Charge carrier dynamics of vapor-deposited small-molecule/fullerene organic solar cells

Although small-molecule organic solar cells (SMOSCs) have shown increasingly promising prospects as a source of solar power, there have been few studies concerning the photophysics of these systems. Here, we report the time scale and efficiency of charge separation and recombination in a vapor-deposited SMOSC material that produces 5.81% power conversion efficiency. Transient absorption and time-resolved photoluminescence (trPL) studies of thin film blends comprising DTDCTB, a narrow-band gap electron donor, and either C ₆₀ or C₇₀ as an electron acceptor show that charge separation occurs in ∼100 fs, while charge recombination takes place over sub-ns and ns time scales. trPL indicates a donor electron-hole pair lifetime of ∼33 ps in the neat film and reveals that ∼20% of donors fail to charge separate in donor-acceptor mixed films, likely owing to some spatially extended donor-rich regions that interact poorly with acceptors. Our results suggest that morphological manipulations of this material could further improve device efficiency.