NIR optoelectronics – organic semiconductors and devices

Resume : The Suzuki-Miyaura cross-coupling reaction is highly desirable for the fabrication of π-conjugated polymers due to its wide functional group tolerance, high regioselectivity, and easy removal of low-toxicity by-products.1 However it remains underexplored as a synthetic tool for producing novel and high-performance semiconducting polymers due to the difficulty of preparing the borylated monomers in high yields and purities, and due to side reactions during the Suzuki-Miyaura polymerization, i.e., monomer protodeboronation, that leads to low molecular weights and poor yields of polymer. Robust boron protecting groups such as N-methyliminodiacetic acid (MIDA) boronate esters have proven to be effective cross-coupling partners even for borylated heterocycles highly prone to deboronation, such as thiophenes and pyridines.2 Likewise, they have demonstrated to be excellent monomers in electron-rich substrates.3 These high-fidelity boron-functionalized monomers, where available, are usually found in donor substrates, and examples with acceptor units (e.g., as trifluoroborate salts) are rare.4 Naphthalene diimides (NDI) are widely used moieties that possess high electron affinities, and are promising candidates as acceptor units for fullerene-free OPVs.5 In this contribution the synthesis of NDI-derived boron-based monomers are investigated via metal-free annulation6 of the parent NDI-bis-ethynyl-MIDA boronate. Thus, naphtho[2,3-b:6,7-b’]dithiophene-4,5,9,10-tetracarboxylic-diimide-2,7-diyl-MIDA-boronate ester, 1, is used to synthesize high molecular weight copolymers with all electron-accepting units and donor-acceptor units. Conditions for hydrolysis of the boronate ester moiety for effective transmetallation during polymerization are discussed. Reactivity of 1 is further explored with a range of electronically distinct dihalogenated comonomers. References. 1. Suzuki, A. Angew. Chem. Int. Ed., 2011, 50, 6722-6737 2. Burke, M.D., Gillis, E.P., Aldrichimica Acta., 2009, 42, 17-27 3. Ayuso Carrillo, J., Turner, M.L., Ingleson, M.J., J. Am. Chem. Soc., 2016, 138, 13361-13368 4. Lee, J.-K., et al., J. Am. Chem. Soc., 2011, 133, 9949-9951 5. Al Kobaisi, M., et al., Chem. Rev., 2016, 116, 11685-11796 6. Nakano, M., Takimiya, K., Chem. Mater., 2017, 29, 256-264

Resume : BODIPY (boron dipyrromethene) gained a lot of attention during the last years due to its outstanding structural and optoelectronic properties as its easily modifiable molecular backbone[1], large molar absorption coefficient[2], high fluorescence quantum yield[3], and excellent chemical and photochemical stability[4]. However, loss of fluorescence intensity in the solid state is limiting any application in organic electronics. A promising approach to improve the photo-physical properties of BODIPY is to initiate Aggregation-Induced Emission (AIE) behavior. This in mind, far-red/NIR emitting BODIPY-tetraphenylethylene hybrids have been synthesized. The effect of varying substituents in different positions of the BODIPY core has been investigated as well as the EL performance of these hybrids in Organic Light Emitting Diodes (OLEDs). [1] A. Loudet, K. Burgess Chem. Rev. 2007,107,4891-4932 [2] T.Rousseau, A. Cravino, T. Bura, G. Ulrich, Z. Ziessel, J.Roncali Chem. Commun. 2009, 19,2298 [3] J. Karolin, L.B.-A. Johansson, L.Strandberg, T. Ny J. Am. Chem. Soc. 1994, 116, 7801-7806 [4] G. Ulrich, R. Ziessel, A.Harriman Angew. Chem. Int. Ed. 2008, 47, 1184-1201

Resume : The threat of terrorism has put safety and surveillance forward as increasingly important aspects in our present society. Infrared image sensors can enhance our safety (feeling) by enabling night vision, thermal imaging and chemical sensing applications. To date, the so-called hybrid technology is used for these image sensors, in which both the photodetector (PD) and the electronic readout are prepared separately and then interconnected at the pixel level. Obviously, this is a time-consuming process, resulting in low throughput and thus a high cost. A monolithic approach is hence more attractive. Attempts to apply this technology to the state of the art inorganic low bandgap absorbers have, however, not been very successful because of compatibility issues. In this respect, organic semiconductors (both polymers and small molecules) are very appealing for their absorptivity and tunability. The near-infrared (NIR) regime up to 2000 nm is targeted by the use of the push-pull concept employing strongly electron rich (e.g. N-alkylated DTP) and electron deficient (e.g. benzo[1,2-c:4,5-c’]bis[1,2,5]thiadiazole, BBT) building blocks. Besides the synthesis protocols of some advanced push-pull type materials with tunable bandgaps, preliminary device specifications of NIR organic PDs will be presented.

Resume : Near-infrared photo-thermo-electric (NIR-PTE) materials convert absorbed light energy into electrical energy via thermal energy. In our previous study, we have synthesized p-type and n-type NIR active conjugated polymers derived from thienoisoindigo or quinoxalineimide backbones, which showed ultra-low energy gap (below 0.60 eV) and relatively high electrical conductivity (σ ≈ 10-2 S cm-1) [1]. In the present study, we have fabricated the NIR-PTE conversion thin-film devices using those polymers, and evaluated their NIR-PTE conversion performances in order to clarify their structure-performance relationship. The PTE devices utilizing top-contact Au electrodes were made of the drop-coated polymer thin film on grass substrate. The channel length and width between Au electrodes were 600 μm and 1000 μm, respectively. Under the NIR laser irradiation, the maximum PTE voltages were 1.4 mV and −12 mV for p and n-type conjugated polymers, respectively. The PTE voltage was dependent on the intrinsic carrier polarity of the conjugated polymers. Interestingly, with changing laser-spot position on the thin film surface, the output PTE voltage was inversely altered both in the intensity and the carrier polarity. These findings demonstrate that ultra-low energy gap polymers are promising for NIR-PTE conversion device applications. Reference [1] T. Hasegawa et al., “An ultra-narrow bandgap derived from thienoisoindigo polymers: structural influence on reducing bandgap and self-organization” Polym. Chem., 7(5), 1181-1190 (2016). (1488 characters)

