Synergistic Contribution of Oligo(ethylene glycol) and Fluorine Substitution of Conjugated Polymer Photocatalysts toward Solar Driven Sacrificial Hydrogen Evolution.

To separately explore the importance of hydrophilicity and backbone planarity of polymer photocatalyst, a series of benzothiadiazole-based donor-acceptor alternating copolymers incorporating alkoxy, linear oligo(ethylene glycol) (OEG) side chain, and backbone fluorine substituents is presented. The OEG side chains in the polymer backbone increase the surface energy of the polymer nanoparticles, thereby improving the interaction with water and facilitating electron transfer to water. Moreover, the OEG-attached copolymers exhibit enhanced intermolecular packing compared to polymers with alkoxy side chains, which is possibly attributed to the self-assembly properties of the side chains. Fluorine substituents on the polymer backbone produce highly ordered lamellar stacks with distinct π-π stacking features; subsequently, the long-lived polarons toward hydrogen evolution are observed by transient absorption spectroscopy. In addition, a new nanoparticle synthesis strategy using a methanol/water mixed solvent is first adopted, thereby avoiding the screening effect of surfactants between the nanoparticles and water. Finally, hydrogen evolution rate of 26 000 µmol g-1  h-1 is obtained for the copolymer incorporated with both OEG side chains and fluorine substituents under visible-light irradiation (λ > 420 nm). This study demonstrates how the glycol side chain strategy can be further optimized for polymer photocatalysts by controlling the backbone planarity.

Design and Synthesis of CdHgSe/HgS/CdZnS Core/Multi-Shell Quantum Dots Exhibiting High-Quantum-Yield Tissue-Penetrating Shortwave Infrared Luminescence.

Cdx Hg1- x Se/HgS/Cdy Zn1- y S core/multi-shell quantum dots (QDs) exhibiting bright tissue-penetrating shortwave infrared (SWIR; 1000-1700 nm) photoluminescence (PL) are engineered. The new structure consists of a quasi-type-II Cdx Hg1- x Se/HgS core/inner shell domain creating luminescent bandgap tunable across SWIR window and a wide-bandgap Cdy Zn1- y S outer shell boosting the PL quantum yield (QY). This compositional sequence also facilitates uniform and coherent shell growth by minimizing interfacial lattice mismatches, resulting in high QYs in both organic (40-80%) and aqueous (20-70%) solvents with maximum QYs of 87 and 73%, respectively, which are comparable to those of brightest visible-to-near infrared QDs. Moreover, they maintain bright PL in a photocurable resin (QY 40%, peak wavelength ≈ 1300 nm), enabling the fabrication of SWIR-luminescent composites of diverse morphology and concentration. These composites are used to localize controlled amounts of SWIR QDs inside artificial (Intralipid) and porcine tissues and quantitatively evaluate the applicability as luminescent probes for deep-tissue imaging.

Non-power-driven organic photodiode via junction engineering.

Here we introduce a junction engineering approach to realize a high performance non-power-driven organic photodiode. To overcome the external power source dependency of conventional photodiodes, in this work, we try not only to implement an inherently large built-in-potential of the junction but also to utilize an inherently low charge carrier concentration of the semiconductor. The strategically designed ITO/plasma-treated ZnO/poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV)/MoO3/Ag geometry showed near-ideal Schottky junction properties with a high zero-bias built-in potential of 0.54 eV, leading to a zero-bias depletion width of 470 nm. As a result, a green-selective polymeric photodiode with high zero-bias detectivity up to 5 × 1011 Jones and a low noise equivalent power of 2.98 × 10-12 W Hz-1/2 are demonstrated, revealing the possibility of a thin film, color-selective and non-power-driven polymeric photodiode for battery-free application.

Polyvinyl alcohol covalently grafted CNT for free-standing, flexible, and high-performance thermoelectric generator film.

