Letter pubs.acs.org/NanoLett
Self-Patterned Nanoparticle Layers for Vertical Interconnects: Application in Tandem Solar Cells Bjoern Niesen,*,† Nicolas Blondiaux,‡ Mathieu Boccard,† Michael Stuckelberger,† Raphael̈ Pugin,‡ Emmanuel Scolan,‡ Fanny Meillaud,† Franz-Josef Haug,† Aïcha Hessler-Wyser,†,§ and Christophe Ballif†,‡ †
Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Institute of Microengineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71, CH-2000 Neuchâtel, Switzerland ‡ Centre Suisse d’Electronique et de Microtechnique (CSEM) SA, Rue Jaquet-Droz 1, CH-2002 Neuchâtel, Switzerland § Centre Interdisciplinaire de Microscopie Electronique (CIME), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: We demonstrate self-patterned insulating nanoparticle layers to define local electrical interconnects in thin-film electronic devices. We show this with thin-film silicon tandem solar cells, where we introduce between the two component cells a solution-processed SiO2 nanoparticle layer with local openings to allow for charge transport. Because of its low refractive index, high transparency, and smooth surface, the SiO2 nanoparticle layer acts as an excellent intermediate reflector allowing for efficient light management. KEYWORDS: Photovoltaics, micromorph, multijunction, planarization, dewetting, self-assembly
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electrically interconnected by highly doped tunneling junctions. These interconnects are generally made from wide-bandgap materials to reduce parasitic light absorption.28 In tandem cells based on thin-film Si,29−32 electrical interconnection is typically provided by the so-called intermediate reflector layer located between the two component cells, which fulfills three additional functionalities.33−35 First, made from a material with a low refractive index, in contrast to the high-index Si material, the intermediate reflector layer reflects weakly absorbed light back into the wide-bandgap component cell. Second, intermediate reflectors of anisotropically conducting materials, such as Si filaments embedded in SiO x, were shown to quench recombination paths by suppressing lateral charge transport.36,37 Finally, it was recently demonstrated that intermediate reflectors can be used to tailor the interface roughness needed in thin-film Si solar cells for light scattering, improving the Si material quality of the component cell grown on top of the intermediate reflector.38 Self-patterned, electrically insulating NP layers with local openings are ideal candidates for intermediate reflectors, as by the choice of NP material and size, they can readily be made highly transparent, with a low refractive index, a smooth surface, and a low lateral conductance, while still allowing for efficient vertical interconnection of the two component cells. A schematic
n recent years, nanoparticle (NP) layers have found application in electronic and optoelectronic devices, in an effort to reduce cost and improve performance. For example, NP layers have been utilized as quantum emitters and absorbers, photonic crystals, and charge-transport layers, for plasmonic and photonic light trapping and for NP-assisted texturing.1−9 Nanoparticle single- and multilayers can readily be deposited by solution-processing methods, such as dip coating, Langmuir−Blodgett deposition, capillarity-assisted particle assembly, and spin coating.10−14 When solution-processed, the deposition of NP layers is governed mainly by the dynamics of the receding liquid contact line during the drying of the NP dispersion film.15,16 Therefore, NP deposition can be influenced by modifying the drying of the liquid film.16−18 Well-controlled, densely packed NP layers have been obtained by texturing the surface with nano- or microstructures that pin the receding liquid contact line.19−23 Interestingly, NP layers deposited on such textured surfaces typically do not cover the top of sharp surface features.22,23 These properties inspired us to realize vertical interconnects in thin-film electronic devices using self-patterned electrically insulating NP layers with local openings. Electrical interconnects are ubiquitous in thin-film electronics, including contacts to transistors in integrated circuits24 and large-area sensor arrays,25 connections between neighboring solar cells in thin-film photovoltaic modules,26 and interconnects between the back-plane circuitry and organic light-emitting diodes in displays.27 In high-efficiency tandem and multijunction solar cells, the component cells are © 2014 American Chemical Society
Received: May 13, 2014 Revised: July 30, 2014 Published: August 7, 2014 5085
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in the trade-off between efficient light trapping and a smoothened template for the growth of the μc-Si cell. Figure 1b shows a schematic drawing of the device architecture of a thin-film Si tandem solar cell in the micromorph p−i−n, or superstrate, configuration. These monolithic tandem cells consist of two component cells, electrically contacted in series: an a-Si cell, with an absorber layer bandgap of 1.7−1.8 eV to harvest visible light, and a μc-Si cell, which absorbs near-infrared light up to wavelengths of λ ∼ 1100 nm, corresponding to a bandgap of 1.1 eV. Both component cells are grown by plasma-enhanced chemical vapor deposition (PE-CVD). Transparent ZnO front and rear electrodes are utilized to electrically contact the tandem cell. The intermediate reflector, in our case a self-patterned insulating SiO2 NP layer with local openings for vertical charge transport, is located between the two component cells. The fabrication of the layer stack begins with the deposition of the transparent ZnO front contact on a glass substrate, through which the solar cell is illuminated. The deposition technique employed for this ZnO layer, low-pressure CVD, yields a rough surface with random pyramidal surface features in the 0.1−1 μm range, as shown by the top-view scanning electron microscopy (SEM) images in Figure 1c for several ZnO layer thicknesses. Such surface features, with sizes comparable to visible−near-infrared wavelengths, are very efficient light scatterers, as shown by the haze spectra in Figure 1d, and enhance light absorption and hence photocurrent in the solar cell by light trapping.39−41 The size of the surface features increases with the ZnO layer thickness, such that thicker layers scatter long-wavelength light more efficiently. Before implementing SiO2 NP layers in thin-film Si solar cells, we assessed their optical and morphological properties. For this purpose, we deposited SiO2 NPs on a polished Si wafer by means of spin coating. As shown in Figure 2a, the NPs form
representation of such a NP-based intermediate reflector is shown in Figure 1a.
Figure 1. (a) Schematic representation of a self-patterned insulating NP intermediate reflector layer with local openings for electrical interconnection between the two component cells in a tandem device. (b) Schematic representation of the thin-film p−i−n a-Si/μc-Si tandem solar cell device architecture. Light enters the cell through the glass substrate. (c) SEM images of ZnO layers with thicknesses from 2.5 to 5 μm. The scale bar corresponds to 2 μm. (d) Haze spectra, defined as the ratio of diffuse transmission to total transmission, of ZnO layers with thicknesses from 2.5 to 5 μm.
Figure 2. (a) SEM image of a SiO2 NP layer deposited on a polished Si wafer by spin coating with its 2D autocorrelation function (inset). The scale bars correspond to 100 nm. (b) Particle size distribution histogram, obtained from 596 particles.
