Investigation of Solvent Vapor Annealing on Highly Efficient Solution-Processed Dithienopyrrole-Based A-D-A Oligothiophene Bulk-Heterojunction Solar Cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Cordula Wessendorf, 217
Publication date: 5th February 2015

Herein two isomers 1 and 2 shown in Figure 1 (top) of highly soluble oligothiophenes as do­nor materials for solution-processed bulk heterojunction solar cells are investigated in more detail. The two isomers differ in the position of the hexyl chains which are either at the “outer” (1) or “inner” (2) positions of the thiophene units.^[1] In a former study, we showed that the position of the hexyl chains has a strong influence of the morphology as well as on the solar cell parameters and that both can be improved by solvent vapour annealing (SVA) with chloroform.^[1] Now we have also fabricated single junction solar cells with PC₇₁BM as acceptor instead of PC₆₁BM. With oligomer 2 as donor, also a strong increase in short circuit current, fill factor and efficiency was found upon SVA (Figure 1, bottom left). Due to the higher absorption of PC₇₁BM, the efficiency could be enhanced and solar cells with 7.1% power con­ver­sion ef­fi­ciency (PCE) at maximum could be obtained after SVA (in comparison: with PC₆₁BM it was 6.1% PCE).^[2] The effects on the morphology during SVA have now been studied in more detail by 2D-grazing-incidence x-ray spectroscopy (GI-XRD), negative secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM) and photoluminescence (PL).^[2] In accordance to former investigations with absorption spectroscopy and AFM,^[1] it was found that for 1:PC₆₁BM there was only a very slightly change upon SVA, whereas for 2:PC₆₁BM an increase in crystallinity and domain size was found after SVA. This was verified by GI-XRD where enhanced p-pstacking of oligomer 2 was found. Also by KPFM increased domains of both, donor and acceptor domains were observed. Additionally, the PL signal for 2:PC₆₁BM is strongly risen, indicating reduced exciton quenching at acceptor-donor interfaces due to the promoted phase segregation upon SVA. Additionally it was found by negative SIMS depths profiles, that for 2:PCBM a vertical gradient is formed during SVA. First, a few monolayers of 2 are on top (verified also by GI-XRD), followed by ~40 nm of enriched PCBM phase (increase of O-signal and decrease of CN- and S-signals) and at the ITO side an oligomer-rich phase is formed (decrease of O-signal, increase of CN- and S-signals) (Figure 1, bottom right).^[2] This vertical gradient facilitates charge transport and is assumed to be the reason besides the enhanced crystallinity for the improved short circuit current density and fill factor upon SVA.
Figure 1: Structures of the solution-processable low band gap A-D-A oligothiophenes 1 and 2 (top); influence of solvent vapour annealing (SVA) on I/V-curves of oligomer 2:PC[71]BM solar cells (bottom, left) and negative SIMS- depths profiles of 2:PC61BM absorber layers without (dash) and with (solid) SVA (bottom, right).[2]
[1] Wessendorf, C. D.; Schulz, G. L.; Mishra, A.; Kar, P.; Ata, I.; Weidelener, M.; Urdanpilleta, M.; Hanisch, J.; Mena-Osteritz, E.; Lindén, M.; Ahlswede, E.; Bäuerle, P. Efficiency Improvement of Solution-Processed Dithienopyrrole-Based A-D-A Oligothiophene Bulk-Heterojunction Solar Cells by Solvent Vapor Annealing, Adv. Energy Mater. 2014, 4, 1400266(1)-1400266(10). [2] Wessendorf, C. D.; Perez, A.; Hanisch, J.; Arndt, A.; Ata, I.; Schulz, G. L.; Quintilla, A.; Bäuerle, P.; Lemmer, U.; Ahlswede, E.; Barrena E. in preparation.

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