Session N16: Organic Electronics II: Doping, Morphology, and Optoelectronic Properties of Conjugated Polymers

Real time mass change and mobility associated with the doping of poly(3-hexylthiophene) for high-performance organic electrochemical transistors

Polythiophene systems have extensive applications in organic electronics especially in organic electrochemical transistors (OECTs) due to the polymer’s tunable conductivity and redox activity. For example, the conductivity and mobility of poly(3-hexylthiophene) (P3HT) is heavily dependent upon the doping level and the dopant type. Quantification of the doping levels in real time of P3HT or similar conjugated polymers associated with switching of electronic states (conductive vs insulation) is essential for high-performance devices. This study shows relationships among electrochemical doping, solvent uptake, and conductivity using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in-situ conductance measurements. Distinct mass transfer regions, quantified as a function of doping level are correlated with spectrochemical response and mobility. The mobility and charge carrier density are calculated, including the real time change in thickness. These experiments are complemented with electrochemical impedance spectroscopy coupled with EQCM-D to identify the time scale at which the doping reaction transitions from kinetic to diffusion control. This work gives valuable insight into the nature of mixed ion-electron transfer, including its time scale, as it relates to the electronic properties of P3HT.   

Effect of Side-Chain Polarity on Conductivity and Thermal Stability in Molecularly Doped Polythiophene Polymers

The need for thermally stable electronic conductivity in doped organic semiconductors is a derisible property for many optoelectronic and energy harvesting applications. Here, we investigate the electronic conductivity (σ) and the corresponding thermal stability of two polythiophene derivatives comprising oligoethylene glycol side-chains with an oxygen directly attached to the thiophene rings (P3MEET) and its analog P3MEEMT that has a methyl spacer between the oxygen and the thiophene rings (P3MEEMT). To modulate the electronic conductivity, thin films were vapor-doped with fluorinated-derivatives of tetracyanoquinodimethane (FnTCNQ, n = 4, 2, 1) to determine the role of dopant strength (electron affinity) on maximum achievable σ. The values of σ are within the same order of magnitude for all three fluorinated dopants while σ of P3MEEMT decreases significantly with the decreasing fluorination level of dopants.  Specifically, when vapor doping with F4TCNQ, P3MEET exhibits substantially higher σ of 37.1 ± 10.1 S/cm compared to σ of 0.82 ± 0.06 S/cm for P3MEEMT. Structural characterization using a combination of X-ray and optical spectroscopies reveals that the higher degree of conformational order of polymer chains in the amorphous domain, which facilitates faster charge carrier mobility, is a major contributing factor for the higher σ of P3MEET. Lastly, vapor-doped P3MEET exhibited superior thermal stability compared to P3MEEMT, which showcases that in contrast to previous belief, the presence of polar side chains alone does not permit higher thermal stability. In fact, the description is more nuanced comprising multiple factors such as dopant-side chain interaction and side chain mobility in addition to the role of oxygen atom near the thiophene rings controls the ionization energy and subsequent stabilization of the charge. Overall, the above summarized observations should be accounted for future design of conductive polymers with high thermal stability.   

Talk Title: Strategies to predict and control semiconducting polymers' dynamics

 

Talk abstract: 

Organic semiconducting polymers were widely studied due to their unique optoelectronic and mechanical property. They are the key component in various functional electronic devices, such as organic photovoltaic devices, flexible displays, wearable sensors, neuromorphic computing, and more recently bioelectronics. Despite tremendous progress were made in improving the charge carrier mobility and optimizing energy bandgap, the conjugated polymer's physical property was not widely studied such as chain rigidly, molecular entanglement behavior, and glass transition phenomenon.  However, they are important for device stability, which is one of the major hurdles for polymer-based organic devices.  Polymer dynamics could play an important role in dictating overall chain mobility and morphology stability in the bulk heterojunction.

 

In my talk, I will provide an overview of our effort in studying the conjugated polymer's dynamics using a wide range of unique characterization tools, from thin-film to bulk. I will discuss the challenge associate with accurately measuring the glass transition temperature for rigid conjugated polymers.  I will discuss our approach to address this challenge using thin-film calorimetry and ellipsometry tools, as well as using molecular dynamic simulation and cheminformatic to accurately predict the glass transition temperature.  Lastly, I will discuss how dynamics could impact thin film morphology and device performance at different operation temperatures and should be carefully considered when design new polymers and devices.   

