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

Dye-sensitized solar cells (DSCs) are well researched globally due to their potential as low-cost photovoltaic (PV) devices especially suited for building and automobile integrated PV (BIPV, AIPV) and portable or indoor light harvesting applications. Since 1991, large monetary and intellectual investments have been made to develop DSCs into deployable technologies, creating a wealth of knowledge about nano-interfaces and devices through an increasing number of research reports. In response to these investments, the dawn of the new millennium witnessed the emergence of a corporate sector of DSC development. Advances in their design, their incorporation on flexible substrates, the development of solid state modules, their enhanced stability in outdoor environments, and their scalable fabrication tools and techniques have allowed DSCs to move from the laboratory to real-life applications. Although photoconversion efficiencies are not on a par with commercially available CIGS or single crystalline silicon solar cells, they possess many features that compel the further development of DSC modules, including transparency, light weight, flexibility, conformability, workability under low-light conditions, and easy integration in buildings as solar windows. In fact, DSC panels have been shown to deliver even more electricity than their silicon and thin film counterparts of similar power ratings when exposed to low light operating conditions due to their workability in such conditions; thus, they are potential market leaders in BIPV and indoor light harvesting photovoltaic technology. However, large area dye-solar modules lack in performance compared to their laboratory scale devices and also suffer from long term stability issues. Herein, we discuss the main factors behind their inferior photovoltaic performance and identify possible opportunities for the design of more efficient DSC modules.

A perspective on the production of dye-sensitized solar modules

Redox electrolytes have proven to be extremely important in determining the performance of dye-sensitized solar cells (DSCs). The design and understanding of the redox couple, especially iodide free systems, has become a recent focus of DSC electrolyte research. In this perspective article, advances in the conception and performance of various redox shuttles including molecular hole transport materials are reviewed with respect to their structure and the charge transfer dynamics at the counter electrode. In particular the discussion will focus on the trends that enable iodide-free redox couples to be the most effective and their feasibility to be used in DSCs in combination with new and innovative sensitizers and counter electrode materials.

Recent developments in redox electrolytes for dye-sensitized solar cells

Porphyrins in bio-inspired transformations: Light-harvesting to solar cell

We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO(2) surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)(3)](3+) species into contact with the TiO(2) surface. This translates into a high probability of intercepting TiO(2)-injected electrons by the oxidized [Co(bpy)(3)](3+) species, lying close to the N719-sensitized TiO(2) surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.

Cobalt electrolyte/dye interactions in dye-sensitized solar cells: a combined computational and experimental study.

Dye-sensitized solar cells with power conversion efficiencies of up to 6.5% have been fabricated using a cobalt tris-bipyridyl redox mediator with the cis-diisothiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907) sensitizer. This represents a significant improvement in efficiency compared with previous reports using ruthenium sensitizers. In situ near-IR transmittance measurements in conjunction with electrochemical impedance spectroscopy have been used to explain the difference in performance between DSCs using Z907 and another benchmark sensitizer cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) bis(tetrabutylammonium) (N719). It is found that the small-perturbation electron diffusion length (Ln) is significantly longer in Z907 cells compared with that in N719 cells, which can explain most of the difference in performance. It is also shown that the longer Ln in Z907 cells is caused by inhibited recombination, as opposed to fa...

Cobalt Redox Mediators for Ruthenium-Based Dye-Sensitized Solar Cells: A Combined Impedance Spectroscopy and Near-IR Transmittance Study

We demonstrate a facile bottom-up growth of CuSCN — a widely used hole conducting material, in mesoscopic TiO 2 films by electrodeposition in the presence of TiO 2 compact layer. The profile-controlled growth of CuSCN in the nanopores of mesoscopic TiO 2 is realized by judiciously optimizing the electrolyte concentration and applied potential, so that the effective electron diffusion length in TiO 2 is much shorter than film thickness and the electrodeposition is strictly confined within very narrow region from the conducting substrate. The TiO 2 /CuSCN heterojunctions exhibit a pore filling ratio of ∼80% and show good rectifying properties, which is anticipated to be an important step forward to the facile fabrication of bulk heterojunction for nanostructured solar cells or 3D batteries.

Yao Huang

Papers

Cobalt Redox Mediators for Ruthenium-Based Dye-Sensitized Solar Cells: A Combined Impedance Spectroscopy and Near-IR Transmittance Study

Fabrication of TiO2/CuSCN Bulk Heterojunctions by Profile-Controlled Electrodeposition