Hyena Lee

Doping Effect of Organosulfonic Acid in Poly(3-hexylthiophene) Films for Organic Field-Effect Transistors

We attempted to dope poly(3-hexylthiophene) (P3HT) with 2-ethylbenzenesulfonic acid (EBSA), which has good solubility in organic solvents, in order to improve the performance of organic field effect transistors (OFET). The EBSA doping ratio was varied up to 1.0 wt % because the semiconducting property of P3HT could be lost by higher level doping. The doping reaction was confirmed by the emerged absorption peak at the wavelength of ~970 nm and the shifted S2p peak (X-ray photoelectron spectroscopy), while the ionization potential and nanostructure of P3HT films was slightly affected by the EBSA doping. Interestingly, the EBSA doping delivered significantly improved hole mobility because of the greatly enhanced drain current of OFETs by the presence of the permanently charged parts in the P3HT chains. The hole mobility after the EBSA doping was increased by the factor of 55-86 times depending on the regioregularity at the expense of low on/off ratio in the case of unoptimized devices, while the optimized devices showed ~10 times increased hole mobility by the 1.0 wt % EBSA doping with the greatly improved on/off ratio even though the source and drain electrodes were made using relatively cheaper silver instead of gold.

Solution-processable all-small molecular bulk heterojunction films for stable organic photodetectors: near UV and visible light sensing

We report stable organic photodetectors with all-small molecular bulk heterojunction (BHJ) sensing layers prepared using solutions of electron-donating and electron-accepting small molecules. As an electron-donating molecule, 2,5-bis(2-ethylhexyl)-3,6-bis(4′-methyl-[2,2′-bithiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EHTPPD-MT) was synthesized via a Stille coupling reaction, whereas [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was used as an electron-accepting component. The devices with the EHTPPD-MT:PC61BM BHJ layer could detect photons at a wavelength of 400–800 nm and exhibited a stable photoresponse under on/off modulation of near UV (405 nm) and visible (532 nm and 650 nm) light even at bias voltage conditions. The corrected responsivity reached ∼175 mA W−1 for the near UV detection at −1 V. An extremely durable photoresponse was measured for the present devices (including flexible devices) under illumination with high intensity green light (133.4 mW cm−2 at 532 nm) which is much stronger than standard sun light (100 mW cm−2, white). The excellent stability has been attributed to the tiny EHTPPD-MT crystals, which are formed in the EHTPPD-MT:PC61BM layers during the coating processes.

Hybrid Phototransistors Based on Bulk Heterojunction Films of Poly(3-hexylthiophene) and Zinc Oxide Nanoparticle

Hybrid phototransistors (HPTRs) were fabricated on glass substrates using organic/inorganic hybrid bulk heterojunction films of p-type poly(3-hexylthiophene) (P3HT) and n-type zinc oxide nanoparticles (ZnO(NP)). The content of ZnO(NP) was varied up to 50 wt % in order to understand the composition effect of ZnO(NP) on the performance of HPTRs. The morphology and nanostructure of the P3HT:ZnO(NP) films was examined by employing high resolution electron microscopes and synchrotron radiation grazing angle X-ray diffraction system. The incident light intensity (P(IN)) was varied up to 43.6 μW/cm², whereas three major wavelengths (525 nm, 555 nm, 605 nm) corresponded to the optical absorption of P3HT were applied. Results showed that the present HPTRs showed typical p-type transistor performance even though the n-type ZnO(NP) content increased up to 50 wt %. The highest transistor performance was obtained at 50 wt %, whereas the lowest performance was measured at 23 wt % because of the immature bulk heterojunction morphology. The drain current (I(D)) was proportionally increased with P(IN) due to the photocurrent generation in addition to the field-effect current. The highest apparent and corrected responsivities (R(A) = 4.7 A/W and R(C) = 2.07 A/W) were achieved for the HPTR with the P3HT:ZnO(NP) film (50 wt % ZnO(NP)) at P(IN) = 0.27 μW/cm² (555 nm).

