Symposium F.EL07—Coulomb Interactions in Functional Organic Materials and Devices—A Curse or a Blessing?

We study the electronic properties of dopants in organic systems on the basis of state-of-the-art many-body GW and Bethe-Salpeter formalisms accounting for proper electrostatic and dielectric environments. [1,2] We show in particular that the absolute energy position of doping levels and band edges of organic semiconductors can be obtained with accuracy both in the bulk and at the surface. Our calculations reveal in particular that the energy levels of a molecular impurity strongly depend on the host environment as a result not only of the proper environment dielectric properties, but crucially as well on electrostatic intermolecular interactions. [3] Concerning doping mechanisms, we demonstrate that ionisation of the dopants can occur even in situations of very deep acceptor (donor) levels thanks to strong electron-hole Coulomb interactions stabilizing charge-transfer states. [4] The increase of conductivity requires from then on to understand the charge separation of these bound electron-hole pairs in order to generate free carriers. An initial blessing, Coulomb interactions turn into a curse that needs to be overcome.

[1] J. Li, G. D'Avino, I. Duchemin, D. Beljonne, X. Blase, J. Phys. Chem. Lett. 2016, 7, 2814.
[2] J. Li, G. D'Avino, I. Duchemin, D. Beljonne, X. Blase, Phys. Rev. B 2018, 97, 035108.
[3] J. Li, I. Duchemin, O. M. Roscioni, P. Friederich et al., Materials Horizons 2019, 6, 107-114.
[4] J. Li, G. D'Avino, A. Pershin, D. Jacquemin, I. Duchemin, D. Beljonne, X. Blase, Phys. Rev. Materials 2017, 1, 025602.

Molecular doping is of central technological relevance for organic (opto-) electronics since it allows enhancement and precise control of electrical properties of organic semiconductors. Nevertheless, it is currently challenging to efficiently p-dope host materials with ionization energies (IEs) higher than 5.5 eV.

Here, we demonstrate the unique potential of an organic salt consisting of a borinium cation (Mes₂B⁺; Mes - mesitylene) and the tetrakis(penta-fluorophenyl)borate anion [B(C₆F₅)₄]^– as p-type dopant for high ionization energy polymer semiconductors, i.e., methylated poly(para-phenylene) (MeLPPP, IE≈5.4 eV) and the donor-acceptor polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT, IE≈5.9 eV). Optical absorption spectroscopy, ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and conductivity measurements were performed in order to verify successful p-type doping. This range of experimental methods allows to identify the doping mechanism, namely electron transfer from the polymer to Mes₂B⁺ combined with stabilization of the positive charge on the polymer chain by [B(C₆F₅)₄]^–. From doping studies with further polymer semiconductors, we estimate the EA of Mes₂B⁺[B(C₆F₅)₄]^– to amount to an impressive 5.9 eV. Especially, our studies on Mes₂B⁺[B(C₆F₅)₄]^–-doped poly(3-hexylthiophene) (P3HT) demonstrate the capability of the organic salt to form bipolarons with a high proportion beyond ca. 10% dopant concentration [1]. Thus, the parameter space for doping of polymer semiconductors is significantly extended.

[1] B. Wegner, D. Lungwitz, A. E. Mansour, C. E. Tait, N. Tanaka, T. Zhai, S. Duhm, M. Forster, J. Behrends, Y. Shoji, A. Opitz, U. Scherf, E. J. W. List-Kratochvil, T. Fukushima, N. Koch, Adv. Sci. 2020, 7, 2001322.

High electrical conductivity is a prerequisite for advancing the performance of organic semiconductors for various applications and can be achieved through molecular doping. However, it is often observed that the conductivity is enhanced only up to a certain optimum doping concentration, beyond which it decreases significantly. This is typically attributed to morphological changes upon increasing dopant loading.

Here we address and elucidate the effect of morphology, dopant-carrier interactions, and carrier-carrier interactions on the conductivity of doped organic semiconductors. We first show that the conductivity increases, maximizes and then decreases with increasing doping density for a molecularly doped fullerene derivative. We use grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements to investigate the microstructural changes upon doping and find that even though the conductivity decreases at high doping density, the microstructure does not deteriorate. In other words, the loss of electrical conductivity in this system cannot be explained by changes to the microstructure.
We combine analytical work and Monte-Carlo simulations to demonstrate that repulsive Coulomb interactions between charge carriers can cause this decrease in conductivity. We use Efros-Shklovskii theory on carrier-carrier interactions to predict the carrier density at which the maximum of conductivity occurs.[1] Next, we show kinetic Monte-Carlo simulations exhibiting a maximum in conductivity at a charge carrier density that corresponds to that observed experimentally. The maximum is observed both in the presence and absence of dopants. Overall, we show that carrier-carrier interactions lower the conductivity by orders of magnitude at high doping densities and therefore propose the suppression of carrier-carrier interactions as a key strategy in order to create higher conductivity organic semiconductors.

[1] A. L. Efros, and B.I. Shklovskii, Coulomb gap and low temperature conductivity of disordered systems, J. Phys. C Solid State Phys. 8, 1 (1975).

Open-shell organic molecules, like free radicals and radical-cations, exhibit magnetic and optical properties due to the presence of an unpaired electron in a Single Occupied Molecular Orbital (SOMO) with an accessible Single Unoccupied Molecular Orbital (SUMO). This characteristics together with their low spin-orbit couplings and weak hyperfine interactions make them good candidates for molecular spintronics insofar the radical character is preserved in solid state and the molecules are properly arranged on a solid state device. To achieve these properties we used functionalized substituted polychlorinated triphenylmethyl (PTM) radicals attached, as self-assembled monolayers (SAMs), on metallic surfaces, like Au, Ag, Cu, or graphene, exploring the transport properties through such spin-containing organic hybrid systems. [1] In all studied metal-organic systems the conductance through the radical molecules is enhanced two orders of magnitude with respect the corresponding non-radical ones, a result that can be explained by a mediated electron transport through the SUMO orbitals in the open-shell systems. [2-3] These results paves the way towards the use of all-organic neutral radical molecules in spintronics.
In this contribution the use of such radical molecules to attain metallic surfaces with rectifying properties [4,5] as well as to manipulate the electronic properties of metallic surfaces, like the work function, will be presented. Remarkable is the possibility to switch reversibly the work function of the metallic surfaces using an external input like NIR light. [6]
Also molecular junctions based on self-assembled monolayers with a strong donor molecule, like benzotetrathiafulvalene (BTTF), linked to Au, Ag, and Pt surfaces have been prepared and fully characterized exhibiting rectifying properties. Interestingly these solid-state molecular tunnel junctions show a switching of the direction of rectification by controlling the direction of charge transfer between the BTTF redox unit and the bottom-electrode that depend on the nature of the metallic surfaces. [7]



References
[1] L. Yuan, C. Franco, N. Crivillers, M. Mas-Torrent, L. Cao, C. S. Suchand Sangeeth, C. Rovira, J. Veciana, C. A. Nijhuis, Nature Comm., 7, 12066 (2016).
[2] M. Souto, L. Yuan, D. C. Morales, L. Jiang, I. Ratera, C. A. Nijhuis, J. Veciana. J. Am. Chem. Soc., 139, 4262, (2017).
[3] F. Bejarano I.J. Olavarria-Contreras, A. Droghetti, I. Rungger, A. Rudnev, D. Gutierrez, M. Mas Torrent, J. Veciana, H.S. J. van der Zant, C. Rovira, E. Burzuri, N. Crivillers, J. Am. Chem. Soc. 140, 1691 (2018).
[4] M. Souto, V. Díez-Cabanes, L. Yuan, A. R. Kyvik, I. Ratera, C. A. Nijhuis, J. Cornil, J. Veciana, Phys.Chem.Chem.Phys.,20, 25638 (2018).
[5] V. Diez-Cabanes, D.C. Morales, M. Souto, M. Paradinas, F. Delchiaro, A. Painelli, C. Ocal, D. Cornil, J. Cornil, J. Veciana, I. Ratera, Adv. Mater. Technol. 2018, 1800152 (2018).
[6] V. Diez-Cabanes, A. Gómez, M. Souto, N. González-Pato, J. Cornil, J. Veciana, I. Ratera, J. Mater. Chem. C, 7, 7418-7424 (2019).
[7] Y. Han, M.S. Maglione, V. Diez Cabanes, J. Casado-Montenegro, X. Yu, S.K. Karuppannan, Z. Zhang, N. Crivillers, M. Mas-Torrent, C. Rovira, J. Cornil,, J. Veciana, Ch.A. Nijhuis, submitted (2020).

Molecular n-dopants can play useful roles in controlling conductivity and charge injection/extraction in organic semiconductors, in lowering the work functions (WFs) of electrode materials, and in controlling the properties of low-dimensional materials such as graphene. However, even molecules that can n-dope C₆₀, which has a fairly high electron affinity of ca. 4.0 eV, through simple one-electron transfer are inevitably sensitive to air and water, while stronger one-electron reductants are even more sensitive. Coupling bond breaking and/or formation to electron transfer provides a way of circumventing this issue. One such approach is based on the dimers, D₂, formed by certain odd-electron species, D, formed on alkali-metal reduction of the corresponding cations, D⁺. Another class of relatively stable n-dopant molecules, DH, can be formally regarded as hydride-reduced derivatives of D⁺ cations. We have synthesized a range of both organic and organometallic D₂ and DH derivatives, in particular, derivatives of 2-substituted-1,3-dimethylbenzimidazolium cations (Y-DMBI⁺) , and of 18-electron cationic sandwich compounds such as 1,2,3,4,5-pentamethylrhodocenium and ruthenium pentamethylcyclopentadienyl mesitylene cations. This talk will describe the diversity of structures and reactivity that we have obtained, and will compare the advantages and disadvantages of the organic and organometallic dopants, and of the D₂ and DH approaches, from the point of view of synthesis, stability, dopant strength, kinetics, dopant ion size and shape, and the extent to which these properties can be tuned.

Doping is a key technology in both organic and inorganic electronics, allowing to control the type and the magnitude of the conductivity over many orders of magnitude. In organic electronics, it is primarily used to reduce injection barriers and voltage drop on the electron/hole transporting layers of organic light-emitting diodes (OLEDs). Doped organic semiconductors, comprising a lot of charged molecules, represent an ideal laboratory for both theoretical and experimental studies on strong Coulomb interactions.
In this talk, I will present several effects, which appear in the doped organic semiconductors due to the cooperative effect of strong Coulomb interactions and energetic disorder: appearance of doping-induced gap states, disorder-driven doping activation [Fediai et al, PCCP, 22, 10256-10264 (2020)] and the superlinear increase of electric conductivity due to the disorder-compensation effect [Fediai et al. Nature Comm. 10, 4547 (2019)]. We predict and consistently explain these experimentally observed features using a kinetic Monte Carlo (kMC) method. In this method, electrons and their interactions are treated individually, so that dynamic Coulomb correlations are taken into account. Moreover, the movement of charge carriers is simulated in the characteristic disordered energy landscape, allowing to study cooperative effects, such as the correlation of the doping-induced and intrinsic material disorder. Remarkably, taking into account only a model morphology and empirical hopping rates, this method qualitatively predicts the vast majority of the experimentally observed doping effects, including the temperature and doping rate dependence of the Fermi level shift, conductivity and ionized dopant rates. It also predicts passivation of the trap states in the ultralow doping regime and other previously reported experimental findings.
Second, I will present a multiscale method that allows to establish a direct link between the type of dopant and host molecules and the properties of the constituent doped material. In this method, the kMC method acts as the last component of the multi-scale approach, in which the morphology of doped materials is generated using the Deposit method (Monte-Carlo protocol that mimics the vacuum deposition [J. Comput. Chem. 2013, 34, 2716-2725]) and the Coulomb interaction in host-dopant charge-transfer complexes are computed using ab-initio methods, based on the previously reported Quantum Patch method [J. Chem. Theory Comput. 2014, 10, 9, 3720-3725]. The latter has been extended here to also treat charge-transfer complexes embedded into morphology. As a use case, we have computed the density of states and the ionized dopant fraction of the prototypical amorphous doped organic material α-NPD:F₄TCNQ. Due to well-balanced approximations, our multiscale approach allows to calculate the Coulomb interaction energies of several tens of host-dopant pairs and obtain the distribution of the charge-transfer states energies, which is of great practical importance for amorphous organic semiconductors, with every host and dopant molecule having a unique conformation and electrostatic environment. In the future these methods offer the opportunities to design materials and devices for optimal performance.