Temperature Dependent Studies of IDT-BT Reveal Liquid Crystalline Phase Behavior

Some of the highest charge transporting conjugated polymers to date such as Indacenodithiophene-co-benzothiadiazole (IDT-BT) have shown weak to no crystallinity which is puzzling as it deviates from traditional evidence that higher order is needed for high charge mobility. While many have hypothesized that an increased order at a molecular scale along with a stiff planar backbone can increase charge transport without the need for long range order, this behavior is still not fully understood. Often, stiffer conjugated polymers can have liquid crystalline phases, indeed many high mobility conjugated polymers, such as PBTTT, have liquid crystallinity. As the polymer chains are not restricted to a certain packing structure dictated by crystallization, the chains in a liquid crystalline polymer are able to adopt conformations that can maximize interchain charge coupling. Using rheology, X-ray scattering, and polarized optical measurements we have investigated the morphology and phase behavior of IDT-BT as a function of temperature. We have identified the backbone and side chain glass transition temperatures and discovered three liquid crystalline phases. We also have fabricated IDT-BT field effect transistors, measured in-situ, to reveal how phase behavior affects hole mobility.   

Blends of conjugated and adhesive polymers for multifunctional organic thin-film transistors

Organic thin-film transistors (OTFTs) have attracted significant attention due to their large area compatibility, potential solution processability, and stretchability. In this work, we present a novel polymer blend that consists of a conjugated polymer (Poly(3-hexylthiophene-2,5-diyl), Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], or Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)]) and an adhesive polymer (Poly[(3,4-dihydroxystyrene)-co-styrene]) as the active layer of OTFTs. The blend films possess both semiconductive and adhesive properties that can be utilized to enable 3D integration and minimize delamination. For a broad range of composition of the blended films, OTFTs maintain comparatively constant field-effect charge carrier mobility. Addition of a conjugated polymer helps improve adhesion strength of the bonded films until an optimal adhesion strength is achieved, up to 1.28 MPa. This strategy for polymer blends is an example of the potential for multifunctional OTFTs.   

Tight-binding model accurately describes frontier orbitals of conjugated oligomer acceptors for organic solar cells.

Conjugated organic oligomers are designed as acceptor materials for organic photovoltaic [OPV] cells. OPV optoelectronic properties, including light absorption, intramolecular and intermolecular charge transfer, depends on the properties of frontier molecular orbitals of these conjugated molecules. Recently, we have shown that tight-binding models can efficiently describe a broad range of optoelectronic properties for copolymers. Tight-binding parameters derived from density functional theory [DFT] calculations on constituent homopolymers reasonably predict copolymer valence and conduction bands. Here, we extend this approach to heterogeneous oligomers designed for non-polymeric acceptors including IDTBR, which gives high-efficiency OPVs in combination with various polymeric donors. IDTBR consists of 7 aromatic moieties: an indacenodithiophene [IDT] core, flanked by benzothiadiazole and 3-ethylrhodadine on either side. We show that tight-binding parameters fitted to frontier orbital energies of alternating oligomers of these constituent moieties can be used without adjustment in a tight-binding model to predict the energies and wavefunctions of IDTBR frontier orbitals.   

Tie Chains and Charge Carrier Transport in Semicrystalline Polymer Semiconductors

It has been hypothesized that the existence of tie chains, i.e., polymer molecules bridging from one crystal domain through the interstitial amorphous region into another crystalline region, is critical for good charge carrier tranport in semicrystalline polymer semiconductors. We examine the validity of this requirement through two experimental polymer physics-based methods and demonstrate that tie molecules are likely unnecessary for good charge carrier transport. The impact of these conclusions on materials design will be further examined.   

Photo-induced Charge Transfer of Fullerene and Non-Fullerene Conjugated Polymer Blends via Density Functional Theory

Donor-acceptor (D-A) type semiconducting conjugated polymers (CPs) are promising candidates for organic photovoltaic (OPV) devices due to their unique tunable mechanical compliance and optoelectronic performance. One of the most important parameters in the PV devices is photo-induced charge transfer (CT) at the interface of the CP and acceptor unit.  It would be largely beneficial to computationally examine the ability of a different molecular configuration serving as an efficient charge transfer device before any synthesis process to narrow down search list of possible high-performance CPs. In this study, we employ density functional theory (DFT) to explore photo-induced charge transfer of diketopyrrolopyrrole (DPP) based polymer as a blend with non-fullerene (ITIC) and fullerene (PCBM) acceptor units.  To evaluate the efficiency of charge transfer, we study the non-radiative relaxation of photoexcited electrons and holes using the reduced density matrix on the basis of the Redfield theory. Non-adiabatic couplings between electronic orbitals are computed based on nuclear trajectories obtained from ab initio calculations. We track the relaxation rates of charge carriers over time, where the derivative of difference between the rate of electron and hole can qualitatively represent the current density at zero voltage. This can be utilized to characterize the charge transfer performance of CPs blended with different acceptor units. Relaxation rate results indicate that CPs blend with ITIC offers a better PV effect, illustrating the potential of the current approach to explore CPs blend electronic performance for OPV devices and narrowing down the list of potential candidate CPs.   

