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Huisu Jeong, Hui Song, Yusin Pak, Il Keun Kwon, Kyubong Jo, Heon Lee,* and Gun Young Jung* In recent years, hybrid solar cells composed of n-type silicon (n-Si)/poly(3,4-ethylene dioxythiophene):poly-styrenesulfonate (PEDOT:PSS) heterojunction[1–8] have gained increased attention because of their easy fabrication and low cost compared to the conventional p-n Si solar cells, which require high temperature (∼1000 °C) processing for ion implantation and dopant diffusion. When the n-Si absorbs solar light, electron-hole pairs are generated and separated at the Schottky barrier of the n-Si/ PEDOT:PSS heterojunction. The photogenerated electrons are transported to the cathode as they pass through the n-Si, while the minority carrier of holes moves to the anode through the PEDOT:PSS layer. For high power conversion efficiency (PCE) in hybrid solar cells with a nanotextured n-Si layer, such as nanorod[2] or nanowire[3–8] arrays, have been studied because of their higher broadband light absorption and better carrier collection ability owing to short carrier transport path along the radial direction. Therefore, more carriers can be generated and collected, resulting in a higher short circuit current (Jsc). Furthermore, nanorods with high aspect ratio of approximately 10 have been examined for increased light absorption. However, such Si nanorods are prone to have cavities between the rods that are not filled with the PEDOT:PSS solution and likely to aggregate at the top of the nanorods, leading to a low fill factor (FF) and a higher chance of carrier recombination.[8,9] To overcome this issue, Si nanocone arrays have been introduced, which can complement the non-conformal coating via spin-coating H. Jeong, H. Song, Y. Pak, Prof. G. Y. Jung School of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju 500-712, Republic of Korea E-mail: [email protected] Prof. I. K. Kwon Department of Maxillofacial Biomedical Engineering & Institute of Oral Biology School of Dentistry Kyung Hee University Seoul 130-701, Republic of Korea Prof. K. Jo Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology Sogang University Seoul 121-741, Republic of Korea Prof. H. Lee Department of Materials Science and Engineering Korea University Seoul 136-701, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201305394
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Enhanced Light Absorption of Silicon Nanotube Arrays for Organic/Inorganic Hybrid Solar Cells
because of their tapered sides. The Si nanocone array with an aspect ratio of less than 2 demonstrates light absorption of over 90%.[10] Cui et. al. reported a Jsc of 29.6 mA/cm2 and the highest value of power conversion efficiency (PCE) of 11.1% from a hybrid solar cell composed of Si nanocone array/ PEDOT:PSS.[11] Besides Si nanowires, rods and cones, there have been studies with nanoholes,[12] pyramids,[13] hemispheres[14] for a better light absorption in solar cells. Si tubular structures are achieved by Si thin layer deposition over various templates and followed by removal of the template.[15–18] But, these structures are non-aligned, close-ended and aggregated, which have mostly been incorporated into the anodes in lithium-ion batteries.[16–18] In this work, more advanced structure of periodically aligned vertical Si nanotube (SiNT) arrays with an aspect ratio of 1 are produced using UV nanoimprint lithography and newly applied as a light absorber in the n-Si/PEDOT:PSS heterojunction hybrid solar cell. This structure has two potential advantages over its Si nanorods counterpart: 1) SiNT array has more light absorbing area from both inner and outer shells without an increase in aspect ratio, and 2) the thin tube wall (∼60 nm) shortens the carrier collection distance in the radial direction, thus more facile charge transport can be anticipated, leading to a higher PCE. A series of hybrid solar cells with various Si structures of planes, nanorods and cones were also fabricated for comparison. Figure 1 shows schematic images of the process to fabricate the SiNTs-based hybrid solar cell. The SiNTs were produced using an etching process of the n-Si substrate (specific resistivity: ∼4 Ω cm) with a periodic chromium (Cr) nanorings mask, which was prepared by two step lift-off process after UV nanoimprint lithography (Supporting Information for detailed procedure, Figure S1). A 50 nm thick Cr nanorings mask was produced on the n-Si substrate with an outer diameter of 360 nm and an inner diameter of 240 nm (Figure 1a). Inductively coupled reactive ion etching (ICP-RIE) was performed using an SF6 and C4F8 plasma gas to etch the Si substrate until the SiNTs had an aspect ratio of 1 (height: 360 nm) and the Cr nanorings mask was eliminated using a Cr etchant. After cleaning with deionized water and drying, heavy n-type doping was completed on the backside of the n-Si substrate to create a built-in field within the substrate to reduce carrier recombination and sheet resistance as well.[11] A 150-nm thick aluminum (Al) film, which functioned as the cathode, was successively deposited onto the n++-Si layer using an electron beam evaporator (Figure 1b) and then the PEDOT:PSS solution was spin-coated onto the SiNT array at a thickness of approximately 400 nm (Figure 1c). Finally, a finger grid type silver anode (100 nm thickness) was
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Figure 1. Schematic of the Si nanotubes fabrication process: a) forming a Cr nanorings mask on the n-Si substrate using UV nanoimprint lithography and two step lift-off process, b) ring pattern transfer by dry etching with the Cr nanorings mask and deposition of Al at the substrate backside as a bottom electrode, c) spin-coating of the PEDOT:PSS solution onto the SiNT array and d) deposition of finger grid-type silver top electrode. e) A tilted optical image of the SiNTs-based hybrid solar cell with finger grid top electrodes.
