Correlation between optical properties and molecular aggregation in conjugated low band gap copolymers : PTB7 and PTB7-Th (Poly thieno [3,4-B] thiophene/ benzodithiophene)

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

Poster, Fatima Bencheikh Aboura, 235
Publication date: 5th February 2015

Solar cells based on interpenetrating blend of polymer and soluble fullerene derivative have recently attracted much attention with recently published power conversion efficiency (PCE) of 10.3% [1]. In order to improve the PCE of organic solar cells, the  molecular engineering mastering (design of materials), interfaces between organic-organic and organic-inorganic materials [2] as well as the morphology of the photo-active layer must be understood and improved. Many studies on the correlation between the morphology and the optical properties were carried out on homopolymers such as P3HT (poly(3-hexylthiophene)). In contrast, studies on new low band gap copolymers are still focused on the design of new molecular structures to optimize energy levels. These copolymers have a complex structure. They consist of an alternation of two units, an electron donor and an electron acceptor. Due to the complexity of the structure of these copolymers, the relationship between the film morphology and the optical properties is less understood than for conjugated homopolymers. In this study, we have chosen two copolymers, the PTB7 [3] and PTB7-Th [1] (Poly thieno [3,4-B] thiophene/ benzodithiophene). The difference between these two copolymers lies in the chemical structure of the side chains attached to the BDT (benzodithiophene) unit. We initially determine the dielectric function of thin film by spectroscopic ellipsometry which permits us to identify the optical transitions in terms of energy and intensity, independently of the thickness. Second, by using complementary characterizations, spectrophotometry and photoluminescence, on solution and thin films, we interprets ellipsometric data. The final goal is to correlate the optical properties with the morphology of the thin films. The analysis used to study the absorption and photoluminescence is based on the work of Spano et al. [4] on H and J aggregates, in other words on inter and intra molecular interactions, respectively. This analysis stipulates that the origins of the absorption and the photoluminescence are differents. While, the absorption is sensitive to the interchain interactions, the photoluminescence is related to the intrachain interactions.  Based on this theory,  we use the intensity ratio between the  first and the second optical transitions in the imaginary part of the dielectric function and photoluminescence spectra to measure the degree of polymer aggregation.
Fig. 1 : Chemical structure of a) PTB7 and b) PTB7-Th
[1] Liao, S.-H.; Jhuo, H.-J.; Yeh, P.-N.; Cheng, Y.-S.; Li, Y.-L.; Lee, Y.-H.; Sharma, S.; Chen, S.-A. Single Junction Inverted Polymer Solar Cell Reaching Power Conversion Efficiency 10.31% by Employing Dual-Doped Zinc Oxide Nano-Film as Cathode Interlayer. Scientific Reports 2014, 4, 6813(7). [2] Ben Dkhil, S.; Duché, D.; Gaceur, M.; Thakur, A.K.; Bencheikh, A. F.; Escoubas, L.; Simon, J.J.; Guerrero, A.; Bisquert, J.; Garcia, B. M.; Bao, Q.; Fahlman, M.; Ackermann, C. V.; Margeat, O.; Ackermann, J. Interplay of Optical, Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Effi cient Polymer Solar Cells. Adv. Energy Mater 2014, 4, 1400805(12). [3]He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nature Photonics 2012, 6, 591-595. [4] Spano, F. C. ; Silva, C. H- and J-Aggregate Behavior in Polymeric Semiconductors. Annual Review of Physical Chemistry 2014, 65, 477-500

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