The Impact of 1,8-Diiodooctane on the Morphology, Performance and Stability of Organic Photovoltaics

Controlling the nanomorphology of the solution-processed blend layer in organic photovoltaic (OPV) technologies is crucial for optimizing both performance and stability. A promising approach is to alter the drying dynamics and consequently, the nanostructure of the blend film using solvent additives such as 1,8-diiodooctane (DIO). However, the morphologies that form are often metastable and can evolve due to kinetic and thermodynamic driving factors resulting in performance changes over time. Understanding this ageing behaviour alongside other variables such as residual solvent, molecular design and the influence of light and oxygen is complex and often difficult to disentangle. This work covers an in-depth study of the impact of DIO on the morphology, performance and stability of two model OPV systems: a fullerene-based system (PBDB-T:PC₇₁BM) and an non-fullerene-based system (PBDB-T:ITIC).^[1,2] A combination of neutron and X-ray scattering techniques are employed to characterize subtle morphological differences across Å to nm length scales. This is correlated to an extensive device stability study under different ageing conditions. It is found that processing with DIO yields higher crystallinity for non-fullerene-based devices but poorer initial device performance when used in excess due to large-scale phase-separation. Additionally, an order of magnitude more residual DIO is retained in non-fullerene-based films than for fullerene-based films. During ageing, devices demonstrate a complex relationship between DIO content and stability under different conditions. The results provide new insights into the processing and optimization of OPV blend films to obtain devices that are simultaneously efficient and long-lasting.

[1] R. C. Kilbride, E. L. K. Spooner, S. L. Burg, B. L. Oliveira, A. Charas, G. Bernardo, R. Dalgliesh, S. King, D. G. Lidzey, R. A. L. Jones, A. J. Parnell, Small 2024, 20, 2311109.
[2] R. C. Kilbride, E. L. K. Spooner, E. J. Cassella, M. E. O’Kane, K. Doudin, D. G. Lidzey, R. Jones, A. J. Parnell, ACS Appl. Energy Mater. 2024, DOI 10.1021/acsaem.4c01272.