Plasmonic dye-sensitized solar cells through collapsible gold nanofingers

The demands for clean, renewable energy sources have become one of the mostly urgent issues for our society. Utilization of solar energy through photovoltaic technology is a solution. A variety of photovoltaic devices are developed in past years, such as, silicon solar cells, dye-sensitized solar cells (DSSCs), perovskite solar cells etc [1–6]. In order to improve the performance of solar cells, the new techniques which mainly include structural design, new materials, efficiently and highly stable dyes, have been developed to improve the light trapping and power conversion efficiency (PCE) further [7–11]. Among them, plasmonic solar cells are an attractive technology because plasmonic nanostructures at resonant excitations can efficiently trap the incident light within nanoscale space and form an extremely strong local electromagnetic (EM) field near the surface of nanostructures, and thus efficiently enhance the Sunlight absorption [12–14]. DSSCs contains photoanode, counter electrode, electrolyte and dyes in which dye-sensitized photoanode absorbs sunlight to generate electrons. At present, plasmonic nanoparticles are broadly investigated to design photoanode of DSSCs so that the extinction of dye can be mostly increased, leading to a high PCE [15–18]. However, the distribution of metal nanoparticles in TiO₂ network is not uniform due to the reproducibility in fabrication over a large area. Therefore a design of ordered plasmonic nanostructure is necessary.

Metallic gap-structures with gap-plasmon resonance can produce an extremely strong EM field within a sub-nanometer gap that is decided by both the classical EM theory and quantum mechanics [19–26]. For example, the aggregates of metal nanoparticles could create nanometer junctions with maximum enhancement of (E/E_in )² up to 10⁶ fold [27, 28]. However, to achieve the optimized enhancement decided by quantum plasmons, the fabrication of ordered nanostructures with atomic precision of tunable gap is still a challenge. Most conventional methods including electron-beam lithography and physical etching for creating sub-nanometer gap are constrained by the lithography resolution limit [29–35]. Thus, precise tailoring of the gap size at sub-nanometer scale over a large area is critical to form strong and reproducible enhanced EM field. On the other hand, emission quenching of dyes is associated with the direct contact between the dye and the metal, which is decided by the separation distance between the metal and the dye [17, 18]. It has been demonstrated that the optimal separation distance is at sub-5 nm scale [36, 37]. Therefore, the construction of ordered plasmonic nanostructures with high EM field enhancements and the control of dye-metal distance are very important to enhance the efficiency of plasmonic DSSCs. Previously we demonstrated the collapsible nanofingers that show strong EM field enhancement by precisely controlling the gap size at the sub-nanometer scale. This collapsible nanofingers could be fabricated over large area by nanoimprint lithography (NIL), reactive-ion etching (RIE) and atomic-layer deposition (ALD) [24]. The flexible Au nanofinger matrix have a high aspect ratio, and each nanofinger is composed of a polymer nanopillar capped with an Au nanodisk. The subsequent dielectric layer coating on the nanofingers and collapse process can realize the precisely tuning of gap size by twice the thickness of dielectric layer.

In present work, the collapsible nanofingers are studied to design a photoanode in DSSCs, which can precisely control the gap size and the dye-metal distance, achieving high reliability and high throughput at low cost [24, 38–41]. First, a thin conformal Al₂O₃ layer of 2.5 nm is deposited uniformly by ALD onto flexible Au nanofingers. Through exposing them to ethanol, a 5 nm nanogap is formed under capillary force [24]. Second, the collapsed Au/Al₂O₃/Au nanofingers are further deposited with a 2 nm TiO₂ film as the photoanode. The configuration of plasmonic DSSCs containing both dye and electrodes is shown in figure 1(a), in which the electrolyte is typically I⁻/I⁻³. Upon the absorption of sunlight, the dye injects an electron into the TiO₂ network. The electron travels through this network until it is collected. The resulting dye radical is reduced to its ground state by a redox process of I⁻/I³⁻, as shown in figure 1(b) [17]. In this plasmonic DSSCs, the resulting extremely strong local EM field through collapsed Au/Al₂O₃/Au nanofingers can improve the absorption of incident sunlight and the extinction of dyes simultaneously, which leads to a high enhancement of PCE. The results show that the PCE is mostly improved as compared to the normal photoanode of Au/Al₂O₃/TiO₂. This new structure with low cost is also applicable to other types of solar cells.

