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Organic Semiconductors for NIR Optoelectronics

Final Report Summary - OSNIRO (Organic Semiconductors for NIR Optoelectronics)

Scientific targets of OSNIRO have been synthesis, characterization and application of NIR absorbing and emitting organic materials for Organic Electronics. More than 80 polymeric and oligomeric materials have been synthesised within the consortium and tested for application in optoelectronic devices.
NIR OLEDs - Organic light emitting diodes emitting in the near infrared region (700-2500 nm) - 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. Additionally, the semitransparency of biological tissue between 700 and 1000 nm makes such light sources appealing for a broad class of biomedical applications, particularly imaging and sensing. For near-infrared OLED devices the consortium followed two approaches: (1) Development of low band gap dyes that are blended with high band gap polymers and (2) development of polymer/polymer guest-host systems. For the first approach, several BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)- derived materials have been synthesised. Researchers at BUW concentrated on the synthesis of BODIPY-based small molecules that show the so-called aggregation induced emission, a phenomenon that leads to very high photoluminescence quantum yields in the solid state, one prerequisite for the fabrication of OLEDs. First test with BODIPY-based small molecules blended into a wide band gap polymer resulted in OLEDs with external quantum efficiency (EQE) of more than 1 % @ 660 nm. Another BODIPY-based molecular material – NIRBDTE – was synthesised at ADVENT. By using NIRBDTE blended into the wide bandgap polymer F8BT a maximum EQE of up to 1.1 % with electroluminescence (EL) emission peaking @ 720 nm could be achieved. With another NIR dye, BTT* blended into the novel host polymer PIDT-2TPD virtually pure (98%) NIR EL peaking @ 840 nm with a turn-on Von of only 1.7 V, an EQE in excess 1.15 % and 1.5 mW/cm2 optical output could be realised. To the best of our knowledge, such values are the highest ever reported in this spectral range for PLEDs with a heavy-metal-free, solution-processed active layer without triplet-assisted emission. With a porphyrin derivative blended into F8BT UCL achieved an EQE up to 2.2 % with Von of 0.6 V and an EL emission (ca. 87 % in the NIR region) of ~ 0.3 mW/cm2 peaking @ 850 nm. With a guest-host system developed at Chalmers PLEDs with NIR EQE values of > 0.1 % @ ~10 mA/cm2 current density could be achieved. These are the highest values reported so far for a purely organic NIR-emitting PLED.
Organic photovoltaic cells (or organic photovoltaic devices OPVDs) with a sensitivity window extended into the NIR region should allow a better coverage of the solar emission spectrum and thus lead to increased photovoltaic performance. Within the consortium several new donor and acceptor polymers have been developed. For the realisation of all-polymer solar cells, ADVENT synthesised new n-type solution processable, conjugated polymer networks. Based on the acceptor polymer PNDI-T introduced by Chalmers an all-polymer solar cell with power conversion efficiency (PCE) of 8 % could be achieved. For the optimisation of bulk heterojunction (BHJ) solar cells FAU investigated the concept of ternary and quaternary polymer/fullerene blends for the active layer of bulk heterojunction solar cells. Hereby, the influence of inorganic quantum dots, organic dyes and polymeric sensitizers on the cell properties have been studied. The incorporation of 15 – 20 wt% dye sensitizer into ternary or quaternary BHJ solar cells led to a20 – 25 % PCE improvement. For polymeric sensitizers an initial set of design rules has been set up. Theoretical calculations and high resolution imaging methods were employed to obtain deeper insight into the nanomorphology of the ternary blends in order to derive reliable the structure-property relationships. Based on the achieved knowledge on morphology and transport in ternary systems, FAU designed and fabricated a novel ternary blend-based device with a PCE exceeding 11 %. Motivated by the ease of device fabrication and the high efficiency of this ternary blend-based solar cell the suitability for fabricating efficient photovoltaic modules was tested. FAU achieved solar module efficiencies as high as 8.16 % on glass and 6.83 % on flexible substrates, which are among the highest values reported to date. At TU/e tandem and triple junction solar cells have been investigated. For the fabrication of tandem solar cells an optimized method for coating the interconnecting layer (ICL) was developed. Hereby, for several combinations of active layers the proper material of this electron transporting layer (ETL) was selected. With polymer PTB7-Th blended with PC70BM as front cell material, PMDP3T blended with PC60BM as back cell active layer, and PEDOT:PSS and ZnO nanoparticles as ICL, a PCE of 8.6 % has been achieved. In triple-junction solar cells several combinations for front, middle and back cell have been tested, and the layer thicknesses have been optimized. The best combination resulted in a PCE of ca. 10 %.
