Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excite state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells is raises the maximum theoretical efficiency of a solar cell form the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials in to functional solar cells, although demonstrating modest efficiency enhancements, have had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult. Therefore we were interested in developing a new range of SF materials that could be solution processed, promoted SF in the solid state, and were compatible with deposition methods used in printed organic solar cells.
We reasoned that materials designed to promote intra-molecular SF [1] would allow us greater control over the SF process. In addition greater control over the pre-organisation of the chromophores would enhance SF yields. Design criteria outlined by Busby et al. [1] suggested an Acceptor-Donor-Acceptor (A-D-A) structure may promote SF, and control of the coupling between the acceptors (triplet hosts), and the donor could mdoify SF yield.
In our own work we have been examining A-D-A chromophores, such as BTR or BQR [2], as p-type organic semiconductors in organic solar cells. However, calculations indicated that the triplet energy levels in these materials were too high. Of interest to this study was that BTR and BQR have a rich phase space with high temperature nematic liquid crystalline phases. Could we use the self-organisation inherent if the BTR or BQR structure to promote self organisation in new SF A-D-A chromophores? In our preliminary studies we coupled the benzodithiophene (BDT) core in BTR or BQR with thiophene substituted diketopyrrolopyrrole (TDPP), which has a known triplet energy of around 1.0 eV. The new material BDT(TDPP)2has a singlet energy level of 1.88 eV and a measured triplet energy of 0.95 eV, which is on the borderline for the energy requirement for SF, that is E(S1) ≥ 2xE(T1).[3] We reported that BDT(TDPP)2 shows modest SF in the solid state, perhaps due to the low singlet energy level.
With BDT(TDPP)₂ there was no evidence self assembly, and the singlet energy level was low. We needed a SF host, with i) a singlet energy around 2.0 eV, ii) stronger self-association through the core, and iii) solution processability. We have already described such a material where we had coupled TDPP to a fluorenyl substituted hexabenzocornene core (FHBC) as a p-type organic semiconductor.[4] The discotic liquid crystalline FHBC(TDPP)2material forms hexagonally packed columns and has a singlet energy level of 2.00 eV, however we did not report it's triplet energy. We report here our SF studies on FHBC(TDPP)2 and demonstrate that a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.
 

Flash photolysis, time-resolved microwave conductivity (fp-TRMC) is an ideal spectroscopic tool for the detection of free, mobile charges in molecular and polymeric thin films, where the inherent sensitivity enables the detection of charge carriers that have extremely low carrier mobilities. This presentation will examine the dissociation of triplet states into separated carriers using low concentrations of acceptor molecules dispersed into the thin films. An investigation of the dependence of driving force on the yield of free carriers generated from triplet states produced from a singlet fission process in thin films of pentacene. The yield from these studies suggests an optimum driving force consistent with a Marcus formulation of photo-induced electron transfer, although the peak yield appears lower than expected.

Organic materials continue to make an impact on a wide variety of optoelectronic applications due to an ever-increasing chemical design space, novel tools to assemble these interesting materials, and the advancement of novel device architectures and devices. One important aspect thereby is how to achieve assembly not only on the molecular- and on the meso-scale, but also on the hundreds of nanometer to micrometer scale. Various devices and applications, indeed, require the active material to be patterned and/or to be deposited at pre-determined locations. In areas, such as the bioelectronics field, step-changes may be achieved when more complex, multidimensional architectures that mimic the hierarchical structures of, e.g., tissues or bones could be man-made using stimuli-responsive functional organic materials. Other potential lies in nano- and micro-engineering multifaceted structures to realize new media with unique interactions with electromagnetic radiation. This would lead to new possibilities to harvest light and manipulate light-matter interactions. Here we use model systems to demonstrate hierarchical assembly of organic nanoparticles, covering a range of systems, from inert polystyrene particles to conjugated polymer emitter particles made, e.g., of polyfluorene-co-divinylbenzene (F8DVB)[1]. We show that surface relief structures can be used to direct this process. Thereby, simple geometrical relationships can be employed to program the particles to deposit into specific sites and patterns: from ordered to disordered arrangements; hexagonally-packed, square-packed or random-packed structures; to single layer vs. multilayer architectures. This opens a versatile design platform in terms of the fabrication of multifunctional nano- and microstructures with hierarchies for use in the field of photonics (e.g., in solar cells, light-emitting diodes and optical display devices over large areas), bioelectronics and beyond.

In this research, we developed organic bioelectronic interfaces based on highly crystalline poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films or microfibers to overcome the trade-off between electrical/electrochemical performance and aqueous stability. Crystalline PEDOT:PSS films exhibits excellent electrical/electrochemical/optical characteristics, long-term aqueous stability without film dissolution/delamination, and good viability for primarily cultured cardiomyocytes and neurons over several weeks. Furthermore, the consequent cellular electrodes based on crystalline PEDOT:PSS films show good cell/tissue adhesion, dramatically enlarged surface areas, and electrochemical capacitance, which are successfully employed for PEDOT:PSS-based multi-electrode arrays (MEAs) to record and stimulate the activities of primarily cultured cardiomyocytes and chicken retinae tissues. In parallel, we developed that crystalline PEDOT:PSS microfibers with high electrical conductivity and aqueous stability can be formed using strong acid-based coagulation media and employed for single-strand wearable sweat sensors in the configuration of organic electrochemical transistor and volumetric ion storage devices with arbitrary 3-D shapes. Finally, we demonstrated that electrochemically active PEDOT:PSS can be incorporated into hydrogel microfibers, which can be beneficial for 3-D neuronal cell cultures.      

Mixed-conductors are materials that conduct both electrons and ions, thereby allowing transduction between biological signals and electronics. Conjugated polymers (CP) have the potential to be high-performing mixed-conductors for bioelectronics due to their redox chemistry, flexibility, processability and biocompatibility.¹ Previous work showed increased electrochemical oxidation in regio-regular poly(3-hexylthiophene) (P3HT) by OH-functionalization of the side chains, underlining the importance of hydrophilicity and ion mobility in mixed-conduction materials.²

This work demonstrates faster electrochemical doping in conjugated polymers by blending with hydrophilic components and their application in organic electrochemical transistors (OECTs). Poly(3-hydroxyhexylthiophene) (P3HHT) and random copolymer P3HT-co-P3HHT are blended with hydrophilic block copolymers (BCPs) of P3HT and polyethylene oxide (P3HT-b-PEO) where the PEO fraction is varied between 1 and 20 kg/mol. Electrochromic spectroscopy and electrochemical impedance spectroscopy are used to investigate the effect of blending the CP with BCPs on the doping kinetics and on the capacitance in thin films, respectively. Addition of hydrophilic BCPs induces higher doping rates and lowers the impedance with respect to neat CPs promoting ion penetration in the bulk of the thin film. Finally, the blends are implemented in OECTs to show the effect on mixed conduction properties. These results show how ion mobility in films of electroactive CP can be modulated by enhancing hydrophilicity through side-chain engineering and blending with hydrophilic components. Both strategies are viable and compatible to improve ionic conductivity in mixed-conductors for bioelectronics.