deposited on the PEDOT:PSS layer using a shadow mask with 0.1 mm wide fingers at 1 mm spacing (Figure 1d). Figure 1e is a tilted optical image of the fabricated hybrid solar cell based on the SiNTs. The reflected colors appear as a result of light scattering from the periodic SiNT array, demonstrating a uniform, high-quality fabrication over the entire 1 cm2 imprinted area. Different Si nanostructured hybrid solar cells with Si nanorod arrays (SiNRs-1, aspect ratio: 1, height: 360 nm; SiNRs-2, aspect ratio: 2.4, height: 860 nm) and nanocone arrays (SiNCs, aspect ratio: 1, height: 360 nm) were fabricated with the same diameter and pitch size as the SiNTs using a Cr dot array mask and different RIE recipes depending on the structure. Figure 2 shows the field emission scanning electron microscope (FE-SEM) images of the fabricated Si nanostructures and their corresponding shapes after coating with the PEDOT:PSS solution. The Si nanostructures have a diameter and spacing of 360 nm and 140 nm, respectively. Conformal coverage of the PEDOT:PSS layer (∼400 nm thickness) was demonstrated onto the Si nanostructures having an aspect ratio of 1, as shown in the right inset figures. Notably, the solution fills the inner space of the SiNT, resulting in a conformal coating that enables a large junction area at both the outer and inner faces. However, in the case of SiNRs-2, an intermediate layer of almost transparent PMMA (poly-methyl methacrylate) was coated and etched to a thickness of ∼550 nm prior to the PEDOT:PSS coating, not only for the conformal coating around the high aspect SiNRs-2 but also for matching the PEDOT:PSS thickness with the other samples. But for the intermediate layer, conformal coating with only the PEDOT:PSS film was not achieved via spin-coating and significant sunlight was unavoidably absorbed by the thick PEDOT:PSS layer rather than
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light-harvesting along the long axial nanorod direction (Supporting Information, Figure S2). Figure 3a compares the reflectance spectra of the Si nanostructures coated with the PEDOT:PSS film; ideally, a lower reflectance is required for higher light-capturing and thus better device performances. All periodic Si nanostructures have a lower reflectance in a wavelength range of 400 – 900 nm, when compared to the planar Si because of their better lightharvesting capability. The SiNR array has a curvy reflectance over the wavelengths. However, the SiNC and SiNT arrays exhibit a relatively straight reflectance behavior. The reflectance of the SiNR array is reduced with increasing height from 11 ∼ 23% (SiNR-1) to 6 ∼ 21% (SiNR-2).[19] The anti-reflection of the SiNC array (4 ∼ 8% reflectance) is enhanced when compared to the SiNRs one. The SiNT array significantly suppresses the reflectance to 3 ∼ 6%, showing the most effective light capturing ability. Reflectance spectra from the Si nanostructures without coating the PEDOT:PSS film are shown in Figure S3 for comparison. The photovoltaic performances were measured over an active area of 1 cm2 using a solar simulator with an air mass 1.5G illumination at 100 mW/cm2. Figure 3b shows the photocurrent density (J) – voltage (V) characteristics of the hybrid solar cells with different Si nanostructures, and their photovoltaic properties are listed in Table 1. The hybrid solar cell containing the SiNTs reveals the best device performance among the fabricated devices: a Jsc of 29.9 mA·cm−2, a Voc of 0.51 V, a FF of 65.7% and an overall PCE of 10.03%, which is an enhancement of 77% when compared to the reference cell using the planar Si. It is noted that the photovoltaic properties improve with the Jsc, which is directly related to the extent of light absorption capability of each Si nanostructure because the discrepancies
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absorption, the Jsc of SiNRs-2 cell is higher by only 1.3 mA/cm2 compared to that of the SiNRs-1 one, because the increased possibility of carrier recombination negates the effect of the higher light trapping ability within the SiNR-2 array. The minority carrier lifetime can be determined by measuring the transient photovoltage decay curve. The solar cells were illuminated by a 525 nm green light emitting diode (LED) with an intensity of 15 mW/cm2 and the generated photovoltage was recorded at 4 µs interval using an oscilloscope (Tektronix, DPO4014B) immediately after turning off the LED. Figure 3d shows the transient photovoltage curves normalized with the initial photovoltage of each sample generated immediately after turning off the light. The minority carrier lifetime (τ) of each Si nanotextured solar cell is calculated by the following equation:[21]
τ = (kT /q)| 1/(dVoc /dt )|
Figure 2. FE-SEM images of Si nanostructure arrays before (left) and after (right) PEDOT:PSS coating; SiNRs-1 (a, e); SiNRs-2 (b, f); SiNCs (c, g) and SiNTs (d, h). The insets are cross-sectional images of Si nanostructures embedded in the PEDOT:PSS layer (a large area image for the SiNT arrays is presented in the Supporting Information (Figure S1)). Two organic layers are sectioned by the dotted line in the SiNRs-2 sample. (scale bars: 500 nm)
