Crystallization Mechanism and Charge Carrier Transport in MAPLE-Deposited Conjugated Polymer Thin Films

MAPLE-Deposited Perylene Diimide Derivative Based Layers for Optoelectronic Applications

Nowadays, the development of devices based on organic materials is an interesting research challenge. The performance of such devices is strongly influenced by material selection, material properties, design, and the manufacturing process. Usually, buckminsterfullerene (C60) is employed as electron transport material in organic photovoltaic (OPV) devices due to its high mobility. However, considering its low solubility, there have been many attempts to replace it with more soluble non-fullerene compounds. In this study, bulk heterojunction thin films with various compositions of zinc phthalocyanine (ZnPc), a perylene diimide derivative, or C60 were prepared by matrix-assisted pulsed laser evaporation (MAPLE) technique to assess the influence of C60 replacement on fabricated heterostructure properties. The investigations revealed that the optical features and the electrical parameters of the organic heterostructures based on this perylene diimide derivative used as an organic acceptor were improved. An increase in the JSC value (4.3 × 10-4 A/cm2) was obtained for the structures where the perylene diimide derivative acceptor entirely replaced C60 compared to the JSC value (7.5 × 10-8 A/cm2) for the heterostructure fabricated only with fullerene. These results are encouraging, demonstrating the potential of non-fullerene compounds as electron transport material in OPV devices.

Meticulous Molecular Engineering of Crystal Orientation and Morphology in Conjugated Polymer Thin Films for Field-Effect Transistors

The ability to precisely tailor molecular packing and film morphology in conjugated polymers offers a robust means to control their optoelectronic properties. This, however, remains a grand challenge. Herein, we report the dependency of molecular packing of an important conjugated polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), on a set of intrinsic parameters and unveil the correlation between their crystalline structures and charge transport characteristics. Specifically, a family of PBTTT with varying side chains (i.e., hexyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl referred to as C6, C8, C10, C12, C14, and C16, respectively) and molecular weights (MWs) with a focus on C14 are judiciously designed and synthesized. Various crystalline structures are yielded by tuning the alkyl chain and MW of PBTTT together with thermal annealing. It reveals that extending the alkyl chain length of PBTTT to C14, along with a larger MW and heating at 180 °C, promotes the formation of edge-on crystallites with significantly improved orientation and ordering. Furthermore, these distinct crystalline structures greatly impact their charge mobilities. This study sheds light on the tailored design of crystalline structures in PBTTT through a synergetic approach, which paves the way for potential applications of PBTTT and other conjugated polymers in optoelectronic devices with enhanced performance.

Macrocyclic Compounds: Metal Oxide Particles Nanocomposite Thin Films Deposited by MAPLE

Nanocomposite films based on macrocyclic compounds (zinc phthalocyanine (ZnPc) and 5,10,15,20-tetra(4-pyridyl) 21H,23H-porphyrin (TPyP)) and metal oxide nanoparticles (ZnO or CuO) were deposited by matrix-assisted pulsed laser evaporation (MAPLE). 1,4-dioxane was used as a solvent in the preparation of MAPLE targets that favor the deposition of films with a low roughness, which is a key feature for their integration in structures for optoelectronic applications. The influence of the addition of ZnO nanoparticles (~20 nm in size) or CuO nanoparticles (~5 nm in size) in the ZnPc:TPyP mixture and the impact of the added metal oxide amount on the properties of the obtained composite films were evaluated in comparison to a reference layer based only on an organic blend. Thus, in the case of nanocomposite films, the vibrational fingerprints of both organic compounds were identified in the infrared spectra, their specific strong absorption bands were observed in the UV-Vis spectra, and a quenching of the TPyP emission band was visible in the photoluminescence spectra. The morphological analysis evidenced agglomerated particles on the composite film surface, but their presence has no significant impact on the roughness of the MAPLE deposited layers. The current density-voltage (J-V) characteristics of the structures based on the nanocomposite films deposited by MAPLE revealed the critical role played by the layer composition and component ratio, an improvement in the electrical parameters values being achieved only for the films with a certain type and optimum amount of metal oxide nanoparticles.

Effect of Aluminum Nanostructured Electrode on the Properties of Bulk Heterojunction Based Heterostructures for Electronics

The properties of organic heterostructures with mixed layers made of arylenevinylene-based polymer donor and non-fullerene perylene diimide acceptor, deposited using Matrix Assisted Pulsed Laser Evaporation on flat Al and nano-patterned Al electrodes, were investigated. The Al layer electrode deposited on the 2D array of cylindrical nanostructures with a periodicity of 1.1 µm, developed in a polymeric layer using UV-Nanoimprint Lithography, is characterized by an inflorescence-like morphology. The effect of the nanostructuring on the optical and electrical properties was studied by comparison with those of the heterostructures based on a mixed layer with fullerene derivative acceptor. The low roughness of the mixed layer deposited on flat Al was associated with high reflectance. The nano-patterning, which was preserved in the mixed layer, determining the light trapping by multiple scattering, correlated with the high roughness and led to lower reflectance. A decrease was also revealed in photoluminescence emission both at UV and Vis excitation of the mixed layer, with the non-fullerene acceptor deposited on nano-patterned Al. An injector contact behavior was highlighted for all Al/mixed layer/ITO heterostructures by I-V characteristics in dark. The current increased, independently of acceptor (fullerene or non-fullerene), in the heterostructures with nano-patterned Al electrodes for shorter conjugation length polymer donors.

