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Antireflective nanosponges are fabricated on poly crystalline silicon (poly-Si) thin films using Ag-nanoparticles (NPs) assisted etching. Crystal orientations and grain sizes of the poly-Si thin films are investigated for the poly-Si nanosponge formation and the resultant optical properties. The Ag-NPs assisted etching preferentially etches the poly-Si thin films along crystal orientation of [110]. A 400 nm thick poly-Si nanosponge reduces effective optical reflection of the poly-Si thin film with substrate crystal orientation of (110) and averaged grain size of 250 nm from 26 % to 3 % at the wavelengths ranging from 400 nm to 1000 nm. Carrier lifetimes were found to be 41 and 36 μs for poly-Si thin film and RTO-passivated nanosponges, respectively.
©2009 Optical Society of America
1. Introduction
Polycrystalline silicon (poly-Si) thin film solar cells are emerging as a promising candidate for photovoltaic (PV) applications [1, 2]. Rising atmosphere temperature as well as soaring oil price stimulates solar energy demand all over the world in recent years. Such strong demand has not been satisfied by crystal silicon solar cells due to the shortage of purified silicon crystals. The poly-Si thin film solar cells produced by vapor deposition not only reduce the consumption of silicon material but meet the future trend of large area manufacturing. The PV efficiencies of the poly-Si solar cells, however, are of desire to further improve, particularly via enhancement of light absorption in the solar spectrum from ultraviolet (UV) to near-infrared (NIR) [3]. With no proper antireflection (AR) treatment nearly 30% of solar irradiation is reflected on the poly-Si surface under AM 1.5 which represents a standard terrestrial solar spectral irradiance under specified atmospheric conditions.
The AR technologies based on thin-film coating are proposed for the poly-Si thin film solar cells. Single-layer AR coating, such as 80 nm thick silicon nitride (SiNx), was reported to reduce effective optical reflection from 35% to 10% and improves 43% in short-circuit current density for crystal silicon solar cells [4–6]. Multi-layer and graded-index AR coatings were also reported to further reduce the effective optical reflection to 2.5%. The drawback is the high intrinsic optical absorption of the multi-layer and graded-index AR coatings which is two-order larger than the single-layer SiNx AR coating. Photon absorption within the AR coatings decreases the short-circuit current density. Hence, the multi-layer and graded-index AR coatings function effectively similar to the single-layer SiNx AR coating.
To achieve low optical reflection as well as to minimize optical absorption in the AR coatings of the solar cells, subwavelength nanostructures, inclusive of nanotips, nanorods and nanosponges, were proposed [7–9]. Metal nanoparticles (NPs) assisted etching [10, 11] emerges as a cost-effective nanostructure fabrication process for large silicon surfaces compared to ICP etching [7], oblique-angle evaporation [8] and electrochemical etching [12]. Silicon nanostructures using the metal-NPs assisted etching are demonstrated in multi-crystalline silicon (mc-Si) and single-crystalline silicon (sc-Si) wafers. From the UV to the near-infrared spectrum, the silicon nanostructures show the effective optical reflection lower than 6% and 2% for the mc-Si wafers and the sc-Si silicon wafers, respectively. Distinct to the sc-Si and mc-Si, poly-Si thin films have various crystal orientations, much smaller silicon grains and grain boundaries. The metal-NPs assisted etching is found to be affected by crystal orientations. The grain size of the poly-Si thin film is close to the pitch of nanostructures in an order of 100 nm. Hence, an etching process must simultaneously overcome the etching resistance difference of grains and grain boundaries in between.
To achieve uniform and effective etching on silicon surfaces, the etching solution must deal with a high density of silicon grains that are associated with different crystal orientation and grain boundaries. Poly-Si nanostructures, namely poly-Si nanosponges, fabricated on thin films using a metal-NPs assisted etching process are reported. Further, the morphologies and the optical properties are investigated for the poly-Si nanosponges.