Controlled doping of semiconductors is crucial for the functionality and efficiency of modern (opto-) electronic devices. For organic semiconductors, the use of strong molecular electron donors and acceptors is meanwhile established as viable approach for doping. However, diffusion and poor thermal stability of most presently used dopants poses a challenge in applications. It turns out that the salt of the mesitylene-borinium cation (Mes₂B⁺) and tetrakis(penta-fluorophenyl)borate anion [B(C₆F₅)₄]^– is a superior p-type dopant for polymer semiconductors, and doping occurs via a charge-exchange reaction. Remarkably, the [B(C₆F₅)₄]^– anion enables the stabilization of polarons and bipolarons in poly(3-hexylthiophene), and the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]^– is estimated to be 5.9 eV. The comparably high molecular weight and bulkiness of [B(C₆F₅)₄]^– is expected to reduce diffusion, and no negative impact on doped polymer conductivity is observed up to 100 °C.
Notably, many of the molecular dopants used for organic semiconductors can also be employed for doping of transition metal dichalcogenides (TMDCs). When thinned to the monolayer, many are direct semiconductors, e.g., MoS₂ and WS₂, with superior optical properties. Due to the pronounced excitonic nature of TMDCs, their exciton energy and band gap depend strongly on the dielectric constant (ε) of the mechanical support used for the monolayer. Moreover, the electrical nature of the substrate plays a critical role for the mechanism by which doping with molecules occurs. Three fundamentally different charge transfer mechanisms between MoS₂ and molecular dopants have been identified and will be discussed.
Finally, the impact of residual water in conductive polymer films on their work function will receive attention. The high ε of water reduces the polymer's work function by screening of local charge transfer induced dipoles, and the work function varies with temperature as does ε of water. This effect should thus be considered when device characterization includes temperature variation.

Molecular doping of conjugated polymers generates polarons and counterions that can act as charge carriers and Coulomb scattering sites. Knowledge about the charge-carrier density is needed to rationalize the electrical properties that are observed for different degrees of doping. This talk with explore how the charge-carrier density can be estimated through a comparison of the UV-vis-IR absorbance spectra of molecularly doped polymers and electrochemically oxidized material, using a combination of spectroelectrochemistry and chronoamperometry. The interplay of charge-carrier density and electrical conductivity will be discussed for a range of doped polymers. P-doping of polymers with ionization energies ranging from about 4.5 eV to 5.5 eV will be evaluated, using a series of dopants with electron affinities up to 5.8 eV in case of the strong oxidant Magic Blue.

In organic device applications, contact doping has been commonly used method for circumventing a high contact resistance between metal electrodes and organic semiconductors prevents an efficient charge injection and extraction, which fundamentally limits the device performance. We have recently employed an efficient doping method based on solid-state diffusion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) [1] to selectively bulk-dope contact regions of PBTTT organic field effect transistors (OFETs), which significantly enhanced the charge injection properties [2]. However, adopting such extensive contact doping method compromises the device stability of OFETs due to dopant diffusion problem, which significantly degrades the device stability by damaging the ON/OFF switching performance despite a . In this presentation, we show a significantly improved stability of the contact doping method by incorporating “dopant-blockade molecules” in PBTTT film in order to suppress the diffusion of the dopant molecules. By carefully selecting the dopant-blockade molecules for effectively blocking the dopant diffusion paths, the ON/OFF ratio of PBTTT OFETs can be maintained over 2 months. This result will maximize the potential of contact doping in OFETs by providing a promising route toward achieving stable and low-contact-resistance OFETs.

References:
[1] K. Kang, S. Watanabe, K. Broch, A. Sepe, A. Brown, I. Nasrallah1, M. Nikolka, Z. Fei, M. Heeney, D. Matsumoto, K. Marumoto, H. Tanaka, S. Kuroda and H. Sirringhaus, Nat. Mater. 15, 896 (2016).
[2] Y. Kim, S. Chung, K. Cho, D. Harkin, W.-T. Hwang, D. Yoo, J.-K. Kim, W. Lee, Y. Song, H. Ahn, Y. Hong, H. Sirringhaus, K. Kang and T. Lee, Adv. Mater. 31, <a href="tel:10 1806697">10 1806697</a> (2019).
[3] Y. Kim, K. Broch, W. Lee, H. Ahn, J. Lee, D. Yoo, J. Kim, S. Chung, H. Sirringhaus, K. Kang and T. Lee, Adv. Funct. Mater. 2000058 (2020)

Organic semiconductors (OSC) hold tremendous potential to address the demand for cheap and sustainable thermoelectric (TE) materials. They are cost-efficient, environmentally friendly, and have a low lattice thermal conductivity well below their inorganic counterparts. Their lightweight and flexibility also make them a promising choice for self-powered wearable medical monitors and sensing electronics.
Polymers are intrinsically insulators and need to be electrochemically doped to induce high levels of carrier density to improve their electrical conductivity. This simple process, however, introduces complexities in the electronic structure because of the charge-charge Coulombic interactions between the dopant and the polymer. Poor screening of the dopant-polymer interactions increases the energetic disorder, alters the width and the shape of the density of states (DOS) [1-2], and suppresses the density of free charge carriers. In this work, we investigate the effect of the dopant-polymer Coulombic interactions on the DOS and subsequently on the charge transport dynamics and the trend of the Seebeck vs. conductivity trade-off curve. Our charge transport model is based on electron hopping between localized sites with a modified Gaussian disorder model to account for the impact of dopant size and density, and the clustering of dopants on the energies. We iteratively solve the non-linear Pauli’s master equation to compute the time-averaged occupational probabilities of the sites from which relevant transport quantities are calculated [3-4].
We find that the energetic disorder caused by doping which results in an increase in the width of the DOS distribution closely follows the Coulomb interaction energy with the nearest dopant. Also, the deep columbic traps cause an additional tail in the otherwise Gaussian DOS which is further enhanced with dopant clustering [1]. The spread in site energies due to a broadened DOS causes a larger difference in energies between neighboring localized sites which reduces the carrier hopping rate and thus the electron mobility [2]. We find that at low doping the carrier mobility first decreases with doping, due to the trap states in the tail of the DOS being filled first, followed by rapidly increasing at high doping levels. Furthermore, our results show that the effect of dopant-polymer Coulombic interactions on the width and tail of the DOS dictate the slope of the Seebeck vs. conductivity plots. We conclude that reducing the dopant-carrier Coulomb interactions and minimizing dopant clustering can have a beneficial impact on the conductivity and the thermoelectric power factor of OSCs.

REFERENCES: [1] V. I. Arkhipov, P. Heremans, E. V. Emelianova, and H. Bässler, Phys. Rev. B 71, 045214 (2005). [2] G. Zuo, H. Abdalla, and M. Kemerink, Phys. Rev. B 93, 235203 (2016). [3] C. J. Boyle, M. Upadhyaya, P. Wang, L. Renna, M. Lu-Díaz, S. P. Jeong, N. Hight-Huf, Lj. Korugic-Karasz, M. Barnes, Z. Aksamija, and D. Venkataraman, Nat. Comm. 10, 2827 (2019). [4] M. Upadhyaya, C. J. Boyle, D. Venkataraman, and Z. Aksamija, Scientific Reports 9, 5820 (2019).

Sensing of quantum mechanical current in DNA is important to the fields of molecular devices, disease detection and next generation of sequencing techniques. Molecular recognition and self-assembly properties exhibited by DNA offer a bottom-up fabrication process as opposed to silicon-based electronics which requires a top-down approach. These properties along with the possibility for long-range charge transport makes DNA an attractive material for nanoelectronics. Further, the self-assembly property is exploited to form complex 3D structures known as DNA Origami. This enables us to potentially create a new class of molecular electronic devices that occupy cross-wired 3D configurations. In addition, the ability to sense changes in current due to modifications in the quantum mechanical interaction between bases as a function of sequence raises the possibility for its use in next generation sequencing techniques.

Quantum tunnel current which flows both along and perpendicular to the helical axis are essential to describing DNA-based materials. For molecular electronics applications involving double strand DNA (ds-DNA), current flow along the helical axis is important. However, for sequencing applications and origami based nanostructures, tunneling both along and perpendicular to the helical axis are crucial.

The fundamental questions in this field relate to the sensitivity of the quantum tunneling rate through a base to (1) changing a single neighboring base as a function of distance and (2) the DNA-substrate or DNA-contact interface. DNA maintains its structure at room temperature in a solvent, so to begin answering these questions it is essential to understand the role of decoherence in altering the coherent quantum tunneling process at room temperature.

We model the effect of decoherence in studying the effect of contact-location and interstrand coupling in a system consisting of a ds-DNA lying on a substrate. Our model allows us to modulate the strength of DNA-substrate binding to simulate DNA lying on a gold surface, which also forms the first electrical contact. The second electrical contact is a gold tip that is perpendicular to the substrate. An example of a simulated sequence is a 15 base pair long, 3’-GGGAAAAAAAAAGGGG-5’ double strand. This sequence allows us to probe the effect of the tunnelling current when the tip makes contact to a single single base while its nearest neighbors gradually change. For example, we calculate the tunnelling current through a tip making contact to an adenine either at distant or close proximity to a guanine.

Our results show that the effect of the contact location on charge transport is dependent on the energy and spatial distribution of the HOMO band between two contacts via multiple trajectories through the DNA. Resonant tunneling peaks when a contact is made to an adenine can increase by ten times depending on the nearest neighbor bases. Moreover, a sufficient coupling between adenine and the neighboring guanine is required to maintain the large resonant tunneling peak. Therefore, by moving the tip to an adenine that is only weakly coupled to a guanine, the resonant tunneling peak almost disappears. We also find that the tunneling current decays more significantly as a function of the neighboring bases in the coherent regime. Our results clearly establish the sensitive nature of tunnelling current to the nearest neighbor bases even in a direction perpendicular to the helical axis. We find that the delocalization of the frontier molecular orbitals responsible for charge transport are significantly affected by the nearest neighbors. To summarize, our results demonstrate the dependence of tunneling current on the local environment which is of relevance to DNA sequencing. We also expect these results to help design DNA nanowires with tolerance to the location of the cross-linking between multiple structures while maintaining the desired conductance value.

Designing doped π-conjugated polymers (CPs) for organic thermoelectric remains a challenging pursuit, in part because of the difficulty in understanding the complete details of charge transport. For example, it is typically assumed that charge-carrier transport in CPs is dominated by one type of charge carrier, either holes or electrons, as determined by whether the dopant reduces or oxidizes the polymer. However, we find that this is not always a correct assumption, as several polymers that are heavily doped with dopants that oxidize the polymer (i.e., p-type dopants) show n-type charge transport, which is supported by negative Seebeck coefficients and negative (n-type) Hall voltages. Here, we show that moderate p-doping of a DPP-containing polymer with FeCl₃ leads to a p-type power factor of 24.5 µW m^-1 K^-2, while further increases in doping concentration lead to an n-type power factor of 9.2 µW m^-1 K^-2, where 9.2 µW m^-1 K^-2 is a record power factor for an n-type donor-acceptor (D-A) conjugated polymer. Notably, n-type behavior upon heavy doping with oxidizing dopants can be realized with multiple dopants (FeCl₃ and NOBF₄) and with all five of the D-A polymers examined. A mechanistic explanation for the origin of n-type charge-carrier transport is presented based on the Seebeck coefficient measurements, Hall effect measurements, density functional theory calculations, ultraviolet and inverse photoelectron spectroscopy (UPS and IPES, respectively), UV-Vis-NIR absorbance, and electron paramagnetic resonance (EPR) spectra. Ultimately, the combination of data points to the presence of hopping-type holes in the amorphous regions and delocalized electrons in the crystalline regions. With two carrier types present the Seebeck coefficient is determined by the energy-weighted contributions of each carrier type to the total electrical conductivity. This work provides fundamental insight to charge-carrier transport in highly doped polymers while presenting a new approach to developing high-performance n-type organic thermoelectrics.