of Voc and FF are not significant among the samples. External quantum efficiency (EQE) experiments (Figure 3c) also show the same phenomena as the Jsc among the samples, in which the SiNTs cell yields 75% at 650 nm and exhibits the highest spectral response over the entire visible range. It is reported that the carrier recombination rate increases with the junction area in nanotextured solar cells, causing a negative effect on both Jsc and Voc.[7–9,20] In our study, the surface areas are calculated and normalized with respect to the planar Si; 1.5 (SiNC), 2.6 (SiNR-1), 3.7 (SiNT) and 5.1 (SiNR2). The SiNRs-2 cell shows the lowest Voc of 0.47 V because it has the highest surface area. Besides, the intermediate PMMA layer disturbs fast minority carrier collection. The highest Jsc is observed from the SiNTs cell, even though it has a larger junction area than the SiNR-1 and SiNC arrays, which can be explained by its superior light trapping capability that overwhelms the carrier losses by recombination at the increased junction area. Despite the considerable difference in light
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where, k is Boltzmann’s constant, T is the temperature, q is the electron charge, and t is the time. The calculated carrier lifetimes are 11.1 µs (planar Si), 9.4 µs (SiNCs), 8.9 μs (SiNRs-1) 8 μs (SiNTs) and 5.7 µs (SiNRs-2). It is evident that the carrier lifetime decreases with the Si surface area because of the increased chance of charge recombination at the surface boundaries.[22] In the case of SiNTs cell, the lower carrier lifetime can be overwhelmed by the superior light trapping ability, leading to the highest photovoltaic performance among the cells. To verify the light trapping location and intensity variation within these different structures, a finite-difference timedomain (FDTD, Lumerical Solutions, Inc.) mathematical technique was utilized. The cross-sectional electric field intensity distribution, |E|2, was simulated at a wavelength of 600 nm using a model (here, represented by each nanostructure which is embedded within the PEDOT:PSS film) on the condition that an incident light, electromagnetic (EM) wave, propagates downward as plane waves from the dashed line to the bottom of each nanostructure. Figure 4 compares the |E|2 distribution within each nanostructure with color index, depicting the magnitude of |E|2. In the case of SiNR, electric fields are located along the core and at the sidewall surface where, light trapping and carrier generation occurs accordingly. The minority carriers generated at the core can be lost through bulk recombination while moving to the junction interfacing the PEDOT:PSS layer. This phenomenon is more serious in the case of SiNR-2. The generated minority carriers at the bottom region (indicated by arrows in Figure 4b), which is surrounded by the PMMA layer, are prone to recombination while transferring towards the PEDOT:PSS interface, inducing short carrier lifetimes and a large Voc drop. The color of free space above the dashed line signifies the reflected light intensity from the nanostructure. Significant amount of light is reflected as a result of inefficient light absorption and scattering at the top surface of the SiNR. In comparison, strong electric fields are located at the core and tapered boundaries of the SiNC. Interestingly, highly intense electric fields are located around the thin tube walls (∼60 nm) in the case of SiNT; hence, most minority carriers are generated at the thin tube boundaries and can move short distance in the radial direction for the efficient charge collection,
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Figure 3. a) Reflectance spectra from the PEDOT:PSS/Si nanostructures of NR, NC and NT array with reference to the PEDOT:PSS/planar Si in the wavelength range of 400 nm to 900 nm. The reflectance spectra of bare Si nanostructure substrate without PEDOT:PSS layer is demonstrated in Supporting Information Figure S3. b) The photocurrent density (J) – voltage (V) curves of hybrid solar cells with different Si nanostructures. c) The EQE characteristics of the fabricated hybrid solar cells. d) The transient photovoltage decay curves normalized with the respective initial photovoltage, indicating the minority carrier lifetime of each cell.
leading to the highest Jsc among the samples. A broadband light trapping with a negligible light reflection in the free space is simulated at wavelengths of 400 nm, 500 nm and 700 nm as shown in Figure 4e. Strong light trapping is simulated inside the SiNT at short wavelengths and located along the thin tube walls at above 600 nm. The simulation results demonstrate that the SiNT array is the most effective light absorbing morphology in the visible wavelengths. In summary, a SiNT array was fabricated using the UV nanoimprint technique and sophisticated two step lift-off process. The periodically aligned Si nanotube array was first introduced as a light absorber in n-Si/PEDOT:PSS heterojunction hybrid solar cells whose device performance is compared to other Table 1. Comparison of the photovoltaic parameters of hybrid solar cells with different Si nanostructures.
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Jsc (mA·cm−2)
Voc (V)
FF (%)
PCE (%)
Planar Si
15.6
0.55
66.1
5.67
SiNR-1
22.5
0.51
63
7.23
SiNR-2
23.8
0.47
59.3
6.63
SiNC
26.7
0.53
62.5
8.84
SiNT
29.9
0.51
65.7
10.03
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cells based on Si plane, nanorod and nanocone structures. A conformal coating of the PEDOT:PSS film was achieved within the nanotubes with an aspect ratio of 1, enlarging the light absorbing area for high efficiency devices. The SiNTs/ PEDOT:PSS hybrid solar cell exhibited a PCE of 10.03%, which is an enhancement of 77% compared to the reference cell with the planar Si and of 13.4% compared to the cell with the SiNC array owing to both the superior light trapping capability at broadband visible wavelengths and the facile collection of minority carriers.
