High Efficiency Thin Upgraded Metallurgical-Grade Silicon Solar Cells

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Letter pubs.acs.org/NanoLett

High Efficiency Thin Upgraded Metallurgical-Grade Silicon Solar Cells on Flexible Substrates Jae Young Kwon,†,# Duck Hyun Lee,†,# Michelle Chitambar,‡ Stephen Maldonado,‡ Anish Tuteja,*,† and Akram Boukai*,† †

Department of Materials Science and Engineering and ‡Department of Chemistry, University of Michigan, Ann Arbor, MI, United States S Supporting Information *

ABSTRACT: We present a thin film ( 8%. In addition, the solar cells are flexible and semitransparent so as to reduce balance of systems costs and open new applications for conformable solar cell arrays on a variety of surfaces. Detailed studies on the optical and electrical properties of the resulting solar cells suggest that both antireflective and light-trapping mechanisms are key to the enhanced efficiency. KEYWORDS: Upgraded metallurgical-grade silicon, flexible solar cell, surface plasmons, nanopillars, block copolymer nanolithography

T

following three mechanisms: (1) scattering incident light at oblique angles thereby increasing the optical path-length,15 (2) substrate-coupled Mie resonances,16 and (3) impedance matching caused by a tapered refractive index.17 In this work, we have combined the absorption enhancement effects of both the nanoparticle and nanopillar arrays, by creating a periodic and uniform metal nanoparticle array on top of a hexagonally close-packed array of silicon nanopillars.23 Various nanopatterning methods such as electron beam lithography24 and nanoimprint lithography25 have been previously employed to create periodic and uniform plasmonic metal nanoparticles. However, the low throughput of e-beam lithography due to serial processing, as well as, the difficulty in reproducing sub-50 nm features have severely limited their applicability.24−26 An alternative emerging approach to prepare a nanopatterned periodic and uniform array of metal particles is through the use of block copolymer lithography.21,27−30 Here, we present low-cost UMG-Si based solar cells that are composed of parallel connected, ultrathin, 17 μm, microcell arrays on flexible substrates. The microcell arrays incorporate Ag nanoparticles and Si nanopillar arrays, which serve to enhance light absorption and increase cell efficiency. Figure 1a schematically illustrates the completed cell prepared from ultrathin, polycrystalline UMG-Si solar microcells. (see Supporting Information, Figure S1). Most of the process steps are adapted from the monocrystilline Si microcell process developed by the Roger’s research group.9−11,31,32 In their technique, the preparation of ultrathin silicon bars using wet

he solar cell market has experienced tremendous growth in past several years, and silicon accounts for more than ∼90% of the market due to advantages of earth abundance, good reliability, performance, and a wealth of silicon materials processing knowledge.1−3 However, the high cost (>$1/W) of solar-grade Si solar cells has hindered their immediate adoption in the commercial market.4−6 Currently, two major approaches have been pursued to reduce the cost of Si-based solar cells per watt: the adoption of low-cost Si such as metallurgical-grade (MG)4,6 or upgraded MG (UMG) Si7,8 and reducing the usage of Si material by preparing ultrathin solar modules.9−11 UMGSi is generally prepared by the chemical refining of raw Si,12 and it costs ∼$10 per kg, which makes it almost 5−10 times less expensive than solar-grade Si.13 The main disadvantage of UMG-Si is the high impurity level of the starting material leading to decreased minority-carrier lifetimes and device efficiency.5,6 Solar cell modules composed of ultrathin microcells allow for increased efficiency with shorter collection pathways to the p-n junction and also decrease the usage of Si. However, the efficiency of ultrathin microcells decreases steeply due to inadequate optical absorption.9,10 Nanoparticle and nanopillar arrays14−18 are two structures that have been utilized for enhancing optical absorption in Si solar cells. Nanoparticles arrays on top of a semiconducting surface can enhance optical absorption through localized surface plasmon resonances (LSPR). LSPR10,19−22 is the collective oscillation of conduction electrons stimulated by incident light at the interface between a metal (Ag, Au, Pt) and a dielectric, which enables the light to be concentrated and absorbed into the Si layer.20 Both the silver nanoparticles and the nanopillar arrays formed on the Si surface can also significantly increase light absorption by one or more of the © XXXX American Chemical Society

