Session L04: Organic Electronics II

Live

In an organic photovoltaic (OPV) cell, excitons dissociate in the donor-acceptor interfacial region. Brute-force electronic calculations in this region are prohibitively expensive. Here, we provide a tractable method to evaluate electronic couplings between adjacent donor and acceptor moieties, with realistic local geometries from atomistic molecular dynamics simulations of mixed donor oligomer and acceptor molecules. In these simulations, we exploit aromatic moieties' stiffness by using virtual sites for many atoms, significantly reducing degrees of freedom while retaining the shapes and dipole moments of moieties. Realistic local geometries of nearby donor-acceptor pairs serve as input to electronic structure calculations. As an example, we have examined the Poly(3-hexylthiophene) P3HT (donor) and O-IDTBR (acceptor) blend. We efficiently obtain the donor-acceptor hopping matrix element for nearly 10,000 nearby pairs of moieties from the splitting between HOMO and HOMO-1 for each pair. In this way, we obtain the histogram of interchain couplings, which exhibit a long tail of rare but strong couplings above 100 meV. By referring to the input geometries, we can identify criteria for strongly coupled pairs.   

Live

Control of exciton energy by externally applied electric fields could lead to dynamically tunable color (allochroism) and photophysics in organic semiconductor devices, such as organic light-emitting devices. Unfortunately, few reports have shown dramatic electric-field induced modulation of exciton energy in organic semiconductors, largely confined to electro-absorption. Here we employ excited states formed across electron donor-acceptor heterojunctions, called charge-transfer (CT) states, as a field-responsive electro-optical medium. CT states exhibiting thermally activated delayed fluorescence provide highly luminescent states that enable optical measurement of excited state energy. We employ biased, non-injection-type devices to show a reversible and linearly field-dependent energy shift of CT state photoluminescence. Further we relate our findings to the more general phenomenon of the Stark effect observed in electro-absorption and electro-emission across other excitonic material systems. Finally, we note the relationship between our findings in photoluminescence to observations and opportunities in organic light-emitting devices exhibiting electroluminescence.   

Live

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during the formation and processing of thin films. During solution deposition solvent vaporizes, forcing molecules and polymers to aggregate and become electronically coupled. Thermal annealing may further change the conformation of polymers and relative intermolecular arrangement of small molecules. This can dramatically change the exciton dynamics and thus the suitability of the material for electronic devices. The exciton dynamics of organic molecules and polymers can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation and processing is unknown since measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra while the systems are changing. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. In this work, a single-shot transient absorption spectroscopy technique is used that can measure transient spectra with up to a 60 ps pump-probe time delay in mere seconds. The exciton dynamics of an organic semiconducting system is revealed during the formation of an organic film. These measurements provide a new way to understand the mechanism of organic film formation and the impact of these complex processes on excited state dynamics.   

Live

Heterojunctions between organic semiconductors are central to the operation of many optoelectronic devices. Here the charge transfer (CT) exciton proves to be a crucial component, fundamental to the operation of organic solar cells. However, the impact on CT excitons of dynamic disorder and permanent dipoles – both common features in organic materials - still lacks a rigorous understanding. We have grown perylene-tetrabromophthalic anhydride (TBPA) - a donor-acceptor cocrystal - to study the effect of disorder on the CT exciton. Perylene-TBPA exhibits dipolar disorder at room temperature which ceases below a phase transition at 250 K, thus providing an ideal system to study the effects of disorder without added mesoscale morphological influences. The CT exciton was studied in both the ordered and disordered phase using optical spectroscopy. We find that the absorption band consists of three separate CT exciton states in the disordered phase – a novel finding in donor-acceptor co-crystals. We use model calculations to attribute this splitting of the CT exciton to nearest-neighbour dipoles flipped from their lowest energy configuration. This work thus highlights the important role of dipole motion and dipole-dipole interactions in the energetics of CT excitons in organic materials.   

