We now report metal-free organic dyes having a new type of indoline structure, which exhibits high efficiencies in dye-sensitized solar cells. The solar energy to current conversion efficiencies with the new indoline dye was 6.51%. Under the same conditions, the N3 dye was 7.89% and the N719 dye was 8.26%. The new indoline dye was optimized for the amount of 4-tert-butyl pyridine in the electrolyte and cholic acid as a coadsorbent. Subsequently, the solar energy to current conversion efficiencies reached 8.00%. This value was the highest obtained efficiency for dye-sensitized solar cells based on metal-free organic dyes without an antireflection layer.

[26] SIMS analysis was carried out in the negative-ion mode with a 25 kV Ga + primary ion gun and a 2 pA ion-beam current.[27] On some substrates, we also purposely denatured the attached laminin using hot water. On those substrates, no PC12 activity could be seen. Dye-sensitized solar cells are an interesting low-cost alternative to conventional solar cells. Efficiencies over 10 % have been achieved. [1,2] Advantageous is the replacement of the liquid electrolyte in these devices with a solid charge carrier material to avoid any sealing and long-term stability problems. In 1998 Bach et al. [3] demonstrated that the electrolyte can be replaced by a hole conductor. Here we report a very efficient solid-state solar cell with the amorphous organic hole transport material 2,2¢,7,7¢-tetrakis-(N,N-di-p-methoxyphenylamine)9,9¢-spirobifluorene (spiro-OMeTAD) as hole conductor and for the first time an organic metal-free indoline dye as sensitizer. Record efficiencies for this type of cell of over 4 % over the solar spectrum were reached. Until now, sensitization using organic dyes has not been as efficient as using ruthenium dyes, which have been successfully applied previously. Metal-free dyes such as perylene derivatives, [4] coumarin dyes, [5] porphyrin dyes, [6] and cyanine and merocyanine dyes [7] have been used as sensitizers, but did not achieve the same solar conversion efficiency as ruthenium dyes. The efficiencies achieved in this work with the indoline dye (D102) are even higher than the highest currently reported values for ruthenium-dye sensitized solid-state cells. In addition, this dye has the advantage that it can be produced at low cost, because it does not contain the expensive rare metal ruthenium and it is easy to synthesize. Up to now the best results reported with spiro-OMeTAD as a hole conductor are efficiencies of 3.2 %. [8] In that work the dye uptake and open-circuit voltage were optimized by a silver complexation. In the case of the indoline dye we were able to reach an efficiency of just over 4 % without further optimization. This shows the extremely high potential of the indoline dye as a sensitizer in solid-state dye-sensitized solar cells. Indoline dyes have previously been used in dye-sensitized solar cells with a liquid electrolyte. [9,10] There also they showed a good performance of up to g = 6.1 % conversion efficiency compared to 6.3 % for a N3 dye-sensitized cell.[9] Solid-state devices usually have a lower performance, which is not the case here with an extraordinary efficiency of 4 %, which approaches that of their liquid electrolyte counterpart. Figure 1a shows the chemical structure of the indoline dye D102 and spiro-OMeTAD (Fig. 1b), the organic compounds used in this work.The dye D102 has a very strong absorption coefficient (55 800 L mol ±1 cm ±1 at 491 nm), which is four times stronger than the previously used ruthenium dye N3 (13 900 L mol ±1 cm ±1 at 541 nm). [9] This makes it of especial interest for solid-state devices, where the film thickness is crucial. Our results show...

A solar-to-electric conversion efficiency of 6.1% is achieved with this new dye, compared to 6.3% for N3 dye under the same experimental conditions. Although these indoline dyes are slightly less efficient than N3 dye, the cost of indoline dye is much less than for N3 due to ease of preparation. Furthermore, indoline dyes are shown to be highly stable to photoredox processes by cyclic voltammogram.

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