Over the last decade, organic solar cells (OSCs) have become a promising technology for next generation solar cells combining novel properties such as light weight, flexibility, or color design with large-scale manufacturing with low environmental impact. However, the main challenge for OSC will be the transfer from lab-scale processes to large-area industrial solar cell fabrication. High efficiencies in the field of OSCs are mainly achieved for devices fabricated under inert atmosphere using small active areas, typically below 0.2 cm². So far, a small lab scale devices have now reached performances above 17% [1].

In this light, inkjet printed organic solar cells and modules with large area were demonstrated. Inkjet printing allows direct patterning of four layers, including the top electrode, offering full freedom of design without the use of masks or structuring by hardware. Inkjet printed large area (>1 cm²) organic solar cells with power conversion efficiency exceeding 6.5 % deposited from environmentally friendly solvents in an air atmosphere are demonstrated using the same printer. To prove the great advantage of inkjet printing as a digital technology allowing freedom of forms and designs, large area organic modules with different artistic shapes were demonstrated
keeping high performance.

The good module performance at low illumination make our OPV modules good candidates for indoor applications, field in full expansion thanks to the Internet of Things (IoT).

Reported results confirm that inkjet printing has high potential for the processing of OPV, allowing quick changes in design as well as the materials.

[1] L. Meng et al., Organic and solution-processed tandem solar cells with 17.3% efficiency, Science 10.1126/science.aat2612 (2018)

In this presentation we report the development of novel semiconductors, as well as the process engineering, for flexible circuits. In particular we show that “ultra-soft” polymers comprising NDI units co-polymerized with “rigid” and “flexible” organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. Furthermore, by fabricating polymer/polymer blends by shear techniques, it provides a new avenue to enhance charge transport and achieve excellent mechanical robustness, which is further increase by modification of the film morphology. Furthermore, we report new “soft” polymer-metal oxide alloys, where the insulating polymer can promote formation of semiconducting metal oxide amorphous phases but with improved charge carrier mobility. By selecting polymers containing a different degree of amine nitrogen content, the charge transport of these blends can be manipulated to a greater extent. Again, these materials, and the corresponding electronic building blocks, can sustain far larger stresses than those based on pure metal oxide matrices. We demonstrate that these materials can enable TFT-based circuits for ultra-flexible displays and sensors on plastics. Finally, new oxide-polymer blends can be used to fabricate high-rectification and stretchable diodes via the self-assembly/phase separation properties of polymers having different surface energy.

Organic electronics, based on semiconducting and conducting polymers, have been extensively investigated in the past decades and have found commercial applications in lighting panels, smartphone and TV screens using OLEDs (organic light emitting diodes) technology. Many other applications are foreseen to reach the commercial maturity in future in areas such as transistors, sensors and photovoltaics.

Organic electronic devices, apart from consumer applications, are paving the path for key applications at the interface between electronics and biology, such as in polymer electrodes for recording and stimulating neural activity in neurological diseases. In such applications, organic polymers are very attractive candidates due to their distinct property of mixed conduction: the ability to transport both electron/holes and ionic species. Additionally, conducting polymers offer the possibility to tune their surface properties (e.g., wettability or chemical reactivity) by changing their oxidation state, thus promoting or hindering the adhesion of biomolecules. This feature can be particularly useful for enhancing the biocompatibility of implantable electrodes.

My talk will deal with processing and characterization of conducting polymer films and devices for flexible, stretchable and healable electronics as well as for implantable electrodes. I will particularly focus on micro-patterning of conducting polymer films for flexible and stretchable devices and on self-healing of conducting polymer films [1, 2, 3].

Our group fabricated water-stable and flexible organic electrochemical transistors based on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) on a plastic substrate using a new process based on a fluorinated photoresist. The PEDOT:PSS films, mixed solely with a biocompatible conductivity enhancer, show robust adhesion on plastic substrates, and exhibit unchanged electrical properties under extreme bending. This simplifies the fabrication of high-performance OECTs and places them in a highly competitive position for flexible electronics and healthcare applications.

The fabrication of stretchable electronic devices is presently rather challenging due to both the limited number of materials showing the desired combination of mechanical and electrical properties and the lack of techniques to process and pattern them.  My group reported on transfer patterning process to fabricate high-resolution metal microelectrodes on polydimethylsiloxane (PDMS) by using ultrathin Parylene films (2 μm thick). By combining transfer patterning of metal electrodes with orthogonal patterning of PEDOT:PSS on a pre-stretched PDMS substrate and a biocompatible “cut and paste” hydrogel, we demonstrated fully stretchable organic electrochemical transistors, relevant for wearable electronics, biosensors and surface electrodes to monitor body conditions.

Self-healing electronic materials are highly relevant for application in biology and sustainable electronics. We observed mechanical and electrical healability of PEDOT:PSS thin films. Upon reaching a certain thickness (about 1 µm), PEDOT:PSS thin films damaged with a sharp blade can be healed by simply wetting the damaged area with water. The process is rapid, with a response time on the order of 150 ms. Significantly, after being wetted, the films are transformed into autonomic self-healing materials without the need of external stimulation. This work reveals a new property of PEDOT:PSS and enables its immediate use in flexible and biocompatible electronics, such as electronic skin and bio-implanted electronics, placing conducting polymers on the front line for healing applications in bioelectronics.

While the demonstration of the first OLED emitting thermally activated delayed fluorescence (TADF) in Advanced Materials in 2009 by our group gave a glimpse of TADF's potential, our successive report in Nature in 2012 of a TADF-based OLED with an internal quantum efficiency of nearly 100% grabbed the attention of researchers world-wide and established TADF as a truly promising technology. After the quick review of recent progress on TADF OLEDs with RGB and NIR emission, we will introduce our recent efforts to inject high current density, reduce rolloff related to Joule heating and various exciton annihilation processes and control excited-state absorption for the realization of electrically pumped organic semiconductor laser diodes. By managing these issues, we report low-threshold surface-emitting organic distributed feedback lasers operating under continuous-wave photoexcitation. Further, we mention future current driven organic semiconductor laser diodes (OSLDs) by engineering fluorescence-based emitters with the combination of DFB structures that can well confine charge carriers, exciton and light simultaneously.

Efficient triplet exciton diffusion in amorphous solid films is essential for triplet–triplet annihilation (TTA) and TTA-mediated photon upconversion (UC). The detailed TTA-UC energy scheme is provided in Fig. 1. 

Fig. 1.  TTA-UC energy scheme. ISC - intersystem crossing, TET - triplet energy transfer, TTA - triplet-triplet annihilation, UC - upconversion.