The development of low-cost and flexible organic electronic and optoelectronic devices is enabled by solution-processed conducting polymer thin films. Residual water present in organic devices and their surrounding environment influences the properties of their polymer thin film components, which accelerates devices degradation and limits their lifetime and reproducibility. For example, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is a high work function conducting polymer, commonly used to achieve Ohmic contacts for holes with many semiconductors. However, it has been shown that residual water in PEDOT:PSS films lowers their work function [1, 2] and is detrimental for device lifetime.[3]
Here, we investigate the fundamental role of residual water in changing the work function of conducting polymers and confirm an earlier proposed model,[1] which involves the dielectric screening of PEDOT:PSS surface dipoles due to the high dielectric constant (ε) of water.
Photoemission spectroscopy and electrical conductivity measurements in the temperature range from -100 °C to 100 °C were conducted on thin films of different formulations of PEDOT, as well as, molecularly doped polymers. Our results reveal that the work function of PEDOT:PSS films containing residual water follows the same trend as a function of temperature as does ε of water, in the range between 25 °C and -100 °C. For instance, the work function at -100 °C decreases by ca. 270 meV from its value at room temperature and recovers its initial value once the thin film is warmed up to room temperature. After removal of residual water from PEDOT:PSS films by annealing in ultrahigh vacuum, the work function of thin films is much higher as before (reaching 6.1 eV) and, notably, independent of temperature. These findings are further supported by investigating a water-free variant of PEDOT, which shows no temperature dependence of the work function even without annealing. In contrast, we find no indication that the presence of residual water has any impact on the electrical conductivity. From a device perspective, the hole injection barrier at the interface with PEDOT:PSS can be lowered by increasing its work function once residual water is removed. However, from studies of PEDOT:PSS interfaces with the molecular semiconductor 2,2',7,7'-tetrakis(carbazol-9-yl)-9,9-spirobifluorene (s-CBP) we find that the water-removing annealing step should be done prior to molecular deposition, since water desorption is significantly reduced in the presence of s-CBP layer as thin as 5 nm. Furthermore, we find a similar correlation between thin film work function and temperature for the molecularly doped conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) films to that seen for PEDOT:PSS, which might thus be caused by a similar dielectric screening mechanism involving unanticipated residual water.

[1] N. Koch, A. Vollmer, A. Elschner, Appl. Phys. Lett. 2007, 90, 043512.
[2] E. S. Muckley, C. B. Jacobs, K. Vidal, J. P. Mahalik, R. Kumar, B. G. Sumpter, I. N. Ivanov, ACS Appl. Mater. Interfaces 2017, 9, 15880
[3] M. P. De Jong, L. J. Van Ijzendoorn, M. J. A. De Voigt, Appl. Phys. Lett. 2000, 77, 2255.

By mixing two molecular materials in thin films the electrical and optoelectronic properties of organic semiconductors can be tailored. Resulting morphologies include phase separation or mixed crystals, which can form either solid solutions or ordered complexes. At the moment it is difficult to predict a priori the resulting morphology for a selected material combination. Here, we study electron transfer between planar, rod-like electron donor molecules and a non-planar electron acceptor molecule in co-evaporated films by analyzing morphological, vibrational and optical properties. [1] For the donor under study here without ground-state electron transfer to the acceptor we find phase separation in the mixed film. If ground-state electron transfer is observed, the relation of the crystal binding energy of the single component materials and the Coulomb attraction between ions formed in the co-deposited film drives these films either into phase separation or mixed crystal formation. The resulting morphology of the co-deposited films can be rationalized within the laws of thermodynamics. Therefore, it is necessary to consider structural incompatibility of the molecules in terms of interaction energies between the molecules as well as the Coulomb attraction between molecular ions after the formation via ground-state electron transfer.
[1] A. Opitz et al. J. Phys. Chem. C 124 (2020) 11023–11031.

Organic semiconductors (OSCs) are gradually integrated into our lives as parts of various optoelectronic devices given their low-cost processing, light weight, and the opportunities that they offer for the versatile design of new materials with “on-demand” properties. These systems undergo considerable electronic and structural transformations during device fabrication and operation which can profoundly impact their performance and stability. Characterization techniques that can elucidate the mechanisms of the time-dependent transformations occurring in these materials and devices could guide the processing and design to yield high performance and stable organic devices. Here we introduce a highly efficient methodology to elucidate the microscopic processes occurring within the OSC when deliberately exposed to different external stimuli by accessing in real-time the trap density of states spectrum of the OSC using organic field-effect transistor (OFET) measurements. In the first example, we monitored the generation/annihilation of charge carrier traps in a high-mobility small molecule OSC, 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (diF TES ADT), as a result of microstructural changes occurring in the film during exposure to solvent vapors. The time-dependent changes in charge carrier mobility are correlated with the density and energetic distribution of the electronic states in the bandgap of the OSC providing unprecedented access to processes taking place in all intermediate states during the microstructural transformations. Given the strong dependence of the electrical properties on the film microstructure, such information is instrumental in identifying performance-limiting processes in devices and subsequently guiding material processing to achieve intrinsic limits. The discovery of defect-tolerant intermediate crystalline motifs can provide pathways for fabricating stable, high-performance devices for the next generation of low-cost electronics. We further employed this methodology to investigate the underlying physical process impacting the operational stability of OFETs based on a small molecule trimer, tri(n-hexyl) silylethynyl benzodithiophene (TnHS BDT) trimer. A large shift in threshold voltage and a drastic decrease in mobility observed during prolonged transistor operation in ambient air coincided remarkably with the establishment of a peak in the DOS spectrum at ca. 0.3 eV from the valence band edge after 20 min of repetitive transistor measurements. The peak is indicative of charge carrier trapping in discrete electronic states generated in the bandgap of the OSC during operation, which in turn is responsible for the device degradation. This information provided us with unparalleled insights into the microscopic mechanisms leading to device degradation which allowed us to devise a strategy that completely suppressed the formation of the discrete peak in the DOS spectrum, subsequently yielding highly stable OFETs with constant mobilities and threshold voltage shifts below 1 V during 500 min of continuous transistor operation in ambient air.

Organic semiconductors bring key advantages to integration into next generation flexible and bendable optoelectronics due to ease of processing and compatibility with arbitrary substrates. These properties are a direct consequence of their van der Waals intermolecular interactions, yet these interactions are also the main cause of the formation of localized electronic states in the band gap, which are responsible for the modest performance and reduced stability of organic semiconductor devices. In this talk, I will discuss the dynamics of trap formation and healing under several different pre, post deposition or in operando stimuli and relate it to the performance and long-term stability of the organic field-effect transistor devices. Charge carrier trapping can also provide a platform for developing sensors and radiation detectors by recording the changes in the trap DOS spectra upon interaction with external factors like impurities (chemical and biological), temperature, light, or radiation. In the second part of my talk, I will introduce radiation dosimeters for medical applications which exploit trap generation/annihilation in organic transistors. These X-ray medical imaging systems are robust and highly sensitive for doses relevant to a variety of medical investigations, including cancer diagnosis and therapy. Placement of the sensor directly onto the human body, coupled with the similarity in the atomic number Z between the electronically active layer and the human tissue, greatly enhances the precision and reduces the complexity of the medical equipment. These findings uncover new opportunities for organic circuits that will not only improve the quality of healthcare, but can have other applications, including wearable personal dosimeters.

Mixed organic ionic and electronic conductors are being explored for a wide range of applications, from bioelectronics to neuromorphic computing, artificial muscles and energy storage applications. These materials exploit the simultaneous transport properties of ionic and electronic carriers to enable novel device functions. Recently, polymer semiconductors have received significant amounts of attention because of their flexibility, biological compatibility and ease of fabrication. These materials, particularly thiophene-based polymers such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS) and related derivatives, have demonstrated significant enhancements in performance in a relatively short amount of time, with transconductance values of PEDOT:PSS transistors surpassing those achieved even with graphene.

We explore how small changes in ethylene-glycol functionalized polythiophenes alter ion injection. Specifically, their thin film morphology, and their ionic and electronic conductivities, and comparisons against theoretical predictions will be discussed. In this talk, the effect on the density of the ethylene-glycol side chains and their pattern of placement on ionic conductivity will be discussed.

The development of multifunctional devices capable to respond to multiple and independent stimuli is one among the grand challenges in organic electronics. The combination of multiple components, with each one conferring a specific function to the ensemble, is a facile strategy to impart a multifunctional nature to electronic devices. The controlled combination of such components and their integration in real devices can be achieved by mastering the supramolecular approach.
In my lecture I will review our recent works on the combination of carbon-based nanomaterials, in particular comprising organic semiconductors, with photochromic molecules (diarylethenes or azobenzenes) in order to fabricate smart, high-performing and light-sensitive (opto)electronic devices such as field-effect transistors and light-emitting transistors and as well as flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels.

References
[1] For reviews see: (a) X. Zhang, L. Hou, P. Samorì, Nat. Commun. 2016, 7, 11118. (b) E. Orgiu, P. Samorì, Adv. Mater. 2014, 26, 1827-1845.
[2] For modulating charge injection at metal-organic interface with a chemisorbed photochromic SAM see: (a) N. Crivillers, E. Orgiu, F. Reinders, M. Mayor, P. Samorì, Adv. Mater. 2011, 23, 1447-1452. (b) T. Mosciatti, M.G. del Rosso, M. Herder, J. Frisch, N. Koch, S. Hecht, E. Orgiu, P. Samorì, Adv. Mater. 2016, 28, 6606.
[3] For hybrid structure combining organic semiconductors blended with Au nanoparticles coated with a photochromic SAM see: C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor, P. Samorì, Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 12375-12380.
[4] For blends energy level phototuning in a photochromic - organic semiconductor blend see: (a) E. Orgiu, N. Crivillers, M. Herder, L. Grubert, M. Pätzel, J. Frisch, E. Pavlica, G. Bratina, N. Koch, S. Hecht, and P. Samorì, Nat. Chem. 2012, 4, 675-679. (b) M. El Gemayel, K. Börjesson, M. Herder, D.T. Duong, J.A. Hutchison, C. Ruzié, G. Schweicher, A. Salleo, Y. Geerts, S. Hecht, E. Orgiu, P. Samorì, Nat. Commun. 2015, 6, 6330.
[5] For the fabrication of memory devices: T. Leydecker, M. Herder, E. Pavlica, G. Bratina, S. Hecht, E. Orgiu, P. Samorì, Nat. Nanotech. 2016, 11, 769–775.
[6] For the novel nanomesh scaffold based photodetector: (a) L. Zhang, X. Zhong, E. Pavlica, S. Li, A. Klekachev, G. Bratina, T.W. Ebbesen, E. Orgiu, P. Samorì, Nat. Nanotech. 2016, 11, 900–906. (b) L. Zhang, N. Pasthukova, Y. Yao, X. Zhong, E. Pavlica, G. Bratina, E. Orgiu, P. Samorì, Adv. Mater. 2018, 30, 1801181.
[7] For optically switchable light-emitting trasnsistors: L. Hou, X. Zhang, G. F. Cotella, G. Carnicella, M. Herder, B. M. Schmidt, M. Pätzel, S. Hecht, F. Cacialli, P. Samorì, Nat. Nanotechnol., 2019, 14, 347–353.

