Minimizing Energy Losses in Dye‐Sensitized Solar Cells Using Coordination Compounds as Alternative Redox Mediators Coupled with Appropriate Organic Dyes

To further boost the efficiency of dye‐sensitized solar cells, a correct evaluation of the real limitations of this type of solar cell should be conducted. From a simple analysis of the photovoltaic data found in the literature, it is evident that the V_oc decrease, through energy losses, seems to be the most crucial factor diminishing the overall system efficiency. In particular, losses arising from the regeneration of the dye by the iodide/triiodide redox mediator are huge. The most convenient manner to recuperate these losses is the use of alternative redox mediators with more positive redox potentials than that of the iodide‐based shuttle. Among various types of redox active compounds, ferrocene/ferrocenium, Cu^+/2+, and Co^2+/3+ complexes are found to be the most effective in solar cells and the obtained efficiencies of more than 12% clearly show that a breakthrough is close, paving revolutionary roads towards making new records.     

TiO2 Nanocrystals Synthesized by Laser Pyrolysis for the Up‐Scaling of Efficient Solid‐State Dye‐Sensitized Solar Cells

A crucial issue regarding emerging nanotechnologies remains the up‐scaling of new functional nanostructured materials towards their implementation in high performance applications on a large scale. In this context, we demonstrate high efficiency solid‐state dye‐sensitized solar cells prepared from new porous TiO₂ photoanodes based on laser pyrolysis nanocrystals. This strategy exploits a reduced number of processing steps as well as non‐toxic chemical compounds to demonstrate highly porous TiO₂ films. The possibility to easily tune the TiO₂ nanocrystal physical properties allows us to demonstrate all solid‐state dye‐sensitized devices based on a commercial benchmark materials (organic indoline dye and molecular hole transporter) presenting state‐of‐the‐art performance comparable with reference devices based on a commercial TiO₂ paste. In particular, a drastic improvement in pore infiltration, which is found to balance a relatively lower surface area compared to the reference electrode, is evidenced using laser‐synthesized nanocrystals resulting in an improved short‐circuit current density under full sunlight. Transient photovoltage decay measurements suggest that charge recombination kinetics still limit device performance. However, the proposed strategy emphasizes the potentialities of the laser pyrolysis technique for up‐scaling nanoporous TiO₂ electrodes for various applications, especially for solar energy conversion.     

Development of Next‐Generation Organic‐Based Solar Cells: Studies on Dye‐Sensitized and Perovskite Solar Cells

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.     

Titanium Dioxide Mesoporous Electrodes for Solid‐State Dye‐Sensitized Solar Cells: Cross‐Analysis of the Critical Parameters

We report a comparative study on the use of four different mesoporous titanium dioxide (TiO₂) photo‐electrodes for the fabrication of solid‐state dye‐sensitized solar cells (sDSSCs). The photovoltaic parameters of the device correlate with several intrinsic properties of the film, based not only on its morphological features, as commonly considered in standard characterizations, but also on the transport and the electronic properties of the photo‐electrode. These properties differ significantly for TiO₂ electrodes processed using different colloidal pastes, and are decisive for the photovoltaic efficiency, ranging from 3.7% up to 5.1%. In particular, the dielectric permittivity of each mesoporous layer (ε_eff) and the number of traps (N_t) determined by the space‐charge‐limited current (SCLC) theory are found to be a bottle‐neck for the charge transport, greatly influencing the fill factor (FF) and open circuit voltage (V_oc) of the cells. In addition, a direct correlation between TiO₂ surface potential with the V_oc was established. Cross‐analysis of key macroscopic parameters of the films prior to integration in the devices, in particular focusing on the determination of the capacitance and surface potential shift of the TiO₂ mesoporous anode, represents a straightforward yet powerful method to screen and select the most suitable TiO₂ for applications in sDSSCs.     

High‐Performance,Transparent, Dye‐Sensitized Solar Cells for See‐Through Photovoltaic Windows

Dye‐sensitized solar cells (DSCs) have attracted great interest as one of the most promising photovoltaic technologies, and transparent DSCs show potential applications as photovoltaic windows. However, the competition between light absorption for photocurrent generation and light transmittance for obtaining high transparency limits the performance of transparent DSCs. Here, transparent DSCs exhibiting a high light transmittance of 60.3% and high energy conversion efficiency (3.66%) are reported. The strategy is to create a cocktail system composed of ultraviolet and near‐infrared dye sensitizers that selectively and efficiently harvest light in the invisible or low‐eye‐sensitivity region while transmitting light in high‐eye‐sensitivity regions. This new design provides a reasonable approach for realizing high efficiency and transparency DSCs that have potential applications as photovoltaic windows.     