Here, we utilize self-patterned insulating NP layers with local openings to define vertical electrical interconnects in thin-film electronic devices. We demonstrate this with a solutionprocessed SiO2 NP layer that acts as an efficient intermediate reflector in thin-film amorphous silicon (a-Si)/microcrystalline silicon (μc-Si) tandem solar cells due to its high transparency and low refractive index. The a-Si cell features a highly textured rear surface, which we employ as a template for self-patterning the NP layer during solution processing. Rupture and dewetting of the insulating SiO2 NP film around the peaks of the texture open local point contacts for efficient vertical electrical interconnection. In contrast, lateral shunt paths are quenched by the low lateral conductivity of the SiO2 NP layer. Moreover, we tune the thickness of the SiO2 NP layer to find an optimum
a dense multilayer with a smooth surface. From this SEM image, an average SiO2 NP size of 38.7 ± 5.4 nm was found (Figure 2b). The NP film exhibits near-range order without long-range periodicity, resulting in a 2D autocorrelation function that shows rings with spacing equal to the NP size, in agreement with a closely packed NP layer (inset to Figure 2a). Moreover, the SiO2 NP layer did not show any significant diffuse reflectance, indicating negligibly weak light scattering, as expected42 for dielectric NPs with a size ≪ λ. The NP layer thus optically behaves like a uniform thin film. According to the effective medium approximation,43 a closely packed multilayer 5086
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Figure 3. Schematic representation and SEM images of (a) a 230-nm-thick a-Si layer deposited on 5-μm-thick low-pressure CVD ZnO, and the same layer stack coated with a (b) thin or (c) thick SiO2 NP layer. SEM images on top row: sample tilted by 20°, scale bar corresponds to 500 nm; bottom row: plan view, scale bar corresponds to 1 μm.
of SiO2 spheres with a refractive index n = 1.45 possesses an effective refractive index neff = 1.33. This value matches the experimentally measured n = 1.33 ± 0.02 of the NP layer (Figure S1 in the Supporting Information), confirming the close packing. Notably, this n value is below that of dense thin films of low-index materials commonly used for optical applications, such as MgF with n = 1.37, making the SiO2 NP layer suitable for index-contrast applications, as demonstrated here for the SiO2 NP layer in combination with the high-index material Si (n ≈ 4 at λ = 600 nm). Moreover, the SiO2 NP layer was found to be highly transparent throughout the whole wavelength range of interest (300 nm < λ < 1200 nm), as evidenced by the imaginary part of the refractive index k ≈ 0 shown in Figure S1. Both the low refractive index and the high transparency are strong advantages of the SiO2 NP layer compared to conventional intermediate reflectors, which consist of conductive oxide layers deposited by sputtering or CVD.33−36 The conducting nature of such layers leads to parasitic light absorption due to free charge carriers.44 In addition, these intermediate reflectors have relatively high refractive indices, typically above 1.8 at λ = 600 nm, which results in a limited index mismatch between the intermediate reflector and the Si absorber layers and therefore in a less efficient reflection. Next, we spin coated the SiO2 NPs onto the a-Si cell to form the self-patterned SiO2 NP intermediate reflector. The pyramidal surface features of the ZnO front electrode are reproduced on the rear surface of the conformally grown a-Si layer, as shown in Figure 3a. The coating behavior of the SiO2 NPs on this textured surface strongly differs from that on the flat Si wafer (Figure 3b and c). Instead of completely covering the substrate, a dense SiO2 NP multilayer is formed in the valleys, whereas the peaks of the pyramids remain uncovered. By varying the spin coating conditions, the thickness of the SiO2 NP layer can be adjusted either to cover only the lowestlying areas of the surface or to fill up larger valleys, such that smaller surface features are completely coated. In the remainder of this article, the former will be referred to as thin SiO2 NP layer (covering ∼36% of the surface area, Figure 3b), whereas the latter will be referred to as thick SiO2 NP layer (covering ∼75% of the surface area, Figure 3c). We employed conductive atomic force microscopy (AFM) on the thick SiO2 NP layer to confirm this valley-filling behavior. As shown in Figure 4, only the tips of the SiO2-NP-coated a-Si cell surface exhibit a high electrical conductance, whereas the valleys are highly resistive with a ten times lower AFM tip−sample current, confirming the
Figure 4. 10 × 10 μm conductive AFM image of the thick SiO2 NP layer on an a-Si (230 nm thick)/ZnO (5 μm thick) layer stack. The color scale indicates the current flow between the AFM tip and the sample. This combined morphology/current flow 3D graph demonstrates the high contrast in electrical conductance between the conductive, uncoated a-Si pyramid tips and the highly resistive, dense SiO2 NP layer.
selective coating of the a-Si surface and efficient electrical insulation by the dense SiO2 NP layer. This self-patterning effect is achieved by exploiting the liquid dynamics of the SiO2 NP dispersion on the rough a-Si surface while spin coating: First, the SiO2 NP dispersion spreads across the entire substrate, as confirmed by an advancing contact angle of 74° (measured from a drop of the SiO2 NP dispersion on a smooth a-Si layer). Then, the liquid volume is reduced due to droplet ejection by centrifugal forces and solvent evaporation, which becomes the dominating mechanism once the liquid film is sufficiently thin.16 When the liquid film thickness becomes comparable to the height of the surface features, the sharp tips of the pyramids act as nucleation points, which initiate the rupture and dewetting of the liquid film.45,46 Once the liquid film has dewetted from the pyramid tips, they can also act as pinning points for the liquid contact line, enhancing the planarization of the liquid film.22,47 The combination of dewetting and planarization directs the NPs toward the bottom of the valleys, which leads to a smoothening of the NP layer surface and the formation of openings around the peaks of the surface features. Liquid film rupture and dewetting at sharp surface features have also been reported for polymer solutions on corrugated Si wafers, albeit on a much smaller length scale than demonstrated here for SiO2 NPs.45 We then completed the thin-film a-Si/μc-Si tandem cell layer stack by depositing the μc-Si cell on top of the self-patterned 5087
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We utilized ZnO front electrodes with thicknesses of 2.5, 3.5, or 5 μm. As elucidated in the Supporting Information, we found that a 3.5-μm-thick ZnO front electrode offers sufficient flexibility when adjusting the SiO2 NP layer thickness while still allowing for a high electrical quality of the tandem cell. In short, for a 2.5-μm-thick ZnO front electrode, complete coverage of the a-Si surface was observed at low spin speeds, and for a 5-μm-thick ZnO front electrode, the electrical quality of the μc-Si layers was significantly reduced. Therefore, we will focus the following detailed discussion on solar cells made with a 3.5-μm-thick ZnO front electrode, whose device characteristics, including external quantum efficiency (EQE) and current density−voltage (J(V)) characteristic, are shown in Figure 6 and summarized in Table 1. The optimization of an intermediate reflector for thin-film Si solar cells involves a delicate interplay between electrical and optical effects. In the following paragraphs, the influence of the SiO2 NP intermediate reflector on light absorption and photocurrent generation within the tandem solar cell will be presented first, followed by its effect on the electrical properties of the tandem cell. Finally, combining both aspects, the optimization of the overall device efficiency will be discussed. One of the main purposes of an intermediate reflector is the enhancement of the a-Si cell photocurrent by partially reflecting nonabsorbed light. That way, only a thin a-Si cell is required, which minimizes its light-induced degradation.48 In order to individually determine the photocurrent generated in each component cell, we employed EQE measurements, shown in Figure 6a. The EQE spectra of the a-Si and μc-Si component cells were measured individually, using near-infrared and blue bias light, respectively, to generate a high photocurrent in the component cell not being measured. The current densities of
SiO2 NP layer, followed by the ZnO rear contact. A crosssectional SEM image of such a tandem cell is shown in Figure 5. The nonconformal SiO2 NP layer leads to a partial
Figure 5. SEM image of a polished cross section of a thin-film a-Si/μcSi tandem cell with a thick SiO2 NP intermediate reflector. The scale bar corresponds to 1 μm. The inset shows a detailed view of the SiO2 NP layer, with a scale bar that corresponds to 200 nm.
planarization of the μc-Si cell. Moreover, direct contact points between the a-Si and μc-Si cells are visible at the uncovered peaks of the a-Si surface features. Therefore, the SiO2 NP layer allows for a decoupling of the two component cell morphologies while still enabling direct electrical contact between the component cells. In addition, the inset to Figure 5 demonstrates that the dense packing of the SiO2 NP layer is preserved after the deposition of the μc-Si and ZnO layers. We fabricated and characterized thin-film Si tandem cells with either a thin or thick SiO2 NP intermediate reflector and tandem cells without an intermediate reflector as references.
