Light absorption enhancement in dye-sensitized solar cells using thin nanostructured photoelectrodes

As thin-film solar cells are the attraction of many research works nowadays, dye-sensitized solar cells (DSSCs) among other types hold the promise of excellent cost/performance ratio because of the relatively low material costs and simple processing conditions. In order to improve its relatively low performance, different approaches are being explored to enhance the photon-to-current conversion efficiency. Notably, one of the most accessible approaches is by modifying the optical characteristics of the photoelectrode of DSSCs to improve light trapping within the cell. Here, we introduce a cost-effective nanopatterning process done on the photo-active TiO2 photoelectrode layer for better light absorption. This simple technique resulted in increased cell short-circuit current density (from 4.664 to 5.963 mA/cm2) which consequently enhanced cell efficiency (from 2.243 to 2.819%). Furthermore, without compromising the absorption significantly, nanopatterned photoelectrodes with a smaller thickness can lead to reduced material costs and possibly allowing the construction of semitransparent cells. This work can contribute to an attainable efficient DSSC manufacturing route for practical applications.


Introduction
Third-generation solar cells based on semiconducting organic macromolecules, inorganic nanoparticles, or hybrids are especially attractive due to their potentially low costs, solution-processed fabrication, and mechanical flexibility.Nevertheless, these technologies have not yet been proven commercially due to the significant limitations in terms of improving efficiency at large scale (modules) and device stability in particular under harsh operating conditions (mainly related to humidity, thermal and light-induced degradations) [1].Among the third-generation photovoltaic technologies, dye-sensitized solar cell (DSSC) is a promising candidate.The operation of a DSSC depends largely on the optical/electrical properties of the photoelectrode, which typically consists of a mesoporous TiO2 layer (covalently bonded by light-absorbing organic dye) deposited on fluorine-doped tin oxide (FTO) glass.
Aiming for high-efficiency and less costly cells, many approaches are actively being explored to increase device power conversion efficiency (PCE).Light-trapping techniques, where light absorption is increased by providing additional light scattering from nanoparticles or by increased absorption at the desired location from textured surfaces, have shown promise in improving the efficiency of many thinfilm solar cells.Notably, the nano-imprint or the nanopatterning method has been reported as an effective light-trapping strategy to generate desired optical nanostructures via applying an external force and thermal energy to a soft polymer transforming the underlying thin film.Using this approach, organic solar cells with nano-imprinted poly (3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS) nanogratings have been reported with enhanced performance [2].The patterned nanostructure was further investigated together with silver nanograting arrays as the active layer in inverted polymer solar cells [3,4].Furthermore, it has been investigated that the nano-imprint method induces light trapping not only in a silicon solar cell [5] and a dye-sensitized solar cell [6] but also in a perovskite solar cell [7].In DSSCs, given the much thicker photoelectrode compared to the active layer in e.g.organic cells (~ 10 μm vs. a few hundred nm), light trapping by nanopatterning will be even more appealing as the material consumption for TiO2 can be considerably reduced using a thinner nanostructured photoelectrode.In addition, thin photoelectrodes with a thickness of a few μm would allow the construction of semi-transparent DSSCs.
In this work, we propose implementing a nanograting structure on the DSSC's photoelectrode layer using an accessible and low-cost nanoimprint technique (instead of more costly methods such as photolithography) [8,9].Increased scattering effect from the nanopatterned mesoporous TiO2 is expected to enhance the light absorption in the photoelectrode and subsequently the cell efficiency.Meanwhile, DSSCs using thinner photoelectrode can potentially realize semi-transparent photovoltaics at a performance level similar to conventional designs.Finally, the approach of nanopatterning thin films using nanograting for enhancing optical absorption is easily applicable to other types of solar cell technology.

PDMS stamp preparation
Polydimethylsiloxane (PDMS) nanopatterning stamp was prepared by mixing Sylgard 184 silicone elastomer components of a base and curing agent with a ratio of (10:1).The mixed elastomer was placed in a vacuum chamber for ~ 15 min to remove air bubbles.A digital versatile disc (DVD) sample was cut into smaller pieces and washed a few times with ethanol to remove the protective layer until becoming completely transparent.The liquid PDMS was poured onto the cleaned DVD pieces placed in a petri dish, which made contact with the patterned side of the DVD.The petri dish was then carefully placed again into the vacuum chamber at 80℃ for 2 hours for the PDMS to harden.Finally, the PDMS stamp was peeled and separated from the DVD, and cut to the desired size.