Resume : Recently, development of pattern / printing technology capable of solution process has been actively carried out to produce photovoltaics, OFETs, displays, and sensors at low cost and large area. Our team first developed pen printing, a patterning technique for solution processes, and fabricated large-area, high-performance off-devices on flexible substrates and curved surfaces. Pen-printing technology is a technique for precise printing of fine solutions, enabling the development of efficient flexible OFETs. In this study, we fabricated a stable OFET arrays for solvent vapor with vertical phase separation layer of insulator / semiconductor polymer materials by applying pen printing technology. The PTAA used as the polymer semiconductor material migrates to the surface part of the substrate due to the high surface energy compared with the blended commercial polymer material PS, and the surface encapsulation is performed by the PS polymer layer existing in the upper layer. The fabricated OFETs exhibited superior operational stability over severe vapor (humidity, Ethyl alcohol) conditions compared with the performance of the previously reported devices. This study is expected to contribute to the development of low cost organic electronics which is stable against the chemical components in the atmosphere and prevents deterioration of the device performance due to the solvent generated during the solution process.

Resume : All-polymer solar cells (all-PSCs), which consist of p-type and n-type semiconducting acceptor polymers have become particularly popular in the field of PSCs due to their tuneable chemical and electronic properties as well as better stabilities. So far, many efforts have been made to develop new n-type acceptor polymers with a donor-acceptor (D-A) structure and among these promising materials, P(NDI2OD-T2) (or Polyera Activeink N2200) is the most widely utilized n-type polymer as it provides high electron mobility, excellent solubility and stability, and strong absorption in the visible range. However, it easily formed large crystals because of its high crystallinity, leading to undesired phase separation and large domain sizes, this characteristic may limit the overall photovoltaic performance. Therefore, we present a new generic approach that alleviates these restrictions and results in efficient all-PSCs. The correlation between crystallinity and the photovoltaic performance were investigated by different characterization tools such as DSC, GIWAXS, AFM, TRPL and so on. In this study, efficiency of all-PSCs over 7% is achieved, which is in the same range as the state of the art performance in all-PSCs

Resume : Organic photovoltaics (OPVs) have attracted much interest during the last decades. This is a result of their considerable amount of advantages compared to inorganic photovoltaics, such us their potential for low cost synthesis through solution processed roll to roll techniques and their production into light weight and flexible devices. [1] For the fabrication of high performance OPVs it is important to fully exploit the sun irradiation from ultraviolent (UV) to near infrared (NIR), with the design of materials exhibiting NIR absorbance being an effective way to enhance the power convention efficiency (PCE) of the solar cells. [2] In this project a number of porphyrin based small molecules owning the Acceptor-Donor-Acceptor (A-D-A) motif will be synthesized aiming to the reduction of the materials band gap and thus extension of the absorbance to the NIR region. Porphyrin based materials have been chosen due to porphyrin’s interesting characteristics, which include high absorption coefficients (ε), broad absorption in both the blue (Soret) and red (Q-bands) part of the visible spectrum and the ease of modification of its photo and electrochemistry. [3] References [1] Eva Bundgaard, Frederik C. Krebs. Low band gap polymers for organic photovoltaics. Solar Energy Materials & Solar Cells 91 (2007) 954–985 [2] Chang Liu, Kai Wang, Xiong Gong, Alan J. Heeger. Low bandgap semiconducting polymers for polymeric photovoltaics. Chem. Soc. Rev., 2016, 45, 4825 [3] Jurgen Kesters , Pieter Verstappen , Mathias Kelchtermans , Laurence Lutsen , Dirk Vanderzande, Wouter Maes. Porphyrin-Based Bulk Heterojunction Organic Photovoltaics: The Rise of the Colors of Life. Adv. Energy Mater. 2015, 5, 1500218

Resume : Photodetectors are key components in every photonic sensors like photonics, bio-signal detection in health care and internet of things, etc.1-3 To serve as the converter from the optical to the electrical signal, the photodetector should offer high responsivity and sensitivity. Avalanche photodiodes are widely used as opto-electronic converter for highly-sensitive optical receivers. For covering the spectral range from 800 nm to 1600 nm wavelength, the avalanche photodiodes are needed sophisticated layer structures and supply voltage values in excess of 20 V. Particularly, the avalanche photodiodes are not appropriate to apply in comfortable applications like wearable devices and smart phones. In addition, the operation of the photodetector should be facilitated by a compact size, a low power supply voltage and temperature insensitivity. To overcome this problem, InGaAs-baed PIN photodiodes are investigated to obtain a high quantum efficiency and low leakage current. To study the optical behaviors according to the intrinsic layer, the thicknesses of the intrinsic layer and the positive-doped layer are changed. Fig. 2 shows the peak intensities of the photoluminescence according to the thickness of PIN epitaxial structures.

