Poly[(9,9-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2″-diyl]:PCBM thin films: Molecular self-assembly, hexyl substituent groups and sheet resistance relationships

•D-A copolymer thin film with molecular self-assembly behavior.•Circular-like cavity morphology analysis.•Bulk heterojunction morphology.•Relationship between thin film morphology and Hall sheet resistance.

Poly[(9,9-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2′′-diyl] (F8TBT) is a conjugated polymer with peculiar characteristics due to its D-A type architecture. F8TBT ambipolar feature has allowed the confection of F8TBT/PCBM ([6,6]−Phenyl-C61-butyric acid methyl ester) and polymer/F8TBT heterojunctions for solar cells. However, the use of PCBM as electron acceptor in photovoltaic devices is still sovereign because of its great electron affinity and compatibility with a large amount of conjugated polymers.The effects of PCBM content (0, 33, 50, 67, 75 and 80 wt%) and amount of hexyl substituent groups in F8TBT polymer backbone on morphology and Hall sheet resistance (Rs) of F8TBT:PCBM thin films are evaluated quantitatively by optical microscopy and image processing. Circular-like cavities are formed on F8TBT films by molecular self-assembly, such structures on F8TBT film surface were identified by AFM. The statistical distributions of the self-assembled cavity diameters can be fitted by Lorentz, Boltzmann sigmoidal and Gauss statistics functions, depending on the amount of hexyl substituent groups and PCBM. The absence of these alkyl groups on the polymer chains causes the following effects: increasing of the Rs value on 69% and modification of the cavity size distribution. Moreover, there is significant elevation of Rs due to the cavity density on polymer and nanocomposite thin films.

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