DSSC fabrication
To prepare the nanostructured photoelectrode, FTO glass substrates (2.2 mm glass thickness and 7 ohm/sq, purchased from Solaronix) were coated with a mesoporous TiO2 paste (purchased from Solaronix) by blade coating.The TiO2 layer was then patterned by the prepared PDMS stamp placed on top for a few minutes to get rid of trapped air bubbles.The pressure was carefully applied between small magnets placed on the top of the stamp and the bottom of the petri dish, which was placed in an oven and heated for 20 minutes at 80℃.After cooling the sample and carefully detaching the magnets and the PDMS stamp, the TiO2 layer was successfully nanopatterned, proven by the multicolored reflection as demonstrated in Figure 1.
For DSSC preparation, platinum-coated FTO glass (purchased from Solaronix) functioning as the counter electrode and the TiO2 photoelectrode (patterned or non-patterned) were annealed at ~ 250℃ for 30 minutes and then the temperature was ramped up to 500℃ for 45 minutes.The cooled photoelectrodes were stored in a container and dipped in an ethanol dye solution (N719 purchased from Solaronix) for 24 hours in dark.The electrodes were then washed with ethanol to remove the excess dye and dried.The photoelectrode and counter electrode were assembled and held together by paperclips while being separated by a square shaped-hole spacer to avoid short-circuit.Iodine electrolyte (purchased from Solaronix) was applied between the two electrodes by capillary force.Copper tapes were attached to the cell contacts for increasing conductivity during testing.

Characterizations
Scanning electron microscope (SEM) images were taken by a VEGA3 TESCAN SEM.Maya 2000-Pro high-resolution spectrometers (Ocean Optics) were used for the optical spectrometry measurements.The current density-voltage (J-V) measurements on DSSCs were conducted by an ABET Sunlite solar simulator and a Keithley 2400 SourceMeter.

Results and Discussion
Nanostructured patterns transferred from DVD gratings to the PDMS stamp and then to the TiO2 photoelectrode are shown in Figure 1 Error!Reference source not found.(a)and (b).The multicolored r eflection proves the complete transfer of the nanopattern onto both samples.As a reference, the opposite side of the PDMS stamp was used to create a flat (non-patterned) TiO2 layer under identical conditions.Figure 1 (c) shows the cross-sectional SEM of a TiO2 photoelectrode.The thickness of the TiO2 layer was found to be 3.57 µm, which demonstrates the successful fabrication of thin TiO2 photoelectrode.The top-down SEM in Figure 1 (d) exhibits a ~350 nm grating period of the imprinted grating pattern with ~200 nm line width.To study the effects of the nanopattern on the optical properties of the electrodes, optical transmission in the visible spectral range was studied using spectrometry measurements.Figure 2 shows the results of  As mentioned above, the nanopattern enhances light trapping.We measured the photovoltaic performance of DSSCs using patterned and flat photoelectrodes.The patterned cell showed an enhanced performance.The photovoltaic parameters of the DSSCs are summarized in Table 1, with JSC of 4.664 and 5.963 mA/cm 2 and VOC of 0.734 and 0.741 V for the reference and grating imprinted cells, respectively.The increase of JSC in the patterned cell confirmed the role of the nanopatterned photoelectrode in improving light trapping, as is also supported by the spectrometry measurements in Figure 2.This resulted in an increased cell PCE from 2.243% to 2.819% with comparable fill factors (FFs) observed (65.5% vs. 63.8%).Further research and experimentation are required to optimize the DSSC performance with thinner nanopatterned photoelectrodes.

Conclusions
In conclusion, the nano-imprint method used for the fabrication of DSSCs with enhanced light trapping yields better photovoltaic performance.The approach of fabricating the patterned photoelectrode is costefficient since only soft polymer stamps were used to replicate the gratings from existing DVD masters, instead of the costly lithography process.The obtained results indicated that efficient light-trapping within the cell was the reason for the cell performance enhancement.This work provides a simple and fast approach and opens many opportunities to fabricate and investigate the effect of light-harvesting with a direct nanopatterning technique, which is easy to up-scale to fabricate large-area DSSCs inexpensively.

Figure 1 .
Figure 1.(a)(b) Photographs of imprinted DVD patterns of a PDMS stamp and a TiO2 photoelectrodes, respectively.(c) Cross-sectional SEM and (d) top-down SEM images of a patterned TiO2 photoelectrode.The thickness of the TiO2 photoelectrode layer is indicated.
the measurements.The transmission of the patterned photoelectrode was found to be overall lower compared to the reference flat sample.The decreased transmission indicates a direct optical improvement resulting from increased light trapping by the nanostructured surface morphology.Related work by Usami demonstrated that the incident solar energy is absorbed effectively in the thinner sensitized porous film due to multiple scattering at the bottom and total reflection at the surface[10].This optical improvement is expected to reflect on the cell performance and therefore on increasing the cell efficiency.

Figure 2 .
Figure 2. Optical transmission of flat and patterned TiO2 photoelectrodes in the visible spectral range.