Resume : Here we report the optical characterisation of a series of near-infrared (NIR) emitting porphyrin oligomers incorporated in polymer light-emitting diodes. We characterised the optical properties both in solution and thin film and the electroluminescence external quantum efficiency (EL EQE) of the oligomers in blends with the commercial conjugated polymers poly(9,9’-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4’-(N-(p-butylphenyl))diphenylamine)] (TFB), that demonstrated high EQEs up to 3.8 %. We investigated different loadings of the porphyrins in the polymer matrices and carried-out optical characterisation by means of UV-VIS absorption, time-resolved emission spectroscopy (time-correlated single photon counting) and PL efficiency measurements using an integrating sphere. We chose the best performing blends, in terms of PL efficiency, and incorporated them in Polymer Light-Emitting Diodes (PLEDs). With a carefully optimised device architecture including an Electron Blocking Layer (EBL), devices with the F8BT polymer matrix and 2.5 w/w % loading of the zinc porphyrin hexamer yielded a maximum EQE of 3.8 % with almost pure NIR emission (>95 % at λ>700 nm). This value is the highest EL EQE reported so far for a NIR heavy-metal-free "fast" emitter (τ < μs). These results prove that efficient NIR emission from PLEDs is possible without the use of toxic heavy metals, while maintaining solution processability and low-cost fabrication.

Resume : Due to their superb properties, diketopyrrolopyrrole (DPP)-containing copolymers are currently one of the most widely used active layer donor materials for high efficiency organic photovoltaics (OPV).[1] The implementation of an additional vinylene bridge between the aromatic substituents and the amide nitrogens of the DPP unit leads to an expanded diketopyrrolopyrrole based π-System (EDPP) resulting in unique optical properties.[2] Moreover the EDPP unit can be used for the formation of new conjugated ladder polymers (cLPs).[3] We have successfully synthesized novel donor-acceptor type polymers containing fluorene- and thiophene-based EDPP building blocks. All polymers show strong fluorescence with high photoluminescence quantum yields and number average molecular weights up to 20000 D. Especially cLPs that contain EDPP units are promising for an application in optoelectronic devices. References: [1] W. Li, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, Acc. Chem. Res., 2015, 49, 78 ? 85 [2] M. Grzybowski, E. Glodkowska-Mrowka. T. Stoklosa, D. T. Gryko, Org. Lett., 2012, 14 (11), 2670 ? 2673 [3] K.-J. Kass, M. Forster, U. Scherf, Angew. Chem., Int. Ed., 2016, 55, 7816?7820

Resume : In a world threatened by terrorism, ensuring safety is crucial for a well-functioning society. One way to do so is monitoring of possible threats through the use of photodetectors. More specifically, nearinfrared photodetectors (NIR-PDs) can be of high value due to their ability to detect infrared light, possibly exposing ‘invisible’ threats. Other applications of NIR-PDs range from thermal and night vision (e.g. as an accessory in smartphones) to remote control. Nowadays, NIR-PDs are mainly based on inorganic semiconductors, but the resulting devices suffer from stress-induced bump failures and limited pixel sizes. The incorporation of organic semiconductors can overcome some of these issues and offer additional advantages such as large area processing by printing, high tunability by chemical engineering and high absorption coefficients. In this contribution, the development of various electron donating copolymers will be presented, to be blended with suitable acceptors in bulk heterojunction organic photodetector (OPD) devices. To achieve NIR absorption, strong donor building blocks have to be polymerized with strong acceptor monomers to obtain a very low bandgap. To this extent, the widely used diketopyrrolopyrrole (DPP) and less well-known bay-annulated indigo (BAI) acceptor units (Figure 1) have been combined with electron rich dithieno[3,2-b;2’,3’-d]pyrrole (DTP), thieno[3,2-b]thiophene (TT) and 3,4- ethylenedioxythiophene (EDOT) monomers. Synthesis as well as preliminary device results will be presented.

Resume : Low band gap (LBG) organic materials that absorb into the near-infrared (NIR) (700-2500 nm) are of great interest in the recent years for a number of potential applications from defense to medical and energy harvesting applications. The use of NIR-absorbing or NIR photovoltaic organic materials (small molecules or polymers) could extend the material’s absorption into the NIR spectral region and even beyond 1000 nm wavelength, which in principle could enhance the current power conversion efficiency (PCE) of organic photovoltaics (OPVs). In addition, there has been also a growing interest in NIR OLEDs. NIR OLEDs are particularly interesting for night vision-readable displays which are unreadable to the naked eye but could be read with night-vision goggles, or as light source for sensors that operate with NIR light. Furthermore, the semitransparency of biological tissue between 700 and 1000 nm makes these applications appealing for a broad class of biomedical applications, and particularly imaging and sensing. Based on these considerations, we have identified the development of a new class of “metal-free” organic NIR materials based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, commonly known as BODIPY. Here we will present the synthesis of two new materials: polymer TBDPTV and the small molecules NIRBDTE Polymer TBDPTV shows a panchromatic absorption between 300 nm and 1100 nm and an optical band-gap (Egopt)of 1.15 eV, suitable for NIR organic photovoltaic applications as electron donor. A preliminary power conversion efficiency (PCE) of 1.1 % in polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) 1:3 weight ratio bulk heterojunction (BHJ) solar cells has been achieved. As regard the small molecule CC42, this shows an interesting emission between 700nm and 800 nm, demonstrating to be suitable for OLED devices. The small molecule has been then characterized in OLED device using F8BT, as host material. The OLED exhibit an electroluminescence emission (~ 65 % in NIR region) peaked at 720 nm with an EQE up to 1.1%, which is the highest value achieved so far in the world by a “metal-free” NIR OLED. References [1] Squeo, B. M.; Gasparini, N.; Ameri, T.; Palma-Cando, A.; Allard, S.; Gregoriou, V. G.; Brabec, C. J.; Scherf, U.; Chochos, C. L. J. Mater. Chem. 2015, 3, 16279–16286. [2] Zampetti, A.; Minotto, A.; Squeo, B. M.; Gregoriou, V. G.; Allard, S.; Scherf, U.; Chochos, C. L.; Cacialli, F. Sci. Reports 2017, 7, 1611.

