticle sample is similar for the two polarizations as expected, and the EQE is increased with respect to the reference solar
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Received 23 Jan 2015; revised 25 Mar 2015; accepted 30 Mar 2015; published 17 Apr 2015 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.00A525 | OPTICS EXPRESS A530
(a)
s−polarization
(b)
p−polarization
(d)
Wavelength (nm)
EQE (%)
60
Ref. NP 0 deg NP 8 deg
40
20
0 (c) Relative EQE
5 4 3 2 1 400 500 600 700 800 900 1000 Wavelength (nm)
400 500 600 700 800 900 1000 1100 Wavelength (nm)
Fig. 3. External quantum efficiency (EQE) measured in s-polarization (a) and in ppolarization (b). The grey lines refer to the reference solar cell while the blue lines and red lines correspond to measurements on the nanoparticle array made at an angle of incidence of 0◦ and 8◦ respectively. The relative EQE with respect to the reference sample is shown in (c) and (d) for s- and p-polarization respectively.
cell from the UV-visible spectral region to the infrared. The photocurrent enhancement is more easily identified from the gain profiles shown in Figs. 3(c) and 3(d), where it is seen that the cross-over point from photocurrent reduction to photocurrent enhancement occurs near 400 nm and the region of enhancement is maintained throughout the measurement range towards longer wavelengths where the gain is increasing to very large values near the Si band gap at 1100 nm. By inspection of the EQE data in Figs. 3(a) and 3(b), and the absorptance data in Fig. 2(c) it is observed that the enhancement appears to be stronger in two regions: In the region from the crossover point to ∼500 nm and between ∼550 nm and ∼1000 nm, where a broad hump can be observed. In the first region the gain curve has a distinct Fano-like profile just below 400 nm [20]. Fano-like profiles in the photocurrent gain curves for solar cells with frontside particles, like that observed below 400 nm, have been shown in the literature to correlate with resonances in the metal nanoparticles. [6, 10, 13]. Moreover, in [15] Uhrenfeldt et al. recently showed that diffractively coupled collective modes in periodic nanoparticle arrays may introduce additional Fano-like profiles in the photocurrent gain. The diffractive coupling of these collective modes is mediated by Rayleigh-Wood anomalies (RA), which is an optical phenomenon related to diffracted light propagating parallel to the array plane [21–23]; this diffracted light leads to a strong coupling of the particles which induces collective resonant modes in the array near the spectral positions of the RA [21–29]. An important hallmark of these collective array modes is a strong dependence on the angle of incidence and the polarization of the incident light [15],
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which enables an easier identification of such modes. For the EQE measurements made at an incidence angle of 8◦ [Fig. 3] it is seen that the gain for s-polarization is almost the same as that for the normal incidence EQE measurements, whereas the EQE has changed in the ultravioletvisual spectral range for p-polarization. Thus it is evident that the Fano-like profile near 400 nm has a clear dependence on the angle of incidence and on the polarization. For later use it is noted that since the EQE measurements in s-polarization at normal incidence and at an 8◦ angle of incidence are almost the same, the transmittance and reflectance measurements made in s-polarization may be considered as approximative normal incidence measurements. For normal incidence light, the spectral position of the Rayleigh Wood anomalies can be calculated from ni p λ0 = √ (1) n2 + m2 where λ0 is the RA free-space wavelength, p is the pitch of the array, (n, m) are integers for the diffraction order, and ni is the refractive index of the material into which the light is scattered. Thus ni = 1 (air) for light reflected from the array into the air superstratum and ni = nSiO2 ≈ 1.46 (at 600 nm) for light scattered from the array into the SiO2 dielectric layer below. The position of the reflected light Rayleigh-Woods anomalies with (n, m) = (±1, 0) and (0, ±1) becomes λ0 =380 nm, which is just the array pitch value for the array studied here. This value may be compared to the s-polarized reflectance measurements shown in Fig. 2(a), since these resemble normal incidence measurements as noted previously. In the figure the λ0 =380 nm position has been marked as a vertical dashed line which can be seen to coincide with the strong drop in reflectance observed between ∼360 nm and ∼420 nm in s-polarization. The correlation of the spectral position of the reflectance drop with the particle pitch combined with the strong dependence of the Fano-profile on the angle of incidence and the polarization strongly supports that the Fano-like profile below 400 nm and the photocurrent enhancement in the ∼400-500 nm spectral range is indeed associated with diffractively coupled collective modes in the Al nanoparticle array in line with the findings in [15]. Turning to the second region of EQE enhancement between ∼550 nm and ∼1000 nm, the “hump” observed in Fig. 2(c) and Figs. 3(a) and 3(b) arises from the convolution of the strongly decreasing absorption efficiency of Si towards the IR, as seen in the reference sample, and the strong increase in the relative gain observed above ∼550 nm in Figs. 3(c) and 3(d). The fact that the gain increases to very large values above 550 nm indicates that the nanoparticle array causes light-trapping beyond this point. It should be noted that the EQE gain is larger than unity for most of the part of the solar spectrum that can be absorbed by Si. This implies that the Al nanoparticle array leads to both a broadband increased in-coupling of light into the solar cell and also facilitates light-trapping in the solar cell. 3.3.
