Role of molecular and interchain ordering in the formation of a δ-hole-transporting layer in organic solar cells

Naresh Chandrasekaran, Cheng Li, Shivam Singh, Anil Kumar, Christopher R. McNeill, Sven Huettner, Dinesh Kabra

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4 Citations (Scopus)

Abstract

Interface engineering, especially the realization of Ohmic contacts at the interface between organic semiconductors and metal contacts, is one of the essential preconditions to achieve high-efficiency organic electronic devices. Here, the interface structures of polymer/fullerene blends are correlated with the charge extraction/injection properties of working organic solar cells. The model system - poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) - is fabricated using two different degrees of P3HT regioregularity to alter the blend interchain order and molecular packing, resulting in different device performances. Investigations by electroabsorption spectroscopy on these devices indicate a significant reduction (≈1 V) in the built-in potential with an increase in the P3HT regioregularity. This observation is also supported by a change in the work function (WF) of high regioregular polymer blends from photoelectron spectroscopy measurements. These results confirm the presence of a strong dipole layer acting as a δ-hole-transporting layer at the polymer/MoO3/Ag electrode interface. Unipolar hole-only devices show an increase in the magnitude of the hole current in high regioregular P3HT devices, suggesting an increase in the hole injection/extraction efficiency inside the device with a δ-hole-transporting layer. Microscopically, near-edge X-ray absorption fine structure spectroscopy was conducted to probe the surface microstructure in these blends, finding a highly edge-on orientation of P3HT chains in blends made with high regioregular P3HT. This edge-on orientation of P3HT chains at the interface results in a layer of oriented alkyl side chains capping the surface, which favors the formation of a dipole layer at the polymer/MoO3 interface. The increase in the charge extraction efficiency due to the formation of a δ-hole-transporting layer thus results in higher short circuit currents and fill factor values, eventually increasing the device efficiency in high regioregular P3HT devices despite a slight decrease in cell open circuit voltage. These findings emphasize the significance of WF control as a tool for improved device performance and pave the way toward interfacial optimization based on the modulation of fundamental polymer properties, such as polymer regioregularity.

Original languageEnglish
Pages (from-to)3806-3814
Number of pages9
JournalACS Applied Materials & Interfaces
Volume12
Issue number3
DOIs
Publication statusPublished - 22 Jan 2020

Keywords

  • interchain ordering
  • interface dipole
  • interface engineering
  • molecular ordering
  • organic solar cells
  • P3HT
  • regioregularity

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