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2.1. Fabrication of Nanofingers array and DSSCs assembly

2.2. SEM and fluorescence measurements

2.3. DSSCs solar cell data measurements

Plasmonic DSSCs based on the collapsed Au nanofingers are mainly described in the experimental sections. In the collapsed Au/Al₂O₃/Au nanofingers, the gap size is precisely defined by twice the Al₂O₃ thickness, which is 5 nm. Since the tunneling barrier height is 2.52 eV that is defined by the difference between the Fermi level of Au (5.1 eV) and the electron affinity of Al₂O₃ layer (2.58 eV), the strongest field enhancement occurs at a 2 nm Al₂O₃ dielectric gap determined by classical EM theory and quantum mechanics [24]. However, by considering fluorescence quenching, we have demonstrated that 5 nm Al₂O₃ dielectric gap is the optimal distance [47]. Thus, a 2.5 nm Al₂O₃ dielectric film is deposited here to realize 5 nm nanogap. It is noted that a 2 nm amorphous TiO₂ layer is successfully proved to afford efficient electron transport in DSSCs and thus 2 nm TiO₂ layer is selected [17]. Furthermore, since the Al₂O₃ layer has the lower permittivity than TiO₂ film, thicker high permittivity TiO₂ film will lead the EM field to be much more confined inside of the dielectric gap, which is not useful to the extinction of dyes. In addition, Al₂O₃ layer can provide a higher tunneling barrier and a stronger screening for the radiation field than TiO₂, which leads to strong fluorescence enhancement for a dye [47, 48]. Therefore, we combine a 2.5 nm Al₂O₃ layer with a 2 nm TiO₂ layer to construct the photoanode in which the 2 nm amorphous TiO₂ layer only affords efficient electron transport network in DSSCs.

