Spectroscopic assessment of charge mobility in organic semiconductors

Rapid progress in organic electronics demands new highly efficient organic semiconducting materials. Nevertheless, only few materials have been created so far that show reliable band-like transport with high charge mobilities, which reflects the two main obstacles in the field: the poor understanding of charge transport in organic semiconductors (OSs) and the difficulty of its quantification in devices. Here, we present a spectroscopic method for assessment of the charge transport in organic semiconductors. We show that the intensities of the low-frequency Raman spectrum allow calculation of the dynamic disorder that limits the charge carrier mobility. The spectroscopically evaluated mobility clearly correlates with the device charge mobility reported for various OSs. The proposed spectroscopic method can serve as a powerful tool for a focused search of new materials and highlights the disorder bottleneck in the intrinsic charge transport in high-mobility organic semiconductors

Real-Time Tracking of Polymer Crystallization Dynamics in Organic Bulk Heterojunctions by Raman Microscopy

State-of-the-art organic photovoltaic active layers typically undergo post-treatment such as thermal or solvent vapor annealing to increase their performance by tuning the bulk heterojunction morphology. Molecular crystallinity is one of the key factors that determine the morphology. Real-time tracking of the crystallinity during the post-treatment is strongly desired for understanding the physics of the crystallization process and for optimizing the post-treatment protocol. Here, we report on the cold crystallization (CC) dynamics of the polymer in the temperature range of 50-150 degrees C in polymer:fullerene blends based on poly(3-hexylthiophene) with various fullerene-based acceptors (C-60, PC61BM, PC71BM, bisPC(61)BM, HBIM, AIM8, and IrC60) in real-time by Raman microscopy. We also reveal how different solvents, fullerene acceptors, and temperatures affect CC during thermal annealing. We further demonstrate a correlation between the fullerene derivative weight and the polymer crystallinity for the as-cast films and also a correlation of the polymer crystallinity before and after annealing. Our findings are essential for developing efficient strategies of morphology optimization in emerging organic photovoltaic devices with real-time Raman microscopy tracking as a valuable tool

Highly Luminescent Solution-Grown Thiophene-Phenylene Co-Oligomer Single Crystals

Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.</p

Long-range exciton transport in brightly fluorescent furan/phenylene co-oligomer crystals

The design of light-emitting crystalline organic semiconductors for optoelectronic applications requires a thorough understanding of the singlet exciton transport process. In this study, we show that the singlet exciton diffusion length in a promising semiconductor crystal based on furan/phenylene co-oligomers is 24 nm. To achieve this, we employed the photoluminescence quenching technique using a specially synthesized quencher, which is a long furan/phenylene co-oligomer that was facilely implanted into the host crystal lattice. Extensive Monte-Carlo simulations, exciton-exciton annihilation experiments and numerical modelling fully supported our findings. We further demonstrated the high potential of the furan/phenylene co-oligomer crystals for light-emitting applications by fabricating solution-processed organic light emitting transistors

Charge-transfer complexes of conjugated polymers as intermediates in charge photogeneration for organic photovoltaics

Ultrafast visible-pump/IR-probe spectroscopy is applied to study the wavelength dependence of charge photogeneration in materials based on donor–acceptor charge-transfer complexes (CTCs) of the conjugated polymer MEH-PPV. In binary polymer–acceptor blends, photoexcitation in the absorption band of either CTC or polymer results in similar dynamics of the charge-associated transient absorption. Likewise, in polymer/CTC–acceptor/fullerene ternary blends, where charge separation occurs via a two-step pathway, the photophysics is also independent of excitation wavelength. These similarities in charge dynamics indicate that CTC excited states serve as an intermediate for charge photogeneration. The conclusions of the ultrafast study are supported by photocurrent spectroscopy.

Ultrafast Charge Photogeneration Dynamics in Ground-State Charge-Transfer Complexes Based on Conjugated Polymers

The charge photogeneration and early recombination in MEH-PPV-based charge-transfer complexes (CTCs) and in MEH-PPV/PCBM blend as a reference are studied by ultrafast visible-pump-IR-probe spectroscopy. After excitation of the CTC band, an immediate (<100 fs) electron transfer is observed from the polymer chain to the acceptor with the same yield as in the MEH-PPV/PCBM blend. The forward charge transfer in the CTCs is followed by an efficient (~95%) and fast (<30 ps) geminate recombination. For comparison, the recombination efficiency obtained in the MEH-PPV/PCBM blend does not exceed a mere 50%. Polarization-sensitive experiments demonstrate high (~0.3) values of transient anisotropy for the CTCs polaron band. In contrast, in the MEH-PPV/PCBM blend the dipole moment orientation of the charge-induced transition is less correlated with the polarization of the excitation photon. According to these data, photogeneration and recombination of charges in the CTCs take place locally (i.e., within a single pair of a polymer conjugation segment and an acceptor) while in the MEH-PPV/PCBM blend exciton migration precedes the separation of charges. Results of the ultrafast experiments are supported by photocurrent measurements on the corresponding MEH-PPV/acceptor photodiodes.