Rethinking Charge Transport and Recombination in Donor-diluted Organic Solar Cells

Abstract

We systematically investigate PM6:Y12 bulk-heterojunction solar cells with donor fractions ranging from 1% to 45%, linking morphology, charge transport, and recombination to device performance. Complementary structural and spectroscopic methods reveal that a percolating PM6 network forms even at below 5% donor content, with lamellar stacking and vertical composition gradients that do not hinder the charge extraction. The reduction of the effective active layer conductivity towards low donor fractions obeys a three-dimensional percolation model, indicating that charge transport is governed by network topology rather without a pronounced percolation threshold. A transition from nongeminate Langevin recombination to a dispersive Smoluchowski-type loss occurs below 5% donor fraction. The latter regime is also nongeminate, i.e., pertains to recombination of the total charge carrier density. Correspondingly, we observe that the Langevin reduction in the higher donor fractions - mostly dominated by redissociation of electron-hole pairs after encounter - changes towards low donor fractions: in these cases, the nongeminate loss rate exceeds the prediction of the Langevin model. This regime coincides with increasing transport resistance due to topology-limited hole conduction, leading to reduced fill factors despite a high retained charge-generation efficiency. Our results demonstrate that strong donor dilution preserves photogeneration if a continuous donor network is maintained, and unveil how topology-controlled transport and non-Langevin recombination jointly define the performance limits of donor-diluted organic solar blends.

Figures (31)

• Figure 1: (a) JV characteristics of PM6:Y12 solar cells with varying PM6 fraction, under 1 sun (i.e., 100 mW cm$^{-2}$ under AM1.5G spectrum) equivalent illumination. (b) Statistical $V_\mathrm{oc}$, $J_\mathrm{sc}$, and $F\!F$ as a function of PM6 content measured using the LS2 solar simulator. Calibrated $J_\mathrm{sc}$ values from AM1.5G spectrum are shown for reference. With decreasing donor fraction, the difference between $pF\!F$ and $F\!F$ increases, showing growing contribution of transport resistance.
• Figure 2: (a) Horizontal line cuts (q$_r$) of the 2D GIWAXS data shown in Figure \ref{['SI_fig:GIWAXS:2D']} for PM6:Y12 blend films with varying PM6 content, measured at an incidence angle of 0.18°. The curves are background-corrected and min–max normalized for clarity. (b) Vertical composition profiles of various donor--acceptor ratios derived from UPS depth profiles. (c) The fraction of PM6 at various cross-sections of UPS depth profile in (b) as a function of input PM6 content. Bulk UPS at 40 nm from the surface aligns well with the expected ratio. At 5 nm, it corresponds to the relative ordered material fraction obtained from GIWAXS line cuts. (d) PL quenching ratio for PM6 and Y12 excitons, and Y12 exciton lifetime in PM6:Y12 blend films as a function of PM6 content.
• Figure 3: (a) IQE spectra of solar cells with different PM6 content. (b) The calculated contributions from individual photogeneration process steps to the IQE losses at 510 nm (top) and 820 nm (bottom).
• Figure 4: The effective conductivity $\sigma_{\mathrm{oc}}$ in PM6:Y12 solar cells with varying PM6 content. (a) The method of $\sigma_{\mathrm{oc}}$ extraction from the slope of JV curve at $V_\mathrm{oc}$, separating transport and recombination contributions. (b) Energetic disorder parameter $E_\mathrm{u}$ extracted at different energetic positions within the DOS. (c) Temperature dependent $\sigma_{\mathrm{oc}}$ normalized to the value of the 45% PM6 device, evaluated at a fixed energetic cross-section of (c) $E_{g} - eV_\mathrm{oc} = 0.66$ eV, where $E_{U}$ is similar for all PM6 compositions, and (d) $E_{g} - eV_\mathrm{oc} = 0.55$ eV.
• Figure 5: Percolation analysis of charge transport in PM6:Y12 blends. (a) Effective conductivity $\sigma_{\mathrm{oc}}$ at 300 K as a function of PM6 fraction, fitted with a classical percolation model, Eq. \ref{['eq:percolation']}. (b) Electron and hole mobilities extracted from SCLC measurements. (c) PM6 fraction-dependent effective mobility $\mu_{\mathrm{eff}}$, calculated as the harmonic mean of the electron and hole mobilities, fitted as in (a).