Figure 2(a) shows a scanning electron microscopy (SEM) image of 2.5 nm Al₂O₃ coated Au nanofingers prior to the collapsing process. The diameter and height of each nanofinger are 70 nm and 300 nm, and the lattice spacing is 130 nm. The lattice spacing and Au cap size can be adjusted by the designed mold with atomic precision, which is fabricated by double interference lithography. Through the collapsing process, the nanofingers with the height of nearly 300 nm tend to form tetramer Au/Al₂O₃/Au nanostructures. The coverage of Al₂O₃ film on the Au tip of nanofinger was demonstrated by cross-sectional TEM in our previous result [24]. Based on this tetramer Au/Al₂O₃/Au nanofingers, a 2 nm TiO₂ layer is coated on their surface, as shown in figure 2(b). In addition, some nanofingers at a small area with the defects of remaining a single nanofinger have not collapsed due to the uniformity of RIE etching process, as shown in figure 2(b). However, for such large area, the effect of these defects on the performance of DSSCs are very small. To study the optical properties of collapsed nanofingers, the reflectance spectra are measured, as shown in figure 2(c). The results show that the collapsed Au/Al₂O₃/Au nanofingers have a resonance band centered at approximately 660 nm, which can be ascribed to the bonding mode of the localized surface plasmon resonance (LSPR) of Au caps coupling each other to form bonding-LSPR [40], as shown by the black line in figure 2(c). After the deposition of TiO₂ film, the TiO₂–Au/Al₂O₃/Au nanostructures show a little redshift and is centered approximately at 670 nm, as shown by the red lines in figure 2(c). This little difference in the central wavelength for the coupled bonding-LSPR is ascribed to high index of TiO₂ films. The resonance can cover a broadband range from ∼550 to ∼800 nm which is consistent with energy spectra of sunlight. For comparison, the photoanode of Au/Al₂O₃/TiO₂ films on the plain glass has small response to the spectra range for the Sunlight, as shown by the blue line in figure 2(c). Therefore, the plasmonic nanostructure can induce scattering and absorption which can enhance the Sunlight absorption. To quantitatively demonstrate EM field enhancements in collapsed nanofingers, we run simulations for near-filed distribution and localized EM field enhancement using a commercial finite-element method-based software package (COMSOL Multiphysics). The permittivity of Au is interpolated from the experimental data [49], and the refractive index of the polymer support is set as 1.48 [39]. The 660 nm plane waves that is at the resonance condition is polarized parallel to the gap direction, which corresponds to bonging-LSPR [50]. The cross-sectional distribution of coupled EM field for the tetramer Au/Al₂O₃/Au nanofingers is depicted in figure 2(d). It shows that the strong EM field between the two approaching Au/Al₂O₃ nanofingers is ascribed to hybridization of the dipole resonances into the bonding-dipole plasmon modes. In this case, the local electric field enhancement ((E/E_inc )²), reaches up to ∼10⁴. In addition, at the non-resonance of 540 nm due to the absorption of N719 dye, the local EM field enhancement ((E/E_inc )²) at the dielectric area is ∼100-fold due to the coupling of surface charge as shown in figure S2 (supporting information) (available online at stacks.iop.org/NANO/32/355301/mmedia). Therefore, by plasmonic nanofingers, the incident photons efficiency can be improved, and the charge carrier density for the sensitizing dyes can be further boosted. It is noted that in our design, since the tunneling barrier heights for Au/Al₂O₃ and Au/TiO₂ are 2.58 eV and 0.9 eV, respectively, and thus the near-field EM enhancement to dyes emissions process dominates the electrons injection to TiO₂ network, and hot electrons injection from Au to TiO₂ network is not major contribution [48].

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To investigate the performance of plasmonic DSSCs, we carried out the measurements. A full list of the solar cells data is shown in table S1 (supporting information). Figure 3(a) shows the picture of assembled DSSCs, in which the red one represents plasmonic DSSCs based on the collapsed Au nanofingers and the blue one is DSSCs based on the photoanode of Au/Al₂O₃/TiO₂ films (noted as normal DSSCs) for comparison. The current of the device can be measured when it is electrical biased and sunlight illuminated [51]. During the current density–voltage (J–V) measurements, the active area of DSSCs is set to 0.25 cm². The J–V characteristics of DSSCs are measured under sunlight conditions (AM1.5 G, 100 mW cm⁻², with a xenon lamp light source). Figure 3(b) shows the J–V curves of DSSCs. To clearly observe, the curve in the green dotted square region in figure 3(b) is zoomed in and shown in figure 3(c). The fluctuation of J–V curve for normal DSSCs may due to the quality of the deposited film. It is clearly seen that the open circuit voltage (V_oc ) of plasmonic DSSCs increases from 0.50 to 0.73 V, and the J_sc increases from 0.058 mA cm⁻² to 0.136 mA cm⁻², as compared to normal DSSCs. Further, for our plasmonic DSSCs, the PCE improves from 0.01% to 0.03% (as shown in table S1), and the maximum power output (P_max) enhances from 2.7 μw to 6.5 μw as shown in figure 3(d). The enhancements of PCE and the P_max are consistent with the trend of the J_sc improvement for plasmonic DSSCs. In our design, since TiO₂ layer is directly deposited on the collapsed nanofingers, the sensitized surface area is not increased by compared to normal DSSCs and the enhancement of J_sc in plasmonic DSSCs is mainly due to the improvement of sunlight absorption. Therefore, the improvements of the PCE and P_max are mainly induced by plasmonic nanofingers.

