Efficient charge generation from triplet excitons in metal-organic heterojunctions

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine–${\mathrm{C}}_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons $D_{\mathrm{CuPc}}=\left(1.8\pm 1.2\right)\times {10}^{-5}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ (where CuPc is copper-phthalocyanine) and their diffusion length $L_{\mathrm{diff}}=\left(8\pm 3\right)\mathrm{nm}$.

Figure 1

Time-resolved XPS spectra for planar heterojunction configurations of (a) $\sim 2\mathrm{MLs}$ of ${\mathrm{C}}_{60}$ deposited on top of a thin film of CuPc and (b) $\sim 2\mathrm{MLs}$ of CuPc atop $\sim 8\mathrm{MLs}$ of ${\mathrm{C}}_{60}$. Pump-probe time delays are indicated in each panel. To highlight the photoinduced spectral changes, red and blue areas indicate missing and additional intensities, respectively, relative to the spectra recorded at a delay of $-328\mathrm{ns}$. Additionally, the difference spectra are shown and they are magnified by a factor of 2 in (b). The dotted and dashed spectra in the $-328-\mathrm{ns}$ panel of (a) are from pristine films of ${\mathrm{C}}_{60}$ and CuPc, respectively. All films were supported by $n$-doped Si(100) substrates.

Figure 2

(a) Temporal evolution of ${\mathrm{C}}_{60}\text{-C}1s$ peak shift for ${\mathrm{C}}_{60}/\mathrm{CuPc}$ (blue squares) and $\mathrm{CuPc}/{\mathrm{C}}_{60}$ (red circles) heterojunctions. Symbols and error bars indicate measurements with $\pm 1\sigma$ uncertainties. Solid lines are the result of a fit to a coupled rate equation model taking into account the energy- and charge-transfer processes indicated in (b) and (c). Note that only ${\tau }_2=1/k_2=\left(280\pm 40\right)\mathrm{ps}$ and ${\tau }_3=1/k_3=\left(9\pm 4\right)\mathrm{ns}$ are derived from the fit while all other rates are known from literature. See text for details.

Figure 3

Photoinduced charge generation in a planar CuPc-${\mathrm{C}}_{60}$ heterojunction. Blue- and red-shaded areas correspond to the time-dependent contributions to the acceptor charges from singlet- and triplet-exciton splitting, respectively. The triplet-exciton contribution is also plotted separately as a dashed line in the inset.