We introduce a strategic approach to synthesize covalntly cross-linked carbon nanotube (CNT)-polymer nanocomposites, which can be applied as a free-standing and flexible organic thermoelectric generator film. Esterification of polyvinyl alcohol (PVA) to render PVA-COOH followed by an amide reaction with single-walled CNTs (SWCNTs) functionalized with amino groups (SWCNT-NH2) yielded a covalently grafted PVA/SWCNT composite film with an excellent dispersion of SWCNTs within the polymer matrix as confirmed using Fourier-transform infrared spectroscopy and scanning electron microscopy. This amide reaction could be further optimized with the addition of a small amount of Triton™ X-100, which resulted in a better dispersion of SWCNT prior to the amide condensation reaction. Consequently, a covalently cross-linked PVA/SWCNT composite film showed better Seebeck coefficients than those of previously reported non-covalently, physically wrapped polymer/CNT composite films, resulting in a high power factor up to 275 µW m-1 K-2. Furthermore, a covalent amide-linking between PVA and SWCNT yielded a free-standing film (30 × 30 mm) with excellent flexibility and notable shelf stability as confirmed by negligible changes in thermoelectric parameters after bending test for 10 000 times with a bending radius of 2 mm and also shelf stability test in ambient condition without any passivation layer for 30 d.

Organic-inorganic hybrid inverted photodiode with planar heterojunction for achieving low dark current and high detectivity.

In this study, the strategy of using an organic-inorganic hybrid planar heterojunction consisting of polymeric semiconductors and inorganic nanocrystals is introduced to realize a high-performance hybrid photodiode (HPD) with low dark current and high detectivity. To prevent undesired charge injection under the reverse bias condition, which is the major dark current source of the photodiode, a well-defined planar heterojunction is strategically constructed via smart solution process techniques. The optimized HPD renders a low dark current of ∼10(-5) mA cm(-2) at -5 V and ∼10(-6) mA cm(-2) at -1 V, as well as a high detectivity ∼10(12) Jones across the entire visible wavelength range. Furthermore, excellent photocurrent stability is demonstrated under continuous light exposure. We believe that the solution-processed planar heterojunction with inverted structure can be an attractive alternative diode structure for fabricating high-performance HPDs, which usually suffer from high dark current issues.

Synergetic Evolution of Diketopyrrolopyrrole-Based Polymeric Semiconductor for High Reproducibility and Performance: Random Copolymerization of Similarly Shaped Building Blocks.

A new random copolymer consisting of similarly shaped donor-acceptor building blocks of diketopyrrolopyrrole-selenophene-vinylene-selenophene (DPP-SVS) and DPP-thiophene-vinylene-thiophene (DPP-TVT) is designed and synthesized. The resulting P-DPP-SVS(5)-TVT(5) with an equal molecular ratio of the two building blocks produced significantly enhanced solubility when compared to that of the two homopolymers, PDPP-SVS and PDPP-TVT. More importantly, despite the maximum segmental randomness of the PDPP-SVS(5)-TVT(5) copolymer, its crystalline perfectness and preferential orientation are outstanding, even similar to those of the homopolymers thanks to the similarity of the two building blocks. This unique property produces a high charge carrier mobility of 1.23 cm2 V-1 s-1 of PDPP-SVS(5)-TVT(5), as determined from polymer field-effect transistor (PFET) measurements. The high solubility of PDPP-SVS(5)-TVT(5) promotes formulation of high-viscosity solutions which could be successfully processed to fabricate large-areal PFETs onto hydrophobically treated 4 in. wafers. A total of 269 individual PFETs are fabricated. These devices exhibit extremely narrow device-to-device deviations without a single failure and demonstrate an average charge carrier mobility of 0.66 cm2 V-1 s-1 with a standard deviation of 0.064. This is the first study to report on successfully realizing large-areal reproducibility of high-mobility polymeric semiconductors.

