Jonggi Kim

Solution-Processable Ambipolar Diketopyrrolopyrrole–Selenophene Polymer with Unprecedentedly High Hole and Electron Mobilities

There is a fast-growing demand for polymer-based ambipolar thin-film transistors (TFTs), in which both n-type and p-type transistor operations are realized in a single layer, while maintaining simplicity in processing. Research progress toward this end is essentially fueled by molecular engineering of the conjugated backbones of the polymers and the development of process architectures for device fabrication, which has recently led to hole and electron mobilities of more than 1.0 cm(2) V(-1) s(-1). However, ambipolar polymers with even higher performance are still required. By taking into account both the conjugated backbone and side chains of the polymer component, we have developed a dithienyl-diketopyrrolopyrrole (TDPP) and selenophene containing polymer with hybrid siloxane-solubilizing groups (PTDPPSe-Si). A synergistic combination of rational polymer backbone design, side-chain dynamics, and solution processing affords an enormous boost in ambipolar TFT performance, resulting in unprecedentedly high hole and electron mobilities of 3.97 and 2.20 cm(2) V(-1) s(-1), respectively.

Synthesis of PCDTBT-based fluorinated polymers for high open-circuit voltage in organic photovoltaics: towards an understanding of relationships between polymer energy levels engineering and ideal morphology control.

The introduction of fluorine (F) atoms onto conjugated polymer backbone has verified to be an effective way to enhance the overall performance of polymer-based bulk-heterojunction (BHJ) solar cells, but the underlying working principles are not yet fully uncovered. As our attempt to further understand the impact of F, herein we have reported two novel fluorinated analogues of PCDTBT, namely, PCDTFBT (1F) and PCDT2FBT (2F), through inclusion of either one or two F atoms into the benzothiadiazole (BT) unit of the polymer backbone and the characterization of their physical properties, especially their performance in solar cells. Together with a profound effect of fluorination on the optical property, nature of charge transport, and molecular organization, F atoms are effective in lowering both the HOMO and LUMO levels of the polymers without a large change in the energy bandgaps. PCDTFBT-based BHJ solar cell shows a power conversion efficiency (PCE) of 3.96 % with high open-circuit voltage (VOC) of 0.95 V, mainly due to the deep HOMO level (-5.54 eV). To the best of our knowledge, the resulting VOC is comparable to the record VOC values in single junction devices. Furthermore, to our delight, the best PCDTFBT-based device, prepared using 2 % v/v diphenyl ether (DPE) additive, reaches the PCE of 4.29 %. On the other hand, doubly-fluorinated polymer PCDT2FBT shows the only moderate PCE of 2.07 % with a decrease in VOC (0.88 V), in spite of the further lowering of the HOMO level (-5.67 eV) with raising the number of F atoms. Thus, our results highlight that an improvement in efficiency by tuning the energy levels of the polymers by means of molecular design can be expected only if their truly optimized morphologies with fullerene in BHJ systems are materialized.

Toward the Realization of A Practical Diketopyrrolopyrrole‐Based Small Molecule for Improved Efficiency in Ternary BHJ Solar Cells

An easily accessible DPP-based small molecule (DMPA-DTDPP) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM-based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA-DTDPP as an additive yields an increase in the short circuit current density (Jsc) because DMPA-DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble-free and cost effective synthesis of DMPA-DTDPP, it may prove very useful in high-performance solar cells.

Ladder-type heteroacene polymers bearing carbazole and thiophene ring units and their use in field-effect transistors and photovoltaic cells

A family of ladder-type π-excessive conjugated monomer (dicyclopentathienocarbazole (DCPTCz)) integrating the structural components of carbazole and thiophene into a single molecular entity is synthesized and polymerized by oxidative coupling to yield poly(dicyclopentathienocarbazole) (PDCPTCz). Moreover, through the careful selection of 2,1,3-benzothiadiazole unit as a π-deficient building block, the dicyclopentathienocarbazole-based donor–acceptor copolymer (poly(dicyclopentathienocarbazole-alt-2,1,3-benzothiadiazole) (PDCPTCz-BT)) is prepared by Suzuki polycondensation. The optical, electrochemical, and field-effect charge transport properties of the resulting polymers (PDCPTCz and PDCPTCz-BT) are not only characterized in detail but also their bulk-heterojunction (BHJ) solar cell in combination with PC71BM are evaluated. The values of field-effect mobility (µ) for PDCPTCz and PDCPTCz-BT are 8.7 × 10−6 cm2 V−1s−1 and 2.7 × 10−4 cm2 V−1s−1, respectively. A power conversion efficiency (PCE) of 1.57% is achieved on the PDCPTCz-BT/PC71BM device, implying that the push–pull copolymers based on ladder-type dicyclopentathienocarbazole as an electron-donating moiety are promising for organic electronic devices.

A First Approach to White Organic Electroluminescence Device from a Single Rod-Coil Poly[thiophene-block-(N-vinylcarbazole)] Diblock Copolymer.

A rod-coil block copolymer consisting of poly(3-hexylthiophene) (P3HT) and poly(N-vinylcarbazole) (PVK) (P3HT-b-PVK) in a single molecular architecture is prepared as the first example for WOLEDs. By obtaining the phase separated domains in thin film of the resulting block copolymer, it is possible to suppress energy transfer from PVK as wide bandgap units to P3HT as low bandgap blocks, yielding dual emissions for white electroluminescence with CIE coordination of (0.34, 0.33).

Replacing 2,1,3-benzothiadiazole with 2,1,3-naphthothiadiazole in PCDTBT: towards a low bandgap polymer with deep HOMO energy level

With the rising interest in using the medium bandgap polymer, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PCDTBT) with deep HOMO energy level for polymer solar cells (PSCs), we have developed an analogous polymer with a lower bandgap, namely, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-naphthothiadiazole) (PCDTNT) by replacing 2,1,3-benzothiadiazole (BT) with 2,1,3-naphthothiadiazole (NT) in PCDTBT. Its optical, electrochemical, and photovoltaic properties are fully characterized in comparison with PCDTBT. Clearly, the λmax position of PCDTNT is significantly red-shifted by ∼30 nm, corresponding to a lower optical bandgap (1.71 eV) from the absorption edge of the thin film than that of PCDTBT (1.88 eV). A bulk-heterojunction (BHJ) PSC that incorporated PCDTNT with the low-lying HOMO energy level as a p-type material delivers a higher VOC value of 0.81 V and a power conversion efficiency (PCE) value of 1.31%.

Organic–Inorganic Nanostructure Architecture via Directly Capping Fullerenes onto Quantum Dots

A new form of fullerene-capped CdSe nanoparticles (PCBA-capped CdSe NPs), using carboxylate ligands with [60]fullerene capping groups that provides an effective synthetic methodology to attach fullerenes noncovalently to CdSe, is presented for usage in nanotechnology and photoelectric fields. Interestingly, either the internal charge transfer or the energy transfer in the hybrid material contributes to photoluminescence (PL) quenching of the CdSe moieties.