Nanoscale Observation of Waveguide Modes Enhancing the

Letter pubs.acs.org/NanoLett

Nanoscale Observation of Waveguide Modes Enhancing the Efficiency of Solar Cells Ulrich W. Paetzold,* Stephan Lehnen, Karsten Bittkau, Uwe Rau, and Reinhard Carius IEK5 − Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany S Supporting Information *

ABSTRACT: Nanophotonic light management concepts are on the way to advance photovoltaic technologies and accelerate their economical breakthrough. Most of these concepts make use of the coupling of incident sunlight to waveguide modes via nanophotonic structures such as photonic crystals, nanowires, or plasmonic gratings. Experimentally, light coupling to these modes was so far exclusively investigated with indirect and macroscopic methods, and thus, the nanoscale physics of light coupling and propagation of waveguide modes remain vague. In this contribution, we present a nanoscopic observation of light coupling to waveguide modes in a nanophotonic thin-film silicon solar cell. Making use of the subwavelength resolution of the scanning near-field optical microscopy, we resolve the electric field intensities of a propagating waveguide mode at the surface of a state-of-the-art nanophotonic thin-film solar cell. We identify the resonance condition for light coupling to this individual waveguide mode and associate it to a pronounced resonance in the external quantum efficiency that is found to increase significantly the power conversion efficiency of the device. We show that a maximum of the incident light couples to the investigated waveguide mode if the period of the electric field intensity of the waveguide mode matches the periodicity of the nanophotonic twodimensional grating. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes in a number of optoelectronic devices concerned with nanophotonic light-trapping as well as nanophotonic light emission. KEYWORDS: Photovoltaics, thin-film solar cells, light trapping, leaky waveguide mode, scanning near-field microscopy

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waveguide modes in nanophotonic solar cells was limited to macroscopic and indirect methods like angular dependent reflectance (R) measurements or external quantum efficiency (EQE) measurements. In this contribution, we present the first nanoscopic observation of light coupling to individual waveguide modes enhancing the power conversion efficiency of nanophotonic solar cells. We make use of scanning near-field optical microscopy (SNOM) to detect the electric field intensity distribution at subwavelength resolution at the surface of nanophotonic solar cells, as illustrated in Figure 1. Based on these measurements, we identify the spectral resonance condition for light coupling to an individual waveguide mode via a two-dimensional grating and associate it to a resonance in the external quantum efficiency of a test thin-film solar cell. This characterization technique establishes experimental access to the local analysis of light coupling to waveguide modes and the associated light management in nanophotonic solar cells. It is uniquely suited to study in future the light coupling to waveguide modes and vice versa in various optoelectronic devices and thereby addresses highly relevant research areas

sustainable future requires a fair, equitable, and affordable access to emission-free electrical energy. Solar cells bear the potential to generate the required amount of electrical energy by harvesting the globally available, inexhaustible, and renewable energy resource of the sun.1,2 Light-trapping concepts are essential building blocks of the economical breakthrough of solar cells, since they allow an increase in the absorption of incident sunlight even in optically thin absorber layers.3−5 This way, the costs of the solar cells are reduced by lowering the material consumption as well as by increasing the photocurrent generation, i.e., the power conversion efficiency. In the past decade, research on nanophotonic light-trapping concepts5−8 has experienced a vast expansion. Among these concepts, periodic nanostructures such as photonic crystals,9−11 nanowire arrays,12−14 grating couplers,15−18 or plasmonic gratings19−22 have raised a particular interest. Prototype solar cells employing these periodic light-trapping concepts have been fabricated with organic materials,23−25 silicon,26−29 or III−V semiconductors.30,31 Most of these devices enable an efficient coupling of light to waveguide modes supported by their thin absorber layers. Since the light coupled to the waveguide modes experiences an enhanced effective optical path length,7,8,32 the absorptance and, in turn, the photocurrent of these solar cells increase. In the past, experimental analysis of light coupling to © 2014 American Chemical Society

Received: August 24, 2014 Revised: October 24, 2014 Published: October 28, 2014 6599

dx.doi.org/10.1021/nl503249n | Nano Lett. 2014, 14, 6599−6605

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Figure 1. (a) Photography of the tuning fork with attached aluminumcoated glass fiber tip several micrometers retracted from the surface of the sample. It shall be noted that the mirror image of the tip on the surface of the sample becomes apparent. (b) Schematic illustration of the scanning near-field optical microscopy measurement setup at the surface of the a-Si:H thin-film solar cell with nanopatterned and flat regions.