Transforming Polymorphs via Meniscus-Assisted Solution-Shearing Conjugated Polymers for Organic Field-Effect Transistors

The ability to tune polymorphs of conjugated polymers affords a robust platform for investigating the processing-structure-property relationship. However, simple and generalizable routes to polymorphs have yet to be realized. Herein, we report a viable meniscus-assisted solution-shearing (MASS) strategy to effectively modulate polymorphs (i.e., polymorphs I and II) of poly(3-butylthiophene) (P3BT) and scrutinize the correlation between the two different polymorphs and charge transport characteristics. Specifically, polymorph II exists solely in drop-cast P3BT films. Intriguingly, confined shearing of P3BT renders efficient transformation of polymorph II to I. The kinetics of polymorph transformation associated with the changes in molecular packing and thus photophysical properties are elucidated. The resulting organic field-effect transistors reveal a strong correlation of device performance to attained polymorphs and crystal orientations of P3BT. Such polymorph transformation via the convenient MASS technique can be readily extended to other conjugated polymers of interest. This study highlights the robustness of MASS in regulating polymorphs of conjugated polymers to interrogate their interdependence of processing, structure, and property for a wide range of optoelectronic applications.

Tunable Properties of MAPLE-Deposited Thin Films in the Presence of Suppressed Segmental Dynamics

Processing polymer thin films by physical vapor deposition has been a major challenge due to material degradation. This challenge has limited our understanding of morphological control by top-down approaches that can be crucial for many applications. Recently, matrix-assisted pulsed laser evaporation (MAPLE) has emerged as an alternative route to fabricate polymer thin films from near-gas phase growth conditions. In this Letter, we investigate how this approach can result in a stable two-phase film structure of semicrystalline polymers via a unique combination of MAPLE and flash calorimetry. In the case of MAPLE-deposited poly(ethylene oxide) (PEO) thin films, we find a 35 °C enhancement in the glass transition temperature relative to melt-crystallized films, which is associated with irreversible chain adsorption in the amorphous region of the film. Remarkably, by varying substrate temperature during deposition, we reveal the ability to significantly tune the crystal orientation, extent of crystallinity, and lamellar thickness of MAPLE-deposited PEO thin films.

Structure Control of a π-Conjugated Oligothiophene-Based Liquid Crystal for Enhanced Mixed Ion/Electron Transport Characteristics

Developing soft materials with both ion and electron transport functionalities is of broad interest for energy-storage and bioelectronics applications. Rational design of these materials requires a fundamental understanding of interactions between ion and electron conducting blocks along with the correlation between the microstructure and the conduction characteristics. Here, we investigate the structure and mixed ionic/electronic conduction in thin films of a liquid crystal (LC) 4T/PEO4, which consists of an electronically conducting quarterthiophene (4T) block terminated at both ends by ionically conducting oligoethylenoxide (PEO4) blocks. Using a combined experimental and simulation approach, 4T/PEO4 is shown to self-assemble into smectic, ordered, or disordered phases upon blending the materials with the ionic dopant bis(trifluoromethane)sulfonimide lithium (LiTFSI) under different LiTFSI concentrations. Interestingly, at intermediate LiTFSI concentration, ordered 4T/PEO4 exhibits an electronic conductivity as high as 3.1 × 10^-3 S/cm upon being infiltrated with vapor of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecular dopant while still maintaining its ionic conducting functionality. This electronic conductivity is superior by an order of magnitude to the previously reported electronic conductivity of vapor co-deposited 4T/F4TCNQ blends. Our findings demonstrate that structure and electronic transport in mixed conduction materials could be modulated by the presence of the ion transporting component and will have important implications for other more complex mixed ionic/electronic conductors.

Self-Assembled Monolayers at the Conjugated Polymer/Electrode Interface: Implications for Charge Transport and Band-Bending Behavior

The role of self-assembled monolayers (SAMs), trichloro(1 H,1 H,2 H,2 H-perfluorooctyl) (FTS) and octadecyltrichlorosilane (OTS), deposited on indium tin oxide (ITO) substrates, on electronic properties of the poly(3-hexylthiophene) (P3HT)/SAM/ITO system is reported. SAMs, well known for modifying the surface energies of materials, are also known to modify the work functions (WFs) of semiconductors. Unsurprisingly, differences between the band-bending behaviors of P3HT/ITO, P3HT/OTS/ITO, and P3HT/FTS/ITO systems were observed because the SAMs modify the WF of ITO. However, the degrees of band bending occurring in these systems could not be attributed solely to the modified WFs of the substrate. This was apparent based on measurements of samples that included P3HT films prepared with different morphological structures. Changes in the morphological structure, due to different deposition methods and surface energies of the substrates, are necessarily connected to changes in the electronic structure, including changes in the electronic density of states (DOS), of P3HT. An association between (i) the WF differences between P3HT, ITO, and SAM/ITO substrates, (ii) the surface energies of the ITO and SAM/ITO substrates, which influence the morphology of the deposited P3HT layer, (iii) the DOS widths of P3HT, and (iv) the degree of band bending is suggested.