Finite difference time domain simulations
To strengthen this point further, numerical finite difference time domain (FDTD) simulations of the nanoparticle and solar cell structure were performed. The calculations were made using the commercial three dimensional FDTD solver provided by Lumerical [30] using a unit cell with periodic boundary conditions on the sides, and perfectly matched layer boundary conditions on the top and bottom. In all the calculations a normal plane wave is incident from the top onto a periodic array of Al nanoparticles placed on a 40 nm thick SiO2 film on top of a slab of 1160 nm Si, which in turn was placed on a SiO2 substrate. The incident wave was polarized along the y-axis [see Fig. 1(d)]. This corresponds to the experimental s-polarization configuration but due to the symmetry of the array at normal incidence the results are identical to polarization along the x-axis. In order to limit the simulation volume the extent of the 300 μ m SiO2 (Pyrex in the experiments) was omitted from the simulation volume. This omission implies that the
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Received 23 Jan 2015; revised 25 Mar 2015; accepted 30 Mar 2015; published 17 Apr 2015 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.00A525 | OPTICS EXPRESS A532
Reflectance (%)
relatively weak reflectance at the pyrex-air backside is not accounted for, which may give slight deviations from the experiments. The optical constants for Al, Si, and SiO2 were taken from Refs. [31–33], respectively, and fitted using the routines provided by Lumerical. In order to include the slight narrowing effect which occurred in the metal evaporation, the particle shape was modeled as truncated cones. As in the experiments, the height and base diameter of the nanodiscs were chosen to be 70 nm and 190 nm, respectively. The top diameter was set to the base diameter minus ∼ 40% of the height to match the experimental conditions. With these parti-
60
Transmittance (%)
Ref.
40 20 0
(b)
1.46× 460 nm 1.46× 460 nm 60 1.46× 380 nm 40 1.46× 360 nm 80
Ref.
20 0
Absorptance (%)
p = 360 nm p = 380 nm p = 400 nm p = 460 nm
(a) 80
(c)
80 60 40 20 0 300
Ref.
400
500
600 700 800 Wavelength (nm)
900
1000
1100
Fig. 4. Simulated normal incidence reflectance (a), transmittance (b), and absorptance (c) spectra for the solar cell with nanoparticles for different array pitch values.
cle dimensions the reflectance, transmittance and absorptance for the composite structure were calculated for four different array pitch values of p=360 nm, p=380 nm, p=400 nm, and p=460 nm, and the results are shown in Fig. 4. By comparison with Fig. 2 it is seen that the calculated reflectance and transmittance for p=380 (green lines in Fig. 4) are in very good agreement with the experimental curves, although the pronounced increase in the transmittance below 550 nm is not observed in the experiments. To highlight the effect of the array pitch on the optical properties of the nanoparticle solar cell, the positions of selected Rayleigh-Woods anomalies have been included in Figs. 4(a) and 4(b). As mentioned previously, the spectral positions of the (±1, 0) and (0, ±1) Rayleigh-Woods anomalies for reflected light is just the array pitch values, which are shown in Fig. 4(a) as vertical lines. In the case of the transmittance measurements the RA spectral positions for transmitted light were calculated using Eq. 1, which for the (±1, 0) and (0, ±1) modes reduces to λ0 = nSiO2 p ≈ 1.46p. The spectral positions of these modes are shown as vertical lines in Fig. 4(b). It is clear that the strong reflectance dip observed in the short wavelength region in the simulations correlates with the (±1, 0) and (0, ±1) RA positions for reflected light, and the corresponding RA values for transmitted light are seen to correlate with the pronounced drops in the simulated transmittance in the 500-700 nm spectral range.
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100
(a)
(c)
Sim. NP Sim. Ref. Exp. NP Exp. Ref.