Integrated Design of Organic Hole Transport Materials for Efficient Solid‐State Dye‐Sensitized Solar Cells

A series of triphenylamine‐based small molecule organic hole transport materials (HTMs) with low crystallinity and high hole mobility are systematically investigated in solid‐state dye‐sensitized solar cells (ssDSCs). By using the organic dye LEG4 as a photosensitizer, devices with X3 and X35 as the HTMs exhibit desirable power conversion efficiencies (PCEs) of 5.8% and 5.5%, respectively. These values are slightly higher than the PCE of 5.4% obtained by using the state‐of‐the‐art HTM Spiro‐OMeTAD. Meanwhile, transient photovoltage decay measurement is used to gain insight into the complex influences of the HTMs on the performance of devices. The results demonstrate that smaller HTMs induce faster electron recombination in the devices and suggest that the size of a HTM plays a crucial role in device performance, which is reported for the first time.     

Rational Design of Metal Oxide–Based Cathodes for Efficient Dye‐Sensitized Solar Cells

Recently, there is an urgent need for alternative energy resources due to the nonrenewable nature of fossil fuels and increasing CO₂ greenhouse gas emissions. The photovoltaic technologies which directly utilize the abundant and sustainable solar energy are critical. Among various photovoltaic devices (solar cells), dye‐sensitized solar cells (DSSCs) have gained increasing attention due to their high efficiency and easy fabrication process in the past decade. The cathode is a critical part in DSSCs while the benchmark Pt cathode suffers from high cost and scarcity. Thus, the development of alternative Pt‐free cathodes has attracted significant attention with the aim to heighten the cost competitiveness of DSSCs. Among various cathodes, metal oxides are of growing interest due to their superior activity, robust stability, and low cost. Simple oxides such as WO₃ and SnO₂ are used as cathodes for DSSCs. Considering the fixed atomic environment in simple oxides, complex oxides are more attractive as cathodes because of their more flexible physical and chemical properties. This review attempts to present the rational design of simple/complex metal oxide–based cathodes in DSSCs and then to provide useful guidance for the future design of Pt‐free cathodes. The demonstrated design strategies can be extended to other electrocatalysis‐based applications.     

Surface‐Plasmon Assisted Energy Conversion in Dye‐Sensitized Solar Cells

In this study, the effect of plasmonic core‐shell structures, consisting of dielectric cores and metallic nanoshells, on energy conversion in dye‐sensitized solar cells (DSSCs) is investigated. The structure of the core‐shell particles is controlled to couple with visible light so that the visible component of the solar spectrum is amplified near the core‐shell particles. In core‐shell particle – TiO₂ nanoparticle films, the local field intensity and light pathways are increased due to the surface plasmons and light scattering. This, in turn, enlarges the optical cross‐section of dye sensitizers coated onto the mixed films. When 22 vol% of core‐shell particles are added to a 5 μm thick TiO₂ film, the energy conversion efficiency of DSSCs increases from 2.7% to 4.0%, in spite of a more than 20% decrease in the amount of dyes adsorbed on the composite films. The correlation between core‐shell particle content and energy conversion efficiency in DSSCs is explained by the balance among near‐field effects, light scattering efficiency, and surface area in the composite films.     

Click‐Functionalized Ru(II) Complexes for Dye‐Sensitized Solar Cells

Three heteroleptic ruthenium complexes incorporating new ancillary ligands synthesized by sequential connection of different alkyl functionalities with triazole as a linker are prepared using click chemistry. These sensitizers exhibit low‐energy metal‐to‐ligand charge transfer bands centered at 540 nm with molar extinction coefficients of up to 1.54 × 10⁴ L mol^?1 cm^?1. The devices using these sensitizers in conjunction with a volatile electrolyte show high photovoltaic conversion efficiencies of 8.7 to 9.9% under standard AM 1.5G sunlight (100 mW cm^?2) conditions. Using an ionic liquid electrolyte, the cells show not only a good power‐conversion efficiency of up to 7.1%, but also promising long‐term stability under full sunlight intensity at 60 °C. The difference in the photovoltaic parameters during the ageing process is investigated by employing transient photoelectrical measurements.     