Figure 6. Device characteristics of a-Si (230-nm-thick intrinsic layer)/μc-Si (1.6-μm-thick intrinsic layer) tandem cells without an intermediate reflector (black), with a thin SiO2 NP layer (blue), or with a thick SiO2 NP layer (red). (a) EQE and 1 − total reflectance spectra. The dashed curves represent the summed EQE spectra of both component cells. (b) J(V) characteristics, measured under AM1.5g irradiance at 1 sun intensity. (c) Fill factor and (d) power conversion efficiency as a function of component cell current mismatch (Ja‑Si − Jμc‑Si), obtained by adding either blue or nearinfrared light to the AM1.5g irradiance spectrum. The crosses indicate the values obtained at AM1.5g irradiance at 1 sun. 5088
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Table 1. Device Parameters of Thin-Film Si Tandem Solar Cells with and without a SiO2 NP Intermediate Reflector Layer, Extracted from the Measurements Shown in Figure 6a no intermediate reflector thin SiO2 NP layer thick SiO2 NP layer
Ja‑Si (mA cm−2)
Jμc‑Si (mA cm−2)
Ja‑Si − Jμc‑Si (mA cm−2)
Voc (mV)
FF (%)
Rsc (Ω cm2)
Roc (Ω cm2)
η (%)
12.00 13.25 14.24
13.86 12.33 10.36
−1.86 0.92 3.88
1358 1363 1381
70.1 73.8 79.8
2800 69300 44900
10.1 10.0 10.2
11.42 12.40 11.42
a
Ja‑Si and Jμc‑Si were obtained from EQE measurements; Voc, FF, Rsc, and Roc values were obtained from J(V) characteristics. For each tandem device, the current of the limiting component cell utilized to calculate the power conversion efficiency (η = Jlimiting cellVocFF/1000 W m−2) is highlighted.
both component cells, Ja‑Si and Jμc‑Si, were then calculated by convolution of the measured EQE spectra with the air mass 1.5 global (AM1.5g) solar irradiance spectrum. The AM1.5g spectrum is equivalent to the solar irradiance at a solar zenith angle of 48.2° and is a commonly used standard spectrum when measuring solar cell performance.49 Ja‑Si increases from 12.00 mA cm−2 without an intermediate reflector to 13.25 and 14.24 mA cm−2 with a thin and a thick SiO2 NP layer, respectively, confirming that the SiO2 NP layers act as efficient intermediate reflectors. The effect of the SiO2 NP layers on the photocurrent distribution between the two component cells is directly visible in the EQE spectra: The a-Si cell EQE is strongly increased by the SiO2 NP layers between λ = 500−800 nm, which is the wavelength range where the a-Si cell does not absorb all the light within a single pass. At the same time, the μc-Si cell EQE is reduced by the SiO2 NP intermediate reflectors. Also, it is clearly visible how the a-Si cell EQE gain increases with the SiO2 NP layer thickness. This can be explained by the higher surface coverage of the a-Si cell surface for the thicker SiO2 NP layer. While the summed EQE (EQEa‑Si + EQEμc‑Si) curves of the tandem cell without an intermediate reflector and that with a thin SiO2 NP layer are nearly identical, the summed EQE is slightly lower at λ > 600 nm for the tandem cell with a thick SiO2 NP layer. This is due to light that is reflected by the intermediate reflector and then lost through the front electrode, as well as the smoothening effect of the thick SiO2 NP layer, which leads to less efficient light trapping in the μc-Si cell in the near-infrared. As a result, the tandem cell with a thick SiO2 NP layer also shows a higher total reflectance in this wavelength range. Recently, spectrally selective intermediate reflectors based on nanostructures such as metal nanoparticles50,51 and photonic crystals2,52 have been reported. These selective reflectors strongly reflect light in the spectral range where the incident light is not completely absorbed by the a-Si cell (550 nm < λ < 800 nm) and efficiently transmit light at wavelengths above the a-Si absorption edge (λ > 800 nm). As a result, reflection losses in the near-infrared are reduced. Therefore, compared to the thick self-patterned SiO2 NP layer, which leads to reflection losses, spectrally selective intermediate reflectors could potentially improve the summed EQE. In contrast, as the thin SiO2 NP layer does not cause substantial reflection losses, only marginal improvements could be expected with spectrally selective reflectors. Moreover, for metal-nanoparticle-based intermediate reflectors, the beneficial effects might be outweighed by parasitic absorption.51 An important feature of the SiO2 NP layer is the smoothening of the a-Si/μc-Si cell interface due to the valleyfilling effect. As a result, the μc-Si cell is grown on a smoother substrate, which is desirable as μc-Si material quality is highly sensitive to substrate morphology.53 Rough substrates result in defective μc-Si layers, reducing the μc-Si cell’s electrical quality, as evidenced by a decrease in open-circuit voltage (Voc). The
SiO2 NP intermediate reflectors lead to an increased Voc, confirming the enhanced electrical performance upon surface smoothening. As expected, the Voc enhancement is stronger for the thicker SiO2 NP layer, which results in the smoothest interface. In addition, we found that the thick SiO2 NP layer makes the Voc nearly independent of the ZnO front electrode roughness, as discussed further in the Supporting Information. Smoothening layers as intermediate reflectors for thin-film Si tandem cells have been proposed before.38 However, these layers were based on complicated and time-consuming multistep protocols, in contrast to the simple one-step preparation of the self-patterned SiO2 NP intermediate reflectors. Typically, the fill factor (FF) of a thin-film Si tandem solar cell is higher when the μc-Si cell is current limiting (Ja‑Si − Jμc‑Si > 0), which is due mainly to the better collection efficiency of μc-Si cells as compared to a-Si cells.54,55 Therefore, as shown in Figure 6b and Table 1, the FF is highest for the tandem cell with the thick SiO2 NP layer, whose current is strongly limited by the μc-Si cell, whereas it is lowest for the tandem cell without an intermediate reflector with a current that is limited by the a-Si cell (Ja‑Si − Jμc‑Si < 0). Closely linked to the FF are the open-circuit resistance (Roc) and short-circuit resistance (Rsc), which can be calculated from the slope of J(V) curves, at Voc and 0 V, respectively (Table 1). A high Roc reflects resistive losses, whereas a low Rsc indicates the presence of shunt paths for state-of-the-art thin-film Si cells.56 In addition, Roc is increased and Rsc reduced in devices with inefficient charge collection. The fact that Roc is not significantly influenced by the presence of a SiO2 NP intermediate reflector indicates that the openings through the NP layer are dense enough even for the thick SiO2 NP layer to allow for efficient charge transport and charge carrier collection. The increased Rsc in the presence of a SiO2 NP layer could indicate a passivation of highly conductive shunt paths but could also be caused by reduced recombination losses. In order to directly compare FF values between tandem cells that exhibit different component cell current mismatches we gradually tuned the Ja‑Si − Jμc‑Si mismatch of each tandem cell by adding blue or red light to the AM1.5g solar irradiation spectrum while measuring J(V) characteristics.55 In a blue-rich spectrum, Ja‑Si − Jμc‑Si will increase, whereas it will decrease in a red-rich spectrum. The FF versus Ja‑Si − Jμc‑Si curves resulting from these measurements are shown in Figure 6c. The crosses indicate the current mismatch under AM1.5g solar irradiance conditions for each tandem cell. Interestingly, both tandem cells containing a SiO2 NP intermediate reflector exhibit nearly identical FF curves, whereas the cell without an intermediate reflector consistently shows ∼2% lower FF values. These results demonstrate that the SiO2 NP intermediate reflectors enhance the FF independent of the current mismatch. Moreover, the tandem cells with a SiO2 NP layer showed higher Voc values at low illumination: At 0.004 sun illumination, the Voc of the cell 5089
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patterned NP layers presented here are suitable for large-area, high-throughput applications.