Experimental Section Nanoimprint Lithography and Two Step Lift-Off Process[23] for Fabricating the Cr Nanoring Etching Mask: Prior to the nanoimprint lithography process, LOL1000 (Shipley) and PVA (poly-vinyl alcohol) solutions were coated sequentially onto the n-Si substrate as under-layers. The spin-coating conditions of these solutions are as follows: LOL1000 (30 s, 3000 rpm, baked at 100 °C for 2 min) and PVA (4 wt% in water, 1 min, 3000 rpm, baked at 80 °C for 30 s). A UV-curable imprint resin composed of a poly(diethylsiloxane) material (Gelest, 87 wt%), a cross-linker (ethylene glycol dimethacrylate, Aldrich, 10 wt%) and a radical initiator (Irgacure 184, Ciba, 3 wt%) were then spin-coated at 6000 rpm for 150 s onto the PVA layer. Immediately after resin coating, a glass stamp with periodic nanopillars was placed in contact with the resin layer at a pressure
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COMMUNICATION Figure 4. Simulated electric field intensity |E|2 distribution in the PEDOT:PSS/Si nanostructures of a) the SiNR-1, b) SiNR-2, c) SiNC and d) SiNT at a wavelength of 600 nm using the numerical finite-difference time-domain (FDTD) method (Version 8.5.0). e) The |E|2 distribution of SiNT at wavelengths of 400, 500 and 700 nm.
of 5 bar and then irradiated with UV light for 10 min. After detaching the stamp from the cured resin, dry etching was performed to remove the residual layer under the trenches using a CF4 plasma (50 sccm, 20 mTorr, 20 W, 30 s) and followed by O2 plasma treatment (50 sccm, 20 mTorr, 20 W, 300 s) to transfer the imprinted patterns to the underlying LOL1000 and PVA layers until the Si surface was exposed, leaving undercuts in the resist profile (Figure S1(a)-iv). Next, Al was deposited by an electron beam evaporator onto the imprinted sample, and then the sample was immersed in water for a few seconds to complete the first lift-off (removal of the PVA layer), generating ringshaped trenches (Figure S1(a)-vi). Cr was then deposited and the sample was immersed in an 1165 remover (Shipley) and an Al etchant successively to remove the surrounding LOL100 layer and the Al cores (second lift-off process), respectively, leaving behind the Cr nanoring array. Fabrication schemes and relevant SEM images are illustrated for easy understanding in the Supporting Information (Figure S1). Dry Etching of Si Nanostructures: All Si nanostructures were achieved using an inductively coupled plasma reactive ion etching (ICP-RIE) (RIE: 100 W, ICP: 300 W, 30 mTorr). For vertical shaped nanostructures, such as SiNR and SiNT, plasma etching was performed using SF6 (10 sccm) and C4F8 (40 sccm) gas. In the case of SiNC, only CF4 gas (50 sccm) was used for isotropic etching. After etching process, the Si substrate containing the newly formed nanostructure was immersed in an HFE7100 solution (3M Novec) to remove the fluoropolymer byproducts formed at the bottom during the dry etching process. N++ Doping at the Backside of Si Substrate: The n-type spinon-dopant (Accuspin phosphorus, Honeywell) was coated on the backside of n-Si substrate with a doping concentration of 2 × 1015 cm−3 and then a diffusion process was performed at 950 °C for 15 min. Next, the n-Si substrate was immersed in a diluted HF solution to remove the phosphosilicate glass formed during the diffusion step. The sheet resistance of the backside Si was reduced from 225 Ω/sq to 3 Ω/sq. PEDOT:PSS Solution Coating: The PEDOT:PSS (PH500, Clevios) solution was mixed with dimethyl sulfoxide (v/v = 1:1) and then thermally treated in an oven at 110 °C to remove water (the solvent of PH500). The solution became viscous, and the total volume was reduced by 20%. Subsequently, this solution was spin-coated onto the Si substrate containing the nanostructures at 3000 rpm for 1 min and then baked at 110 °C for 30 min.
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Acknowledgements This work was supported by the Basic Science Research program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF, No. R15-2008-00603002-0, CLEA, NCRC), the Pioneer Research Center Program (NRF, No. 2013M3C1A3063046) and a grant funded through the MEST (2011-0029414). Received: October 30, 2013 Revised: January 8, 2014 Published online: February 18, 2014
[1] L. He, C. Jiang, H. Wang, H. Lei, D. Lai, Rusli, in Proc. 38th IEEE Photovoltaic Spec. Conf., 2012, 002785. [2] L. He, C. Jiang Rusli, D. Lai, H. Wang, Appl. Phys. Lett. 2011, 99, 021104. [3] L. He, C. Jiang, H. Wang, D. Lai, Y. H. Tan, C. S. Tan, Rusli, Appl. Phys. Lett. 2012, 100, 103104. [4] S. C. Shiu, J. J. Chao, S. C. Hung, C. L. Yeh, C. F. Lin, Chem. Mat. 2010, 22, 3108. [5] W. Lu, C. Wang, W. Yue, L. Chen, Nanoscale 2011, 3, 3631. [6] H. J. Syu, S. C. Shiu, C. F. Lin, Sol. Energy Mater. Sol. Cells 2012, 98, 267. [7] F. Zhang, T. Song, B. Sun, Nanotechnology 2012, 23, 194006. [8] L. He, Rusli, C. Jiang, H. Wang, D. Lai, IEEE Electron Device Lett. 2011, 32, 1406. [9] L. He, D. Lai, H. Wang, C. Jiang, Rusli, Small 2012, 8, 1664. [10] J. Zhu, Z. Yu, G. F. Burkhard, C. M. Hsu, S. T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, Y. Cui, Nano Lett. 2009, 9, 279. [11] S. Jeong, E. C. Garnett, S. Wang, Z. Yu, S. Fan, M. L. Brongersma, M. D. McGehee, Y. Cui, Nano Lett. 2012, 12, 2971. [12] K. Q. Peng, X. Wang, L. Li, X. L. Wu, S. T. Lee, J. Am. Chem. Soc. 2010, 132, 6872. [13] S. E. Han, A. Mavrokefalos, M. S. Branham, G. Chen, Proc. of SPIE 2011, 8031, 80310T-1. [14] Y. Li, H. Yu, J. Li, S. M. Wong, X. W. Sun, X. Li, C. Cheng, H. J. Fan, J. Wang, N. Singh, P. G. Lo, D. L. Kwong, Small 2011, 22, 3138.
