Byung-Kwan Yu

Evolution of nanomorphology and anisotropic conductivity in solvent-modified PEDOT:PSS films for polymeric anodes of polymer solar cells

A highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film, obtained by addition of a polar solvent, dimethylsulfoxide (DMSO), to an aqueous solution of PEDOT:PSS, was thoroughly investigated to gain a deeper understanding of the fundamental characteristics of the solvent-modified PEDOT:PSS film. Use of the DMSO-modified PEDOT:PSS film as a transparent anode to achieve low-cost and high-efficiency ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) was also examined. Changes in the conductivity, morphology, surface composition, work-function, and anisotropic conductivity in both the parallel and perpendicular directions of solvent-treated PEDOT:PSS films that resulted from the addition of various amounts of DMSO were investigated to better understand the nature of the solvent-modified PEDOT:PSS film and the origin of its dramatically enhanced conductivity. Furthermore, the effects of using the modified PEDOT:PSS films as polymer anodes on solar cell performance were investigated by addition of various amounts of DMSO and by the use of PEDOT:PSS films with different thicknesses. The ITO-free OSCs with optimized PEDOT:PSS anodes had a high power conversion efficiency that was comparable to that of conventional ITO-based devices.

Factors to be Considered in Bulk Heterojunction Polymer Solar Cells Fabricated by the Spray Process

One of the attractions of solution-processed polymer solar cells (PSCs) is the potential for low-cost production by high-throughput roll-to-roll processes. The spray process can be suggested as an elegant alternative for PSCs fabrication due to its high production speed. We highlight the effects of sprayed droplet sizes on the film topography and drying time of droplets. We relate the latter to the PSCs device performances. The transfer efficiency and deposition rate of the spray process as a function of the droplet size are investigated for the high production speed and lowering the cost. PSC device fabrication with an optimized spray-operating condition yielded a power conversion efficiency of 3.4%. The comparable efficiency of PSCs fabricated by a roll-to-roll compatible process compared to spin-coated one achieved in this paper brings these devices one step closer to the realization of low-cost PSCs production.

Spray-printed organic field-effect transistors and complementary inverters

We report the fabrication of high-performance organic field-effect transistors (OFETs) and complementary inverters using spray-printed films of n-type small-molecule semiconductors and p-type conjugated polymers. Highly crystalline organic semiconductor films could be obtained by controlling the droplet size, nozzle-to-substrate distance, and solvent drying speed during the printing process. After the optimisation of the spray-printing process, the performances of the spray-printed OFETs were comparable to those of spin-coated and inkjet-printed OFETs. In addition to excellent device-to-device uniformity, the spray-printed n- and p-channel OFETs also exhibited high field-effect mobilities, which were ∼0.3 (ActivInk™ N1450, Polyera), ∼0.01 (regioregular-poly(3-hexylthiophene) (rr-P3HT)), and ∼0.25 cm2 V−1 s−1 (ActivInk™ P2100, Polyera). Organic complementary inverters were fabricated by spray printing and shadow-mask patterning while using ActivInk™ N1450 and P2100 as the n- and p-type semiconductors, respectively. The complementary inverters exhibited a large voltage gain (∼17) and a low power consumption (∼0.02 mW) at VDD = 60 V.

Synthesis and characterization of low-band-gap poly(thienylenevinylene) derivatives for polymer solar cells

A series of conjugated polymers containing thienylenevinylene moieties as the electron donor for polymer solar cells were synthesized through Yamamoto and Stille coupling. The structure and device properties of the homopolymer (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene] (PEBT), and two donor–acceptor-type copolymers (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PEBTBT) and (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PEBTTBT) were investigated. The vinylene group inserted between two thiophene rings has an important role in lowering the band gap of thienylenevinylene derivatives. In addition, the donor–acceptor-type copolymers including a benzothiadiazole unit showed even lower band gap than their own homopolymer due to intramolecular charge transfer (ICT) through their conjugated backbones. As a result, PEBT, PEBTBT and PEBTTBT showed band gaps of 1.77, 1.37 and 1.57 eV, respectively. Bulk heterojunction photovoltaic devices were fabricated using blends of the polymers with [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and PEBTBT gave the best power conversion efficiency among them, at 1.07% under AM 1.5, 100 mW cm−2.

Sequent spray deposition of secondary solvent for efficient polymer solar cells

The spray process is compatible with the roll-to-roll process, which has the highest productivity and the lowest costs among the developed processing techniques. In this study, we demonstrated a technique involving the spraying of a secondary solvent after the formation of an active layer, by using a spray process with a relatively lowboiling- point solution. With this technique, we obtained the much smoother surface topography and more ordered bulk heterojunction morphology through enhanced self-organization of the composite materials. Consequently, the device with additionally sprayed high-boiling-point solvent allowed to achieve fill factor (FF) and power conversion efficiency (PCE) values as high as 67% and 3.76%, respectively.

Building a hybrid nanocomposite assembly of gold nanowires and thienyl-derivative fullerenes to enhance electron transfer in photovoltaics

We report the preparation and characterization of a novel nanocomposite assembly comprising metallic gold nanowires (Au NWs) and thienyl-C61-butyric acid methyl ester (ThCBM) fabricated by adsorbing ThCBM groups, via coordinate bonds involving sulfur functionalities, onto the Au(111) surfaces of Au NWs. The ThCBM-adsorbed Au NW nanocomposites displayed clear shifts in the plasmonic absorption band and in the binding energy of the gold relative to the pure gold NWs. A bulk heterojunction solar cell using the ThCBM-adsorbed Au NW nanocomposites displayed a good power conversion efficiency (PCE) of 3.12%, and a short circuit current density and fill factor that were significantly better than those of a reference cell prepared using ThCBM without Au NWs (with a PCE of 2.67%). The electron mobility in an electron-only device prepared using the ThCBM-adsorbed Au NW nanocomposites was significantly higher (by 35% or more) than the electron mobility in a device prepared with pure ThCBM. In conclusion, the hybrid composites proposed here, i.e., ThCBM-adsorbed Au NW nanocomposites, provide a promising approach to improving the performance of bulk heterojunction photovoltaic cells via improved electron transfer through the metallic nanowires as well as by enhanced ordering among the fullerenes assembled along the longitudinal axes of the gold nanowires.