Naphtho[2,1-b:3,4-b']dithiophene-based bulk heterojunction solar cells: how molecular structure influences nanoscale morphology and photovoltaic properties.

Organic bulk heterojunction photovoltaic devices based on a series of three naphtho[2,1-b:3,4-b']dithiophene (NDT) derivatives blended with phenyl-C71-butyric acid methyl ester were studied. These three derivatives, which have NDT units with various thiophene-chain lengths, were employed as the donor polymers. The influence of their molecular structures on the correlation between their solar-cell performances and their degree of crystallization was assessed. The grazing-incidence angle X-ray diffraction and atomic force microscopy results showed that the three derivatives exhibit three distinct nanoscale morphologies. We correlated these morphologies with the device physics by determining the J-V characteristics and the hole and electron mobilities of the devices. On the basis of our results, we propose new rules for the design of future generations of NDT-based polymers for use in bulk heterojunction solar cells.

Achieving high sensitivity in hybrid photodetectors based on an organic single crystal and an inorganic nanocrystal array.

We demonstrate an approach to enhance the photosensitivity of an organic single-crystal photodetector by combining it with a nanocrystal (NC) array. A systematic study of the dependence of the photodetector performance on illumination wavelength and light power together with the charge carrier mobility reveals that charge separation at the single-crystal/CdSe NC interface and subsequent electron trapping in the CdSe NCs generates effective photoconductive gain by hole circulation through the high-mobility single crystal. As a result, the responsivity and specific detectivity reached values up to 50 A W(-1) and 2 × 10(9) cm Hz(1/2) W(-1), respectively.

Interface engineering of a highly sensitive solution processed organic photodiode.

We report on tuning of the interfacial properties of a highly sensitive organic photodiode by introducing a buffer layer between the anode and the semiconductor layer. The effects of different buffer layers consisting of a self-assembled monolayer (SAM), PEDOT:PSS, and pentacene on the morphology and crystallinity of the upper-deposited bulk heterojunction semiconductor layer are carefully analyzed combined with electrical analysis. The active layer is controlled to be nearly homogeneous and to have low crystallinity by using a SAM or PEDOT:PSS buffer layers, whereas a highly crystalline morphology is realized by using the pentacene buffer layer. When exposed to light pulses, the external quantum efficiency and thus the photocurrent are slightly higher for the PEDOT:PSS-based photodiode; however the dark current is the lowest for the pentacene-based photodiode. We discuss the origin of the high sensitivity (a detectivity of 1.3 × 10(12) Jones and a linear dynamic range of 95 dB) of the pentacene-based photodiode, particularly in terms of the morphology-driven low dark current.

The effect of branched versus linear alkyl side chains on the bulk heterojunction photovoltaic performance of small molecules containing both benzodithiophene and thienopyrroledione.

To evaluate the effect of side chain characteristics on the photovoltaic performance of small molecules containing both benzodithiophene (BDT) and thienopyrroledione (TPD), we designed and synthesized two such molecules, one containing a branched 2-ethylhexyl (2EH) side chain on the BDT unit (BDTEH-TTPD) and the other containing a linear n-octyl (C8) side chain on the BDT unit (BDTO-TTP). The optical and electrochemical properties and crystalline structures of these molecules were examined. Compared to BDTO-TTPD, BDTEH-TTPD, showed stronger light absorption, longer-range ordering and shorter π-π stacking distances between backbones. As a result, the power conversion efficiency of a bulk heterojunction solar cell based on BDTEH-TTPD (2.40%) was substantially higher than that of the BDTO-TTPD device (1.12%).

Shortwave Infrared Organic Photodiodes Realized by Polaron Engineering.