Understanding how dipolar, non-centrosymmetric organic semiconductors self-assemble, nucleate, and crystallize is indispensable for designing new molecular solids with unique physical properties and light-matter interactions. However, some intermolecular interactions, such as dipole-dipole and van der Waals interactions compete to direct the assembly of these compounds, making them difficult to predict how solids are formed from individual molecules. Here, we investigate four small molecules (TpCPD, TpDCF, AcCPD, and AcDCF) featuring anisotropic with large dipole moments, and establish robust algorithms to control their molecular self-assembly via simple physical vapor deposition technique. Each molecule contains a central polar moiety, consisting of either a cyclopentadienone (CPD, ca. 3.5 D dipole moment) or dicyanofulvene (DCF, ca. 7.0 D dipole moment) core, that is surrounded by either four twisted phenyl (tetraphenyl, Tp) groups or a fused aromatic (acenaphthene, Ac) ring system. We find that only molecules possessing extended π system form one-dimensional (1D) nanowires due to the stronger van der Waals associations, originating from the planar acenaphthene moieties. We examine the kinetics of self-assembly for AcDCF and create diverse 1D morphologies, including both curved and linear nanostructures. Finally, conductive AFM (c-AFM) measurements show that 1D AcDCF wires support their higher current densities relative to randomly-oriented clusters lacking long-range order.

Organic electrochemical transistors (OECTs) are ionic-to-electronic transducers that rely on the ability of the organic active layer to transport both ions and electrons. These devices consist of an aqueous electrolyte in direct contact with a π-conjugated polymer channel. In order to maximize both electronic and ionic transport, a balance between ordered π-stacking and the swelling promoted by the glycolated side chain must be achieved.¹
Molecular dynamics (MD) simulations offer a unique perspective into the microstructure of conjugated polymers. In this work, we provide an atomistic picture of the polymer-electrolyte interface in the ‘off’ state of an OECT. Using a combination of MD simulations and X-ray fluorescence, we show how different anions effectively tune the coordination and chelation of cations by the glycolated polymer p(g2T-TT). At the same time, softer and hydrophobic anions such as TFSI and ClO₄ are found to preferentially interact with the p(g2T-TT) phase, further enhancing polymer-cation coordination. These trends are confirmed by X-ray Fluorescence experiments on p(g2T-TT) thin films undergoing passive swelling.²
We then use MD to simulate the swelling of another glycolated polymer, p(g3T2-T), mapping the phase diagram of a water-glycolated polymer mix as a function of the water content. This allows us to predict the microstructure changes associated to passive swelling and how different electrolytes penetrate the polymer morphology.
Finally, we discuss the challenges associated to the modelling of ionic doping of glycolated polymers, and analyse the interactions between polymer-electrolyte using a combination of electronic structure and classical simulations.

1. Rivnay, J.; Inal, S.; Salleo, A.; Owens, R. M.; Berggren, M.; Malliaras, G. G. Organic Electrochemical Transistors. Nat. Rev. Mater. 2018, 3 (2), 17086.

2. Matta, M.; Wu, R.; Paulsen, B. D.; Petty, A.; Sheelamanthula, R.; McCulloch, I.; Schatz, G. C.; Rivnay, J. Ion Coordination and Chelation in a Glycolated Polymer Semiconductor: Molecular Dynamics and X-ray Fluorescence Study, 2020. https://doi.org/10.26434/chemrxiv.12264308.v2

Increasing the charge carrier mobility of organic semiconductors has been a dominating challenge in the field for over a decade, as typical values of mobility of organic materials are orders of magnitude lower than their inorganic counterparts. Various advances in molecular design as well as device fabrication have enabled mobility of organic materials to reach 10 cm²V^-1s^-1 in recent times. However, the innovations in molecular design have focused on modifying main chain properties. Side chain groups have been considered to play a role in solubility and thin film morphology rather than charge transport energetics. Charge transport in organic semiconductor thinfilms occurs by thermally assisted hopping mechanism, wherein charge carriers are temporally localized on sites, such as a repeat unit of polymer chain, and subsequently hop to energetically favorable neighboring sites. A high mobility represents a higher speed of hopping as applied electric field is increased. Hopping rate is enhanced when orbital overlap between neighboring sites is high, which is achieved by repeat units of high area of conjugation, and when activation energy of hopping is low. Accommodating an incoming hopping charge carrier causes stretching and rotation of the chemical bonds in the newly charged polymer repeat unit. Energy consumed for this physical rearrangement, known as the reorganization energy, acts as an activation barrier to charge hopping process. We present the first report of how changing pendant group connecting the side chain to backbone chain may drastically change the field-effect-mobility of a polymer via reduction of its reorganization energy.^[1]

We demonstrate this with the case of two structurally similar polymers with only pendant group different. Polymers PTB7 and PTB7-Th are common solar cell donor polymers.^[2] Structure of both polymers are identical, except the pendant group is an oxygen atom in PTB7 while it is a thiophene unit in PTB7-Th.^[3] In spite of only a minor difference in the polymer structure, we found charge carrier mobility of PTB7-Th (0.11 cm²V^-1s^-1) to be a magnitude higher than PTB7 (0.01 cm²V^-1s^-1) in respective thin film transistors. We show this to be due to the lower reorganization energy of PTB7-Th (0.157 eV) as compared to PTB7 (0.346 eV) as found by calculating from first principles. The values of reorganization energy predict the ratio of rate of charge carrier hopping between the two polymers to be ~10, consistent with our observation of experimental field effect mobility. We find this to be related to rigidity of the thiophene unit of PTB7-Th. During cation formation of polymer segment, side chains in PTB7-Th rotate less (~9^°) as compared to PTB7(~19^°), thus requiring less energy to attain charged state molecular configuration. This lowers the activation barrier, and hence increases rate of hopping, and consequently the mobility.

We assert the role of reorganization energy as a dominant parameter controlling mobility of conjugated polymer semiconductors, and provide a key to tuning it via molecular design: an efficient pendant group.

References
[1] Patrikar, K., Jain, N., Chakraborty, D., Johari, P., Rao, V. R., Kabra, D., Adv. Funct. Mater. 2019, 29, 1805878.
[2] Nakul Jain, Naresh Chandrasekaran, Aditya Sadhanala, Richard H. Friend, Christopher R. McNeill, Dinesh Kabra, J. Mater. Chem. A, 2017, 5, 24749-24757
[3] Kedar D. Deshmukh, Shyamal K. K. Prasad, Naresh Chandrasekaran, Amelia C. Y. Liu, Eliot Gann, Lars Thomsen, Dinesh Kabra, Justin M. Hodgkiss, and Christopher R. McNeill Chemistry of Materials 2017, 29 (2), 804-816

Organic semiconductors systems offer many new exciting device applications with attractive features such as flexibility, low cost, low-temperature processing etc. One key difference to inorganic semiconductors is the usually quite small dielectric constant of organics, leading to much stronger Coulomb interactions. For instance, the exciton binding energy in organic semiconductors is significantly greater than the room temperature thermal energy, thus having profound influence on the optical properties. In this talk, I will discuss the influence of these strong Coulomb interactions on two phenomena, both related to blends of organic materials: First, I will discuss controlled electrical doping where the strong Coulomb interaction might be a curse and should, at least in simple hydrogen-like model, make efficient doping unlikely. I will discuss recent results which clarify that doping is indeed efficient, mainly because of disorder effects. Second, I will discuss how the Coulomb interaction can be a blessing by using it for “band structure engineering” of organic semiconductors: Long range Coulomb interactions allow to continuously tune the energy levels, allowing finely adjustable energy levels without synthesis of new molecules. The electrostatic nature of these phenomena is directly proven by experiments which show the strong influence of these quadrupole effects at interfaces. Finally, I will discuss recent results which show that the energy gap of organic semiconductors can be tuned in a similar way.

In most presently known organic semiconductors (OSCs), the relative dielectric constant (ε_r) is low (ε_r ≈ 3-4) compared to crystalline inorganic semiconductors such as silicon (ε_r ≈ 11). As a result, charges in the material experience strong Coulombic forces which can have a profound effect on the performance of OSC based devices. For instance, at high carrier densities such as found in organic field effect transistors (OFETs), simulations suggest that Coulomb interactions between charge carriers significantly reduce their mobility. Meanwhile, in doped organic semiconductors, the low ε_r can lead to the formation of Coulomb traps and to lower doping efficiencies due to the formation of stable host-dopant charge-transfer complexes. For organic photovoltaics (OPVs), the low ε_r has a significant effect on the exciton binding energy, which represents a crucial barrier to charge separation and extraction. Despite the relevance for many OSC based devices, the effect of ε_r on the electronic and optoelectronic properties of organic materials is largely unexplored due to the scarcity of known organic semiconductors with ε_r higher than 6. Furthermore, ε_r is a frequency dependent property, and it remains unclear which frequency regime (timescale) is most relevant for the different device applications.
In this presentation, a homologous series of fullerene derivatives with varying length and geometrical arrangement of polar ethylene glycol (EG) side chains is investigated and the low-frequency ε_r is measured using impedance spectroscopy. The low frequency ε_r is found to correlate with length for the symmetrical adducts whereas for the unsymmetrical branched adducts, there is no significant difference. For BTrEG-2, the ε_r reaches 10, which is unprecedented in an intrinsic molecular organic semiconductor film. The temperature dependence of the low frequency dielectric response and the optical constants (high frequency ε_r ) are also investigated.

Organic semiconductors are emerging in printable and flexible electronics. Nevertheless, the charge transport in crystal organic semiconductors is attained at the cost of the deformability. Superelastic organic semiconductors (SOSs) is a recently discovered phenomenon that provides a possibility to achieve the high mechanical deformability yet retaining the molecular ordering. Compared to the well-studied martensitic phase transition in metallic alloys, superelasticity and ferroelasticity in solid-state organic molecules through polymorphic changes are much less understood. Here we leverage the phase transformation theory, genetic algorithm refined molecular modeling, and experimental validation to study the versatile cooperative transitions in bis(triisopropylsilylethynyl)-pentacene semiconductor crystal. The molecular rotation governed thermoelasticity, interconvertible super- and ferroelastic transitions, and molecular twinning are systematically studied by integrating both the lattice crystallography and molecular motions. The fundamental understanding promotes polymorphism to modulate the optoelectronic properties of organic semiconductors and can breed a new avenue of environmentally responsive organic devices.

Organic ferroelectrics are in the focus of current research for their potential technological applications as materials to address global challenges. These lightweight, flexible and nontoxic ferroelectrics have potential applications as ferroelectric random access memories in computers, super capacitors for energy storage, transducers for medical ultrasound imaging and functional materials in flexible electronics [1-4]. As a result of their excellent dielectric, pyro- and piezo-electric properties comparable to traditional inorganic ferroelectrics, and with unique coexistence of flexibility and ferroelectricity, organic ferroelectrics are candidates of intense research promising of designing new bespoke functional materials. These materials are often hydrogen or halogen bonded molecular crystals [5,6] and dynamics or movements of protons from the centro symmetric to non-centrosymmetric positions are considered as the key mechanisms of their functionality. In case of inorganic ferroelectrics, large LO-TO splitting of normal modes helps to identify modes responsible for the ferroelectric functionality. A large LO-TO splitting is a manifestation of strong coupling with the electric field and contributes more to the dynamical effective charge. The effect of electric field on long range Coulomb interaction can also be identified by inspecting the splitting of LO-TO vibrational modes. Thus to understand the microscopic origin of ferroelectricity the knowledge of these normal modes are crucial.

We have done inelastic neutron scattering (INS) spectroscopy experiments on a number of hydrogen and halogen bonded ferroelectric materials, such as croconic acid (C₅O₅H₂), 1-cyclobutene-1,2-dicarboxylic acid (CBDC, C₆H₈O₄) and 2-phenylmalondialdehyde (PhMDA, C₉H₈O₂) and dichloromethylimidazole (C₄H₄Cl₂N₂ ). Calculations are done using plane wave pseudo potential density functional theory (DFT) using CASTEP code. Temperature dependent INS spectra are interpreted using first principles lattice dynamics within quasi harmonic approximations. Structures and dynamics of these functional materials are correlated to understand the role of hydrogen bonds for ferroelectric polarisation at room temperatures. A large LO-TO splitting of the O-H stretching mode around 2500 cm^-1 is identified as the mode that has significant contribution to the ferroelectric polarisation. Although this long range Coulomb interaction can be considered as blessings for ferroelctricity in this class of materials, any defect or grain boundary affecting this interacction can kill the ferroelectricity.

In this talk I will present structure and dynamics of few organic ferroelectrics using vibrational neutron spectroscopy and first principles DFT based simulations to find out the finger print of ferroelectricity in this class of materials [7-10]. From INS experiments and DFT simulations, modes sensitive to long range Coulomb interactions and responsible for ferroelectric properties of the material are identified. I would propose vibrational spectroscopy combined with DFT calculations as a characterisation tool to investigate ferroelectric properties in the these molecular crystals.

References:
[1] S. Horiuchi et. al, Nat. Mat. 2008, 7, 267,
[2] S. Horiuchi et. al, Nature 2010, 463, 789.
[3] W. Qin et. al., Nanoscale, 2015, 7, 9122.
[4] F. Kagawa et. al., Nat. Phys. 2010, 6, 169.
[5] A. Stroppa et. al., Phys. Rev. B, 2011, 84, 014101.
[6] M. Owczarek et al. Nature Commn 2016, 7, 1308.
[7] S. Mukhopadhyay et. al., Phys. Chem. Chem. Phys. 2014, 16, 26234.
[8] F. Fernandez-Alonso et. al, J. Phys. Soc. Japan, 2013, 82, SA001,.
[9] S. Mukhopadhyay et. al., Chem Phys., 2013, 427, 95.
[10] S. Mukhopadhyay et. al., Phys. Chem. Chem. Phys. 2017, 19, 32147.

In organic semiconductor devices, extraction barriers are usually undesired since they lead to a reduced device performance. In this work, by intentionally introducing an extraction barrier for holes, near infrared (NIR) organic photodetectors (OPDs) dedicated for distance measurement are realized. The extraction barrier is introduced by replacing the hole transporting layer (HTL) material with a deeper highest occupied molecular orbital (HOMO) HTL material within a well-performing organic solar cell device. Holes pile up at the extraction barrier introducing additional charge recombination. With increasing irradiance, achieved by decreasing the illumination spot area on the OPD, the probability of charge recombination is rising. This allows to determine the distance between the OPD and light source from the irradiance-dependent, non-linear response of the OPD. We extend here this principle to the NIR: We demonstrate fully vacuum-deposited organic NIR optical distance photodetectors with a detection area up to 2.56 cm² and detection wavelengths at 850 nm and 1060 nm. Such NIR OPDs have high potential to be utilized as robust, low-cost and simple optical distance measurement setup.

Thermally-activated delayed fluorescence (TADF) organic molecules undergo more efficient light emission than traditional organic fluorescent emitters, making them attractive materials for OLEDs. In TADF devices, indirect emission from dark triplet states via bright singlet excitons is activated and leads to boosted electroluminescence efficiencies.¹ There is general agreement that TADF emitters should be designed to have small singlet-triplet exchange energies that promote forward and reverse intersystem crossing (ISC) between singlet and triplet excited states. However, there is not agreement on the spin interactions and spin-flip mechanisms responsible for singlet-triplet interconversion, with suggestions of hyperfine- and spin-vibronic-based design rules.^2,3 Most studies of TADF use optical spectroscopies which can examine the photophysics and interconversion rates but do not shed light on the critical spin physics.⁴ Here we use transient electron spin resonance spectroscopy to study TADF triplets and the role of hyperfine and spin-vibronic couplings on the critical ISC for efficient TADF.

We designed a series of TADF emitters with molecular modifications that led to systematic changes of TADF efficiencies. Solution-based studies of this series revealed the importance of low-lying triplet states and the spin-vibronic mechanism was dominant for ISC. However, in device-relevant thin-films, we find additional signatures of hyperfine coupling mediated triplet formation, proposed to arise from solid-state intermolecular interactions. Our work shows that the singlet-triplet-ground state picture for TADF should be expanded to understand the overall luminescent efficiencies of these materials. Understanding the interplay between molecular design and spin interactions for efficient spin interconversion is integral in establishing the structure-function relationships for the rational design of new TADF emitters.

1. Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).
2. Evans, E. W. et al. Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules. J. Phys. Chem. Lett. 9, 4053–4058 (2018).
3. Ogiwara, T., Wakikawa, Y. & Ikoma, T. Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules. J. Phys. Chem. A 119, 3415–3418 (2015).
4. Etherington, M. K., Gibson, J., Higginbotham, H. F., Penfold, T. J. & Monkman, A. P. Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence. Nat. Commun. 7, 13680 (2016).

Charge transfer (CT) states at the interface between electron-donating (D) and electron-accepting (A) materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive, or generate free carriers at high yield. The former can result in rather efficient organic light emitting diodes, via thermally activated delayed fluorescence, while the latter property is exploited in organic photovoltaic (OPV) devices. In this talk, I will show that proper control of CT state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D–A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination.

Radical materials have unpaired electrons, leading to unusual physics that could be exploited in next-generation organic electronics. We have previously shown that luminescent radicals can be used for overcoming the spin statistical efficiency limit for electroluminescence (EL) in organic light-emitting diodes (OLEDs) due to their quantum-mechanical doublet-spin character^1,2. However, suitable radical emitters for OLEDs are generally limited in terms of chemistry and color to tris[2,4,5-trichlorophenyl]methyl derivatives and the near-infrared (> 700 nm). Additionally, the presence of the singly-occupied molecular orbital (SOMO) leads to potential efficiency losses in charge trapping mechanisms for EL. Here we present general design rules to increase the absorption and luminescence yields of organic radicals³. These insights were used to design a series of radical molecules with pure-red emission, up to 90% photoluminescence quantum yields and over 12% external quantum efficiency in OLEDs. Doublet exciton quenching processes were probed for radical OLEDs in operation by optical spectroscopy. This work may be beneficial for the rational design and discovery of highly luminescent radical materials with new colors and motifs, as well as informing new device strategies for more efficient radical optoelectronics.

¹Ai, X., Evans, E.W., Dong, S., Gillett, A.J., Guo, H., Chen, Y., Hele, T.J.H., Friend, R.H. & Li, F. Efficient radical-based light-emitting diodes with doublet emission. Nature (2018).
²Guo, H., Peng, Q., Chen, X.-K., Gu, Q., Dong, S., Evans, E.W., Gillett, A.J., Ai, X., Zhang, M., Credgington, D., Coropceanu, V., Friend, R.H., Bredas, J.-L. & Li, F. High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle. Nature Materials (2019).
³Abdurahman, A., Hele, T.J.H., Gu, Q., Zhang, J., Peng, Q., Zhang, M., Friend, R.H., Li, F. & Evans, E.W. Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes. Nature Materials (2020).

Molecular aggregates are non-covalent self-assemblies of chromophores where long range coulomb coupling of transition dipole moments leads to delocalized excitons with extreme blue or red shifts, called as H- or J-aggregates respectively. Cyanine dye aggregates have garnered tremendous interest due to their robust excitons, chemical tunability and biocompatibility (1). As a result, these materials have exciting applications in shortwave infrared (SWIR) imaging, plexitonics and excitation energy transfer (2–4). Our research aims to chemically control the excitons in cyanine dye aggregates by leveraging the long range coulomb coupling, which in turn, can be controlled via aggregate morphology as well as energetic disorder. In our previous work, we uncovered design principles for tuning the photophysics of 2-dimensional (2D) aggregates by chemically controlling the position of bright state within the excitonic band (5). However, control over the supramolecular self-assembly of cyanine dyes to push the aggregates to a desired H- or J-aggregated structure is still a challenge. Here, we give general principles to tune the self-assembly of cyanine dyes across a vast range of conditions enabling new experimental tools such as diffusion ordered spectroscopy. We predictably stabilize H- or J-aggregate state by independent control of solvent to non-solvent ratio, dye concentration and ionic strength. We observe universal trends in aggregation that can be understood using a simple three component equilibrium model where the monomers go via an H-aggregated dimer state to form extended 2D J-aggregates. Comparing the model with our vast experimental aggregation space, we provide thermodynamic insights into the underlying principles that govern the self-assembly. We further show the universal applicability of these principles across the broad set of cyanine dyes, including benzothiazole (THIATS) and benzimidazole cyanines (TDBC), and stabilize H-aggregated dimers or extended J-aggregated sheets with absorptions across the visible and SWIR. Additionally, we perform temperature dependent absorption of sugar matrix stabilized 2D aggregates across this library and shed light on the effect of static and dynamic disorder in 2D aggregates. Overall, we will provide structure-property relationships that employ the ubiquitous coulomb coupling to chemically control the excitons in molecular aggregates.
References:
1. J. L. Bricks et al., Methods Appl. Fluoresc. 6, 012001 (2017).
2. X. Zhong et al., Angew. Chemie Int. Ed. 55, 6202–6206 (2016).
3. W. Chen et al., J. Am. Chem. Soc. 141, 12475–12480 (2019).
4. C. Wang, E. A. Weiss, Nano Lett. 17, 5666–5671 (2017).
5. A. P. Deshmukh et al., J. Phys. Chem. C. 123, 18702–18710 (2019).

In this work, we report isostructural systems of mixed-stack CT complexes focusing on the D-A structure-property correlation under identical morphology condition with an aim to unveil the pure electronic mechanism of CT emission. For this, isometric motifs of two donors (D) and two acceptors (A) were rationally designed, of which cocrystals form bright luminescent D:A CT complexes with record high PL quantum yields of 83 %. Notably, four different CT pairs made of isometric D and A molecules all showed the quasi-isostructural intra-stack crystal structure, which allowed us to correlate the electronic CT interaction and their photophysical properties aside from the complicated morphological effect. Our isostructural concept of CT crystals is thus shown to be a crucial strategy for the targeted design of highly emissive CT cocrystals.

Photon recycling (PR), whereby photons generated by radiative recombination in semiconductors result in the regeneration of electron-hole pairs, plays an important role in the study of optoelectronic semiconductor materials and significantly affects the properties of their applications. Excitons are expected to play important roles in PR because they promote high luminescence quantum yields, which are necessary for the observation of PR. However, PR in low-dielectric-constant exitonic materials, such as organic semiconductors, has not been studied to date. In this work, we investigate the mechanisms of PR in organic semiconductor thin films. We study PR in three different organic semiconducting polymer media (regiorandom poly(3-hexylthiophene-2,5-diyl) (RRa-P3HT), regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT), and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)) because of their different quantum efficiencies, morphologies and broadband absorptions. In order to study PR, we fabricate different nanostructured surfaces including silver nanoparticle (AgNP) metasurfaces and silver nanograting (AgNG) metasurfaces. In addition, we also use glass, planar Ag and PMMA/Ag surfaces as control groups to analyze the results of PR in the organic thin films compared to the results from nanostructured plasmonic metasurfaces. Surface photoluminescence (PL) emission spectra and edge PL emission spectra are employed to characterize PR because they show strongly red-shifted peaks from surface emission to edge emission when PR is strong. Surface and edge PL emission spectra also play important roles in calculating photon recycling efficiencies and in evaluating reabsorption coefficients. Finally, PL lifetime and PL quantum yield results are used to further explain the PR effect in the different organic semiconductor media. This work is expected to lead to greater understanding of the role of PR in affecting the performance of organic optoelectronic devices.

We report on quantum dynamical studies of ultrafast photo-induced energy and charge transfer in functional organic materials, complementing time-resolved spectroscopic observations that underscore the coherent nature of the ultrafast elementary transfer events in these molecular aggregate systems. Our approach combines first-principles parametrized Hamiltonians [1], with accurate quantum dynamics simulations using the Multi-Layer Multi-Configuration Time-Dependent Hartree (MCTDH) method [2], along with semiclassical approaches [3]. The talk will focus on (i) exciton dissociation and free carrier generation in regioregular donor-acceptor assemblies [1,4], and (ii) the elementary mechanism of exciton migration [3,5-7] and creation of charge-transfer excitons [8] in polythiophene and poly(para-phenylene vinylene) type materials. Special emphasis is placed on the interplay of trapping due to high-frequency phonon modes, and thermal activation due to low-frequency "soft" modes which drive a diffusive dynamics [6].

[1] M. Polkehn, P. Eisenbrandt, H. Tamura, I. Burghardt, Int. J. Quantum Chem. 118:e25502. (2018).
[2] H. Wang, J. Phys. Chem. A 119, 7951 (2015).
[3] R. Liang, S. J. Cotton, R. Binder, I. Burghardt, W. H. Miller, J. Chem. Phys. 149, 044101 (2018).
[4] M. Polkehn, H. Tamura, I. Burghardt, J. Phys. B: At. Mol. Opt. Phys. 51, 014003 (2018).
[5] R. Binder, D. Lauvergnat, I. Burghardt, Phys. Rev. Lett., 120, 227401 (2018).
[6] R. Binder, I. Burghardt, Faraday Discuss., 221, 406 (2020).
[7] R. Binder et al., J. Chem. Phys. 152, 204119 (2020), ibid. 152, 204120 (2020).
[8] W. Popp, M. Polkehn, R. Binder, I. Burghardt, J. Phys. Chem. Lett., 10, 3326 (2019).

The notion that optical environment can change the nature of a chemical reaction emerges in the strong coupling regime when molecular electronic (or vibrational) transitions hybridize with light to form polariton states that have different energies, coherence, and dynamical characteristics than the bare (uncoupled) molecules themselves. Photoinduced charge transfer is arguably one of the most important areas to explore such modification since it underlies processes ranging from photolithography to photosynthesis. Here, we explore this possibility by measuring the photoconductivity of a p-doped organic semiconductor in which charged, rather than neutral molecules (i.e. polaronic rather than excitonic states) are strongly coupled to a Fabry-Perot microcavity.
We confirm the existence of polaron polariton modes in the photoconductivity action spectrum and find that their magnitude evolves differently with applied bias than photocurrent that originates from exciting uncoupled polarons in the same cavity. Importantly, the difference in functional dependence changes systematically with the polariton detuning. We explain these observations on the basis of Onsager theory, where strong coupling effectively changes the thermalization length of photoexcited holes that must escape their dopant counter-ions to contribute to the photoconductivity. We propose that this increase originates from an acceleration of the initial electron transfer event associated with the delocalized nature of the polaritonic excitation, which can effectively sample many more intermolecular electronic couplings in the disordered thin film than a localized excitation can. Access to a wider range of intermolecular electronic couplings in the film allows the polariton to benefit from the largest, thereby speeding up the overall rate of charge transfer.¹
Reference:
¹N. Krainova, A.J. Grede, D. Tsokkou, N. Banerji, and N.C. Giebink, Phys. Rev. Lett. 124, 177401 (2020)

In this talk, I will overview some applications of Non-adiabatic EXcited-state Molecular Dynamics (NEXMD) framework recently developed at LANL. The NEXMD code is able to simulate tens of picoseconds photoinduced dynamics in large molecular systems with hundreds of atoms in size and dense manifold of interacting and crossing excited states. In particular, I will exemplify ultrafast excitonic dynamics guided by intermolecular conical intersections (CoIns). Both simulations and time-resolved two-dimensional electronic spectroscopy track the coherent motion of a vibronic wave packet passing through CoIns within 40 fs, a process that governs the ultrafast energy transfer dynamics in molecular aggregates. Our results suggest that intermolecular CoIns may effectively steer energy pathways in functional nanostructures. Observed relationships between spatial extent/properties of electronic wavefunctions and resulting electronic functionalities allow us to understand and potentially manipulate excited state dynamics and energy transfer pathways toward optoelectronic applications.

Despite the ability of TADF materials to fully utilize triplet excitons, their inherent limitation is the broad emission spectrum. The torsional mobility about the twist angle between donor and acceptor is considered the principal cause of color impurity, since conventional TADF molecules involve large dihedral angles to facilitate intramolecular charge transfer. However, according to the Franck-Condon principle, electronic transitions are instantaneous compared to nuclear motions, and the emission characteristics should not be subject to molecular rotation. Conversely, there is ample experimental evidence that emission from charge transfer states is broad and diffuse. We submit that the dramatic change of electron configuration between ground and charge-transfer excited states causes the broad emission, as confirmed by our computational and experimental findings. Accordingly, to constrict emission broadening it is preferable to control the charge-transfer character by introducing chromophores with localized emission (LE) character, exhibiting minimal change in electron configuration upon emission. However, molecules with LE emitter states tend to have large singlet-triplet energy gaps (ΔE_ST), making TADF pathway inefficient. We have shown that ISC can be further enhanced with a process called “heavy atom oriented El-Sayed rule”, which allows us to design TADF emitters with rather large ΔE_ST and still result in sharp emission. In this design approach, we incorporate a n-π^* triplet state below the emissive singlet state to boost the reverse ISC, and compensate for the larger ΔE_ST of the LE emitter state. Our results can be used to design molecular organic alternatives with sharp emissions for LED applications.

Ultrafast photo-induced processes in organic materials are crucial to several applications, including organic solar cells and LEDs. Such processes include exciton transport and singlet fission. Understanding the microscopic mechanism of these processes from a theoretical perspective is crucial in order to formulate molecular design rules and efficiently guide the experimental search for those structures that optimise them. However, the simulation of the real-time dynamics of ultrafast processes poses a significant challenge, due to the hundreds of molecular vibrations that interact very strongly with the excited charge carriers, often constituting traditional perturbative methods insufficient, and causing commonly employed approximations such as the Markov and Born-Oppenheimer approximations to break down.

In this talk, I will present a novel theoretical framework based on tensor network methods, for the simulation of the full quantum mechanical non-perturbative dynamics of ultrafast processes, which does not rely on any of the aforementioned approximations. This methodology is applied to very large molecules with up to 108 atoms. The large size of the studied molecules has allowed us to study them experimentally, subjecting our theoretical approach to a rigorous comparison with experimental results, and revealing its accuracy. We uncover the full microscopic mechanism of singlet fission in a covalent pentacene dimer and find that two different groups of vibrational modes coordinate in a precise manner in order to enable efficient fission through an avoided crossing [1].

For a dimer of tetracene, we reveal the impact of the specific pathway of vibrational relaxation on charge transfer, and show that selective excitation of vibrations can increase its efficiency by up to 26% [2].

Finally, we combine the insights from the above studies to build a simple model for singlet fission, which combines the effects of excess energy photoexcitation, molecular vibrations and the polarity of the molecular environment. We show that utilising the subtle interplay between these factors allows for switching into the coherent regime of singlet fission in a controlled manner, accelerating the process by an order of magnitude, as also confirmed experimentally [3].

[1] A molecular movie of ultrafast singlet fission, Nature Communications (2019)10:4207;
[2] Non-equilibrium relaxation of hot states in organic semiconductors: impact of mode-selective excitation on charge transfer, J. Chem. Phys. 151, 084104 (2019)
[3] Switching between coherent and incoherent singlet fission via solvent-induced symmetry-breaking, Journal of the American Chemical Society 2019, 141, 44, 17558-17570

Long-persistent luminescence (LPL), also known as the glow-in-the-dark effect, noctilucent or long afterglow, is a self-sustained light emission phenomenon by which a material emits light for a very long time (longer than several minutes) after removal the excitation sources which are typically visible light, ultraviolet (UV) light, electron beams or high energy radiation such as X-, α-, β- or γ-rays. Although the LPL phenomenon is often confused with phosphorescence owing to the confusing terminology in history, the LPL is different with phosphorescence reflected in two apparent features: (i) the almost identical spectra between photoluminescence and LPL; (ii) their decay follows the different law: the LPL (refer in particular to the isothermal luminescence) obeys a power law (I(t) ∼ t^−m, with m = 0.5∼2, mostly 1, hyperbola) in the long time scale, while the phosphorescence follows an exponential law. Therefore, LPL materials can realize long emission duration from minutes to even days after cutoff excitation sources, which is quite longer than phosphorescence. Organic LPL (OLPL) materials have significant superiority over inorganic LPL materials in terms of functionality, flexibility, transparency, and solution-processability. OLPL excited by UV-visible light can be regarded as a recombination luminescence in a special organic solar cell. However, the material design strategies for the OLPL is still not clear. Here, we systematically investigate changes in the LPL duration and spectra for three donor materials having similar molecular structures but different energy levels and find that the LPL performance is significantly influenced by the energy gap between the lowest localized triplet excited-state and the lowest singlet charge-transfer excited-state in the exciplex system. When the energy level of the lowest localized triplet excited-state is much lower than that of the charge-transfer excited state, the system exhibits a short LPL duration under the same excitation condition and clear two distinct emission features originating from exciplex fluorescence and donor phosphorescence.

Fullerene-based organic solar cells (OSCs), which consist of a dilute amount of donor molecules (< 5 wt.%) in a fullerene matrix, exhibit high open-circuit voltage (V_oc) and short circuit current density (J_sc) simultaneously, thus overcoming the intrinsic J_sc-V_oc trade-off observed in traditional bulk heterojunction OSCs. The high V_oc in fullerene-based OSCs has been attributed to reduced bimolecular recombination or the Schottky barrier height between fullerene matrix and the anode. However, the origin of the high J_sc in these OSCs with a donor concentration much lower than the percolation threshold is still unclear. To investigate the photocurrent generation mechanism, we design and synthesize four variations of thienothiophene (TT) molecules with different center and end groups, tuning their HOMO levels from -6.1 to -5.2 eV. We perform experimental studies and density functional theory (DFT) calculations to understand TT:PC₇₁BM devices with donor concentration varying from 0 to 10 wt.%. We show that photocurrent generation depends strongly on the center groups, which are either cyano (CN) or hexyloxy (OHex). With 1 wt.% donor, TTOHex:PC₇₁BM devices show ten times higher photocurrent than neat PC₇₁BM devices. In contrast, TTCN:PC₇₁BM devices do not generate additional photocurrent even with 10 wt.% donor. The results are the same for both end group, dicyanovinylene (DC) or rhodamine (RH). DFT results also show TTOHex-PC₇₁BM dimers have a shallower HOMO energy than PC₇₁BM monomers, while TTCN-PC₇₁BM dimers have a deeper HOMO energy than PC₇₁BM monomers. Using photoelectron spectroscopy in air (PESA) to measure HOMO levels, we find that the difference between photocurrent in TTOHex:PC₇₁BM and TTCN:PC₇₁BM devices are strongly correlated with the HOMO offset between donor and acceptor. Since HOMO offset is required to dissociate excitons at the donor-acceptor interface, these results indicate that the first requirement of photocurrent generation is efficient exciton dissociation. We then analyze the possibility of three possible hole transport mechanisms—tunneling, percolation pathways, and hole back transfer—for the TT:PC₇₁BM systems. Following exciton dissociation, we find that hole back transfer from donor molecules to PC₇₁BM is primarily responsible for photocurrent generation for the TT:PC₇₁BM devices.

Semiconducting polymers are considered highly promising for next generation large area organic solar cell (OSC) devices because of their potential for solution processing and film forming ability. This work explores the effect of increasing side chain steric bulk (methyl, isopropyl, isobutyl, and cyclohexylmethyl) on the acceptor unit of a family of donor-acceptor polymers based on thienopyrroledione (TPD) and investigates the steric effects on electronic properties and device performance. Steric effects on solution-state aggregation properties, solid-state microstructures, optical properties, and thermal properties are also explored. Electronic properties (i.e. hole mobilities, ionization potentials, and electron affinities) were found to be relatively invariant, an effect due to the distance of the side chains from the main backbones of the polymers. When tested in photovoltaic devices with PC₇₁BM, the devices fabricated from the polymers demonstrated subtle changes in short-circuit current, fill factor, and OSC device efficiency. In contrast to the electronic properties and photovoltaic statistics, the aggregation, thermal, and morphological properties demonstrated pronounced differences. By UV-vis absorption spectroscopy, we found that changes in steric bulk caused substantial differences in solution-state aggregation, with the bulkier side chain TPD polymers aggregating to a lesser extent. The thermal and morphological properties of neat thin films and blend thin films with PC₇₁BM were investigated. By differential scanning calorimetry (DSC), melting/crystallization point depression could be observed with increasing acceptor unit side chain steric bulk, while the heats of fusion/crystallization remained the same. By grazing incidence wide angle x-ray scattering (GIWAXS), out-of-plane lamellar spacing distances (d) correlated directly with acceptor-unit side chain bulk – TPD-methyl (d = 24.4 Å) and TPD-cyclohexylmethyl (d = 26.6 Å). In this work, we demonstrate a unique example of side chain engineering that alters a polymer’s solution and solid-state properties but ultimately has little effect on device performance.

Owing to the low dielectric constants, the hole and electron pairs in functional organic materials are strongly bound by the Coulomb interaction. It is therefore crucial to measure precisely the Coulomb bounding energies. This energy is the difference between the bound state and the free hole/electron state. While the bound state energy can often be measured by optical spectroscopies, the energy of the free charge state was difficult to determine in organic materials. The free hole and electron energies correspond to the ionization energy and electron affinity, respectively. Thus, the energy of the free hole-electron pair without mutual interaction can be determined as the difference of the ionization energy and electron affinity measured separately. The ionization energies of organic solid materials are routinely examined by ultraviolet photoelectron spectroscopy (UPS). Therefore, the precise value of the electron affinity was the missing piece for the hole-electron interaction energy.

In 2012, we developed low-energy inverse photoelectron spectroscopy (LEIPS) which enable to determine the electron affinity of organic material as precise as the ionization energy determined by UPS. Using the combination of UPS/LEIPS for the free hole-electron energy together with the existing methods for the bound state energy, we are able to measure the hole-electron interaction energy with the uncertainty of 0.1 eV. We have applied this procedure to examine the Coulomb interaction energy of various functional organic materials. In this presentation, I focus on the following two topics:

1. Exciton binding energy
When the hole-electron pair stays in the same material, the interaction energy is the exciton binding energy. We have determined the exciton binding energies of organic materials used for organic light emitting diode (OLED) and organic solar cell (OSC) as well as prototypical organic semiconductors. The exciton binding energies of OLED materials exceeds 1 eV while those of the OSC materials are less than 0.5 eV. The large exciton binding energy may facilitate binding of the free holes and electrons to generate photons in OLED. In contrast, the small exciton binding energy is preferential to the free carrier generation in OSC. Interestingly, we found no distinct differences in the exciton binding energies among the state-of-the-art non-fullerene acceptors, conventional fullerene acceptors and low-bandgap polymers.

2. Charge dissociation energy in organic solar cell (OSC)
If the hole and electron stay in the donor and acceptor, respectively, at the donor/acceptor interface, the Coulomb interaction energy corresponds to the charge dissociation energy in OSC. A simple consideration based on the Coulomb interaction in continuum dielectric media gives a charge dissociation energy of around 0.5 eV. As this estimated value is much larger than the thermal energy, there has been a long-standing debate how the strongly bound hole-electron pair at the donor/acceptor interface spontaneously dissociate into the free carriers. However, the reliable dissociation energy has never been determined experimentally. We applied the above-mentioned method to the bulk-heterojunction of the low-bandgap polymer/fullerene acceptor and low-bandgap polymer/non-fullerene acceptor. The determined charge dissociation energies (enthalpy difference) are always less than 0.3 eV which is small enough to dissociate the hole/electron pair by the thermal energy with the assistance of the entropy.

In the field of organic photovoltaics, efficiencies beyond 17% have recently been achieved by combining low bandgap conjugated polymers with small-molecule non-fullerene acceptors (NFAs). To understand what differentiates non-fullerenes from conventional fullerenes in terms of the charge separation and transport processes, we have used ultrafast transient absorption spectroscopy (TAS), electro-modulated differential absorption spectroscopy (EDA) and terahertz measurements (THz). We have thus investigated how the charge generation and separation processes depend on parameters such as the charge-transfer and recombination energetics, the short-range charge mobility and the morphology. Moreover, to simplify the complexity of the process caused by the phase morphology of bulk heterojunction blends, we have worked with bilayers of the donor polymer with the NFA, and with blends containing dilute concentrations of the acceptor. Our results are combined into a kinetic model that explains the efficiency of free charge generation in various polymer:NFA blends.

For highly efficient organic photovoltaics (OPVs), control of molecular orientation is of prime importance. While molecular orientation within a film influences charge conduction, relative configuration at donor/accepter interface determines energy level alignment as well as electron-hole separation. Here, we report the change in molecular orientation of planar-shape donor molecules, metal phthalocyanine (CuPc or ZnPc) on ITO by adding CuI templating layer. Near edge X-ray absorption fine structure measurement can estimate average tilt angle of the planar molecules with and without CuI underlayer. CuPc reveals a noticeable change from standing-up to lying-down orientation by CuI, but ZnPc keeps standing-up geometry. Ionization potential for both molecular films measured by ultraviolet photoelectron spectroscopy shows consistent results as well. To explain the different templating effect of CuI, we compare chemical interaction between CuI and the two different metal phthalocyanine molecules by DFT calculation. The stronger interaction between Cu-Cu atoms in CuPc and CuI, respectively, might be reason for the distinct influence on the molecular orientation change of CuPc film. Furthermore, the exciton dynamics at MPc/C₆₀ (donor/accepter) interface is studied by time-resolved two-photon photoemission spectroscopy. On CuI layer, the lying-down CuPc molecules show a clear charge transfer (CT) exciton characteristics at the CuPc/C₆₀ interface, while ZnPc molecules only show long-lived singlet peak even at the ZnPc/C60 interface. Thus, face-on interface of CuPc/C₆₀ induces CT exciton by strong π-π overlap between donor and acceptor. Finally, we discuss the influence of molecular orientation on the enhanced carrier transport and overall OPV cell efficiency by measuring device performance.

Understanding optical absorption and excitonic properties is crucial for organic solar cells (OSCs). We study the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs [1]. We simulate low-energy absorption features including molecular excitons, CT-excitons and hybrids including multi-exciton hybridization as well as low-frequency molecular vibrations. For model donor-acceptor blends featuring charge-transfer excitons, our numerical calculations agree very well with temperature-dependent experimental absorption spectra. We are able to predict the low-energy spectral lineshape, which we connect to radiative voltage losses that limit the solar cell's performance.
In addition, we unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction plays a significant role. It causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially reduce the OSC’s efficiency.
To circumvent such radiative voltage losses, we elaborate on a better understanding of their connection to OSC parameters such as the electronic coupling or the driving force for CT exciton formation. Radiative open circuit voltages are affected by exciton hybridization and delocalization on the order 100 mV or more, depending on the system.
We finally extend our theoretical predictions for a set of different donor-acceptor systems including non-fullerene donor-acceptor blends and different heterojunction and interface geometries. Our proposed strategies to reduce the radiative voltage losses are supported by excellent agreement of the present simulation approach to existing experimental data for the low-energy absorption.

[1] Panhans, M., Hutsch, S., Benduhn, J., Schellhammer, K. S., Nikolis, V. C., Vangerven, T., Vandewal, K., Ortmann, F. Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells. Nat Commun 11, 1488 (2020).

Compared with inorganic or perovskite photovoltaics, the key limiting factor for organic solar cells (OSCs) is large voltage loss, which is usually over 0.7 V. A significant contribution of the large voltage loss in OSCs is due to strong non-radiative recombination, the origin of which has been puzzling the community for a long time, limiting rational design of materials to overcome this critical issue. We systematically investigate a range of low voltage loss systems, where the voltage loss is around or below 0.6 V. We find that the quantum efficiency of electroluminescence in these systems have been significantly increased, leading to decreased non-radiative voltage losses below 0.20 V. Combining spectroscopic and quantum chemistry approaches, we formulate the key rules for minimizing voltage losses: (i) low energy offset between donor and acceptor molecular states, and (ii) high photoluminescence yield of the low-gap material in the blend. We further demonstrate that there is no intrinsic limit for efficient charge separation in OSCs with small voltage losses.

Organic solar cells offer the unique opportunity to prepare flexible photovoltaic films based exclusively on abundantly available raw materials in an energy-efficient low-temperature process compatible with low-cost PET as a substrate foil. Due to their light weight and their attractive appearance, the films are especially suitable for building integration in facades and roofs of buildings that cannot bear the weight of traditional solar cells.
Heliatek has chosen the approach to prepare organic solar cells by a clean and solvent-free vacuum based on oligomer absorbers. It enables to manufacture p-i-n-type tandem or triple junction architectures in a fully integrated additive roll-to-roll process including laser-structuring and encapsulation. After the ramp of a 30cm wide pilot line using tailored organic conjugated oligomer absorbers developed within Heliatek, a production line for 120cm wide PET web is currently set up and starts to become productive within 2020.
To get optimum efficiency, we need strong absorption with steep absorption edges, efficient charge separation and good charge transport perpendicular to the substrate. This requires a nanocrystalline morphology with suitable preferential orientation, a close p-stacking mode within the donor domains and a phase separation on the order of at least 10-20nm between donor and acceptor domains. Moreover, the degree of slipping in x and y direction within pairs of p -stacked molecules plays a critical role both for the steepness of the absorption spectra and for the distribution of subgap states (Urbach tail) which strongly influences the achievable open circuit voltage. We will discuss the influence of non-classical hydrogene bridges and specific interactions such as sulfur-nitrogene attractions on self-organization processes. It turns out that these partly Coulombic interactions are essential for the desired growth mode.
We will summarize the state of the art of organic vacuum deposited photovoltaics reaching currently up to 13.2% efficiency and discuss the potential for further breakthroughs. Finally we report on a new small molecule absorber that combines fill factors above 70%, a very steep absorption edge, a low Urbach energy (22meV) and optimum energetic alignment to fullerene C₆₀ with excellent processability at high deposition rates.

Blended junctions are indispensable for organic solar cells. However, the fabrication of electron and hole transport routes in blended junction remains quite challenging. In this work, we proposed a novel concept of the junction structure of organic solar cell, namely, a lateral alternating multilayered junction [1]. An essential point is that the photogenerated holes and electrons are laterally transported and extracted to the respective electrodes.
Minimum units of proposed junction are hole highway and electron highway. At first, the lateral extraction of photogenerated holes and electrons of the order of 1 mm in the hole and electron highways using ultra-high mobility organic films was demonstrated. Observed macroscopic value of milli-meter order is surprising long compared to the conventional value below 1 mm. Next, the successful operation of organic solar cell having a lateral alternating multilayered junction by combining the hole highway and electron highway was demonstrated. A total of 93% of the photogenerated electrons and holes are laterally collected over a surprising long distance (0.14 mm). The exciton-collection efficiency reaches 75% in a lateral alternating multilayered junction with a layer thickness of 10 nm. Therefore, the lateral junction is proved to have an ability to collect both excitons and carriers almost completely.
A lateral alternating multilayered junction can be regarded to be an alternative blended junction for organic solar cells. Advantage of a lateral cell is its unlimited thickness in the vertical direction. Therefore, tandem solar cells that can utilize the full solar spectrum can be freely designed.