Resume : Membrane phase transitions are crucial for the transport of materials, energy and information between the interior and exterior cell environment. However, membrane order is a delicate balance, and unsought changes in membrane permeability via phase transitions are thought to be behind a number of degenerative diseases.[1] As such, the development of optical probes that are able to accurately detect subtle changes in phase or order in model and live membranes at the nanoscale is crucial for the development of targeted diagnostic and therapeutic platforms. This study addresses the interaction of a family of red-emitting polythiophenes bearing zwitterionic, cationic or anionic terminal groups with model cell membranes prepared from the zwitterionic phospholipid dipalmitoylphosphatidylcholine (DPPC). A combination of optical, small-angle scattering and microscopy studies have been used to evaluate the binding affinity, localization and local environment of each probe within the membrane. The global data indicate three distinctive modes of interaction for the polythiophenes within DPPC vesicles, whereby the nature of the association is ultimately controlled by the pendant charged group on each CPE chain.[2] The selective localisation of the anionic polythiophene within the surface head groups of the model membrane enabled detection of subtle phase transitions (e.g. pre-transition, gel-to-liquid) using photoluminescence spectroscopy.[3] Our results present a novel approach to the selective detection of phase transition changes across bilayer membranes, demonstrating the potential application of these CPEs as biologically relevant sensors. References: 1. R. Marin, J. A. Rojo, N. Fabelo, C. E. Fernandez and M. Diaz, Neuroscience, 2013, 245, 26. 2. J. E. Houston, M. Kraft, U. Scherf and R. C. Evans, Phys. Chem. Chem. Phys, 2016, 18, 12423. 3. J. E. Houston, M. Kraft, I. Mooney, A. E. Terry, U. Scherf and R. C. Evans, Langmuir, 2016, 32, 8141.

Resume : Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and bio-integrated electronics, is hampered by the use of bulky dispersive optics. Here, we demonstrate solution- and vacuum deposited organic donor-acceptor blends, implemented in a resonant optical cavity device architecture, providing compact and wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, this cavity device exploits weak subgap absorption of intermolecular charge-transfer states. This allows to extend the wavelength detection range further into the NIR as compared to neat material absorption. We present devices exhibiting a narrowband photocurrent response down to 1650 nm, i.e. more than 500 nm longer than the optical gap of the neat donor and acceptor materials. For the best performing devices, we demonstrate a spectral resolution of 14 nm at 900 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this new device concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors is fabricated by using a blade-coated organic thin films with a thickness gradient.

Resume : Porous organic polymers found an important place among other inorganic, inorganic organic hybrid porous materials due to the numerous alternative synthetic pathways of wet polymer chemistry which allow the easier functionality over a (hydro)thermally stable backbone.[1] Indeed, this have been a promoting factor for POPs in traditional applications of porous materials such as gas storage and heterogeneous catalysis.[1] Nevertheless, use of POPs in light driven and (opto)electronic applications have been more intriguing with respect to abovementioned classical ones since high dimensionality and porosity in POPs are found to be highly effective as compared to their 1D linear analogues.[2-7] Hence, recently POPs were started to be integrated to organic photovoltaics (OPVs) and power conversion efficiencies (PCEs) up to 5% were reported in this novel area.[8-10] On the other hand, the processability of such porous structures are limited due to their non soluble nature, and introducing alkyl chains have been avoided not to reduce accessibility of the pores along the skeleton. However, the applications take place in electronic level does not require tremendously high surface areas. Thus, here we present a soluble porous polymer analogue (PPA) produced from benzotrithiophene and naphtalenediimide (NDI) units (B3TNDI-PPA) bearing alkyl chains. Several different synthetic conditions were employed to yield the soluble material from a A3 B2 type Stille polycondensation, and a high molecular weight, hyperbranched final material was obtained and introduced to bulk heterojunction OPV devices as an alternative polymeric electron acceptor. References: [1] N. Chaoui, M. Trunk, R. Dawson, J. Schmidt, A. Thomas, Chem. Soc. Rev. 2017, DOI 10.1039/C7CS00071E. [2] V. W. H. Lau, M. B. Mesch, V. Duppel, V. Blum, J. Senker, B. V. Lotsch, J. Am. Chem. Soc. 2015, 137, 1064?1072. [3] R. S. Sprick, J. X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M. A. Zwijnenburg, D. J. Adams, A. I. Cooper, J. Am. Chem. Soc. 2015, 137, 3265?3270. [4] V. S. Vyas, V. W. H. Lau, B. V. Lotsch, Chem. Mater. 2016, 28, 5191?5204. [5] Y.-L. Wong, J. M. Tobin, Z. Xu, F. Vilela, J. Mater. Chem. A 2016, 4, 18677?18686. [6] H. Bildirir, I. Osken, T. Ozturk, A. Thomas, Chem. - A Eur. J. 2015, 21, 9306?9311. [7] S. Dadashi-Silab, H. Bildirir, R. Dawson, A. Thomas, Y. Yagci, Macromolecules 2014, 47, 4607?4614. [8] M. Dogru, M. Handloser, F. Auras, T. Kunz, D. Medina, A. Hartschuh, P. Knochel, T. Bein, Angew. Chemie - Int. Ed. 2013, 52, 2920?2924. [9] C. Gu, N. Huang, Y. Chen, L. Qin, H. Xu, S. Zhang, F. Li, Y. Ma, D. Jiang, Angew. Chemie - Int. Ed. 2015, 54, 13594?13598. [10] H. Bildirir, V. Gregoriou, A. Avgeropoulos, U. Scherf, C. L. Chochos, Mater. Horiz. 2017, DOI: 10.1039/C6MH00570E.