Generally, poor triplet diffusion is believed to cause low UC quantum yield (< 3%) of rigid solid UC systems typically consisting of sensitizer and emitter molecules dispersed in a polymer matrix.[1] Since the diffusion relies on short-range Dexter-type energy transfer, UC systems containing high concentrations of chromophores or ultimately matrix-free systems are preferred. However, our study on high-emitter-content UC films shows that enhanced triplet diffusion can severely reduce TTA-UC performance in the presence of triplet quenching sites, which can be related to unintentionally introduced impurities or some other defects.[2] Evaluation of UC temperature dynamics and triplet diffusivity of matrix-free solid UC systems based on rationally-designed bisfluorene-anthracene (BFA) emitters and standard octaethylporphyrin (PtOEP) sensitizer revealed that the diffusion-facilitated triplet quenching constitutes major energy losses. Even above glass transition temperature of BFA/PtOEP films, whereby translational molecular motions are sufficient to promote TTA, these losses outcompete TTA resulting in degraded overall UC performance. Elimination of the triplet quenching channel is estimated to boost UC quantum yield in the amorphous BFA/PtOEP films well above 7% at room temperature. The current study implies that the UC performance of a particular matrix-free UC system with the triplet quenchers present can be optimized by slightly increasing the intermolecular separation. This is expected to restrict the access of the triplets to the quenching sites while enabling the domination of TTA.

Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around these single bonds, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers. Organic electrochemical transistors (OECTs) have been shown to be promising devices for amplification of electrical signals and selective sensing of ions and biologically important molecules in an aqueous environment, and thus have potential to be utilised in bioelectronic applications. The sensitivity, selectivity and intensity of the response of this device is determined by the organic semiconducting polymer employed as the active layer. This work presents the design of new organic semiconducting materials which demonstrate significant improvements in OECT performance, through operation in accumulation mode, with high transconductance and low operating voltage.

Triplet states are commonly observed in most optoelectronic devices that rely on conjugated organic small molecules or polymers. Previously, triplets have often been considered a loss mechanism, but the properties of triplets are now being manipulated in order to achieve beneficial photophysical pathways in conjugated small molecules and polymers, such as photon upconversion, singlet fission, and thermally-activated delayed fluorescence. Despite these recent advances, the properties of conjugated polymer triplets are significantly less well-understood than their singlet counterparts, owing largely to the fact that triplet states are not directly produced upon photoexcitation. As such, key parameters such as triplet absorption cross-sections, quantum yields, population densities, and generation dynamics are much harder to probe accurately. It is therefore vital to establish a greater fundamental understanding of triplet behaviour in such polymers.

In this talk I will cover a couple of examples where the triplet energy levels are manipulated in organic materials and show the consequences of this on the spectroscopy and photophysics. One polymer, for example, has a reduced exchange energy by introducing a large degree of orthogonality to the electron-withdawing moiety, which has the effect of producing a suprisingly high yield of charges, even in solution. Additional examples are a porphyrin-F8BT hybrid polymer in which a dual energy transfer mechanism is active[1], and a small molecule DPP system that, when blended with fullerene, undergoes ultra-fast spin-mixing in a charge transfer state[2].    

The importance of studying biological systems in 3D as opposed to 2D is now clear. The difficulty lies with standard biological techniques and assays that are unable to adapt to 3D formats. Polymeric electroactive materials and devices can bridge the gap between hard inflexible materials used for physical transducers and soft, compliant biological tissues. In this presentation, I will discuss our recent progress in adapting conducting polymer devices, including simple electrodes and transistors, to integrate with 3D cell models. We go further, by generating 3D electroactive scaffolds capable of hosting and monitoring cells. Alongside the monitoring we attempt to add to the repertoire of tissue engineers by integrating electrical cues alongside the biochemical and mechanical cues. Electrical cues have a demonstrated role in development, not just for electrogenic tissues but for all tissues. To enable the trifecta of stimuli necessary for recreating tissues in vitro, we have generated conducting polymer scaffolds blended with biopolymers such as collagen. I will show evidence that these structures can not only monitor tissue formation but impact differentiation of the cells on the structures. Zooming in at the subcellular level I will also talk about our recent work on biomimetic lipid membranes integrated with organic electronic devices to readout out transmembrane protein function.

Organic electronic materials exhibit an array of desired characteristics making them excellent as the signal translator across the gap between biology and technology. These biocompatible materials, often complexed with polyelectrolytes and other functional materials, can be included in device structures, which are flexible, stretchable and even gelled, and can also process electronic, ionic and charged biomolecules in combination. This makes the organic electronic materials unique in several respects to record and regulate functions and physiology of biological systems.

Here, a short review of some of the recent progresses from the Laboratory of Organic Electronics is given. In the BioComLab effort, a body area network is used to “connect” electronic skin patches with drug delivery components. This system provides a feedback system, also connected to the cloud for future healthcare. Sensors, converting biochemical signals into electric ones, are typically built up from organic electrochemical transistors and selectivity is typically provided from receptor mediation and oxidase approaches. Conversely, the organic electronic ion pump, converts an electronic addressing signal into the delivery of specific biomolecules, such as a neurotransmitter, to actuate and control functions of for instance the neuronal system. With the BioComLab technology the wide array of neuronal disorders and diseases are targeted, such as epilepsy, Parkinson’s disease and chronical pain.

In the e-Plant effort, the BioComLab technology is applied to the plant kingdom to record and impact the signaling pathways of phytohormones, thus allowing us to regulate the growth and expression of specific components of flowers and trees. Further, organic electronic materials can also be applied from aqueous solution directly into the biological system, thus enabling a unique approach to manufacture devices and electrodes in vivo. We are currently exploring this in vivo-manufacturing concept in several settings to define devices and circuits in various plants, to generate a seamless interface between Organic Bioelectronics and biological systems, in general.

While mixed conduction polymer materials demonstrate a wide range of potential applications, understanding of the connection between morphology, structure and ion transport in these materials is quite limited. Herein we present a computationally driven study of two polythiophene derivatives with oligoethylene glycol side chains, investigating the effects of morphology and monomer structure on ionic conduction. Two repeat unit structures were synthesized, one with an oxygen atom directly conjugated to the polythiophene backbone (P3EGT), and one with a methylene spacer between the initial oxygen atom and the polythiophene backbone (P3MEGT). Using molecular dynamics (MD) simulations, we demonstrate that amorphous P3MEGT has a higher ionic conduction than P3EGT, whereas P3EGT has the higher crystalline conduction. The lower crystalline conduction in P3MEGT is due to ion caging, a feature not present in P3EGT. To investigate their structural evolution with Li⁺ introduction, the polymers were blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and studied using grazing incidence wide angle X-ray scattering. From this, it was determined that introduction of LiTFSI results in both a reduction in crystallinity and expansion of the side-chain stacking direction. Further, it is shown that LiTFSI is present in both the crystalline and amorphous regions at low loadings, though the crystalline region saturates at high loadings. The ionic conduction was measured using electrochemical impedance spectroscopy, determining that ionic conduction occurs predominantly in the amorphous domains for both polymers. Further, the measurements show that P3MEGT has a higher ionic conduction for all conditions, a result consistent with MD simulations. By using MD simulations to augment our experimental results we have deepened our understanding of the effect of monomer structure on ionic conductivity of conjugated polymers.

