Resonant Raman-Active Polymer Dot Barcodes for Multiplex Cell Mapping.

Resonance Raman spectroscopy is an efficient tool for multiplex imaging because of the narrow bandwidth of the electronically enhanced vibrational signals. However, Raman signals are often overwhelmed by concurrent fluorescence. In this study, we synthesized a series of truxene-based conjugated Raman probes to show structure-specific Raman fingerprint patterns with a common 532 nm light source. The subsequent polymer dot (Pdot) formation of the Raman probes efficiently suppressed fluorescence via aggregation-induced quenching and improved the dispersion stability of particles without leakage of Raman probes or particle agglomeration for more than 1 year. Additionally, the Raman signal amplified by electronic resonance and increased probe concentration exhibited over 103 times higher relative Raman intensities versus 5-ethynyl-2'-deoxyuridine, enabling successful Raman imaging. Finally, multiplex Raman mapping was demonstrated with a single 532 nm laser using six Raman-active and biocompatible Pdots as barcodes for live cells. Resonant Raman-active Pdots may suggest a simple, robust, and efficient way for multiplex Raman imaging using a standard Raman spectrometer, suggesting the broad applicability of our strategy.

Simplified Y6-Based Nonfullerene Acceptors: In-Depth Study on Molecular Structure-Property Relation, Molecular Dynamics Simulation, and Charge Dynamics.

Two new Y6 derivatives of symmetrical YBO-2O and asymmetrical YBO-FO nonfullerene acceptors (NFAs) are prepared with a simplified synthetic procedure by incorporating octyl and fluorine substituents onto the terminal 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) moiety. By moving the alkyl substituents on the Y6 core to the terminal INCN moiety, the lowest unoccupied molecular orbital of the YBO NFAs increases without decreasing solubility, resulting in high open-circuit voltages of the devices. Molecular dynamics simulation shows that YBO-2O/-FO preferentially form core-core and terminal-terminal dimeric interactions, demonstrating their tighter packing structure and higher electron mobility than Y6, which is consistent with 2D grazing incidence X-ray scattering and space charge limited current measurements. In blend films, the hole transfer (HT) from YBO-2O/-FO to the polymer donor PM6 is studied in detail by transient absorption spectroscopy, demonstrating efficient HT from YBO-FO to PM6 with their suitable energy level alignment. Despite the simplified synthesis, YBO-FO demonstrates photovoltaic performance similar to that of Y6, exhibiting a power conversion efficiency of 15.01%. Overall, this design strategy not only simplifies the synthetic procedures but also adjusts the electrical properties by modifying the intermolecular packing and energy level alignment, suggesting a novel simplified molecular design of Y6 derivatives.

Effect of Terminal Electron-Withdrawing Group on the Photovoltaic Performance of Asymmetric Fused-Ring Electron Acceptors.

The terminal electron-withdrawing group is thought to strongly influence π-π stacking interactions and thereby the charge transport properties of fused-ring electron acceptors (FREAs). In this work, we designed and synthesized three asymmetric/symmetric small molecule FREAs (LC301, LC302, and LC303), in which three electron-withdrawing functional groups with different polarities (phenyl-fused indanone < thienyl-fused indanone < F-modified phenyl-fused indanone) were selected as the terminal groups. Photophysical properties, electrochemistry, charge transport, and crystalline properties of the materials were studied to investigate the effect of electron-withdrawing abilities of the terminal groups on the properties of FREAs. Starting from the symmetric LC302 (two thienyl-fused indanone terminal groups), we have found that by simply replacing only one thienyl-fused indanone terminal group in symmetric LC302 with one difluorinated phenyl-fused indanone terminal group, the asymmetric LC301-based organic solar cells (OSCs) can yield a high power conversion efficiency (PCE) of 17.21% and a promising fill factor (FF) of 78.1%, which are higher than those of symmetric LC302-based OSCs (PCE = 15.18%, FF = 73.3%) and the asymmetric LC303-based OSCs bearing one thienyl-fused indanone terminal group and one nonfluorinated phenyl-fused indanone terminal group (PCE = 14.28%, FF = 70.3%). The results indicate that the rational selection of terminal groups with different electron-withdrawing capabilities is highly desirable for designing high-performance asymmetric FREAs.

Recent Advances in Nonfullerene Acceptor-Based Layer-by-Layer Organic Solar Cells Using a Solution Process.