without a SiO2 NP intermediate reflector was 660 mV, compared to 880 mV and 900 mV for the cells with a thin and a thick SiO2 NP layer, respectively. As the presence of shunt paths strongly reduces Voc at low illumination, these results suggest that the SiO2 NP layers indeed act as shuntquenching layers. From these current mismatch measurements, it is also possible to determine the optimal mismatch to maximize the power conversion efficiency (η), as shown in Figure 6d. A tandem cell with a current that is strongly limited by the μc-Si cell might be desirable to reach a high FF, but at the same time, the short-circuit current density decreases with increasing current mismatch as the current-limiting component cell determines the current flow through the tandem cell. This trade-off leads to an optimal Ja‑Si − Jμc‑Si of 1−1.5 mA cm−2. Under AM1.5g irradiance, the current of the tandem cell without an intermediate reflector is too strongly limited by the a-Si cell, whereas the current of the cell with the thick SiO2 NP layer is too strongly limited by the μc-Si cell. Both tandem cells show an efficiency of 11.42%. The efficiency−current mismatch curve of the cell with the thick SiO2 NP layer lies below that of the cell without an intermediate reflector due to reduced light trapping. In contrast, the tandem cell with the thin SiO2 NP layer shows an enhanced η = 12.40%, with an improved efficiency−current mismatch curve due to the combination of the enhanced FF and Voc values and only marginal losses in light trapping compared to the cell without an intermediate reflector. In addition, its Ja‑Si − Jμc‑Si mismatch under AM1.5g conditions is optimal to reach a maximized efficiency. Therefore, the thin SiO2 NP layer represents the best choice for this a-Si/μc-Si cell combination. Finally, we assessed the influence of the SiO2 NP layers on the light-induced degradation of thin-film Si tandem cells. For this purpose, we protected the cells with an epoxy/glass encapsulation and exposed them to an AM1.5g light spectrum with 1 sun intensity at 50 °C. The presence of the SiO2 NP intermediate reflector did not alter the relative degradation, as shown by Figure S4 in the Supporting Information. After 1000 h of light soaking, the optimized cell with the thin SiO2 NP intermediate reflector reached a stabilized efficiency of 11.15%. In summary, we implemented electrical interconnects based on self-patterned insulating NP layers with local openings into thin-film electronic devices. We demonstrated this by applying SiO2 NP layers as intermediate reflectors in thin-film a-Si/μc-Si tandem solar cells. Local openings in the NP layers were obtained during solution processing by employing the rough rear surface of a-Si cells to rupture the liquid film formed by the NP dispersion. We found that self-patterned SiO2 NP layers act as excellent intermediate reflectors, leading to strong light reflection, minimal parasitic absorption losses, and lateral shunt quenching while allowing for efficient vertical charge transport. Furthermore, the NP layers filled in the valleys of the textured a-Si cell surface and thus provided a smoothened template for the growth of the μc-Si cell, enhancing its electrical quality. It is noteworthy that the interconnection concept presented here with SiO2 NP layers can be readily adapted to other NP materials, offering a wide range of optical and electrical properties. Moreover, in addition to the randomly textured substrates utilized in this work, specifically engineered surface textures could be employed, enabling a large variety of applications. Finally, as the deposition as well as the patterning of the NP layers is achieved by a simple one-step solutionprocessing method, the electrical interconnects based on self-
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, the refractive index of the SiO2 NP layers, and the influence of the ZnO front electrode thickness and light-induced degradation on tandem cell performance. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: Bjorn.Niesen@epfl.ch. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mustapha Benkhaira (CSEM) for ZnO front contact deposition, Danièle Laub and Tomasz Płociński (EPFL CIME) for cross section TEM sample preparation, and Massoud Dadras and Mireille Leboeuf (CSEM) for their support during conductive AFM measurements. The authors gratefully acknowledge funding from the Velux Stiftung and the Swiss Federal Office of Energy under grant number SI/50075001. This publication is dedicated to Mona Klein.
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(21) Kraus, T.; Malaquin, L.; Schmid, H.; Riess, W.; Spencer, N. D.; Wolf, H. Nat. Nanotechnol. 2007, 2, 570−576. (22) Kim, M. R.; Im, S. H.; Kim, Y.-S.; Cho, K. Y. Adv. Funct. Mater. 2013, 23, 5700−5705. (23) Gao, J.; Pei, K.; Sun, T.; Wang, Y.; Zhang, L.; Peng, W.; Lin, Q.; Giersig, M.; Kempa, K.; Ren, Z.; Wang, Y. Small 2013, 9, 733−737. (24) Weste, N.; Harris, D. CMOS VLSI Design: A Circuits and Systems Perspective; Addison-Wesley Publishing Company: Boston, MA, 2010. (25) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 9966−9970. (26) Green, M. A.; Basore, P. A.; Chang, N.; Clugston, D.; Egan, R.; Evans, R.; Hogg, D.; Jarnason, S.; Keevers, M.; Lasswell, P.; O’Sullivan, J.; Schubert, U.; Turner, A.; Wenham, S. R.; Young, T. Sol. Energy 2004, 77, 857−863. (27) Steudel, S.; Myny, K.; Schols, S.; Vicca, P.; Smout, S.; Tripathi, A.; van der Putten, B.; van der Steen, J.-L.; van Neer, M.; Schütze, F.; Hild, O. R.; van Veenendaal, E.; van Lieshout, P.; van Mil, M.; Genoe, J.; Gelinck, G.; Heremans, P. Org. Electron. 2012, 13, 1729−1735. (28) Law, D. C.; King, R. R.; Yoon, H.; Archer, M. J.; Boca, A.; Fetzer, C. M.; Mesropian, S.; Isshiki, T.; Haddad, M.; Edmondson, K. M.; Bhusari, D.; Yen, J.; Sherif, R. A.; Atwater, H. A.; Karam, N. H. Sol. Energy Mater. Sol. Cells 2010, 94, 1314−1318. (29) Guha, S.; Yang, J.; Yan, B. Sol. Energy Mater. Sol. Cells 2013, 119, 1−11. (30) Meillaud, F.; Billet, A.; Battaglia, C.; Boccard, M.; Bugnon, G.; Cuony, P.; Charriere, M.; Despeisse, M.; Ding, L.; Escarre-palou, J.; Hanni, S.; Lofgren, L.; Nicolay, S.; Parascandolo, G.; Stuckelberger, M. E.; Ballif, C. IEEE J. Photovolt. 2012, 2, 236−240. (31) Matsui, T.; Sai, H.; Saito, K.; Kondo, M. Prog. Photovolt: Res. Appl. 2013, 21, 1363−1369. (32) Shah, A. Thin-film silicon solar cells; EPFL Press: Lausanne, 2010. (33) Kirner, S.; Calnan, S.; Gabriel, O.; Neubert, S.; Zelt, M.; Stannowski, B.; Rech, B.; Schlatmann, R. Phys. Status Solidi C 2012, 9, 2145−2148. (34) Bugnon, G.; Söderström, T.; Nicolay, S.; Ding, L.; Despeisse, M.; Hedler, A.; Eberhardt, J.; Wachtendorf, C.; Ballif, C. Sol. Energy Mater. Sol. Cells 2011, 95, 2161−2166. (35) Veneri, P. D.; Mercaldo, L. V.; Usatii, I. Prog. Photovolt: Res. Appl. 2013, 21, 148−155. (36) Cuony, P.; Alexander, D. T. L.; Perez-Wurfl, I.; Despeisse, M.; Bugnon, G.; Boccard, M.; Söderström, T.; Hessler-Wyser, A.; Hébert, C.; Ballif, C. Adv. Mater. 2012, 24, 1182−1186. (37) Lambertz, A.; Smirnov, V.; Merdzhanova, T.; Ding, K.; Haas, S.; Jost, G.; Schropp, R. E. I.; Finger, F.; Rau, U. Sol. Energy Mater. Sol. Cells 2013, 119, 134−143. (38) Boccard, M.; Battaglia, C.; Blondiaux, N.; Pugin, R.; Despeisse, M.; Ballif, C. Sol. Energy Mater. Sol. Cells 2013, 119, 12−17. (39) Sai, H.; Jia, H.; Kondo, M. J. Appl. Phys. 2010, 108, 044505. (40) Paetzold, U. W.; Zhang, W.; Prömpers, M.; Kirchhoff, J.; Merdzhanova, T.; Michard, S.; Carius, R.; Gordijn, A.; Meier, M. Mater. Sci. Eng., B 2013, 178, 617−622. (41) Boccard, M.; Battaglia, C.; Hänni, S.; Söderström, K.; Escarré, J.; Nicolay, S.; Meillaud, F.; Despeisse, M.; Ballif, C. Nano Lett. 2012, 12, 1344−1348. (42) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (43) Choy, T. C. Effective Medium Theory: Principles and Applications; Oxford University Press: New York, 1999. (44) Faÿ, S.; Steinhauser, J.; Oliveira, N.; Vallat-Sauvain, E.; Ballif, C. Thin Solid Films 2007, 515, 8558−8561. (45) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261− 302. (46) Bonn, D.; Eggers, J.; Indekeu, J.; Meunier, J.; Rolley, E. Rev. Mod. Phys. 2009, 81, 739−805. (47) Forsberg, P. S. H.; Priest, C.; Brinkmann, M.; Sedev, R.; Ralston, J. Langmuir 2009, 26, 860−865. (48) Stuckelberger, M.; Despeisse, M.; Bugnon, G.; Schüttauf, J.-W.; Haug, F.-J.; Ballif, C. J. Appl. Phys. 2013, 114, 154509.