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[15] S. Y. Jeong, J. Y. Kim, H. D. Yang, B. N. Yoon, S. H. Choi, H. K. Kang, C. W. Yang, Y. H. Lee, Adv. Mater. 2003, 15, 1172. [16] T. Song, J. Xia, J. H. Lee, D. H. Lee, M. S. Kwon, J. M. Choi, J. Wu, S. K. Doo, H. Chang, W. I. Park, D. S. Zang, H. Kim, Y. Huang, K. C. Hwang, J. A. Rogers, U. Paik, Nano Lett. 2010, 10, 1710. [17] M. H. Park, M. G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui, J. Cho, Nano Lett. 2009, 9, 3844. [18] H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Nat. Nanotechnol. 2012, 7, 310.
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[19] L. Hu, G. Chen, Nano Lett. 2007, 7, 3249. [20] J. Oh, H. C. Yuan, H. M. Branz, Nat. Nanotechnol. 2012, 7, 743. [21] J. E. Mahan, T. W. Ekstedt, R. I. Frank, R. Kaplow, IEEE Trans. Electron Devices 1979, 5, 733. [22] J. E. Allen, E. R. Hemesath, D. E. Perea, J.L. Lensch-Falk, Z. Y. Li, F. Yin, M. H. Gass, P. Wang, A. L. Bleloch, R. E. Palmer, L. J. Lauhon, Nat. Nanotechnol. 2008, 3, 168. [23] S. S. Song, E. U Kim, H. S. Jung, K. S. Kim, G. Y. Jung, J. Micromech. Microeng, 2009, 19, 105022.
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Huisu Jeong, Hui Song, Yusin Pak, Il Keun Kwon, Kyubong Jo, Heon Lee,* and Gun Young Jung* In recent years, hybrid solar cells composed of n-type silicon (n-Si)/poly(3,4-ethylene dioxythiophene):poly-styrenesulfonate (PEDOT:PSS) heterojunction[1–8] have gained increased attention because of their easy fabrication and low cost compared to the conventional p-n Si solar cells, which require high temperature (∼1000 °C) processing for ion implantation and dopant diffusion. When the n-Si absorbs solar light, electron-hole pairs are generated and separated at the Schottky barrier of the n-Si/ PEDOT:PSS heterojunction. The photogenerated electrons are transported to the cathode as they pass through the n-Si, while the minority carrier of holes moves to the anode through the PEDOT:PSS layer. For high power conversion efficiency (PCE) in hybrid solar cells with a nanotextured n-Si layer, such as nanorod[2] or nanowire[3–8] arrays, have been studied because of their higher broadband light absorption and better carrier collection ability owing to short carrier transport path along the radial direction. Therefore, more carriers can be generated and collected, resulting in a higher short circuit current (Jsc). Furthermore, nanorods with high aspect ratio of approximately 10 have been examined for increased light absorption. However, such Si nanorods are prone to have cavities between the rods that are not filled with the PEDOT:PSS solution and likely to aggregate at the top of the nanorods, leading to a low fill factor (FF) and a higher chance of carrier recombination.[8,9] To overcome this issue, Si nanocone arrays have been introduced, which can complement the non-conformal coating via spin-coating H. Jeong, H. Song, Y. Pak, Prof. G. Y. Jung School of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju 500-712, Republic of Korea E-mail: [email protected] Prof. I. K. Kwon Department of Maxillofacial Biomedical Engineering & Institute of Oral Biology School of Dentistry Kyung Hee University Seoul 130-701, Republic of Korea Prof. K. Jo Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology Sogang University Seoul 121-741, Republic of Korea Prof. H. Lee Department of Materials Science and Engineering Korea University Seoul 136-701, Republic of Korea E-mail: [email protected]
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Enhanced Light Absorption of Silicon Nanotube Arrays for Organic/Inorganic Hybrid Solar Cells
because of their tapered sides. The Si nanocone array with an aspect ratio of less than 2 demonstrates light absorption of over 90%.[10] Cui et. al. reported a Jsc of 29.6 mA/cm2 and the highest value of power conversion efficiency (PCE) of 11.1% from a hybrid solar cell composed of Si nanocone array/ PEDOT:PSS.[11] Besides Si nanowires, rods and cones, there have been studies with nanoholes,[12] pyramids,[13] hemispheres[14] for a better light absorption in solar cells. Si tubular structures are achieved by Si thin layer deposition over various templates and followed by removal of the template.[15–18] But, these structures are non-aligned, close-ended and aggregated, which have mostly been incorporated into the anodes in lithium-ion batteries.[16–18] In this work, more advanced structure of periodically aligned vertical Si nanotube (SiNT) arrays with an aspect ratio of 1 are produced using UV nanoimprint lithography and newly applied as a light absorber in the n-Si/PEDOT:PSS heterojunction hybrid solar cell. This structure has two potential advantages over its Si nanorods counterpart: 1) SiNT array has more light absorbing area from both inner and outer shells without an increase in aspect ratio, and 2) the thin tube wall (∼60 nm) shortens the carrier collection distance in the radial direction, thus more facile charge transport can be anticipated, leading to a higher PCE. A series of hybrid solar cells with various Si structures of planes, nanorods and cones were also fabricated for comparison. Figure 1 shows schematic images of the process to fabricate the SiNTs-based hybrid solar cell. The SiNTs were produced using an etching process of the n-Si substrate (specific resistivity: ∼4 Ω cm) with a periodic chromium (Cr) nanorings mask, which was prepared by two step lift-off process after UV nanoimprint lithography (Supporting Information for detailed procedure, Figure S1). A 50 nm thick Cr nanorings mask was produced on the n-Si substrate with an outer diameter of 360 nm and an inner diameter of 240 nm (Figure 1a). Inductively coupled reactive ion etching (ICP-RIE) was performed using an SF6 and C4F8 plasma gas to etch the Si substrate until the SiNTs had an aspect ratio of 1 (height: 360 nm) and the Cr nanorings mask was eliminated using a Cr etchant. After cleaning with deionized water and drying, heavy n-type doping was completed on the backside of the n-Si substrate to create a built-in field within the substrate to reduce carrier recombination and sheet resistance as well.[11] A 150-nm thick aluminum (Al) film, which functioned as the cathode, was successively deposited onto the n++-Si layer using an electron beam evaporator (Figure 1b) and then the PEDOT:PSS solution was spin-coated onto the SiNT array at a thickness of approximately 400 nm (Figure 1c). Finally, a finger grid type silver anode (100 nm thickness) was