Recently, sequential layer-by-layer (LbL) organic solar cells (OSCs) have attracted significant attention owing to their favorable p-i-n vertical phase separation, efficient charge transport/extraction, and potential for lab-to-fab large-scale production, achieving high power conversion efficiencies (PCEs) of over 18%. This review first summarizes recent studies on various approaches to obtain ideal vertical D/A phase separation in nonfullerene acceptor (NFAs)-based LbL OSCs by proper solvent selection, processing additives, protecting solvent treatment, ternary blends, etc. Additionally, the longer exciton diffusion length of NFAs compared with fullerene derivatives, which provides a new scope for further improvement in the performance of LbL OSCs, is been discussed. Large-area device/module production by LbL techniques and device stability issues, including thermal and mechanical stability, are also reviewed. Finally, the current challenges and prospects for further progress toward their eventual commercialization are discussed.

Vertically optimized phase separation with improved exciton diffusion enables efficient organic solar cells with thick active layers.

The development of organic solar cells (OSCs) with thick active layers is of crucial importance for the roll-to-roll printing of large-area solar panels. Unfortunately, increasing the active layer thickness usually results in a significant reduction in efficiency. Herein, we fabricated efficient thick-film OSCs with an active layer consisting of one polymer donor and two non-fullerene acceptors. The two acceptors were found to possess enlarged exciton diffusion length in the mixed phase, which is beneficial to exciton generation and dissociation. Additionally, layer by layer approach was employed to optimize the vertical phase separation. Benefiting from the synergetic effects of enlarged exciton diffusion length and graded vertical phase separation, an efficiency of 17.31% (certified value of 16.9%) is obtained for the 300 nm-thick OSC, with a short-circuit current density of 28.36 mA cm-2, and a high fill factor of 73.0%. Moreover, the device with an active layer thickness of 500 nm also shows an efficiency of 15.21%. This work provides valuable insights into the fabrication of OSCs with thick active layers.

Completely foldable electronics based on homojunction polymer transistors and logics.

An increase in the demand for completely foldable electronics has motivated efforts for the development of conducting polymer electrodes having extraordinary mechanical stability. However, weak physical adhesion at intrinsic heterojunctions has been a challenge in foldable electronics. This paper reports the completely foldable polymer thin-film transistors (PTFTs) and logic gate arrays. Homojunction-based PTFTs were fabricated by selectively doping p-type diketopyrrolopyrrole-based semiconducting polymer films with FeCl3 to form source/drain electrodes. The doping process caused a gradual work function change with depth, which promoted charge injection to semiconducting regions and provided a low contact resistance. In addition, the interfacial adhesion in the PTFTs was improved by interfacial cross-linking between adjacent component layers. The electrical performance of the resulting PTFTs was maintained without noticeable degradation even after extreme folding, suggesting that the proposed fabrication strategy can further be applied to various semiconducting polymers for the realization of foldable electronics.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency.

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve Jsc and FF without sacrificing Voc .

Hysteresis Behavior of the Donor-Acceptor-Type Ambipolar Semiconductor for Non-Volatile Memory Applications.

Donor-acceptor-type organic semiconductor molecules are of great interest for potential organic field-effect transistor applications with ambipolar characteristics and non-volatile memory applications. Here, we synthesized an organic semiconductor, PDPPT-TT, and directly utilized it in both field-effect transistor and non-volatile memory applications. As-synthesized PDPPT-TT was simply spin-coated on a substrate for the device fabrications. The PDPPT-TT based field-effect transistor showed ambipolar electrical transfer characteristics. Furthermore, a gold nanoparticle-embedded dielectric layer was used as a charge trapping layer for the non-volatile memory device applications. The non-volatile memory device showed clear memory window formation as applied gate voltage increases, and electrical stability was evaluated by performing retention and cycling tests. In summary, we demonstrate that a donor-acceptor-type organic semiconductor molecule shows great potential for ambipolar field-effect transistors and non-volatile memory device applications as an important class of materials.

Solution-Processed Organic Solar Cells with High Open-Circuit Voltage of 1.3 V and Low Non-Radiative Voltage Loss of 0.16 V.

