Deposition of polyaniline via molecular self-assembly on TiO2 and its uses as a sensitiser in solid-state solar cells

https://doi.org/10.1016/j.jphotochem.2003.12.026Get rights and content

Abstract

Volatile solvent free quasi-solid solar cells were fabricated using acid-doped polyaniline (PANI) covalently grafted on surface-modified nanocrystalline TiO2 substrates via self assembled monolayer (SAM) of silane-bearing aniline compound (C6H5NHC3H6Si(OMe)3) (SAM) and investigated their photovoltaic performances with different additives. PANI was able to sensitize TiO2 efficiently in the presence of SAM. Introduction of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluroromethyl)sulfonylamid and LiTf2N enhanced the photocurrent of TiO2/PANI/Au cell by more than two-fold. The cell treated with 1-methyl-3-n-hexylimidazolium iodide delivered a photocurrent of ∼450 μA cm−2 with photovoltage of ∼565 mV, giving an efficiency of ∼0.12% (FF=0.47).

Introduction

There has recently been great interest in the development of dye-sensitized solar cells (DSCs) owing to their potential low cost alternative to the conventional inorganic counter part [1]. In general these DSCs consist of sensitisers (dyes), which absorb light and inject electrons to the semiconductor where the dye is chemically attached on the surface. In order to complete the charge transfer mechanism, these dye-sensitized electrodes are combined either with a hole conducting material or a redox couple usually iodide/triiodide in an electrolyte [2], [3]. However, due to some limitations, such as encapsulation problems, practical applications of these cells have not been fully realized yet. Therefore, to overcome these problems it is important to fabricate efficient dye-sensitized solid-state solar cells (DSSSCs) by replacing the liquid electrolyte with suitable hole conductor like CuI, CuSCN, PPY, OMeTAD, etc. [4], [5], [6], [7], [8], [9].

On the other hand, since the sensitisers used in these DSCs are expensive and generally they employed noble metal complexes, investigations have been widened to explore the possibilities of replacing these by various materials such as organic pigments, conducting polymers (CP’s), etc [10], [11]. In this context, conducting polymers with extended π-conjugated electron systems such as polypyrrole, polythiophenes, polyanilines, etc have shown great promises due to their high absorption coefficients in the visible part of the spectrum and high mobility of charge carriers. Further, many CP’s in their undoped or partially doped states are electron donors upon photo-excitation and they are known as good hole conductors, which can carry current with several milliamperes [11]. Therefore, CPs may replace, in principle, the dye and the electrolyte, bringing together the function of the sensitiser and hole conductor within a single material. To use these CP’s effectively as sensitisers in these devices, rigid bondings, such as by carboxylic moieties in the ruthenium based dyes, with semiconducting metal oxide substrate are required [1]. However, due to the difficulties encountered in the synthesize of these materials with suitable carboxylic moieties, it is generally accepted that self-assembled monolayers are highly promising to construct such a molecular architecture on metals and semiconducting surfaces. This approach has several advantageous such as; it permits the fabrication of highly ordered, appropriately oriented 2D and 3D structures at a fraction of the cost over traditional band-gap engineering like molecular beam epitaxy [12], [13], [14]. In this context, several publications revealed that ployaniline could be successfully grafted on to glass substrates (SiO2) via self-assembled monolayers (SAM) compounds due to their suitable structural compatibilities [14], [15].

On the other hand, recently, some ammonium salts like imidazolium salts have attracted much attention as ionic liquids or room temperature molten salts and are reported to improve the physical properties of the conducting polymers [16], [17], [18]. In addition, one of the authors (S.Y.) has clarified that the adsorption of cationic species like imidazolium cations to nanoporous TiO2 electrodes enhances electron diffusion coefficients of the TiO2 electrodes in dye-sensitized solar systems [19].

Therefore, by considering these facts, here we investigated the capability to inject electrons to TiO2 upon excitation of covalently grafted polyaniline via (C6H5NHC3H6Si(OMe)3) (SAM) to TiO2. Relatively high photovoltaic performances than the reported values for the polymer sensitized cells were obtained when the ionic liquids, 1-methyl-3-n-hexylimidazolium iodide and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfone)imide (EMImTf2N) with LiTf2N were added to the cell.

Section snippets

Preparation of mesoporus nanocryastalline TiO2 films

Dense less porous (compact) nanoporous films of TiO2 were coated on fluorine doped conducting tin oxide (FTO) glasses (sheet resistance ∼10 Ω/□) by the following method. Prior to use, FTO glasses were cleaned in an ultrasonic bath using detergent, distilled water and isoproponol and dried with dry air-flow and followed by UV-O3 treatments for 18 min (Technovision Inc., Model 208). An ethanolic solution of titaniumtetra-isoporopoxide Ti[OCH(CH3)3]4 containing a few drops of HNO3 (pH 2) was spin

PANI on TiO2

Fig. 2 shows the IR spectra of (a) SAM and TiO2 before (b) and after (c) the treatments of SAM. Chemical anchoring of SAM onto TiO2 surfaces can be identified by the disappearance of bands assigned to the various functional groups as follows. Complete disappearance of the peaks at 2837 and 2933 cm−1 in the SAM/TiO2 shows splitting of methoxy groups during grafting processes. In addition to that disappearance of the broad band at 3440 cm−1 in the SAM treated TiO2, which is assigned to the

Conclusion

An organic solvent free heterojunction of n-TiO2/p-PANI with enhanced photovoltaic properties could be constructed by chemical polymerisation of aniline followed by covalent grafting via C6H5NHC3H6Si(OMe)3 compound. Further improvement could be obtained by employing another conducting polymer such as derivatives of thiophenes as hole conductor in this system because PANI film can transfer the holes effectively and rapidly to the counter electrode side when an another hole conductor is employed.

Acknowledgements

The authors would like to thank Mr. Seiji Watase, Inorganic Chemistry Department, Osaka Municipal Technical Research Institute, Japan, for providing XPS data and Ministry of Education, Culture, Sports, Science and Technology of Japan, and the New Energy and Industrial Technology Development Organisation (NEDO) under Ministry of Economy, Trade and Industry of Japan. G.K.R.S. acknowledges the Postdoctoral Research Fellowship from Japan Society for the Promotion of Science (JSPS). This work was

References (25)

  • G.P Smestad et al.

    Sol. Energy Mater. Sol. Cells

    (2003)
  • K Murakoshi et al.

    Sol. Energy Mater. Sol. Cells

    (1998)
  • G.K.R Senadeera et al.

    Sol. Energy Mater. Sol. Cells

    (2002)
  • Z.F Li et al.

    Synth. Met.

    (2002)
  • Y Saito et al.

    Synth. Met.

    (2002)
  • Y Li et al.

    Synth. Met.

    (1998)
  • Y Hao et al.

    Sol. Energy Mater. Sol. Cells

    (1998)
  • B O’Regan et al.

    Nature

    (1991)
  • M Grätzel

    Prog. Photovolt. Res. Appl.

    (2000)
  • K Tennakone et al.

    Semicond. Sci. Technol.

    (1995)
  • U Bach et al.

    Nature

    (1998)
  • B O’Regan et al.

    Chem. Mater.

    (2002)
  • Cited by (0)

    View full text