Resume : Lanthanide(III) complexes with organic ligands present a promising class of emissive materials for organic light-emitting diodes (OLEDs) [1,2]. The unique properties of lanthanide(III) ions arise from their electronic configuration with many spectroscopic terms and energetically close f-levels. Moreover, the energy distances between respective levels give a combination suitable for many up and down conversion processes and some Ln(III) complexes exhibit narrow-line emission in the near infrared range. Although these properties make them suitable for several applications, the number of reliable NIR luminescent lanthanide complexes available for OLED devices is negligible [3]. In this context, we have synthetized new Er3+ and Nd3+ complexes, featuring acetylacetonate as the counterion and different N,N´-donors as complementing coordination ligands. The compounds where obtained by solvo-thermal method, in methanol solution under stirring and heating, giving products in 90-95% yield (based on Ln). The crystal structure of the complexes will be shown as determined by single crystal X-ray diffraction, with the geometry of the coordination sphere fully characterized. Full structural and thermal characterization was performed by powder X-ray diffraction, FTIR and NMR spectroscopy, DSC and thermomicroscopy. Absorption of light by the lanthanide-ligand ensemble was revealed by diffuse reflectance and luminescence properties of the powder sample, measured under different excitation energies, confirms the ligand-to-lanthanide energy transfer. The magnetic properties of the polycrystalline samples – as a function of temperature and field– confirms the single-lanthanide-ion behaviour and show slow magnetic relaxation behavior. The potential use of this new family as multifunctional materials will be debated. [1] K. Binnemans, Chem. Rev. 109 (2009) 4283-4374; [2] S.V. Eliseeva and J.C.G. Bunzli. Chem. Soc. Rev. 39 (2010) 189-227; [3] J.G. Bunzli, Acc. Chem. Res., 39 (2006) 53-61;

Resume : The recently re-discovered class of hybrid metal-halide perovskite semiconductors hold great promise for solar cells, LEDs and lasers.[1] Wet-chemical patterning techniques to create resonator structures, waveguides etc. are typically not applicable.[2] Crystal binding in these perovskites includes significant contributions of weak van der Waals interactions and hydrogen bonding.[3] The formation enthalpy per unit cell is only about 0.1eV in MAPbI3.[4] Here, we take advantage of the “soft-matter properties” of hybrid metal-halide perovskites and demonstrate that photonic nano-structures can be prepared by direct thermal nano-imprint lithography in MAPbI3 and MAPbBr3 at relatively low temperatures (<150°C). The resulting periodic patterns provide distributed feedback (DFB) resonators, which afford NIR lasing in MAPbI3 between 778.8 and 800.8 nm with ultra-low threshold levels on the order of 1 μJ/cm2.[5] Furthermore we will also present the first DFB lasers based on MAPbBr3, which emit in the visible spectrum. [1] B. R. Sutherland and E. H. Sargent, Nat Photon 2016, 10, 295. [2] D. Lyashenko, et al., physica status solidi (a) 2017, 214, 1600302. [3] D. A. Egger and L. Kronik, The Journal of Physical Chemistry Letters 2014, 5, 2728. [4] A. Buin, et al. Nano Letters 2014, 14, 6281. [5] N. Pourdavoud, et al., Advanced Materials 2017, 29, 1605003.

Resume : One key advantage of solution-processable organic semiconductors is the opportunity of blending different materials in order to attain novel material properties and applications. The concept of ternary blend organic solar cells makes use of exactly that idea: three (or more) organic chromophores are combined to better match the solar irradiance spectrum and thus increase the amount of light absorbed, which, in turn, will increase the power output of the solar cell. However, the charge transport limitations of many current generation polymer blends typically require rather low active layer thicknesses (around 100 nm) for optimum performance. Here, in addition to extending the absorption window of organic solar cells by adding a near infrared polymer, we demonstrate devices with unusually thick active layers (~300 nm) and power conversion efficiencies beyond 11%. Motivated by the possibility to process thick-film devices based on ternary blends, we demonstrate solar modules consisting of three solar cells connected is series, delivering 8.2% and 6.8% power conversion efficiency on the glass and flexible substrates, respectively. These results underscore the relevance of ternary photovoltaic polymer blends for future upscaling technologies. Reference: N. Gasparini, T. Ameri et al, Energy Environ. Sci., 10, 885, 2017.

Resume : We report the control of spontaneous emission in a flexible all-polymeric microcavity (FAPMC) embedding a layer of a novel porphyrin polymer (l-Pn) near-infrared (NIR) emitter blended with poly(9,9-dioctylfluorenyl-alt-benzothiadiazole) (F8BT) as fluorescent material. The optical characterization of the blend in solid films was carried out by UV-Vis absorption, ellipsometry and fluorescence spectroscopies, as a function of the weight ratio between the guest (l-Pn) and the host (F8BT) (1-10% w/w). The best performing blend (1% w/w, quantum yield QY = 20%) was incorporated in a plastic 1D photonic crystal, fabricated by spin coating 50 bilayers of poly(9-vinyilcarbazole) (n = 1.68) and cellulose acetate (n = 1.46) and tuned on the photoluminescence (PL) band of the NIR emitter, peaking at 890 nm. We investigate the polarized angle-resolved transmittance spectra of the FAPMC, which exhibit a peak within the photonic band gap (PBG), and a blue-shift of ~ 100 nm when increasing the incidence angle from 0° to 80°. Furthermore, we demonstrate a fifteen-fold enhancement of the l-Pn fluorescence at 0° incidence. The linewidth of the l-Pn PL centred at 890 nm shrinks from 50 nm outside the FAPMC down to 8 nm (at normal incidence) and defines the quality factor of the FAPMC, Q = λ/Δλ ~ 110, similarly to other FAPMCs reported in the literature.[1] Similarly, angle-resolved PL spectra display a reshaping of the PL blue-shifts with the increasing incidence angle from 0° to 40° in agreement with the PBG trend. This flexible, narrow and tunable light source could be usefully implemented in inexpensive lab-on-a-chip for photonic and sensing applications in NIR spectral region. [1] F. Scotognella S. Varo, L. Criante, S. Gazzo, G. Manfredi, R. J. Knarr III, D. Comoretto, 'Spin-Coated Polymer and Hybrid Multilayers and Microcavities', in Organic and Hybrid Photonic Crystals, ed. by Comoretto D. (Springer International Publishing, 2015).