A novel approach for developing shortwave IR (SWIR) organic photodiodes (OPDs) using doped polymers is presented. SWIR OPDs are challenging to produce because of the limitations in extending the absorption of conjugated molecules and the high dark currents of SWIR-absorbing materials. Herein, it is shown that the conversion of bound polarons to free polarons by light energy can be utilized as an SWIR photodetection mechanism. To maximize the bound-polaron density and bound-to-free polaron ratio of the doped polymer film, the doping process is engineered and dopant molecules are diffused into the crystalline domain of the polymer matrix and a direct correlation between the bound-to-free polaron ratio and device performance is confirmed. The optimized double-doped SWIR OPD exhibits a high external quantum efficiency of 77 100% and specific detectivity of 1.11 × 1011 Jones against SWIR. These findings demonstrate the application potential of polarons as alternatives for Frenkel excitons in SWIR OPDs.

Photophore-Anchored Molecular Switch for High-Performance Nonvolatile Organic Memory Transistor.

Over the past decade, molecular-switch-embedded memory devices, particularly field-effect transistors (FETs), have gained significant interest. Molecular switches are integrated to regulate the resistance or current levels in FETs. Despite substantial efforts, realizing large memory window with a long retention time, a critical factor in memory device functionality, remains a challenge. This is due to the inability of an isomeric state of a molecular switch to serve as a stable deep trap state within the semiconductor layer. Herein, the study addresses this limitation by introducing chemical bonding between molecular switch and conjugated polymeric semiconductor, facilitating closed isomer of diarylethene (DAE) to operate as a morphologically stable deep trap state. Azide- and diazirine-anchored DAEs are synthesized, which form chemical bonds to the polymer through photocrosslinking, thereby implementing permanent and controllable trapping states nearby conjugated backbone of polymer semiconductor. Consequently, when diazirine-anchored DAE is blended with F8T2 and subjected to photocrosslinking, the resulting organic FETs exhibit remarkable memory performance, including a memory window of 22 V with a retention time over 106 s, a high photoprogrammable on/off ratio over 103, and a high operational stability over 100 photocycles. Further, photophore-anchored DAEs can achieve precise patterning, which enables meticulous control over the semiconductor layer structure.

Boosting the Performance of Photomultiplication-Type Organic Photodiodes by Embedding CsPbBr3 Perovskite Nanocrystals.

In this study, it is demonstrated that CsPbBr3 perovskite nanocrystals (NCs) can enhance the overall performances of photomultiplication-type organic photodiodes (PM-OPDs). The proposed approach enables the ionic-polarizable CsPbBr3 NCs to be evenly distributed throughout the depletion region of Schottky junction interface, allowing the entire trapped electrons within the depletion region to be stabilized, in contrast to previously reported interface-limited strategies. The optimized CsPbBr3 -NC-embedded poly(3-hexylthiophene-diyl)-based PM-OPDs exhibit exceptionally high external quantum efficiency, specific detectivity, and gain-bandwidth product of 2,840,000%, 3.97 × 1015 Jones, and 2.14 × 107  Hz, respectively. 2D grazing-incidence X-ray diffraction analyses and drift-diffusion simulations combined with temperature-dependent J-V characteristic analyses are conducted to investigate the physics behind the success of CsPbBr3 -NC-embedded PM-OPDs. The results show that the electrostatic interactions generated by the ionic polarization of NCs effectively stabilize the trapped electrons throughout the entire volume of the photoactive layer, thereby successfully increasing the effective energy depth of the trap states and allowing efficient PM mechanisms. This study demonstrates how a hybrid-photoactive-layer approach can further enhance PM-OPD when the functionality of inorganic inclusions meets the requirements of the target device.

Visible-Light Communication with Lighting: RGB Wavelength Division Multiplexing OLEDs/OPDs Platform.

