Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.

We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance "ion gel" gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylateb-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 µF/cm 2 at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gelgated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 10 14 carriers/ cm 2 , with associated saturation hole mobilities that are also remarkably large, ∼1 cm 2 /(V s), likely because of the large gate-induced carrier densities. Examination of the frequency response of GEL-OTFTs indicates that increases in the OFF current with frequency ultimately limit switching speed; the cutoff frequency correlates with the ionic conductivity versus frequency response of the gel dielectric. Further, attenuated total internal reflection infrared (ATR-IR) spectroscopy of the ion gel/polymer semiconductor gate stack reveals that the conductance switching mechanism in GEL-OTFTs spans both electrochemical and electrostatic (field effect) regimes. Specifically, modeling of the time dependence of the near-infrared polaron absorption in gated GEL-OTFTs indicates that the [TFSI]anion diffusivity in regioregular poly(3-hexylthiophene) is on the order of 10 -12 cm 2 /s at room temperature. This diffusivity implies that, for time scales greater than 1 ms, there is significant penetration (>1 nm) of [TFSI]anion into the polymer semiconductor at the gel/polymer semiconductor interface, corresponding to an electrochemical doping process. On the other hand, for time scales shorter than 1 ms (i.e., for GEL-OTFT switching frequencies >1 kHz), the device switching mechanism can be viewed as primarily electrostatic as average ion penetration depths are less than 1 nm.

We aim to understand the origins of intrinsic charge carrier traps in organic and polymeric semiconductor materials from a physical chemistry perspective. In crystalline organic semiconductors, we point out some of the inadequacies in the description of intrinsic charge traps using language and concepts developed for inorganic semiconductors. In π-conjugated polymeric semiconductors, we suggest the presence of a two-tier electronic energy landscape, a bimodal majority landscape due to two dominant structural motifs and a minority electronic energy landscape from intrinsic charged defects. The bimodal majority electronic energy landscape results from a combination of amorphous domains and microcrystalline or liquid-crystalline domains. The minority tier of the electronic density of states is comprised of deep Coulomb traps embedded in the majority electronic energy landscape. This minority electronic energy landscape may dominate transport properties at low charge carrier densities, such as those expected for organic photovoltaic devices, while the bimodal majority electronic energy landscape becomes significant at high carrier densities, that is, in organic field effect transistors.

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Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

We apply attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs). The OTFT studied uses the n-type organic semiconductor N-N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from poly(ethylene oxide) and LiClO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). There are two distinctive doping regions: a reversible and electrostatic doping region for VG 2 V. On the basis of intensity loss of vibrational peaks attributed to neutral PTCDI-C8, we obtain a charge carrier density of 2.9 x 10(14)/cm2 at VG=2 V; this charge injection density corresponds to the conversion of slightly more than one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film can convert all PTCDI-C8 molecules in a 30-nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) is used, the maximum charge carrier density achievable at VG=200 V is approximately 4.5 x 10(13)/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions.

The final step in the preparation of p-DTS(PTTh 2 ) 2 involves end capping of the PT-DTS-PT core with 2-hexylbithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of 2-hexylbithiophene transfer) can occur leading to the formation of (MePT)DTS(PTTh 2 ). Although (MePT)DTS(PTTh 2 ) is difficult to separate from the p-DTS(PTTh 2 ) 2 product via column chromatography, it is readily extracted using hexanes solvent to give absolute p-DTS(PTTh 2 ) 2 . Trace impurities of (MePT)DTS(PTTh 2 ) in BHJ solar cells fabricated from synthesis batches of p-DTS(PTTh 2 ) 2 significantly influence the photovoltaic properties, causing a $50% reduction in efficiency and affecting all of the relevant device parameters (J sc , V oc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data presented in this publication, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination.