A-D-A Oligothiophenes as Donors for Solution-Processed Organic Solar Cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Poster, Cordula Wessendorf, 041
Publication date: 1st March 2014

A series of six highly soluble oligothiophenes as shown in Figure 1 (top) have been developed as do­nor material for solution-processed bulk heterojunction solar cells.^[1,2] The low band gap con­jugated oligomers (E_g=1.6 eV) consist of an acceptor-donor-acceptor (A-D-A) structure, with dicyanovinyl (DCV) end groups as terminal acceptor units and an electron-rich dithie­no­pyr­role (DTP) in the centre of the donor moiety. In contrast to vacuum-processed A-D-A oli­go­thiophenes shown in the literature,^[3] here the conjugated oligomer backbone carries long alkyl chains, which guarantee good solubility in organic solvents such as chloroform.

The impact of length of the DTP-substituents and position of the hexyl chains (“outer” (1-3) or “inner” (4-6) positions of thiophene units) on the solar cell parameters has been in­ves­tigated. Single junction solar cells with the standard configuration ITO/PEDOT:PSS/oli­go­mer 1-6:PCBM/LiF/Al were produced by spin-coating in ambient atmosphere. After opti­mi­­za­tion of the blend ratio, absorber thickness, and annealing methods it was shown that the best soluble compounds 5 and 6 gave the best solar cell results. The highest power con­ver­sion ef­­fi­ciency (PCE) of 6.1% and a remarkable high fill factor of 72% were obtained with oli­go­mer 6 and PC₆₁BM as acceptor (V_OC=843 mV, I_SC=10.1 mA/cm², FF=72 %).^[2] The use of 6 and PC₇₁BM as acceptor raised the PCE to 6.8% (V_OC=810 mV, I_SC=11.9 mA/cm², FF=70 %).

It was found that solvent vapour annealing (SVA) applied after the coating step had a very positive effect on the morphology and therefore on the solar cell parameters for most of the investigated oligomers. As an example, the PCE increased from 1.1% without SVA to 6.1% with SVA 6:PC₆₁BM solar cells (Figure 1, bottom, left). The influence of SVA on the layer mor­­pho­lo­gy was further investigated by atomic force microscopy, X-ray diffraction, and ab­sorp­tion spec­troscopy. Solar cells with oligomer 6:PC₇₁BM showed external quan­tum effi­cie­n­cies (EQE) of 50-60% over a broad range spanning from 340 to 700 nm after treatment with SVA (Figure 1, bottom, right).

Besides single junction solar cells, the most promising A-D-A oligothiophene 6 was fur­ther in­cor­porated in first solution-processed homo-tandem cells, which were assembled of two identical sub­cells of oligomer 6 and PC₆₁BM (ITO/PEDOT:PSS/6:PC₆₁BM/ZnO-nanopar-ticles/PEDOT:PSS/6:PC₆₁BM/LiF/Al), resulting in a PCE of 5.7% (V_OC=1556 mV, I_SC=5.4 mA/cm², FF=68 %).

Tandem cells with complementary solution-processable oligomeric ab­sor­bers are currently under way in our laboratories. In the near future, we aim at the up-scaling of the coating pro­cess by slot die-coating and at the fabrication of larger modules.

Figure 1: Structures of the novel solution-processable low band gap A-D-A oligothiophenes 1-6 (top); influence of solvent vapour annealing (SVA) on I/V-curves of oligomer 6:PC[61]BM solar cells (bottom, left) and EQE-spectra of 6:PC[71]BM solar cell (bottom, right).
[1] M. Weidelener, C. D. Wessendorf, J. Hanisch, E. Ahlswede, G. Götz, M. Lindén, G. Schulz, E. Mena-Osteritz, A. Mishra, P. Bäuerle, Chem. Commun. 2013, 49, 10865. [2] C. D. Wessendorf, G. L. Schulz, P. Kar, I. Ata, M. Weidelener, M. Urdanpilleta, J. Hanisch, E. Mena-Osteritz, A. Mishra, M. Lindén, E. Ahlswede, P. Bäuerle, Adv. Energy Mat. submitted. [3] R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, M. Weil, C. Körner, H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich, P. Bäuerle J. Am. Chem. Soc. 2012, 134, 11064.

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