(49) IEC (International Electrotechnical Commission). IEC 60904-3 Ed. 2: Photovoltaic devicesPart 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data; IEC: Geneva, 2006. (50) Fahr, S.; Rockstuhl, C.; Lederer, F. Appl. Phys. Lett. 2009, 95, 121105. (51) Feltrin, A.; Meguro, T.; Ichikawa, M.; Sezaki, F.; Yamamoto, K. J. Photon. Energy 2011, 1, 017003. (52) O’Brien, P. G.; Yang, Y.; Chutinan, A.; Mahtani, P.; Leong, K.; Puzzo, D. P.; Bonifacio, L. D.; Lin, C.-W.; Ozin, G. A.; Kherani, N. P. Sol. Energy Mater. Sol. Cells 2012, 102, 173−183. (53) Bugnon, G.; Parascandolo, G.; Söderström, T.; Cuony, P.; Despeisse, M.; Hänni, S.; Holovský, J.; Meillaud, F.; Ballif, C. Adv. Funct. Mater. 2012, 22, 3665−3671. (54) Nakajima, A.; Ichikawa, M.; Sawada, T.; Yoshimi, M.; Yamamoto, K. Jpn. J. Appl. Phys. 2004, 43, 7296−7302. (55) Bonnet-Eymard, M.; Boccard, M.; Bugnon, G.; Sculati-Meillaud, F.; Despeisse, M.; Ballif, C. Sol. Energy Mater. Sol. Cells 2013, 117, 120−125. (56) Merten, J.; Asensi, J. M.; Voz, C.; Shah, A. V.; Platz, R.; Andreu, J. IEEE Trans. Electron Devices 1998, 45, 423−429.
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Self-Patterned Nanoparticle Layers for Vertical Interconnects: Application in Tandem Solar Cells Bjoern Niesen,*,† Nicolas Blondiaux,‡ Mathieu Boccard,† Michael Stuckelberger,† Raphael̈ Pugin,‡ Emmanuel Scolan,‡ Fanny Meillaud,† Franz-Josef Haug,† Aïcha Hessler-Wyser,†,§ and Christophe Ballif†,‡ †
Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Institute of Microengineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71, CH-2000 Neuchâtel, Switzerland ‡ Centre Suisse d’Electronique et de Microtechnique (CSEM) SA, Rue Jaquet-Droz 1, CH-2002 Neuchâtel, Switzerland § Centre Interdisciplinaire de Microscopie Electronique (CIME), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: We demonstrate self-patterned insulating nanoparticle layers to define local electrical interconnects in thin-film electronic devices. We show this with thin-film silicon tandem solar cells, where we introduce between the two component cells a solution-processed SiO2 nanoparticle layer with local openings to allow for charge transport. Because of its low refractive index, high transparency, and smooth surface, the SiO2 nanoparticle layer acts as an excellent intermediate reflector allowing for efficient light management. KEYWORDS: Photovoltaics, micromorph, multijunction, planarization, dewetting, self-assembly
I
electrically interconnected by highly doped tunneling junctions. These interconnects are generally made from wide-bandgap materials to reduce parasitic light absorption.28 In tandem cells based on thin-film Si,29−32 electrical interconnection is typically provided by the so-called intermediate reflector layer located between the two component cells, which fulfills three additional functionalities.33−35 First, made from a material with a low refractive index, in contrast to the high-index Si material, the intermediate reflector layer reflects weakly absorbed light back into the wide-bandgap component cell. Second, intermediate reflectors of anisotropically conducting materials, such as Si filaments embedded in SiO x, were shown to quench recombination paths by suppressing lateral charge transport.36,37 Finally, it was recently demonstrated that intermediate reflectors can be used to tailor the interface roughness needed in thin-film Si solar cells for light scattering, improving the Si material quality of the component cell grown on top of the intermediate reflector.38 Self-patterned, electrically insulating NP layers with local openings are ideal candidates for intermediate reflectors, as by the choice of NP material and size, they can readily be made highly transparent, with a low refractive index, a smooth surface, and a low lateral conductance, while still allowing for efficient vertical interconnection of the two component cells. A schematic
n recent years, nanoparticle (NP) layers have found application in electronic and optoelectronic devices, in an effort to reduce cost and improve performance. For example, NP layers have been utilized as quantum emitters and absorbers, photonic crystals, and charge-transport layers, for plasmonic and photonic light trapping and for NP-assisted texturing.1−9 Nanoparticle single- and multilayers can readily be deposited by solution-processing methods, such as dip coating, Langmuir−Blodgett deposition, capillarity-assisted particle assembly, and spin coating.10−14 When solution-processed, the deposition of NP layers is governed mainly by the dynamics of the receding liquid contact line during the drying of the NP dispersion film.15,16 Therefore, NP deposition can be influenced by modifying the drying of the liquid film.16−18 Well-controlled, densely packed NP layers have been obtained by texturing the surface with nano- or microstructures that pin the receding liquid contact line.19−23 Interestingly, NP layers deposited on such textured surfaces typically do not cover the top of sharp surface features.22,23 These properties inspired us to realize vertical interconnects in thin-film electronic devices using self-patterned electrically insulating NP layers with local openings. Electrical interconnects are ubiquitous in thin-film electronics, including contacts to transistors in integrated circuits24 and large-area sensor arrays,25 connections between neighboring solar cells in thin-film photovoltaic modules,26 and interconnects between the back-plane circuitry and organic light-emitting diodes in displays.27 In high-efficiency tandem and multijunction solar cells, the component cells are © 2014 American Chemical Society
Received: May 13, 2014 Revised: July 30, 2014 Published: August 7, 2014 5085
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in the trade-off between efficient light trapping and a smoothened template for the growth of the μc-Si cell. Figure 1b shows a schematic drawing of the device architecture of a thin-film Si tandem solar cell in the micromorph p−i−n, or superstrate, configuration. These monolithic tandem cells consist of two component cells, electrically contacted in series: an a-Si cell, with an absorber layer bandgap of 1.7−1.8 eV to harvest visible light, and a μc-Si cell, which absorbs near-infrared light up to wavelengths of λ ∼ 1100 nm, corresponding to a bandgap of 1.1 eV. Both component cells are grown by plasma-enhanced chemical vapor deposition (PE-CVD). Transparent ZnO front and rear electrodes are utilized to electrically contact the tandem cell. The intermediate reflector, in our case a self-patterned insulating SiO2 NP layer with local openings for vertical charge transport, is located between the two component cells. The fabrication of the layer stack begins with the deposition of the transparent ZnO front contact on a glass substrate, through which the solar cell is illuminated. The deposition technique employed for this ZnO layer, low-pressure CVD, yields a rough surface with random pyramidal surface features in the 0.1−1 μm range, as shown by the top-view scanning electron microscopy (SEM) images in Figure 1c for several ZnO layer thicknesses. Such surface features, with sizes comparable to visible−near-infrared wavelengths, are very efficient light scatterers, as shown by the haze spectra in Figure 1d, and enhance light absorption and hence photocurrent in the solar cell by light trapping.39−41 The size of the surface features increases with the ZnO layer thickness, such that thicker layers scatter long-wavelength light more efficiently. Before implementing SiO2 NP layers in thin-film Si solar cells, we assessed their optical and morphological properties. For this purpose, we deposited SiO2 NPs on a polished Si wafer by means of spin coating. As shown in Figure 2a, the NPs form
representation of such a NP-based intermediate reflector is shown in Figure 1a.
