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Organic sensitizers with different thiophene units as conjugated bridges: molecular engineering and photovoltaics

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  • ReceivedApr 2, 2016
  • AcceptedMay 5, 2016
  • PublishedJul 19, 2016

Abstract

Three structural modifications with incorporation of alkyl, alkoxy and vinyl bond into the skeleton of thiophene bridge in D-π-A featured organic sensitizers are specifically developed for insight into their influences on photophysical, electrochemical as well as photovoltaic properties in nanocrystalline TiO2-based dye sensitized solar cells (DSSCs). The insertion of vinyl bond into the conjugation bridge leads to the molecular planar configuration, and the conjugation bridge of 3,4-ethylenedioxythiophene (EDOT) is prone to positively shift its highest occupied molecular orbital (HOMO). The electrochemical impedance spectroscopy (EIS) results indicate that the grafted long alkyl chain onto thiophene is favorable to suppress dye aggregation when adsorbed onto TiO2 film and modification on interface of TiO2/dye/electrolyte, resulting in a relatively high open-circuit voltage (Voc). Under optimized conditions, dye LS-4 bearing hexylthiophene as the conjugation bridge shows a relatively high overall conversion efficiency of 5.45%, with a photocurrent of 11.61 mA cm–2, Voc of 744 mV.


Funded by

Science Fund for Creative Research Groups(21421004)

Distinguished Young Scholars(21325625)

NSFC/China

and Oriental Scholarship

Programme of Introducing Talents of Discipline to Universities

Science and Technology Commission of Shanghai Municipality(14YF1410500,15XD1501400)

Shanghai Young Teacher Supporting Foundation(ZZEGD14011)

School Funding of Shanghai Second Polytechnic University(EGD14XQD08)

“Shu Guang” Project(13SG55)


Acknowledgment

This work was supported by the Science Fund for Creative Research Groups (21421004), Distinguished Young Scholars, the National Natural Science Foundation of China (21325625), Oriental Scholarship, Programme of Introducing Talents of Discipline to Universities, Science and Technology Commission of Shanghai Municipality (14YF1410500 and 15XD1501400), Shanghai Young Teacher Supporting Foundation (ZZEGD14011), School Funding of Shanghai Second Polytechnic University (EGD14XQD08), and “Shu Guang” project (13SG55).


Interest statement

The authors declare that they have no conflict of interest.


Supplement

The supporting information is available online at chem.scichina.com and link.springer.com/journal/11426. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.


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  • Figure 1

    Chemical structures of organic sensitizers LS-1LS-4 with rational different thiophene units as conjugated bridge.

  • Figure 2

    Absorption spectra of LS-1LS-4 in CH2Cl2 (a) and on 3 μm transparent TiO2 films, dipping time for 30 min (b).

  • Figure 3

    Oxidative cyclic voltammetry plots of LS-1LS-4 in CH2Cl2.

  • Figure 4

    Optimized ground-state geometries for LS-1LS-4 (color online).

  • Figure 5

    Calculated frontier orbitals of dyes LS-1LS-4 (isodensity=0.020 a.u.) (color online).

  • Figure 6

    IPCE action spectra of LS-1LS-4 co-adsorbed with 20 mM chenodeoxycholic acid (CDCA).

  • Figure 7

    Photocurrent-voltage characteristics of DSSCs sensitized by LS-1LS-4 co-adsorbed with CDCA (color online).

  • Figure 8

    EIS Nyquist (a) and Bode (b) plots for DSSCs based on LS-1, LS-3 and LS-4 measured under dark.

  • Table 1   Photophysical and electrochemical properties of sensitizers in CHClsolution and adsorbed on TiO films

    Dyes

    Absorption

    HOMO c) (V) (vs. NHE)

    E0-0 d) (V)

    LUMO e) (V) (vs. NHE)

    λmax a) in CH2Cl2 (nm)/ε (M–1 cm–1)

    λmax b) on TiO2 (nm)

    LS-1

    483/20100

    442

    0.85

    2.17

    –1.32

    LS-2

    529/22800

    467

    0.84

    2.00

    –1.16

    LS-3

    523/31500

    445

    0.80

    2.12

    –1.32

    LS-4

    528/28200

    442

    0.84

    2.20

    –1.36

    , b) Absorption in CH2Cl2 and coated onto 3 μm TiO2 film; c) HOMO level in CH2Cl2 with ferrocene as internal reference; d) E0-0 is deduced from the absorption spectrum on 3 μm nanocrystalline TiO2 film; e) LUMO is obtained according to LUMO=HOMO–E0-0.

  • Table 2   Critical dihedral angles (in degrees) in – optimized at B3LYP/6-31G(d) level

    Dyes

    |C1–C2–C3–C4|

    LS-1

    16.8

    LS-2

    0

    LS-3

    19.48

    LS-4

    17.86

  • Table 3   Effect of CDCA on photovoltaic performances of –. Each value in this work was an average of five samples, while the standard deviations were lower than ±3%

    Dyes

    CDCA

    Jsc (mA cm–2) a)

    Voc (V)

    ffb)

    η (%) c)

    LS-1

    0 d)

    10.98

    612

    0.66

    4.41

    10 mM e)

    10.90

    644

    0.67

    4.56

    20 mM f)

    10.58

    650

    0.69

    4.72

    Saturated g)

    10.25

    644

    0.69

    4.54

    LS-2

    0 d)

    7.98

    608

    0.71

    3.42

    10 mM e)

    8.97

    632

    0.74

    4.22

    20 mM f)

    9.24

    646

    0.75

    4.50

    Saturated g)

    7.53

    592

    0.73

    3.24

    LS-3

    0 d)

    11.39

    705

    0.64

    5.17

    10 mM e)

    11.26

    707

    0.60

    4.77

    20 mM f)

    8.86

    704

    0.63

    3.94

    Saturated g)

    7.98

    679

    0.64

    3.48

    LS-4

    0 d)

    11.86

    719

    0.61

    5.22

    10 mM e)

    11.61

    744

    0.63

    5.45

    20 mM f)

    10.56

    745

    0.64

    5.07

    Saturated g)

    10.44

    721

    0.62

    4.66

    Jsc, short-circuit current density; b) ff, fill factor; c) η, quantum yield; d)–g) TiO2 films were pretreated with ethanol solution with 0, 10, 20 mM and saturated CDCA, respectively.

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