Compared with inorganic or perovskite solar cells, the relatively large non-radiative recombination voltage losses (ΔVnon-rad ) in organic solar cells (OSCs) limit the improvement of the open-circuit voltage (Voc ). Herein, OSCs are fabricated by adopting two pairs of D-π-A polymers (PBT1-C/PBT1-C-2Cl and PBDB-T/PBDB-T-2Cl) as electron donors and a wide-bandgap molecule BTA3 as the electron acceptor. In these blends, a charge-transfer state energy (ECT ) as high as 1.70-1.76 eV is achieved, leading to small energetic differences between the singlet excited states and charge-transfer states (ΔECT ≈ 0.1 eV). In addition, after introducing chlorine atoms into the π-bridge or the side chain of benzodithiophene (BDT) unit, electroluminescence external quantum efficiencies as high as 1.9 × 10-3 and 1.0 × 10-3 are realized in OSCs based on PBTI-C-2Cl and PBDB-T-2Cl, respectively. Their corresponding ΔVnon-rad are 0.16 and 0.17 V, which are lower than those of OSCs based on the analog polymers without a chlorine atom (0.21 and 0.24 V for PBT1-C and PBDB-T, respectively), resulting in high Voc of 1.3 V. The ΔVnon-rad of 0.16 V and Voc of 1.3 V achieved in PBT1-C-2Cl:BTA3 OSCs are thought to represent the best values for solution-processed OSCs reported in the literature so far.

Fluorination Position: A Study of the Optoelectronic Properties of Two Regioisomers Using Spectroscopic and Computational Techniques.

The influence of the fluorination positions on the optical and electronic properties of a pair of donor-acceptor (DA)-based regioisomers is explored. In the regioisomers, the fluorination was varied between the 2 and 3 positions on benzothiadiazole (BTD) acceptor units. Although the structural variation between the regioisomers is small, significant variations in the electronic properties of the two compounds were observed. These were observed most markedly in the excited-state properties with a 15 nm (280-390 cm-1) red shift of the emission between the regioisomers. The combination of resonance Raman spectroscopy (RRS) and density functional theory (DFT) calculations was used to probe the possible causes of the observed variations. The analysis suggested a F···S through-space interaction as being responsible for tuning both the electronic properties and rigidity of the compounds. Shifting the fluorine atoms shifted the location of the F···S interaction, changing which part of the molecule was locked down, and showed a variation in the overall rigidity of the molecule. In this series, the influence could be varied between the core and periphery. This study adds to a growing body of work demonstrating the effectiveness of selective fluorination in tailoring the properties of organic molecules.

Fuller-Rylenes: Paving the Way for Promising Acceptors.

The hybridization of different acceptors remains a fertile ground awaiting exploration, to fully promote the properties of both components. The concept of this work is to exploit a new form of fuller-rylene hybrids as promising acceptors by integrating planar rylene dye and spherical fullerene for boosting the power conversion efficiency. The synthesis of the fuller-rylenes via a straightforward synthetic strategy by one-pot Pd-catalyzed cyclization can be scaled-up. Specifically, our strategy allows the supplements and enhancement of absorption in the visible region, much wider structural and electronic variations by installing R1 groups as well as decorating R2 on the perylene core at will, and good processability without compromising the superior characteristics of fullerene. Thus, bay-decorated fuller-rylene S-Fuller-PMI revealed a ground-breaking efficiency as high as 8.01%, even outperforming [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as a parallel comparison (7.09%). Our exploration paves a new way for the design of high-efficiency acceptors, which are promising alternatives to PC61BM in photovoltaic devices.

Recent progress in indoor organic photovoltaics.

Among various potential applications of organic photovoltaics (OPVs), indoor power generation has great potential because of several advantages over outdoor light harvesting under 1 sun conditions. Commonly used indoor light sources have narrower emission spectra with lower intensity (by 3 orders of magnitude) as compared to the solar spectrum. Highly tunable optical absorption, large absorption coefficients, and small leakage currents under dim lighting conditions make OPVs promising candidates for indoor applications. For optimizing indoor photovoltaic materials and devices, several key issues (different from those under 1 sun conditions), such as developing new indoor photovoltaic materials and devices with suitable absorption spectra, large open-circuit voltages with low energy loss, minimized trap-mediated charge recombination and leakage currents, and device stability under indoor conditions, should be considered carefully. In this review, the recent progress in optimization of indoor photovoltaic materials and devices, and the key strategies to optimize the indoor photovoltaic characteristics will be summarized and discussed.

