Jongsun Lim

Highly Efficient Inkjet-Printed Organic Photovoltaic Cells

In this article, we report the development of inkjet printing technique for the fabrication of bulk heterojunction organic photovoltaic cells (OPVs). We have demonstrated highly efficient OPV with a mixture of poly(3-hexylthiophene)s (P3HT) and 1-[3-(methoxycarbonyl)propyl]-1-phenyl[6.6]C61 (PCBM) as an inkjet-printed photoactive layer. Especially, 3.8% of power conversion efficiency (PCE) and 0.68 V of open circuit voltage (Voc) can be obtained by using the chlorobenzene as a solvent. This value is the highest performance in P3HT:PCBM based OPV system by now. We reveal that substantial improvements can be realized by nano-structured heterojunction after inkjet printing and this technique is one of the promising fabrication methods for organic photovoltaic cells. Also, we have designed and fabricated five-series connected large area (18 cm2) inkjet-printed OPVs using the patterned indium tin oxide (ITO) with Au bus-electrode to minimize the efficiency drop due to the surface resistance of ITO. Consequently, 1.25% of power conversion efficiency can be obtained with a short circuit current of 1.24 mA/cm2, a fill factor of 0.32, and an open circuit voltage of 3.2 V.

Synthesis of Polythiophenes with Electron-Donating Side-Chain and their Application to Organic Thin-Film Transistors

Thiophene-based π-conjugated polymers have been widely used in p-channel OTFTs due to their high charge carrier mobility. SPQT-12 which has dodecylsulfanyl side-chain was synthesized by Stille polymerization. The polymer was characterized by 1H NMR, GPC, UV-Vis spectroscopy, and cyclic voltammetry. Molecular weight (Mn) of SPQT-12 was 10,000 (PDI = 1.7), and SPQT-12 were readily soluble in common organic solvents. In the film state, SPQT-12 showed absorption with λmax at 553 nm and optical band gap of 1.87 eV. A mobility of 1.0 × 10−3 cm2/Vs, an on/off ratio of 5.8 × 103 and threshold voltage of −11 V have been obtained with OTFTs using SPQT-12.

An Amorphous Polythiophene as a Binder Material for Organic Thin-Film Transistor Channel Applications

Blending of crystalline low molecular-weight organic semiconductors, e.g., 6,13-bis (triisopropylsilylethynyl)pentacene (TIPS-Pentacene), and binder polymers has recently been known to be an effective way to achieve high performance organic thin-film transistors (OTFTs). However, only limited number of examples has been reported as binder materials for OTFT applications, e.g., poly(triarylamine), poly(α-methylstyrene), and etc. In this work, an amorphous polythiophene derivative, poly(3-(2-ethylhexyl)thiophene) (P3EHT), was prepared by introducing 2-ethylhexyl group as a side-chain to the polythiophene. The P3EHT was synthesized by Grignard metathesis (GRIM) polymerization to give a molecular weight (Mn) of 4,100 (PDI = 3.4), and it was readily soluble in common organic solvents, such as chloroform, tetrahydrofuran, toluene, and etc. An amorphous nature of the P3EHT was confirmed from differential scanning calorimetry (DSC) and ultraviolet-visible spectroscopy results. A mobility of 2.5 × 10−4 cm2/Vs, an on/off ratio of 0.57 × 103 and a threshold voltage of 1.6 V have been obtained with OTFTs using a P3EHT/TIPS-Pentacene blend.

Silyl Substituted Methanofullerenes as Electron Acceptors in Organic Photovoltaic Cells

The electron-accepting capability of fullerenes has made them interesting materials for organic photovoltaic cells (OPVCs). In this study, we propose the new methanofullerene derivatives incorporated with trimethylsilyl moiety to increase the solubility without electron transporting property decrease. Methanofullerene acceptor materials having trimethylsilyl substituent (PCB-Sis) were synthesized from the reaction of fullerene with trimethylsilyl substituted hydrazide type derivatives, electrochemical propertie was examined by cyclicvoltametry. OPVCs fabricated by using the mixture of P3HT and mono-adducts of PCB-Sis (Compound 7–9) as active layer showed 2.82% of power conversion efficiency under 100 mW/cm2 AM1.5 simulated light after annealing at 150°C for 10 min. Also, we have synthesized bis-adduct type PCB-Sis as moderate yield and obtained increased Voc about 0.5–0.14 V from P3HT and bis-adduct of PCB-Sis (Compound 10–12) based OPVCs. Unfortunately, these bis-adducts show slightly lower power conversion efficiency compare to mono-adducts due to the numerous region-isomers of bis-adducts. Although it was revealed our new PCB-Sis were candidates for acceptor materials in OPVCs.