The process on what controls the photogeneration of free charges in organic donor-acceptor films is still not well understood. In this presentation I shall show that the primary step of electron transfer from a photoexcited donor to the acceptor in an organic donor-acceptor type film is not suitable described by Marcus theory, an in particular independent of the driving force.[1] In contrast, the subsequent step of dissociating the thus-formed charge-transfer (CT) state strongly depends on energetics, and is assisted by delocalization. Further, I shall show that a description of the absorption and emission spectra of CT states is not consistent with both Marcus’ electron transfer theory and the original Marcus-Levich-Jortner (MLJ) theory. Instead, the inclusion of disorder effects is crucial for a suitable and consistent description[2]. Both, PL and EQE spectra of CT states can be rationalized in terms of the classic Franck-Condon picture of electronic transitions that couple to intra-molecular vibrations as well as low frequency modes of the donor-acceptor pair that forms the CT state.

Organic molecular and polymer semiconductors are usually regarded as disordered materials where weak intermolecular interactions and strong coupling to molecular and lattice vibrations drive localization of the primary charge and energy carriers in space and where transport occurs via a sequence of incoherent hopping events. In contrast, thanks to the much stronger bonding interactions and the presence of long-range order, inorganic semiconductors sustain extended electronic states and band-like transport. On the basis of recent computational studies based on a mutifacted modeling approach, we will challenge this simplified view by showcasing examples of organic semiconductors: (i) where side-chain engineering allows freezing out vibrational degrees of freedom that are detrimental to charge transport in conjugated molecular materials thereby prompting hole delocalization and a power-law behavior for the temperature-dependent carrier mobility; and (ii) where backbone rigidity yields long persistence lengths along polymer chains that, together with the presence of interchain contact points, results in high-mobility but also high-luminescent conjugated polymers. In a second part, we will turn to 3D and 2D organic-inorganic lead halide perovskites showing evidence for large polaronic and excitonic effects together with the formation of color centers associated with point defect induced traps.      

In the field of organic photovoltaics, efficiencies beyond 13% have recently been achieved by combining conjugated polymers with small-molecule non-fullerene acceptors (NFAs). Besides remarkable efficiencies, intriguing properties for these NFAs have been reported. Polymer:NFA devices typically show low open circuit voltage (V_OC) loss, which is linked to the small energy offset between the excited state of the low bandgap component and the charge transfer state. This might lead to slow charge transfer rates and it must be elucidated why photocurrent generation is still efficient. To understand what differentiates non-fullerenes from fullerenes in terms of the charge generation, we have used ultrafast transient absorption spectroscopy (TAS) on blends containing m-ITIC (NFA) with different conjugated polymers (different driving forces for charge transfer). Both the NFA and the polymers were selectively excited to differentiate hole and electron transfer processes. Moreover, to disentangle the driving force from structural/morphological effects, the ratio in the polymer:NFA blends was varied from optimal to dilute concentrations, and we have worked with bilayers. Varying the thickness of the layers, the direction of irradiation and the excitation wavelength, we could determine the exciton dissociation rates upon electron and hole transfer, and gain insight how those processes are limited by exciton diffusion.

Many organic photovoltaic devices suffer from an irreversible deterioration in performance when illuminated by 1 sun irradiation in inert atmospheres. This is typically a biphasic degradation with a rapid initial drop in the first few 10’s of hours, followed by a slower phase of degradation that continues indefinitely during irradiation. Recently, with the development of new non-fullerene acceptors (NFAs), burn-in free devices have been demonstrated and T80 lifetimes of almost 10 years have been reached.[1,2] Yet, it is not fully understood why these systems show superior stability. In this study we have systematically tested a range of benchmark NFAs in combination with several high-performance donor polymers and found that the device photostability is highly dependent on the chosen acceptor. Photoluminescence, electroluminescence, UV-vis and Raman spectroscopy were used alongside transient photovoltage measurements to investigate the degradation processes. Of the investigated NFAs, Eh-IDTBR and O-IDTBR showed the most promise with excellent photostability when used in combination with PCE10, PCE11 and PCE12. This was attributed to exceptional photochemical and morphological stability. Other devices based on ITIC and M-ITIC were much less stable and lost up to 40% of their initial performance in less than 5 days of light soaking in a nitrogen atmosphere. Chemical degradation of the acceptor and morphological evolution were both demonstrated to be contributing factors to this loss of performance. Whilst it may be possible to resolve morphological instability by modifying device fabrication procedures, photochemical stability is a more fundamental property of a material. We show that by utilising Raman spectroscopy, it may be possible to identify the degradation location on NFA molecules. This information could allow for improved molecular design and the development of more photochemically stable NFAs.

Organic and hybrid semiconductors offer many advantages for energy conversion, saving and storage applications. However, the process of separating photo-generated charge carriers in organic photovoltaics (OPV) is generally less efficient than in conventional inorganic PV technologies due to the large binding energy of the photogenerated excitons and the spatially localized charge carriers in molecular semiconductors. This has motivated intense research into the basic processes that govern charge separation in OPV devices. An increasing number of experimental and theoretical reports show a correlation between molecular vibrational modes, charge delocalization, and the separation of photogenerated excitons.  As a result, many researchers have recognised that the electrostatic band diagram of the donor-acceptor system is not sufficient to explain charge separation in OPV.

In this talk, evidence demonstrating dynamic charge separation in emerging photovoltaic devices, such as OPV and perovskite solar cells, is discussed more generally in light of recent results from the theory of open quantum systems. These theoretical results point to the need for a self-oscillating internal degree of freedom, acting as a microscopic piston in order to explain how a solar cell behaves as a heat engine, producing an inexhaustible photovoltage and photocurrent under illumination. The dynamic picture of photovoltaic energy conversion opens up new possibilities for the design and development of efficient new energy transducers.

Metal halide perovskites are in the focus of research as this material class allowed realizing photovoltaic cells (PVCs) with already over 20% energy conversion efficiency. Despite tremendous progress made in PVC performance, the fundamental understanding of material properties is rather limited. Particularly, the alignment of energy levels at interfaces within devices determines their function and efficiency, and thus needs to be known and controlled. Using angle-resolved photoemission spectroscopy, we provide a solid understanding of prototypical perovskites’ electronic band structure, and link results obtained from single crystals to electronic properties of solution-deposited thin films. This is done by contructing thin film spectra from appropriate cuts throug the Brillouin zone of single crystal data. The influence of oxygen and water exposure on perovskite surface electronic properties is unravelled, and the impact of Pb-based defects on surface properties and the level alignment in PVCs exemplified. Furthermore, it is demonstrated how molecular agents can be employed to reduce the density of defect states, which are detrimental for efficient photovoltaic cells.

Because of their promising properties as semiconducting materials for optoelectronic applications, like solar cells, three-dimensional (3D) hybrid perovskites have been in the spotlight during the last years. This is due to two main factors: low temperature and precursor solution based synthesis and impressive certified photovoltaic efficiency larger than 20%. In light of this, 3D hybrid perovskites have been claimed as the new big thing in photovoltaics^[1]. However, in spite of their photovoltaics performances, hybrid 3D perovskites still suffer from significant material stability issues, which result in the degradation of real devices^[2].