Resume : Recently, NIR polymers have been employed as an electron donor material in organic solar cells because they can harvest many more photons in the solar light. As a result, short-circuit current density (JSC) is effectively increased and hence power conversion efficiency (PCE) is also improved by using NIR polymers. However, most of NIR polymer solar cells still suffer from a low fill factor (FF) because of inefficient charge collection. In this study, we focus on the FF in NIR polymer solar cells based on naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole (NTz)-based polymers (PNTz4T, PNTz4TF2, and PNTz4TF4) and a C70 fullerene derivative (PC71BM). For these solar cells, the FF decreases with increasing number of fluorine atoms attached in the polymer main chain. In order to discuss the origin of the lower FFs, we evaluated the bimolecular recombination rate krec by measuring transient photovoltage (TPV) and transient photocurrent (TPC). By comparing krec with the Langevin recombination rate kL, we found that the reduction factor ζ = krec/kL increases as the number of fluorine atoms increases. Furthermore, we analyzed the J–V curves of these devices on the basis of these recombination parameters obtained from the TPV/TPC measurements. As a result, we found that the FF can be quantitatively explained by the　parameters. We conclude that the bimolecular recombination should be suppressed to improve the charge collection efficiency and hence PCE of NIR polymer solar cells furthermore. Reference: "Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in Naphthobisthiadiazole-Based Semiconducting Polymers'', K. Kawashima, T. Fukuhara, Y. Suda, Y. Suzuki, T. Koganezawa, H. Yoshida, H. Ohkita, I. Osaka, K. Takimiya, J. Am. Chem. Soc., 138[32], 10265-10275 (2016).

Resume : Two-photon absorption (2PA) brightness is a non-linear optical phenomenon with broad scope of applications. It has already been applied, or is under intensive investigation, in fields such as: optical limiting, polymerization-microfabrication, 3D-data storage, photodynamic therapy, two-photon excited fluorescence etc. Structurally unique π-expanded diketopyrrolopyrroles were designed and synthesized. Strategic placement of a fluorene scaffold at the periphery of a diketopyrrolopyrrole via tandem Friedel-Crafts-dehydratation reactions, resulted in dyes with supreme solubility. Despite the extended ring system, the dye still preserved good solubility and was further functionalized using Pd-catalyzed coupling reactions, such as Buchwald-Hartwig amination. By placing two amine groups at peripheral positions of the resulting dyes, we have achieved values of two-photon absorption cross-section on the level of 2000 GM around 1000 nm, which generated a two-photon brightness of ~1600 GM. These characteristics in combination with red emission (665 nm) make these new π-expanded diketopyrrolopyrroles of major promise as two-photon dyes for bioimaging applications.

Resume : Oligothiophenes represent an important class of compounds in the field of organic materials and have successfully been used as active components in organic solar cells (OSC).(1) On the basis of thiophenes, we are currently synthesizing and investigating novel organic semiconductors comprising various conjugated architectures and shapes, such as linear, macrocyclic, dendritic, or fused. In particular, we have recently developed a series of novel S,N-heteroacenes, which combine the stability of oligothiophenes and the planar extended pi-system of oligoacenes. By multiple fusions of thiophene and pyrrole rings, a full series up to a 13-mer has been synthesized leading to a novel class of conjugated materials with highly interesting optoelectronic properties.(2) X-ray structure and theoretical analyses contribute to the elucidation of their particular geometric and electronic structure. The small bond length alternation, planarity, optoelectronic, and good charge transport properties(3) qualify the S,N-heteroacenes for application in organic electronics, in particular for highly efficient organic(4) and hybrid (perovskite) solar cells.(5) 1. C. Wetzel, E. Brier, A. Vogt, A. Mishra, E. Mena-Osteritz, P. Bäuerle, Angew. Chem. Int. Ed. 2015, 54, 12334; 2. C. Wetzel, A. Mishra, E. Mena-Osteritz, A. Liess, M. Stolte, F. Würthner, P. Bäuerle, Org. Lett. 2014, 16, 362; 3. A. Mishra, D. Popovic, A. Vogt, H. Kast, T. Leitner, K. Walzer, M. Pfeiffer, E. Mena-Osteritz, P. Bäuerle, Adv. Mater. 2014, 26, 7217; 4. A. Mishra, P. Qin, H. Kast, M. Nazeeruddin, S. Zakeeruddin, P. Bäuerle, M. Grätzel, Energy & Environ. Sci. 2014, 7, 2981. 5. A. Mishra, P. Bäuerle, Angew. Chem. Int. Ed. 2012, 51, 2020-2067.

Resume : Perylene bisimide J-aggregates (PEH-PBI) are highly attractive as active materials in plastic optoelectronic devices owing to outstanding optical properties such as superradiance, high absorbance, and sharp emission bands.[1] Despicably, their high tendency to form low emitting H-type aggregates obstacles their compatibility with processable polymer matrices. In this work, we report on the efficient compatibilization of a functionalized PEH-PBI and amorphous polypropylene, and on the use of the new nanocomposite as active medium in all-polymer planar microcavities.[2] The new structure provides a strong and directional spectral redistribution of the PEH-PBI photoluminescence due to the modification of photonic states. A systematic characterization of the fluorescence enhancement effects, including photoluminescence decay and quantum yields, shows that the microcavity structure does not introduce any new non-radiative de-excitation pathways in addition to those already found in the J-aggregate nanocomposite film. These results are promising for the integration of solid-state PEH-PBI in polymer photonic devices such as in lasing microcavity [1] F. Würthner, C. R. Saha-Möller, B. Fimmel, S. Ogi, P. Leowanawat, D. Schmidt, Chem. Rev. 2016, 116, 962. [2] D. Comoretto, Organic and Hybrid Photonic Crystals, Springer, Cham, Switzerland 2015.