Hybrid Dye‐Titania Nanoparticles for Superior Low‐Temperature Dye‐Sensitized Solar Cells

In this work, a new strategy to design low‐temperature (≤200 °C) sintered dye‐sensitized solar cells (lt‐DSSC) is reported to enhance charge collection efficiencies (η_coll), photoconversion efficiencies (η), and stabilities under continuous operation conditions. Realization of lt‐DSSC is enabled by the integration of hybrid nanoparticles based on TiO₂‐Ru(II) complex (TiO₂_Ru_IS)—obtained by in situ bottom‐up construction of Ru(II) N3 dye‐sensitized titania—into the photoelectrode. Incentives for the use of TiO₂_Ru_IS are i) dye stability due to its integration into the TiO₂ anatase network and ii) enhanced charge collection yield due to its significant resistance toward electron recombination with electrolytes. It is demonstrated that devices with single‐layer photoelectrodes featuring blends of P25 and TiO₂_Ru_IS give rise to a 60% η_coll relative to a 46% η_coll for devices with P25‐based photoelectrodes. Responsible for this trend is a better charge transport and a reduced electron recombination. When using a multilayered photoelectrode architecture with a top layer based only on TiO₂_Ru_IS, devices with an even higher η_coll (74%) featuring a η of around 8.75% and stabilities of 600 h are achieved. This represents the highest values reported for lt‐DSSC to date.     

Triblock‐Terpolymer‐Directed Self‐Assembly of Mesoporous TiO2: High‐Performance Photoanodes for Solid‐State Dye‐Sensitized Solar Cells

A new self‐assembly platform for the fast and straightforward synthesis of bicontinuous, mesoporous TiO₂ films is presented, based on the triblock terpolymer poly(isoprene ‐ b ‐ styrene ‐ b ‐ ethylene oxide). This new materials route allows the co‐assembly of the metal oxide as a fully interconnected minority phase, which results in a highly porous photoanode with strong advantages over the state‐of‐the‐art nanoparticle‐based photoanodes employed in solid‐state dye‐sensitized solar cells. Devices fabricated through this triblock terpolymer route exhibit a high availability of sub‐bandgap states distributed in a narrow and low enough energy band, which maximizes photoinduced charge generation from a state‐of‐the‐art organic dye, C220. As a consequence, the co‐assembled mesoporous metal oxide system outperformed the conventional nanoparticle‐based electrodes fabricated and tested under the same conditions, exhibiting solar power‐conversion efficiencies of over 5%.     

Dominating Energy Losses in NiO p‐Type Dye‐Sensitized Solar Cells

Nickel oxide based p‐type dye‐sensitized solar cells (DSCs) are limited in their efficiencies by poor fill factors (FFs). This work explores the origins of this limitation. Transient absorption spectroscopy identifies fast recombination between the injected hole and the dye anion under applied load as one of the predominant reasons for the poor FF of NiO‐based DSCs. A reduced hole injection efficiency, η_INJ, under applied load is found to play an equally important role. Both, the dye regeneration yield, Φ_REG, and η_INJ decrease by approximately 40%–50% when moving from short‐ to open‐circuit conditions. Spectroelectrochemical measurements reveal that the electrochromic properties of NiO are a further limiting factor for the device performance leading to variable light‐harvesting efficiencies, η_LH, under applied load. The peak light‐harvesting efficiency decreases from 63% at short circuit to 57% at 600 mV reducing the FF of NiO DSCs by 5%. This effect is expected to be more pronounced for future devices with higher operating voltages. Incident, photon‐to‐electron conversion efficiency front–back analysis at applied bias is utilized to characterize the interfacial charge recombination. It is found that the recombination between the injected hole and the redox mediator has a surprisingly small effect on the FF.     

Indolizine‐Based Donors as Organic Sensitizer Components for Dye‐Sensitized Solar Cells

Strong electron‐donating functionality is desirable for many organic donor‐π‐bridge‐acceptor (D‐π‐A) dyes. Strategies for increasing the electron‐donating strength of common nitrogen‐based donors include planarization of nitrogen substituents and the use of low resonance‐stabilized energy aromatic ring‐substituted nitrogen atoms. Organic donor motifs based on the planar nitrogen containing heterocycle indolizine are synthesized and incorporated into dye‐sensitized solar cell (DSC) sensitizers. Resonance active substitutions at several positions on indolizine in conjugation with the D‐π‐A π‐system are examined computationally and experimentally. The indolizine‐based donors are observed to contribute electron density with strengths greater than triarylamines and diarylamines, as evidenced by UV/Vis, IR absorptions, and oxidation potential measurements. Fluorescence lifetime studies in solution and on TiO₂ yield insights in understanding the performance of indolizine‐based dyes in DSC devices.