Spin-dependent recombination of photoinduced carriers in phthalocyanine/C60heterojunctions

Abstract

Spin-dependent recombination of photoinduced carriers in ${\mathrm{H}}_{2}$-phthalocyanine $({\mathrm{H}}_{2}{\mathrm{P}\mathrm{c})/\mathrm{C}}_{60}$ heterojunctions is studied by electrically detected electron-spin resonance (EDESR) spectroscopy. The EDESR spectrum of the ${\mathrm{H}}_{2}{\mathrm{P}\mathrm{c}/\mathrm{C}}_{60}$ consists of two components A and B, the g values of which are 2.0018$\ifmmode\pm\else\textpm\fi{}$0.0002 and 2.0010$\ifmmode\pm\else\textpm\fi{}$0.0002, respectively. The two components are attributed to exchange-coupled localized electron-hole pairs trapped at different types of recombination centers. Component A has spin-flip satellites due to an interaction between the electron (or hole) spin, and its surrounding nuclear spins of protons which belong to the ${\mathrm{H}}_{2}\mathrm{Pc}$ rings. From the satellite intensity, the distance between the electron (or hole) and the protons is estimated to be 4.33$\ifmmode\pm\else\textpm\fi{}$0.25 \AA{}, indicating that the localized pairs for the component A locate close to the ${\mathrm{H}}_{2}\mathrm{Pc}$ rings. The spin dynamics of the localized pairs for the component A is studied by a microwave recovery experiment, in which the time dependence of the EDESR signal intensity is measured after turning the resonant microwave on and off. A theoretical model of the spin-dependent recombination of the exchange-coupled electron-hole pair is proposed with which the experimental results of the microwave recovery are explained. By a theoretical analysis, it is found that $R\ensuremath{\gtrsim}1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and ${D+W}_{\mathrm{sl}}=6.2(\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for component A at room temperature, where R is the recombination rate of the localized pair in the ${S}_{z}=0$ triplet sublevel, and D and ${W}_{\mathrm{sl}}$ are the dissociation and the spin-lattice relaxation rates, respectively, of the pairs in the triplet sublevels. The photocurrent ${I}_{2}$ that is caused by the dissociation of the localized pairs for component A is about 5% of the total photocurrent.