Live

Utilization of singlet fission (SF) in organic photovoltaics requires detailed knowledge of the triplet-triplet (TT) spin biexciton. We report many-body calculations of excited states, including TT, in 2 hypothetical phenylene-linked anthracene dimers. These systems are candidates for intra-molecular SF (iSF) as opposed to inter-molecular SF (xSF), but our results should apply to both types of systems. Although the molecules in question do not exist in nature, the results prove valuable due to the high accuracy of the calculations given our ability to retain a large active space of molecular orbitals, and they translate to larger systems. We obtain the unexpected result that slower TT formation may ultimately lead to greater yield of free triplets than systems in which TT formation is truly ultrafast. This finding is consistent with previous theoretical and experimental results obtained for real dimers that exhibit iSF. Finally, we present future avenues for identifying and characterizing systems that may be strong candidates for singlet fission.   

Live

The dependence of electron mobility of a n-type semiconducting polymer on polymer chain length is investigated and thereby factors limiting efficient electron transport are pinpointed. Both field-effect and space-charge limited current electron mobilities are found to reach maxima at a critical degree of polymerization (DP_c). This trend is shown to be similar for other n-type semiconducting polymers, suggesting a nearly universal dependence of electron mobility on polymer chain length. The underlying physics are deconvoluted: the bottleneck for DP<DP_c is intercrystallite transport due to insufficient domain connectivity whereas the limiting factor for DP>DP_c is intracrystallite transport due to intrachain charge localization, poor interchain hopping rate, and intracrystallite disorder. Our results argued that the basis to achieve efficient multiscale electron transport in n-type semiconducting polymers lie in the control of polymer chain length to concurrently optimize intracrystallite and intercrystallite charge transport.   

Live

Thin film polymer dielectrics with controllable electronic properties are of considerable interest for applications in organo-electronic systems. We use aryl-amines in thin film polystyrene dielectrics to modify electron and hole trap states and densities. When introduced as free additives in blends as well as when chemically bound to polymers, these moieties induce changes in static potential differences that modify the behavior of adjacent electronic materials. The aryl-amines contribute to the charge storage capacity of the system and lower operational threshold voltages, on the order of thirty volts ΔV_th =O(30V), for devices after the application of static charging fields as measured in pentacene-based organic field effect transistors. Films with free additives showed a strongly concentration dependent stability in ΔV_th and this behavior was corroborated by polymers with tethered amines. Stability is higher at lower amine-concentrations suggesting that inter-amine distance is an important effect in these systems.   

Live

Organic thin film transistors (TFTs) can be made with materials that allow them to be mechanically stretched during electrical operation. Circuits made with stretchable elements have uses ranging from bioelectronics to strain sensors. In practice, it is critical to understand circuit behavior in a variety of deformation modes. We describe the application of mechanical models of the elasticity of polymers to predict the electrical characteristics of elastic TFTs. The model predicts the current-voltage behavior of TFTs under uniaxial and biaxial deformation assuming stretchable elements for contacts, dielectrics, and the semiconducting layer. The behavior of complementary inverters using elastic TFTs is presented along with criteria for stable operation as digital circuit elements. The mechanical model was also applied to organic electrochemical transistors (OECTs). The behavior of elastic OECTs differs substantially from TFTs and the model predicts that they can provide benefits for the stability of simple digital circuits.   

Live

Recently, organic electrochemical transistors (OECTs) have received lots of attention for variety of applications, particularly in the areas of bioelectronics, due to attractive characteristics, such as low voltage operation, flexibility, high transconductance and biocompatibility. [1–3] Our present study is motivated by ionic electroactive polymers (iEAPs) [4,5] and the new class of material-ionic liquid crystal elastomers (iLCEs) [6]. Here, we report a novel flexible OECT based on iLCE solid electrolyte. More importantly, this transistor does not require a substrate as traditional transistor does. The effect of molecular alignment (planar, homeotropic, hybrid and isotropic) of the solid electrolyte (iLCE) on the steady-state (transfer and output) and transient behavior of the transistor is studied.

[1] J. Rivnay et al., Nat. Rev. Mater. 3, (2018).
[2] V. Kaphle et al., Nat. Commun. 11, (2020).
[3] P. R. Paudel et al., Adv. Funct. Mater. 11, 2004939 (2020).
[4] C. P. H. Rajapaksha et al., Macromol. Rapid Commun. 41, 1900636 (2020).
[5] C. R. Piedrahita et al., ACS Appl. Mater. Interfaces 12, 16978 (2020).
[6] C. Feng et al., Macromol. Rapid Commun. 40, 1900299 (2019).   

