Influence of triple bonds as π-spacer units in metal-free organic dyes for dye-sensitized solar cells

Ulf Anders Hagfeldt, Chao Yang
2010
Article

Résumé

Four metal-free org. sensitizers (TC101-TC104) with triple bonds in π-spacers and 5 ref. dyes (TC, TC105, TPC1, D5, and TH208) without triple bonds were applied in dye-sensitized solar cells to study the influence of triple bonds as π-spacer units on their photoelectrochem. properties and dye-sensitized solar cells (DSCs) performance. The introduction of triple bond could red shift the dye's absorption spectrum due to the enhancement of the π-spacer. However, the spectrum red shift is much less than that of the introduction of double bond because of more electronegativity of triple bond. The incident photon-to-current conversion efficiency reveals that the electron transfer yield (Φ(ν)ET) of DSCs becomes larger with the introduction of triple bond. Electrochem. impedance spectroscopy anal. reveals that the introduction of triple bond almost does not change the electron lifetimes in TiO2 films but decreases the effective diffusion lengths.

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Molecular Design of Anthracene-Bridged Metal-Free Organic Dyes for Efficient Dye-Sensitized Solar Cells

Ming Cheng, Ulf Anders Hagfeldt, Chao Yang

Metal-free org. dyes bridged by anthracene-contg. π-conjugations were designed and synthesized as new chromophores for dye-sensitized solar cells (DSSCs). Studies on the relation between the dye structure, photophys. properties, electrochem. properties and performances of the DSSCs are described. With the introduction of the anthracene moiety, together with a triple bond for the fine-tuning of mol. planar configurations and to broaden absorption spectra, the Jsc and Voc of DSSCs improved. The improvement of Jsc is attributed to a broader absorption spectra of the dyes with the anthracene moiety. EIS anal. reveals that the anthracene moiety suppresses charge recombination arising from electrons in TiO2 films with I3- ions in the electrolyte, thus improving Voc. From optimized mol. structures and DSSC test conditions, the dye TC501 shows a prominent solar energy conversion efficiency up to 7.03% (Jsc = 12.96 mA/cm2, VOC = 720 mV, ff = 0.753) under simulated AM 1.5 irradn. (100 mW/cm2).

2010

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We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim]+[B(CN)4]- using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6 + radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S0 â S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (Ï ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6+ radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6 + absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 Î¼s, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S1 quickly decays to S0 (time constants 1 and 35 ps). Â© 2014 the Partner Organisations.

2014

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Harvesting UV Photons for Solar Energy Conversion Applications

Magdalena Anna Marszalek, Jacques-Edouard Moser

We report the synthesis and characterization of five new donor–π–spacer–acceptor dye molecules with a diphenylamine donor, fluorene–1,2,5-oxadiazole spacers and a range of acceptor/anchor groups (carboxylic acid 1, cyanoacrylic acid 2 and 3, alcohol 4 and cyano 5) to facilitate electron injection from the excited dye into the TiO2 photoanode in dye-sensitized solar cells (DSSCs). Detailed photophysical studies have probed the dyes' excited state properties and revealed structure–property relationships within the series. Density functional theory (DFT) and time dependent DFT (TDDFT) calculations provide further insights into how the molecular geometry and electronic properties impact on the photovoltaic performance. A special feature of these dyes is that their absorption features are located predominantly in the UV region, which means the dye-sensitized TiO2 is essentially colorless. Nevertheless, DSSCs assembled from 1 and 2 exhibit photovoltaic power conversion efficiencies of η = 1.3 and 2.2%, respectively, which makes the dyes viable candidates for low-power solar cells that need to be transparent and colorless and for applications that require enhanced harvesting of UV photons.

Royal Soc Chemistry2014

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2014