Recently, two-dimensional (2D) hybrid perovskites got the attention of the scientific community, as they show largely improved stability against water and moisture, still retaining positive photovoltaic efficiencies^[3]. Together, 2D hybrid perovskites allow for much wider flexibility in their chemical composition. In fact, while in the 3D case the size of the cation is subject to the spatial constraint imposed by the octahedral corner-shared network, in 2D perovskites this is no more the case. As result, a huge library of organic cations becomes available. With these materials, we are hence expanding the field of possibilities of hybrid halide perovskites, but a clear connection is needed, at this point, linking the choice of the organic component and its effect on the opto-electronic properties of the material itself.

We report here density functional theory (DFT) calculations on alkyl-ammonium lead iodide perovskites, to study the influence of the alkyl chain length on the electronic structure and optical properties of the material. The reciprocal role of spin-orbit coupling and electronic correlation has been adressed, highlighting important differences with respect to the 3D case^[4]. We predict a significant change in the electronic structure (opening of the band gap and higher effective masses, in particular) using long against short chains. Indeed, if the inorganic layers fix the electronic properties, the length of the organic cation is shown to have an indirect effect. In the case of long, dodecyl chains, an opening of the electronic band gap occurs, due to the influence of the supramolecular packing of the organic spacers on the structure organization of the octahedra network. In this materials, in fact, the organic chains adopt a polyethylene-like packing, causing angle distortions in the inorganic framework and leading to the observed electronic band gap opening. These theoretical results are in agreement with experimental data and demonstrate that organic saturated chains can modify the electronic properties of layered halide perosvkite seminconductors^[5].

Hybrid organic-inorganic perovskite materials, particular, the CH₃NH₃PbI₃ have attracted significant attention because of their strong absorption, high carrier mobility, long diffusion lengths, and low fabrication costs. Due to all these advantages, the certified power conversion efficiency (PCE) of perovskite solar cells (PSC) reached over 20% during last years [1]. It is also known that PSC performance demonstrate a specific dependence under varied light intensity from 10 to 1000 W/m² [2]. At the same time the problems of PSC photostability when operating under high humidity conditions are still unsolved.

A possible way to increase the performance and stability of PSC is the modification of the interface properties using surface passivation of mesoporous TiO₂-photoelectrode. The process leads to the decrease of oxygen vacancies at the TiO₂/photoelectrode interface and results in the reducing the interfacial recombination processes. The modification of the perovskite/TiO₂ interface can also impact to the optimization of interface optoelectronic structure and improve the charge transfer efficiency. For this purpose the low cost cadmium sulfide (CdS) interlayer with high electron conductivity could be introduced to the PSC structure. It should be mentioned that CdS is also an excellent hole-blocking material that is sufficient for the improvement of PSC performance.

In this paper we report the application of thin CdS layers between TiO₂-based photoelectrode and perovskite material prepared using chemical bath deposition (CBD) method. It is low cost and low temperature method, which allows to control CdS infliction by alteration of the process parameters such as reagent concentrations, temperature profile, and the deposition time. We have prepared and characterized a series of CdS/TiO₂/compact layer/FTO/glass samples, fabricated at different process parameters using home-made CBD deposition system. The structural, morphological, and optical properties of TiO₂ layers were studied before and after CdS deposition using X-ray diffraction, scanning electron microscopy, and UV-vis NIR analysis. The band alignment at CdS/TiO₂ interface was estimated using X-ray photoelectron spectroscopy. EIS measurements were also conducted. The J-V characteristics and energy conversion efficiencies of PSCs under investigation were measured at AM1.5 incident light illumination (1000 W/m²) using Abet Technologies Solar Simulator and Semiconductor Characterization System 4200-SCS (Keithley, USA). Neutral-density filters were used to perform the characterization of the PSCs at varying illumination intensities ranging from 10 to 1000 W/m². All PSC samples were fabricated under ambient conditions. The effect of the thickness of CdS interlayer on the PSC efficiency was investigated.

We have found that insertion of CdS interlayer in PSCs leads to the increase of the efficiency by around 15% and to the improvement of photostability and stability under ambient conditions. The effects were assigned to the favorable electronic structure of perovskite/CdS/TiO₂ interface and the passivation of the mesoporous TiO₂-photoelectrode surface. The PCE values of PSCs under varied light intensities (> 100 W/m²) were noticeably increased with insertion of sulfide-based interlayer (see picture below). The obtained results are of great interest because they open the opportunity to improve PSC performance under the outdoor conditions.

The last five years have witnessed remarkable progress in the field of lead halide perovskite materials and devices. Examining the existing body of literature reveals staggering inconsistencies in the reported results among different research groups with a particularly wide spread in the photovoltaic performance and stability of devices. Focusing on two model systems, namely methylammonium lead triiodide (MAPbI₃) and triple cation Cs_0.05(FA_0.83MA_0.17)_0.95Pb(Br_0.1I_0.9)₃ perovskite, we investigate how minor, likely unintentional, variations in the fabrication procedure of the perovskite layer may affect its properties and consequently those of the complete photovoltaic devices.

In the case of MAPbI₃, we demonstrate that fractional deviations in precursor stoichiometry result in significant changes to the surface composition and energetics, crystallinity, emission efficiency and the energetic disorder of the perovskite layers. These variations result in a spread of photovoltaic performance and significant disparities in device stability.[1] Additionally, layers deposited from the same precursor solution may exhibit large-scale lateral inhomogeneities in their composition and electronic structure, resulting in variations in photovoltaic performance of devices fabricated on the same substrate.[2]

In the case of triple cation perovskites, we find that it is the anti-solvent quenching step that determines the reproducibility of the fabricated devices. We also find that variations in device performance and stability are not always coupled, with devices of equal initial performance showing drastically different lifetimes depending on slight changes in the fabrication procedure.

Organic-inorganic lead halide perovskites (CH₃NH₃PbI₃) are of utmost interest for the development of cost effective and efficient next generation solar cells [1]. Perovskite family is generally adopting the ABX₃ structure, where X is an anion (Br^-, I^-, Cl^-), A (MA⁺, FA⁺, Cs⁺) and B (Pb²⁺, Sn²⁺) are cations of different sizes. The PSC’s outstanding performance is determined by strong optical absorption of the perovskite layer in a visible range of solar spectrum. However the key challenge of the PSCs is associated with their poor operational stability, mainly caused by the rapid degradation in perovskite light absorbing layer which remains sensitive to air, high temperature and humidity [2]. Thus, numerous studies have been carried out aimed to overcome the issue of the long term device performance stability, particularly concerning the ion doping of the each of A-, B- and X-sites of the system. For alkali metal doped perovskites, several studies have been reported previously [3-4]. It was reported elsewhere [5] that the alkali metal halides could affect the perovskite layer crystallinity, due to re-crystallization of the small grains and passivation of the grain boundaries.