Photosensors with NIR sensitivity are especially important for integration into optoelectronic devices for chemical/biological sensing. Therefore, UCL and SHC GmbH investigated NIR photodetectors (OPDs) and image sensors. Both partners investigated strategies for the increase of EQE and lowering of dark currents in NIR photodetectors. At SHC GmbH a OPD with dark current densities lower than 5x10-5 mA/cm2 @ -5 V bias and a quantum efficiency of 67 % @ 800 nm wavelength and -5 V bias could be realised. Work at UCL resulted in the optimisation of the deposition process of donor/acceptor blends by spin coating and blade coating, showing the opportunity of transferring the active layer fabrication from lab-scale coating technique (spin coating) to a large scale roll-to-roll compatible coating technique (blade coating). Moreover, an appropriate processing method was found for PEDOT:PSS and NiO nanoparticle interlayers. Image sensors with an EQE of more than 60 % @ 850 nm have been fabricated. The fabrication and integration of OPDs on various substrates, in particular silicon TFTs backplanes on glass and printed circuit boards (PCBs) have been realised. On both kinds of substrates, working OPDs have been processed in standard as well as in inverted architecture. Hereby, interlayer effects and the influence of a smoothing layer have been studied.
Main goal of OSNIRO was the training of young high profile European research fellows from the field Science and Engineering. Therefore, in OSNIRO young scientists (chemists, physicists, engineers, and materials scientists) have been trained in a very topical, strongly interdisciplinary area: optoelectronic devices (organic solar cells, photodetectors and image sensors, organic light-emitting diodes) based on organic materials that operate in the near infrared (NIR) region. The interdisciplinary training programme involved materials synthesis and characterisation, device fabrication & characterisation as well as the integration of such devices into demonstrators. OSNIRO thus covered the full value chain from design of NIR absorbing and emitting organic materials to organic electronics applications: device design, prototyping, and commercialisation. The consortium is placed around leading academia groups in materials science and potent industrial players, including device manufacturers (Siemens Healthcare GmbH) and materials suppliers (ADVENT Technologies/SME). The consortium members were supported by the associated partner Bavarian Research Alliance (BayFor), through specific workshops on funding activities and Europe-wide dissemination activities.
Within OSNIRO 17 fellows have been recruited, 8 of them female scientist. All of them have been working on their own scientific projects, closely interconnected with the projects of the other fellows. Networking between the fellows was boosted by their own "What's App" group as efficient way for an intense contact between the fellows. During secondments at partner institutions all fellows have been trained in fields complementary to the training in their host institutions. Secondments also provided insights into work at University and Industry in other, especially foreign, locations and countries. Moreover, secondments also facilitated the interaction and collaboration between the fellows and the transfer of knowledge between fellows and partner institutions. All fellows had the opportunity to present their results at international conferences and in peer-reviewed publications. Until now, 29 articles in high ranked international journals have been published, ten more have been submitted, and further publications are in preparation. Most of the publications are joint, collaborative publications of two or more partners of the project. Moreover, one patent application has been filed by partner SHC GmbH. Future applications arising from project results could be photodetectors for the visible and NIR range as well as sensors.
During workshops and schools organised within the project, the fellows have been trained in relevant scientific topics as well as in transferable skills. During the past four years, 6 workshops and three International Schools have been organised, one of them, the International School on Devices was a joint event of OSNIRO and the ITN projects iSwitch and SYNCHRONICS, which took place between Mach 1st and March 3rd, 2016 at UCL in London.
The Final Meeting of OSNIRO has been executed as Symposium "NIR Optoelectronics – Organic Semiconductors and Devices" at the E-MRS Fall Meeting 2017 in Warsaw between September 18th and September 21st, 2017. Here, six speakers from outside the consortium with high reputation in the field of organic electronic devices provided invited lectures. The Symposium allowed the fellows to present and discuss their results with scientists from inside and outside the consortium.
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