Apsorptance(%)
80 60 40 20 EQE (b)
(d)
400 500 600 700 800 900 1000 Wavelength (nm)
400 500 600 700 800 900 1000 Wavelength (nm)
Gain
8 6 4 2
Fig. 5. (a) Comparison of the measured s-polarized absorptance (blue lines) and simulated normal incidence absorptance (red lines) for the nanoparticle sample as well as the experimental (black line) and simulated (grey line) absorptance for the reference solar cell. (b) Relative gain in the measured (blue line) and simulated (red line) absorptance shown in (a). (c) Comparison of the s-polarized normal incidence external quantum efficiency measurements with the simulated normal incidence absorptance in the 1160 nm Si layer for the nanoparticle sample (blue and red lines) and the reference sample (black and grey lines). (d) Comparison of the relative gain in the measured EQE and the simulated Si layer absorptance shown in (c).
3.4.
Comparison between experiment and simulation
To allow a more detailed comparison of the simulations with the experiments, the experimental absorptance measured in s-polarization for the nanoparticle (blue line) and reference samples (black line) are shown in Fig 5(a) together with the simulated values for p=380 nm. Furthermore, the gain in absorptance of the nanoparticle sample relative to that of the reference sample were also derived for both the experiment and the simulation and the results are shown in Fig. 5(b). As can be seen from Figs. 5(a) and 5(b) the simulations are in excellent agreement with the experimental data except from a few minor deviations: Some of the peaks in the simulations are less distinct in the measurements and the nanoparticle induced modulation of the absorptance in the ∼350 - 500 nm range is less pronounced in the experiments. This may be due to experimental non-idealities, such as corner rounding, surface roughness, and grains in the Al nanoparticles, which are not included in the simulation and which may affect the optical properties [34]. To be able to compare the simulations with the experimental EQE, the amount of light absorbed solely in the Si layer was extracted from the simulations since this quantity can be considered an ideal EQE. The simulated Si layer absorptance for the nanoparticle sample with
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Received 23 Jan 2015; revised 25 Mar 2015; accepted 30 Mar 2015; published 17 Apr 2015 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.00A525 | OPTICS EXPRESS A534
p=380 nm and the reference sample are shown in Fig. 5(c) as red and grey lines, respectively. By comparing Figs. 5(a) and 5(c) it is seen that the Si absorptance is less than the total absorptance in the nanoparticle sample which is due to parasitic absorption in the nanoparticles. The experimental EQE measured in s-polarized at normal-incidence for the nanoparticle and reference sample are also shown in Fig. 5(c) for comparison. It can be seen that the measured EQE data of both the nanoparticle cell and the reference cell are markedly reduced relative to the simulated ideal EQE. Since the absorptance data in Figs. 5(a) and 5(c) show that the simulations are fairly accurate with respect to the optical absorption, this difference with respect to the ideal EQE may be ascribed to a reduced electron-hole collection efficiency in the experimental samples, caused by interface recombination at the front and back interfaces of the Si absorption layer. This effect of recombination is largely eliminated when comparing the relative gain in EQE from the simulations with the experiments. This is seen in Fig. 5(d) where the gain in experimental EQE and simulated Si absorptance derived from the data in Fig. 5(b) are compared. As can be seen the experimental and simulated EQE gain curves are in very good agreement apart from the same minor deviations commented on previously. An ideal shortcircuit current density Jsc may be obtained from the simulations by convoluting the simulated ideal EQE shown in Fig. 5(c), with the solar spectrum. For the reference sample with a 40 nm SiO2 layer used here this ideal Jsc yields ∼9.8 mA/cm2 while a more conventional SiO2 antireflective layer thickness of 100 nm gives 11.6 mA/cm2 . In the case of the Al nanoparticle sample cell investigated here the idealized Jsc yields ∼ 14.6 mA/cm2 which constitutes an enhancement of 49% relative to reference sample and 26% relative to the antireflective layer configuration. This is contrary to the findings of Massa et al. in [37] where Al nanoparticle arrays on the front were also studied numerically but where collective resonances were not utilized. From Fig. 4 it is clear that the Rayleigh-Woods anomalies have a strong influence on the absorptance profile of the solar cell as it also appears from Fig. 4(c). A full analysis of the modes and coupling efficiencies of the compound structure is beyond the scope of the present experimental work, since these are likely to depend on the array pitch as well as on the solar cell layer configuration in a complex manner [23,35,36]. The latter may be hinted by observing that the detailed spike structure in the simulated absorptance depends on the particle pitch in a nontrivial manner [see Fig. 4(c)]. Nevertheless some important implications of the array pitch influence can still be given based on a more qualitative account as follows: At wavelengths below the (±1, 0) and (0, ±1) RA positions for