Poly(3-hexylthiophene-co-benzothiadiazole) (THBT) as an electron-accepting polymer for normal and inverted type all-polymer solar cells

A new n-type polymer, poly(3-hexylthiophene-co-benzothiadiazole) (THBT), which contains the same molar ratio of p-type (3-hexylthiophene) and n-type (benzothiadiazole) units in the main chain, was synthesized by Suzuki coupling reaction. Optical and photoelectron measurements showed that the THBT polymer features an optical band gap energy of 1.8 eV, a highest occupied molecular orbital energy of 5.7 eV and a lowest unoccupied molecular orbital energy of 3.9 eV. Synchrotron-radiation grazing incidence angle X-ray diffraction measurements disclosed a strong chain stacking effect in the THBT polymer film, which is similar to the poly(3-hexylthiophene) (P3HT) film case, but the chain stacking distance in the THBT film was different from that in the P3HT film. The zero-field electron mobility of the THBT film was measured as ca. 10−6 cm2 V−1 s−1 when a space-charge limited current method was employed. Based on the characteristics of the THBT polymer, two types (normal and inverted types) of all-polymer solar cells were fabricated using the P3HT:THBT films as an active layer. The result showed that the maximum open circuit voltage reached ca. 0.86 V and the solar cell performance was found to be strongly dependent on the nanomorphology of the P3HT:THBT films upon changing the composition of co-solvents. The fill factor of the inverted solar cells was higher than that of the normal type solar cells, leading to a higher power conversion efficiency for the inverted type P3HT:THBT solar cells.

Device Performance and Lifetime of Polymer:Fullerene Solar Cells with UV‐Ozone‐Irradiated Hole‐Collecting Buffer Layers

We report the influence of UV-ozone irradiation of the hole-collecting buffer layers on the performance and lifetime of polymer:fullerene solar cells. UV-ozone irradiation was targeted at the surface of the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layers by varying the irradiation time up to 600 s. The change of the surface characteristics in the PEDOT:PSS after UV-ozone irradiation was measured by employing optical absorption spectroscopy, photoelectron yield spectroscopy, and contact angle measurements, while Raman and X-ray photoelectron spectroscopy techniques were introduced for more microscopic analysis. Results showed that the UV-ozone irradiation changed the chemical structure/composition of the surface of the PEDOT:PSS layers leading to the gradual increase of ionization potential with irradiation time in the presence of up-and-down variations in the contact angle (polarity). This surface property change was attributed to the formation of oxidative components, as evidenced by XPS and Auger electron images, which affected the sheet resistance of the PEDOT:PSS layers. Interestingly, device performance was slightly improved by short irradiation (up to 10 s), whereas it was gradually decreased by further irradiation. The short-duration illumination test showed that the lifetime of solar cells with the UV-ozone irradiated PEDOT:PSS layer was improved due to the protective role of the oxidative components formed upon UV-ozone irradiation against the attack of sulfonic acid groups in the PEDOT:PSS layer to the active layer.

A pronounced dispersion effect of crystalline silicon nanoparticles on the performance and stability of polymer:fullerene solar cells.

We investigated the dispersion effect of crystalline silicon nanoparticles (SiNP) on the performance and stability of organic solar cells with the bulk heterojunction (BHJ) films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM). To improve the dispersion of SiNP in the BHJ films, we attached octanoic acid (OA) to the SiNP surface via esterification reaction and characterized it with Raman spectroscopy and high-resolution transmission electron microscopy. The OA-attached SiNP (SiNP-OA) showed improved dispersion in chlorobenzene without change of optical absorption, ionization potential and crystal nanostructure of SiNP. The device performance was significantly deteriorated upon high loading of SiNP (10 wt %), whereas relatively good performance was maintained without large degradation in the case of SiNP-OA. Compared to the control device (P3HT:PC(61)BM), the device performance was improved by adding 2 wt % SiNP-OA, but it was degraded by adding 2 wt % SiNP. In particular, the device stability (lifetime under short time exposure to 1 sun condition) was improved by adding 2 wt % SiNP-OA even though it became significantly decreased by adding 2 wt % SiNP. This result suggests that the dispersion of nanoparticles greatly affects the device performance and stability (lifetime).

