The challenge of developing efficient deep red emitters is addressed by the design of donor-acceptor chromophores which afford high fluorescence quantum yields through thermally activated delayed fluorescence (TADF). Alas, harmonizing TADF with the general design of deep red emitters proves difficult. One strategy to improve the (TADF) emission properties entails the design and comparison of regioisomers, i.e. systems with altered donor-acceptor configurations. In the presented work, dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzCN) served as an acceptor template for the design of four regioisomers. These materials were computationally simulated using density functional theory (DFT). Following synthesis, time-resolved emission spectroscopy was utilized to probe the time-dependent emission in (doped) films, and to derive the TADF properties. Isomer 4-TPA-DBPzCN combines the favorable TADF characteristics of 2-TPA-DBPzCN and the higher fluorescence quantum yield of 3-TPA-DBPzCN. Via ‘isomeric modulation’, we were able to analyze the effect of geometry, orientation, and electronic interaction on the underlying emission characteristics.
The current state-of-the-art infrared (IR) systems use inorganic semiconductors for so-called photonic detection in the near (few μm) and mid-IR (10’s of μm) range and switch to thermal detection systems such as bolometers for the large wavelength regimes (up to mm range). In order to lower the cost of IR detectors, much effort is put into designing low-gap polymers or molecules for organic photo-detectors, but their detection range is currently still limited to 1500-1600 nm wavelengths, with modest detectivities in comparison to existing inorganic technologies in the short wave infrared (SWIR) range. In this work, we look at the possibility of using organic semiconductors in a bolometric device for infrared detection over an extensive wavelength range, spanning from the SWIR to the mid-IR. We find advantages of using organic materials compared to inorganic materials for the fabrication of bolometers and use theoretical modeling to guide us on which parameters we can use to optimize our devices. We find two key parameters determining the device performance: the polymers’ thermal conductivity and the overall device thickness.
Transparent photovoltaics, a burgeoning field at the intersection of materials science and renewable energy technology, have the potential to revolutionize the way in which we harness sunlight and integrate solar power into our daily lives. The performance of transparent photovoltaics is evaluated by the light utilization efficiency and necessitates a trade-off between optimizing efficiency and ensuring transparency. In this work, a comparative analysis is performed of two donor polymer materials that differ only in the presence of a single fluorine atom among the polymer backbone, examining their performance in transparent solar cells in relation to both of these critical aspects.
State-of-the-art alternating semiconducting polymers, synthesized via established academic protocols, often contain homocoupling defects, causing the true structure to deviate from the anticipated perfectly alternating polymer backbone. These structural defects significantly hinder the reproducibility across different polymer batches, posing a challenge to the commercial viability of the organic semiconductor field, while simultaneously imposing performance limitations in different applications by creating defected chains, limiting the attainable molecular weight and increasing the dispersity. In this study, two synthesis methods – conventional Stille polymerization and a novel defect-free route – are employed to create the p-type accumulation mode OECT (organic electrochemical transistor) benchmark material pgBTTT. The effect of homocoupling, and its absence, is investigated by comparing the bulk properties of the two polymers and evaluating their respective OECT performances.
The true structure of alternating conjugated polymers – the state-of-the-art materials for a number of organic electronics technologies – often deviates from the idealized picture but this gets relatively limited attention. Here, we quantify the amount of homocoupling defects resulting from Stille polymerization and shed new light on the actual distribution of these structural defects in a prototype polymer material. Further, when compared to a homocoupling-free variant, these defects hinder fullerene intercalation, with a clear implication on charge-transfer absorption. This demonstrates that molecular defects may (strongly) impact polymer and blend properties and calls for increased attention for defect-free materials.
At this moment, we are still far away from understanding many neurologic diseases. However, the emergence of different bioelectronic technologies opens the pathway to fathom them once and for all. Organic electrochemical transistors (OECTs) can assist here, thanks to their superior recording of neurological signals. Moreover, their flexible nature allows application in non-invasive scalp electrode devices. Nevertheless, the field of OECTs is still in its infancy, and clear, rational design criteria are lacking. Hence, dedicated backbone and side-chain engineering could enlighten the structure-performance relationships for next-generation conjugated polymers that are able to transport both ions and electrons in future OECTs.
Semiconducting organic polymers are most often synthesized by linking an electron poor and an electron rich (hetero)aromatic building block via a transition metal catalyzed cross-coupling copolymerization. Researchers aiming at exploring applications and fundamental performance limits, for example for organic photovoltaics, organic photodetectors, and organic electrochemical transistors, often assume that the obtained material consists strictly of a perfect repetition of the depicted polymeric repeating unit, whereas this is likely not the case. In this contribution, we demonstrate a synthesis approach to obtain the depicted “perfect” structure of these types of polymers and the influence of material defects on the optoelectronic properties and device performance.
Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime.[1] However, near-infrared photodetection at a high detectivity has been proven to be more challenging and, to date, the true potential of organic semiconductors in this spectral range (800‒2500 nm) remains largely unexplored. We have recently shown that the main factor limiting the specific detectivity ( ) is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses in organic photovoltaics.[2] Based on this finding we concluded that OPDs have the potential to be a useful technology up to 2 μm, given that high external quantum efficiencies can be maintained at these low photon energies. To further elaborate on the fundamentals defining these limitations, a next-generation of ‘defect-free’ organic semiconductors is synthesized. This allows us to investigate the influence of material imperfections (end-capping, homocoupling defects, and other impurities) on the material properties and device performance. In this contribution, we give an overview of the synthetic approaches applied and the latest organic semiconductors for high performance NIR OPDs, approaching their intrinsic limits.