[1] M. Kikuchi, S. Izawa, M. Hiramoto et al., ACS Appl. Energy Mater., 2, 2087 (2019).

Organic Solar Cells (OSCs) have been gaining more and more momentum, starting to exhibit efficiencies close to the ones of their inorganic and perovskite counterparts, with recent achievements of certified Power Conversion Efficiency (PCE) above 17% for single-junction devices. The great flexibility in tailoring organic molecules, allowed to design systems that achieve strong absorption with External Quantum Efficiency (EQE) of 80% over whole visible spectrum, while still maintaining efficient charge generation with Fill Factor (FF) above 75%. On the other hand, the open-circuit voltage is still much lower as compared to the most efficient technologies, mainly due to large nonradiative losses, which were identified as intrinsic to devices based on organic molecules. The origin of those losses was proposed to be mainly due to electronic and molecular parameters of the donor and the acceptor molecules.
To study the effect of molecular parameters on nonradiative losses, we built Planar-Heterojunction (PHJ) devices carefully choosing material systems with identical Charge Transfer (CT) state energies while providing different Open-Circuit Voltages (V_oc). Moreover, by keeping the optical gap constant thanks to red-absorbing SubNc donor, driving force for all material systems is kept the same. This allows to exclude several mechanisms influencing the overall nonradiative losses and focus only on molecular parameters such as reorganization energies and static dipole moment.
Calculated nonradiative voltage losses are indeed larger for the material systems with smaller V_oc, varying by about 100 mV. Subsequent Transient Photovoltage (TPV) measurement further confirm those results, indicating faster decay of free charges in systems with larger voltage losses. To make sure that the TPV is probing bulk recombination kinetics we make use of the novel Transient Photocharge (TPQ) technique, which allows to reliably measure the recombination lifetime of the charge carriers in the bulk.

Controlled doping of semiconductors is crucial for the functionality and efficiency of modern (opto-) electronic devices. For organic semiconductors, the use of strong molecular electron donors and acceptors is meanwhile established as viable approach for doping. However, diffusion and poor thermal stability of most presently used dopants poses a challenge in applications. It turns out that the salt of the mesitylene-borinium cation (Mes₂B⁺) and tetrakis(penta-fluorophenyl)borate anion [B(C₆F₅)₄]^– is a superior p-type dopant for polymer semiconductors, and doping occurs via a charge-exchange reaction. Remarkably, the [B(C₆F₅)₄]^– anion enables the stabilization of polarons and bipolarons in poly(3-hexylthiophene), and the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]^– is estimated to be 5.9 eV. The comparably high molecular weight and bulkiness of [B(C₆F₅)₄]^– is expected to reduce diffusion, and no negative impact on doped polymer conductivity is observed up to 100 °C.
Notably, many of the molecular dopants used for organic semiconductors can also be employed for doping of transition metal dichalcogenides (TMDCs). When thinned to the monolayer, many are direct semiconductors, e.g., MoS₂ and WS₂, with superior optical properties. Due to the pronounced excitonic nature of TMDCs, their exciton energy and band gap depend strongly on the dielectric constant (ε) of the mechanical support used for the monolayer. Moreover, the electrical nature of the substrate plays a critical role for the mechanism by which doping with molecules occurs. Three fundamentally different charge transfer mechanisms between MoS₂ and molecular dopants have been identified and will be discussed.
Finally, the impact of residual water in conductive polymer films on their work function will receive attention. The high ε of water reduces the polymer's work function by screening of local charge transfer induced dipoles, and the work function varies with temperature as does ε of water. This effect should thus be considered when device characterization includes temperature variation.

Molecular n-dopants can play useful roles in controlling conductivity and charge injection/extraction in organic semiconductors, in lowering the work functions (WFs) of electrode materials, and in controlling the properties of low-dimensional materials such as graphene. However, even molecules that can n-dope C₆₀, which has a fairly high electron affinity of ca. 4.0 eV, through simple one-electron transfer are inevitably sensitive to air and water, while stronger one-electron reductants are even more sensitive. Coupling bond breaking and/or formation to electron transfer provides a way of circumventing this issue. One such approach is based on the dimers, D₂, formed by certain odd-electron species, D, formed on alkali-metal reduction of the corresponding cations, D⁺. Another class of relatively stable n-dopant molecules, DH, can be formally regarded as hydride-reduced derivatives of D⁺ cations. We have synthesized a range of both organic and organometallic D₂ and DH derivatives, in particular, derivatives of 2-substituted-1,3-dimethylbenzimidazolium cations (Y-DMBI⁺) , and of 18-electron cationic sandwich compounds such as 1,2,3,4,5-pentamethylrhodocenium and ruthenium pentamethylcyclopentadienyl mesitylene cations. This talk will describe the diversity of structures and reactivity that we have obtained, and will compare the advantages and disadvantages of the organic and organometallic dopants, and of the D₂ and DH approaches, from the point of view of synthesis, stability, dopant strength, kinetics, dopant ion size and shape, and the extent to which these properties can be tuned.

Organic semiconductors systems offer many new exciting device applications with attractive features such as flexibility, low cost, low-temperature processing etc. One key difference to inorganic semiconductors is the usually quite small dielectric constant of organics, leading to much stronger Coulomb interactions. For instance, the exciton binding energy in organic semiconductors is significantly greater than the room temperature thermal energy, thus having profound influence on the optical properties. In this talk, I will discuss the influence of these strong Coulomb interactions on two phenomena, both related to blends of organic materials: First, I will discuss controlled electrical doping where the strong Coulomb interaction might be a curse and should, at least in simple hydrogen-like model, make efficient doping unlikely. I will discuss recent results which clarify that doping is indeed efficient, mainly because of disorder effects. Second, I will discuss how the Coulomb interaction can be a blessing by using it for “band structure engineering” of organic semiconductors: Long range Coulomb interactions allow to continuously tune the energy levels, allowing finely adjustable energy levels without synthesis of new molecules. The electrostatic nature of these phenomena is directly proven by experiments which show the strong influence of these quadrupole effects at interfaces. Finally, I will discuss recent results which show that the energy gap of organic semiconductors can be tuned in a similar way.

The development of multifunctional devices capable to respond to multiple and independent stimuli is one among the grand challenges in organic electronics. The combination of multiple components, with each one conferring a specific function to the ensemble, is a facile strategy to impart a multifunctional nature to electronic devices. The controlled combination of such components and their integration in real devices can be achieved by mastering the supramolecular approach.
In my lecture I will review our recent works on the combination of carbon-based nanomaterials, in particular comprising organic semiconductors, with photochromic molecules (diarylethenes or azobenzenes) in order to fabricate smart, high-performing and light-sensitive (opto)electronic devices such as field-effect transistors and light-emitting transistors and as well as flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels.

References
[1] For reviews see: (a) X. Zhang, L. Hou, P. Samorì, Nat. Commun. 2016, 7, 11118. (b) E. Orgiu, P. Samorì, Adv. Mater. 2014, 26, 1827-1845.
[2] For modulating charge injection at metal-organic interface with a chemisorbed photochromic SAM see: (a) N. Crivillers, E. Orgiu, F. Reinders, M. Mayor, P. Samorì, Adv. Mater. 2011, 23, 1447-1452. (b) T. Mosciatti, M.G. del Rosso, M. Herder, J. Frisch, N. Koch, S. Hecht, E. Orgiu, P. Samorì, Adv. Mater. 2016, 28, 6606.
[3] For hybrid structure combining organic semiconductors blended with Au nanoparticles coated with a photochromic SAM see: C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor, P. Samorì, Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 12375-12380.
[4] For blends energy level phototuning in a photochromic - organic semiconductor blend see: (a) E. Orgiu, N. Crivillers, M. Herder, L. Grubert, M. Pätzel, J. Frisch, E. Pavlica, G. Bratina, N. Koch, S. Hecht, and P. Samorì, Nat. Chem. 2012, 4, 675-679. (b) M. El Gemayel, K. Börjesson, M. Herder, D.T. Duong, J.A. Hutchison, C. Ruzié, G. Schweicher, A. Salleo, Y. Geerts, S. Hecht, E. Orgiu, P. Samorì, Nat. Commun. 2015, 6, 6330.
[5] For the fabrication of memory devices: T. Leydecker, M. Herder, E. Pavlica, G. Bratina, S. Hecht, E. Orgiu, P. Samorì, Nat. Nanotech. 2016, 11, 769–775.
[6] For the novel nanomesh scaffold based photodetector: (a) L. Zhang, X. Zhong, E. Pavlica, S. Li, A. Klekachev, G. Bratina, T.W. Ebbesen, E. Orgiu, P. Samorì, Nat. Nanotech. 2016, 11, 900–906. (b) L. Zhang, N. Pasthukova, Y. Yao, X. Zhong, E. Pavlica, G. Bratina, E. Orgiu, P. Samorì, Adv. Mater. 2018, 30, 1801181.
[7] For optically switchable light-emitting trasnsistors: L. Hou, X. Zhang, G. F. Cotella, G. Carnicella, M. Herder, B. M. Schmidt, M. Pätzel, S. Hecht, F. Cacialli, P. Samorì, Nat. Nanotechnol., 2019, 14, 347–353.

In this talk, I will overview some applications of Non-adiabatic EXcited-state Molecular Dynamics (NEXMD) framework recently developed at LANL. The NEXMD code is able to simulate tens of picoseconds photoinduced dynamics in large molecular systems with hundreds of atoms in size and dense manifold of interacting and crossing excited states. In particular, I will exemplify ultrafast excitonic dynamics guided by intermolecular conical intersections (CoIns). Both simulations and time-resolved two-dimensional electronic spectroscopy track the coherent motion of a vibronic wave packet passing through CoIns within 40 fs, a process that governs the ultrafast energy transfer dynamics in molecular aggregates. Our results suggest that intermolecular CoIns may effectively steer energy pathways in functional nanostructures. Observed relationships between spatial extent/properties of electronic wavefunctions and resulting electronic functionalities allow us to understand and potentially manipulate excited state dynamics and energy transfer pathways toward optoelectronic applications.

Charge transfer (CT) states at the interface between electron-donating (D) and electron-accepting (A) materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive, or generate free carriers at high yield. The former can result in rather efficient organic light emitting diodes, via thermally activated delayed fluorescence, while the latter property is exploited in organic photovoltaic (OPV) devices. In this talk, I will show that proper control of CT state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D–A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination.

Compared with inorganic or perovskite photovoltaics, the key limiting factor for organic solar cells (OSCs) is large voltage loss, which is usually over 0.7 V. A significant contribution of the large voltage loss in OSCs is due to strong non-radiative recombination, the origin of which has been puzzling the community for a long time, limiting rational design of materials to overcome this critical issue. We systematically investigate a range of low voltage loss systems, where the voltage loss is around or below 0.6 V. We find that the quantum efficiency of electroluminescence in these systems have been significantly increased, leading to decreased non-radiative voltage losses below 0.20 V. Combining spectroscopic and quantum chemistry approaches, we formulate the key rules for minimizing voltage losses: (i) low energy offset between donor and acceptor molecular states, and (ii) high photoluminescence yield of the low-gap material in the blend. We further demonstrate that there is no intrinsic limit for efficient charge separation in OSCs with small voltage losses.