Phenanthroline diimide as an organic electron-injecting material for organic light-emitting devices

We report a diimide-type organic electron-injecting material, bis-[1,10]phenanthrolin-5-yl-pyromellitic diimide (Bphen-PMDI), for organic light-emitting devices (OLEDs), which was synthesized from its monomers, pyromellitic dianhydride (PMDA) and 1,10-phenanthrolin-5-amine (PTA). The vacuum-purified Bphen-PMDI powder showed high glass transition (∼230 °C) and thermal decomposition (∼400 °C) temperatures, whereas neither melting point nor particular long-range crystal nanostructures were observed from its solid samples. The optical band gap energy and the ionization potential of the Bphen-PMDI film were 3.6 eV and 6.0 eV, respectively, leading to the lowest unoccupied molecular orbital (LUMO) energy of 2.4 eV. Inserting a 1 nm thick Bphen-PMDI layer between the emission layer and the cathode layer improved the device current density by 10-fold and the luminance by 6-fold, compared to the OLED without the Bphen-PMDI layer. The result suggests that an effective electron tunnel injection process occurs through the Bphen-PMDI layer.

Strong Photo-Amplification Effects in Flexible Organic Capacitors with Small Molecular Solid-State Electrolyte Layers Sandwiched between Photo-Sensitive Conjugated Polymer Nanolayers

We demonstrate strong photo-amplification effects in flexible organic capacitors which consist of small molecular solid-state electrolyte layers sandwiched between light-sensitive conjugated polymer nanolayers. The small molecular electrolyte layers were prepared from aqueous solutions of tris(8-hydroxyquinoline-5-sulfonic acid) aluminum (ALQSA3), while poly(3-hexylthiophene) (P3HT) was employed as the light-sensitive polymer nanolayer that is spin-coated on the indium-tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) film substrates. The resulting capacitors feature a multilayer device structure of PET/ITO/P3HT/ALQSA3/P3HT/ITO/PET, which were mechanically robust due to good adhesion between the ALQSA3 layers and the P3HT nanolayers. Results showed that the specific capacitance was increased by ca. 3-fold when a white light was illuminated to the flexible organic multilayer capacitors. In particular, the capacity of charge storage was remarkably (ca. 250-fold) enhanced by a white light illumination in the potentiostatic charge/discharge operation, and the photo-amplification functions were well maintained even after bending for 300 times at a bending angle of 180o.

Resorcinol-functionalized carbon nanoparticles with a stick-out nanostructure for stable hydrogen bonding with polyester microfibers

We report new functionalized carbon nanoparticles (CNPs) featuring a stick-out nanostructure that delivers strong hydrogen bonding with polar groups in polymer chains. To make the stick-out nanostructures with hydroxyl groups, bromoresorcinols were used to react with the double bonds of CNPs by atom transfer radical reaction (ATRR). The resorcinol-bound CNPs (CNP-RCs) were characterized by solid-state proton nuclear magnetic spectroscopy and Raman spectroscopy. CNP-RCs were extremely well dispersed and stable (>6 months) compared to the original CNPs which precipitated in less than 60 h. High resolution transmission electron microscopy measurements disclosed that the characteristic particle shape and fine nanostructure of the original CNPs were almost unchanged during the ATRR process. We found that the surface of poly(ethylene terephthalate) (PET) microfibers was almost fully covered with the CNP-RC material by employing a double soaking process at 130 °C. The resulting CNP-RC-bound PET microfibers in fabrics exhibited excellent washing and rubbing resistance and outstanding deodorizing properties against ammonia gas. Hence, the present stick-out nanostructured CNP-RCs are expected to be a breakthrough in widening the application of carbon nanoparticles in terms of environmentally friendly processes using water and product-level stability.

Wide band gap triarylamine derivative doped with organosulfonic acid and its application for organic light-emitting devices

We report a bis-type wide band gap triarylamine derivative, N, N′-diphenyl-N, N-bis(3-methylphenyl)-[1,1′-diphenyl]-4,4′-diamine (TPD), that was chemically doped with camphorsulfonic acid (CSA) in chloroform. Optical measurements showed that new optical absorption peak, leading to a broadband (480–620 nm) photoluminescence, was created in the TPD–CSA after doping, while the redox peak of TPD was shifted by the CSA doping. A tuneable (from green to red) electroluminescence (EL) was measured from organic light-emitting devices with the TPD–CSA-dispersed polymer layer and a green emission layer (tris(8-hydroxyquinolinato)aluminium – Alq3). The red EL, which is actually impossible from the Alq3 layer that intrinsically emits typical green light, has been ascribed to the contribution from both newly generated gap state in the TPD–CSA molecules and shifted charge recombination zone owing to hole mobility changes in the TPD–CSA-dispersed polymer layers.