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However, as shown in figure 3(d), the fill factor (FF) for plasmonic DSSCs decreases from 40 to 26 for comparison to normal DSSCs. The FF is mostly dependent on the separation of electron and hole from a dye under the excitation. The photo-excited electrons rapidly transferring into the TiO₂ network will cause a high FF. In our plasmonic photoanode of TiO₂–Au/Al₂O₃/Au nanofingers, since the small dye molecules are mostly trapped on the top surface of nanofingers, the electron does not easily transfer into the feedthrough in photoanode. In addition, the polymer base has lower conductivity while the conductive wire connects to the photoanode to export the current from the Al₂O₃ coated polymer, which is confirmed by the measurement of high resistance at I_sc (short circuit current), as shown in table S1. Furthermore, the dark current measurement also proves this high resistance, as shown in figure S1. These factors decrease the separation of electron and hole from the dyes and cause a relative low FF. Here, we note that though the performance of our plasmonic DSSCs has an obvious improvement, the PCE of solar cell is still small as compared to the published data [17], which can be ascribed to the simple assembly of the solar cells. In our assembly, the conductive wire directly connects to the TiO₂ layer of photoanode, and thus causing low conductivity. By optimizing the fabrication flow, for example, using ITO as base and precoating an Au layer on the polymer base, the performance can be further improved. Except for low conductivity of photoanode, as compared to DSSCs with thicker porous TiO₂ films, our deposited TiO₂ film is only 2 nm by the consideration of the incident light to excite the LSPR mode of Au nanofinger. The load of dye on this ultrathin TiO₂ film is very low, which make the PCE lower. It is also consistent with the previous report for DSSCs with thinner amorphous TiO₂ films [17]. Here, we only consider a proof of concept for our coupled nanofingers in the application of plasmonic solar cells.

In order to further verify plasmon induced DSSCs performance enhancement, we investigate the fluorescence enhancement of N719 dye on the nanofingers because the fluorescence enhancement is a direct evidence for the absorption enhancement in plasmonic DSSCs [37, 52]. In the experiment, the photoluminescence (PL) of the N719 dye on the 2 nm TiO₂ film coated Au/Al₂O₃/Au nanofingers which is the same as the photoanode of plasmonic DSSCs, and the Au/Al₂O₃/TiO₂ films on the plain glass are measured, as shown in figure 4. The emission peak position of N719 dye almost does not shift and only the fluorescence intensity increases. The results show that the PL enhancement of N719 dye on the plasmonic nanofingers is nearly 10 times higher than the normal photoanode, which is the main reason for the strong plasmonic EM field enhancement. In addition, the LSPR position is only partially overlapped with the emission peak position of N719 dye. By the optimization between the plasmonic resonance and the emission band of dyes, the efficiency of DSSCs can be further improved.

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In summary, we have demonstrated the plasmon enhanced DSSCs through coupled nanofingers. In plasmonic DSSCs, considering the local field enhancement and dye quenching which is both dependent on the separation distance, the collapsed Au/Al₂O₃/Au nanofingers with 5 nm Al₂O₃ gap are deposited with 2 nm TiO₂ as photoanode to enhance the absorption of sunlight. The results show that, for our designed plasmonic DSSCs, the PCE can be mostly improved, and the maximum power output enhances from 2.7 to 6.5 μW as compared to that based on the photoanode of Au/Al₂O₃/TiO₂ films. The improvement of plasmonic DSSCs performance can be ascribed to the coupled EM field enhancement within the sub-nanometer gap. In addition, fluorescence enhancement of N719 dyes on the plasmonic nanofingers is measured with nearly 10 times higher than that on the plain Au films, which further proves the strong plasmonic effect on DSSCs performance. This plasmonic nanostructure enables the precise engineering of the gap-plasmon modes and can be successfully fabricated by NIL into large areas with low cost, which has huge potentials to be used as photoanode for other types of solar cells, such as organic solar cells, perovskite solar cells.

This work was financially supported by the National Key R&D Program of China (No. 2018YFA0306202), the National Natural Science Foundation of China (Nos. 11574270, 11974015 and 11674166, 91963211). W W acknowledges the support from the NSF (No. CMMI-1635612).

All data that support the findings of this study are included within the article (and any supplementary files).