Extracting Disorder Parameters from Optical Spectra of Non-Fullerene Acceptors

The performance of organic opto-electronic devices is crucially dependent on the morphology and crystallinity of the organic semiconductors used, which affects the energetic landscape and disorder experienced by charge carriers and excitons. Despite its importance, energetic disorder has been difficult to characterize.^[1] When evaluating new non-fullerene acceptor (NFA) materials for efficient organic solar cells or near-infrared detectors, the sub-bandgap region of the absorption spectrum is frequently characterized using the empirical Urbach rule, with the resulting Urbach energy serving as a measure of energetic disorder.^[1] However, the exact meaning of the Urbach energy and its relevance in quantifying disorder has recently been questioned for organic semiconductors.^[2]
In this study, we therefore introduce a method to extract both static and dynamic excitonic energetic disorder from the absorption spectrum of neat non-fullerene acceptor thin films. Four NFAs were investigated to explore how structural variations influence the disorder of the films. First, EH-IDTBR and O-IDTBR were analyzed, where the two molecules differ by their sidechains. From the extracted static disorder, we conclude that the linear n-octyl sidechains in O-IDTBR promotes superior film morphology compared to the branched 2-ethylhexyl sidechains in EH-IDTBR.
Additionally, ITIC and COi8DFIC were studied, both of which share phenylhexyl sidechains and an acceptor-donor-acceptor (A-D-A) core structure. However, COi8DFIC has a more electron-donating D block and a fluorinated A unit, resulting in a lower bandgap compared to ITIC. Despite these differences, the aggregation behavior of the two compounds was found to be minimally affected, as indicated by the similar disorder parameters. Collectively, these findings underscore the critical role that molecular design, particularly sidechain engineering, plays in determining the morphology of NFA thin films and consequently the energetic disorder.
[1] P. Hartnagel, S. Ravishankar, B. Klingebiel, O. Thimm, T. Kirchartz, Adv Energy Mater 2023, 13, DOI 10.1002/aenm.202300329.
[2] N. Zarrabi, O. J. Sandberg, P. Meredith, A. Armin, J Phys Chem Lett 2023, 14, 3174. DOI 10.1021/acs.jpclett.3c00021.