RG Dyes for organic solar cells

RG-Leader:      Dr. Amaresh Mishra (amaresh.mishra(at)uni-ulm.de)
Members:        Semih Atasever (semih.atasever(at)uni-ulm.de)
                       Stefan Haid (stefan.haid(at)uni-ulm.de)
                       Dr. Prasenjit Kar (prasenjit.kar(at)uni-ulm.de) A. von Humboldt Stipendiat)
                       Hannelore Kast (hannelore.kast(at)uni-ulm.de)
                       Mirjam Löbert (mirjam.loebert(at)uni-ulm.de)
                       Martin Weidelener (martin.weidelener(at)uni-ulm.de)
                      

Our research group is focusing on synthesis and development of special dyes and materials with tailor–made electronic and redox properties as active components suitable for application in organic solar cells, in particular, dye-sensitized solar cells. The most important goal is the design of materials with good solubility and thermal stability. Towards this direction, several functionalized oligothio­phenes bearing electron-donating and accepting groups attached at different locations of the π-conjugated systems were synthesized to evaluate their effects in organic electronic devices.[1,2]

Development of Dyes for Dye-Sensitized Solar Cell (DSSCs)

Dye-sensitized solar cell (DSSC) technology appears to be a highly promising and cost-effective alternative in the photovoltaic energy sector. Currently, DSSC technology is based on three different classes of electrolyte systems (liquid, ionic liquid and solid-state). The general operating principle of a dye-sensitized solar cell comprising a photoanode (TiO2) and a passive cathode is depicted in Figure 1.

Figure 1. Fundamental processes and energy level diagram of a dye-sensitized solar cell.

The sensitizers for DSSCs can be grouped into two broad areas: 1) Functional ruthenium(II)–polypyridyl complexes with power conversion efficiencies approaching 11% in liquid and ≥5% in solid state devices and 2) metal-free organic donor–acceptor (D–A) dyes, cur­rent­ly run­ning at power conver­sion efficiencies of 8-9% in liquid, about 7% in ionic liquid, and nearly 5% in solid state cells.[2]

The very general de­sign principle for metal-free organic dyes or sensitizers consists of a donor-acceptor-substituted π‑conju­ga­ted “bridge” and an an­cho­ring group to the TiO2 which is attached at the side of the acceptor (Figure 2).   

 

Figure 2: Design principle of organic dyes for TiO2 photoanodes in DSSCs.

   

We are presently active in designing and synthesizing thiophene-based dyes for DSSCs. Just as examples, the following novel developments are shown in Figure 3. A RuII-complex including thiophene as an extended π‑conju­ga­ted system in which the carboxylic acid groups are attached to the thiophene units was developed. The complex shows power conversion efficiencies of up to 9.1% in volatile electrolyte, and 8.3 % in low-volatile electrolyte and 3.6 % in solid state devices. The cells display remarkable stability retaining >95 % of its efficiency under continuous light soaking over 1000h.[3,4,7]

Click-chemistry approach was used to develop a series of new 1,2,3-triazolyl pyridine ligands. A Ru-complex comprising 1,2,3-triazolyl pyridine ligand shows efficiency of 7.8 %.[8]

A metal-free dye comprising a diphenylamine-terthiophene dyad as donor and cyanoacrylic acid as acceptor unit gave efficiencies of up to 6.8% in non-volatile electrolyte and >3% in solid state.

The implementation of these novel dyes into DSSCs is achieved with our cooperation partners at the EPFL Lausanne, Prof. M. Grätzel, Dr. S. M. Zakeeruddin, and Dr. M. K. Nazeeruddin.

 

Figure 3: Structure of some RuII-complexes and a thiophene-based metal-free organic dye used in DSSCs.

Dyes in p-DSSCs with Photocathodes and Tandem Devices

Recently, DSSCs comprising photocathodes (NiOx) came into play. For the first time, tandem cells become available comprising two photoelectrodes at the same time, a photoanode (TiO2) and a photocathode (NiOx) (Figure 4).

