Performance enhancement of dye-sensitized solar cells using an ester-functionalized imidazolium iodide as the solid state electrolyte

Insight into the structure and transport properties of pyrrolidinium-based geminal dicationic-organic ionic crystals: inravelling the role of alkyl-chain length

The present study has been undertaken with an aim to design and develop safer and more efficient all solid-state electrolytes, so that the issues associated with the use of conventional room temperature ionic liquid-based electrolytes can be tackled. To fulfil this objective, a series of geminal di-cationic Organic Ionic Crystals (OICs), based on C₃-, C₆-, C₈- and C₉-alkylbridged bis-(methylpyrrolidinium)bromide are synthesized, and the structural features, thermal properties and phase behaviours of these as synthesized OICs have been investigated. Additionally, a number of electro-analytical techniques have been employed to assess their suitability as an efficient electrolyte composite (OIC:I₂:TBAI) for all solid-state dye sensitised solar cells (DSSCs). The structural analysis has revealed that along with excellent thermal stability and well-defined surface morphology, all thsese OICs exhibit a well-ordered three-dimensional network of cations and anions that can serve as a conducting channel for the diffusion of iodide ions. Electrochemical investigations have shown that OICs with an intermediate length of alkyl bridge (C₆- and C₈-alkyl bridged) show better electrolytic performance than those that are based on OICs with a relatively shorter (C₃-) or longer (C₉-) alkyl-bridge chain. A careful analysis of the above data has essentially demonstrated that the length of the alkyl bridge chain plays a significant role in determining the structural organisation, morphology and eventually the ionic conductivity of OICs. Overall, the comprehensive knowledge on OICs that has been extracted from the current study is expected to be helpful to explore further new types of OIC-based all solid-state electrolytes with improved electrolytic performance for targeted applications.

Flexible energy generation and storage devices: focus on key role of heterocyclic solid-state organic ionic conductors

This review addresses the vital role of solid-state electrolytes to develop highly efficient, customizable flexible energy generation and storage devices.

Iodine induced 1-D lamellar self assembly in organic ionic crystals for solid state dye sensitized solar cells

A novel saturated heterocyclic organic ionic crystal, piperidinium iodide (PiHI), is synthesized by a facile route and applied as a solid electrolyte in Dye Sensitized Solar Cells (ss-DSSCs). Upon addition of a small quantity of iodine, PiHI self-assembles into a 1D lamellar micro crystalline structure that shows anisotropic conductivity. The two-component PiHI was characterized by using electrochemical impedance spectroscopy, cyclic voltammetry, steady state voltammetry, FT-IR, and Raman spectroscopy. Wide angle X-ray diffraction (XRD) measurement confirms the presence of long range 1D lamellar channels that pave the way for the diffusion of the redox couple I^-/I₃^- and exhibit high anisotropic conductivity. The ionic conductivity of 1D PiHI (with I₂) aligned perpendicular to the electrode, σ_⊥ (15.46 mS cm^-1), is 1.5 times higher than that aligned parallel to the electrode σ_∥ = 10.32 mS cm^-1. The ss-DSSC devices with these self-assembled ordered ionic crystals with a carbazole based sensitizer (SK1) achieved a power conversion efficiency (PCE) of 4.2% and 5.2% for ∥^al and ⊥^ar arrangement, respectively. The reported PCEs are better than that obtained from a classical liquid electrolyte with SK1 sensitizers. The electron kinetics at various interfaces of ss-DSSC devices was evaluated using Electrochemical Impedance Spectroscopy (EIS) measurements. The presence of a saturated cyclic structure promotes close packing through H-bonding and electrostatic interactions, which make ss-DSSC devices more stable up to 600 h under illumination of 1 sun.

Conjugated or Broken: The Introduction of Isolation Spacer ahead of the Anchoring Moiety and the Improved Device Performance

Acceptors in traditional dyes are generally designed closed to TiO₂ substrate to form a strong electronic coupling with each other (e.g., cyanoacrylic acid) to enhance the electron injection for the high performance of the corresponding solar cells. However, some newly developed dyes with chromophores or main acceptors isolated from anchoring groups also exhibit comparable or even higher performances. To investigate the relatively untouched electronic coupling effect in dye-sensitized solar cells, a relatively precise method is proposed in which the strength is adjusted gradually by changing isolation spacers between main acceptors and anchoring groups to partially control the electronic interaction. After an analysis of 3 different groups of 11 sensitizers, it is inferred that the electronic coupling should be kept at a suitable level to balance the electron injection and recombination. Based on a reference dye LI-81 possessing a cyanoacrylic acid as acceptor and anchoring group, both photocurrent and photovoltage are synergistically improved after the properties of isolation spacers were changed through the adjustment of the length, steric hindrance, and push-pull electronic characteristic. Accordingly, the rationally designed dye LI-87 with an isolation spacer of thiophene ethylene gives an efficiency of 8.54% and further improved to 9.07% in the presence of CDCA, showing a new way to develop efficient sensitizers.

Unprecedentedly targeted customization of molecular energy levels with auxiliary-groups in organic solar cell sensitizers

Lowering the LUMOs and decreasing energy “waste” is targeted through inserting an auxiliary group from an electron donor or acceptor into D–π–A organic sensitizers, and the photovoltaic efficiency increases 38 fold from 0.24 to 9.46%.

In dye-sensitized solar cells (DSSCs), the HOMO–LUMO energy gap of organic sensitizers should be large enough to enable efficient electron injection and dye regeneration. However, the LUMOs of most practical organic dyes are always too high, making energy “waste”. In order to deepen the LUMOs, we focus on the targeted modulation of the molecular energy levels by embedding an electron donor or acceptor into the skeleton of a typical D–π–A model. The electron-rich group of 3,4-ethylenedioxythiophene (EDOT) lifts up the HOMO level with little influence on the LUMO, while the electron-deficient group of benzothiadiazole (BTD) or benzooxadiazole (BOD) mainly lowers the customized LUMO level. As a consequence, the auxiliary group change from EDOT (dye WS-53) to BOD (dye WS-55) brings forth a huge photoelectric conversion efficiency (PCE) increase by 38 fold from 0.24 to 9.46% based on an I^–/I₃^– redox couple, and even reaching a high PCE of 10.14% with WS-55 under 0.3 sunlight irradiation.

Stable, High-Efficiency Pyrrolidinium-Based Electrolyte for Solid-State Dye-Sensitized Solar Cells

We synthesized a series of pyrrolidinium based dicationic ionic crystals with high melting point and good thermal stability. Research on the crystal structure shows that there are ordered three-dimensional ionic channels in these crystals which is favorable for the ionic conductor to achieve high conductivity and diffusion coefficient. These ionic crystals are applied to electrolyte as matrix in dye sensitized solar cells, and the influence of crystal structure (including the alkylene chain separating two pyrrolidinium rings and anion) versus the device performances are studied by steady-state voltammography, current-voltage trace, and electrochemical impedance spectroscopy. As the solid state electrolyte, an optimized efficiency of 6.02% have achieved under full sunlight irradiation using ionic crystal [C6BEP][TFSI]2. And the device based on this solid electrolyte shows the excellent long-term stability, maintaining 92% of the initial efficiency after 960 h. This study elucidates fundamental the structure of dicationic crystal and provide useful clues for further improvement of solid-state electrolytes in DSSC.