A multichannel/multicolor visible light communication (VLC) system using entirely organic components, including organic light emitting diodes (OLEDs) and organic photodiodes (OPDs), is developed to demonstrate indoor lighting applications where the integration of OLEDs and OPDs has significant potential. To achieve this, tricolor (Red/Green/Blue(R/G/B))-selective OPD arrays for the receiver and tricolor OLED arrays for the emitter are developed. For (R/G/B)-selective OPDs, a Fabry-Pérot electrode to enhance color selectivity and a thick junction structure to effectively accommodate a wide range of driving voltages are introduced. For tricolor OLEDs, fluorescent-emitting materials are used to enhance the operating frequency in addition to introducing a cavity structure to achieve narrow emission. Utilizing these spectrally refined tricolor OPDs/OLEDs, a VLC system is designed for indoor lighting applications, and a systematic analysis of their signal-to-interference ratio dependence on the distance or angle between the transmitter and receiver is performed. The study's findings indicate the importance of emission angle-dependent wavelength shift of the OLED and the luminosity function, which varies with wavelength, in the R/G/B mixed-white-light-based VLC systems. Finally, the feasibility of VLC using tricolor OPDs/OLEDs in the real-life context of indoor white-color lighting is demonstrated, showing that the transmitted data patterns well-matched the received data patterns.

Advancing Fab-Compatible Color-Selective Organic Photodiodes: Tailored Molecular Design and Nanointerlayers.

High-performance organic photodiodes (OPDs) and OPD-based image sensors are primarily realized using solution processes based on various additives and coating methods. However, vacuum-processed OPDs, which are more compatible with large-scale production, have received little attention, thereby hindering their integration into advanced systems. This study introduces innovations in the material and device structures to prepare superior vacuum-processed OPDs for commercial applications. A series of vacuum-processable, low-cost p-type semiconductors is developed by introducing an electron-rich cyclopentadithiophene core containing various electron-accepting moieties to fine-tune the energy levels without any significant structural or molecular weight changes. An additional nanointerlayer strategy is used to control the crystalline orientation of the upper-deposited photoactive layer, compensating for device performance reduction in inverted, top-illuminated OPDs. These approaches yielded an external quantum efficiency of 70% and a specific detectivity of 2.0 × 1012 Jones in the inverted structures, which are vital for commercial applications. These OPDs enabled visible-light communications with extremely low bit error rates and successful X-ray image capture.

Vertically Phase Separated Photomultiplication Organic Photodetectors with p-n Heterojunction Type Ultrafast Dynamic Characteristics.

Photomultiplication (PM)-type organic photodetectors (OPDs), which typically form a homogeneous distribution (HD) of n-type dopants in a p-type polymer host (HD PM-type OPDs), have achieved a breakthrough in device responsivity by surpassing a theoretical limit of external quantum efficiency (EQE). However, they face limitations in higher dark current and slower dynamic characteristics compared to p-n heterojunction (p-n HJ) OPDs due to inherent long lifetime of trapped electrons. To overcome this, we have developed a new PM-type OPD that demonstrates ultrafast dynamic properties through a vertical phase separation (VPS) strategy between the p-type polymer (host) and n-type acceptor (dopant), referred to as VPS PM-type OPDs. Notably, VPS PM-type OPDs show a remarkable increase (by three orders of magnitude) in -3 dB cut-off frequency (120 kHz) and over a 200-fold faster response time (rising time = 4.8 µs, falling time = 8.3 µs) compared to HD PM-type OPDs, while maintaining high EQE of 1,121% and specific detectivity of 2.53 × 1013 Jones at -10 V. The VPS PM OPD represents a groundbreaking advancement by demonstrating the coexistence of p-n HJ and PM modes within a single photoactive layer for the first time. This innovative approach holds the potential to enhance both static and dynamic properties of OPDs. This article is protected by copyright. All rights reserved.

Hydrophilic Photocrosslinkers as a Universal Solution to Endow Water Affinity to a Polymer Photocatalyst for an Enhanced Hydrogen Evolution Rate.

