Isomerization and Aggregation of the Solar Cell Dye D149

Abstract

D149, a metal-free indoline dye, is one of the most promising sensitizers for dye-sensitized solar cells (DSSCs) and has shown very high solar energy conversion efficiencies of 9%. Effective electron injection from the excited state is a prerequisite for high efficiencies and is lowered by competing deactivation pathways. Previous investigations have shown surprisingly short-lived excited states for this dye, with maximum lifetime components of 100–720 ps in different solvents and less than 120 ps for surface-adsorbed D149. Using steady-state and time-resolved fluorescence, we have investigated the photochemical properties of D149 in nonpolar and polar solvents, polymer matrices, and adsorbed on ZrO2, partially including a coadsorbent. In solution, excitation to the S2 state yields a product that is identified as a photoisomer. The reaction is reversible, and the involved double-bond is identified by NMR spectroscopy. Our results further show that lifetimes of 100–330 ps in the solvents used are increased to more than 2 ns for D149 in polymer matrices and on ZrO2. This is in part attributed to blocked internal motion due to steric constraint. Conversely, concentration-dependent aggregation leads to a dramatic reduction in lifetimes that can affect solar cell performance. Our results explain the unexpectedly short lifetimes observed previously. We also show that photochemical properties such as lifetimes determined in solution are different from the ones determined on semiconductor surfaces used in solar cells. The obtained mechanistic understanding should help develop design strategies for further improvement of solar cell dyes.

Improvement of light-harvesting and electron injection efficiencies by lithium ion in D149-sensitized nanocrystalline TiO2 films

“…This is consistent with Horiuchi et al's [14] original paper on D149-sensitized DSCs. In that paper and later papers [18,22], the spectral red shift was claimed to be due to aggregate formation; a theoretical calculation was also reported [23]. In our opinion, the aggregate formation model proposed by these researchers is not supported by direct experimental evidence.…”

“…A possible interpretation is that the molecular structures in solution and on the semiconductor surface are different. Recently, El-Zohry et al [18] reported that photoisomerization of D149 in solution is suppressed when dyes are adsorbed on semiconductor films. This finding implies that the structure of D149 is restricted on the TiO 2 surface.…”

“…D149 ( Figure 1) is a promising sensitizing dye, and DSC devices based on this dye show a high solar light-to-energy conversion efficiency (Á > 8%) [14,15]. To understand the origin of the high performance, various characterization methods have been employed [10,[16][17][18][19]. However, in reality, the origin is still controversial because the synergy effect between a sensitizing dye and the electrolyte solution plays an important role.…”

Enhanced Photovoltaic Performance via Co-sensitization of Ruthenium (II)-Based Complex Sensitizers with Metal-Free Indoline Dye in Dye-Sensitized Solar Cells

“…Rutheniumbased sensitizers such as the N3 dye/ N719 dye [9,10,23] and black dye [24] were rigorously tested as light har-vesters and reached remarkable overall power conversion efficiency (η) in the range of 11% under Air Mass 1.5 (AM 1.5) irradiation while organic photosensitizers have reached power conversion efficiency (PCE) in the range of 5-8% [25][26][27][28][29][30][31]. Metal-free indoline dye (D149) is the most effective photosensitizer for DSSCs and has attained very high efficiencies of 9% [32]. Recently, devices fabricated by using ruthenium (II) complexes with cobalt (3+/2+) based electrolytes have achieved efficiencies of 9.4% [33].…”

Ultrafast Charge‐Transfer Reactions of Indoline Dyes with Anchoring Alkyl Chains of Varying Length in Mesoporous ZnO Solar Cells

“…Detailed studies regarding the electron injection from the excited state of D149 to ZnO [10,[16][17][18][19][20][21] revealed that the electron injection occurs on a femtosecond to picosecond timescale and that electron injection into ZnO surface states is a non-negligible path that slows down electron injection dynamics. [11,22] Transient absorption spectroscopy (TAS) measurements performed by us previously on complete D149/ZnO cells with a variety of electrolytes based on the I À /I 3 À redox couple yielded characteristic times for primary charge injection, parasitic recombination, and regeneration rates.…”