Figure 1. (a) Schematic representation of a self-patterned insulating NP intermediate reflector layer with local openings for electrical interconnection between the two component cells in a tandem device. (b) Schematic representation of the thin-film p−i−n a-Si/μc-Si tandem solar cell device architecture. Light enters the cell through the glass substrate. (c) SEM images of ZnO layers with thicknesses from 2.5 to 5 μm. The scale bar corresponds to 2 μm. (d) Haze spectra, defined as the ratio of diffuse transmission to total transmission, of ZnO layers with thicknesses from 2.5 to 5 μm.
Figure 2. (a) SEM image of a SiO2 NP layer deposited on a polished Si wafer by spin coating with its 2D autocorrelation function (inset). The scale bars correspond to 100 nm. (b) Particle size distribution histogram, obtained from 596 particles.
Here, we utilize self-patterned insulating NP layers with local openings to define vertical electrical interconnects in thin-film electronic devices. We demonstrate this with a solutionprocessed SiO2 NP layer that acts as an efficient intermediate reflector in thin-film amorphous silicon (a-Si)/microcrystalline silicon (μc-Si) tandem solar cells due to its high transparency and low refractive index. The a-Si cell features a highly textured rear surface, which we employ as a template for self-patterning the NP layer during solution processing. Rupture and dewetting of the insulating SiO2 NP film around the peaks of the texture open local point contacts for efficient vertical electrical interconnection. In contrast, lateral shunt paths are quenched by the low lateral conductivity of the SiO2 NP layer. Moreover, we tune the thickness of the SiO2 NP layer to find an optimum
a dense multilayer with a smooth surface. From this SEM image, an average SiO2 NP size of 38.7 ± 5.4 nm was found (Figure 2b). The NP film exhibits near-range order without long-range periodicity, resulting in a 2D autocorrelation function that shows rings with spacing equal to the NP size, in agreement with a closely packed NP layer (inset to Figure 2a). Moreover, the SiO2 NP layer did not show any significant diffuse reflectance, indicating negligibly weak light scattering, as expected42 for dielectric NPs with a size ≪ λ. The NP layer thus optically behaves like a uniform thin film. According to the effective medium approximation,43 a closely packed multilayer 5086
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Figure 3. Schematic representation and SEM images of (a) a 230-nm-thick a-Si layer deposited on 5-μm-thick low-pressure CVD ZnO, and the same layer stack coated with a (b) thin or (c) thick SiO2 NP layer. SEM images on top row: sample tilted by 20°, scale bar corresponds to 500 nm; bottom row: plan view, scale bar corresponds to 1 μm.
of SiO2 spheres with a refractive index n = 1.45 possesses an effective refractive index neff = 1.33. This value matches the experimentally measured n = 1.33 ± 0.02 of the NP layer (Figure S1 in the Supporting Information), confirming the close packing. Notably, this n value is below that of dense thin films of low-index materials commonly used for optical applications, such as MgF with n = 1.37, making the SiO2 NP layer suitable for index-contrast applications, as demonstrated here for the SiO2 NP layer in combination with the high-index material Si (n ≈ 4 at λ = 600 nm). Moreover, the SiO2 NP layer was found to be highly transparent throughout the whole wavelength range of interest (300 nm < λ < 1200 nm), as evidenced by the imaginary part of the refractive index k ≈ 0 shown in Figure S1. Both the low refractive index and the high transparency are strong advantages of the SiO2 NP layer compared to conventional intermediate reflectors, which consist of conductive oxide layers deposited by sputtering or CVD.33−36 The conducting nature of such layers leads to parasitic light absorption due to free charge carriers.44 In addition, these intermediate reflectors have relatively high refractive indices, typically above 1.8 at λ = 600 nm, which results in a limited index mismatch between the intermediate reflector and the Si absorber layers and therefore in a less efficient reflection. Next, we spin coated the SiO2 NPs onto the a-Si cell to form the self-patterned SiO2 NP intermediate reflector. The pyramidal surface features of the ZnO front electrode are reproduced on the rear surface of the conformally grown a-Si layer, as shown in Figure 3a. The coating behavior of the SiO2 NPs on this textured surface strongly differs from that on the flat Si wafer (Figure 3b and c). Instead of completely covering the substrate, a dense SiO2 NP multilayer is formed in the valleys, whereas the peaks of the pyramids remain uncovered. By varying the spin coating conditions, the thickness of the SiO2 NP layer can be adjusted either to cover only the lowestlying areas of the surface or to fill up larger valleys, such that smaller surface features are completely coated. In the remainder of this article, the former will be referred to as thin SiO2 NP layer (covering ∼36% of the surface area, Figure 3b), whereas the latter will be referred to as thick SiO2 NP layer (covering ∼75% of the surface area, Figure 3c). We employed conductive atomic force microscopy (AFM) on the thick SiO2 NP layer to confirm this valley-filling behavior. As shown in Figure 4, only the tips of the SiO2-NP-coated a-Si cell surface exhibit a high electrical conductance, whereas the valleys are highly resistive with a ten times lower AFM tip−sample current, confirming the
Figure 4. 10 × 10 μm conductive AFM image of the thick SiO2 NP layer on an a-Si (230 nm thick)/ZnO (5 μm thick) layer stack. The color scale indicates the current flow between the AFM tip and the sample. This combined morphology/current flow 3D graph demonstrates the high contrast in electrical conductance between the conductive, uncoated a-Si pyramid tips and the highly resistive, dense SiO2 NP layer.
