Jooyeok Seo

Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers.

Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but their efficiency is still less than required for commercial application as lightweight/flexible modules. Forming a dipole layer at the electron-collecting interface has been suggested as one of the more attractive approaches for efficiency enhancement. However, only a few dipole layer material types have been reported so far, including only one non-ionic (charge neutral) polymer. Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot morphology, increases the effective work function at the electron-collecting interface within inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74% efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor.

Organic solar cells based on conjugated polymers : History and recent advances

Organic solar cells have attracted huge attention because of their potential in the low-cost manufacturing of plastic solar modules featuring flexible, lightweight, ultrathin, rollable and bendable shapes. The power conversion efficiency of organic solar cells is now passing ~10%, which is a critical sign toward commercialization because organic solar cells surpass any other types of solar cells in terms of development speed. The encouraging efficiency enhancement could be realized by introducing a ‘bulk heterojunction’ concept that overcomes the weakness of organic semiconductors by minimizing their charge transport paths through making effective p-n junctions inside bulk organic films. However, there are several hurdles for commercialization, including stability and lifetime issues, owing to the bulk heterojunction concept. This review summarizes the important aspects of organic solar cells, particularly focusing on conjugated polymers as an active layer component.

Significant stability enhancement in high-efficiency polymer:fullerene bulk heterojunction solar cells by blocking ultraviolet photons from solar light

Achievement of extremely high stability for inverted‐type polymer:fullerene solar cells is reported, which have bulk heterojunction (BHJ) layers consisting of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM), by employing UV‐cut filter (UCF) that is mounted on the front of glass substrates. The UCF can block most of UV photons below 403 nm at the expense of ≈20% reduction in the total intensity of solar light. Results show that the PTB7‐Th:PC71BM solar cell with UCF exhibits extremely slow decay in power conversion efficiency (PCE) but a rapidly decayed PCE is measured for the device without UCF. The poor device stability without UCF is ascribed to the oxidative degradation of constituent materials in the BHJ layers, which give rise to the formation of PC71BM aggregates, as measured with high resolution and scanning transmission electron microscopy and X‐ray photoelectron spectroscopy. The device stability cannot be improved by simply inserting poly(ethylene imine) (PEI) interfacial layer without UCF, whereas the lifetime of the PEI‐inserted PTB7‐Th:PC71BM solar cells is significantly enhanced when UCF is attached.

Broadband All-Polymer Phototransistors with Nanostructured Bulk Heterojunction Layers of NIR-Sensing n-Type and Visible Light-Sensing p-Type Polymers.

We report ‘broadband light-sensing’ all-polymer phototransistors with the nanostructured bulk heterojunction (BHJ) layers of visible (VIS) light-sensing electron-donating (p-type) polymer and near infrared (NIR) light-sensing electron-accepting (n-type) polymer. Poly[{2,5-bis-(2-ethylhexyl)-3,6-bis-(thien-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-diyl}-co-{2,2′-(2,1,3-benzothiadiazole)]-5,5′-diyl}] (PEHTPPD-BT), which is synthesized via Suzuki coupling and employed as the n-type polymer, shows strong optical absorption in the NIR region (up to 1100 nm) in the presence of weak absorption in the VIS range (400 ~ 600 nm). To strengthen the VIS absorption, poly(3-hexylthiophene) (P3HT) is introduced as the p-type polymer. All-polymer phototransistors with the BHJ (P3HT:PEHTPPD-BT) layers, featuring a peculiar nano-domain morphology, exhibit typical p-type transistor characteristics and efficiently detect broadband (VIS ~ NIR) lights. The maximum corrected responsivity (without contribution of dark current) reaches up to 85 ~ 88% (VIS) and 26 ~ 40% (NIR) of theoretical responsivity. The charge separation process between P3HT and PEHTPPD-BT components in the highest occupied molecular orbital is proposed as a major working mechanism for the effective NIR sensing.

Liquid Crystal-on-Organic Field-Effect Transistor Sensory Devices for Perceptive Sensing of Ultralow Intensity Gas Flow Touch

We demonstrate liquid crystal-on-organic field-effect transistor (LC-on-OFET) sensory devices that can perceptively sense ultralow level gas flows. The LC-on-OFET devices were fabricated by mounting LC molecules (4-cyano-4′-pentylbiphenyl – 5CB) on the polymer channel layer of OFET. Results showed that the presence of LC molecules on the channel layer resulted in enhanced drain currents due to a strong dipole effect of LC molecules. Upon applying low intensity nitrogen gas flows, the drain current was sensitively increased depending on the intensity and time of nitrogen flows. The present LC-on-OFET devices could detect extremely low level nitrogen flows (0.7 sccm–11 μl/s), which could not be felt by human skins, thanks to a synergy effect between collective behavior of LC molecules and charge-sensitive channel layer of OFET. The similar sensation was also achieved using the LC-on-OFET devices with a polymer film skin, suggesting viable practical applications of the present LC-on-OFET sensory devices.

