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.

A new molecular design strategy for tuning the energy levels of cis-configured squaraine sensitizers for dye-sensitized solar cells is described. The Hammett substituent constant and the π-conjugation length are used as quantitative indicators to modify the central squarate moiety of the sensitizer dyes; specifically, novel near-infrared squaraine dyes HSQ3 and HSQ4 are synthesized by incorporation of an electron-withdrawing and π-extending ethyl cyanoacetate unit on the central squarate moiety. The solution absorption maximum of HSQ4 occurs at 703 nm, and the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital are in the ideal range for energetically efficient electron injection and regeneration of the oxidized dye. A solar cell sensitized with HSQ4 exhibits a broad incident photo­n-to-current conversion efficiency spectrum, extending into the near-infrared region with a maximum value of 80% at 720 nm, which is is the highest value reported for a squaraine dye–based dye-sensitized solar cell. The HSQ4-sensitized solar cell also exhibits excellent durability during light soaking, owing to the double anchors attaching the dye to the TiO₂ surface and to the long alkyl chains extending outward from the surface.

Hierarchical ZnO nanorods composed of interconnected nanoparticles, which were synthesized by controlling precursor concentrations in a solvothermally assisted process, were exploited as photoanodes in dye-sensitized solar cells (DSCs). The as-prepared hierarchical nanorods showed greatly enhanced light scattering compared to ZnO nanoparticles for boosting light harvesting while maintaining sufficient dye-adsorption capability. The charge-transfer characteristics were studied by electrochemical impedance measurements, and reduced electron recombination and longer electron lifetime were observed for the ZnO nanorods. Photovoltaic characterization demonstrated that DSCs utilizing the hierarchical nanorods significantly improved the overall conversion efficiency by 34 % compared to nanoparticle-based DSCs.

Hierarchical SnO₂ microspheres with exposed {101} facets are synthesized by a one-pot solvothermal reaction and explored as a scattering layer on top of a SnO₂ nanoparticle film for dye-sensitized solar cells (DSCs). The microspheres not only enhance photon harvesting by effective light scattering/reflecting but also improve the energy level of the conduction band edge of the SnO₂ photoanode, which leads to a simultaneous increase of the short-circuit photocurrent by 17 % and open-circuit photovoltage by 20 %. An improved overall conversion efficiency of 3.46 % is achieved for DSCs using the SnO₂ microspheres, which is the highest value reported so far for pure-SnO₂ DSCs. Moreover, the SnO₂-microsphere DSCs exhibit a better photostability than the conventional TiO₂ DSCs under simulated full sunlight.

A cis-configured squaraine dye (HSQ1), synthesized by incorporation of a strongly electron-withdrawing dicyanovinyl group into the central squaric acid moiety, is employed in dye-sensitized solar cells (DSCs). In solution, HSQ1 displays an intense absorption in the near-infrared region with a maximum at 686 nm and when the dye is adsorbed on a TiO₂ surface, the absorption spectrum broadens in both the blue and the near-infrared regions, which is favorable for efficient light harvesting over a broad wavelength range. A solar cell sensitized with HSQ1 shows a broader incident photon-to-current conversion efficiency (IPCE) spectrum (from 400 to 800 nm) and a higher IPCE in the long-wavelength region (71% at 700 nm) than a cell sensitized with squaraine dye SQ1. Furthermore, a solar cell co-sensitized with HSQ1 and N3 dye shows remarkably improved short-circuit current density and open-circuit voltage compared to those of a DSC based on N3 alone and fabricated under the same conditions. The energy-conversion efficiency of the co-sensitized DSC is 8.14%, which is the highest reported efficiency for a squaraine dye–based co-sensitized DSC without using Al₂O₃ layer.

A novel Ru π-expanded terpyridyl sensitizer, referred to as HIS-2, is prepared based on the molecular design strategy of substitution with a moderately electron-donating 4-methylstyryl group onto the terpyridyl ligand. The HIS-2 dye exhibits a slightly increased metal-to-ligand charge transfer (MLCT) absorption at around 600 nm and an intense π–π* absorption in the UV region compared with a black dye. Density functional theory calculations reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine and 4-methylstyryl moieties, which enhances the light-harvesting capability and is appropriate for smooth electron injection from the dye to the TiO₂ conduction band. The incident photon-to-electricity conversion efficiency spectrum of HIS-2 exhibits better photoresponse compared with black dye over the whole spectral region as a result of the extended π-conjugation. A DSC device based on black dye gives a short-circuit current (J_SC) of 21.28 mA cm⁻², open-circuit voltage (V_OC) of 0.69 V, and fill factor (FF) of 0.72, in an overall conversion efficiency (η) of 10.5%. In contrast, an HIS-2 based cell gives a higher J_SC value of 23.07 mA cm⁻² with V_OC of 0.68 V, and FF of 0.71, and owing to the higher J_SC value of HIS-2, an improved η value of 11.1% is achieved.

Hierarchical TiO₂ ellipsoids 250–500 nm in size have been synthesized on a large scale by a template-free hydrothermal route. The submicrometer-sized hierarchitectures are assembled from highly crystallized anatase nanorods about 17 nm in diameter with macroporous cavities on the outer shells. Based on the time-dependent morphological evolution under hydrothermal conditions, an oriented attachment process is proposed to explain formation of the hierarchical structures. Such hierarchical TiO₂ not only adsorbs large amounts of dye molecules due to high surface area, but also shows good light scattering caused by the submicrometer size. The TiO₂ hierarchitectures were deposited on top of a transparent TiO₂ nanocrystalline main layer to construct a double-layered photoanode for dye-sensitized solar cell (DSC) application, exhibiting enhanced light harvesting and power-conversion efficiency compared to a commercial TiO₂-based electrode.