The Energy Level Conundrum of Organic Semiconductors in Solar Cells

Jules Bertrandie, Jianhua Han, Catherine S.P. De Castro, Emre Yengel, Julien Gorenflot, Thomas Anthopoulos, Frederic Laquai, Anirudh Sharma*, Derya Baran*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

69 Scopus citations

Abstract

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC. It is demonstrated that high-performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5 eV, determined by PES. Using various D–A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property–performance relationships in organic solar cells.

Original languageEnglish (US)
Article number2202575
JournalAdvanced Materials
Volume34
Issue number35
DOIs
StatePublished - Sep 1 2022

Bibliographical note

Funding Information:
This publication was based upon the work supported by King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award no: OSR‐2018‐KAUST‐KAU Initiative‐3902, OSR‐2019‐CARF/CCF‐3079, and OSR‐CRG2018‐3746.

Publisher Copyright:
© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.

Keywords

  • bandgap
  • electron affinity
  • energetic offset
  • ionization energy
  • organic photovoltaics
  • organic semiconductors
  • redox potentials

ASJC Scopus subject areas

  • General Materials Science
  • Mechanics of Materials
  • Mechanical Engineering

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