Figure 2. (a) Scanning electron image of a cross section of a periodically nanostructured a-Si:H solar cells prepared in substrate configuration onto the periodically nanostructured substrate. (b) External quantum efficiency (EQE) and absorptance (A) of the periodically structured solar cells (red lines) as well as the EQE of a flat reference solar cell (black lines). The wavelengths of coupling of incident light to waveguide modes are indicated by light blue bars.

such as the light emission in nanopatterned light emitting diodes, light trapping in optical detectors, as well as other types of photovoltaic devices. The nanophotonic test solar cell used in this work to demonstrate the nanoscopic characterization of the light coupling to waveguide modes is a periodically nanopatterned thin-film solar cell made of hydrogenated amorphous silicon (aSi:H) (see Figure 2a). The two-dimensional grating structure has a lateral size of 5 × 5 mm2. Outside this nanopatterned region, it is flat. This type of solar cell is very suitable for this analysis since it exhibits a broad spectral region (550−800 nm) of intermediate absorption. In this spectral region, the absorptance of the test solar cell is not sufficient to absorb all incident photons within a single passage, while the absorptance remains sufficiently high to provide a significant contribution to the overall photocurrent generation. In consequence, light trapping as induced for example by the periodic nanopattern of the nanophotonic test solar cell increases the absorptance (A) and the external quantum efficiency (EQE) in this wavelength range compared to a flat reference solar cell (see Figure 2b). The nanophotonic light trapping of our test solar cell is caused by two effects:19−21 (i) increased light incoupling, i.e., the reduction of reflection at the textured front side, and (ii) coupling of incident light to waveguide modes in the absorber layers of the solar cell. While the light incoupling is a broad band effect, the efficient coupling of incident light to individual waveguide modes is strongly spectrally dependent. In Figure 2b, four pronounced and spectrally narrow maxima in EQE and A are highlighted which are caused by coupling of incident light to waveguide modes in the nanophotonic test solar cell. The light coupling to such waveguide modes was experimentally observed with macroscopic methods for a number of similar types of solar cells.19,26,33−37 Three-dimensional electro-

magnetic simulations of two-dimensionally nanopatterned solar cells were used to explain the improved light trapping in prototype solar cells.19,21,38 Furthermore, in a large number of studies, the absorptance in the photoactive layer is maximized by optimizing the light coupling at the nanopattern to a spectrally dense distribution of waveguide modes.16,39−41 Novel strategies to improve further the broad band light coupling to waveguide modes are multilattices,42 quasicrystals,43 and disordered photonic crystals.34,44,45 Additionally, the light coupling to waveguide modes in nanophotonic solar cell was treated analytically,46−48 including limits for nanophotonic light management.33,49 It is explained that the coupling of incident light to waveguide modes in nanophotonic test solar cells requires a transfer of horizontal wave vector k∥ to match the dispersion relation of the waveguide mode. This wave vector is provided by light diffraction at the periodic grating by multiples of the reciprocal lattice constant, i.e., k∥ = 2π/dmn, where dmn is the lattice constant. For normal incidence, this leads to spectrally discrete resonances in the EQE and A. Since the horizontal wave vector provided by diffraction at the grating is directed, light can only couple in the two-dimensional nanostructured solar cells to waveguide modes which propagate in discrete direction, although propagation in any direction is physically allowed. In this contribution, we focus on the analysis of the light coupling to the waveguide mode causing the maximum in EQE and A at around 750 nm in the test solar cell (compare Figure 2b). Since the absorption in the a-Si:H layers decreases with increasing wavelengths, the maxima in A and EQE are more pronounced for longer wavelengths. At a wavelength of 750 nm, the absorptance of the a-Si:H absorber layer is sufficiently low to enable a propagation of the waveguide mode at low 6600


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Figure 3. (a) SNOM measurement of the a-Si:H thin-film silicon solar cell setup. (b) Surface topography of the area of investigation of the nanophotonic solar cell which includes a nanopatterned region and a flat region. Measured electric field intensity distributions at the surface of the area of investigation for incident monochromatic light of 750 nm and for incident polarization parallel (c) [pol. 0°] and orthogonal (d) [pol. 90°] to the borderline of the nanostructured region. Two-dimensional FFT of the electric field intensity distribution for incident polarization parallel (e) [pol. 0°] and orthogonal (f) [pol. 90°] to the borderline of the nanostructured region.

damping. At the same time, the absorption is high enough to induce a significant photocurrent generation, i.e., a detectable EQE of the solar cell. The absolute difference in the EQE and A

is caused by parasitic absorption losses in the contact layers. Overall, the nanophotonic light trapping improves the shortcircuit current density (JSC) of our test solar cell from 11.6 to 6601


