Titanium dioxide (TiO2) has been the most intensively investigated binary transition metal oxide in the past four decades as indicated by Figure S1. Furthermore, the annual number of papers published on TiO2 has seen a continuous increase, particularly since the beginning of this century (Figure S2). This is understandable when one considers the wide range of applications of TiO2 from the conventional areas (i.e., pigment, cosmetic, toothpaste, and paint) to the later developed functional areas such as photoelectrochemical cell,(1-3) dye-sensitized solar cells (DSSCs),(4-11) photocatalysis,(12-24) catalysis,(25-31) photovoltaic cell,(32-34) lithium ion batteries,(35-41) sensors,(42-46) electron field emission,(47-51) microwave absorbing material, biomimetic growth, and biomedical treatments.(52-57) Nearly all these functional applications of TiO2 fall in the scope of energy, environment, and health, which are definitely the three most important and challenging themes facing the Human race that need to be addressed in this century. Besides the apparent merits including nontoxicity, elemental abundance, good chemical stability, and easy synthesis, TiO2 has attracted strong research interest worldwide due to its physicochemical properties for realizing various functions.(15, 58, 59) Especially, very encouraging progresses in photocatalysis and DSSCs with the involvement of TiO2 have greatly stimulated the rapid development of TiO2 crystals with controllable phase, size, shape, defect, and heteroatom.(58, 60-68)

Titanium dioxide crystals with tailored facets

The carbon neutral process for energy conversion is of utmost importance, in this context photovoltaics, especially dye sensitized solar cells (DSSCs) are a viable alternative. Few terawatts of carbon free renewable energy can be produced by DSSCs. However the judicious use of platinum group free metals may further enhance the limit of energy production by making it further cost effective. In this context a metal free sensitizer and electrocatalyst related to DSSCs are reviewed for their merits. The current state-of-the-art sensitizer as well as carbon based materials for its chemistry and photovoltaic characterization are discussed. The present article combines recent progress and its emerging behavior from our laboratories and from other groups working on this perennial topic.

Metal free sensitizer and catalyst for dye sensitized solar cells

The application of polymer solar cells (PSCs) with n-type organic semiconductor as acceptor requires further improving powder conversion efficiency, increasing stability and decreasing cost of the related materials and devices. Here we report a simplified synthetic route for 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b'] dithiophene by using the catalyst of amberlyst15. Based on this synthetic route and methoxy substitution, two low cost acceptors with less synthetic steps, simple post-treatment and high yield were synthesized. In addition, the methoxy substitution improves both yield and efficiency. The high efficiency of 13.46% was obtained for the devices with MO-IDIC-2F (3,9-bis(2-methylene-5 or 6-fluoro-(3-(1,1-dicyanomethylene)-indanone)-4,4,9,9-tetrahexyl-5,10-dimethoxyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b'] dithiophene) as acceptor. Based on the cost analysis, the PSCs based on MO-IDIC-2F possess the great advantages of low cost and high photovoltaic performance in comparison with those PSCs reported in literatures. Therefore, MO-IDIC-2F will be a promising low cost acceptor for commercial application of PSCs.

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Simplified synthetic routes for low cost and high photovoltaic performance n-type organic semiconductor acceptors.

Dye-sensitized solar cells (DSSCs) feature low cost, stability, and environment friendliness and are thus a promising substitute for traditional silicon solar cells. DSSCs have received intensive research attention and have been rapidly developing in the last two decades. The efficiency of DSSCs should be increased to promote their commercialization and large-scale application. This brief review summarizes the major progress in advanced nano/micromaterials to improve photoanodes and enhance the conversion efficiencies of DSSCs. Commonly used methods to improve photoanodes include semiconductor film nanoarchitecture, light-scattering material application, compositing, doping, interfacial engineering, and TiCl4 post-treatment. This review provides insights into DSSC improvement and development of other photovoltaics, such as perovskite solar cells and photoelectrochemical cells.

Improving photoanodes to obtain highly efficient dye-sensitized solar cells: a brief review

A series of tetrafluorine-substituted small molecules with a D1-A-D2-A-D1 linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution-processed small-molecule organic solar cells. The impacts of thiophene π-bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open-circuit voltage, and short-circuit current density are dramatically improved in comparison with fluorinated-free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very-high FF of 0.76 for BIT-4F-T/PC71BM is achieved without additional additive, which is among one of the highest reported for small-molecules-based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A nanocrystalline TiO2 (anatase) nanosheet exposing mainly the (001) crystal faces was tested as photoanode material in dye-sensitized solar cells. The nanosheets were prepared by hydrothermal growth in HF medium. Good-quality thin films were deposited on F-doped SnO2 support from the TiO2 suspension in ethanolic or aqueous media. The anatase (001) face adsorbs a smaller amount of the used dye sensitizer (C101) per unit area than the (101) face which was tested as a reference. The corresponding solar cell with sensitized (001)-nanosheet photoanode exhibits a larger open-circuit voltage than the reference cell with (101)-terminated anatase nanocrystals. The voltage enhancement is attributed to the negative shift of flatband potential for the (001) face. This conclusion rationalizes earlier works on similar systems, and it indicates that careful control of experimental conditions is needed to extract the effect of band energetic on the current/voltage characteristics of dye-sensitized solar cell.

https://link.springer.com/content/pdf/10.1007%2Fs10008-012-1729-0.pdf

Voltage enhancement in dye-sensitized solar cell using (001)-oriented anatase TiO2 nanosheets

4,9-Dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b']dithiophene as a π-spacer of donor-π-acceptor dye and its photovoltaic performance with liquid and solid-state dye-sensitized solar cells.

Liu Bin

Papers

Voltage enhancement in dye-sensitized solar cell using (001)-oriented anatase TiO2 nanosheets

4,9-Dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b']dithiophene as a π-spacer of donor-π-acceptor dye and its photovoltaic performance with liquid and solid-state dye-sensitized solar cells.