High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium−Tin−Oxide Mie Scatterers Dick van Dam,† Niels J. J. van Hoof,†,‡ Yingchao Cui,† Peter J. van Veldhoven,† Erik P. A. M. Bakkers,†,§ Jaime Gómez Rivas,†,‡ and Jos E. M. Haverkort*,† †
Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Dutch Institute for Fundamental Energy Research DIFFER, P.O. Box 6336, 5600 HH Eindhoven, The Netherlands § Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands ‡
S Supporting Information *
ABSTRACT: Photovoltaic cells based on arrays of semiconductor nanowires promise efficiencies comparable or even better than their planar counterparts with much less material. One reason for the high efficiencies is their large absorption cross section, but until recently the photocurrent has been limited to less than 70% of the theoretical maximum. Here we enhance the absorption in indium phosphide (InP) nanowire solar cells by employing broadband forward scattering of self-aligned nanoparticles on top of the transparent top contact layer. This results in a nanowire solar cell with a photovoltaic conversion efficiency of 17.8% and a short-circuit current of 29.3 mA/cm2 under 1 sun illumination, which is the highest reported so far for nanowire solar cells and among the highest reported for III−V solar cells. We also measure the angle-dependent photocurrent, using time-reversed Fourier microscopy, and demonstrate a broadband and omnidirectional absorption enhancement for unpolarized light up to 60° with a wavelength average of 12% due to Mie scattering. These results unambiguously demonstrate the potential of semiconductor nanowires as nanostructures for the next generation of photovoltaic devices. KEYWORDS: nanowires, solar cells, Mie scattering, absorption, efficiency, photovoltaics, InP very important for achieving a high absorption.13,14 However, until recently the highest photocurrent of a III−V nanowire solar cell (24.6 mA/cm2)15 was still significantly below that of the best planar cells of the same material (30.5 mA/cm2).16 All InP nanowire solar cells use indium−tin−oxide (ITO) as a transparent front contact. This material has a relatively high refractive index of more than 2 in the visible wavelength range. This results in a reflectance of over 10%, which lowers the maximum absorption significantly and makes it very difficult to achieve photocurrents close to 30 mA/cm2. In this article, we demonstrate a nanowire solar cell with an efficiency of 17.8% and a short-circuit current as large as 29.3 mA/cm2, calculated on the basis of the exposed area. The ITO
P
hotovoltaic cells based on high-index planar material suffer from high reflection losses. Antireflection coatings and micro- or nanosized scattering particles have been widely used to reduce this reflection.1−3 Nanoimprinted dielectric particles have been employed to enhance the incoupling of light in planar material. This enhanced incoupling has been explained in terms of Mie resonances, which scatter light into the forward direction.4−6 Mie theory7,8 describes the interaction of light with scatterers, such as spheres or cylinders of dimensions comparable to the wavelength of light, and provides a solution to Maxwell’s equations for electromagnetic waves obeying specific boundary conditions. Mie theory has proven to be very useful for dielectric and semiconductor nanoparticles5 and nanowires.9,10 Nanowire solar cells have become an important field of study due to their intrinsic advantages over planar solar cells, leading to both a higher photocurrent and a higher photovoltage.11,12 It has been shown that the shape and size of the nanowire are © 2016 American Chemical Society
Received: October 12, 2016 Accepted: November 29, 2016 Published: November 29, 2016 11414
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ACS Nano reflection loss was reduced and the nanowire absorption was enhanced with respect to a planar contact by using Mie scattering of the nanostructured transparent contact layer. This efficiency, which is the highest so far reported for nanowire solar cells and among the highest for III−V solar cells, unambiguously demonstrates the high potential of nanowire solar cells as a next generation of photovoltaic devices. Generally, omnidirectional absorption is an important property of solar cells since the incident sunlight is often not (only) coming from normal incidence. Strong absorption up to large angles with respect to normal incidence has been reported for black silicon17 and also for nanowire arrays, both in simulations18 and in measurements.13,14,19 However, until a recent report,20 no studies on the angle-dependent photocurrent of processed solar cells based on nanowire arrays have been reported. Here we demonstrate that forward scattering of the nanostructured transparent contact layer allows for omnidirectional absorption (up to at least 70° incident angle), thus providing additional proof for the application potential of nanowire solar cells.
