Color‐Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

Future lightweight, flexible, and wearable electronics will employ visible-lightcommunication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >10² mA W⁻¹ and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals

Kohärente Kristalle und ihre Bedeutung in organischen Solarzellen

In organischen Halbleiterschichten aus Donor- und Akzeptormaterialien besteht einenger Zusammenhang zwischen der Dynamik photoelektrischer Prozesse und den vor-handenen Energiezuständen. Diese werden durch die Eigenschaften der Materialienselbst bestimmt, sind aber auch von der Wechselwirkung der Moleküle miteinanderabhängig und damit von deren relativen Orientierung sowie deren räumlichen Di-stanz. Die Energiezustände sind also direkt von der Morphologie der organischenSchicht mitbestimmt. In dieser Arbeit wird die Morphologie funktioneller Schichtenunter anderem von organischen Solarzellen visualisiert, welche aus einem Polymerund einem Nicht-Fulleren Akzeptor, im Speziellen PBDB -T und ITIC, gemischtwerden. Die Visualisierung der Morphologie erfolgt mittels Hellfeld Transmissions-elektronenmikroskopie (TEM) sowie analytischer TEM und wird mit den jeweili-gen Bauteilparametern korreliert. Durch Kryo-TEM Tomographie wurden kohären-te PBDB -T und ITIC Kristalle am Donor-Akzeptor-Übergang von PBDB -T:ITICMischschichten gefunden. Dabei dominieren ITIC Kristallstrukturen eines spezifi-schen Polymorphs. Mit diesen Resultaten demonstriert die vorliegende Arbeit Mög-lichkeiten zum visuellen Nachweis spezifischer morphologischer Eigenschaften vonorganischen Funktionsmaterialien. Diese Visualisierung ist grundlegend für die Mo-dellierung der genauen Anordnung von PBDB -T und ITIC Molekülen in der Misch-schicht und damit wegweisend für eine Simulation und Optimierung des Modellsys-tems PBDB -T:ITIC und darauf aufbauender Systeme

Visualizing morphological principles for efficient photocurrent generation in organic non-fullerene acceptor blends

International audienceThe efficiency of organic solar cells with donor polymers and non-fullerene acceptors depends on a complex morphology. Similar chemical and electronic structures impede generating in-depth insights in morphological details. We visualise molecular arrangements and the nanomorphology in PBDB-T:ITIC blends by correlating transmission electron micrographs and material distribution maps. Material phases are identified by machine learning on hyperspectral data from electron spectroscopic imaging. We observe a specific polymorph of ITIC after thermal annealing. During annealing, enhanced by the presence of additives, PBDB-T acts as nucleation site for ITIC due to strong p-p-interactions of the electron withdrawing groups of both molecules. This leads to efficient charge transport paths in ITIC phases with direct p-p-contact to PBDB-T at the interface. We conclude that p-p-stacking between donor and acceptor molecules facilitates charge carrier generation within mixed interface regions. Broader context A crucial step for the advancement of organic solar cells was made by introducing novel, versatile non-fullerene molecules. These materials still provide specific advantages, such as solution processing, or strongly reduced toxicity in processing and use. However, the nanoscale structural landscape that determines performance and understanding of photophysics is still not fully known. Elucidating this relation by applying electron microscopy (EM)-one of the major tools offering the required resolution-is hampered by the similarity of donor and acceptor molecules regarding chemical composition and electronic structure. We demonstrate that analytical EM enables materials phase identification at the nanometer scale. Interpretation of morphological details is augmented by correlating phase distribution maps with high-resolution information about crystallinity. Using the system PBDB-T:ITIC we experimentally show that ITIC acceptor molecules adopt the periodic spacing of the PBDB-T repetition units. This happens already in mixed interface regions and is facilitated by polymorphism of ITIC. Here, we demonstrate that these visualized crystallites only form under thermal treatment in the presence of PBDB-T. We were able to correlate these structural features to improved performance. We envision an in-depth understanding of both the influence of processing parameters and the relation between molecular structure and photophysics

Color‐Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

Future lightweight, flexible, and wearable electronics will employ visible-light- communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >102 mA W−1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals

Improved ultraviolet stability of fullerene-based organic solar cells through light-induced enlargement and crystallization of fullerene domains