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13.7 mA/cm2 causing a relative power conversion efficiency η from 7.3% (flat reference solar cell) to 8.4% (nanophotonic solar cell). It shall be noted that several research groups have recently investigated comparable types of nanophotonic solar cells and reported on their remarkable improvements by nanophotonic light management even in comparison to stateof-the-art commercially available random textures.33,34,50−52 To improve the understanding and reveal the nanoscale physics of light coupling to waveguide modes in nanophotonic solar cells, local probing with SNOM is applied in this work. This technique allows for the detection of the electric field intensity distribution at the surface of the nanophotonic solar cell via an aluminum-coated tapered glass fiber tip which scans at a constant distance of approximately 20 nm by means of a shear-force distance control. The short distance between the tip and the surface allows for the tunneling of photons guided in the nanophotonic thin-film solar cell into the tip, i.e., the detection of the evanescent fields of waveguide modes propagating in the nanophotonic thin-film solar cell. This way, the SNOM breaks the far-field resolution limit and the subwavelength electric field intensity distribution of waveguide modes at the surface of our nanophotonic thin-film silicon solar cells can be detected53,54 (for more details on our SNOM setup, see the experimental details). During the measurement, the nanophotonic test solar cell was illuminated at approximately normal incidence with a spectrally tunable titanium− sapphire laser at discrete wavelengths between 700 and 800 nm. The area of illumination was much larger than the area of investigation (ca. 23 μm × 23 μm) which include the borderline of the periodically nanostructured area of the substrate of the nanophotonic thin-film solar cells (see Figure 3a and b). The setup is able to detect the electric field intensity distribution of the waveguide modes which couple to light in the nanopatterned region and propagate away from the borderline into the flat region of the solar cell. Moreover, this setup has experimental advantages: (i) the electric field distribution of the waveguide modes can be studied in the absence of local near-field effects of the nanopatterns or diffraction of light at the periodically nanostructured front interface (see schematic illustration in Figure 3a), and (ii) the propagating waveguide mode in the flat region is nominally trapped perfectly. Aside of some minor radiative losses at small surface features of the nominal flat region of the solar cell, the coupling of waveguide modes to propagating modes outside the solar cell is not possible. This is constituted by the horizontal wave vector of the waveguide mode which is too large to allow coupling to propagating modes in air. In the nanopatterned region of the solar cell, the horizontal wave vector of the waveguide mode can be provided by light diffraction at the periodic grating causing a discrete value given by a multiple of the reciprocal lattice constant, i.e., k∥ = 2π/dmn, where dmn is the lattice constant.55−57 This coupling process will result in a discrete maximum in the two-dimensional FFT image of the electric field distribution. This discrete maximum reflects the absolute horizontal wave vector, i.e., the period of the grating, as well as the directionality defined by the orientation of the grating. Note that light coupling to a specific waveguide mode by light diffracting at the grating is only possible for distinct wavelengths, where the horizontal wave vector of the waveguide modes matches multiples of the horizontal wave vector transferred by the lattice. Another possible coupling process is the light diffraction at the borderline of the finite nanopatterned region or light scattering

at unintentional small surface features. Both processes allow for a broad range of horizontal wave vectors. Therefore, light coupling to waveguide modes is possible for any wavelength. While light diffraction at the borderline of the nanopatterned region is expected to show a preferential direction perpendicular to the borderline, light scattering at small surface features is not expected to show any directionality. For the case of light scattering at small surface features, a ring-like structure in the FFT image of the electric field distribution is expected, as the absolute horizontal wave vector is still defined by the dispersion relation of the waveguide mode. However, the surface area of the two-dimensional grating structure significantly exceeds the effective cross sections of the borderline or possible small surface features. Therefore, the intensity of the waveguide mode is expected to show a resonance at the wavelength where the two-dimensional grating allows for light coupling. Each waveguide mode fulfills the Bloch boundary condition in the direction of the transferred horizontal wave vector. Therefore, a periodic light intensity pattern is expected. The period of this pattern allows determining the horizontal wave vector of the waveguide mode. In Figure 3c and d, two-dimensional electric field intensity distributions measured by SNOM at the surface of the nanophotonic solar cell are shown for monochromatic incident light (λ = 750 nm) at almost normal incidence and polarization parallel as well as polarization orthogonal to the borderline of the nanostructured region, respectively. For incident light of polarization parallel to the borderline (Figure 3c), a strongly pronounced periodic pattern of the electric field intensity with wave fronts parallel to the borderline is apparent, while the electric field intensity distribution of the incident light with polarization orthogonal to the borderline (Figure 3d) shows no order. Translating the electric field intensity distribution at the surface of the flat regions of the solar cell into the spatial frequency domain via a two-dimensional fast Fourier transform (FFT) leads to a Fourier transform image in the reciprocal space (called FFT image in the following). As shown exemplary in Figure 3e, the periodic pattern of the electric field intensity distribution transforms into distinct maxima. Such maxima are not apparent in the FT image of the nonordered electric field intensity distribution (Figure 3f). This means that, for a polarization orthogonal to the borderline, light coupling to waveguide modes is dominated by scattering at small surface features, whereas for light polarization parallel to the borderline, diffraction at the borderline or the nanopattern is obviously the main coupling process. Moreover, the maxima in Figure 3e are located at the spatial frequency ( f) of 2 μm−1. This spatial frequency corresponds to a period pWG of 500 nm. Therefore, light diffraction at the periodic grating is responsible for light coupling to waveguide modes at this specific wavelength. Since the period is smaller than the actual wavelength of incident light in air (λ = 750 nm) and since it matches the grating period pGeo = 500 nm of the nanophotonic solar cell, the periodic pattern of the measured electric field intensity distribution in the flat regions of the solar cell is a direct observation of light guided inside the solar cell propagating away from the borderline of the nanostructured region of the solar cell. This observation is in agreement with the existing theory on light management with light coupling to waveguide modes in nanophotonic solar cell. In fact it can be distinguished as a direct validation of the theoretical prediction by Yu et al.32,49 It shall also be noted, that the periodic pattern cannot be induced by superposition of reflected or diffracted 6602


