Impact of Self-Assembled Monolayers on the Morphology and Optoelectronic Properties of Conjugated Polymer Films

The efficiency and stability of organic optoelectronic devices rely on the high quality of their constituent layers and interfaces. Conventional hole transport layers (HTLs) based on the highly efficient conjugated polymer, PEDOT:PSS, have led to instabilities at the transparent conducting oxide interface, impacting the long-term stability of devices. In recent years, self-assembled monolayers (SAMs) have re-emerged as alternatives to conventional PEDOT:PSS-based HTLs. SAMs benefit from their facile solution-based processing, ability to bond covalently to transparent conducting oxides, tunability of the electrode work function, and their vanishingly low parasitic absorption. Blending of SAMs with different frontier molecular orbitals can also lead to having fully tunable energy level alignment between the active layer and electrode [1]. One SAM molecule in particular, (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (i.e., 2PACz), and its derivatives have stood out as leading to the highest improvements in device efficiencies.

While there have been many studies on the impact SAMs have on device efficiency and stability from the perspective of typical HTLs (i.e., energy level alignment, reduced parasitic absorption, greater chemical stability), the impact of SAMs on the active layer morphology and properties has not been fully explored. The functional groups of SAM molecules can vary in their degree of electronegativity, impacting the surface energy of the SAM-modified transparent conducting oxide. As such, it is expected that SAMs with different functional groups should have some impact on the physical properties of the active layers subsequently deposited.

In this work, we investigate the impact of carbazole-based SAMs on the morphology and optoelectronic properties of conjugated polymer active layers. Using a combination of spectroscopic and morphological characterization techniques, we observe the impact that 2PACz and its derivative have on thin films of the high-efficiency conjugated polymer, PBDB-T-2F (i.e., PM6). Using steady-state photoluminescence (PL) spectroscopy and PL quantum yield (PLQY) measurements, we demonstrate that the PL emission intensity from PM6 is enhanced by nearly 1 order of magnitude when deposited on MeO-2PACz compared to pristine PM6 films, and by a factor of ~2.6 compared to PM6 deposited on PEDOT:PSS. We show that the PL lifetime increases from 67 ps for pristine PM6 films to nearly 220 ps for PM6/MeO-2PACz, arising from reduced non-radiative recombination from conformational defects. Using atomic force microscopy (AFM) and grazing-incidence X-ray scattering (GIWAXS), we show that both 2PACz and MeO-2PACz result in PM6 films having higher degrees of crystallinity compared to pristine PM6 films. Finally, we discuss the impact that these morphological changes of the polymer film have on final device characteristics.

[1] L. V. Torres Merino, C. E. Petoukhoff, et al., “Impact of the valence band energy alignment at the hole-collecting interface on the photostability of wide band-gap perovskite solar cells,” Joule, 8, 2585 (2024).