Blending Poly(3-hexylthiophene) for Controlled Thermal Conductivity

Semiconducting polymers have transport properties that can be tuned by both the synthetic design and processing techniques. Their thermopower, electronic conductivity, and low lattice thermal conductivity make them attractive in thermal management and thermoelectric applications, especially in form factors unfit for comparable inorganic ceramics. The effects of blending differing molecular weights and regioregularities of poly(3-hexylthiophene) (P3HT) on the thermal conductivity of films of P3HT doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane were investigated in order to develop design rules for the co-optimization of thermal and electronic properties. The thermal conductivity of blended P3HT films at room temperature was found to be controlled by the distribution of molecular weights and regioregularities of chains. The resulting thermal conductivity of P3HT at room temperature was found to span 0.2–0.85 W/mK without specialized processing methods. Upon electrical doping, a significant decrease in thermal conductivity was found at all blending compositions despite each composition having a comparable electronic conductivity. These results suggest the blending of molecular weights and regioregularities as a rational means to optimize thermal conductivity while maintaining desired electronic properties in semiconducting polymers

Charge-Carrier Dynamics and Crystalline Texture of Layered Ruddlesden–Popper Hybrid Lead Iodide Perovskite Thin Films

Solution-processable organic metal halide Ruddlesden–Popper phases have shown promise in optoelectronics because of their efficiencies in solar cells along with increased material stability relative to their three-dimensional counterparts (CH₃NH₃PbI₃). Here, we study the layered material butylammonium methylammonium lead iodide (C₄H₉NH₃)₂(CH₃NH₃)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 for values of <i>n</i> ranging from 1 to 4. Thin films cast from solution show a gradual change in the crystalline texture of the two-dimensional lead iodide layers from being parallel to the substrate to perpendicular with increasing <i>n</i>. Contactless time-resolved microwave conductivity measurements show that the average recombination rate order increases with <i>n</i> and that the yield–mobility products and carrier lifetimes of these thin films are much lower than that of CH₃NH₃PbI₃, along with increased higher-order recombination rate constants

Thermoelectric Properties of Poly(3-hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) by Vapor-Phase Infiltration

Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly­(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F₄TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F₄TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the electrical conductivity is significantly higher, improving the thermoelectric power factor

Phase Separated Morphology of Ferroelectric–Semiconductor Polymer Blends Probed by Synchrotron X‑ray Methods

Control of the domain size and morphology of ferroelectric-semiconductor polymer blend thin films is essential for producing working organic ferroelectric resistive switches that can be used for low-cost, flexible memory applications. However, improvements in characterization techniques that can selectively probe these polymers are still needed. The unique core-level absorption profiles of these polymers make synchrotron based soft X-ray techniques ideal to achieve contrast and chemical sensitivity between polymers and characterize thin film morphology. Transmission soft X-ray microscopy and scattering reveal that a phase separated structure exists through the bulk for a blend of a semicrystalline semiconducting polythiophene with a functionalized side chain and a well-studied ferroelectric polymer. Surface sensitive soft X-ray spectroscopy and wide-angle X-ray scattering suggest a potential enhancement of polythiophene at the film surface, and that the surface layer is more amorphous in character. This work demonstrates the utility of soft X-rays to characterize ferroelectric-semiconductor polymer blends both in the bulk and at the film surface. Understanding differences in composition and morphology between the bulk and thin film interfaces is critical to further improve organic-based memory technology

Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase

We demonstrate how processing methods affect the thermoelectric properties of thin films of a high mobility semiconducting polymer, PBTTT. Two doping methods were compared: vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)­trichlorosilane (FTS) or immersion in a solvent containing 4-ethylbenzenesulfonic acid (EBSA). Thermally annealed, thin films doped by FTS deposited from vapor yield a high Seebeck coefficient (α) at high electronic conductivity (σ) and, in turn, a large power factor (PF = α²σ) of ∼100 μW m^–1 K^–2. The FTS-doped films yield α values that are a factor of 2 higher than the EBSA-doped films at comparable high value of σ. A detailed analysis of X-ray scattering experiments indicates that perturbations in the local structure from either dopant are not significant enough to account for the difference in α. Therefore, we postulate that an increase in α arises from the entropic vibrational component of α or changes in scattering of carriers in disordered regions in the film

Quadrites and Crossed-Chain Crystal Structures in Polymer Semiconductors

Many high-performance conjugated polymers for organic photovoltaics and transistors crystallize such that chains are parallel, resulting in significant anisotropy of the nanoscale charge transport properties. Here we demonstrate an unusual intercrystallite relationship where thin lamellae adopt a preferred epitaxial relationship with crossed-chains at the interface. The crossed-chains may allow either crystal to use the other as an “electronic shunt”, creating efficient quasi-three-dimensional transport pathways that reduce the severity of grain boundaries and defects in limiting transport