Figure 1. Characterization of nanowire solar cell. (a) SEM image of a cleaved nanowire solar cell device, acquired under a 30° angle. The scale bar is 500 nm. (b) Schematic image of solar cell design. (c) Microscope image of the solar cell device. The scale bar is 0.5 mm. The red square is the edge of the full solar cell, and the green square is the edge of the area that is exposed to light. (d) J−V curve of the best nanowire solar cell device, measured at 1 sun (AM1.5G) illumination. The photocurrent and efficiency are calculated on the basis of the exposed area (green square in panel c). (e) External quantum efficiency (EQE) measurement of one of the cells. The JSC of this cell is 26.5 mA/cm2 (calculated on the basis of exposed area). A bias light of 7.2 mA/cm2 was used.
RESULTS We have fabricated solar cells that are based on tapered InP nanowires. The nanowires were dry-etched from epitaxially grown n- and p-doped InP layers on top of an InP substrate. The thickness and dopant densities are listed in the Supporting Information. The nanowires were covered with silicon oxide (SiO2) to improve adhesion with the surrounding material. A layer of benzocyclobutene (BCB) was used for insulation and planarization, and etched back to expose the tips of the nanowires. We used BCB because of its good electrical insulation and transparency in the visible and infrared. Finally, indium−tin−oxide (ITO) was sputtered as a transparent front contact. The ITO forms spherically shaped particles21 due to the fact that the deposited material sticks better on the InP than on the BCB surface. Therefore, diffusion toward the InP tips causes the formation of spherical particles, embedded in the layer that simultaneously grows on the BCB. More information on the fabrication can be found in Methods. We have fabricated the solar cell device as depicted in a tilted SEM image (Figure 1a) with a schematic of a cross-cut of the cell (Figure 1b). Extra cross-sectional SEM imaging is provided in the Supporting Information. A microscope image of the device is shown in Figure 1c. The current density−voltage (J−V) curve of our highest efficiency nanowire solar cell under 1 sun illumination (AM 1.5) was independently measured at NREL. The certified parameters are a VOC of 0.765 V, a fill factor (FF) of 0.794, and a JSC of 10.1 mA/cm2, based on the total cell area including gold bus bar (red square in Figure 1c). If we calculate the photocurrent based on the exposed area of the cell that absorbs light (green square in Figure 1c), we get JSC = 29.3 mA/cm2, which leads to a power conversion efficiency of 17.8%. This J− V curve is shown in Figure 1d (cell parameters of all measured cells are listed in the Supporting Information). The external quantum efficiency (EQE) spectrum shown in Figure 1e demonstrates high efficiency for all wavelengths below the optical band gap of InP. The JSC and power conversion efficiency are the highest achieved for nanowire solar cells so far. Specifically, the photocurrent of 29.3 mA/cm2 is among the highest for single junction solar cells based on III−V semiconductors.22 To quantify the absorption enhancement induced by the ITO nanoparticles, we simulate the absorption in the nanowire
solar cells. We consider cells with a planar ITO layer on top, both with and without the half-embedded (so hemispherical) ITO nanoparticles, which have a diameter of 350 nm. The simulations use a finite-difference time-domain (FDTD) method in the commerical software package Lumerical. We model one unit cell of a device, consisting of a tapered InP nanowire on top of an InP substrate, embedded in BCB, with a planar ITO layer on top. The light is incident as a plane wave from the top. More details on the simulations are given in Methods. The absorptance of the cell without ITO nanoparticle is displayed by the black circles in Figure 2a and has an average value of 0.80 between 350 and 900 nm. The absorptance of the cell with ITO hemisphere is plotted by the red squares in Figure 2a. In this case the absorptance is clearly enhanced over the entire wavelength range, resulting in an average absorptance of 0.90 between 350 and 900 nm. From ellipsometry measurements (not shown) we know that the absorptance in the ITO is negligible beyond 470 nm, which means that the absorption in the InP is enhanced. In Figure 2b,c we show the simulated near-field intensity profiles of a unit cell, both without (b) and with (c) ITO hemisphere, for incident light at 532 nm. The fields are the calculated average between the two orthogonal polarizations of the incident light. Clearly, the electric field in the top of the nanowire is enhanced, which is caused by the enhanced forward scattering of the ITO hemisphere. The absolute wavelength-dependent absorptance enhancement is defined as ahemis − aplan, in which ahemis is the absorptance of the cell including the ITO hemispheres and aplan is the absorptance of the cell with the planar ITO layer. This absorptance enhancement is displayed by the black circles in 11415
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Figure 2. FDTD simulations of solar cell absorptance. (a) Simulated absorptance in the nanowire solar cell with a planar ITO layer (black circles) and a nanowire solar cell with a planar ITO layer and ITO hemispheres on top (red squares). The lines are guides to the eye. (b, c) Near field amplitude profiles of one unit cell, simulated by FDTD, for both without (b) and with ITO hemispheres (c). The simulations were performed using periodic boundary conditions, and for incident light with a wavelength of 532 nm. The profiles are an average of the two profiles simulated for incident light polarized parallel and perpendicular with respect to the displayed profile.
Figure 3. Mie scattering and simulated absorptance. (a) Absolute absorptance enhancement ahemis − aplan (black circles). The solid line is a guide to the eye. The blue curve is the scattering efficiency by an ITO sphere with a diameter of 350 nm in air, calculated by Mie theory. (b) Simulated absorptance in nanowire solar cells without (black squares) and with (red circles) hemispherical ITO particle on top, for incident light at 532 nm. Filled symbols indicate p-polarization, and open symbols indicate s-polarization. The solid lines are guides to the eye.
wavelength range, illustrating the role of the particles in the absorption enhancement. At this point we should note that scattering in the forward direction by individual subwavelength dielectric particles has been experimentally studied recently,23−25 but has not been applied in nanowire devices yet. From the good agreement between the absorptance enhancement and scattering efficiency we additionally conclude that the individual particle resonances dominate, as the Mie theory considers individual particles. There might also be collective resonances in the nanostructured ITO layer, but their contribution appears to be minor. The size of the ITO particles is important. We have calculated the scattering efficiencies of ITO spheres in air for varying diameter, the results are displayed in Figure S6. Additionally, we note that our results clearly show that the enhanced absorption should not be interpreted as a light concentration effect.21 As the angle-dependent behavior of solar cells is important for their performance, we also simulate the angle-dependent absorptance of the nanowire solar cells, both with and without the ITO hemisphere (Figure 3b). The FDTD simulations use incident light at 532 nm. From the simulations we learn that the two angle-dependent absorption series follow the same trend.
Figure 3a. In the same plot, we show the scattering efficiency of an ITO sphere (blue curve) with a diameter of 350 nm, resembling the size of the hemispheres in the simulation. As an approximation, we take air as the embedding medium, since the light is incident from air. The scattering efficiency is larger than 1 for the full wavelength range that is relevant for InP solar cells, and exceeds 3.5 for wavelengths from 450 to 800 nm. This broadband scattering is advantageous since it enhances the coupling of the light to the absorber layer of the cell. We observe a remarkable qualitative correspondence between the scattering efficiency and the absorptance enhancement. The absorptance enhancement shows more peaks than the scattering efficiency, which we attribute to Fabry−Pérot resonances in the structure. The Mie model is only an approximation, but the simulated absorptance enhancement clearly peaks at the scattering resonances of the ITO spheres, which allows us to draw conclusions from these graphs. From this observation, together with the simulated field profiles in Figure 2, we conclude that Mie scattering is responsible for the absorptance enhancement. We have calculated the scattering efficiencies in both the forward and backward direction, shown in Figure S5. The forward scattering dominates over the entire 11416
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Figure 4. Angle-dependent photocurrent measurements. (a) Schematic of the setup. (b) Angle-dependent photocurrent measured using a laser scanning one quarter of the back focal plane, as indicated in panel a. The photocurrent is normalized to the value at (θ,ϕ) = (0,0). The polarization of the incident laser is shown by the black double arrow. (c) Profiles (blue and red lines in panel b) of the normalized photocurrent with polarized incident light, for p-polarization (blue) and s-polarization (red). The measurements (symbols) are shown with the absorption simulations acquired by FDTD (solid curves). The black curve and squares are the average between p- and s-polarization.
