Session L50: Focus Session: Organic Electronics and Photonics - Morphology

Real-time observation of optoelectronic properties during poly(3-alkylthiophene) crystallization

Poly(3-alkylthiophenes) (P3ATs) are commonly used semiconducting polymers in organic electronic applications where the nanoscale morphology of the active layer largely dictates device performance. However, during solution processing, a large driving force for crystallization results in thin film morphologies that are kinetically trapped and cannot be easily controlled. We show that rational side chain design, particularly the use of branched alkyl chains, can reduce the melting transition increasing thermal control relative toP3HT. Importantly, the lower crystallization transition temperature provides the opportunity to monitor crystallization in real-time using grazing-incidence x-ray diffraction and UV-vis absorption spectroscopy, and the relative degree of crystallinity is observed to increase gradually over the course of crystallization. In contrast to the gradual increase in crystallinity and optical properties, the field-effect mobility monitored during crystallization exhibits a sharp increase of approximately two orders of magnitude. We propose that the difference in time scales may be due to the formation of a percolated network between electrodes, and that increases in the degree of crystallinity beyond this point are not probed by the transistor geometry.   

In Situ Annealing Study of Organic Photovoltaic Morphology via Non-invasive Polarized Neutron Reflectivity

Polarized neutron reflectivity, a non-invasive technique, allows the unambiguous density distribution within a thin film to be determined. By utilizing this technique with organic photovoltaics is it possible to study the same device pre- and post-annealing. We studied a bulk herterojunction cell consisting of an organic semiconductor (P3HT) and a nanoparticle electron acceptor (PCBM). We found a shift in the location of PCBM within the organic film which migrates toward the anode and cathode following annealing. However, while some PCBM can reach the substrate interface it never fully blooms to the air interface and pure P3HT resides at the surface for the thick (200 nm) films used in this study. These results differ from previous research in that the same device was characterized allowing a true study on the effect of annealing to be performed.   

Solvent Studies for Solution Processing of Polymer-Fullerene Bulk Heterojunctions

A major advantage of polymer solar cells over higher-efficiency photovoltaic alternatives is the low cost of printing technologies. Solution-based film preparation relies on the self-assembly of the polymer and fullerene phases as the layer dries from the casting ink into a bulk heterojunction morphology. The nanoscale morphology must facilitate optimal charge separation and transport; thus solution processing parameters heavily influence the device performance. Current technology uses chlorinated aromatic solvents and small processing windows with stringent requirements on ink properties. In this study, the stability of various ink formulations is investigated to develop more reliable, sustainable alternatives. Model solutions of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are prepared using a range of solvents and solvent mixtures. P3HT solution crystallization behavior is investigated using differential scanning calorimetry, UV-visible absorbance measurements and neutron and light scattering. The behavior of the solutions under shear is investigated in order to predict the success of disparate printing and coating techniques.   

Morphology in Hybrid Oxide/Polymer Core-Shell Nanowire Photovoltaics

We use first principles theory (DFT) to gain insight into the Poly(3-hexylthiophene) (P3HT)-ZnO nanowire binding morphologywhere the P3HT is bound to ZnO via SiO$_3$ linkers [1]: understanding the experimentally realized morphology is the first step in trying to predict and enhance electronic energy alignments conducive to photovoltaic operation. For large diameter ZnO nanowires, the morphology is that of P3HT bound to the ($10\overline{1}0$) surface of ZnO. The existence of a lattice mismatch of the ZnO surface and the polymer backbone creates a competition between linker binding and strain energy. We solve this realization of the classic Frenkel-Kontorova model based on ab initio parameters to obtain the lowest energy binding morphology on the surface. For small ZnO nanwire diameters, curvature effects become important: the P3HT polymer must bend around the nanowire, and the reduction of linker spacing due to curvature enlarges the lattice mismatch for helical wrappings. We use DFT to estimate these curvature energies, and we predict a morphology change from helical wrapping to linear alignment with the nanowire axis at a diameter close to 25 nm. [1] S. Zhang, Adv. Mater. (in press, 2011).   

An Improved Forcefield for Molecular Modeling Oof Crystalline Poly(3-Hexyl Thiophene)

A fundamental understanding of molecular structure and dynamics of poly(3-hexyl thiophene) (P3HT), one of the most promising organic solar cell materials, is crucial for improving the efficiency of organic solar cells containing P3HT as the donor. Molecular dynamics (MD) simulations can produce the correct structures and dynamics of P3HT provided that robust forcefields are employed for this system. The forcefields that are currently used for MD simulations of P3HT are mostly taken from the analogous thiophene molecule, bi-thiophene. However, such forcefields may lack to produce the correct morphology and stacking properties of P3HT. We present the results of MD simulations using an improved forcefield for P3HT. In the improved forcefield the torsional and partial atomic charge parameters for both the alkyl side chains and backbone were derived from ab initio calculations. Our results from MD simulations are compared with available experimental and theoretical data and the range of accessible structures is explored.   

Device simulation of morphologies that are consistent with small angle scattering and reflectometry

Through the use of a dynamic Monte Carlo simulation, we are able to evaluate the efficiency of bulk heterojunction morphologies of P3HT/PCBM based photovoltaic devices that are consistent with neutron reflectometry and SANS data. We have developed a method to efficiently generate simulated small angle scattering data from hypothetical nanoscale systems such as polymer-fullerene bulk heterojunctions found in organic photovoltaic devices. We will show how this method can be used to accurately calculate the scattering information of well known systems such as a polydisperse collection of hard and soft spheres. We will then demonstrate this method on the calculated device morphologies, and show how the simulated scattering can grant insight into the validity of assumptions based on traditional fitting methods.   

