Intramolecular Charge-Transfer Tuning of Perylenes:  Spectroscopic Features and Performance in Dye-Sensitized Solar Cells

Abstract

Five novel perylene molecules with different intramolecular charge-transfer (ICT) characters have been synthesized. The relation between the ICT character for different donating groups and the results for their electro- and photochemical properties as well as their performance in nanostructured dye-sensitized solar cells (nDSC) are reported. With the stronger donors, we obtain a shift of the lowest unoccupied molecular orbital (LUMO) to more negative potential versus normal hydrogen electrode (NHE) as well as an increase the charge separation in the dye upon excitation. Ab initio calculations were used to analyze the effects on orbital energies and electron distribution with the different donors. Incorporating the dyes in nDSCs, we see a drastical improvement in the performance for the more polar dyes. In particular, we find a much improved photovoltage because of higher LUMO levels, allowing conduction band tuning in the TiO2 as well as a contribution from the permanent dipoles in the dyes. The photocurrent improves remarkably with increasing ICT character of the dyes. The external quantum efficiency reached over 70%, and the overall solar-to-electrical energy conversion efficiency was improved to almost 4% for the dye with highest ICT character, which can be compared with devices with the standard N719 dye (Ru(dcbpy)2(NCS)2) showing 6% under similar conditions. The performance is a significant improvement compared to previous reports for perylenes and lifts the performance from modest to promising. Initial stability tests show that the dye with the highest performance was spectrally stable after more than 2000 h of irradiation in a solar-cell device.

An Improved Perylene Sensitizer for Solar Cell Applications

“…We have also demonstrated the possibility to utilize perylenes for solid-state solar cells yielding 1.8 % power conversion efficiency under standard illumination conditions. Comparing these results with other perylene sensitizers, [6] we conclude that the thiophenol groups in 5 contribute strongly to the obtained high efficiencies. Work to extend the spectral response of these sensitizers further into the red and near-IR spectral region is in progress.…”

“…This phenomenon is well known for perylene anhyride sensitizers. [5,6] Figure 2 shows the cyclic voltammogram [11] of 5 which exhibits reversible waves both in the oxidation and reduction regions, indicating the electrochemical stability of 5, a vital parameter for the durability of solar cells. The HOMO and LUMO are observed at 0.48 V and À1.08 V (vs Fc/Fc + ), respectively.…”

Functionalized pentacenes for dye-sensitized solar cells

“…Most organic sensitizers absorb light below 600 nm while reasonably efficient near-IR/IR dyes used in DSSCs are only occasionally seen in literature. [24][25][26][27][28][29] Pentacene chromophores, with potential absorption out to 750 nm, 30 could extend the window of performance for DSSCs, provided that there is efficient coupling between the acene and the inorganic matrix. We report here the synthesis of a series of pentacene dyes ( Fig.…”

Geometrical, electronic structure, nonlinear optical and spectroscopic investigations of 4-(phenylthio)phthalonitrile dye sensitizer for solar cells using quantum chemical calculations

“…Benefited from systematic device engineering and continuous material innovation, a state of the art DSSC with a ruthenium sensitizer has achieved a validated efficiency of 11.1% [4] measured under the air mass 1.5 global (AM1.5G) conditions. In view of the limited ruthenium resource and the heavy-metal toxicity, metal-free organic dyes have received surging research interest in recent years [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Because of their high molar absorption coefficient, relatively simple synthesis procedure, various structures and lower cost in contrast to a ruthenium dye and the flexibility in molecular tailoring of an organic sensitizer provides a large area to explore [22][23][24].…”