Han Young Woo

(Semi)ladder-Type Bithiophene Imide-Based All-Acceptor Semiconductors: Synthesis, Structure–Property Correlations, and Unipolar n-Type Transistor Performance

Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTI n and semiladder-type homopolymers PBTI n ( n = 1-5) were synthesized, and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (μe). Due to their all-acceptor backbone, these polymers all exhibit unipolar n-type transport in organic thin-film transistors, accompanied by low off-currents (10-10-10-9 A), large on/off current ratios (106), and small threshold voltages (∼15-25 V). The highest μe, up to 3.71 cm2 V-1 s-1, is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the μe drops by 2 orders to 0.014 cm2 V-1 s-1 for PBTI5. This monotonic decrease of μe was also observed in their homologous BTI n small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor-type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.

The crucial role of intermolecular π–π interactions in A–D–A-type electron acceptors and their effective modulation

Here, the crucial role of intermolecular π–π interactions in A–D–A-type acceptors is examined and a simple yet effective modulation method is presented. By side chain manipulation, two small molecule acceptors, IDT-C6 and IDT-PhC6, are designed and synthesized with different crystalline morphologies. Since n-hexyl side chains have less steric hindrance than hexylphenyl side chains, IDT-C6 exhibits enhanced intermolecular π–π interactions relative to IDT-PhC6. Benefiting from the improved charge transport properties of the IDT-C6-based device, a high PCE of 12.5% is recorded, whereas those of the IDT-PhC6-based device delivers a moderate PCE of just 5.6%.

Engineering the morphology via processing additives in multiple all-polymer solar cells for improved performance

In this contribution, we report the working mechanisms of several processing additives for controlling the morphology of four all-polymer systems. The optical and electrical properties, photovoltaic performance, morphology and the dynamic process of film formation of these all-polymer systems were thoroughly examined. We revealed that the effect of additives is largely dependent on the aggregation behaviors of the polymers used. Here, the polymer acceptors with large planar structures have stronger inter-chain interactions, which make their morphology more susceptible to additive treatment compared to the donors. 1,8-Di(R)octane (R = Cl, Br, and I) additives can be applied to multiple all-polymer devices with improved efficiency due to their general capability to increase the crystallinity and extend the effective time during the film formation. Interestingly, DBrO outperforms the widely used DIO, obtaining a highest efficiency of 8.23% for the PTzBI/P(NDI2OD-T2) based all-polymer solar cells, indicating finer morphology control by a subtle change of the additive structure. In contrast, the addition of chloronaphthalene (CN) can alleviate the inter-chain interaction of polymers to prevent the formation of oversized domains, which make it especially efficient for systems using strongly aggregated polymers like P(NDI2OD-T2). Our results provide insight into processing additives and suggest guidelines to rationally select additives for nonfullerene solar cells.

Cyano-substituted benzochalcogenadiazole-based polymer semiconductors for balanced ambipolar organic thin-film transistors

Due to their high-lying lowest unoccupied molecular orbitals (LUMOs), π-conjugated polymers based on benzothiadiazole and its derivatives typically are p-type. We report here the successful development of two narrow bandgap, ambipolar donor–acceptor copolymers, PDCNBT2T and PDCNBSe2T, which are based on new cyano-substituted strong electron acceptors, 4,7-dibromo-5,6-dicyano-2,1,3-benzothiadiazole (DCNBT) and 4,7-dibromo-5,6-dicyano-2,1,3-benzoselenadiazole (DCNBSe), respectively. Compared to their polymer analogues with fluorine substituents, the LUMO was lowered by a big margin of ca. 0.6 eV and the bandgap was reduced by 0.2–0.3 eV for the cyano-substituted polymers. Therefore, the cyano-substituted benzothiadiazole polymers showed very low-lying LUMO levels of ca. 4.3 eV. Benefiting from their narrow bandgaps of 1.1–1.2 eV and appropriately positioned LUMO levels, both polymers exhibit well balanced ambipolar transport characteristics in organic thin-film transistors, which differ from the p-type dominating transport properties of their fluorinated polymer analogues. A balanced hole/electron mobility of 0.59/0.47 cm2 V−1 s−1 was achieved for polymer PDCNBT2T, and a reduced hole/electron mobility of 0.018/0.014 cm2 V−1 s−1 was observed for the benzoselenadiazole-based PDCNBSe2T due to its lower crystallinity. These results show that the electron mobility can be enhanced by approximately two orders versus the electron mobility of the previously reported 4,7-di(thiophen-2-yl)-5,6-dicyano-2,1,3-benzothiadiazole-based polymer. This improvement was achieved by using the new acceptor units without additional electron-rich thiophene flanks, which allow a higher degree of freedom in selecting the donor co-unit and more effective tuning of energy levels of frontier molecular orbitals.