In this study we have fabricated air-processed perovskite solar cells based on pristine and potassium-doped (K-doped) CH₃NH₃PbI₃. We have provided long term and thermal stability tests of the PSCs which have been exposed to ambient conditions in the dark and to higher temperature of 60°C. We have shown that the incorporation of the additive metal ions into perovskite structure improved either the PV performance, or stability of PSCs.

PSCs were fabricated using a common one-step spin-coating deposition method. K-doping of CH₃NH₃PbI₃ was provided by adding 0.02M KI to 1.2M perovskite precursor solution. The long-term device performance of the PSCs was investigated by periodical measurements of the J-V curves under simulated light (AM1.5G, 1000 W/m²) using Keithley 4200-SCS Semiconductor Characterization System (USA).

The structure investigations of CH₃NH₃(Pb:K)I₃ layer have shown that the alkai doping results in the rearrangement of perovskite layer morphology, reducing grain boundaries and trap states, thus, retarding surface recombination processes.

Within the aging tests the first series of both K-doped and pristine PSC samples was exposed to normal temperature and pressure conditions (NTP) in the dark with the average humidity of 50%. The second series was exposed to higher temperature of 60°C in the dark. A drop in the PCE (resulting from J_SC and FF decrease) was observed for pristine PSCs. Due to the moisture influences, perovskite layer bonds are disrupted with the formation of highly corrosive HI and PbI₂. Rapid thermal degradation is probably attributed to the Au diffusion processes in PSCs. The results have shown that the incorporation of the additive metal ions into perovskite structure has a positive effect in terms of PSCs operation stability. K-doped PSCs exhibit improved stability both in air and when exposed to higher temperatures. Resistance to degradation in the doped structure was found to be 3 times greater than that in the pristine structure.

To conclude, the incorporation of alkai metal ions can affect the formation, phase stability and charge transport characteristics of the perovskite structures. Thus, we have succeeded in the development of PSCs with optimized materials, which led to the improvement of the PV performance and stability of the devices.

Recently organic-inorganic perovskite hybrid materials have been developed for solar cell applications reaching record efficiencies of more than 23%. Furthermore, these materials allow to move from a fundamentally 3D structure using small organic building blocks to essentially 2D layered structures using larger organic building blocks. This opens an avenue towards a quite new class of organic-inorganic nano-composites in which the inorganic perovskite sheet acts as a template for the self-assembly of organic chromophores confined between the sheets of the inorganic layer. Thus the complexity of the organic interlayer in organic-inorganic hybrid perovskites can be increased by introducing additional secondary interactions between different organic components, e.g. pi-interactions. Potentially an organization can be obtained for the organic chromophores that resemble the order observed in a single crystal or by sterical constraints induced by the inorganic sheet to a different type of structures and potential properties. The number of inorganic sheets can easily be tuned by changing the stoichiometry of the organic small and large building blocks. In this way, a fluent transition of electro-optical properties can be achieved of the inorganic part from confined 2D structures to strongly delocalized quasi-3D structures.  We will present the concept and discuss the structures obtained so far. The use of carbazole ammonium salts in 2D hybrid perovskites leads to materials for solar cells with enhanced photoconductivity and stronger resistance toward moisture yielding solar cells with enhanced stability [1]. Also, the use of pyrene ammonium salts to synthesize 2D hybrid perovskites has been explored and initial results on the structure and optoelectronic properties will be discussed. Transitions between 1D and 2D structures were observed by specific modification of the stoichiometry of the components and said phase changes are observed also in function of temperature. In combination with the introduction of extra secondary interactions in the organic layer, a material is obtained with an exceptionally low bandgap. These systems are explored extensively toward structural, spectroscopic and optoelectronic properties. In this way, a new group of truly organic-inorganic hybrid materials is disclosed with possible new applications for thin film electronics.

In recent years several new classes of conjugated polymer and small molecule organic semiconductors have shown promise as materials for organic field-effect transistors. Many of these recently discovered high mobility conjugated polymers, in particular donor-acceptor copolymers, are characterised by a puzzling lack of pronounced crystalline order, while the best molecular semiconductors are highly crystalline but exhibit strong electron-phonon coupling. In this presentation I will present our current understanding of the charge transport and spin physics of these materials and of the reasons why these van-der-Waals bonded materials can exhibit such high carrier mobilities. We are also interested in investigating the thermoelectric properties of these materials and understanding of the physics that governs their Seebeck coefficients, as well as electrical and thermal conductivities. Finally, I will also give an introduction to some of the status of applications of organic FETs in large-area flexible sensors and active-matrix addressing of flexible liquid crystal and OLED displays. 

Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >23%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off.

Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on energy and charge transfer dynamics, as well as charge recombination and trapping processes happening over a time scale from fs to ms. As shown in Fig.1, when 2D perovskite is used on top of the 3D, charge transfer happens, while electron hole recombination at the perovskite/hole transporter interface is prevented. This results in improved device efficiency. In concomitance, the stable 2D perovskite is used as a sheath to physically protect the 3D underneath, with the aim to enhance the device stability. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.

Improving the performance of organic semiconductors further is crucial for next generation organic thin-film transistor (OTFTs) technologies and their application in the rapidly expanding sector of large-area electronics. Beside design and development of advanced new organic semiconductors, molecular doping represents a new powerful approach that promises to address various technical hurdles that are currently preventing OTFT commercialisation. Yet the addition of a molecular dopant into a host organic semiconductor often disrupts its microstructure, introducing defects and therefore harming long-range charge transport. Here I will discuss different doping strategies that can be used to enhance the charge carrier mobility in OTFTs based on a wide range of organic semiconductors including, small-molecules, polymers and small-molecule:polymer blends, with the latter system exhibiting record performance characteristics. Emphasis will be placed on the “all-round” positive effects that the inclusion of a suitable dopant has on key OTFT characteristics including carrier transport, contact resistance, bias-stress stability and device-to-device parameter variation.

Conducting polymers, known for  many years for their interesting electrical, optoelectronic, and mechanical properties, and good processability even on flexible substrates, are nowadays studied for their thermoelectrical properties. Among them, poly(3,4-ethylenedioxythiophene) (PEDOT) is certainly the most investigated and the most used conductive polymer and recent studies have led to high conductivity enhancements.[1] However, an exhaustive understanding of the mechanisms governing such enhancement is still lacking, hindered by the semi-crystalline nature of the material itself. In this lecture, we will present the development of highly conductive PEDOT films by controlling the crystallization of the PEDOT chains and by a subsequent dopant engineering approach using iron (III) trifluoro-methanesulfonate as oxidant, N-methyl-pyrrolidone as polymerization rate controller and organic or sulfuric acid as dopant.[2] XRD, HRTEM, Synchrotron GIWAXS analyses and conductivity measurements down to 3 K allowed us to unravel the organization, doping and transport mechanism of these highly conductive PEDOT materials. We propose a charge transport model that fully corroborates our experimental observations. Our PEDOT-based materials exhibit conductivities up to 5400 S cm^-1 and transmittance at 550 nm between 80 and 96 %. In the last part of this lecture we will discuss their thermoelectric properties and we will show their high potential for application in thermoelectric devices and all-polymeric flexible transparent heaters.[3]

Organic semiconductors are highly attractive for large-area, low-cost, lightweight, flexible and bendable electronic applications, but their transition to market place is delayed by inadequate performance. Inefficient charge injection represents a significant hurdle in the pursuit of the promised potential of organic semiconductors. This issue becomes even more severe with increasing the effective mobility of the organic layer and reducing the channel dimensions. In this talk, I will discuss the origin and characterization of contact effects in organic field-effect transistors (OFETs) and their impact on device performance and accuracy in extraction of charge carrier mobility. I will present a strategy for reducing contact resistance in small molecule and polymeric OFETs consisting of developing high work function surface domains at the surface of the injecting electrodes to promote channels of enhanced injection.[1] This led to contact resistances of 200 Ωcm and device charge carrier mobilities of 20 cm²/Vs independent of the applied gate voltage. In addition to allowing the demonstration of high-mobility transistors with near ideal current-voltage characteristics, the use of this method has lead to accurate measurement of the charge carrier mobility, a critical step in a rational material design.

