Seok-In Na

Significant vertical phase separation in solvent-vapor-annealed poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) composite films leading to better conductivity and work function for high-performance indium tin oxide-free optoelectronics.

In the present study, a novel polar-solvent vapor annealing (PSVA) was used to induce a significant structural rearrangement in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films in order to improve their electrical conductivity and work function. The effects of polar-solvent vapor annealing on PEDOT:PSS were systematically compared with those of a conventional solvent additive method (SAM) and investigated in detail by analyzing the changes in conductivity, morphology, top and bottom surface composition, conformational PEDOT chains, and work function. The results confirmed that PSVA induces significant phase separation between excess PSS and PEDOT chains and a spontaneous formation of a highly enriched PSS layer on the top surface of the PEDOT:PSS polymer blend, which in turn leads to better 3-dimensional connections between the conducting PEDOT chains and higher work function. The resultant PSVA-treated PEDOT:PSS anode films exhibited a significantly enhanced conductivity of up to 1057 S cm(-1) and a tunable high work function of up to 5.35 eV. The PSVA-treated PEDOT:PSS films were employed as transparent anodes in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs). The cell performances of organic optoelectronic devices with the PSVA-treated PEDOT:PSS anodes were further improved due to the significant vertical phase separation and the self-organized PSS top surface in PSVA-treated PEDOT:PSS films, which can increase the anode conductivity and work function and allow the direct formation of a functional buffer layer between the active layer and the polymeric electrode. The results of the present study will allow better use and understanding of polymeric-blend materials and will further advance the realization of high-performance indium tin oxide (ITO)-free organic electronics.

In Situ Synthesis of Thermochemically Reduced Graphene Oxide Conducting Nanocomposites

Highly conductive reduced graphene oxide (GO) polymer nanocomposites are synthesized by a well-organized in situ thermochemical synthesis technique. The surface functionalization of GO was carried out with aryl diazonium salt including 4-iodoaniline to form phenyl functionalized GO (I-Ph-GO). The thermochemically developed reduced GO (R-I-Ph-GO) has five times higher electrical conductivity (42,000 S/m) than typical reduced GO (R-GO). We also demonstrate a R-I-Ph-GO/polyimide (PI) composites having more than 10(4) times higher conductivity (~1 S/m) compared to a R-GO/PI composites. The electrical resistances of PI composites with R-I-Ph-GO were dramatically dropped under ~3% tensile strain. The R-I-Ph-GO/PI composites with electrically sensitive response caused by mechanical strain are expected to have broad implications for nanoelectromechanical systems.

Efficient work-function engineering of solution-processed MoS2 thin-films for novel hole and electron transport layers leading to high-performance polymer solar cells

The work-function of MoS2 interfacial layers can be efficiently modulated by p- and n-doping treatments. As a result, the PCE of devices with a p-doped MoS2-based HTL is increased from ∼2.8 to ∼3.4%. Particularly, after n-doping the PCE was dramatically increased due to the change in work-function compared with un-doped MoS2 thin-films.

Sulfonic acid-functionalized, reduced graphene oxide as an advanced interfacial material leading to donor polymer-independent high-performance polymer solar cells

A r-GO with sulfonic acid groups (sr-GO) was newly developed and it showed dramatically concentrated aqueous dispersions with high film conductivity. With the aid of sulfonic acid groups, good compatibility with various HOMO materials was achieved, resulting in PCEs over 7% for sr-GO-based cells with superior device stability to PEDOT:PSS-based devices.

Highly transparent and flexible InTiO/Ag nanowire/InTiO films for flexible organic solar cells

We investigated characteristics of Ti-doped In2O3 (TIO)/Ag nanowire (NW)/TIO films as potential transparent and flexible electrodes for flexible organic solar cells. Effective insertion of metallic Ag NW between TIO layers led to improvement of electrical and mechanical properties of sputtered TIO film. At optimized conditions, we achieved a flexible TIO/Ag NW/TIO multilayer with a low sheet resistance of 9.01 Ω/square and a high optical transmittance of 85.14%. In addition, the TIO/Ag NW/TIO multilayer electrode showed small inner and outer bending radius of 5 and 6.5 mm indicating good flexibility. This indicates that hybridization of sputtered TIO films and printed Ag NW network is a promising solution to solve critical problems of high resistance and brittle TIO film and Ag NW network with poor adhesion.

