Rapid Room-Temperature Synthesis of Mesoporous TiO2 Sub-Microspheres and Their Enhanced Light Harvesting in Dye-Sensitized Solar Cells

Submicron sized mesoporous spheres of TiO₂ have been a potential alternative to overcome the light scattering limitations of TiO₂ nanoparticles in dye-sensitized solar cells (DSSCs). Currently available methods for the growth of mesoporous TiO₂ sub-microspheres involve long and relatively high temperature multi-stage protocols. In this work, TiO₂ mesoporous sub-microspheres composed of ~5 nm anatase nanocrystallites were successfully synthesized using a rapid one-pot room-temperature CTAB-based solvothermal synthesis. X-Ray Diffraction (XRD) showed that the grown structures have pure anatase phase. Transmission electron microscopy (TEM) revealed that by reducing the surfactant/precursor concentration ratio, the morphology could be tuned from monodispersed nanoparticles into sub-micron sized mesoporous beads with controllable sizes (50–200 nm) and with good monodispersity as well. The growth mechanism is explained in terms of the competition between homogeneous nucleation/growth events versus surface energy induced agglomeration in a non-micelle CTAB-based soft templating environment. Further, dye-sensitized solar cells (DSSCs) were fabricated using the synthesized samples and characterized for their current-voltage characteristics. Interestingly, the DSSC prepared with 200 nm TiO₂ sub-microspheres, with reduced surface area, has shown close efficiency (5.65%) to that of DSSC based on monodispersed 20 nm nanoparticles (5.79%). The results show that light scattering caused by the agglomerated sub-micron spheres could compensate for the larger surface areas provided by monodispersed nanoparticles.

Extremely robust photocurrent generation of titanium dioxide photoanodes bio-sensitized with recombinant microalgal light-harvesting proteins

Bio-dyes for light harvesting in dye-sensitized solar cells (DSSC) have the advantage of being environmentally-friendly, non-toxic alternatives, which can be produced in a sustainable fashion. Free photosynthetic pigments are unstable in the presence of light and oxygen, a situation which can hardly be avoided during the operation of DSSCs, especially in large-scale applications. We therefore investigated the recombinant light-harvesting protein LHCBM6, which naturally occurs in the photosynthetic apparatus of the green microalga Chlamydomonas reinhardtii as a bio-dye in DSSCs. Photocurrent densities of up to 0.87 and 0.94 mA·cm-2 were determined for the DSSCs and solar energy to electricity conversion efficiencies (η) reached about 0.3% (100 mW·cm-2; AM 1.5 G filter applied). Importantly, we observed an unprecedented stability of LHCII-based DSSCs within long DSSC operation times of at least 7 days in continuous light and show that operation times are restricted by electrolyte decomposition rather than reduced dye performance, as could be demonstrated by DSSC reactivation following re-supplementation with fresh electrolyte. To the best of our knowledge, this is the first study analysing bio-dye sensitized DSSCs over such long periods, which revealed that during illumination an activation of the DSSCs occurs.

Efficient Energy Harvesting in SnO2-Based Dye-Sensitized Solar Cells Utilizing Nano-Amassed Mesoporous Zinc Oxide Hollow Microspheres as Synergy Boosters

