Preparation and Characterization of Transferable Encapsulated Dye-Sensitized Solar Cells

Abstract

The increasing demand for sustainable energy as a means to combat the impact of climate change is addressed via a novel concept in the present work. Herein presented are developed transferable encapsulated dye-sensitized solar cells, canonically “solar capsules”, for photovoltaic applications on alternative surfaces, such as facades. The solar capsule assembly houses all the components necessary for photovoltaic energy conversion, enclosed within a semiconductor nanotubular array, making them truly unique in their construction. This capsule-style unit enables an easy transfer and draft onto a wide range of materials and surfaces for photovoltaic functionalization and applications. This type of dye-sensitized solar cell typically consists of transferred solar capsules and two additional electrodes. The design and construction of solar capsules means they have a high economic viability as they can seamlessly be up-scaled using commercially established techniques such as anodization and subsequent functionalization. This work demonstrates a working model of such transferable solar capsules by fabricating TiO₂ nanotubes that are functionalized via facile dip- and spin-coating techniques in a wet lab at ambient conditions. These prototypes are characterized in bulk and are thoroughly investigated at the nanoscale for information on the chemical distribution of the constituents, as they may be influenced during the manufacturing process.

1. Introduction

Climate change-driven trends in energy utilization and the rapid expansion in global consumption are urging societies to mitigate the use of available resources. A main task of today’s society in managing energy production is the changeover from fossil fuels to environmentally friendly and sustainable strategies. In this light, photovoltaic applications have become mainstream and focus on increasing device performance and efficiency. Product relevance is often measured in terms of solar efficiency; however, a well-rounded means of measuring novel systems is to combine a high throughput with techno-economical robustness. This essentially offers insights into developmental costs and techniques. In this work, we develop uniquely designed solar capsules bearing photovoltaic components housed within anodic oxide nanotubes. Such solar capsules are especially beneficial for applications relying on larger areas, e.g., building façades. A large-scale immobilization compensates for any inadequacies within individual components by maintaining high overall efficiency. A recent study published by Benish et al. reported that in Germany, a capacity of approximately 12,000 km² of facade surfaces is suitable for photovoltaic use [1]. The viability of such surfaces lies in successfully exploiting ‘surface-transfer’ technologies by utilizing available space. An ergonomic photovoltaic assembly could tremendously contribute to overall application and energy production capabilities in comparison to traditional solar panel parks. Additionally, such solar capsules bring both a harmonious functional effect and an esthetic influence to surfaces.

Since, O’Regan and Graetzel invented the first working dye-sensitized solar cell (DSSC), the working principle and setup have not changed drastically (see Figure 1a) [2,3,4]. Typical DSSCs often face challenges due to using a liquid redox mediator, requiring a tight area around the solar cell. Commonly used thermoplastic sealing materials were unfortunately not optimal for long-term stability. Therefore, improvements involved using UV-curable adhesives for sealing the liquid electrolyte and preventing the intrusion of oxygen and water [5]. Nevertheless, the question of proper sealing still remains a source of concern. To address this, the conceptual construction of DSSCs enclosed the photosensitive part here, meaning the dye and redox system. Enhancements were made by encapsulating these within semiconducting titanium dioxide (TiO₂) nanotubes and using a conductive polymer as the sealant and electrode connector (see Figure 1b) [3]. The shape of the nanotubes acts as a blocking layer, ideally preventing short circuiting [5].

Using TiO₂ nanotubes for DSSC applications has been widely studied. It has been concluded that the overall photoconversion efficiency is low due to their relatively low surface area compared to nanoparticles. Even though the photoconversion efficiency is low, electron transport is more efficient in tubular structures because of the reduced charge recombination [6,7], meaning the overall performance of nanotube structures exceeds that of nanoparticles. Furthermore, tubular nanostructures are predominantly synthesized via easily scalable techniques like electrochemical anodization. This process yields a well-ordered array of nanotubular structures on a metal substrate, leading to a good processability during dye sensitization as well as the filling and sealing of the tubes [8,9]. Although there have been reports on the transferability of nanotubes [10,11,12], little to no data exist on the demonstration of the process of transferring complete cells. This work is a first of its kind in describing a feasible and scalable process of fabricating DSSCs for a broad number of cases, unlike as discussed before.