First-Principles Predictions of Near-Edge X‑ray Absorption Fine Structure Spectra of Semiconducting Polymers

The electronic structure and molecular orientation of semiconducting polymers in thin films determine their ability to transport charge. Methods based on near-edge X-ray absorption fine structure (NEXAFS) spectroscopy can be used to probe both the electronic structure and microstructure of semiconducting polymers in both crystalline and amorphous films. However, it can be challenging to interpret NEXAFS spectra on the basis of experimental data alone, and accurate, predictive calculations are needed to complement experiments. Here, we show that first-principles density functional theory (DFT) can be used to model NEXAFS spectra of semiconducting polymers and to identify the nature of transitions in complicated NEXAFS spectra. Core-level X-ray absorption spectra of a set of semiconducting polymers were calculated using the excited electron and core-hole (XCH) approach based on constrained-occupancy DFT. A comparison of calculations on model oligomers and periodic structures with experimental data revealed the requirements for accurate prediction of NEXAFS spectra of both conjugated homopolymers and donor–acceptor polymers. The NEXAFS spectra predicted by the XCH approach were applied to study molecular orientation in donor–acceptor polymers using experimental spectra and revealed the complexity of using carbon edge spectra in systems with large monomeric units. The XCH approach has sufficient accuracy in predicting experimental NEXAFS spectra of polymers that it should be considered for design and analysis of measurements using soft X-ray techniques, such as resonant soft X-ray scattering and scanning transmission X-ray microscopy

Polymer Side Chain Modification Alters Phase Separation in Ferroelectric-Semiconductor Polymer Blends for Organic Memory

Side chain modification of a semiconducting polythiophene changes the resulting phase separation length scales when blended with a ferroelectric polymer for use in organic ferroelectric resistive switches. The domain size of the semiconducting portion of blends of poly­[3-(ethyl- 5-pentanoate)­thiophene-2,5-diyl] (P3EPT) and poly­(vinylidene fluoride-<i>co</i>-trifluoroethylene) (PVDF-TrFE) in thin film blends are smaller than previously reported and easily controllable in size through simple tuning of the weight fraction of the semiconducting polymer. Furthermore, P3EPT has a relatively high degree of crystallinity and bimodal crystallite orientations, as probed by wide-angle X-ray scattering. Resistive switches fabricated from blends of P3EPT and PVDF-TrFE show memristive switching behavior over a wide range of polythiophene content and good ON/OFF ratios

Energy Transfer Directly to Bilayer Interfaces to Improve Exciton Collection in Organic Photovoltaics

Ternary blends and energy cascades are gaining popularity as ways to engineer absorption as well as exciton and charge collection in organic solar cells. Here, we use kinetic Monte Carlo simulations to investigate energy cascade designs for improving exciton collection in bilayer solar cells via a Förster energy transfer mechanism. We determine that an interfacial monolayer (C) between the donor and acceptor with a D → A → C energy cascade will lead to good exciton collection, allowing for >90% collection, even for energy donor layers up to 75 nm thick. We further examine how roughening the interface, increasing the exciton diffusion length, and using other energy cascade designs affect the enhancement from the energy transfer. We also propose using the inherent charge transfer states at the interfaces as energy acceptors and estimate that the Förster radius could be as large as 3.4 nm, leading to nearly 70% improvement in exciton collection, without the need for a third material

Temperature Dependence of the Diffusion Coefficient of PCBM in Poly(3-hexylthiophene)

Interest in new functional small molecule and polymer blends, such as polymer–fullerene bulk heterojunction (BHJ) organic solar cells motivates the development of new methods to measure the diffusion coefficient of molecular species (e.g., PCBM) in polymers. The aim of this study is to systematically improve our understanding of the relevant material and processing parameters needed to control the microstructure of BHJ organic solar cells in order to develop a more complete understanding of how to improve its power conversion efficiency. Here, we fabricate a terraced monolayer–bilayer sample of P3HT and P3HT/PCBM and use this structure to quantify both the volume fraction of miscible PCBM in P3HT and the diffusion coefficient of disordered PCBM in disordered P3HT. Our findings reveal that the diffusion coefficient for disordered PCBM in P3HT is strongly dependent on the annealing temperature (i.e., increasing by 3 orders of magnitude when doubling the annealing temperature) and weakly dependent on the PCBM concentration. The temperature-dependent diffusion coefficients were fit with an Arrhenius relationship to determine an activation energy for the diffusion of disordered PCBM through P3HT. Ultimately, this report demonstrates that the self-assembly of the P3HT:PCBM BHJ solar cell during annealing and cooling is not limited by the diffusion of deuterated PCBM in P3HT with the nanostructure of PCBM being controlled by the relative volume fractions of ordered and disordered P3HT