An iminostilbene functionalized benzimidazole derivative for improved solution n-doping of P(NDI20D-T2)

Pietro Rossi^a^,^b, Giulia Coco^a, Francesca Pallini^c, Marco Cassinelli^a, Christopher McNeill^d, Guglielmo Lanzani^a^,^b, Luca Beverina^c, Mario Caironi^a

^aCenter for NanoScience and Technology, Italian Institute of Technology, Via Pascoli 70/3, 20133 Milano, Italy

^bDipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano, Italy

^cDipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via Roberto Cozzi 55, I-20125, Milano, Italy

^dDepartment of Materials Science and Engineering, Monash University, Clayton, Victoria, 3800 Australia, Wellington Rd, Clayton, Australia

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2021 (NFM21)

#ThermoElect21. New concepts in organic/hybrid thermoelectrics

Online, Spain, 2021 October 18th - 22nd

Organizers: L. Jan Anton Koster and Derya Baran

Contributed talk, Pietro Rossi, presentation 121
DOI: https://doi.org/10.29363/nanoge.nfm.2021.121
Publication date: 23rd September 2021

The lack of highly conducting n-type materials greatly hampers the realization of performing organic thermoelectric generators (OTEGs); molecular doping (e.g. blending with electron-donating compounds) has stemmed as one of the possible solutions to enhance charge transport. Despite the actual interaction mechanism has not yet been fully disclosed [1, 2], benzimidazoles derivatives have so far emerged has one of the most promising class of organic n-type dopants due to their solution-processability, generality and relatively good shelf-life when stored in air [3].

Here we report on a modification of the widely used and commercially available 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) obtained by replacing the two methyl end-groups with an iminostilbene moiety, creating an innovative derivative named IStBI. The introduction of a bulkier and more sterically hindered substituent bears significant changes in the crystallinity (shown through DSC and Thermogravimetric analysis) and packing in the dopant-host matrix, with only a marginal effect on the compound electronic structure, therefore preserving its electron-donating character. The study is conducted using the widely spread n-type polymer P(NDI2OD-T2), ensuring a large platform of comparable results available in the literature [4, 5, 6]. All the processing was carried out in solution and under N2-atmosphere due to the lack of air and moisture stability of the obtained semiconducting blends. Thin films, casted through spin-coating technique, have been extensively characterized in their electrical conductivity and Seebeck coefficient. After optimizing the processing conditions (i.e. solvent, dopant concentration and mixing temperature) and post-deposition thermal treatment, we obtained an electrical conductivity of 1.14 x 10^-2 S cm^-1, which is to the best of our knowledge among the highest values ever reported for doped P(NDI2OD-T2) [7]. This resulted in also an improved power factor of 8 x 10^-3 mW/mK². To support these findings structural and morphological analysis have been performed through GIWAXS analysis and AFM imaging techniques respectively, suggesting reduced disruption of the pristine polymer crystalline structure compared to other reported dopant molecules leading to improved charge transport properties.

References:

[1] Jhulki, S.; Un, H.I.; Ding, Y.F.; Risko, C.; Mohapatra, S.K.; Pei, J.; Barlow, S.; Marder, S.R. Reactivity of an air-stable dihydrobenzoimidazole n-dopant with organic semiconductor molecules. Chem, 2021, 7(4), 1050-1065.

[2] Zeng, Y.; Zheng, W.; Guo, Y.; Han, G.; Yi, Y. Doping mechanisms of N-DMBI-H for organic thermoelectrics: hydrogen removal vs. hydride transfer. Journal of Materials Chemistry A, 2020, 8(17), 8323-8328.

[3] Pallini, F.; Mattiello, S.; Cassinelli, M.; Rossi, P.; Mecca, S.; Sassi, M.; Lanzani, G.; Caironi, M.; Beverina, L. A New Look into 4-(2, 3-Dihydro-1, 3-Dimethyl-1H-Benzimidazol-2-Yl)-N, N-Dimethylbenzenamine (DMBI-H) N-Type Dopant: Not so Stable After All, but It Does Not Necessarily Matter. ChemRxiv, 2021. This content is a preprint and has not been peer-reviewed.

[4] Saglio, B.; Mura, M.; Massetti, M.; Scuratti, F.; Beretta, D.; Jiao, X.; McNeill, C.R.; Sommer, M.; Famulari, A.; Lanzani, G.; Caironi, M.; Bertarelli, C. N-Alkyl substituted 1 H-benzimidazoles as improved n-type dopants for a naphthalene-diimide based copolymer. J. Mater. Chem A, 2018, 6, 15294–15302.

[5] Al Kurdi, K.; Gregory, S.A.; Jhulki, S.; Conte, M.; Barlow, S.; Yee, S.K.; Marder, S.R. Electron transport in a sequentially doped naphthalene diimide polymer. Materials Advances, 2020, 1(6), 1829-1834.

[6] 6. Riera-Galindo, S.; Orbelli Biroli, A.; Forni, A.; Puttisong, Y.; Tessore, F.; Pizzotti, M.; Pavlopoulou, E.; Solano, E.; Wang, S.; Wang, G.; Ruoko, T.P.; Chen, W.M.; Kemerink, M.; Berggren, M.; Di Carlo, G.; Fabiano, S. Impact of singly occupied molecular orbital energy on the n-doping efficiency of benzimidazole derivatives. ACS applied materials & interfaces, 2019, 11(41), 37981-37990.

[7] Tam, T. L. D.; Xu, J. Strategies and concepts in n-doped conjugated polymer thermoelectrics. Journal of Materials Chemistry A, 2021, 9(9), 5149-5163.

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