Herein, it is conceptionally shown that the encapsulation technique opens an opportunity for transferring the assembled solar capsules to external structures. The fabrication steps of the solar capsule and a report on the nanoanalytical study of the cell are described in detail. Additionally, a successful transfer onto glass and a subsequent comprehensive characterization is demonstrated. It is noteworthy that the use of materials like Ag nanowires [13,14] or graphene-based [15] conductive transparent electrodes was not part of the present research. The material selection process involved using commercial, in-circulation basic materials for the better comparability of transferable solar capsules to already-reported non-transferable DSSCs.

2. Materials and Methods

2.1. Synthesis of TiO₂ Nanotubes

The experimental details for the synthesis of the TiO₂ nanotube structures are based on a fast anodization setup discussed in several sources and are visualized in Figure 2a–e. Titanium sheets (99.6% purity, 0.125 mm thickness, Advent, Oxford, UK) were cut into 20 × 30 mm² pieces and ultrasonicated in ethanol (99.5%; VWR Chemicals, Darmstadt, Germany) and deionized water (<0.16 µS conductivity) successively for five minutes each and dried in a N₂ stream. For anodization, a two-electrode setup utilizing a platinum sheet with the same surface area as the counter electrode was used. The bath electrolyte consists of ammonium fluoride (0.1 mol/L; analytical grade, Merck, Taufkirchen, Germany), lactic acid (1.5 mol/L; 90% in water, VWR Chemicals, Darmstadt, Germany), and deionized water (5 vol.%; <0.16 µS) dissolved in ethylene glycol (analytical grade, VWR Chemicals, Darmstadt, Germany). The temperature-controlled anodization (45–50 °C) was performed with 100 V (LAB-SMP, ET System electronic GmbH, Altlußheim, Germany) applied for 20 min supported by a magnetic stirrer. To remove electrolyte residues, the anodized samples were soaked in ethanol for two hours directly after anodization, followed by a rinse with 2-propanol (technical grade, VWR Chemicals Darmstadt, Germany), and subsequently air dried for 24 h. These samples were finally annealed at 450 °C for 4 h to convert amorphous TiO₂ into anatase.

In order to enable a robust transfer of nanotubes, these annealed samples were anodized a second time using the same electrolyte composition as stated above. The second anodization was performed at 60 V for 5 min at room temperature. The second anodized samples were then immersed in a hydrogen peroxide solution (30%, Merck, Taufkirchen, Germany) at room temperature for 30 min to promote the detachment of the nanotube arrays from the metal surface. The detached nanotube arrays were rinsed with 2-propanol and air dried afterwards.

2.2. Preparation of Solar Capsules

For dye sensitization, a commercially available ruthenium dye, N719 (cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bis(tetrabutylammonium), solid state, Solaronix SA, Aubonne, Switzerland) was diluted into a 0.5 mmol/L solution, using ethanol (technical grade, VWR Chemicals, Darmstadt, Germany) as a solvent. The annealed samples were immersed into the dye solution for 72 h and subsequently rinsed with ethanol and dried in air. During the soaking experiment, the evaporation of the solvent was prevented by proper sealing. After sensitization, the samples were rinsed with ethanol and dried in a N₂ stream. These samples were then stored in a dark place until further processing.

The remaining components, such as the redox mediator, based on an ${\mathrm{I}}^{-}/{\mathrm{I}}_3^{-}$ redox couple (Idololyte 50-Z, Solaronix SA, Aubonne, Switzerland), and the conductive sealant PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, HTL Solar 3, Ossila, Sheffield, UK), were coated on the capsules by static spin-coating (Spin Coater, Ossila, Sheffield, UK). The sensitized nanostructure sample was fixed with double-sided tape during the spin-coating step, which was followed by the application of the solution, maintaining a fixed volume-to-surface area ratio, with respect to the active area of the sensitized nanostructure. The spin-coating stage consisted of a two-step program using 500 rpm for 30 s, directly followed by 2000 rpm for 120 s. Each solution was coated in a separate step. The redox couple was applied with a volume-to-surface (v/s) ratio of 15 µL/cm² with respect to the anodized surface. For PEDOT:PSS, the solution was filtered (<0.45 µm filter) and coated at a surface ratio of 25 µL/cm², prior to post-treatment at 120 °C for 15 min on a heating plate for drying. Any excess redox electrolyte was removed from the interspaces between the nanotubes by immersing the solar capsules in ethanol for 5 min, rinsing with isopropyl alcohol, and air drying afterwards. At this stage, the completed assembly of the solar capsules was characterized and analyzed.