Received: May 30, 2012 Revised: August 27, 2012

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Table 1. Comparison between the Physical Properties of Solar Grade (SG) Si and Upgraded Metallurgical-Grade (UMG) Sia materials SG-Si* UMG-Si

type p p

ρ (Ω-cm)

ne (cm−3)

μ (cm2/V·s)

1 0.868

1.46 × 10 1.42 × 1017

428.1 50.8

16

Here ne is the carrier concentration, and μ is the mobility. The SG-Si data was taken from previous work.40,41

a

shows the current density−voltage (J−V) measurements of 180 μm thick bulk UMG-Si cell and the completed microcell module under forward bias in dark at room temperature. Note that the current density of the microcell module was calculated using the active area of microcells rather than the total surface area. The diode ideality factor (n) for our 180 μm thick bulk UMG-Si cell is 2.9, while that for the microcell is 2.1. This low value indicates improved performance and compares favorably to monocrystalline Si microcell cells (n = 1.85).9 The J−V measurements under simulated AM 1.5 illumination conditions and 100 mW/cm2 (1 sun) also clearly show that this shorter current pathway increases the short circuit current (JSC) and efficiency (η) for our microcell (see Supporting Information Figure S2d). Although the small thickness for the microcell improves the current collection of UMG-Si cells, it has the disadvantage of relatively low absorptance especially at longer wavelengths (see Supporting Information Figure S3). Both nanoparticle and nanopillars arrays are effective strategies to increase the absorption of light for an indirect bandgap material such as silicon.17,18,22,33 Figure 2a schematically illustrates the process steps for the preparation of plasmonic Ag nanoparticles and Si nanopillar arrays through block copolymer lithography and reactive ion etching (RIE). Block copolymers are self-assembling polymeric materials that provide a variety of periodic nanoscale morphologies having feature sizes ranging from 5 to 50 nm. Unlike other nanopatterning techniques such as focused E-beam lithography or scanning probe lithography, block copolymer assembly provides a high throughput patterning process, thereby enabling ease of scalability.21,27−30,34−36 Here, for the block copolymer lithography process, we chose polystyrene-blockpoly(methyl methacylate) (PS-b-PMMA) block copolymers that yield perpendicular self-assembled domains of cylinder forming PMMA.29,30,37 Figure 2b shows the nanoporous PS template on the UMGSi surface after selective removal of the PMMA. Masked deposition of Ag with a thickness (t) of 30 nm over the PS template and the subsequent lift-off process enabled the formation of a periodic and uniform Ag nanoparticle array (Figure 2c), whose diameter (D) and pitch were determined by the block copolymer template. We utilized these Ag nanoparticles as an etching mask to etch into the UMG-Si by RIE. The Si nanopillar arrays were subsequently formed on the microcell. This results in an array of Ag nanoparticles sitting on top of the Si nanopillar arrays (see Figure 2d). The flexibility and semitransparency of the microcell module were maintained after the formation of Si nanopillar arrays (see Supporting Information, Figure S2f). Note that the side-walls of the prepared Si nanopillar arrays were slightly tapered due to the RIE etching process (Figure 2d). Figure 2e shows the J−V measurement curves under 100 mW/cm2, AM 1.5, for three different cells: (1) a bare microcell with no Ag nanoparticles, (2) Ag nanoparticles on a microcell,

Figure 1. (a) A Schematic illustration of the prepared microcell module. (b,c) Tilted SEM images of suspended microcell arrays prepared with isotropic XeF2 etching and supported by two narrow anchors. (d) SEM image of the microcell arrays retrieved from the raw substrate and transferred on a polymer substrate (NOA61). (e) An optical image of a completed microcell module consisting of ultrathin microcells interconnected by metal (Cr/Au, 30/400 nm) lines. (f) Representative J−V curves for the 180 μm thick bulk upgraded metallurgical-grade Si cell and the prepared microcell module under dark conditions.