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light in air with the incident light (λ = 750 nm) which would result in interference patterns of periods equal or larger than the wavelengths of incident light, itself. Having proven that our SNOM setup is able to detect the electric field intensity variations of a waveguide mode which couples to incident light in the nanostructured region of the solar cell and propagates in the flat region, we study in detail the spectral dependence of light coupling to this mode. To this end, the wavelength of excitation is varied between 700 and 800 nm in steps of 5 nm. The polarization is set parallel to the borderline of the nanostructured region. For each wavelength, the electric field intensity distribution at the surface of the solar cell is measured with our SNOM setup and corrected for variations of the excitation intensity. From these normalized electric field intensity distributions, restricted to the flat region of the solar cell, corresponding FFT images are calculated (examples in Figure 4a), with a complete set of data in the Supporting Information. In a wavelength range from 710 to 770 nm, we observed pronounced maxima within the FFTs. The intensities Ic of these maxima in the FFTs provide a figure of merit for the coupling intensity of incident light to the waveguide modes. Plotting the intensity Ic as a function of the excitation wavelength (in Figure 4b) unveils a pronounced maximum at wavelengths around 750 nm. This maximum is an excellent match to the maximum of the EQE of the investigated nanophotonic solar cell illustrated in Figure 4d as the quantum efficiency enhancement factor f EQE, i.e., the ratio between EQEper of the nanophotonic and EQEflat of the flat reference solar cell. The agreement proves that the waveguide mode studied with our SNOM setup provides a significant contribution to the enhancement of the EQE. Since the efficiency of the solar cell scales directly with the short-circuit current density JSC, which is given as a spectral integral of the product of the solar flux at the surface of the earth and the EQE, the waveguide mode in focus of our studies leads straightforward to an efficiency enhancement of the solar cell. Beyond the light coupling intensity, the analysis of the distinct maxima in the FFT images for different excitation wavelengths also provides their spatial frequencies ( f) or the spatial periods (pWG). These values correspond to the respective waveguide mode propagating in the flat region of the solar cell. In Figure 4c, the period (pWG) is shown to increase from around 350 nm to around 580 nm with increasing excitation wavelengths from 710 to 770 nm. This increase is caused by the dispersion of the waveguide mode excited at the semi-infinite grating of the nanostructured region of the solar cell. Most noticeable for our analysis is the fact that the measured period pWG of the propagating waveguide mode matches the geometrical period (pGeo = 500 nm) of the nanopatterned region of the solar cell precisely at wavelengths close to 750 nm, i.e., the maximum of the coupling efficiency shown in Figure 4b. As the theory of waveguide coupling in nanopatterned solar cell describes,32,49 the light coupling to the waveguide mode must be most efficient for the case where incident light couples efficiently into the waveguide mode if it has the same period as the nanopattern of the solar cell. For this condition, a maximum of incident light intensity couples to the investigated waveguide mode and thereby induces a maximum in the EQE, i.e., photocurrent generation of the solar cell. Altogether, the SNOM measurements presented here provide a self-consistent picture of enhanced EQE in the nanopatterned region of the solar cell at wavelengths of efficient coupling of incident light to a waveguide mode. Considering