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Figure 1. Schematic of the Si nanotubes fabrication process: a) forming a Cr nanorings mask on the n-Si substrate using UV nanoimprint lithography and two step lift-off process, b) ring pattern transfer by dry etching with the Cr nanorings mask and deposition of Al at the substrate backside as a bottom electrode, c) spin-coating of the PEDOT:PSS solution onto the SiNT array and d) deposition of finger grid-type silver top electrode. e) A tilted optical image of the SiNTs-based hybrid solar cell with finger grid top electrodes.
deposited on the PEDOT:PSS layer using a shadow mask with 0.1 mm wide fingers at 1 mm spacing (Figure 1d). Figure 1e is a tilted optical image of the fabricated hybrid solar cell based on the SiNTs. The reflected colors appear as a result of light scattering from the periodic SiNT array, demonstrating a uniform, high-quality fabrication over the entire 1 cm2 imprinted area. Different Si nanostructured hybrid solar cells with Si nanorod arrays (SiNRs-1, aspect ratio: 1, height: 360 nm; SiNRs-2, aspect ratio: 2.4, height: 860 nm) and nanocone arrays (SiNCs, aspect ratio: 1, height: 360 nm) were fabricated with the same diameter and pitch size as the SiNTs using a Cr dot array mask and different RIE recipes depending on the structure. Figure 2 shows the field emission scanning electron microscope (FE-SEM) images of the fabricated Si nanostructures and their corresponding shapes after coating with the PEDOT:PSS solution. The Si nanostructures have a diameter and spacing of 360 nm and 140 nm, respectively. Conformal coverage of the PEDOT:PSS layer (∼400 nm thickness) was demonstrated onto the Si nanostructures having an aspect ratio of 1, as shown in the right inset figures. Notably, the solution fills the inner space of the SiNT, resulting in a conformal coating that enables a large junction area at both the outer and inner faces. However, in the case of SiNRs-2, an intermediate layer of almost transparent PMMA (poly-methyl methacrylate) was coated and etched to a thickness of ∼550 nm prior to the PEDOT:PSS coating, not only for the conformal coating around the high aspect SiNRs-2 but also for matching the PEDOT:PSS thickness with the other samples. But for the intermediate layer, conformal coating with only the PEDOT:PSS film was not achieved via spin-coating and significant sunlight was unavoidably absorbed by the thick PEDOT:PSS layer rather than
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light-harvesting along the long axial nanorod direction (Supporting Information, Figure S2). Figure 3a compares the reflectance spectra of the Si nanostructures coated with the PEDOT:PSS film; ideally, a lower reflectance is required for higher light-capturing and thus better device performances. All periodic Si nanostructures have a lower reflectance in a wavelength range of 400 – 900 nm, when compared to the planar Si because of their better lightharvesting capability. The SiNR array has a curvy reflectance over the wavelengths. However, the SiNC and SiNT arrays exhibit a relatively straight reflectance behavior. The reflectance of the SiNR array is reduced with increasing height from 11 ∼ 23% (SiNR-1) to 6 ∼ 21% (SiNR-2).[19] The anti-reflection of the SiNC array (4 ∼ 8% reflectance) is enhanced when compared to the SiNRs one. The SiNT array significantly suppresses the reflectance to 3 ∼ 6%, showing the most effective light capturing ability. Reflectance spectra from the Si nanostructures without coating the PEDOT:PSS film are shown in Figure S3 for comparison. The photovoltaic performances were measured over an active area of 1 cm2 using a solar simulator with an air mass 1.5G illumination at 100 mW/cm2. Figure 3b shows the photocurrent density (J) – voltage (V) characteristics of the hybrid solar cells with different Si nanostructures, and their photovoltaic properties are listed in Table 1. The hybrid solar cell containing the SiNTs reveals the best device performance among the fabricated devices: a Jsc of 29.9 mA·cm−2, a Voc of 0.51 V, a FF of 65.7% and an overall PCE of 10.03%, which is an enhancement of 77% when compared to the reference cell using the planar Si. It is noted that the photovoltaic properties improve with the Jsc, which is directly related to the extent of light absorption capability of each Si nanostructure because the discrepancies
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absorption, the Jsc of SiNRs-2 cell is higher by only 1.3 mA/cm2 compared to that of the SiNRs-1 one, because the increased possibility of carrier recombination negates the effect of the higher light trapping ability within the SiNR-2 array. The minority carrier lifetime can be determined by measuring the transient photovoltage decay curve. The solar cells were illuminated by a 525 nm green light emitting diode (LED) with an intensity of 15 mW/cm2 and the generated photovoltage was recorded at 4 µs interval using an oscilloscope (Tektronix, DPO4014B) immediately after turning off the LED. Figure 3d shows the transient photovoltage curves normalized with the initial photovoltage of each sample generated immediately after turning off the light. The minority carrier lifetime (τ) of each Si nanotextured solar cell is calculated by the following equation:[21]