Finding the material characteristics satisfying most of the photovoltaic conditions is difficult. In contrast, utilization of foreign materials that can contribute to light harvesting and charge transfers in the devices is now desirable/thought-provoking. Herein, a binary hybrid photoanode utilizing nano-amassed micron-sized mesoporous zinc oxide hollow spheres (meso-ZnO HS) in conjunction with SnO₂ nanoparticles (NPs), i.e., SnO₂ NP_ZnO HS (for an optimized weight ratio (8:2)), displayed a nearly ∼4-fold increase in the efficiency (η) compared to that of bare SnO₂ nanoparticle device. Enhanced device efficacy in the composite photoanode-based device can be accredited to the dual function of nano-amassed meso-ZnO HS. Nano-amassed micron-sized ZnO HS embedded in the photoanode can increase the light-harnessing capability without sacrificing the surface area as well as optical confinement of light by multiple reflections within its cavity and enhanced light-scattering effects. Electrochemical impedance spectroscopy analysis revealed an extended lifetime of electron (τ_e) and a higher value of R _ct2 at the working electrode/dye/redox mediator interface, indicating a minimum photoinduced electron interception. The open-circuit voltage decay reveals a slower recombination kinetics of photogenerated electrons, supporting our claim that the nano-ammased meso-ZnO HS can serve as an energy barrier to the photoinjected electrons to retard the back-transfer to the electrolyte. Moreover, the improvement in the fill factors of the composite-based devices is endorsed to the facile penetration of the electrolyte through the pores of nano-amassed meso-ZnO HS, which increases the regeneration probability of oxidized dyes.

Tailoring the Mesoscopic TiO2 Layer: Concomitant Parameters for Enabling High-Performance Perovskite Solar Cells

Architectural control over the mesoporous TiO₂ film, a common electron-transport layer for organic-inorganic hybrid perovskite solar cells, is conducted by employing sub-micron sized polystyrene beads as sacrificial template. Such tailored TiO₂ layer is shown to induce asymmetric enhancement of light absorption notably in the long-wavelength region with red-shifted absorption onset of perovskite, leading to ~20% increase of photocurrent and ~10% increase of power conversion efficiency. This enhancement is likely to be originated from the enlarged CH₃NH₃PbI₃(Cl) grains residing in the sub-micron pores rather than from the effect of reduced perovskite-TiO₂ interfacial area, which is supported from optical bandgap change, haze transmission of incident light, and one-diode model parameters correlated with the internal surface area of microporous TiO₂ layers. With the templating strategy suggested, the necessity of proper hole-blocking method is discussed to prevent any direct contact of the large perovskite grains infiltrated into the intended pores of TiO₂ scaffold, further mitigating the interfacial recombination and leading to ~20% improvement in power conversion efficiency compared with the control device using conventional solution-processed hole blocking TiO₂. Thereby, the imperatives that originate from the structural engineering of the electron-transport layer are discussed to understand the governing elements for the improved device performance.

Wrinkled silica/titania nanoparticles with tunable interwrinkle distances for efficient utilization of photons in dye-sensitized solar cells

Efficient light harvesting is essential for the realization of high energy conversion efficiency in dye-sensitized solar cells (DSCs). State-of-the-art mesoporous TiO₂ photoanodes fall short for collection of long-wavelength visible light photons, and thus there have been efforts on introduction of scattering nanoparticles. Herein, we report the synthesis of wrinkled silica/titania nanoparticles with tunable interwrinkle distances as scattering materials for enhanced light harvesting in DSCs. These particles with more than 20 times larger specific surface area (>400 m²/g) compared to the spherical scattering particles (<20 m²/g) of the similar sizes gave rise to the dye-loading amounts, causing significant improvements in photocurrent density and efficiency. Moreover, dependence of spectral scattering properties of wrinkled particles on interwrinkle distances, which was originated from difference in overall refractive indices, was observed.

Micron-scale rod-like scattering particles for light trapping in nanostructured thin film solar cells

Micron-scale rod-like particles scatter light stronger than their spherical counterparts do; hence leading to a more efficient light trapping in solar cells.

Photonic design of embedded dielectric scatterers for dye sensitized solar cells

A photonic design approach is proposed to determine the optimal size and concentration of dielectric scatterers for nanostructured solar cells.