International audienceOrganic solar cells (OSCs) are a promising technology with the potential for low-cost manufacturing. However, to translate to economically viable applications, long-term stability is a fundamental requirement. Amongst intrinsic degradation pathways the sensitivity of OSC to ultraviolet (UV) light severely limits their photostability. Here, we focus on the impact of UV on the stability of solar cells based on well-known fullerene-based blends processed with 1,8-diidooctane (DIO) as additive. The post-annealed devices resulting in DIO-free blends are directly compared to as-cast devices containing residual DIO. After a pronounced initial burn-in, as-cast devices demonstrate a self-healing effect leading to stable solar cells under prolonged exposure to UV light. This initial burn-in can be considerably reduced in annealed devices with a suitable heating process, resulting in very stable solar cells under UV-containing light over a long time period. Under UV-free LED light, solar cells are stable, which implies a direct impact of UV on the performance evolution of devices. Advanced characterization techniques were used for in-depth morphological analyses under light exposure to distinguish the observed UV-related processes in the polymer blends. Our results point thus towards the presence of two processes occurring under UV-light within as-cast devices involving fullerenes, one causing a performance degradation and the other allowing a repair tending towards a performance stability. Due to an improved initial crystal order within annealed devices, the process related to the degradation is in the minority. The UV stability of devices can be attributed to the UV light-induced diffusion of fullerenes, leading jointly to the enlargement of the initial existing fullerene domains and to their crystallization under UV light. These results path the way for a better understanding of the stability of efficient normal OSCs under simulated sunlight

Compensation of Oxygen Doping in p‐Type Organic Field‐Effect Transistors Utilizing Immobilized n‐Dopants

Poly(3‐hexyl‐thiophene‐2,5‐diyl) (P3HT) is one of the most commonly used materials in organic electronics, yet it is considered to be rather unattractive for organic field‐effect transistors (OFETs) due to its tendency to oxidize under aerobic conditions. Strong p‐doping of P3HT by oxygen causes high off‐currents in such devices opposing the desired high on/off‐ratios. Herein, a new application‐oriented method involving the recently developed immobilizable organic n‐dopant 2‐(2‐((4‐azidobenzyl)oxy)phenyl)‐1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol (o‐AzBnO‐DMBI) is presented allowing to process and operate P3HT OFETs in air. The n‐dopants compensate oxygen doping by trapping generated free holes, thereby rediminishing OFET off‐currents by approximately two orders of magnitude. At the same time, field‐effect mobilities remain high in the order of up to 0.19 cm² V⁻¹ s⁻¹. Due to the covalent attachment of the dopants to the host matrix after photochemical activation, a drift of the otherwise mobile ions within the device is prevented even at high operating voltages and, thus, hysteresis in the corresponding transfer characteristics is kept low. In this manner, the air instability of P3HT OFETs is successfully resolved paving an auspicious way toward OFET mass production. As the immobilization process employed here is nonspecific with respect to the host material, this strategy is transferable to other p‐type semiconductors

Insights into the relationship between molecular and order-dependent photostability of ITIC derivatives for the production of photochemically stable blends

International audienceWe present a systematic study of the intrinsic photostability of ITIC derivative acceptors, namely ITIC,ITIC-Th and ITIC-4F, in solution, in layers and in blends with donor polymers such as PBDB-T (alsoknown as PCE12) or its fluorinated derivative PBDB-T-2F (known as PM6). We followed the evolution ofthe absorption spectra of ITIC derivative acceptors and blends under constant irradiation using differentlight sources (AM1.5 solar simulator, SUNTest or LED). Any interaction with oxygen under illuminationwas avoided by placing the solutions and thin films under inert conditions, i.e. nitrogen or vacuum.While donor polymers are highly photostable, all three NFAs show a common photodegradation processwith the formation of photoproducts and molecular structure modifications under light exposure.Compared to the solutions where the degradation kinetics are very fast, the degradation in films issignificantly slowed down. The corresponding photodegradation process including a cis–trans isomeri-sation of end groups is found to be common for all ITIC derivatives and independent of the light source.In blends, photodegradation is directly attributable to the acceptor, but varies according to the deriva-tive. In fact, ITIC-4F based blends will benefit from a stabilizing mechanism due to a favorable molecularpacking inside the blend. Thereafter, we have studied the photostability of blend layers post-annealed atdifferent temperatures, and we show that the thermally induced packing and crystallinity of the ITICderivatives dominate the photostability of the blends. We finally investigate the photostability of organicsolar cells based on thermally annealed PM6:ITIC-4F blend layers

High‐Efficiency Digital Inkjet‐Printed Non‐Fullerene Polymer Blends Using Non‐Halogenated Solvents

International audienc