Solvent-vapor-annealed A–D–A-type semicrystalline conjugated small molecules for flexible ambipolar field-effect transistors

This paper reports a series of acceptor–donor–acceptor (A–D–A)-type small molecules (named P3T4-VCN, P3T4-RCN, and P3T4-INCN) based on an oligothiophene–phenylene core with three different electron-accepting terminal groups—dicyanovinyl (VCN), cyano-rhodanine (RCN), and cyano-indanone (INCN), respectively—for application to flexible ambipolar organic field-effect transistors (OFETs). Intrachain noncovalent coulombic interactions (via S–F and H–F interactions) were incorporated into the design of the P3T4 backbone to enhance the chain planarity. All the P3T4-based OFETs exhibited ambipolar behavior with hole-dominant transport, and the OFET performances were strongly dependent on the terminal groups. The P3T4-INCN OFET exhibited the highest carrier mobility owing to the extended π-conjugation via the INCN moiety, which enhanced the intermolecular cofacial π–π stacking and generated an efficient carrier pathway in the transistor channel. Room temperature solvent vapor annealing resulted in a dramatic increase in the carrier mobility of the OFETs without causing any damage to a polyethylene naphthalate (PEN) plastic substrate. The effects of both the terminal groups of the P3T4 small molecules and solvent vapor annealing were systematically investigated by UV-vis absorption spectroscopy, two-dimensional grazing incidence X-ray diffraction, and atomic force microscopy. In addition, a flexible OFET array with solvent-vapor-annealed P3T4-INCN was successfully fabricated on a PEN substrate. These OFET devices exhibited a hole mobility of 0.15 cm2 V−1 s−1, an electron mobility of 0.05 cm2 V−1 s−1, an on–off current ratio of ∼105, and excellent mechanical stability even after 300 bending cycles.

Enhanced Electron Transfer Mediated by Conjugated Polyelectrolyte and Its Application to Washing-Free DNA Detection

Direct electron transfer between a redox label and an electrode requires a short working distance (<1-2 nm), and in general an affinity biosensor based on direct electron transfer requires a finely smoothed Au electrode to support efficient target binding. Here we report that direct electron transfer over a longer working distance is possible between (i) an anionic π-conjugated polyelectrolyte (CPE) label having many redox-active sites and (ii) a readily prepared, thin polymeric monolayer-modified indium-tin oxide electrode. In addition, the long CPE label (∼18 nm for 10 kDa) can approach the electrode within the working distance after sandwich-type target-specific binding, and fast CPE-mediated oxidation of ammonia borane along the entire CPE backbone affords high signal amplification.

Measuring the competition between bimolecular charge recombination and charge transport in organic solar cells under operating conditions

The rational design of new high performance materials for organic photovoltaic (OPV) applications is largely inhibited by a lack of design rules for materials that have slow bimolecular charge recombination. Due to the complex device physics present in OPVs, rigorous and reliable measurement techniques for charge transport and charge recombination are needed to construct improved physical models that can guide materials development and discovery. Here, we develop a new technique called impedance-photocurrent device analysis (IPDA) to quantitatively characterize the competition between charge extraction and charge recombination under steady state operational conditions. The measurements are performed on actual lab scale solar cells, have mild equipment requirements, and can be integrated into normal device fabrication and testing workflows. We perform IPDA tests on a broad set of devices with varying polymer:fullerene blend chemistry and processing conditions. Results from the IPDA technique exhibit significantly improved reliability and self-consistency compared to the open-circuit voltage decay technique (OCVD). IPDA measurements also reveal a significant negative electric field dependence of the bimolecular recombination coefficient in high fill factor devices, a finding which is inaccessible to most other common techniques and indicates that many of these techniques may overestimate the value that is most relevant for describing device performance. Future work utilizing IPDA to build structure–property relationships for bimolecular recombination will lead to enhanced design rules for creating efficient OPVs that are suitable for commercialization.

Conjugated Polyelectrolytes Bearing Various Ion Densities: Spontaneous Dipole Generation, Poling‐Induced Dipole Alignment, and Interfacial Energy Barrier Control for Optoelectronic Device Applications

Conjugated polyelectrolytes (CPEs) with π-delocalized main backbones and ionic pendant groups are intensively studied as interfacial layers for efficient polymer-based optoelectronic devices (POEDs) because they facilitate facile control of charge injection/extraction barriers. Here, a simple and effective method of performing precise interfacial energy level adjustment is presented by employing CPEs with different thicknesses and various ion densities under electric poling to realize efficient charge injection/extraction of POEDs. The effects of the CPE ion densities and electric (positive or negative) poling on the energy level tuning process are investigated by measuring the open-circuit voltages and current densities of devices with the structure indium tin oxide/zinc oxide/CPE/organic active layer/molybdenum oxide/gold while changing the CPE film thickness. The performances of inverted polymer light-emitting diodes and inverted polymer solar cells are remarkably improved by precisely controlling the interfacial energy level matching using optimum CPE conditions.

Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

We synthesized three semicrystalline polymers (PTTBTBO, PDTBTBO, and P2FDTBTBO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (Rs) and high shunt (Rsh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm-2). Considering these factors, among three polymers, PDTBTBO polymer solar cells (PSCs) exhibited the most desirable characteristics, with peak power conversion efficiencies (PCE) of 7.52 and 9.60% by being blended with PC71BM under standard and dim light (2.5 mW cm-2), respectively. P2FDTBTBO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high Rs (9.42 Ω cm2). However, the PCE was remarkably improved by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to an exceptionally high Rsh of over 1000 kΩ cm2. Rs is less significant under dim light because the generated current is too small to cause noticeable Rs-induced voltage losses. Instead, high Rsh becomes more important to avoid leakage currents. This work provides important tips to further optimize PSCs for indoor applications with low-power electronic devices such as Internet of things sensors.