the reflected light, the light scattered by the nanoparticle array back into the air can couple to radiative diffractive modes [21]. On the high wavelength side above the (±1, 0) and (0, ±1) RA positions these modes are evanescent on the air side and as a consequence they are confined to the vicinity of the array. This can account for the abrupt drop in the reflectance spectra observed at the RA positions in Fig. 4(a). The amplitude of this reflectance reduction is most pronounced near the RA which indicate the resonant nature of the diffractive coupling. The light scattered by the array into the SiO2 layer can couple to radiative diffractive modes in the SiO2 if the wavelength is below the (±1, 0) and (0, ±1) RA positions for transmitted light at ∼ 555 nm. These radiative modes in the SiO2 layer can in turn couple to radiative modes in the Si slab which are partially transmitted into the pyrex substrate employed here, and hence this light is not trapped in the Si. However, at wavelengths larger than 555 nm the transmitted RA related modes in the glass layer are evanescent in nature. Since the SiO2 is very thin these evanescent modes may couple via mode overlap to internal guided modes in the nearby Si slab and this can lead to light trapping inside the Si layer. This is in good agreement with the strong increase in EQE gain above ∼ 550 nm observed in Figs. 3(c) and 3(d) and is further evidenced by the strong and narrow peaks that starts to dominate the simulated Si absorptance in Fig. 5(b) at wavelengths
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above 555 nm. A nanoparticle array may support collective resonances in one spectral range and still display single-particle like resonances in other parts of the spectrum if these are detuned from the RA anomalies that mediate the diffractive coupling [22]. In order to elucidate the role of such single-particle-like resonances in the EQE enhancement the scattering cross sections and absorption cross sections were calculated for a single Al nanoparticle placed on a Si surface with a 40 nm SiO2 spacer layer using FDTD. The calculations are made for normal incidence using the ”Total-Field Scattered-Field” source provided by Lumerical [30], where the scattering and absorption cross sections are calculated by placing frequency domain power monitors outside and inside the region of the source respectively and by using perfectly matched layers on all boundaries. The scattering and absorption efficiencies Q, which are obtained by normalizing the respective cross sections to the geometric cross section, are shown in Fig. 6(a) as full and dashed lines, respectively, for three different particle geometries; for a base width of 190 nm and a height of 70 nm as in the experiments (blue lines), for an increased base width of 220 nm (red lines) as well as for an increased height of 110 nm (green lines). The Si absorptance was also calculated for a p=380 nm nanoparticle array with these particle geometries using the simulation settings as those used in Figs. 4 and 5, and the results are shown in Fig. 6(b).
5
(a)
4 Q
3 2 1
Absorptance (%)
0
(b)
Wavelength (nm)
80 60
Reference Exp. geometry NP D190 h110 NP D220 h70
40 20 0 300
400
500
600
700
800
900
1000
1100
Fig. 6. (a) Calculated normal incidence scattering efficiency (full lines) and absorption efficiencies (dashed lines) for the experimental nanoparticle geometry (blue), for a nanoparticle with a larger basediameter of 220 nm (red) and for a nanoparticle with a height of 110 nm (green), all other structural dimensions being similar. (b) The simulated normal incidence absorptance for nanoparticle solar cells using the nanoparticle geometries in (a) (blue,red,green) as well as for the reference cell (grey).
It can be seen that the Al nanoparticles with the experimental geometry have a strong scattering efficiency in most of the investigated spectral range with a broad dipole-like resonance centered around 650 nm and an additional resonance-like peak structure near 300 nm, which may be a quadropole-like resonance [13]. The corresponding absorption efficiency is seen to be substantially smaller with a faint peak structure centered near 830 nm, which coincides with the narrow interband transition in Al at 1.5 eV [38]. For fairly large nanoparticles radiative damping of the single particle resonances becomes pronounced and leads to a broadening of dipolar modes and increases the scattering-to-absorption ratio, which is in agreement with the #233121 - $15.00 USD (C) 2015 OSA
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trends observed. It is also noted that the RA spectral positions near 400 nm correspond to regions where the single particle scattering is still pronounced, which is a necessary condition for an efficient coupling. By comparing the single particle resonances with the corresponding Si absorptance in Fig. 6(b), it can be seen that the profile of the dipole-like mode coincides with the light-trapping hump in the Si absorptance enhancement, although this is truncated at wavelengths below 555 nm due to the influence of the array structure as discussed above. Thus, while the diffractive coupling accounts for the steep increases in absorptance near 400 nm and 555 nm, the broad absorption enhancement region in the IR is due to light-trapping caused by scattering from the single particle dipole-like resonances. 4.