2.3. Solar Capsule Transfer

To prepare the solar capsules for the subsequent transfer, a proper electrical connection to both sides of the nanotubular backbone structure needs to be installed, while preserving the transparency of at least one side. Therefore, an electrically conductive Ag adhesive (ACHESON 1415, Plano GmbH, Wetzlar, Germany) was coated onto the sealed solar capsules, connecting the coated PEDOT:PSS layer. After 20 min of air drying, the solar capsules were wrapped entirely in a flexible sheet made of water-soluble methyl cellulose ether (MC). This sheet was partially wet in water, resulting in it being completely adhered to the solar capsules. After an additional 24 h drying in standard condition, solar capsules remained attached to the MC sheet and were easily peeled from the metal substrate. By re-wetting the MC from the top, the solar capsules were attached to a glass substrate as the main transfer step. A Cu metal sheet was connected to the Ag adhesive layer by electrical contact and indium-doped tin oxide (ITO) sputtered glass (Solaronix SA, Aubonne, Switzerland) was mechanically attached onto the solar capsules, serving as the transparent conductive electrode. The Cu electrode was connected to the ITO surface for electrical contact (see Figure 2f–i). An illustrated outline of the procedure is attached to the Supplementary Materials (Video S1).

2.4. Characterization Methods

For structural and compositional characterization, the prepared solar capsules were pretreated in a cross-section polisher (IB-19500 CP Cross-section polisher, JEOL, Freising, Germany) using a constant argon ion beam adjusted to an 8 kV acceleration voltage and a beam current of 200 µA for 4 h, followed directly by fine milling using the interval mode (1 s beam on, 1 s beam off) and an additional 30 min at beam settings of 3 kV and 50 µA. The back side of the sample was directed towards the ion beam, in order to protect the solar capsules from a direct ion-beam exposure. The cross-section was analyzed by scanning electron microscopy (SEM; Quanta FEG 250, FEI, Eindhoven, The Netherlands) and energy-dispersive X-ray spectroscopy (EDX). For time-of-flight secondary ion mass spectrometry analysis (TOF-SIMS; TOF-SIMS 4, IonTOF, Münster, Germany), spectra of all the initial materials were recorded to distinguish characteristic fragments during depth profiling. Finally, a depth profile of the solar capsules was performed, measuring the detected negative fragments with a dual-beam setup (analysis: Bi³⁺ ions, 25 keV; area of interest: 50 × 50 µm²; sputtering: Cs⁺ ions, 5 keV; sputter area: 500 × 500 µm²). The depth profile was normalized to the Cs⁻ fragment signal to remove artifacts caused by variations in the ion beam. The sputter depth was estimated by the thickness of the solar capsules, as estimated by SEM.

For measuring the final photovoltaic characterization of the solar capsules, current density–voltage measurements (j-U-plots) were performed under standardized AM1.5 (1000 W/m²) conditions and additionally without illumination, in darkness, utilizing an Oriel LCS-100 solar simulator, a Keithley 4200-SCS semiconductor characterization system, and a Suss Microprobe Station. The incident photon-to-converted electron (IPCE) characterization was performed employing a monochromator (Acton Research, Thousand Oaks, CA, USA) in conjunction with a tungsten halogen lamp in the wavelength range of 380 nm to 900 nm. The device short-circuit photocurrent was converted into a voltage using transimpedance amplifiers (FEMTO DLCPA-200), prior to data recording with the lock-in technique (Princeton 5210). A silicon photodetector (Hamamtsu S1337) was used for measuring the optical power density [16].

3. Results and Discussion

For the solar capsules to be physically functional, it was necessary to achieve a proper filling of the nanostructure with DSSC constituents. Detailed morphological and chemical information on the depth distribution of the architecture was achieved using SEM and ToF-SIMS analysis. As shown in Figure 3, a well-ordered layer of nanotubes was synthesized. The tube thickness varied between 10 and 13 µm. From the back-scattered electron (BSE) information, a strong material contrast was observed in the transition zone, visually confirming the successful material uptake into the nanotubes. Similar results to this BSE characterization have been previously reported, yielding valuable information on material uptake into titanium dioxide nanotubes [17]. EDX analysis (c.f. Figure 4) exhibited more detailed information on the qualitative elemental distribution of the major elements within the nanostructures, showing an increased carbon content in the transition zone, likely due to the PEDOT:PSS layer and the electrolyte. The limited lateral resolution as well as the low detection limit of the method [18], accompanied by the comparatively low concentration of the constituents (N719, redox system, and PEDOT:PSS), lead to a poor signal/noise ratio; no accurately distinguishable distribution of minor elements (S, Ru, I) was observable with EDX.