Linkage dependence on charge transfer in organic dimers with application to intra-molecular singlet fission

Singlet fission (SF) photophysics is currently being investigated intensively because of the possibility that the process can be utilized to double the photoconductivity of organic solar cells. Interest in SF materials has shifted from intermolecular to intramolecular singlet fission (iSF) in recent years. The iSF compounds currently being investigated are mostly dimers of tetracene and pentacene linked by other entities. Experimental work has shown that the rate of singlet fission depends very strongly on the actual connectivity between the outer chromophores and the linker molecules. Thus, for example, with the phenyl group as the linker, and tetracene and pentacene as the chromophores, the rate is two orders of magnitude faster with para linkage than with meta linkage. Similar results are obtained with naphthalene as the linker molecule; different connectivities can give SF rates different by factor of about 20 or greater. We explain this enormous dependence of the SF rates on connectivity within fully many-body calculations. In particular we show that there is a one-to-one correlation between SF rates and the oscillator strength of a charge-transfer absorption from the ground state that occurs at wavelengths intermediate between the monomer absorptions.   

Donor-Acceptor Alternating Copolymer-Type Compatibilizer Enables High Efficiency, Thermal Stability, and Mechanical Robustness of Organic Solar Cells

Small molecule acceptor (SMA)-based organic solar cells (OSCs) have achieved high power conversion efficiencies (PCEs), while their long-term stabilities remain to be improved to meet the requirements for real applications. Herein, we demonstrate the use of donor−acceptor alternating copolymer-type compatibilizers (DACCs) into high-performance SMA-based OSCs, enhancing their PCE, thermal stability, and mechanical robustness simultaneously. The addition of DACCs to polymer donor (P_D)–SMA blends effectively reduces P_D–SMA interfacial tensions and stabilizes the interfaces, preventing the coalescence of the phase-separated domains. As a result, desired morphologies with high thermal stability and mechanical robustness are obtained for the P_D–SMA blends. The addition of 20 wt% DACCs_­ affords OSCs with a PCE of 17.1% and a cohesive fracture energy (G_c) of 0.89 J m^–2, higher than those (PCE= 13.6% and G_c= 0.35 J m^–2) for the control OSCs without DACCs. Moreover, at an elevated temperature of 120 °C, the OSCs with 20 wt% DACC exhibit excellent thermal morphological stability, retaining over 95% of the initial PCE after 300 hr.   

Visualizing Complex Multi-Length-Scale Morphology in Non-Fullerene Photovoltaics with Nitrogen K-Edge Resonant Soft X-Ray Scattering

Multi-length-scale morphology in organic photovoltaics (OPVs) and other functional soft materials leads to high complexity, but often dictates functionality. Such mesoscale complexity in OPVs originates from the kinetically trapped nonequilibrium morphology, which determines the device charge generation and transport. Resonant soft x-ray scattering (RSoXS) has been revolutionary on the exploration of OPV morphology in the past decade. However, for non-fullerene OPVs, RSoXS analysis near the carbon K-edge (CK-RSoXS) has been difficult, due to the chemical similarity of materials used in active layers. An innovative approach is provided by nitrogen K-edge RSoXS (NK-RSoXS), utilizing the spatial and orientational contrasts from the cyano groups in the acceptor material, which allows for determination of the phase separation. Of particular importance is that NK-RSoXS, for the first time, clearly visualizes the combined feature sizes in PM6:Y6 blends from crystallization and liquid-liquid demixing, determining optoelectronic properties. NK-RSoXS also reveals that PM6:Y6:Y6-BO ternary blends with reduced phase separation size and enhanced material crystallization can lead to current amplification in devices.   

Beyond conformational control: effects of noncovalent interactions on molecular electronic properties of conjugated polymers

Tuning the electronic properties of polymers is of great importance in designing highly efficient organic solar cells. Noncovalent intramolecular interactions have been often used as conformational control to enhance the planarity of polymers or molecules and tune the electronic properties. However, it is little known if noncovalent interactions may further alter the electronic properties of polymers with high planarity through some mechanism other than the conformational control. Here, we studied the effects of various noncovalent interactions, including S-N, S-O, S-F, O-N, O-F, and N-F, on the electronic properties of polymers with planar geometry using unconstrained and constrained density functional theory. We found that the sulfur-nitrogen intramolecular interaction may reduce the band gaps of coplanar polymers and enhance the charge transfer more obviously than other noncovalent interactions. For the first time, our study shows that the sulfur-nitrogen noncovalent interaction may further affect the electronic structure of coplanar conjugated polymers, which cannot be only explained by the enhancement of molecular planarity. Our work suggests a new mechanism to manipulate the electronic properties of polymers to design high-performance small-molecule-polymer and all-polymer solar cells.