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We present tailor-made imprinted nanostructures for light management in liquid phase crystallized silicon thin-film solar cells providing both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values >640mV. All superstrate textures successfully enhance light in-coupling in 10-20µm thick liquid phase crystallized silicon thin-films. Moreover, the effect of combining imprinted textures at the front side with individually optimized light trapping schemes at the rear side of the absorber layers on the optical properties is analyzed. With a silicon absorber layer thickness of 17µm maximum achievable short-circuit current density of 37.0mA/cm2 is obtained, an increase by + 1.8mA/cm2 (or 5.1%) compared to the optimized planar reference.
© 2017 Optical Society of America
1. Introduction
In recent years liquid phase crystallized (LPC) silicon thin-film solar cells on glass have emerged as promising silicon thin-film solar cell technology. A key characteristic of this technology are high Voc-values of up to 650mV being comparable to multi-crystalline silicon wafer solar cells [1–3]. During liquid-phase crystallization (LPC) a line-shaped laser beam is scanned across the sample heating the silicon precursor layer above its melting temperature. The silicon recrystallizes from the melt into grains with a few millimeters in width and up to a few centimeters in length [1]. With absorber layer thicknesses of 10µm short-circuit current densities above 30mA/cm2 [4] and efficiencies up to 12.1% [5] were reached. Optical losses were identified as one major efficiency limiting factor [5]. On the one hand, incomplete light trapping of long-wavelength light reaching the rear side of the silicon absorber limits the efficiency. Technologically this kind of loss can be addressed by texturing the comparably easily accessible silicon rear side in combination with a suitable back reflector [4–6]. On the other hand, reflection losses at the sun-facing glass-silicon interface can amount to a short-circuit current density reduction of at least −3.4mA/cm2 if only planar antireflective layers are used [5]. Therefore, we recently developed a method for the implementation of nanotextured glass superstrates, either prepared by nanoimprint lithography in combination with high-temperature stable silica sol-gels [3,7,8] or by chemical wet-etching [2,6], into LPC silicon solar cells resulting in double sided structured absorbers layers . Here, the use of SiOx capping layers on the silicon layer during LPC allows preserving the superstrate texture also at the rear side of the absorber [8].
However, the implementation of such antireflective nanotextures at the buried glass-silicon interface maintaining the electronic silicon material quality after the LPC process remains a challenge [7]. Further, the dimensions of the nanotextures of front and rear side of the silicon absorber ideally have to be optimized independent from each other for their respective purposes, light in-coupling and light trapping [9–12].
In this paper, we first review the impact of diverse types of glass superstrate nanotextures - namely a pillar-like structure (“Pillar”, [7]), a sinusoidal grating (“Sine”, [7]), a SMooth Anti-Reflective Three-dimensional texture (“SMART”, [3]), and a planar references (“planar”, [7]) – on the optical and electrical properties of LPC silicon thin-film solar cells. In the second part, the effect of combining individually optimized light-trapping schemes at the front and rear side of LPC silicon thin-films on the absorption behavior is analyzed.
2. Experiment
The samples were prepared on 5cm x 5cm Corning Eagle XGTM glasses with a thickness of 1.1mm. On top, 750nm-periodic pillar-like, sinusoidal and SMART nanotextures are fabricated. The sinusoidal grating matches the pillar structure besides featuring smoothed texture flanks. The SMART texture fills the voids between the SiOx pillar-features with TiOx and features an almost flat glass-silicon interface. Further details on the fabrication of nano-textured superstrates can be found in the respective publications [3,7]. Textured superstrates as well as planar references were either coated with a 10nm thick SiOx-layer (SMART) or 70nm SiNx / 10nm SiOx (Pillar, Sine, planar) interlayer stack. The SiNx-layer acts as anti-reflection coating [13], which in case of the SMART texture is obsolete since its SiOx/TiOx/Si layer system (Fig. 1) provides an optimized graded index effect [3]. On top 8µm-20µm liquid phase crystallized (LPC) silicon absorber layers were fabricated preserving the superstrate texture at the rear side of the absorber layer as described in [8]. Trapping of long- wavelength light at the rear side of the absorber layers was realized either by wet-chemical etching with a KOH solution, by applying a white paint reflector, or a combination of both. The superstrate surface morphology was determined by atomic force microscopy (AFM) imaging using a Park Systems XE-70. AFM images of the different superstrate textures featured in this study and schematic sample stacks are depicted in Fig. 1.