A universal approach for enhancing water affinity in polymer photocatalysts by covalently attaching hydrophilic photocrosslinkers to polymer chains is presented. A series of bisdiazirine photocrosslinkers, each comprising bisdiazirine photophores linked by various aliphatic (CL-R) or ethylene glycol-based bridge chains (CL-TEG), is designed to prevent crosslinked polymer photocatalysts from degradation through a safe and efficient photocrosslinking reaction at a wavelength of 365 nm. When employing the hydrophilic CL-TEG as a photocrosslinker with polymer photocatalysts (F8BT), the hydrogen evolution reaction (HER) rate is considerably enhanced by 2.5-fold compared to that obtained using non-crosslinked F8BT photocatalysts, whereas CL-R-based photocatalysts yield HER rates comparable to those of non-crosslinked counterparts. Photophysical analyses including time-resolved photoluminescence and transient absorption measurements reveal that adding CL-TEG accelerates exciton separation, forming long-lived charge carriers. Additionally, the in-depth study using molecular dynamics simulations elucidates the dual role of CL-TEG: it enhances water penetration into the polymer matrix and stabilizes charge carriers after exciton generation against undesirable recombination. Therefore, the strategy highlights endowing a high-permittivity environment within polymer photocatalyst in a controlled manner is crucial for enhancing photocatalytic redox reactivity. Furthermore, this study shows that this hydrophilic crosslinker approach has a broad applicability in general polymer semiconductors and their nanoparticulate photocatalysts.

Record high hole mobility in polymer semiconductors via side-chain engineering.

Charge carrier mobility is still the most challenging issue that should be overcome to realize everyday organic electronics in the near future. In this Communication, we show that introducing smart side-chain engineering to polymer semiconductors can facilitate intermolecular electronic communication. Two new polymers, P-29-DPPDBTE and P-29-DPPDTSE, which consist of a highly conductive diketopyrrolopyrrole backbone and an extended branching-position-adjusted side chain, showed unprecedented record high hole mobility of 12 cm(2)/(V·s). From photophysical and structural studies, we found that moving the branching position of the side chain away from the backbone of these polymers resulted in increased intermolecular interactions with extremely short π-π stacking distances, without compromising solubility of the polymers. As a result, high hole mobility could be achieved even in devices fabricated using the polymers at room temperature.

Effect of solvent on detectivity of solution-processed polymer photodetectors.

We demonstrate a high-performance solution-processed polymer photodetector (PPD) whose detectivity (D*) exceeds 10(11) Jones over almost the entire range of visible light. PPDs processed from a chlorobenzene solution using 1,8-diiodooctane (DIO) as an additive exhibit large crystalline domains with long-range order, in contrast to the featureless morphology of PPDs processed with pure solvent. Moreover, PPDs processed with DIO exhibit higher charge carrier mobility, and thus high photoconductive gain without compromising long carrier lifetime. Consequently, a high-loading ratio of DIO (3%) yielded a PPD whose D* in the visible range was over threefold higher than that of PPD without DIO.

Synthesis of poly(benzothiadiazole-co-dithienobenzodithiophenes) and effect of thiophene insertion for high-performance polymer solar cells.

We describe herein the synthesis of novel donor-acceptor conjugated polymers with dithienobenzodithiophenes (DTBDT) as the electron donor and 2,1,3-benzothiadiazole as the electron acceptor for high-performance organic photovoltaics (OPVs). We studied the effects of strategically inserting thiophene into the DTBDT as a substituent on the skeletal structure on the opto-electronic performances of fabricated devices. From UV/Vis absorption, electrochemical, and field-effect transistor analyses, we found that the thiophene-containing DTBDT derivative can substantially increase the orbital overlap area between adjacent conjugated chains and thus dramatically enhance charge-carrier mobility up to 0.55 cm(2) V(-1) s(-1). The outstanding charge-transport characteristics of this polymer allowed the realization of high-performance organic solar cells with a power conversion efficiency (PCE) of 5.1 %. Detailed studies on the morphological factors that enable the maximum PCE of the polymer solar cells are discussed along with a hole/electron mobility analysis based on the space-charge-limited current model.