The high performance and low cost of dye-sensitized solar cells (DSSCs) have drawn great interest from both academic and industrial circles. The research on exploring novel efficient sensitizers, especially on inexpensive metal-free pure organic dyes, has never been suspended. The donor–π bridge–acceptor (D–π–A) configuration is mainstream in the design of organic sensitizers due to its convenient modulation of the intramolecular charge-transfer nature. Recently, it has been found that incorporation of additional electron-withdrawing units (such as benzothiadiazole, benzotriazole, quinoxaline, phthalimide, diketopyrrolopyrrole, thienopyrazine, thiazole, triazine, cyanovinyl, cyano- and fluoro-substituted phenyl) into the π bridge as internal acceptors, termed the D–A–π–A configuration, displays several advantages such as tuning of the molecular energy levels, red-shift of the charge-transfer absorption band, and distinct improvement of photovoltaic performance and stability. We apply the D–A–π–A concept broadly to the organic sensitizers containing additional electron-withdrawing units between electron donors and acceptors. This review is projected to summarize the category of pure organic sensitizers on the basis of the D–A–π–A feature. By comparing the structure–property relationship of typical photovoltaic D–A–π–A dyes, the important guidelines in the design of such materials are highlighted.

Organic sensitizers from D-π-A to D-A-π-A: effect of the internal electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances.

Arylamine organic dyes with donor (D), π-bridge (π) and acceptor (A) moieties for dye-sensitized solar cells (DSCs) have received great attention in the last decade because of their high molar absorption coefficient, low cost and structural variety. In the early stages, the efficiency of DSCs with arylamine organic dyes with D–π–A character was far behind that of DSCs with ruthenium(II) complexes partly due to the lack of information about the relationship between the chemical structures and the photovoltaic performance. However, exciting progress has been recently made, and power conversion efficiencies over 10% were obtained for DSCs with arylamine organic dyes. It is thus that the recent research and development in the field of arylamine organic dyes employing an iodide/triiodide redox couple or polypyridyl cobalt redox shuttles as the electrolytes for either DSCs or solid-state DSCs has been summarized. The cell performance of the arylamine organic dyes are compared, providing a comprehensive overview of arylamine organic dyes, demonstrating the advantages/disadvantages of each class, and pointing out the field that needs to reinforce the research direction in the further application of DSCs.

Arylamine organic dyes for dye-sensitized solar cells

High energy conversion efficiency is one of the most important keys to the commercialization of dye-sensitized solar cells (DSCs) in the huge electricity generation market. According to our experience in the persistent efforts that helped to achieve high efficiency DSCs, we selectively review the major progress of improving the energy conversion efficiency of DSCs which may be useful for future applications. We start the discussion from modelling the device by macroscopic equivalent circuit and then highlight some approaches to improve the device performance, such as the molecular engineering of novel dye sensitizers and light trapping effect, tuning the potential of redox shuttles and surface passivation of photoelectrodes, and optimizing the resistance. Finally, we illustrate a roadmap of possible future directions of DSCs with the challenges of how to further improve the efficiency to accelerate the progress in the commercialization of DSCs.

Highly efficient dye-sensitized solar cells: progress and future challenges

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

This paper reports a new D–A–π-A organic dye WS-9, which is derived from the known dye WS-2 by incorporating an n-hexylthiophene unit into the π-conjugation. Due to the presence of a strong electron-withdrawing benzothiadiazole unit in the π-bridge, the specific D–A–π-A organic dyes show more complicated electronic transition absorption bands than traditional D–π-A dyes. The origins of the absorption bands in D–A–π-A organic dyes are analysed by density functional theory (DFT). The calculated results in combination with the deprotonation experiments suggest that the spectral response range of D–A–π-A organic dyes is superior to that of D–π-A ones. When employed in dye-sensitized solar cells (DSSCs), the two dyes show a large difference in aggregation behaviour. It was found that WS-2 forms the unfavourable aggregates more easily. High performance of WS-2 strongly depends on the coadsorbent and suitable dye bath solvent. In contrast, WS-9 shows strong anti-aggregation ability, and always exhibits high performance regardless of the coadsorbent and dye bath solvent. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the injected electron lifetime and charge recombination resistance are largely increased due to the introduction of a hexylthiophene unit, resulting in the high photovoltage based on WS-9. The optimized power conversion efficiency of WS-9 reaches 9.04% with high photocurrent (18.00 mA cm−2) and photovoltage (696 mV). The accelerating dye photo-stability was tested upon light irradiation of a dye-adsorbed TiO2 film in the absence of redox electrolyte, and a WS-9-based DSSC device with ionic liquid redox electrolyte. These results suggest that the structural engineering of organic dyes is important for highly efficient photovoltaic performance of solar cells, and our research will pave a novel way to design new efficient D–A–π-A organic dye sensitizers.

High-conversion-efficiency organic dye-sensitized solar cells: molecular engineering on D–A–π-A featured organic indoline dyes

Two novel benzotriazole-containing organic dyes based on D–A−π–A configuration, WS-5 with octyl group and WS-8 with methyl group, have been designed and synthesized for use in dye-sensitized solar cells (DSSCs). Compared with the traditional D−π–A sensitizers, the benzotriazole unit as an additional acceptor has several merits: (i) essentially facilitating the electron transfer from the donor to the acceptor/anchor; (ii) conveniently tailoring the solar cell performance with a facile structural modification on 2-position in the benzotriazole unit; and (iii) the nitrogen-containing heterocyclic group of benzotriazole being expected to improve the open-circuit photovoltage. The analysis of controlled intensity modulated photovoltage spectroscopy reveals that the replacement of methyl with octyl group enhances electron lifetime by 4-fold and retards charge recombination rate constant by 4-fold. The two dye-loaded TiO2 films possess almost the same conduction band position under the same condition, as reveale...

Incorporating Benzotriazole Moiety to Construct D–A−π–A Organic Sensitizers for Solar Cells: Significant Enhancement of Open-Circuit Photovoltage with Long Alkyl Group

Nanoparticle chains of anatase TiO2 composed of ∼5 nm nanoparticles with a high surface area (309 m2 g−1) are prepared by slow hydrolysis of titanium tetraisopropoxide modified with glacial acetic acid at a temperature of 50 °C. These nanoparticle chains without hydrothermal treatment can be used directly to fabricate photoanodes for highly efficient dye-sensitized solar cells (DSSCs), which achieve a power conversion efficiency of 9.80% at simulated AM1.5 G illumination (100 mW cm−2). The approach presented in this study demonstrates a facile synthesis of anatase TiO2 chains for use in DSSCs, which avoid hazards arising from a hydrothermal process that is typically adopted to prepare crystalline TiO2 nanoparticles for use as the photoanode in DSSCs.

Low temperature preparation of TiO2 nanoparticle chains without hydrothermal treatment for highly efficient dye-sensitized solar cells

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Incorporating Benzotriazole Moiety to Construct D–A−π–A Organic Sensitizers for Solar Cells: Significant Enhancement of Open-Circuit Photovoltage with Long Alkyl Group

Low temperature preparation of TiO2 nanoparticle chains without hydrothermal treatment for highly efficient dye-sensitized solar cells