Fig. 1 AFM images of the textured superstrate surfaces and schematic sample stacks of a planar reference (black), a pillar (blue), a sinusoidal (red) and a SMART (green) textured device. In case of the sinusoidal superstrate texture the period (P = 750nm) and height (h = 200nm), determining the texture’s aspect ratio (h/P), are denoted. A scale bar set for all AFM images is depicted along with the planar reference.
Optical characterization was carried out in a wavelength range of 280nm – 1200nm with a step size of 5nm using a Perkin Elmer LAMBDA 1050 spectrometer featuring an integrating sphere with a diameter of 15cm. During analysis the samples were placed within the integrating sphere. Based on measured absorptance spectra the maximum achievable short-circuit current density (jsc,max) was calculated based on the assumption that every photon absorbed generates an electron-hole pair via
where q represents the elementary charge, hν the photon energy, S(λ) the spectral intensity under AM1.5g, and A(λ) the measured absorptance.
For electronic characterization solar cell devices with an n-type absorber doping concentration of 5·1016-1·1017 and a a-Si:H/c-Si hetero-junction are prepared on all superstrate types as described in [1–3]. Open-circuit voltages (Voc) were obtained by Suns-Voc measurements carried out at room temperature using a Suns-Voc unit of a WCT-100 photo-conductance lifetime tool from Sinton Instruments. Short-circuit current densities (jsc) were obtained by external quantum efficiency measurements using a custom-made setup featuring a probe beam size of 3mm × 2mm and halogen-lamp based bias light. All measurements were conducted in superstrate configuration, i.e. with light impinging through the glass side.
3. Anti-reflective properties of glass-silicon interface textures
The optical and electronic properties of sinusoidal and pillar textured devices in comparison with a planar reference were already studied in the scope of an earlier publication [7]. To allow for a qualitative comparison a similar processed SMART-textured device [3] is added to this data set (Fig. 2). A quantitative comparison is not possible because the SMART texture was processed in a different sample batch resulting in different deposition conditions, e.g. a lower silicon absorber layer thickness and a higher doping density.
Fig. 2 (a) External quantum efficiency (EQE, solid lines) and 1-Reflectance (1-R, dashed lines) data of SMART (green), sinusoidal (red), and pillar (blue) textured LPC silicon thin-film solar cells compared to a planar reference device (black). (b) Corresponding maximum obtained solar cell parameters measured by EQE (jsc ) and Suns-Voc (Voc). This graph summarizes previous work [3,7].
