A comparative study of a polyene-diphenylaniline dye and Ru(dcbpy)2(NCS)2 in electrolyte-based and solid-state dye-sensitized solar cells

doi:10.1016/j.tsf.2007.12.035

Thin Solid Films

Volume 516, Issue 20, 30 August 2008, Pages 7214-7217

https://doi.org/10.1016/j.tsf.2007.12.035 Get rights and content

Abstract

A small organic sensitizer, the polyene-diphenylaniline dye D5, was compared with the standard sensitizer N719 (Ru(dcbpy)₂(NCS)₂) in a dye-sensitized solar cell investigation. In solar cells with relatively thin layers of mesoporous TiO₂ (< 3 μm) D5 outperformed N719 because of its high extinction coefficient. D5 showed also better performance than N719 in the case of sensitization of mesoporous ZnO. In solid-state solar cells, where the iodide/triiodide electrolyte was replaced by an amorphous hole conductor (spiro-OMeTAD), D5 gave promising preliminary results. The hole conductivity, observed in monolayers of D5 adsorbed at TiO₂, may possibly lead to improved performance in such cells.

Introduction

To date, best efficiency and stability data for dye-sensitized solar cells (DSCs) have been obtained with Ru-based sensitizers [1], [2], [3]. There is, however, great interest in fully organic dyes as sensitizers, as they can have very high extinction coefficients, and can potentially be produced at a lower cost.

A series of dye molecules with the general structure donor – conjugated linker – acceptor has been synthesized in our lab and was characterized and tested in dye-sensitized solar cells [4], [5], [6], [7]. These dyes display an intramolecular charge separation upon photoexcitation: the electron moves towards the acceptor group, which is attached at the surface of TiO₂, while positive charge moves towards the donor group. We have systematically changed the length of the conjugated linker and have investigated the effect of the replacing the acceptor group [6], [7]. Energy-level matching between the dye, TiO₂ and redox electrolyte is crucial: LUMO levels of the dye should be sufficiently high in energy to allow electron injection from the excited dye. Additives in the electrolyte can be used to fine-tune the energy of the conduction band edge of the TiO₂. On the other hand, HOMO levels of the dye should be sufficiently low in energy to promote fast reduction of the oxidized dye by the iodide in the electrolyte [5].

In this short paper we will limit ourselves to the description of one successful organic dye, called D5 [4], and compare its performance with the standard dye in DSC research, the Ru-complex called N719 (bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2′-bypiridine-4,4′-dicarboxylato)ruthenium(II)). The polyene-diphenylaniline dye D5 is relatively easily synthesized and shows good solar cell performance. Its chemical structure is shown in Fig. 1. Power conversion efficiencies exceeding 5% have been obtained using an iodide/triiodide-based redox electrolyte [4]. Promising results are also obtained when the liquid electrolyte is replaced by a solid hole conductor, which will be presented here.

Section snippets

Solar cell preparation

Mesoporous TiO₂ and ZnO films, with nanocrystal size of ~ 20 nm, were deposited onto transparent conducting oxide (TCO) coated substrates and used in electrolyte-based dye-sensitized solar cells as described elsewhere [8], [9]. D5, available from a previous study [4], was adsorbed at the mesoporous films from a 0.5 mM solution in acetonitrile (ethanol in case of ZnO), while N719 dye (Solaronix, Switzerland) was adsorbed from a 0.5 mM solution in ethanol. Dye adsorption time was typically 16 h.

Electrolyte-based solar cells (el-DSC)

D5 is readily adsorbed at mesoporous TiO₂ and displays good results in dye-sensitized solar cells. In Fig. 1a, the incident photon-to-current conversion efficiency (IPCE) spectra of D5 and N719 are compared. In this comparison thin films of TiO₂ were used (< 3 μm). Solar cells with D5 give a much higher IPCE (72% at 430 nm) than those with N719 (29% at 510 nm). This difference is mainly caused by the much higher extinction coefficient of D5 (33,000 M^− 1 cm^− 1 at 427 nm) compared to N719 (14,000 M^− 1

Conclusions

Small organic dye molecules like the polyene-diphenylaniline dye D5 show competitive performance in dye-sensitized solar cells with relatively thin layers of mesoporous TiO₂, because of their high extinction coefficients. D5 outperformed N719 in the case of sensitization of ZnO electrodes. In solid-state solar cells, where the redox electrolyte was replaced by an amorphous hole conductor (spiro-OMeTAD), D5 showed promising preliminary results. The hole conductivity, observed in monolayers of D5

Acknowledgement

We acknowledge the Swedish Energy Agency, the Swedish Research Council and the Knut and Alice Wallenberg foundation for financial support.

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A comparative study of a polyene-diphenylaniline dye and Ru(dcbpy)2(NCS)2 in electrolyte-based and solid-state dye-sensitized solar cells

Abstract

Introduction

Section snippets

Solar cell preparation

Electrolyte-based solar cells (el-DSC)

Conclusions

Acknowledgement

J. Photochem. Photobiol. C: Photochem.Rev.

Coord. Chem. Rev.

Thin Solid Films

Jpn. J. Appl. Phys.

Prog. Photovolt: Res. Appl.

Chem. Commun.

J. Phys. Chem. C

J. Org. Chem.

A comparative study of a polyene-diphenylaniline dye and Ru(dcbpy)₂(NCS)₂ in electrolyte-based and solid-state dye-sensitized solar cells