Efficient Fused-Ring Extension of A-D-A-Type Non-Fullerene Acceptors by a Symmetric Replicating Core Unit Strategy.

The extension of fused aromatic ring core structures is beneficial for enhancing intramolecular charge transfer and effective π conjugation in A-D-A-type (A=acceptor; D=donor) non-fullerene acceptors (NFAs). In this work, a novel strategy involving the extension of a fused-ring core by symmetrically replicating the core unit has been developed, and a novel symmetric fused-12-ring NFA, LC81, has been synthesized. When paired with the wide-bandgap polymer donor PBT1-C, the corresponding organic solar cells (OSCs) showed a high power conversion efficiency of 12.71 %, much higher than that of the device based on the reference NFA, TPTT-4F. Moreover, the LC81-based OSC displayed a lower energy loss and a better ambient stability than the TPTT-4F-based device. Our results indicate that the extension of the fused-ring core by the symmetric replicating core unit strategy is an effective approach to promoting the photovoltaic characteristics of A-D-A-type NFAs.

Synthesis, Molecular Packing, and Electrical Properties of New Regioisomeric n-type Semiconducting Molecules with Modification of Alkyl Substituents Position.

We design and synthesize a series of regioisomeric n-type small molecules, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by two-dimensional NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed organic field-effect transistors (OFETs). Interestingly, a minor change in the substitution position of the octyl side chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 °C, the kinked DPP(EH)-INCNO1 (EH = 2-ethylhexyl) structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ∼0.6 cm2V-1 s-1 compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 (HD = 2-hexyldecyl) molecules become more crystalline with thermal treatments, but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.

High-Performance Near-Infrared-Selective Thin Film Organic Photodiode Based on a Molecular Approach Targeted to Ideal Semiconductor Junctions.

A molecular approach to achieve wide linear dynamic range (LDR) and near-infrared (NIR)-selective thin film organic photodiodes (OPDs) with high detectivity is reported. Comparative studies based on two NIR-selective polymers are systematically investigated: the commercially available poly[(4,4'-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b']dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTBT) and the synthesized poly[(4,4'-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b']-dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT). The introduction of sp2-hybridized side chains in the PCPDTSBT structure can improve chain planarity and thus intermolecular interactions, as confirmed by Raman spectroscopy and grazing incidence X-ray diffraction studies. The favorable crystalline orientation of PCPDTSBT leads to enhanced photocurrent and suppressed noise current, compared to that of PCPDTBT, followed by a sharp increase in the specific detectivity of PCPDTSBT-based NIR OPDs by 1.54 × 1012 Jones. The physics behind PCPDTSBT is analyzed employing optical simulation, temperature-dependent junction analyses, and Mott-Schottky analysis. Furthermore, it is found that PCPDTSBT possesses an exceptional nonsaturation photocurrent, which leads to a wide LDR of 128 dB. This study shows the possibility of realizing thin film NIR-selective OPDs using synthetic approaches.

High-Performance Photomultiplication Photodiode with a 70 nm-Thick Active Layer Assisted by IDIC as an Efficient Molecular Sensitizer.

Here, a smart strategy for decreasing the active layer thickness of the organic photodiode down to 70 nm is demonstrated by utilizing a trap-assisted photomultiplication mechanism with the optimized chemical composition. Despite the presence of a high dark current, dramatically enhanced external quantum efficiency (EQE) via photomultiplication can allow significantly reduced active layer thickness, yielding high detectivity comparable to that of conventional Si. To achieve this, a spatially confined and electrically isolated optical sensitizer, 2,2'-((2 Z,2' Z)-((4,4,9,9-tetrahexyl-4,9-dihydro- s-indaceno[1,2- b:5,6- b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1 H-indene-2,1-diylidene))dimalononitrile (IDIC) was introduced strategically between a hole transport active layer and a cathode. A nonfullerene acceptor, IDIC, turned out to be a much more efficient sensitizer than the conventional fullerene-based acceptors, as confirmed by the effective lowering of the Schottky barrier under illumination, as well as the highest EQE exceeding 130 000%. Due to its favorable electronic structure as well as two-dimensional molecular structure, a high detectivity over 1012 Jones was successfully demonstrated while maintaining the active layer thickness as 70 nm.