selective coating of the a-Si surface and efficient electrical insulation by the dense SiO2 NP layer. This self-patterning effect is achieved by exploiting the liquid dynamics of the SiO2 NP dispersion on the rough a-Si surface while spin coating: First, the SiO2 NP dispersion spreads across the entire substrate, as confirmed by an advancing contact angle of 74° (measured from a drop of the SiO2 NP dispersion on a smooth a-Si layer). Then, the liquid volume is reduced due to droplet ejection by centrifugal forces and solvent evaporation, which becomes the dominating mechanism once the liquid film is sufficiently thin.16 When the liquid film thickness becomes comparable to the height of the surface features, the sharp tips of the pyramids act as nucleation points, which initiate the rupture and dewetting of the liquid film.45,46 Once the liquid film has dewetted from the pyramid tips, they can also act as pinning points for the liquid contact line, enhancing the planarization of the liquid film.22,47 The combination of dewetting and planarization directs the NPs toward the bottom of the valleys, which leads to a smoothening of the NP layer surface and the formation of openings around the peaks of the surface features. Liquid film rupture and dewetting at sharp surface features have also been reported for polymer solutions on corrugated Si wafers, albeit on a much smaller length scale than demonstrated here for SiO2 NPs.45 We then completed the thin-film a-Si/μc-Si tandem cell layer stack by depositing the μc-Si cell on top of the self-patterned 5087
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We utilized ZnO front electrodes with thicknesses of 2.5, 3.5, or 5 μm. As elucidated in the Supporting Information, we found that a 3.5-μm-thick ZnO front electrode offers sufficient flexibility when adjusting the SiO2 NP layer thickness while still allowing for a high electrical quality of the tandem cell. In short, for a 2.5-μm-thick ZnO front electrode, complete coverage of the a-Si surface was observed at low spin speeds, and for a 5-μm-thick ZnO front electrode, the electrical quality of the μc-Si layers was significantly reduced. Therefore, we will focus the following detailed discussion on solar cells made with a 3.5-μm-thick ZnO front electrode, whose device characteristics, including external quantum efficiency (EQE) and current density−voltage (J(V)) characteristic, are shown in Figure 6 and summarized in Table 1. The optimization of an intermediate reflector for thin-film Si solar cells involves a delicate interplay between electrical and optical effects. In the following paragraphs, the influence of the SiO2 NP intermediate reflector on light absorption and photocurrent generation within the tandem solar cell will be presented first, followed by its effect on the electrical properties of the tandem cell. Finally, combining both aspects, the optimization of the overall device efficiency will be discussed. One of the main purposes of an intermediate reflector is the enhancement of the a-Si cell photocurrent by partially reflecting nonabsorbed light. That way, only a thin a-Si cell is required, which minimizes its light-induced degradation.48 In order to individually determine the photocurrent generated in each component cell, we employed EQE measurements, shown in Figure 6a. The EQE spectra of the a-Si and μc-Si component cells were measured individually, using near-infrared and blue bias light, respectively, to generate a high photocurrent in the component cell not being measured. The current densities of
SiO2 NP layer, followed by the ZnO rear contact. A crosssectional SEM image of such a tandem cell is shown in Figure 5. The nonconformal SiO2 NP layer leads to a partial
Figure 5. SEM image of a polished cross section of a thin-film a-Si/μcSi tandem cell with a thick SiO2 NP intermediate reflector. The scale bar corresponds to 1 μm. The inset shows a detailed view of the SiO2 NP layer, with a scale bar that corresponds to 200 nm.
planarization of the μc-Si cell. Moreover, direct contact points between the a-Si and μc-Si cells are visible at the uncovered peaks of the a-Si surface features. Therefore, the SiO2 NP layer allows for a decoupling of the two component cell morphologies while still enabling direct electrical contact between the component cells. In addition, the inset to Figure 5 demonstrates that the dense packing of the SiO2 NP layer is preserved after the deposition of the μc-Si and ZnO layers. We fabricated and characterized thin-film Si tandem cells with either a thin or thick SiO2 NP intermediate reflector and tandem cells without an intermediate reflector as references.
Figure 6. Device characteristics of a-Si (230-nm-thick intrinsic layer)/μc-Si (1.6-μm-thick intrinsic layer) tandem cells without an intermediate reflector (black), with a thin SiO2 NP layer (blue), or with a thick SiO2 NP layer (red). (a) EQE and 1 − total reflectance spectra. The dashed curves represent the summed EQE spectra of both component cells. (b) J(V) characteristics, measured under AM1.5g irradiance at 1 sun intensity. (c) Fill factor and (d) power conversion efficiency as a function of component cell current mismatch (Ja‑Si − Jμc‑Si), obtained by adding either blue or nearinfrared light to the AM1.5g irradiance spectrum. The crosses indicate the values obtained at AM1.5g irradiance at 1 sun. 5088
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Table 1. Device Parameters of Thin-Film Si Tandem Solar Cells with and without a SiO2 NP Intermediate Reflector Layer, Extracted from the Measurements Shown in Figure 6a no intermediate reflector thin SiO2 NP layer thick SiO2 NP layer
Ja‑Si (mA cm−2)
Jμc‑Si (mA cm−2)
Ja‑Si − Jμc‑Si (mA cm−2)
Voc (mV)
FF (%)
Rsc (Ω cm2)
Roc (Ω cm2)
η (%)
12.00 13.25 14.24
13.86 12.33 10.36
−1.86 0.92 3.88
1358 1363 1381
70.1 73.8 79.8
2800 69300 44900
10.1 10.0 10.2
11.42 12.40 11.42
a
Ja‑Si and Jμc‑Si were obtained from EQE measurements; Voc, FF, Rsc, and Roc values were obtained from J(V) characteristics. For each tandem device, the current of the limiting component cell utilized to calculate the power conversion efficiency (η = Jlimiting cellVocFF/1000 W m−2) is highlighted.