In the second part of my talk, I will present a method for contact deposition and patterning that allows fabrication of all-printed OFETs on conventional paper. The method relies on depositing contacts using aerosol spray and patterning them with a digitally printed mask from an office laser printer, at ambient temperature and pressure. This technique, which we have denoted aerosol spray laser lithography, is cost-effective and extremely versatile in terms of material choice and electrode geometry. The method was successfully adopted for manufacturing different types of electrode materials, that showed an excellent tolerance to extreme bending, confirming its potential for emerging printed electronics applications.

Although significant effort has been devoted to developing high-performance mixed conductors, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) is still regarded as one of the most promising materials for this purpose due to its favorable electrical/electrochemical properties, microstructural versatility, and the relatively low production cost. Herein, we establish a correlation between the composition of PEDOT:PSS films, their degree of crystallinity, their electrochemical transistor performance, and their operational stability. We demonstrate that specific sulfuric acid treatments efficiently remove excess PSS and simultaneously induce crystallization of the PEDOT into an anisotropic ‘edge-on’ texture. Such microstructural changes lead to a significant improvement in electrochemical transistor performance (i.e., [μC^*] ~ 490 F·cm^-1V^-1s^-1) and operational stability via an improvement in electronic transport, an increase of the content of conducting polymer chain per unit volume, and a reduced swelling of the films in the aqueous electrolyte. All these results suggest that crystallized PEDOT:PSS is a promising material for future bioelectronics applications. Insights gained on the PEDOT:PSS’s strucutre/property/performance interrelations will also be key in designing the next-generation organic bioelectronic material.

Organic and organohalide perovskite solar cells and photodetectors share several common electro-optical operating principles [1]. Both families of devices operate within the thin film, low finesse cavity limit and there are also commonalities in electrodes and ancillary layer materials and structures [2]. Generally speaking, photodetectors are optimized with respect to their external quantum efficiency (Responsivity) and noise characteristics under small reverse bias voltages as a function of frequency [3], and solar cells the maximum power that can be extracted under equilibrium conditions of AM1.5G illumination. In both cases, a clear understanding of photogeneration and extraction efficiency via simulation and measurement leads to informed design and robust structure-property relationships [4].  

In my talk I will describe some of the most recent thinking in regard establishing these structure-property relationships for both organic and organohalide perovskite material systems. In particular, I will discuss how device architecture can be used to manipulate the photo-generated charge spatial profile and extraction efficiency, and explore the concept of creating narrowed spectral response via cavity enhancements and charge collection narrowing [5,6]. Finally, I will summarize new protocols recently established to solidify the emerging field of thin film photodetectors based upon a clear understanding of key electro-optical phenomena [3].  

[1] Lin et al. Nature Photonics, 9, 106-112 (2015); Lin et al. Nature Photonics, 9, 687-694 (2015);

[2] Armin et al. ACS Photonics, 1(3), 173-181 (2014);

[3] Fang, et al. Nature Photonics, doi.org/ NPHOT-2018-06-00806B (2019);

[4] Stolterfoht, et al. Nature Communications, 7, 11944 (2016);

[5] Yazmaciyan et al. Advanced Optical Materials, In Press (2019);

[6] Armin et al. Nature Communications, 6, 6343 (2015).

Transparent Conductive Electrodes (TCE), optically transparent to visible light and electrically conductive, begin to have a great industrial interest for the development optoelectronic devices, such as, liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), organic photovoltaic (OPV) cells and touch panels. Currently, Indium Tin Oxide (ITO) is the most widely used thanks to its excellent opto-electrical properties, including the low sheet resistance (R_s) (10–15 Ω/sq) and the high transparency (>85% 550 nm) [1].

To achieve the current optoelectronic devices market requirements, industrially scalable production methods of high quality electrodes with advanced functional properties such as high optical transparency and electrical conductivity is vastly demanded via low-cost approaches. Besides the functional properties (conductivity and transparency), flexibility begins to be an important criteria. This needs is even more pronounced for the new competitive applications such as stretchable electronics. They require low-cost, flexible, transparent electrodes that may be employed at large-scale with printing or roll-to-roll coatings. In this case, ITO becomes less competitive because of its high intrinsic and processing costs (Physical Vapor Deposition). New constituents have thus emerged as alternative to ITO such as metal nanowires, graphene, carbon nanotube and intrinsically conductive polymers [2, 3]. Silver nanowires (AgNWs) exhibit  functional properties comparable to that of ITO combined with a high flexibility, and the ability to be deposited by printing technologies at room temperature and air atmosphere.  They are adaptable to different substrates nature (glass or polymers) with dissimilar mechanical properties (brittle or flexible) and roughness via low-cost and scalable manufacturing techniques. Importantly, inkjet printing is a promising technique for large-scale printed flexible and stretchable electronics. However, the development of printable optoelectronics devices based on AgNWs for commercialization, still has to overcome several processing challenges.

The current work concerns organic photovoltaic on cells flexible polymer substrates, and with innovative transparent electrodes. Efficient inkjet printing of highly concentrated AgNW ink was successfully realized with large active surface (100*100 mm²). Ink formulations and printing conditions have been studied and optimized for electrical and transparence properties.