Compared to the planar device (black), the pillar structure (blue) offers vital optical anti-reflective properties enhancing light in-coupling over the entire wavelength range but exhibits an impaired electronic material quality. This might be caused by structural defects arising at the steep texture flanks [7]. The sinusoidal structure (red) preserves the optical anti-reflective properties of the pillar structure despite the smoothed texture flanks. The external quantum efficiency of the double-sided sinusoidally textured device could be enhanced compared to pillar textured devices and compared to the planar reference device with exception of a wavelength range between 400nm and 700nm. This might at least partially be attributed to an increased surface recombination at the textured glass-silicon interface and highlights the demand for an improved surface passivation on textured glass superstrates [2]. The SMART-texture (green) increases light in-coupling for the entire wavelength range ≥400nm. At shorter wavelengths parasitic absorption occurs in the TiOx layer [3]. For wavelengths >400nm the optical benefit is translated into an enhanced external quantum efficiency. Hence, the morphologically flat SMART texture is found to overcome the trade-off between optical gain and nanotexture-caused electronic losses best. The small decline of the EQE of the SMART texture for wavelengths >800nm compared to the planar reference can be attributed to the 1µm lower silicon layer thickness (Fig. 2(b)). The decline compared to the double-sided textured pillar and sinusoidal samples is additionally caused by increased transmission losses at the un-textured rear side of the SMART textured sample. Based on the EQE measurements the corresponding short-circuit current densities (jsc) are calculated and summarized in Fig. 2(b) together with maximum achieved open-circuit voltages (Voc) obtained from Suns-Voc measurements. While pillar-like nanostructures exhibit a strongly reduced Voc and jsc, sinusoidal and SMART structures yield Voc-values well above 600mV highlighting the high electronic material quality of absorber layers being grown on both texture types. Even in terms of maximum obtained jsc both textures outperform their respective planar references.
4. Combining light-trapping schemes at different interfaces
To further enhance light absorption in the device, samples with the two most promising antireflective front side textures – Sine and SMART – were fabricated for optical characterization as depicted in Fig. 1. The 15-17µm thick silicon absorbers were combined with light management schemes at the rear side of the absorber layer optimized to trap long-wavelength light. Pyramidal rear side textures produced by wet-chemical etching with KOH and dielectric back reflectors have proven their suitability to realize light trapping at the rear side of planar LPC silicon thin film devices [4,5]. In Fig. 3 the effect of combining individual tailor-made sub-micrometer sized front side textures with larger scaled light trapping schemes at the absorber layer rear side on the absorption behavior of LPC silicon thin-film absorber layers is analyzed.
Fig. 3 (a) Exemplary absorptance spectra of a 17.1µm thick sinusoidally textured LPC silicon thin film absorber with alternated rear side for light trapping: as crystallized (black), with white-paint reflector (green), with pyramidal texture (blue) and with pyramidal texture and reflector (red) as well as absorptance of the white paint on glass (dotted line). (b) Maximum achievable short-circuit current density (jsc,max) calculated from measured absorptance for planar (black), sinusoidal (red), and SMART (green) textured absorber layers with alternated rear side: as crystallized (filled square), with reflector (open square), with pyramidal texture (filled triangle) and with pyramidal texture combined with a white paint reflector (open triangle). (c) SEM images of the absorber layer rear side before and after ( + Pyr.) pyramidal texturing resulting in characteristic etch pyramids.
In Fig. 3(a) exemplarily the absorptance spectra of a sinusoidal front side texture with alternated absorber layer rear side are shown. The sample without additional light trapping scheme at the rear side (as crystallized, black) is compared to the same sample with white paint reflector (green), with pyramidal texture (blue), and with a combination of both, pyramidal texture and reflector (red). Since the white paint reflector and the pyramidal rear side texture are both applied at the rear side of the absorber layer, the absorptance spectra are only alternated for wavelengths being long enough to encounter the rear side of the absorber layer. Remaining absorption for wavelengths longer than 1100nm is likely to be attributed to defect absorption caused by sinusoidal and pyramidal texturing. Since in this wavelength range parasitic absorption in the white paint (dotted line) can be excluded.
The corresponding jsc,max-values are depicted in Fig. 3(b) (red) in comparison to results obtained on a planar (black) and a SMART structured (green) device. Please note that due to being deposited in different sample runs the silicon absorber layer thickness of the SMART structure device is slightly lower (15.0µm) compared to the depicted planar and sinusoidal textured devices (17.1µm) leading to a lower absorptance in the long wavelength range. In any case, using a textured superstrate enhances absorption and, thus, the maximum achievable short-circuit current density (jsc,max) compared to the planar reference device. Before an additional rear side light trapping scheme is applied (filled squares), an enhancement from 29.5mA/cm2 in the planar case to 36.0mA/cm2 in the sinusoidal double-sided textured case, and to 32.9mA/cm2 in the SMART textured case is obtained. This absorption enhancement is based on an increased in-coupling of light at the front side (Sine and SMART texture) as well as light trapping effect at the rear side (in case of the double-sided sinusoidal texture). Also applying a white paint back reflector (open symbols compared to filled symbols) generally increases light trapping. In case of the devices with planar rear side (planar and SMART) applying a pyramidal rear side texture enhances light absorption.