Figure 4. (a) Two-dimensional fast Fourier transforms (FFT) of the electric field intensity distribution measured for incident polarization parallel to the borderline of the nanostructured region at wavelengths of 735, 750, and 765 nm. (b) Light coupling intensity Ic of incident light to a waveguide mode derived from a series of FFTs with a wide range of wavelengths (polarization as above). (c) Period pWG of the dominant waveguide mode identified as the inverse of the spatial frequency at the maxima in the FFT image. (d) External quantum efficiency enhancement factor f EQE = EQEper/EQEflat defined as the ratio between EQEper of the nanophotonic and EQEflat of the flat reference solar cell.

the directionality of the oscillations in the electric field intensities along the lattice of the nanopattern, the spectral agreement of maxima in the coupling strength Ic with the maxima of EQE, and the match of the pGeo and pWG at the 6603


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material is available free of charge via the Internet at http:// pubs.acs.org.

maximum in EQE, no alternative explanation than that given in our manuscript is feasible. For example, light scattering at nanostructures or perturbations would not show the same directional dependence of the electric field oscillations. The light diffraction at the borderline would not show a maximum in Ic at the same period as the grating. Also, any effect related to the geometry of our tip or the SNOM measurement (e.g., the Tien effect) would very unlikely show a maximum in Ic precisely at a maximum of EQE and A of the solar cell. The experimental method introduced in this contribution is suitable to study the nanoscale physics of light coupling to waveguide modes in nanophotonic solar cells. Our measurements determine the propagation direction and periods pWG of the electric field intensities of waveguide modes as well as the coupling intensities Ic of light coupling to waveguide modes. Based on this information, we can identify efficient light coupling; i.e., we gain some degree of predictive power on the quantum efficiency enhancement caused by the nanophotonic light trapping concepts for solar cells (see Figure 4d). Our method is ready to be applied to a broad range of nanophotonic light-trapping concepts, including photonic crystals, nanowires arrays, and grating couplers. In this field, the overarching goal is to achieve optimal light coupling to a large density of waveguide modes in the region of low absorptance of the solar cell.32,39,49 Our method will help to identify in future the optimal shapes and dimensions of nanostructures and/or geometries and dimensions of the lattice spacing with regard to the optimal light coupling to waveguide modes in the region of low absorption in thin-film solar cells. New types of gratings such as multilattices, quasicrystals, and disordered photonic crystals will be studied with regard to their potential to yield efficient light coupling to a maximum of waveguide modes in the solar cells. The method can be expanded to many other solar cell technologies which deal with low absorption such as organic photovoltaics or thin crystalline silicon solar cells. Moreover, based on our method, simulated electric field intensity distributions of nanophotonic solar cells but also lightemitting diodes or sensors can be validated experimentally for the first time on a nanoscale. Thereby, the quality and predictive power of the three-dimensional electromagnetic simulations are expected to improve significantly. In conclusion, we demonstrated the nanoscopic observation of a single waveguide mode enhancing the light-trapping effect and, in turn, the power conversion efficiency of a nanophotonic thin-film silicon solar cell. Making use of the nanoscale resolution of the scanning near-field optical microscopy, we measured the electric field intensities of waveguide modes which couple to incident light in the periodically nanostructured solar cell. The conditions for efficient light coupling to an individual waveguide mode by a two-dimensional nanostructure were identified and associated with an individual resonance in the external quantum efficiency of a state-of-theart thin-film solar cell. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes, i.e., nanophotonic light-trapping and nanophotonic light emission concepts in a large variety of optoelectronic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

U.W.P.: IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium. Author Contributions

U.W.P. developed the idea of this project and fabricated the nanophotonic light-trapping concept. S.L. developed the scanning near-field optical microscopy setup. U.W.P., S.L. and K.B. conducted jointly the measurements and post processing of the data. R.C. and U.R. assisted in all aspects of the work and supervised the project. All authors discussed the results and contributed to writing the manuscript. The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Smeets, M. Meier, U. Aeberhard, M. Ermes, M. Hülsbeck, J. Klomfaß, A. Hoffmann, V. Smirnov, F. Finger, T. Kirchartz, M. Prömpers, D. Michaelis, and C. Wächter for technical support and fruitful discussions. This work was supported by the German Federal Ministry of Education and Research (BMBF) within the project “PhoNa” (support code 03IS2101F).

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ABBREVIATIONS SNOM, scanning near-field microscopy; FFT, fast Fourier transform REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Description of fabrication details of the solar cells, applied characterization techniques, the scanning near-field optical microscopy setup and a full set of the experimental data. This 6604


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