[1] N. Li et al., Mater. Sci. Adn Eng., 146, 100643 (2021)
[2] S. Gielen et al., Adv. Mater., 32, 2003818 (2020)
KEYWORDS: Cameras, Imaging systems, Sensors, Unmanned aerial vehicles, Control systems, System integration, Data processing, Visualization, Data acquisition, UAV imaging systems
Multispectral imaging technology analyzes for each pixel a wide spectrum of light and provides more spectral
information compared to traditional RGB images. Most current Unmanned Aerial Vehicles (UAV) camera systems are
limited by the number of spectral bands (≤10 bands) and are usually not fully integrated with the ground controller to
provide a live view of the spectral data.
We have developed a compact multispectral camera system which has two CMV2K 4x4 snapshot mosaic sensors
internally, providing 31 bands in total covering the visible and near-infrared spectral range (460-860nm). It is compatible
with (but not limited to) the DJI M600 and can be easily mounted to the drone. Our system is fully integrated with the
drone, providing stable and consistent communication between the flight controller, the drone/UAV, and our camera
payload. With our camera control application on an Android tablet connected to the flight controller, users can easily
control the camera system with a live view of the data and many useful information including histogram, sensor
temperature, etc. The system acquires images at a maximum framerate of 2x20 fps and saves them on an internal storage
of 1T Byte. The GPS data from the drone is logged with our system automatically. After the flight, data can be easily
transferred to an external hard disk. Then the data can be visualized and processed using our software into single
multispectral cubes and one stitched multispectral cube with a data quality report and a stitching report.
KEYWORDS: Organic photovoltaics, Solar cells, Dielectric spectroscopy, Dielectrics, Heterojunctions, Renewable energy, Chemical engineering, Physics, Current controlled current source
Organic photovoltaics (OPV) show strong potential for a number of renewable energy applications because of some specifically appealing features (light weight, flexibility, color, …). Over the past decade, the power conversion efficiencies of organic solar cells have strongly risen to values surpassing the 10% threshold, mainly due to strong efforts in chemical engineering of the photoactive components, architectural device optimization and acquisition of fundamental insights in the underlying device physics. As part of the device optimization, the use of conjugated polyelectrolyte (CPE) interfacial layers has been introduced as a popular and powerful way to boost the inherent I-V characteristics. In the presented work, we applied impedance spectroscopy to probe the dielectric permittivity of a series of polythiophene-based CPE interlayer materials as a means to postulate design rules toward novel generation interfacial layers. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is that the material’s affinity with respect to the underlying photoactive layer diminishes. To enhance the interlayer-photoactive layer compatibility, copolymer structures containing a certain amount of non-ionic side chains are found to be beneficial.
When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.
In this Proceedings paper, we report on the synthesis of a family of polythiophene-based conjugated polyelectrolytes, both homopolymers and random copolymers varying in the building block ratio and counter ions, toward a better fundamental understanding of the structure-property relations of these ionic derivatives in organic photovoltaics. One of the ionic homopolymers was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells (up to 1.6% PCE in combination with PC71BM) prepared by the low impact meniscus coating technique. On the other hand, these imidazolium-substituted polythiophenes were also applied as materials for electron transport layers (ETLs), boosting the I-V properties of PCDTBT:PC71BM solar cell devices up to average PCE values of 6.2% (~20% increase), which is notably higher than for previously reported ETL materials. Advanced scanning probe microscopy techniques were used to elucidate the efficiency enhancing mechanism.
Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration
planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to DTU and characterized
simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun
simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally
identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for
a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the
devices. Characterization includes current-voltage curves, light beam induced current (LBIC) imaging, dark lock-in thermography (DLIT),
photoluminescence (PL), electroluminescence (EL), in situ incident photon-to-electron conversion efficiency (IPCE), time of flight secondary
ion mass spectrometry (TOF-SIMS), cross sectional electron microscopy (SEM), UV visible spectroscopy, fluorescence microscopy, and
atomic force microscopy (AFM). Over 100 devices with more than 300 cells were used in the study. We present here design of the device
sets, results both on individual devices and uniformity of device sets from the wide range of characterization methods applied at different
stages of aging under the three illumination conditions. We will discuss how these data can help elucidate the degradation mechanisms as well
as the benefits and challenges associated with the unprecedented size of the collaboration.
Long alkyl chain ligands such as oleic acid (OLA) which cover the as-prepared PbS nanodots act as an insulating layer
that impedes efficient charge transfer in PbS nanodots:polymer hybrid solar cells. The replacement of OLA with tailored
ligands of an appropriate chain length is needed to achieve a noticeable enhancement of photovoltaic performance.
Several studies have centered on the ligand exchange prior to casting the PbS film1,2,3. However, this post synthesis
approach requires careful consideration for the choice of a ligand as clustering of the nanodots has to be avoided.
Recently, a new approach that allows direct chemical ligand replacement in a blended mixture of PbS:P3HT has been
demonstrated 4,5,6. In this contribution, the latter approach (post-fabrication) was compared with the post-synthesis ligand
exchange. We investigated the effect of the ligand exchange processes to the charge separation dynamics in the
P3HT:PbS blends by steady-state and time-resolved photoluminescence (PL). Hexanoic acid and acetic acid were used
as a short-length ligand for the post fabrication approach while decylamine, octylamine and butylamine were used for the
post-synthesis approach. As expected, decreasing the chain length of the ligand led to an increase of the P3HT
fluorescence quenching. The absence of enhancement of PbS luminescence due to energy transfer from P3HT and the
dependence of the quenching efficiency on the bulkiness of the ligands coating the QDs suggest that the quenching of the
P3HT fluorescence is dominated by electron transfer to PbS quantum dots (QDs). In addition, the fluorescence
quenching is also less prominent in the P3HT with higher regioregularity (RR) suggesting an enhanced phase separation
in the blend due to more densely packed nature of conjugated polymer with higher RR.
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