Live

Wearable strain sensors are essential for the realization of applications in the broad fields of remote healthcare monitoring, soft robots, immersive gaming, among many others. These flexible sensors should be comfortably adhered to skin and capable of monitoring human motions with high accuracy, as well as exhibiting excellent durability. However, it is challenging to develop electronic materials that possess the properties of skin. The presented skin-like electronic material exhibits ultrahigh stretchability, repeatable autonomous self-healing ability, quadratic response to strain, and linear response to flexion bending. This conductive organic system, under ambient conditions, synergistically constructs a regenerative dynamic polymer complex crosslinked by hydrogen bonds and electrostatic interactions, which enables these unique properties. Sensitive strain-responsive mechanisms owing to the homogenous and viscoelastic nature provide omnidirectional tensile strain and bending deformations. Furthermore, this material is scalable and simple to process in an environmentally-friendly manner, paving the way for the next generation wearable sensors.   

Live

The magnet-field modulated response of electronic devices gives us insight into the spin dynamics in materials. In organic semiconductors, the dynamics of spontaneous singlet fission/triplet fusion can be observed using the magnetic field modulated electro-luminescence (MEL) or photocurrent (MPC). In this talk, we present results which show singlet fission and triplet fusion processes though the observation of MEL and MPC signals from organic light emitting diodes and organic transistors. For the semiconductor diF-TES-ADT (2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene) we observe that a polycrystalline film in an OLED produces an MEL signal which is similar to the angle-averaged MPC of a single crystal transistor. We use a simple model to calculate response and find that signal from both devices is directly related to the overlap in spin character of the triplet and singlet states that drives both the singlet fission in the MPC response and triplet fusion in the MEL response. The MEL/MPC response between 0 mT and 200 mT is primarily dependent on the zero-field splitting (magnetic dipole) of the triplet excitons.   

Live

Coherent spin processes like singlet-fission (SF) and triplet fusion (TF) are gaining in interest to the scientific community, as SF could lead to an increase in performance of organic photovoltaics and TF in the emission efficiency of OLED devices. Recently reported results have suggested that higher order effects such as TF lead to a reduction of the luminescence turn-on voltage and is necessary for what was referred to as “sub-band gap turn-on”. However, we have shown that heterojunction band alignment can describe the low voltage turn-on. In this presentation we will present electro-reflection (ER) studies that elucidate the built-in potentials in 2 prototypical OLED devices based on Rubrene and DiFTES-ADT as the emitting layer. The results are put into context of the low luminescence turn-on, which is observed in both cases, and potential higher order effects. Furthermore, differences in the luminescence - current density-voltage-characteristics, L-J-V and magneto-electro-luminescence, MEL, between the two systems will be presented. A kinetic model captures zero field splitting and various exciton and charge carrier recombination pathways.   

Live

The Organic Electrochemical Transistor (OECT) is a key device for the bioelectronic field as it operates in a very low voltage range and transduces ionic into electronic signals. The transconductance of OECTs, which is a parameter that represents the sensitivity of transistors, is very high compared to silicon-based transistors, and hence facilitates effective transduction of ionic signals into electronic signals essential for efficient bio-sensing. However, a convincing model describing its working mechanism is still lacking as many observations are not well explained. We discuss the experimental results with the help of simulation results using 2D model [1] correctly accounting for lateral drift and diffusion of ions inside polymer channel, resulting in a better understanding of the working mechanisms of OECTs in general and more specifically a better understanding of the scaling laws of this new technology [2].

References
[1] V. Kaphle, P. R. Paudel, D. Dahal, R. K. Radha Krishnan, and B. Lüssem, Nat. Commun. 11, 1 (2020).
[2] P. R. Paudel, V. Kaphle, D. Dahal, R. K. Radha Krishnan, and B. Lüssem, Adv. Funct. Mater. 11, 2004939 (2020).