τ = (kT /q)| 1/(dVoc /dt )|
Figure 2. FE-SEM images of Si nanostructure arrays before (left) and after (right) PEDOT:PSS coating; SiNRs-1 (a, e); SiNRs-2 (b, f); SiNCs (c, g) and SiNTs (d, h). The insets are cross-sectional images of Si nanostructures embedded in the PEDOT:PSS layer (a large area image for the SiNT arrays is presented in the Supporting Information (Figure S1)). Two organic layers are sectioned by the dotted line in the SiNRs-2 sample. (scale bars: 500 nm)
of Voc and FF are not significant among the samples. External quantum efficiency (EQE) experiments (Figure 3c) also show the same phenomena as the Jsc among the samples, in which the SiNTs cell yields 75% at 650 nm and exhibits the highest spectral response over the entire visible range. It is reported that the carrier recombination rate increases with the junction area in nanotextured solar cells, causing a negative effect on both Jsc and Voc.[7–9,20] In our study, the surface areas are calculated and normalized with respect to the planar Si; 1.5 (SiNC), 2.6 (SiNR-1), 3.7 (SiNT) and 5.1 (SiNR2). The SiNRs-2 cell shows the lowest Voc of 0.47 V because it has the highest surface area. Besides, the intermediate PMMA layer disturbs fast minority carrier collection. The highest Jsc is observed from the SiNTs cell, even though it has a larger junction area than the SiNR-1 and SiNC arrays, which can be explained by its superior light trapping capability that overwhelms the carrier losses by recombination at the increased junction area. Despite the considerable difference in light
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where, k is Boltzmann’s constant, T is the temperature, q is the electron charge, and t is the time. The calculated carrier lifetimes are 11.1 µs (planar Si), 9.4 µs (SiNCs), 8.9 μs (SiNRs-1) 8 μs (SiNTs) and 5.7 µs (SiNRs-2). It is evident that the carrier lifetime decreases with the Si surface area because of the increased chance of charge recombination at the surface boundaries.[22] In the case of SiNTs cell, the lower carrier lifetime can be overwhelmed by the superior light trapping ability, leading to the highest photovoltaic performance among the cells. To verify the light trapping location and intensity variation within these different structures, a finite-difference timedomain (FDTD, Lumerical Solutions, Inc.) mathematical technique was utilized. The cross-sectional electric field intensity distribution, |E|2, was simulated at a wavelength of 600 nm using a model (here, represented by each nanostructure which is embedded within the PEDOT:PSS film) on the condition that an incident light, electromagnetic (EM) wave, propagates downward as plane waves from the dashed line to the bottom of each nanostructure. Figure 4 compares the |E|2 distribution within each nanostructure with color index, depicting the magnitude of |E|2. In the case of SiNR, electric fields are located along the core and at the sidewall surface where, light trapping and carrier generation occurs accordingly. The minority carriers generated at the core can be lost through bulk recombination while moving to the junction interfacing the PEDOT:PSS layer. This phenomenon is more serious in the case of SiNR-2. The generated minority carriers at the bottom region (indicated by arrows in Figure 4b), which is surrounded by the PMMA layer, are prone to recombination while transferring towards the PEDOT:PSS interface, inducing short carrier lifetimes and a large Voc drop. The color of free space above the dashed line signifies the reflected light intensity from the nanostructure. Significant amount of light is reflected as a result of inefficient light absorption and scattering at the top surface of the SiNR. In comparison, strong electric fields are located at the core and tapered boundaries of the SiNC. Interestingly, highly intense electric fields are located around the thin tube walls (∼60 nm) in the case of SiNT; hence, most minority carriers are generated at the thin tube boundaries and can move short distance in the radial direction for the efficient charge collection,
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Figure 3. a) Reflectance spectra from the PEDOT:PSS/Si nanostructures of NR, NC and NT array with reference to the PEDOT:PSS/planar Si in the wavelength range of 400 nm to 900 nm. The reflectance spectra of bare Si nanostructure substrate without PEDOT:PSS layer is demonstrated in Supporting Information Figure S3. b) The photocurrent density (J) – voltage (V) curves of hybrid solar cells with different Si nanostructures. c) The EQE characteristics of the fabricated hybrid solar cells. d) The transient photovoltage decay curves normalized with the respective initial photovoltage, indicating the minority carrier lifetime of each cell.
leading to the highest Jsc among the samples. A broadband light trapping with a negligible light reflection in the free space is simulated at wavelengths of 400 nm, 500 nm and 700 nm as shown in Figure 4e. Strong light trapping is simulated inside the SiNT at short wavelengths and located along the thin tube walls at above 600 nm. The simulation results demonstrate that the SiNT array is the most effective light absorbing morphology in the visible wavelengths. In summary, a SiNT array was fabricated using the UV nanoimprint technique and sophisticated two step lift-off process. The periodically aligned Si nanotube array was first introduced as a light absorber in n-Si/PEDOT:PSS heterojunction hybrid solar cells whose device performance is compared to other Table 1. Comparison of the photovoltaic parameters of hybrid solar cells with different Si nanostructures.