Conclusion
Broadband photocurrent enhancement and light trapping in the IR spectral range have been demonstrated using periodic arrays of large Al nanoparticles on the front side of a test solar cell. It is demonstrated that the steep increase observed in the EQE enhancement in the ultravioletblue region is caused by diffractively coupled collective modes in the array, while the broad absorption enhancement region in the IR is due to light-trapping caused by scattering from the single particle dipole-like resonances. The results demonstrate that an apparent paradox between broadband photocurrent enhancement and light-trapping for nanoparticle arrays on the front side of a solar cell may be avoided if the properties of the particle arrays are tailored to support multiple resonances. A.
Appendix
The procedure to fabricate the thin crystalline Si solar cell is illustrated in Fig. 7. A commercial silicon-on-insulator (SOI) wafer (SOITEC) is used as a starting substrate presenting a 290 nm top silicon layer and a 400 nm buried oxide layer [Fig. 7(a)]. The diode stack (DS) is grown by molecular beam epitaxy (MBE) on the top Si layer of the SOI wafer [Fig. 7(b)]. The DS which is also shown in Fig. 1(a) consists of a highly B doped ∼60 nm p++ Si layer (1 × 1020 B/cm3 ), a 500 nm intrinsic Si layer and a Sb doped 600 nm n+ Si layer (5 × 1018 Sb/cm3 ). The highly B doped layer is utilised as an etch stop in one of the fabrication steps described below. The pressure under growth is 1.5 × 10−10 mbar. After careful cleaning, the structure is anodically bonded in vacuum to a borosilicate glass substrate [Fig. 7(c)] at a temperature of 400◦ C, with an applied voltage of 700 V modulated at 1 Hz [39]. The structure is then flipped up-side down [Fig. 7(d)], and the SOI wafer is then removed in a series of chemical wet-etching steps. The silicon substrate of the SOI wafer is removed by a 30% KOH wet-etch at 85◦ C where the buried oxide layer acts as an etch stop [Fig. 7(e)]. The second etch step uses a 30% HF solution to remove the 400 nm oxide layer [Fig.7(f)]. The remaining inactive Si layer from the SOI wafer is then etched away in a solution of ethylene diamine pyrocatechol (EDP) at 100◦ C utilizing the highly B doped p++ Si layer of the DS as a very efficient etch stop [Fig. 7(g)] [40]. The EDP etchant combined with the B etch stop leaves a very uniform surface confirmed by atomic force microscopy (roughness of ∼1.2 nm RMS). The DS has hereby been transferred onto a glass substrate. The native oxide has been removed using a 30% HF solution prior to defining the contacts. Ring shaped top electrodes were made by photolithography masking followed by evaporation of a ∼300 nm thick Al layer. Subsequently the diode and the top electrodes were masked with black wax [Fig. 7(i)] in order to define the mesa diode using a 1 : 9 : 1 HNA wetetching solution [Fig. 7(j)]. The wax is stripped off [Fig. 7(k)] and backside Al contacts were made by a second photolithography process. The resulting Si test solar cell with the electrode pattern can be seen in Fig. 1(b).
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Fig. 7. (a) Schematic illustrating the different fabrication steps to obtain a thin mono crystalline Si solar cell on a glass substrate starting with (a) a SOI substrate on which (b) a DS is grown by MBE. (c) This structure is bonded to a borosilicate glass substrate and (d) the whole structure is flipped up-side down. (e)(f)(g) Chemical wet-etching is used to remove the SOI wafer: KOH is used to remove the thick Si substrate, HF is used to remove the SiO2 layer, and ethylene diamine pyrocathecol is used to remove the top Si inactive layer. (h) The thermally evaporated top Al electrodes are (i) masked with wax to isolate the diode with (j) a HNA wet-etch. (k) The wax is removed and (l) the back Al electrodes are thermally evaporated.
Acknowledgments The authors greatly acknowledge the financial support from the project Localized surface plasmons and silicon thin-film solar cells-PLATOS financed by the Villum Foundation.
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