                       
Figure 4: Energy level diagram of dye-sensitized tandem DSSCs.
Figure 5: Design principle of organic dyes  NiOx photo­anodes in DSSCs.

In principle, dyes for adsorption on photocathodes should be designed on the reverse way, i.e., the anchoring group is attached to the donor (D) part of the D-π-A system (Figure 5). In this case, the ac­ceptor unit of the dye molecule should be remote from the semiconductor surface. The basic requirements for the use of this ‘inverted’ type of dye adsorbed on photo­cathodes are firstly that the HOMO level of the dye must be sufficiently below the valence band of semi­con­doctor.

Se­cond­ly, the LUMO level must be sufficiently above the redox potential of the I3–/I– system. Upon light excitation, the elec­trons flow from the excited dye to the electrolyte and the dye ground sta­te is re­generated by effi­cient electron tra­ns­­fer from the valence band of a p-type semiconductor.



 

         

In this respect, we develop and synthesize thiophene-based dyes (Figure 6), and record efficiencies for DSSCs with NiOx photocathodes (0.4%) and tandem DSSCs (2.42%) have been  achieved in collaboration with Dr. Udo Bach and his group (Monash University, Melbourne).[5]

Figure 6: Structure of Dyes used for NiO-based photocathodes.

References

[8] I. Stengel, A. Mishra, N. Pootrakulchote, S.-J. Moon, S. M. Zakeeruddin, M. Gratzel, P. Bäuerle; “Click-chemistry” approach in the design of 1,2,3-triazolyl-pyridine ligands and their Ru(II)-complexes for dye-sensitized solar c ells, J. Mater. Chem. 2011, 21, 3726-3734.
[7] A. Mishra, N. Pootrakulchote, M. Wang, S.-J. Moon, S. M. Zakeeruddin, M. Grätzel, P. Bäuerle; A Thiophene-based Anchoring Ligand and its Heteroleptic Ru(II)- complex for Efficient Thin Film Dye-Sensitized Solar Cells, Adv. Funct. Mater. 2011, 21, 963-970.
[6] A. Nattestad, A. J. Mozer, M. K. R. Fischer, Y.-B. Cheng, A. Mishra, P. Bäuerle, U. Bach; Highly Efficient Photocathodes for Dye-Sensitized Tandem Solar Cells, Nat. Material. 2010, 9, 31-35.
[5] M. K. R. Fischer, S. Wenger, M. Wang, A. Mishra, S. M. Zakeeruddin, M.  Grätzel, P. Bäuerle, D-π-A Sensitizers for Dye-Sensitized Solar Cells: Linear vs Branched Oligothiophenes, Chem. Mater. 2010, 22, 1836-1845.
[4]  A. Mishra, N. Pootrakulchote, M. K. R. Fischer, C. Klein, M. K. Nazeeruddin, S. M. Zakeeruddin, P. Bäuerle, M. Grätzel; Design and Synthesis of a Novel Anchoring Ligand for Highly Efficient Thin Film Dye-Sensitized Solar Cells, Chem. Commun. 2009, 7146-7148.
[3] F. Sauvage, M. K. R. Fischer, A. Mishra, S. M. Zakeeruddin, M. K. Nazeeruddin, P. Bäuerle, and M. Grätzel; A Dendritic Oligothiophene Ruthenium Sensitizer for Stable Dye-Sensitized Solar Cells, Chemsuschem 2009, 2, 761-768.
[2]  A. Mishra, M. K. R. Fischer, P. Bäuerle; Metal-free Organic Dyes for Dye-Sensitized Solar Cells: From Structure Property Relationships to Design Rules, Angew. Chem. Int. Ed. 2009, 48, 2474-2499.
[1] A. Mishra, C.-Q. Ma, P. Bäuerle, Functional Oligothiophenes: Molecular Design for Multi-Dimensional Nanoarchitectures and their Applications, Chem. Rev. 2009, 109, 1141-1276.