Revealing electron transport connectivity as an important factor influencing stability of organic solar cells

Abstract

In the pursuit of advancing the commercialization of organic solar cells (OSCs), stability emerges as a paramount challenge. Herein, we show that the electron transport connectivity is a key factor determining the electron transport and device stability of OSCs. When compared to small molecular acceptors (SMAs), the larger-size polymeric acceptors (PAs) are likely to establish an electron transport network with superior connectivity. This enhanced connectivity enables more robust electron transport during potential device degradation. Our findings indicate that PA-integrated devices sustain elevated electron mobilities, even under reduced acceptor ratios (or higher impurity doping) over prolonged device operation. Furthermore, we employ the refined Su-Schrieffer-Heeger tight-binding model, in tandem with a random electron passing test and algebraic connectivity evaluations of molecular configurations, to conclusively validate the pivotal role played by the electron transport connectivity. These revelations are poised to offer new perspectives for material choices and methodologies for improving stability of OSCs.

Fig. 1. Chemical structures, energy levels, and morphology illustrations.

Chemical structures of (a) polymer acceptors, (b) single-component materials, (c) oligomer acceptor, (d) small molecular acceptors, (e) fullerene acceptor, and f polymer donors. g Energy level diagrams of photovoltaic materials investigated in this study.

Fig. 2. The trend of electron transport properties with different composition content.

a–c Electron current density × film thickness (J × d) of devices as a function of the applied electric field for PM6:PY-V-γ, PM6:QM1 and PM6:Y6 devices with different PS addition and D:A composition; d–f logarithm of electron mobilities as a function of acceptor weight fraction and various χ values for PM6:PY-V-γ, PM6:QM1 and PM6:Y6 devices, the PS was added at the optimized compositions for the BHJ devices. Source data are provided as a Source Data file.

Fig. 3. Degradation of electron transport and photovoltaic performance during light exposure process.

a Electron current density × film thickness (J × d) of devices as a function of the applied electric field for fresh and aged (standard 1-Sun illumination for 70 h) of PM6:PY-V-γ and PM6:Y6 electron-only devices; b PM6:PY-V-γ and PM6:Y6 electron-only devices in (a) as a function of the standard 1-Sun illumination time; c normalized PCE of PM6:PY-V-γ and PM6:Y6 organic solar cells as a function of the standard 1-Sun illumination time; and d fresh and aged (standard 1-Sun illumination for 70 h) PM6:PY-V-γ and PM6:Y6 electron-only devices with various acceptor weight fractions.

Fig. 4. Electron transport mechanism analysis of the blend films.

a Mobility change ratio of devices with different PS content, (b) the electron mobility of devices with different acceptor weight fractions, and c electron percolation thresholds as a function of impurity tolerance for small-molecule acceptor-based polymer acceptor-based films of PBDB-T:Y6, PM6:Y6, PM6:N3, and PM6:BTP-eC9, polymer acceptor-based films of PM6:PY-IT, PM6:PYF-T-o, and PM6:PY-V-γ, oligomer acceptor-based films of PM6:QM1 and the PBDB-T-b-PTY6 and DCPY2 single-component devices. Source data are provided as a Source Data file.

Fig. 5. Morphology of thin films.

a, c Two-dimensional GIWAXS diffraction patterns of corresponding BHJ films. q_xy, scattering vector in the in-plane (IP) direction; q_z, scattering vector in the out-of-plane (OOP) direction. b, d IP and OOP line cut profiles of the 2D GIWAXS data based on binary and ternary blended films. e 2D GISAXS patterns and f the 1D intensity profiles along the q_xy direction of corresponding BHJ films.

Fig. 6. The dynamical simulations for the effect of the connectivity of the electron transport network in BHJs.

The evolution of the charge center position x_c of the negative polaron in the (a) polymer, (b) oligomer, and c small molecular acceptor-based devices with A-to-A-type J-aggregation structural models under an applied electric field of E₀ = 4 × 10⁵V/cm. The time evolutions of the total net charge quantity Q_m (t) (unit: e) in (d) polymer, (e) oligomer, and f small molecular acceptor-based devices, after an applied electric field for E₀ = 4 × 10⁵ V/cm. g The statistics of the transport rate ratios of the polaron centers of the three systems as a function of d values. h The evolution of the charge center position x_c of the negative polaron in the oligomer acceptor-based devices with different transfer integrals between linking units under an applied electric field of E₀ = 4 × 10⁵ V/cm. i The second smallest eigenvalue of the Laplacian matrix corresponding to the graph structures of different molecules is used to represent the algebraic connectivity of molecular structures as a function of weak interaction forces. j Schematic diagram of the remaining electrons after being trapped, with the change in the proportion of acceptors. Red (yellow in small molecular cases) dots represent the donor and acceptor moieties weight ratio of 0.6:0.4, while blue dots represent the donor and acceptor moieties weight ratio of 0.4:0.6.

Fig. 7. Microstructure-stability-charge carrier transport relationships.

Schematic illustrations of changes in morphology and charge carrier transport properties of the blend films based on small-molecule and polymer acceptors with before and after illumination.