Three dimensional device simulations to model conducting probe AFM measurements of organic semiconductors with fibrous morphologies

Organic semiconductors offer a promising material for many optoelectronic devices, with device performance depending significantly on the nanoscale morphology. Atomic force microscopy (AFM) is one of the major instruments for investigating the dependence of current-voltage response on nanoscale structures. We are developing computational methods for fundamental study of charge transport probed by these measurements, using continuum device models and Kinetic Monte Carlo simulations. The simulations are performed on complex three dimensional model morphologies that are consistent with the topology observed in AFM measurements. The calculated IV response of these models is compared with CP-AFM measurements.   

Self-Limiting Crystal Microstructure in Poly(3-hexylthiophene)

Polymeric semiconductors, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), hold great potential for a variety of technologies. These solution processable materials are promising as active layers for low cost or large area flexible electronic or optoelectronic devices that can be prepared through high-throughput deposition processes, such as inkjet or roll-to-roll printing. Of the myriad of materials currently under examination, P3HT is one of the most widely studied materials because of its electronic properties and commercial availability. However, most, if not all, commercially available P3HT is produced with some non-negligible level of regiodefects that has a predefined impact on its crystalline fraction. We examine the effect of regiodefects in P3HT on the semicrystalline microstructure, i.e., lamellar thickness and distribution, and overall crystallinity, and discuss the impact of these effects on potential device performance and bulk heterojunction morphology.   

X-ray and neutron reflectivity and electronic properties of PCBM-poly(bromo)styrene blends and bilayers with poly(3-hexylthiophene)

We used neutron reflectivity to complement x-ray reflectivity characterization of PCBM-based layers formed on poly(3-hexylthiophene) (P3HT). Single-layer analyses were used to provide reliable scattering length density values for bilayer fitting. Atomic force microscopy analyses showed trends similar to the reflectivity experiments when observing upper surfaces. Styrene polymers added to PCBM in small concentrations (ca. 10 percent) led to processing advantages while retaining substantial electron mobility, about 0.001 cm$^{2}$/V s. The further introduction of a relatively heavy bromo atom substituent on the styrene rings greatly increased the film smoothness, as revealed by increases of the oscillation amplitudes in the reflectivity. In addition, the bromine heavy atom increased x-ray reflectivity scattering length density of the upper layer. Finally, we confirm that P3HT is capable of extracting PCBM from a subsequently deposited overlying layer, consistent with predictions based on published phase diagrams of the P3HT-PCBM system.   

Controlled phase separation in conjugated polymer blends using quadruple hydrogen bonding interactions

All-polymer photovoltaic blends have significant promise for low-cost, solution processed photovoltaics, but large-scale phase separation can lead to non-optimal active layer structures for charge-separation and transport. We propose a novel strategy for controlling phase separation in conjugated polymer blends using quadrupole hydrogen bonding interactions. We investigate a series of end-modified conjugated and coil-like polymers in blends, including poly(3-hexyl thiophene) (P3HT), poly(styrene), poly(ethylene glycol), poly(9,9-dioctyl fluorene). Polymers are end-terminated with the multiple hydrogen bonding group 2-ureido-4(1H)-pyrimidinone (UPy) using isocyanate chemistry. Atomic force microscopy and grazing incidence x-ray scattering show phase separation is suppressed and, in some cases, P3HT crystallite orientation is improved in films for blends of UPy functionalized polymers. These results show the quadruple hydrogen bonding groups can prevent large-scale phase separation and direct the orientation of polymer chains.   

Synthesis and Characterization of All-Conjugated Block Copolymers Prepared via Click Chemistry

All-conjugated block copolymers with both hole-conducting and electron-conducting polymer blocks can be used to address fundamental questions regarding the structure, optoelectronic properties, and photovoltaic performance of organic photovoltaic blends, but synthetic challenges have precluded comprehensive studies on such systems. Here, we present a novel synthetic approach for preparing all-conjugated block copolymers and detailed studies of their nanoscale structure and optical properties. Our synthetic approach is based on copper-catalyzed azide-alkyne ``click'' chemistry and enables us to prepare block copolymers with a poly(3-alkylthiophene) block covalently linked to a conjugated polymer prepared by Suzuki polycondensation polymerization, including poly(9,9-dioctyl fluorene), poly(9,9-dioctyl fluorene-alt-benzothiadiazole) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFOTBT). A combination of x-ray diffraction, grazing-incidence x-ray scattering, atomic force microscopy, and fluorescence quenching measurements give insight into their microstructure and potential for use in high-performance all-polymer photovoltaics.   

Undulation instability in drop-cast poly(3-hexylthiophene) film originated from self-assembly

In this study, we characterize the undulated structures which appear at the edge of drop-cast regio-regular poly(3-hexylthiophene) (rr-P3HT, head-to-tail $>$ 95{\%}) film using optical microscopy and atomic force microscopy. We propose that these periodic structures originate from the undulations of the layered structure of liquid crystal-air interface. Evidence of rr-P3HT solution forming liquid crystalline phases at higher concentrations was obtained by the observation of distinct birefringence and characteristic textures under crossed polarizers using an optical microscope. Synchrotron x-ray diffraction pattern provides additional structural information at the undulated area compared with those at the area without undulated pattern. Based on these experimental results, we propose rr-P3HT solution can form a lyotropic liquid crystal at specific concentrations. This work was partially supported by NSF funding (DMR-0706235).