Resume : Commercially available and most efficient organic light-emitting diodes (OLEDs) are based on heavy and costly phosphorescent metal complexes as light-emitting materials. This strongly limits the sustainability of OLEDs and raises concerns for applications in which biocompatibility is essential. The choice of suitable emitting materials is further reduced if the emission is tuned in the near-infrared (NIR) spectral region, so as to allow penetration through biological tissues. Notably, due to both the so-called energy gap law and aggregation quenching, above 800 nm no OLEDs reported so far exceed 1 % efficiency. Here we report electroluminescence external quantum efficiency reaching up to 1.16% from polymer light-emitting diodes (PLEDs) based on a blend of a triazolobenzothiadiazole-based molecule and indacenodithiophene-based transport polymer matrix, exhibiting NIR EL peaked at 840 nm and turn-on voltages as low as 1.7 V. Such values are the best ever reported from a NIR PLED with purely organic and solution-processed active layer.

Resume : Organic light-emitting transistors (OLETs) combine the current modulating function of a transistor with light emission, and offer new simplified solutions for nanoscale light sources and highly integrated organic optoelectronics. By benchmarking the planar transistor structure of OLETs with the vertical diode architecture of OLEDs, it is clear that OLETs have the potential to achieve similar optoelectronic and photonic performances with much simpler architectures and lower production costs. Given the recent results in the fabrication of bright, efficient and reliable devices, it is expected in the near future that the full compatibility of OLETs with well-established electronic and photonic planar technologies will allow the development of viable technological solutions in various application field, including display technology and sensing. However, the practical implementation of the OLET technology requires the development of carefully engineered device structures capable of reducing driving voltages and enhance the photonic performances. We will analyze the electronic, optoelectronic and photonic properties of semiconducting materials, which make them suitable for single-layer or multi-layer OLETs. In addition, we report on the realization of highly integrated multifunctional optoelectronic organic device by introducing a high-capacitance photonic crystal as a gate dielectric into a transparent single-layer ambipolar organic light-emitting transistor (OLET). These results have the potential to enable the realization of highly integrated optoelectronic smart systems based on organic light-emitting transistors. References 1. M. Muccini, Nature Materials 2006, 5, 605. 2. R. Capelli, S. Toffanin, G. Generali1, H. Usta, A. Facchetti, M. Muccini, Nature Materials 2010, 9, 496. 3. W. Koopman, et al. Nano Lett., 2014, 14, 1695. 3. M. Muccini and S. Toffanin, Organic Light Emitting Transistors: towards the next generation display technology, John Wiley & Sons, New York, 2016. 4. M. Natali, S. D. Quiroga, L. Passoni, L. Criante, E. Benvenuti, G. Bolognini, L. Favaretto, M. Melucci, M. Muccini, F. Scotognella, F. Di Fonzo, S. Toffanin, Adv. Funct. Mater. 2017, 1605164.

Resume : NIR detectors are important devices for quality control, biochemistry, infrared imaging as well as sens-ing and control in manufacturing processes. If they can be made transparent in the visible domain, new fields of application would open up. Photovoltaic devices using solely NIR light could be used for power generation in windows or on any other objects where small electricity generation is required while leaving the underlying surface apparent. While most NIR detectors use silicon or InGaAs semi-conductors for photon to electron conversion, organic materials with selective absorption in the NIR are quite rare. Polymethine dyes are very interesting with this respect, since the maximum of the ab-sorption band can be tuned via the length of the conjugated chain connecting the two end-standing moieties of the chromophore. This presentation gives an overview of recent developments in NIR sen-sitive solar cells and photodiodes using cyanine dyes. The fact that these organic semiconductors can be deposited from solution opens up applications in the raising field of printed electronics. References: 1) H. Zhang et al., Solar Energy Mater Solar Cells 2013, 118, 157-164. 2) H. Zhang et al., Scientific Reports 2015, 5, 9439. 3) M. Makha et al., Science and Technology of Advanced Materials, 2017; 18.68-75.

Resume : Organic photodetectors (OPDs) offer clear advantages related to their low processing cost and the production of large active areas; additionally, they offer the possibility to address a wide range of optical applications by tailoring the absorption spectra from the Ultraviolet (UV) – visible to the near infrared (NIR). These applications include image sensing, communications, environmental monitoring, remote control, day- and night-time surveillance, and chemical/biological sensing. The big challenge for the entire organic opto-electronics community is the production of NIR sensors with a solution processed organic semiconductor that presents similar performance to silicon. Important figures of merit involve the reduction of the dark current as well as a high external quantum efficiency (EQE) . We present the positive influence of 1,8-diiodooctane (DIO) as additive in the performance of NIR OPDs: it optimizes donor - acceptor interfaces and improves electron/hole transport pathway to attain higher efficiencies up to 35% at 900nm. Also, it decreases dark currents one order of magnitude down to 100 nA/cm2 at -5V. The photoactive layer is a spray-coated blend of diketopyrrolopyrrole-terthiophene (PDPP3T) and [6, 6]-phenyl C61 butyric acid methyl ester (PCBM) plus the additive. The use in image sensors will be discussed as well.

Resume : The properties of low band gap (LBG) polymers in donor-acceptor based bulk heterojunction (BHJ) solar cells are strongly influenced by their morphology and ability for self-organization in thin-films. We studied the molecular orientation of selected LBG polymers and the reference material P3HT using Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS) at the sulfur K edge. For a few of the polymers the experimental spectra are compared to DFT calculations. For two related polymer pairs the influence of the introduction of additional (hexyl-) thiophene moieties on both the electronic structure and ability for self-organization in thin films is studied using Ultraviolet photoelectron spectroscopy (UPS) and NEXAFS. We find that the introduction of additional (hexyl-) thiophene moieties in the polymer structure does not affect significantly the electronic structure of the polymers in thin films, but has a strong impact on their self-organization properties [1]. This will affect the behavior of such polymers in devices. We thank the group of Prof. Scherf (University of Wuppertal, Germany) for the synthesis of LBG polymers. [1] M. Ivanović, U. Aygül, U. Dettinger, A. Tournebize, M. Polek, D. Batchelor, S. Mangold, M. Forster, U. Scherf, H. Peisert, T. Chassé, Solar Energy Materials and Solar Cells, 157 (2016) 286-294.