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).

5 V driving organic non-volatile memory transistors with poly(vinyl alcohol) gate insulator and poly(3-hexylthiophene) channel layers

Organic non-volatile memory devices were fabricated by employing organic field-effect transistors (OFETs) with poly(vinyl alcohol) (PVA) and poly(3-hexylthiophene) as a gate insulating layer and a channel layer, respectively. The 10-nm-thick nickel layers were inserted for better charge injection between the channel layer and the top source/drain electrodes. The fabricated PVA-OFET memory devices could be operated at low voltages (≤5 V) and showed pronounced hysteresis characteristics in the transfer curves, even though very small hysteresis was measured from the output curves. The degree of hysteresis was considerably dependent on the ratio of channel width (W) to channel length (L). The PVA-OFET memory device with the smaller W/L ratio (25) exhibited better retention characteristics upon 700 cycles of writing-reading-erasing-reading operations, which was assigned to the stability of charged states in devices.

Organic Phototransistors With All-Polymer Bulk Heterojunction Layers of p -Type and n -Type Sulfur-Containing Conjugated Polymers

All-polymer phototransistors were fabricated using both glass and flexible plastic film substrates by employing bulk heterojunction channel layers of p-type polymer (P3HT) and n-type polymer (THBT-ht). The devices could detect the entire visible light because the n-type polymer could sense photons in the deep red parts (>650 nm). The responsivity of devices was higher at the lower light intensity, while it could be controlled by varying the gate and/or drain voltages. Similar performances were measured for flexible all-polymer phototransistors with a bottom-source/drain and top-gate electrode configuration.

Touch sensors based on planar liquid crystal-gated-organic field-effect transistors

We report a tactile touch sensor based on a planar liquid crystal-gated-organic field-effect transistor (LC-g-OFET) structure. The LC-g-OFET touch sensors were fabricated by forming the 10 μm thick LC layer (4-cyano-4′-pentylbiphenyl - 5CB) on top of the 50 nm thick channel layer (poly(3-hexylthiophene) - P3HT) that is coated on the in-plane aligned drain/source/gate electrodes (indium-tin oxide - ITO). As an external physical stimulation to examine the tactile touch performance, a weak nitrogen flow (83.3 μl/s) was employed to stimulate the LC layer of the touch device. The LC-g-OFET device exhibited p-type transistor characteristics with a hole mobility of 1.5 cm2/Vs, but no sensing current by the nitrogen flow touch was measured at sufficiently high drain (VD) and gate (VG) voltages. However, a clear sensing current signal was detected at lower voltages, which was quite sensitive to the combination of VD and VG. The best voltage combination was VD = −0.2 V and VG = −1 V for the highest ratio of signal ...

Stable Protein Device Platform Based on Pyridine Dicarboxylic Acid-Bound Cubic-Nanostructured Mesoporous Titania Films

Here we shortly report a protein device platform that is extremely stable in a buffer condition similar to human bodies. The protein device platform was fabricated by covalently attaching cytochrome c (cyt c) protein molecules to organic coupler molecules (pyridine dicarboxylic acid, PDA) that were already covalently bound to an electron-transporting substrate. A cubic nanostructured mesoporous titania film was chosen as an electron-transporting substrate because of its large-sized cubic holes (∼7 nm) and highly crystalline cubic titania walls (∼0.4 nm lattice). Binding of PDA molecules to the mesoporous titania surface was achieved by esterification reaction between carboxylic acid groups (PDA) and hydroxyl groups (titania) in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) mediator, whereas the immobilization of cyt c to the PDA coupler was carried out by the EDC-mediated amidation reaction between carboxylic acid groups (PDA) and amine groups (cyt c). Results showed that the 2,4-position isomer among several PDAs exhibited the highest oxidation and reduction peak currents. The cyt c-immobilized PDA-bound titania substrates showed stable and durable electrochemical performances upon continuous current-voltage cycling for 240 times (the final current change was less than 3%) and could detect superoxide that is a core indicator for various diseases including cancers.