chemical etching (KOH) is highly dependent on the orientation of silicon, and as a result the starting material for this process has always been constrained to (111)-oriented Si wafer. However, our UMG-Si (obtained from Calisolar, 99.999% purity) is polycrystalline, so we utilized an isotropic XeF2 dry etching method (see Supporting Information, Figure S2b,c), which is independent of crystal orientation, for the undercut etching of our polycrystalline UMG-Si. Figure 1b,c shows tilted scanning electron microscopy (SEM) images of suspended microcell arrays prepared with isotropic XeF2 etching and supported by two narrow anchors. The suspended UMG-Si microcells were retrieved and transferred to a photocurable polymer, NOA61, (Figure 1d) and interconnected by metal (Cr/Au, 30/400 nm) electrodes. Figure 1e shows that the optical image of a completed microcell module, which has excellent flexibility, is semitransparent and demonstrates robust device performance (see Supporting Information, Figure S2e). In the case of Solar grade-Si, the diffusion length of the minority-carriers ranges from hundreds of micrometers to several millimeters due to low trap density. UMG-Si, however, has larger trap densities and polycrystalline interfaces that decrease minority-carrier diffusion lengths and result in diminished photocurrent6 (Table 1). Therefore, an ultrathin microcell, which has a shorter current collection pathway to the junction, may allow for a high efficiency photovoltaic module as long as a large optical absorption can be achieved. Figure 1f B

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Figure 3. Optical images (a), absorptance spectra (b), quantum efficiency curves (c), and J−V curves (d) for the as-doped upgraded metallurgical-grade (UMG) Si (black), Ag nanoparticles (Ag nanoparticles, D, 18 nm; t, 30 nm) on UMG-Si (magenta), Ag nanoparticles/Si nanopillar arrays prepared with 18 nm Ag nanoparticles on UMG-Si (blue), Ag nanoparticles (D, 34 nm; t, 30 nm) on UMG-Si (green), and Ag nanoparticles/Si nanopillar arrays prepared with 34 nm Ag nanoparticles on 180 μm thick UMG-Si (red). (e) Comparison of JSC acquired from quantum efficiency measurement and J−V measurement.

Figure 2. (a) Schematic illustrations for the preparation of periodic and uniform Ag nanoparticles and Ag nanoparticle/Si nanopillar arrays. (b) SEM images of the as-prepared PS template on a polymer substrate (NOA61) and Si. The block copolymer (PS-b-PMMA) thin film was selectively self-assembled on the Si surface. (c) An SEM image of Ag nanoparticles (D, 34 nm; t, 30 nm) prepared by block copolymer lithography on top of a Si surface. (d) A tilted SEM image of Ag nanoparticle/Si nanopillar array prepared by reactive ion etching. The prepared Ag nanoparticles (D, 34 nm; t, 30 nm) were used as a mask for Si etching. (e) Representative J−V curves for the bare microcell module (green), Ag nanoparticle on microcell module (blue), and Ag nanoparticle/Si nanopillar array on microcell module (red). The microcells were 17 μm thick.

larger, the peak absorptance difference undergoes a “blue shift”21,38 (see Supporting Information, Figure S4c), and the absorptance enhancement became more pronounced. This trend is consistent with theoretical predictions for plasmonicenhanced absorption of Ag nanoparticles.39 In contrast to the LSPR effect, Si nanopillar arrays increase the absorptance across the entire wavelength spectrum. The Ag nanoparticles/Si nanopillar arrays prepared with 34 nm diameter Ag nanoparticles on a UMG-Si cell showed excellent absorptance of greater than 90% between wavelengths of 300−950 nm. Quantum efficiency measurements in the visible and near IR were also performed on the UMG-Si microcell devices. Figure 3c,d shows the quantum efficeincy curves and J−V curves for the five cells. At a wavelength λ ∼ 700 nm, the bare doped UMG-Si cell exhibits ∼42% quantum efficiency, whereas the quantum efficiency for the Ag nanoparticle/Si nanopillar array cell prepared with 34 nm Ag nanoparticles reaches ∼81%. Since the VOC (∼0.48) and FF (∼0.55) is similar for all of the cells, η for those cells increases due to an enhanced JSC. The JSC can be calculated from the quantum efficiency with the following equation:

and (3) Ag nanoparticle/Si nanopillar array on a microcell. The JSC for the bare, Ag nanoparticle, and Ag nanoparticle/Si nanopillar array cells were 19.55, 24.32, and 31.77 mA/cm2, respectively, and their efficiencies (η) were 4.69, 5.93, and 8.08%, respectively. The VOC (∼0.4 V) and FF (∼0.6) among all three changed negligibly. For a detailed evaluation of the effects of Ag nanoparticles and Si nanopillar arrays, two different diameters (18 and 34 nm) of Ag nanoparticles and Ag nanoparticles/Si nanopillar arrays were prepared on 180 μm thick bulk UMG-Si cells. Figure 3a shows the optical images of the five cells: (1) asdoped UMG-Si, (2) Ag nanoparticles (D, 18 nm; t, 30 nm) on UMG-Si, (3) Ag nanoparticles/Si nanopillar arrays prepared with 18 nm Ag nanoparticles on UMG-Si, (4) Ag nanoparticles (D, 34 nm; t, 30 nm) on UMG-Si, and (5) Ag nanoparticles/Si nanopillar arrays prepared with 34 nm Ag nanoparticles on UMG-Si. It is clear that the color of the cells gets darker with the presence of the Ag nanoparticles and the Ag nanoparticles/ Si nanopillar arrays. For quantitative analysis, diffuse and specular reflectance of these samples was measured between wavelengths of 300−950 nm (see Supporting Information, Figure S4a,b). The absorptance was calculated by assuming that the transmittance through 180 μm of Si for wavelengths smaller than 950 nm is negligible. Figure 3b shows the measured absorptance curves for the five cells. Both the 18 and 34 nm Ag nanoparticles increased the absorptance of UMG-Si in the short wavelength region. As the size of Ag nanoparticles becomes

JSC = q

∫0

∞

QE(E)bs(E) dE

(1)

where QE(E) is the quantum efficiency spectrum of the cell, and bs(E) is the photon flux density of the solar spectrum at AM 1.5 and 1 sun. Figure 3e shows close agreement between the calculated JSC (eq 1) using the quantum efficiency measurement and the AM 1.5 solar spectrum (downloaded from NREL) and the experimentally derived JSC from the measurement of J−V curves. The JSC data showed that the Ag C

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enhanced diode ideality factor and JSC when compared with bulk cells. The efficiency of the thin microcells was further increased due to plasmonic, antireflective, and light trapping effects induced by the presence of Ag nanoparticles and Si nanopillar arrays. The resulting microcells exhibited η > 8% efficiency with flexibility and semitransparency. Detailed studies on the optical and electrical properties of UMG-Si solar cells further elucidated the effects of Ag nanoparticle and Si nanopillar arrays on the overall solar cell efficiency. Further efficiency enhancement may be possible with surface passivation and back-side reflection. The results reported here may provide useful design considerations for the future work in MG-Si and other classes of solar cell systems.

nanoparticle/Si nanopillar array structures prepared from 34 nm Ag nanoparticles increased JSC of the UMG-Si cell from 14.63 to 29.98 mA/cm2 and η from 3.78 to 8.08%, which is an increase >100%. It is clear that the silver nanoparticles and the silicon nanoarrays enhance the optical absorption over a broad range of wavelengths, from the visible to near IR15−17 (Figure 3). We estimate that Ag nanoparticles contributed a 0.9% increase in the overall efficiency, while the Si nanopillar arrays contributed a 3.4% increase in the overall efficiency of our solar cells (see Supporting Information, Figure S5). Thus, we conclude that the Si nanopillar arrays are more effective than silver nanoparticles in improving the absorptance of light on our Si solar cells. To further elucidate the effects of Ag nanoparticles and Si nanopillar arrays on the efficiencies of our solar cells, the electromagnetic field intensities for Ag nanoparticles on Si and Ag nanoparticles/Si nanopillar arrays on Si were calculated using a commercial finite-difference time-domain (FDTD) software (Lumerical Solutions, Inc.). Figure 4 shows the