2. Experiment
The poly-Si nanosponges are fabricated using the metal-NPs assisted etching on 2 μm thick poly-Si thin films. The thin films are deposited using low pressure chemical vapor deposition (LPCVD) on 6″ oxidized polished silicon wafers at 250 mTorr at 670°C. The etching is conducted in a solution of HF(10%), H2O2(39%) and AgNO3(0.34%) (90:40:6 in v/v) at 20°C. Silver nanoparticles (Ag-NPs) with averaged diameters of 15 nm that are deposited on the surface of the poly-Si thin films function as catalysts in the etching process to produce poly-Si nanosponges and are removed after the nanosponge formation by H2O2. The deposition of the Ag-NPs is formed on the poly-Si surface via electron transfer between silver ions and HF in the solution. Etch rate is calculated to be 200 nm per minute. To further investigate the effects of crystal orientation and grain size on the poly-Si nanosponge formation, both the as-deposited and the annealed poly-Si thin films are processed. Annealing at 1000°C for an hour alters the morphology of the as-deposited poly-Si thin films. To reduce the surface defect density of the poly-Si nanosponge, rapid thermal oxidation (RTO, Kornic KORONA 800 RTP) is used at 900°C for 15 s.
The crystal orientation of the poly-Si thin films is analyzed using X-ray diffraction (XRD, Rigaku RHU-30). The morphology of the poly-Si nanosponges is observed by a field-emission scanning electron microscope (FESEM, Carl Zeizz, Ultra-55). The optical reflection of the poly-Si nanosponges in the spectrum range from 400 nm to 1000 nm is characterized by an integrated sphere (Ocean Optics ISP 30-6-R) and a UV-VIS spectrometer (BWtek BRC111A). Effective carrier lifetimes of the poly-Si thin film and the poly-Si nanosponge are measured by microwave photo-conductance decay (μ-PCD, Semilab WT-2000).
3. Results and discussion
Figure 1 indicates that the as-deposited poly-Si thin film grows preferentially along (220) and hence (110) orientation at the deposition temperature of 670°C. Annealing at 1000°C alters the primary crystal orientation of the poly-Si film from (110) to (111) due to recrystallization. Figure 2 shows the grain sizes of the as-deposited and the annealed poly-Si thin films, revealed by Wright etching [13]. The as-deposited poly-Si thin film is found to have the grain size from 150 nm to 350 nm while the grain size of the annealed film falls in the range of 250 nm to 450 nm. Both the substrate crystal orientation and the grain size of the poly-Si thin film change after 1000°C annealing.
Figures 3(a) and 3(c) show the top view and the cross-section view of a poly-Si nanosponge fabricated on the as-deposited thin film. The as-deposited poly-Si nanosponge consists of nanoholes with averaged diameter of 15 nm and averaged depth of 400 nm. The averaged pitch of the nanoholes is 70 nm. Since the sponge structures are much smaller than the light wavelength, the light wavelength averages over sponge structures revealing a specific refractive index of the AR layer.
The majority of nanoholes in the as-deposited poly-Si nanosponge prefer to form perpendicular to the poly-Si surface. This result implies that the metal-NPs assisted etching can uniformly etch the as-deposited poly-Si thin film unlike Wright etching mainly attacking silicon grain boundary. The metal-NPs assisted etching used prefers to etch silicon along the orientation [110].
Fig. 2. The FESEM photos show (a) the as-deposited poly-Si thin film and (b) the annealed poly-Si thin film. The grain sizes of the as-deposited poly-Si thin film and the annealed poly-Si thin film range from 150 nm to 350nm and from 250 nm to 450 nm, respectively. The scalar bars represent 200nm, each.
Fig. 3. (a) and (b) show the top view of the poly-Si nanosponges which are formed on (a) the as-deposited and on (b) the annealed poly-Si thin films, respectively. (c) and (d) show the cross-section view of the poly-Si nanosponges on (c) the as-deposited and on (d) the annealed poly-Si thin films, respectively. The scalar bars represent 200nm, each.