One-step process for the synthesis and deposition of anatase, two-dimensional, disk-shaped TiO₂ for dye-sensitized solar cells

We report a one-step process for the synthesis and deposition of anatase, two-dimensional (2D), disk-shaped TiO2 (DS-TiO2) using titanium isopropoxide (TTIP), ethyl cellulose (EC), and solvents. The planar structure of EC plays a pivotal role as the sacrificing template to generate the 2D disk-shaped structure with a thickness of 1.5-3.5 μm, while a disk-like structure was well developed in the tetrahydrofuran (THF)/toluene mixed solvent. The quasi-solid-state dye-sensitized solar cells (qssDSSCs), fabricated with a nanogel electrolyte and a DS-TiO2 layer on a nanocrystalline (NC)-TiO2 photoanode, showed an energy conversion efficiency of 5.0% without any TiCl4 post-treatment, which is higher than that fabricated without DS-TiO2 (4.2%). When utilizing a poly((1-(4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII) as the solid electrolyte, a high efficiency of 6.6% was achieved due to the combination of high mobility PEBII and a bifunctional DS-TiO2 layer with a 2D structure and anatase phase. The bifunctionality of the DS-TiO2 layer allows greater light scattering back into the device and provides additional surface area for improved dye adsorption, resulting in short circuit current density (Jsc).

In-situ fabrication of macroporous films for dye-sensitised solar cells: formation of the scattering layer and the gelation of electrolytes

Dye-sensitised solar cells (DSCs) are a promising substitute for conventional silicon solar cells. A scattering layer of submicrometer pores or particles has been widely introduced to achieve a high light-harvesting efficiency. However, many such fabrication processes require high temperatures and multiple steps to prepare the scattering layer. Here, we have developed an in-situ fabrication process for a macroporous (MP) scattering film. The macropores were formed inside the assembled cell via the dissolution of polystyrene (PS) spheres from a PS/TiO₂ composite layer caused by exposure to an electrolyte solution. Specifically, the in-situ MP scattering layer decreased the transmittance of the electrode film from 58% to below 1%. The DSCs using these MP scattering layers exhibited an increase in the efficiency of 22%. Moreover, the dissolution of the PS improved the cell stability because of the gelation of the electrolyte solution; the efficiency of the DSCs was maintained at 80% of its initial value after ageing for 20 days, whereas the efficiency of the bare-electrode DSCs was found to have decreased by 50%. We believe that in-situ porous scattering layers show great promise for next-generation flexible DSCs. Moreover, this approach can be extended to various applications that utilize porous film/liquid systems.

Multi-functionality of macroporous TiO2 spheres in dye-sensitized and hybrid heterojunction solar cells

Micron-sized macroporous TiO2 spheres (MAC-TiO2) were synthesized using a colloidal templating process inside emulsions, which were then coated on a nanocrystalline TiO2 light absorption film to prepare a bilayered photoanode for liquid-based dye-sensitized solar cells (DSSC) and hybrid heterojunction solid-state solar cells. MAC-TiO2 layers can enhance light scattering as well as absorption, because their pore size and periodicity are comparable to light wavelength for unique multiple scattering and a porous surface can load dye more. Moreover, due to the bicontinuous nature of macropores and TiO2 walls, electrolyte could be transported much faster in between the TiO2 spheres rather than within the small TiO2 nonporous architectures. Electron transport was also facilitated along the interconnected TiO2 walls. In DSSCs with these MAC-TiO2 scattering layers, efficiency was higher than conventional DSSCs incorporating a commercial scattering layer. The unique geometry of MAC-TiO2 results in strong improvements in light scattering and infiltration of hole-transporting materials, thereby the MAC-TiO2-based solid-state device showed comparatively higher efficiency than the device with conventional nanocrystalline TiO2.

Carbon nanohorns as integrative materials for efficient dye-sensitized solar cells

Different nanocarbons, that is, single-wall carbon nanotubes, graphene, single-wall carbon nanohorns (SWCNHs), and their respective oxidized analogs have been used to fabricate novel doped TiO2 electrodes for DSSCs. Our results indicate that all of the nanocarbons significantly enhance the device characteristics when compared to standard TiO2 electrodes. Overall, our most outstanding finding is that SWCNH derivatives are also a plausible material for developing highly-efficient DSSCs.