For a better characterization of the overall distribution of the constituents within the structure, a depth profile was measured with ToF-SIMS. Characteristic ion fragments were determined and assigned to the specific components of the solar capsule: TiO⁻ (63.95 m/z) for TiO₂ nanotubes, CNS⁻ (57.98 m/z) for the dye N719, I⁻ (126.90 m/z) for the redox system, and SO₄H⁻ (96.93 m/z) for PEDOT:PSS. Figure 5 reveals the resulting depth distribution of the constituents, indicating an initial layer on the top of the nanotubes reaching approximately 300 nm into the structure, which mainly consists of PEDOT:PSS and is further enriched by dye and redox system fragments. This is justified by the possible accumulation within and on top of a titanium dioxide initiation layer, which is typically formed during Ti anodization. At a depth of 500 nm, the nanotube signal stabilizes further into the bulk of the structure, aligning with the geometry of the nanostructure, congruent with reported results [19]. Furthermore, an accumulation of the constituents within the first 1 µm of the nanotube can be observed, fitting well with the discovery of a transition zone by EDX. However, in comparison to the EDX, it represents an elaborately detailed and distinguishable distribution. Furthermore, a homogeneous filling with all the constituents is indicated for the remaining nanotubular structure.

Overall, the morphological and chemical characterization assessments confirm that all components were successfully introduced into the TiO₂ nanotubes and a good distribution was achieved throughout the material. A uniform functionalization within the device structure is required for the favorable characteristics needed for solar-cell application.

As the j-U plot (see Figure 6a) reveals, a significant difference between the illuminated and the dark current can be observed, showing the clear photovoltaic behavior of the solar capsules under AM 1.5 illumination even after the transfer of the cell. Due to the relatively short length of 10 µm titanium dioxide nanotubes, comparatively low currents were detectable, resulting in a maximum power of only P_max = 35 ± 5 µW/m², a fill factor of FF = 0.25, and an efficiency of η = 0.35 × 10⁻⁶ (35 × 10⁻⁶%). Compared to other solar-cell types, the measured efficiency is significantly lower. Data reported, e.g., by Green et al., give efficiencies for crystalline silicon solar cells (27.3%), organic solar cells (15.8%), or DSSCs (11.9%) [20]. As reported by Roy et al. [21] and So et al. [22], the efficiency of comparable TiO₂ nanotube-based DSSCs is influenced by their length. An increasing length offers a higher surface area, and hence more capacity for dye molecules, leading to increased light absorption. A negative effect of longer nanotubes is the increasing risk of charge recombination. Tube lengths between 8 and 20 µm were reported [21] to have efficiencies of the same order of magnitude. For the reported DSSC-utilizing TiO₂ nanotubes of a 15 µm or 30 µm length, the cell characteristics of solar capsules exhibited values three orders of magnitude lower; e.g., J_SC,15µm = 11.5 × 10⁻³ A/cm² or J_SC,30µm = 12.7 × 10⁻⁶ A/cm² [22], as compared to the short-circuit current of solar capsules J_SC = 3.0 × 10⁻⁶ A/cm² presented here. An additional risk of a low efficiency within the solar capsule architecture might also occur, due to the direct contact of sealing material and nanotubes possibly leading to a direct competitive charge transfer between these materials, instead of a transfer of electrons from the sealant into the electrolyte and back to the dye molecules, as is described within the setup of So et al. [22]. A superficial applied insulation layer via µ-contact printing might reduce these undesired charge transfers; these are the subject of ongoing research. Nevertheless, the determined IPCE characterization of the fabricated solar capsules, as shown in Figure 6b, matches the observed j-U characteristics, ranging in the same order of magnitude. Moreover, it clearly reveals an absorption edge at approx. 620 nm corresponding to an absorption of N719 [23], showing the typical and expected trend in these cell types at ongoing shorter wavelengths. The peak efficiency of this cell is located at around 500 nm, making it suitable for semitransparent applications.