Resume : Since 20 years ago, photonic crystals have been extensively employed for many applications ranging from sensing to optical communications. The wide spreading of these structures is related to the possibility to control the flow of light and light-matter interactions by exploiting the properties of simple dielectric lattices where the refractive index is periodically modulated in the submicrometric scale. Intuitively, the ability to allow or to forbid light propagation, to localize light and to control spontaneous emission is directly related to the dielectric contrast provided by the lattice building blocks. Therefore, thanks to the high dielectric contrasts available with inorganic materials, polymer structures have often been restricted to decorative purposes.1 In the last decades, the fast rising of organic electronics made the fabrication of flexible and light photonic structures increasingly pursued. Moreover, the possibility to reduce both fabrication time and costs increased the attractiveness of polymer materials. In this work, we demonstrate that different polymer families as well as their colloidal building blocks possess properties that can be exploited for several different photonic devices ranging from photovoltaics, lasers, optical switches and sensors tunable in any desired spectral range 1-6. 1. D. Comoretto, Organic and Hybrid Photonic Crystals. Springer: 2015. 2. P. Lova, G. Manfredi, L. Boarino, et al., ACS Photonics 2015, 2, 537. 3. R. J. Knarr III, G. Manfredi, E. Martinelli, et al., Polymer 2016, 84. 4. L. Frezza, M. Patrini, M. Liscidini, et al., J. Phys. Chem. C 2011, 115. 5. G. Canazza, F. Scotognella, G. Lanzani, et al., Laser Phys. Lett. 2014, 11. 6. A. Bozzola, V. Robbiano, K. Sparnacci, et al., Adv. Optical Mater. 2016, 4, 147.

Resume : It is almost a decade that Organic Photodetectors (OPDs) are developed and fabricated in the labs of Siemens Healthcare’s Technology Center. Particularly in the last years, their development experienced a boost towards industrial maturity thanks to a new class of semiconducting polymers which appeared on the market. This development also culminated in several recent publications and press releases by renowned institutions. In particular the field of medical imaging could benefit strongly from the potential of the OPDs as reported recently by Gelinck et al.. Medical imaging requires large area X-ray detectors due to the limited ability to focus X-ray radiation and the relatively large area of the human body. OPDs offer clear advantages related to their low processing cost, large active areas and the possibility to tailor the absorption spectra from the Ultraviolet (UV) – visible to the near infrared (NIR). A Status quo and future perspectives for organic and hybrid photodetectors will be main focus of this lecture.

Resume : Multi-junction device configurations represent a realistic strategy to achieve high power conversion efficiencies (PCEs) in polymer solar cells. Fabricating multi-junction polymer solar cells from solution is generally achieved using orthogonal processing conditions for the subsequent layers in the device stack. Of special interest is the layer that optically and electrically interconnects consecutive absorber layers. This interconnecting layer (ICL) has to be transparent, have different work functions at top and bottom, and should present resilience against organic solvents that are used to deposit the photoactive layers on top. We have now developed a universal procedure to fabricate multi (i.e. tandem and triple) junction polymer solar cells in an inverted device configuration from solution only, using a combination of well-known materials, PEDOT:PSS and ZnO or PEIE. The essence of the method involves creating solvent mixtures to deposit these materials that provide good wetting properties during the entire drying process, but that also do not interact strongly with the previously deposited layer to prevent dissolution. Using a variety of absorber layers we successfully fabricated 9 different tandem and 3 different triple-junction polymer solar cells, without the need of changing any parameter in the processing of the ICL. These multi-junction devices showed very good power conversion efficiencies up to 10% and were obtained in a good yield (16/16 at best) despite their complex structure with up to 8 solution processed layers. A good match of the performance with combined optical-electrical modelling validated the robustness and essentially loss less nature of the ICL. The power conversion efficiencies achieved are limited only by the quality of the active layer materials available for this study. Considering that the development of new photoactive materials has recently resulted in examples where the PCE exceeds 12%, it appears that PCEs for multi-junction cells on the order of 15% are a realistic target for the near future.

Resume : Organic semiconductors emitting in the near-infrared (NIR) range of the electromagnetic spectrum have emerged in the last twenty years as a novel class of materials with useful opto-electronic properties. Although efficiencies up to 19% in the NIR region have been achieved by phosphorescent organic light-emitting diodes (OLEDs), the active materials used in these are toxic because of the heavy-metal content, in addition to being subject to triplet-triplet annihilation, and therefore to the external quantum efficiency (EQE) roll-off at high current density. Attempts to circumvent this issue by using fluorescent emissive layers have mostly resulted in OLEDs with EQEs below 0.5%. Among the different “metal-free” organic materials investigated so far, one of the most emerging class of emitters consists of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) -based red/NIR oligomer. Herein, we take advantage of a recent breakthrough in the synthesis of α,β-unfunctionalised BODIPY moieties, which we symmetrically conjugated with oligothienyls in an unexpectedly stable form, and produce a “metal-free” A-D-A (acceptor-donor-acceptor) oligomer emitting in the NIR range (referred to as NIRBDTE, for brevity). By incorporating this oligomer in a wider gap polyfluorene matrix, we are able to retain a photoluminescence (PL) efficiency of 20% in the solid state and demonstrate OLEDs with electroluminescence spectra peaking at 720 nm. We achieve EQEs up to 1.1%, the highest value achieved so far by a “metal-free” NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.