Localization-dependent charge separation efficiency at an organic/inorganic hybrid interface

By combining complementary optical techniques, photoluminescence and time-resolved excited state absorption, we achieve a comprehensive picture of the relaxation processes in the organic/inorganic hybrid system SP6/ZnO. We identify two long-lived excited states of the organic molecules of which only the lowest energy one, localized on the sexiphenyl backbone of the molecule, is found to efficiently charge separate to the ZnO conduction band or radiatively recombine. The other state, most likely localized on the spiro-linked biphenyl, relaxes only by intersystem crossing to a long-lived, probably triplet state, thus acting as a sink of the excitation and limiting the charge separation efficiency. An efficient light harvesting device relies on high carrier mobilities, low charge injection and ejection barriers and strong light-matter interaction. Inorganic semiconductors, on which current technology is predominantly based, fulfill the first two requirements (1) but their optical bandgap limits the amount of energy that can be converted (2). Strong light-matter coupling, instead, occurs in organic semiconductors and chemical design allows for flexible adjustment of absorption and emission spectra (3). Unfortunately, organic-based devices often suffer from low mobilities (4,5) and charge recombination can occur before extraction. The combination of inorganic and organic semiconductors into hybrid structures promises to lead to a new generation of devices that exploit the advantages of both material classes to increase light conversion efficiency (6,7). Efficient hybrid devices are based on a careful choice of the two materials; the energy level alignment at the interface determines the occurrence of charge and energy transfer processes. Furthermore, the device performance is affected by the relative probability of photoinduced energy relaxation processes that, by competing with the desired energy or charge transfer process at the hybrid interface, lead to an overall loss of harvested energy. In the molecular film, for example, these processes include a) intramolecular vibrational relaxation (IVR), b) internal conversion (IC), c) (non-) radiative recombination to the electronic ground state, d) triplet formation via intersystem crossing (ISC) or e) separation into a charge transfer state, among others. By reducing the exciton lifetime, these mechanisms shorten the exciton diffusion length and lower the probability for charge and energy transfer to occur. Similarly, the diffusion is likely to be less efficient in the case of strongly localized long-lived excited states such as charge transfer and triplet excitons. The relevance of a given relaxation pathway with respect to the others depends on the relative rate of the processes, which is in turn affected by the energetic separation of the involved energy levels. One prominent example is given by Kasha spectroscopic rule stating that light emission always occurs from the lowest electronic excited state independent of the excitation density, since for all higher excited states nonradiative decay pathways to the lowest excited state occur on faster timescales than radiative recombination to the electronic ground state (8). The complete understanding of all competing energy relaxation pathways and the identification of the related electronic states on a fundamental level is therefore crucial for the design and optimization of hybrid devices. Since these relaxation mechanisms occur typically on timescales ranging from hundreds of femtoseconds (IC and IVR) to picoseconds (fluorescence) or even microseconds (triplet recombination), their investigation requires the application of time-resolved techniques that are able to access excited state dynamics on such a variety of timescales. In this paper, we investigate the balance of relaxation pathways in the organic/inorganic hybrid system formed by the spirobifluorene derivative 2,7-bis(biphenyl-4-yl)ditertbutyl-spirobifluorene (SP6) and the non-polar (10-10) surface of ZnO and, for comparison, in the dye film deposited on an inert glass substrate. SP6 is constituted by a sexiphenyl backbone and a spiro-linked biphenyl, further decorated by tertbutyl groups. Both, the spiro-linkage and the tertbutyl groups, are used to improve the morphology of the films and their optical properties by the prevention of crystallization (9). SP6 is characterized by a broad absorption spectrum starting at 3.2 eV and peaking at 3.6 eV (10,11), and exhibits very high fluorescence yields, amplified spontaneous emission and lasing (12,13). The wide band gap, large exciton binding energy and metallic surface of ZnO make it a desirable material for transparent electrodes and substrates in optoelectronic applications. Together, they form a type II junction, i.e. the lowest unoccupied molecular orbital (LUMO) is aligned with the ZnO conduction band while the highest occupied molecular orbital (HOMO) lies within the ZnO band gap. Both, charge separation (CS) with electron transfer from the LUMO to the conduction band and Förster type resonant energy transfer from ZnO to SP6, were previously demonstrated by time-resolved photoluminescence (PL) measurements (10,11). In particular, the CS process was found to be limited by exciton diffusion in the organic layer to the ZnO interface, with an estimated diffusion length of 10 nm at room temperature. Once the exciton has reached the interface, though, charge separation occurs very efficiently within an estimated timescale of 10 ps (11). Here, the relaxation pathways in SP6 and their influence on CS at the ZnO interface are addressed by the combination of two complementary all-optical techniques: PL and timeresolved excited state transmission (tr-EST) spectroscopy. While the former addresses the radiative recombination from the lowest excited state into the electronic ground state, the latter is sensitive to higher excited state electronic transitions that are activated after photoexcitation, accessing the dynamics of those excited states that are dark to PL. Together, they allow to gain a comprehensive picture of the excited state population dynamics in this system and the related energy loss channels. Several relaxation pathways are found to compete with CS. After initial IVR occurring on a few picoseconds timescale, two distinct excited states of comparable lifetime of hundreds of picoseconds, X6P and X2P, form. X6P is localized on the sexiphenyl molecular backbone while X2P is most likely a charge transfer exciton with the electron localized in the biphenyl. The two -systems are decoupled by the spiro-link that seems to hinder internal conversion between X6P and X2P. Therefore, two independent relaxation pathways evolving either as a X6P or X2P species are determined either at the photoexcitation stage or directly after, on the ultrafast timescales of IVR. Only X6P is found to radiatively recombine and efficiently charge separate at the hybrid interface. X2P, instead, decays exclusively by the formation of a long-lived, most likely triplet state and therewith represents the most important loss channel for the charge separation process. These findings highlight the fundamental influence of exciton localization and excited state (de)coupling to the efficiency of hybrid organic/inorganic systems.