Physically Stable Polymer‐Membrane Electrolytes for Highly Efficient Solid‐State Dye‐Sensitized Solar Cells with Long‐Term Stability

A 3D polymer‐network‐membrane (3D‐PNM) electrolyte is described for highly stable, solid‐state dye‐sensitized solar cells (DSCs) with excellent power‐conversion efficiency (PCE). The 3D‐PNM electrolyte is prepared by using one‐pot in situ cross‐linking polymerization on the surface of dye‐sensitized TiO₂ particles in the presence of redox species. This method allows the direct connection of the 3D‐PNM to the surface of the TiO₂ particles as well as the in situ preparation of the electrolyte gel during device assembly. There are two junction areas (liquid and solid‐state junctions) in the DSCs that employ conventional polymer electrolytes, and the major interface is at the liquid‐state junction. The solid‐state junction is dominant in the DSCs that employ the 3D‐PNM electrolyte, which exhibit almost constant performance during aging at 65 °C for over 700 h (17.0 to 17.2 mA cm^–2). The best cell performance gives a PCE of 9.1%; this is slightly better than the performance of a DSC that employs a liquid electrolyte.     

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.     

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.     

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.     

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.     

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.     

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.     

Surface State‐Mediated Charge Transfer of Cs2SnI6 and Its Application in Dye‐Sensitized Solar Cells

A vacancy‐ordered double perovskite, Cs₂SnI₆, has emerged as a promising lead‐free perovskite in the optoelectronic field. However, the charge transfer kinetics mediated by its surface state remains unclear. Here, the charge transfer mechanism of Cs₂SnI₆ is reported and the role of its surface state in the presence of a redox mediator is clarified. Specifically, charge transfer through the surface state of Cs₂SnI₆ and its subsequent surface state charging are demonstrated by cyclic voltammetry and Mott–Schottky measurements, respectively. Because it is expected that the surface state of Cs₂SnI₆ is capable of regenerating oxidized organic dyes, a Cs₂SnI₆‐based regenerator is developed for a dye‐sensitized solar cell composed of fluorine‐doped tin oxide (FTO)/dyed mesoporous TiO₂/regenerator/poly(3,4‐ethylenedioxythiophene)/FTO. As expected, the performance of the Cs₂SnI₆‐based regenerator is strongly dependent on the highest occupied molecular orbital of the dyes. Consequently, Cs₂SnI₆ shows efficient charge transfer with a thermodynamically favorable charge acceptor level, achieving a 79% enhancement in the photocurrent density (14.1 mA cm^?2) compared with that of a conventional liquid electrolyte (7.9 mA cm^?2). The results suggest that the surface state of Cs₂SnI₆ is the main charge transfer pathway in the presence of a redox mediator and should be considered in future designs of Cs₂SnI₆‐based devices.     

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.     

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.     

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.     

Atomic Layer Deposition of Highly Transparent Platinum Counter Electrodes for Metal/Polymer Flexible Dye‐Sensitized Solar Cells

Atomic layer deposition (ALD) is used to deposit Pt nanoparticles at low temperature (25–150 °C) to fabricate highly transparent counter electrodes (CEs) for flexible dye‐sensitized solar cells (DSCs). The Pt nanoparticles (NPs) are deposited for different number of ALD cycles on indium tin oxide (ITO)/polyethylene naphthalate (PEN) substrates. Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) are used to assess the Pt NP loading, density, and size. There is a trade‐off between transparency and catalytic activity of the CE, and the best cell performances of back‐side‐illuminated DSCs (≈3.7% efficiency) are achieved for Pt ALD at temperatures in the range of 100–150 °C, even though deposition at 25 °C is also viable. The best cell produced with ALD platinized CE (100 cycles at 100 °C) outperforms the reference cells fabricated with electrodeposited and sputtered Pt CEs, with relative improvements in efficiency of 19% and 29%, respectively. In addition, these parameters are used to fabricate a large area CE for a sub‐module (active area of 17.6 cm²), resulting in an efficiency of 3.1%, which demonstrates the scalability of the process.     

Recent Progress in Dye‐Sensitized Solar Cells Using Nanocrystallite Aggregates

Nanocrystallite aggregates are spherical assemblies of nanometer‐sized crystallites and feature a size on the order of sub‐micrometers. This paper reports and summarizes recent progress in nanocrystallite aggregates for applications in dye‐sensitized solar cells. It emphasizes that nanocrystallite aggregates are a promising class of materials with the capability to generate light scattering, enhance electron transport, retain high specific surface area for dye adsorption, and facilitate electrolyte diffusion while serving as the photoelectrode film of a dye‐sensitized solar cell. In the Perspectives section, it is suggested that optimization of the porosity of the aggregates, the facets of nanocrystallites forming the aggregates, and the structure of photoelectrode film could possibly lead to breakthroughs in improving the power conversion efficiency of the current state‐of‐the‐art dye‐sensitized solar cells.