Figure 4b shows the measured angle-dependent photocurrent for the polarization of the laser oriented along the horizontal axis. This measurement corresponds to pure ppolarized excitation along the horizontal (blue) axis and pure spolarized excitation along the vertical (red) axis. We see that the p-polarized excitation causes an increasing photocurrent for larger oblique angles θ while s-polarized excitation results in a decreasing photocurrent when θ increases. In Figure 4c we have depicted the measured profiles along the horizontal and vertical axes, together with their average (symbols). In Figure 4c we have also plotted the simulated angle-dependent absorption for both p- and s-polarized incident light and their average (solid curves). Measurement and simulations have been normalized to the value at θ = 0° to allow direct comparison. We find good agreement between measurements and simulation, which proves the omnidirectional photocurrent enhancement constituted by the ITO scattering nanoparticles. In our Mie calculation, we assume spherical particles, although we observed a hemispherical shape in the SEM image. The effect of the particle’s shape and aspect ratio has been investigated recently,25 concluding that the resonance has similar spectral behavior, apart from a spectral shift. Additionally, the forward-to-backward scattering ratio is affected. However, in our case, the light is coming from top. In that case, the projected shape of the particle is equal to a spherical particle, which validates our Mie scattering calculations (from Figure 3) as a first approximation.
However, the cell with the ITO hemisphere on top shows an enhanced polarization-averaged absorption for the entire angular range up to an incident angle of about 65°. We observe increasing absorption for p-polarized light when the incident angle increases, while the absorption of s-polarized light decreases. We attribute this to the relative orientation of the field oscillations of the polarized light with respect to the nanowire. The field oscillations of p-polarized light at increasing angles have an increasingly large overlap with the nanowire. Therefore, the coupling strength of the p-polarized light with the nanowires increases. This is not the case for the s-polarized light: the field oscillations of s-polarized light are perpendicular to the nanowire. In fact, the absorption of s-polarized light even decreases due to the increase in the surface reflection at increasing incident angle. We have measured the angle-dependent photocurrent of the cell that has the geometry depicted in Figure 1a,b. Measuring the angle-dependent photocurrent of solar cells is usually done using a goniometer or rotational stage. However, small cells need to be examined under a microscope, which makes tilting of the sample impossible. Therefore, we utilize time-reversed Fourier microscopy to photoexcite the cell.26 This method uses a focused light source in the back focal plane of a microscope objective, in order to photoexcite the cell with a plane wave under the objective. By scanning the focused beam, we illuminate the sample with a plane wave under angles varying from 0 to 72°. The setup is described in more detail elsewhere.26 We scan one quarter of the back focal plane with a polarized beam from a diode laser at 532 nm, and perform a current−voltage scan for each position of the laser. The schematic of the setup is displayed in Figure 4a. Using a laser instead of calibrated AM1.5 light source affects the spatial distribution of generated carriers, and for a full angular calibration also other wavelengths need to be taken into account. However, our time-reversed Fourier microscopy method ensures a plane wave, for all incident angles. In principle this method can also be used using a white light source, although this is complicated due to chromatic aberration of the optics. We use a laser at a fixed wavelength to express the potential of this method for angle-dependent photocurrent measurements.