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Jsc (mA·cm−2)
Voc (V)
FF (%)
PCE (%)
Planar Si
15.6
0.55
66.1
5.67
SiNR-1
22.5
0.51
63
7.23
SiNR-2
23.8
0.47
59.3
6.63
SiNC
26.7
0.53
62.5
8.84
SiNT
29.9
0.51
65.7
10.03
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cells based on Si plane, nanorod and nanocone structures. A conformal coating of the PEDOT:PSS film was achieved within the nanotubes with an aspect ratio of 1, enlarging the light absorbing area for high efficiency devices. The SiNTs/ PEDOT:PSS hybrid solar cell exhibited a PCE of 10.03%, which is an enhancement of 77% compared to the reference cell with the planar Si and of 13.4% compared to the cell with the SiNC array owing to both the superior light trapping capability at broadband visible wavelengths and the facile collection of minority carriers.
Experimental Section Nanoimprint Lithography and Two Step Lift-Off Process[23] for Fabricating the Cr Nanoring Etching Mask: Prior to the nanoimprint lithography process, LOL1000 (Shipley) and PVA (poly-vinyl alcohol) solutions were coated sequentially onto the n-Si substrate as under-layers. The spin-coating conditions of these solutions are as follows: LOL1000 (30 s, 3000 rpm, baked at 100 °C for 2 min) and PVA (4 wt% in water, 1 min, 3000 rpm, baked at 80 °C for 30 s). A UV-curable imprint resin composed of a poly(diethylsiloxane) material (Gelest, 87 wt%), a cross-linker (ethylene glycol dimethacrylate, Aldrich, 10 wt%) and a radical initiator (Irgacure 184, Ciba, 3 wt%) were then spin-coated at 6000 rpm for 150 s onto the PVA layer. Immediately after resin coating, a glass stamp with periodic nanopillars was placed in contact with the resin layer at a pressure
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COMMUNICATION Figure 4. Simulated electric field intensity |E|2 distribution in the PEDOT:PSS/Si nanostructures of a) the SiNR-1, b) SiNR-2, c) SiNC and d) SiNT at a wavelength of 600 nm using the numerical finite-difference time-domain (FDTD) method (Version 8.5.0). e) The |E|2 distribution of SiNT at wavelengths of 400, 500 and 700 nm.
of 5 bar and then irradiated with UV light for 10 min. After detaching the stamp from the cured resin, dry etching was performed to remove the residual layer under the trenches using a CF4 plasma (50 sccm, 20 mTorr, 20 W, 30 s) and followed by O2 plasma treatment (50 sccm, 20 mTorr, 20 W, 300 s) to transfer the imprinted patterns to the underlying LOL1000 and PVA layers until the Si surface was exposed, leaving undercuts in the resist profile (Figure S1(a)-iv). Next, Al was deposited by an electron beam evaporator onto the imprinted sample, and then the sample was immersed in water for a few seconds to complete the first lift-off (removal of the PVA layer), generating ringshaped trenches (Figure S1(a)-vi). Cr was then deposited and the sample was immersed in an 1165 remover (Shipley) and an Al etchant successively to remove the surrounding LOL100 layer and the Al cores (second lift-off process), respectively, leaving behind the Cr nanoring array. Fabrication schemes and relevant SEM images are illustrated for easy understanding in the Supporting Information (Figure S1). Dry Etching of Si Nanostructures: All Si nanostructures were achieved using an inductively coupled plasma reactive ion etching (ICP-RIE) (RIE: 100 W, ICP: 300 W, 30 mTorr). For vertical shaped nanostructures, such as SiNR and SiNT, plasma etching was performed using SF6 (10 sccm) and C4F8 (40 sccm) gas. In the case of SiNC, only CF4 gas (50 sccm) was used for isotropic etching. After etching process, the Si substrate containing the newly formed nanostructure was immersed in an HFE7100 solution (3M Novec) to remove the fluoropolymer byproducts formed at the bottom during the dry etching process. N++ Doping at the Backside of Si Substrate: The n-type spinon-dopant (Accuspin phosphorus, Honeywell) was coated on the backside of n-Si substrate with a doping concentration of 2 × 1015 cm−3 and then a diffusion process was performed at 950 °C for 15 min. Next, the n-Si substrate was immersed in a diluted HF solution to remove the phosphosilicate glass formed during the diffusion step. The sheet resistance of the backside Si was reduced from 225 Ω/sq to 3 Ω/sq. PEDOT:PSS Solution Coating: The PEDOT:PSS (PH500, Clevios) solution was mixed with dimethyl sulfoxide (v/v = 1:1) and then thermally treated in an oven at 110 °C to remove water (the solvent of PH500). The solution became viscous, and the total volume was reduced by 20%. Subsequently, this solution was spin-coated onto the Si substrate containing the nanostructures at 3000 rpm for 1 min and then baked at 110 °C for 30 min.
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Acknowledgements This work was supported by the Basic Science Research program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF, No. R15-2008-00603002-0, CLEA, NCRC), the Pioneer Research Center Program (NRF, No. 2013M3C1A3063046) and a grant funded through the MEST (2011-0029414). Received: October 30, 2013 Revised: January 8, 2014 Published online: February 18, 2014
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