This concept of solar capsules delivers a novel approach of a fully functional and working prototype of transferable solar cells on well-known and scalable basic materials. This concept may be applied for transfers on nearly every substrate in buildings. Research which is under way is focused on optimizing, sealing, and material selection for an enhanced photovoltaic efficiency paralleling the commercial standard. As development on photovoltaics is advancing, the use of transparent conductive electrodes made from Ag nanowires or graphene-based electrodes are especially interesting in solar capsule applications, as these materials are commonly applicable from solutions, leading to a shape-independent coverage of the surface. Thus, their use might possibly reduce transition resistances between different layers, in comparison to the mechanical bonding of an ITO layer to the solar capsule surface, as was performed in this work. Those alternative electrode materials need good adjustment regarding transparency and conductivity to have beneficial effects on efficiency. For further enhancement of the solar capsule concept, a switch from DSSCs to perovskite solar cells (PSCs) is going to be investigated as they promise a high reported photo-conversion efficiency. Min et al. [24] reported a PCE of 25.5%. In the use of solar capsules, a proper filling of the perovskite material into the nanotubular structure is crucial and might be obtained by a well-adjusted sol–gel process to infiltrate the nanotubes completely. The ease of synthesis and fabrication of a fully functional solar capsule is achieved in ambient, non-cleanroom conditions, making the process highly viable and scalable. Solar capsules encourage the use of alternative materials for assembly and also enable the use of a wide range of constituents commonly used in state-of-the-art DSSCs. The design simultaneously features a structural independence, and opens up possible pathways towards alternative areas of applications for such photovoltaics, potentially promoting systems developed by using differently functionalized nanotube arrays.

4. Conclusions

This work presents the first successful approach of constructing a transferable encapsulated dye-sensitized solar cells based on titanium dioxide nanotubes, which can be used to equip surfaces with a photovoltaic functionality. It offers a uniquely facile route to address limitations in design by employing spin- and dip-coating processes under ambient conditions to produce these solar capsules. The solar capsule assembly consists of TiO₂ nanotubes housing commercially available components such as N719, an ${\mathrm{I}}^{-}/{\mathrm{I}}_3^{-}$ redox couple, and PEDOT:PSS. Their photovoltaic properties were demonstrated successfully. Ongoing research will focus on the optimization of the electronic coupling of the solar capsules, with the aim of improving solar-cell efficiency. As anodization is a scalable process, this multistep approach for developing TiO₂ nanotube housing units for solar capsule production is highly interesting for the future of alternative applications.

Author Contributions

Conceptualization, P.H.; methodology, P.H. and M.M.; investigation, P.H.; writing—original draft, P.H.; writing—review and editing, S.N.V.R., C.P., P.H.B. and M.S.K.; visualization, P.H.; supervision, M.S.K.; project administration, M.S.K. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the MNaF Siegen, were parts of this work were conducted.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. (a) A simplified schematic of a common dye-sensitized solar cell (left): two conductive layers for electron transport: hole-transport material (HTM) and electron-transport layer (ETL); the nanostructures consist of TiO2 nanoparticles covered with a dye. The redox system commonly consists of an [Forumla omitted. See PDF.] couple in organic solvents. (b) Solar capsules featuring all the compartments within a nanotube, replacing the use of nanoparticles (right).

Figure 2. Schematic principle of production process: (a) anodized and annealed nano structures, (b) dye sensitization of nanotubes via bulk immersion, (c) application of iodide-based electrolyte. It is incorporated within nanotubes and their interspace (15 µL/cm2). (d) Application of PEDOT:PSS (25 µL/cm2). Electrolyte serves as blocking layer and prevents nanostructure from being fully covered with polymer. (e) After heat treatment, solar capsules are rinsed with ethanol to wash off excess electrolyte from intertubular spacing. (f) Finished solar capsules are covered with electrically conductive layer of Ag adhesive. (g) Nanostructure is adhered onto sheet of methylcellulose ether (MC). (h) Detached and ready-to-transfer solar capsules. (i) Ready-to-use solar cell consisting of Cu sheets as electrodes connected to Ag adhesive layer on solar capsules and ITO sputtered glass mounted onto other end. Short circuiting is prevented by sufficient MC–layer overlap.

Figure 3. (a) SEM measurements of polished cross-section showing BSE image, indicating material contrast within nanotubular structure due to their filling. (b) Secondary electron (SE) image reveals tubular nature of structure and thin sealing layer on top of nanotubes.

Figure 4. EDX line scan across tubular length of solar capsule (30 kV, 10,000×, 200 µs dwell time, 30 nm/measuring point).

Figure 5. (a) ToF-SIMS depth profile (read right to left). Bottom and top of nanotubes are indicated. (b) Magnified representation of gray region in (a) indicates thickness of initiation layer arising from anodization process. Scale is valid for both diagrams.

Figure 6. Performance of transferred solar capsules. (a) j-U plot with absolute value of AM 1.5 illumination photocurrent and dark current. (b) Incident photon-to-converted electron rate equivalent to external quantum efficiency of solar cell, exhibiting an absorption edge of 620 nm. Determined efficiency in (a,b) perfectly match, showing reliable transfer technique.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/pr12112546/s1, Video S1: Assembly of transferable DSSC.

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