CONCLUSION In conclusion, we have shown that properly designed hemispherical particles on top of the transparent top contact of nanowire solar cells enhance the absorption for the full relevant wavelength and incident angle range. Simulations and calculations show that forward Mie scattering is the mechanism behind this improvement. The absorption enhancement, which is wavelength-averaged 12% at normal incidence, makes the self-aligned ITO hemispheres important design ingredients for solar cells based on nanowires. Using this approach, we have demonstrated a nanowire solar cell with an efficiency of 17.8% and a short-circuit current of 29.3 mA/cm2, measured under 1 11417
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ASSOCIATED CONTENT
sun incident light. This is record efficiency for nanowire solar cells, and it is among the highest of III−V solar cells.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06874. Solar cell layer stack, cross-sectional SEM, solar cell measurements, ellipsometry results, Mie scattering calculations, and reflectance FDTD simulations (PDF)
METHODS Solar Cell Fabrication. To fabricate the cells, n- and p-doped InP layers were grown on top of an InP substrate. An etching mask was made from a silicon nitride layer by nanoimprint lithography. Afterward, the InP nanowires were etched in an inductively coupled plasma (ICP) etching chamber. The sidewalls were digitally etched by surface oxidation followed by a chemical etching step to remove the oxide. This sequence was repeated 10 times to smoothen the surface. The resulting nanowires have a length of 1.6 μm and a diameter of 150 nm at the top and 350 nm at the bottom. The nanowires were covered with silicon oxide, and the space in between was filled with the transparent polymer benzocyclobutene (BCB), until only the tips of the nanowires were exposed. Finally, an indium−tin−oxide (ITO) layer was deposited as transparent conductive layer. A p+-InGaAs layer with Ti/Au was used as a contact at the back side of the substrate. More details on the fabrication can be found in refs 11 and 27. The cells were patterned into squares of 0.5 × 0.5 mm and, thus, have a total area of 0.25 mm2. Only the central 0.3 × 0.3 mm is exposed to the incident light, since a border of 0.2 mm is covered with gold, acting as bus bar. The active semiconductor layer is etched away outside the device and thus cannot contribute to the photocurrent. The full cell is exposed to light, and the photocurrent density is calculated by dividing the photocurrent over the area, which is 0.09 mm2. FDTD Simulations. For the FDTD simulations we model one unit cell of the device in the commercially available package Lumerical FDTD Solutions. We apply Bloch periodic boundary conditions. We use the complex refractive index of InP from Palik,28 for BCB the values delivered by the manufacturer (Dow Chemical), and for ITO the complex refractive index that was measured by ellipsometry (values plotted in Figure S6). We excite a plane wave incident from the top, in the direction parallel to the long axis of the nanowire. We then simulate the electric fields in the unit cell, and calculate the absorption fraction a over the volume V of the cell, using a=
∫
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Dick van Dam: 0000-0002-3265-8822 Erik P. A. M. Bakkers: 0000-0002-8264-6862 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We thank Luca Gagliano for SEM imaging, Alberto Perrotta for the ellipsometry on ITO, Tom Moriarty and Keith Emery from NREL for the solar cell measurements, and Lourens van Dijk from Utrecht University for providing a silicon reference cell. This research is supported by the Dutch technology foundation STW, which is part of the “Netherlands Organisation for Scientific Research (NWO)”, and partially funded by the Dutch Ministry of Economic Affairs. This work is also part of the research program of the “Foundation for Fundamental Research on Matter (FOM)”, which is also financially supported by the NWO. REFERENCES
1 ε0ω |E|2 Im(n)2 dV 2
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where ε0 is the vacuum permittivity, ω the frequency, E the electric field, and n the refractive index at each position in the cell. Mie Scattering Efficiency. The Mie scattering efficiency Qsca of an ITO sphere in an air environment was calculated using the formalism described by Bohren and Huffman,8 and is defined as Q sca =
2 q2
∞
∑ (2l + 1)(|al|2 + (|bl|2 ) l=1
Here, q is the size parameter, which is equal to q = 2πr/λ, where r is the sphere radius and λ the incident wavelength. al and bl are the electric and magnetic scattering amplitudes for nonmagnetic materials, which are defined as al = m2jl (mq)[qjl − 1 (q) − ljl (q)] − jl (q)[mqjl − 1 (mq) − ljl (mq)] m2jl (mq)[qhl − 1(q) − lhl(q)] − hl(q)[mqjl − 1 (mq) − ljl (mq)] and
bl =
jl (mq)[qjl − 1 (q) − ljl (q)] − jl (q)[mqjl − 1 (mq) − ljl (mq)] jl (mq)[qhl − 1(q) − lhl(q)] − hl(q)[mqjl − 1 (mq) − ljl (mq)]
Here, m is the ratio between the refractive indices of the sphere nsphere and of the environment nenv, m = nsphere/nenv. jl and hl are the spherical Bessel and Hankel functions of the first kind. 11418
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