Table 2. J−V Characteristics of Bare Microcell Module, Ag Nanoparticles on Microcell Module, and Ag Nanoparticle/Si Nanopillar Arrays on Microcell Module (Figure 2e) 17 μm thick microcells

JSC (mA/ cm2)

VOC (V)

FF

η (%)

bare microcell module Ag nanoparticles on microcell Ag nanoparticle/Si nanopillar arrays on microcell

19.55 24.32 31.77

0.40 0.40 0.41

0.60 0.61 0.62

4.69 5.93 8.08

Table 3. J−V Characteristics of Doped UMG-Si, Ag Nanoparticles (18 nm) on UMG-Si, Ag Nanoparticle/Si Nanopillar Arrays Prepared with 18 nm Ag Nanoparticles on UMG-Si, Ag Nanoparticles (34 nm) on UMG-Si, and Ag Nanoparticle/Si Nanopillar Arrays Prepared with 34 nm Ag Nanoparticles on 180 μm Thick UMG-Si (Figure 3d) 180 μm thick cells doped UMG-Si Ag nanoparticles, 18 nm Ag nanoparticle/Si nanopillar arrays, 18 nm Ag nanoparticles, 34 nm Ag nanoparticle/Si nanopillar arrays, 34 nm

Figure 4. Field intensity maps (E2) calculated by FDTD model under an incident light of 700 nm for x−y plane (a,b) and x−z plane (c,d) of Ag nanoparticles on Si (a) and (c) and Ag nanoparticles/Si nanopillar arrays on Si (b) and (d). Note the incident light is propagating along the z-axis and is polarized along the x-axis.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

JSC (mA/ cm2)

VOC (V)

FF

η (%)

14.63 17.94 23.79

0.47 0.47 0.49

0.55 0.55 0.55

3.78 4.64 6.41

21.28 29.98

0.49 0.49

0.54 0.55

5.63 8.08

* Supporting Information S

Experimental information and supporting figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

calculated field intensity maps under an incident light of wavelength λ = 700 nm, the wavelength which shows the maximum quantum efficiency in experiments (Figure 3c), for the x−y plane (a and b) and x−z plane (c and d). The FDTD calculations clearly show light trapping at the interface of the Ag nanoparticles and Si. Perhaps more interestingly, the data also shows that the electric field intensity significantly increases on the surface of the Si nanopillar arrays for the Ag nanoparticle/Si nanopillar array structure. These simulated results indicate a pronounced electric field enhancement due to both the plasmonic Ag nanoparticles and Ag nanoparticles/Si nanopillar array structures on the Si surface. The results of this work illustrate that low grade, low cost polycrystalline Si can be used to develop novel photovoltaic cells that match the energy conversion efficiency of thin amorphous Si cells but have superior long-term stability. Our low-cost UMG-Si based ultrathin, 17 μm, solar microcell modules were fabricated on flexible substrates and exhibit

Corresponding Author

*E-mail: [email protected]; [email protected] Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Blosse for the Si raw materials. We acknowledge financial support from the National Science Foundation (Grant CBET-1066447). A.T. acknowledges financial support from the Air Force Office of Scientific Research (AFOSR) under Grant FA9550-11-1-0017. M.C. acknowledges support from the D

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Department of Energy Office of Science Graduate Fellowship program. This work used the Lurie Nanofabrication Facility at University of Michigan, a member of the National Nanotechnology Infrastructure Network funded by the National Science Foundation.

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