However, for the double-sided sinusoidally textured device a decline in absorptance and jsc,max is found (blue line Fig. 3(a) and red filled triangle in Fig. 3(b)). Figure 3(c) demonstrates that pyramidal texturing was successful irrespective of whether a flat (planar, SMART) or a double-sided textured (Sine) absorber layer was etched. Thus, insufficient etching can be excluded as a possible reason for the decline in absorption. The reason might be the material removed from the silicon absorber rear side during the etching process. While in the planar and SMART textured cases the gain in absorptance caused by the pyramidal texture is pronounced enough to compensate for the loss of absorber layer material, in the sinusoidal textured case this material loss outweighs the optical benefits. The decline of absorption if a sinusoidal front side texture is combined with a pyramidal rear side texture (blue) highlights that in the scope of future work deeper understanding of the optical light path between both textured interfaces has to be gained. This will be key prerequisite to fully exploit the optical potential in LPC silicon thin-films. Experimentally, a suitable method for texturing the front and rear side independently is needed. Before then, the rear side texturing step is expendable in case of double-sided sinusoidally textured LPC-Si solar cells.
While the SMART texture provided the most promising electronic properties (Fig. 2), highest absorption values without and with additional light trapping schemes at the rear side (Fig. 3) were found for the sinusoidal superstrate texture. Only if combining the SMART front side texture with a pyramidal texture and white paint reflector at the rear side, similar absorption values are achieved. This highlights the necessity of a suitable rear side texture if a SMART structured superstrate with otherwise flat absorber layer rear side is used.
In summary, if applying a tailor-made light management approach of combining imprinted superstrate textures with reflectors and pyramidal textures at the absorber layer rear side, jsc,max-values of 37.0mA/cm2 (Sine) and 36.4mA/cm2 (SMART) are achieved in comparison to 35.2mA/cm2 if a planar glass-silicon interface is used. Hence, state-of-the-art LPC silicon thin-film solar cells exhibiting a planar anti-reflective SiNx layer at the front side and a pyramidal texture with reflector at the rear side of the absorber can be outperformed if textured superstrates maintaining the electronic silicon material quality are used.
5. Conclusion
We implemented various nano-structures for light management in liquid phase crystallized silicon thin-film solar cells enabling both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values up to 649 mV. For increasing efficiencies in future device designs light absorption needs to be increased highlighting the demand for sophisticated light management. The optical potential of a tailor-made light management approach combining nano-textured superstrates for enhanced light in-coupling, namely a double-sided sinusoidal texture and the optically rough but morphologically flat SMART texture, with larger scaled light trapping measures, a white paint reflector and a pyramidal texture, was analyzed. The approach of individually optimized front and rear side light management enabled jsc,max-values up to 37.0mA/cm2 compared to 35.2mA/cm2 in the optimized planar case, an increase by 1.8mA/cm2 or 5.1%. Thus, a tailor-made light management approach with individually optimized light in-coupling and light trapping constitutes the next crucial step towards high efficiency LPC silicon thin-film solar cells.
Funding
Bundesministerium für Bildung und Forschung (501100002347) (No. 03X5520).
Acknowledgments
The authors gratefully acknowledge the support of M. Krüger, C. Klimm and H. Rhein in solar cell preparation and SEM imaging.