both component cells, Ja‑Si and Jμc‑Si, were then calculated by convolution of the measured EQE spectra with the air mass 1.5 global (AM1.5g) solar irradiance spectrum. The AM1.5g spectrum is equivalent to the solar irradiance at a solar zenith angle of 48.2° and is a commonly used standard spectrum when measuring solar cell performance.49 Ja‑Si increases from 12.00 mA cm−2 without an intermediate reflector to 13.25 and 14.24 mA cm−2 with a thin and a thick SiO2 NP layer, respectively, confirming that the SiO2 NP layers act as efficient intermediate reflectors. The effect of the SiO2 NP layers on the photocurrent distribution between the two component cells is directly visible in the EQE spectra: The a-Si cell EQE is strongly increased by the SiO2 NP layers between λ = 500−800 nm, which is the wavelength range where the a-Si cell does not absorb all the light within a single pass. At the same time, the μc-Si cell EQE is reduced by the SiO2 NP intermediate reflectors. Also, it is clearly visible how the a-Si cell EQE gain increases with the SiO2 NP layer thickness. This can be explained by the higher surface coverage of the a-Si cell surface for the thicker SiO2 NP layer. While the summed EQE (EQEa‑Si + EQEμc‑Si) curves of the tandem cell without an intermediate reflector and that with a thin SiO2 NP layer are nearly identical, the summed EQE is slightly lower at λ > 600 nm for the tandem cell with a thick SiO2 NP layer. This is due to light that is reflected by the intermediate reflector and then lost through the front electrode, as well as the smoothening effect of the thick SiO2 NP layer, which leads to less efficient light trapping in the μc-Si cell in the near-infrared. As a result, the tandem cell with a thick SiO2 NP layer also shows a higher total reflectance in this wavelength range. Recently, spectrally selective intermediate reflectors based on nanostructures such as metal nanoparticles50,51 and photonic crystals2,52 have been reported. These selective reflectors strongly reflect light in the spectral range where the incident light is not completely absorbed by the a-Si cell (550 nm < λ < 800 nm) and efficiently transmit light at wavelengths above the a-Si absorption edge (λ > 800 nm). As a result, reflection losses in the near-infrared are reduced. Therefore, compared to the thick self-patterned SiO2 NP layer, which leads to reflection losses, spectrally selective intermediate reflectors could potentially improve the summed EQE. In contrast, as the thin SiO2 NP layer does not cause substantial reflection losses, only marginal improvements could be expected with spectrally selective reflectors. Moreover, for metal-nanoparticle-based intermediate reflectors, the beneficial effects might be outweighed by parasitic absorption.51 An important feature of the SiO2 NP layer is the smoothening of the a-Si/μc-Si cell interface due to the valleyfilling effect. As a result, the μc-Si cell is grown on a smoother substrate, which is desirable as μc-Si material quality is highly sensitive to substrate morphology.53 Rough substrates result in defective μc-Si layers, reducing the μc-Si cell’s electrical quality, as evidenced by a decrease in open-circuit voltage (Voc). The
SiO2 NP intermediate reflectors lead to an increased Voc, confirming the enhanced electrical performance upon surface smoothening. As expected, the Voc enhancement is stronger for the thicker SiO2 NP layer, which results in the smoothest interface. In addition, we found that the thick SiO2 NP layer makes the Voc nearly independent of the ZnO front electrode roughness, as discussed further in the Supporting Information. Smoothening layers as intermediate reflectors for thin-film Si tandem cells have been proposed before.38 However, these layers were based on complicated and time-consuming multistep protocols, in contrast to the simple one-step preparation of the self-patterned SiO2 NP intermediate reflectors. Typically, the fill factor (FF) of a thin-film Si tandem solar cell is higher when the μc-Si cell is current limiting (Ja‑Si − Jμc‑Si > 0), which is due mainly to the better collection efficiency of μc-Si cells as compared to a-Si cells.54,55 Therefore, as shown in Figure 6b and Table 1, the FF is highest for the tandem cell with the thick SiO2 NP layer, whose current is strongly limited by the μc-Si cell, whereas it is lowest for the tandem cell without an intermediate reflector with a current that is limited by the a-Si cell (Ja‑Si − Jμc‑Si < 0). Closely linked to the FF are the open-circuit resistance (Roc) and short-circuit resistance (Rsc), which can be calculated from the slope of J(V) curves, at Voc and 0 V, respectively (Table 1). A high Roc reflects resistive losses, whereas a low Rsc indicates the presence of shunt paths for state-of-the-art thin-film Si cells.56 In addition, Roc is increased and Rsc reduced in devices with inefficient charge collection. The fact that Roc is not significantly influenced by the presence of a SiO2 NP intermediate reflector indicates that the openings through the NP layer are dense enough even for the thick SiO2 NP layer to allow for efficient charge transport and charge carrier collection. The increased Rsc in the presence of a SiO2 NP layer could indicate a passivation of highly conductive shunt paths but could also be caused by reduced recombination losses. In order to directly compare FF values between tandem cells that exhibit different component cell current mismatches we gradually tuned the Ja‑Si − Jμc‑Si mismatch of each tandem cell by adding blue or red light to the AM1.5g solar irradiation spectrum while measuring J(V) characteristics.55 In a blue-rich spectrum, Ja‑Si − Jμc‑Si will increase, whereas it will decrease in a red-rich spectrum. The FF versus Ja‑Si − Jμc‑Si curves resulting from these measurements are shown in Figure 6c. The crosses indicate the current mismatch under AM1.5g solar irradiance conditions for each tandem cell. Interestingly, both tandem cells containing a SiO2 NP intermediate reflector exhibit nearly identical FF curves, whereas the cell without an intermediate reflector consistently shows ∼2% lower FF values. These results demonstrate that the SiO2 NP intermediate reflectors enhance the FF independent of the current mismatch. Moreover, the tandem cells with a SiO2 NP layer showed higher Voc values at low illumination: At 0.004 sun illumination, the Voc of the cell 5089
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Letter
patterned NP layers presented here are suitable for large-area, high-throughput applications.
without a SiO2 NP intermediate reflector was 660 mV, compared to 880 mV and 900 mV for the cells with a thin and a thick SiO2 NP layer, respectively. As the presence of shunt paths strongly reduces Voc at low illumination, these results suggest that the SiO2 NP layers indeed act as shuntquenching layers. From these current mismatch measurements, it is also possible to determine the optimal mismatch to maximize the power conversion efficiency (η), as shown in Figure 6d. A tandem cell with a current that is strongly limited by the μc-Si cell might be desirable to reach a high FF, but at the same time, the short-circuit current density decreases with increasing current mismatch as the current-limiting component cell determines the current flow through the tandem cell. This trade-off leads to an optimal Ja‑Si − Jμc‑Si of 1−1.5 mA cm−2. Under AM1.5g irradiance, the current of the tandem cell without an intermediate reflector is too strongly limited by the a-Si cell, whereas the current of the cell with the thick SiO2 NP layer is too strongly limited by the μc-Si cell. Both tandem cells show an efficiency of 11.42%. The efficiency−current mismatch curve of the cell with the thick SiO2 NP layer lies below that of the cell without an intermediate reflector due to reduced light trapping. In contrast, the tandem cell with the thin SiO2 NP layer shows an enhanced η = 12.40%, with an improved efficiency−current mismatch curve due to the combination of the enhanced FF and Voc values and only marginal losses in light trapping compared to the cell without an intermediate reflector. In addition, its Ja‑Si − Jμc‑Si mismatch under AM1.5g conditions is optimal to reach a maximized efficiency. Therefore, the thin SiO2 NP layer represents the best choice for this a-Si/μc-Si cell combination. Finally, we assessed the influence of the SiO2 NP layers on the light-induced degradation of thin-film Si tandem cells. For this purpose, we protected the cells with an epoxy/glass encapsulation and exposed them to an AM1.5g light spectrum with 1 sun intensity at 50 °C. The presence of the SiO2 NP intermediate reflector did not alter the relative degradation, as shown by Figure S4 in the Supporting Information. After 1000 h of light soaking, the optimized cell with the thin SiO2 NP intermediate reflector reached a stabilized efficiency of 11.15%. In summary, we implemented electrical interconnects based on self-patterned insulating NP layers with local openings into thin-film electronic devices. We demonstrated this by applying SiO2 NP layers as intermediate reflectors in thin-film a-Si/μc-Si tandem solar cells. Local openings in the NP layers were obtained during solution processing by employing the rough rear surface of a-Si cells to rupture the liquid film formed by the NP dispersion. We found that self-patterned SiO2 NP layers act as excellent intermediate reflectors, leading to strong light reflection, minimal parasitic absorption losses, and lateral shunt quenching while allowing for efficient vertical charge transport. Furthermore, the NP layers filled in the valleys of the textured a-Si cell surface and thus provided a smoothened template for the growth of the μc-Si cell, enhancing its electrical quality. It is noteworthy that the interconnection concept presented here with SiO2 NP layers can be readily adapted to other NP materials, offering a wide range of optical and electrical properties. Moreover, in addition to the randomly textured substrates utilized in this work, specifically engineered surface textures could be employed, enabling a large variety of applications. Finally, as the deposition as well as the patterning of the NP layers is achieved by a simple one-step solutionprocessing method, the electrical interconnects based on self-
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, the refractive index of the SiO2 NP layers, and the influence of the ZnO front electrode thickness and light-induced degradation on tandem cell performance. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: Bjorn.Niesen@epfl.ch. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mustapha Benkhaira (CSEM) for ZnO front contact deposition, Danièle Laub and Tomasz Płociński (EPFL CIME) for cross section TEM sample preparation, and Massoud Dadras and Mireille Leboeuf (CSEM) for their support during conductive AFM measurements. The authors gratefully acknowledge funding from the Velux Stiftung and the Swiss Federal Office of Energy under grant number SI/50075001. This publication is dedicated to Mona Klein.
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