On the other hand, submicron holes are frequently observed in the annealed poly-Si nanosponges (see Fig. 3(b) and 3(d).) The holes result from the etching preference of crystal orientation. The substrate crystal orientation of the annealed poly-Si thin film is (111) while that of the as-deposited one are (110). The averaged diameter, depth and pitch of the nanoholes in the annealed poly-Si nanosponge are similar to those in the as-deposited poly-Si nanosponge.
Figure 4(a) shows that the 400 nm thick poly-Si nanosponge reduces the effective optical reflection of the as-deposited poly-Si thin film from 26 % to 3 %. Figure 4(b) shows that the poly-Si nanosponge reduces the effective optical reflection of the annealed poly-Si thin film to 6 % in average, which is 3% higher than the as-deposited poly-Si thin film. The difference of the effective optical reflection results from the morphology of the poly-Si nanosponges (see Fig. 3). The submicron holes degrade the optical antireflection of the poly-Si nanosponges since they are too large to maintain the effects of subwavelength structures for visible light [14]. In addition to the optical antireflection of the poly-Si nanosponge, theoretical works predict such kind of grating-like subwavelength nanostructures could cause 3-fold enhancement in optical absorption in the spectral range from 920 nm to 1040 nm. The enhancement can be maintained even the incident angle varies up to ±40° [15].
Fig. 4. The optical reflection of the poly-Si nanosponge on (a) the as-deposited and on (b) the annealed poly-Si thin films
The effect of RTO surface passivation is evaluated using the effective carrier lifetime. The effective carrier lifetimes are 41, 1 and 36 μs for the as-deposited poly-Si thin film, the as-deposited poly-Si nanosponge, and the as-deposited poly-Si nanosponge passivated by RTO, respectively (see Fig. 5). The results show that the surface defect density increased by the poly-Si nanosponge can be effectively reduced by RTO and hence the RTO-passivated poly-Si nanosponge can function as an effective AR layer. Figure 6 shows the measured optical reflection of the RTO-passivated nanosponge remain 3%. The negligible variation in the optical reflection results from a mere 1 nm SiO2 layer grown on the nanosponge by RTO [14].
Fig. 5. The effective carrier lifetime mappings of (a) the as-deposited poly-Si thin film, (b) the as-deposited poly-Si nanosponge and (c) the RTO-passivated poly-Si nanosponge. The effective carrier lifetimes are 41, 1 and 36 μs for (a), (b) and (c), respectively. Red color and black color refer to effective carrier lifetimes of 1 μs and 100 μs, respectively.
Fig. 6. The optical reflection of the as-deposited poly-Si nanosponge (red line) and the as-deposited poly-Si nanosponge after RTO passivation (dot).
4. Conclusion
In this study, a 400 nm thick poly-Si nanosponge fabricated using Ag-NPs assisted etching is proposed to function as broadband optical antireflection for poly-Si solar cells. To obtain minimum optical reflection, a poly-Si thin film with the substrate crystal orientation of (110) should be used for the poly-Si nanosponge formation. The minimum optical reflection of the poly-Si nanosponge is only 3 % while that of the single-layer SiNx AR coating is up to 10 % at the spectrum ranging from 400 nm to 1000 nm. The poly-Si nanosponge not only provides the optical reflection similar to the multi-layer and graded-index AR coatings but eliminates the intrinsic optical loss due to the AR coatings. However, short circuit current could be evaluated when the optical reflection, the absorption coefficient of poly-Si, the absorption in the back reflector, and the percentage of the photons escaped from cells are known. Poly-Si photovoltaic cells embedded with nanosponge is under development.
Acknowledgement
The authors gratefully thank to support of National Science Council and Sino-American Silicon Products incorporation.
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