References and links
1. J. Haschke, D. Amkreutz, and B. Rech, “Liquid phase crystallized silicon on glass: technology, material quality and back contacted heterojunction solar cells,” Jpn. J. Appl. Phys. 55(4S), 04EA04 (2016). [CrossRef]
2. G. Köppel, D. Amkreutz, P. Sonntag, G. Yang, R. Van Swaaij, O. Isabella, M. Zeman, B. Rech, and C. Becker, “Periodic and random substrate textures for liquid-phase crystallized Silicon thin-film solar cells,” IEEE J. Photovoltaics 7(1), 85–90 (2017). [CrossRef]
3. D. Eisenhauer, G. Köppel, K. Jäger, D. Chen, O. Shargaieva, P. Sonntag, D. Amkreutz, B. Rech, and C. Becker, “Smooth anti-reflective three-dimensional textures for liquid phase crystallized silicon thin-film solar cells on glass,” submitted (2016), arXiv:1609.06997.
4. P. Sonntag, J. Haschke, S. Kühnapfel, T. Frijnts, D. Amkreutz, and B. Rech, “Interdigitated back-contact heterojunction solar cell concept for liquid phase crystallized thin-film silicon on glass,” Prog. Photovolt. Res. Appl. 20(1), 716–724 (2015).
5. T. Frijnts, S. Kühnapfel, S. Ring, O. Gabriel, S. Calnan, J. Haschke, B. Stannowski, B. Rech, and R. Schlatmann, “Analysis of photo-current potentials and losses in thin film crystalline silicon solar cells,” Sol. Energy Mater. Sol. Cells 143, 457–466 (2015). [CrossRef]
6. M. Z. Pakhuruddin, J. Huang, J. Dore, and S. Varlamov, “Light absorption enhancement in laser-crystallized Silicon thin films on textured glass,” IEEE J. Photovoltaics 6(1), 159–165 (2016). [CrossRef]
7. G. Köppel, B. Rech, and C. Becker, “Sinusoidal nanotextures for light management in silicon thin-film solar cells,” Nanoscale 8(16), 8722–8728 (2016). [CrossRef] [PubMed]
8. C. Becker, V. Preidel, D. Amkreutz, J. Haschke, and B. Rech, “Double-side textured liquid phase crystallized silicon thin-film solar cells on imprinted glass,” Sol. Energy Mater. Sol. Cells 135, 2–7 (2015). [CrossRef]
9. K. X. Wang, Z. Yu, V. Liu, Y. Cui, and S. Fan, “Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings,” Nano Lett. 12(3), 1616–1619 (2012). [CrossRef] [PubMed]
10. W.-C. Hsu, J. K. Tong, M. S. Branham, Y. Huang, S. Yerci, S. V. Boriskina, and G. Chen, “Mismatched front and back gratings for optimum light trapping in ultra-thin crystalline silicon solar cells,” Opt. Commun. 377, 52–58 (2016). [CrossRef]
11. M. Peters, M. Rüdiger, H. Hauser, M. Hermle, and B. Bläsi, “Diffractive gratings for crystalline silicon solar cells optimum parameters and loss mechanisms,” Prog. Photovolt. Res. Appl. 20(7), 862–873 (2012). [CrossRef]
12. M. Boccard, C. Battaglia, S. Hänni, K. Söderström, J. Escarré, S. Nicolay, F. Meillaud, M. Despeisse, and C. Ballif, “Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells,” Nano Lett. 12(3), 1344–1348 (2012). [CrossRef] [PubMed]
13. J. Dore, R. Evans, U. Schubert, B. D. Eggleston, D. Ong, K. Kim, J. Huang, O. Kunz, M. Keevers, R. Egan, S. Varlamov, and M. A. Green, “Thin-film polycrystalline silicon solar cells formed by diode laser crystallisation,” Prog. Photovolt. Res. Appl. 21(6), 1377–1383 (2013). [CrossRef]
