Picosecond Charge-Transfer-State Dynamics in Wide Band Gap Polymer–Non-Fullerene Small-Molecule Blend Films Investigated via Transient Infrared Spectroscopy

Abstract

Organic solar cells based on wide band gap polymers and nonfullerene smallmolecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad electronic charge carrier absorption and the molecular vibrations of the electron acceptor molecule to monitor the charge-transfer state dynamics. In our time-resolved infrared spectroscopy experiments, we find that in the small-molecule acceptor, they have additional dynamics on the order of a few picoseconds. A change in the solvent used in thin film deposition, leading to different morphologies, influences this time further. We interpret our findings as the dynamics of the charge-transfer state at the interface of the electron donor and the electron-acceptor. The additional mid-infrared transient signal is generated in this state, as both electron and hole polarons can interact with small-molecule acceptor vibrational modes.

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“…To better understand how exciton utilization occurs in blend films, Figure S16 depicts the steady-state fluorescence (PL) spectra of the optimized films . To observe the films and measure the efficiency of charge transfer at the donor/acceptor interface, PM6 was excited at 620 nm.…”

“…As indicated in Figure c, cascade frontier molecular orbital (FMO) energy alignment is formed between the HOMOs and the LUMOs of these active layer components. To improve device performance, the cascade FMO energy levels are advantageous for lowering the charge transfer barrier and promoting charge transit and collection. , According to the density functional theory (DFT) calculations, the HOMO and LUMO for BTA-ERh were −5.01 and −2.83 eV, respectively (Figure S6). These results are reliable with the CV measurements.…”

“…To better understand how exciton utilization occurs in blend films, Figure S16 depicts the steady-state fluorescence (PL) spectra of the optimized films. 42 To observe the films and measure the efficiency of charge transfer at the donor/acceptor interface, PM6 was excited at 620 nm. The PL quenching in the PM6:Y7:PC 71 BM and PM6:Y7:PC 71 BM:BTA-ERh blend films demonstrated active electron transfer from the donor to the acceptors.…”