This step was essentially performed with a design of experiments. The AgNW layer has also been considered with a series of physico-chemical (UV-vis, IRTF, Raman) and morphological (OM, SEM, AFM) characterizations to define the properties/structure interrelationships and develop a more comprehensive optimization. To conclude, the optimal conditions lead to thin functional electrodes with high reproducibility.  The solar cells based on these electrodes exhibit promising power conversion efficiencies exceeding 4 %.

the doping of organic semiconductors to improve device performance is an important topic in organic electronics. Recently, we reported a new powerful p-dopant hexacyano-trimethylene-cyclopropane, CN6-CP [1]. Interestingly, its single electron reduction product CN6-CP•- has a high enough doping strength (the second reduction peak of CN6-CP peaks is at -5.1 eV) which is comparable with the strength of the state-of-the-art F4TCNQ [2]. Herein we demonstrate that highly efficient 2D (interfacial) doping of organic semiconductors, poly(3-hexylthiophene) (P3HT) and TIPS-pentacene, can be achieved by a polyelectrolyte-supported layer-by-layer assembly of the dual-mode functional dopant CN6-CP^•-K⁺, having an anionic group for its fixation onto oppositely charged surfaces/molecules as well as electron-deficient groups providing its p-doping ability [Kiriy et al., under review]. Polyelectrolyte-supported dopant layers were used to generate conductive channels at the bottom or at the top of semiconducting films. Unlike to the case of sequentially processed P3HT films doped by F4TCNQ, the use of more polar CN6-CP^•-K⁺ dopant and ultra-thin polycation separation interlayer enables predominantly interfacial kind of doping placement with no detectable intercalation of the dopant into the semiconductor bulk. The layered structure of the doped film was proved by transmission electron microscopy of the cross-section and it agrees well with other data obtained in this work. The interfacial doping enabled an impressive conductivity of 13 S/cm even for ultra-thin P3HT films. We propose to explain the superior efficiency of the interfacial doping compared to the bulk doping in terms of unperturbed morphology of the semiconductor and high mobility of charge carriers, which are spatially-separated from the dopant phase.

[1] Karpov, Y.; Erdmann, T.; Raguzin, I., Al-Hussein, M.; Binner, M.; Lappan, U.; Stamm, M.; Gerasimov, K. L.; Beryozkina, T.; Bakulev, V.; Anokhin, D. V.; Ivanov, D. A.; Günther, F.; Gemming, S.; Seifert, G.; Voit, B.; Di Pietro, R.; Kiriy, A. High Conductivity in Molecularly p‐Doped Diketopyrrolopyrrole‐Based Polymer: The Impact of a High Dopant Strength and Good Structural Order. Adv. Mater. 2016, 28, 6003-6010.

[2] Karpov, Y.; Kiriy, N.; Al-Hussein, M.; Hambsch, M.; Beryozkina, T.; Bakulev, V.; Mannsfeld, S. C. B.; Voit, B.; Kiriy, A. Hexacyano-[3]-radialene Anion-radical Salts: A Promising Family of Highly Soluble p-Dopants. Chem. Commun. 2018, 54, 307-310.

Current organic electronics research aims at exploiting the unique property matrix of “plastic” semiconductors, including their chemical tunability, straightforward processability and mechanical flexibility, to create new applications. While much knowledge has been developed in the synthesis of semiconducting conjugated organic materials, there is still an immense need for establishing broadly applicable design guidelines towards highly conductive macromolecular matter [1,2]. Moreover, the multitude of possibilities for “plastic” semiconductors and dopants to assemble over different length scales creates a daunting task to establish comprehensive and relevant correlations between structure, processing and properties. We will present here a multidisciplinary approach towards a framework to predict such structure/property interrelation in semiconducting “plastics”. We will focus on polythiophene derivatives as model systems, particularly poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) [3], and discuss how the spatial arrangement, manipulated through intercalation and co-crystal formation with dopants, affects charge transport. We will conclude with providing a tentative picture of the complex correlation of structure and electronic landscape for the understanding of conducting “plastics” of metallic-type transport.

Neuromorphic computing has attracted enormous attention in recent years due to its impressive capabilities to address the inherent limitations of conventional integrated circuit technology, ranging from perception, pattern recognition to memory and decision-making. Inspired by the morphology and function of 10¹⁴ synapses in mammalian brain, silicon-based asynchronous spiking neural networks (SNNs) have ushered in an era of developing non-volatile resistive switching as memristors for artificial intelligence systems. Despite its low power consumption, high endurance and fast read/write time, two-terminal memristor suffers from the singular function and the lack of heterosynaptic plasticity, similar to the conditioned reflex of human beings. Inspired by advanced unconditioned reflex, multi-terminal memristive transistors (memtransistors) were developed to realize complex functions, such as multi-factors modulation and heterosynaptic plasticity. Here, we report the development of hybrid memtransistors, modulable by multiple physical inputs (light, field and electric bias), using hybrid perovskite and conjugated polymer heterojunction channels processed from solution-phase at room temperature. The devices exhibit non-volatile but highly reversible conductance modulation due to I^-(Br^-) ions migration which result to the formation of a tunable Schottky barrier at the injecting electrode interface. Meanwhile, the memtransistors show excellent gate tunability of six orders of magnitudes with large switching ratios (10²), high endurance (>10² times) and long-term retention. Furthermore, photo-induced halides redistribution results in light plasticity of the polymer passivated perovskite memtransistor, which may be applicable in image identification and pattern recognition. Lastly, use of in situ scanning Kelvin probe microscopy and variable temperature measurement reveal the kinetics of the all-important bias- and photo-induced halide ions migration across the channel. Overall, the multi-physical input parameter memtransistor could enable the development of complex neuromorphic system capable of combining electrical with optical signals from the near-infrared to gamma rays.

Surface-confined supramolecular self-assembly has been the focus of extensive research in the past decade.¹ A plethora of strategies have been developed and reported for surface patterning leading to novel applications in molecular electronics, photonics and nano-mechanical devices.^{2, 3} Nonetheless, despite of these numerous examples we are still at the early stages of fully exploiting them in viable, practical technologies since the main difficulty encountered upon applying nanomaterials in functional devices is that the functionalities and properties intrinsic to these materials are greatly limited by their inability to form ordered, integrated systems.⁴

       In view of this, we propose a new molecular design approach based on the concept of ‘Nanoarchitectonics’ to demonstrate the growth of a nanoscale-range, well-ordered, fluorescent monolayer on graphene via the use of a ‘smart’ molecular building block. The design approach taken towards this ‘smart’ tecton is discussed, together with the studies of the resulting graphene-confined assemblies by scanning tunnelling microscopy (STM), and the optical properties of these fluorescent nano-surfaces by absorption and fluorescence spectroscopy.

Coordination polymers have attracted a considerable amount of interest as a new class of organic–inorganic hybrid materials, owing to their unique infinite structures and electronic states formed by the combination of metal ions with versatile coordination architectures and a variety of organic bridging ligands.  Until now, crystal engineering of coordination polymers greatly contributed to the development of functional materials, but it would be also important to develop new functional coordination polymers with amorphous structures for opto-electronic devices because of their advantages in the synthesis, control of physical properties and thin-films fabrication processes.  We developed amorphous semiconducting coordination polymers consisting of copper(I) halides and hexaazatriphenylene.  These complexes show semiconducting properties due to the small band-gaps.  We also tried to apply the coordination polymers to bulk heterojunction organic solar cells, and found these coordination polymers worked as a buffer layer of the bulk heterojunction organic solar cells as a substitute for MoO3.  In this symposium, we will present the electronic states and conducting properties of bulk samples and thin films of the amorphous semiconducting coordination polymers studied by UV-Vis-NIR spectroscopies, photoelectron spectroscopies, DC conductivity measurements and impedance measurements.  In addition, we will also report the photovoltaic properties of the organic solar cells including the amorphous semiconducting coordination polymers.