A feasible random copolymer approach for high-efficiency polymeric photovoltaic cells

Two random copolymers based on (2,5-difluorophenylene)dithiophene and dialkoxybenzothiadiazole with benzodithiophene (P1) or thiophene (P2) as the third conjugated bridge having sulfur and fluorine (S⋯F) and/or oxygen (S⋯O) non-covalent intramolecular interaction are synthesized and characterized. In spite of a molecular weight difference over three times between both polymers, P1 and P2 possess similar solubility in organic solvents and thermal stability (Td ∼ 320 °C), which means probably due to that P1 with bulky alkylthiophene substituted benzodithiophene as a third conjugated bridge has less non-covalent intramolecular interaction than that of P2 with thiophene as a bridge. Both polymers were used as electron donors in bulk heterojunction organic photovoltaics (BHJ OPV) with PC71BM as an acceptor. From the photovoltaic measurements it was revealed that P2 shows higher power conversion efficiency (PCE) of up to 6.82% than that of P1 (2.44%). After 1,8-diiodooctane (DIO) treatments as a processing additive, the P1 and P2 devices show a significantly improved PCE of 5.95% for P1 and 7.71% for P2. The surface morphology analysis of the blend films using the atomic force microscope (AFM) reveals that the P1:PC71BM film shows macrophase separation, while the P2 film has a smooth morphology. After DIO treatments, the morphology of both polymer blend films is improved with better bi-continuous nanoscale networks. Charge carrier mobilities through the space charge limited current (SCLC) method demonstrate that P2 with the thiophene bridge has higher charge carrier mobilities than P1. In particular, an inverted structured BHJ OPV with P2 exhibits a PCE of 8.50%, which is the highest PCE reported in the literature regarding random copolymers.

Efficient organic solar cells with solution-processed carbon nanosheets as transparent electrodes

We demonstrate that solution-processed carbon nanosheet (CNS) films can efficiently serve as transparent electrodes for organic solar cells (OSCs). The CNS was obtained by spin-coating of polyacrylonitrile (PAN) dissolved in dimethylformamide on quartz substrates, followed by stabilization and carbonization processes to convert polymer into CNS. The thickness of the newly developed CNS films was easily controlled by varying the PAN solution concentration. The polymer-converted CNS films were intensively examined for the feasibility of the use as transparent anodes in solar cells. This approach could be highly desirable for all-solution-processed or printed OSCs.

Organic photovoltaic devices with low resistance multilayer graphene transparent electrodes

The authors report the characteristics of organic solar cells (OSCs) fabricated on HNO3-treated multilayer graphene (MLG) transparent electrodes. MLG transparent electrodes were prepared using chemical vapor deposition and a multitransfer process. Compared to untreated-MLG electrodes with a fairly high sheet resistance (274 ± 1 Ω/sq) even though three layers of graphene were stacked together, HNO3-treated MLG electrode shows a lower sheet resistance of 119 ± 1 Ω/sq. OSCs (MLG/poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Poly(3-hexylthiophere)/1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61/Ca/Al) fabricated on untreated-MLG transparent electrodes had an open-circuit voltage of 0.575 V, a short-circuit current of 8.08 mA/cm2, a fill factor of 43.5%, and a power conversion efficiency (PCE) of 2.02%. In contrast, application of HNO3-treated MLG films as transparent electrodes for OSCs led to improved performance with an open-circuit voltage of 0.602 V, a short-circuit current of 8.26 mA/cm2, a fil...

Simple brush painted Ag nanowire network on graphene sheets for flexible organic solar cells

The authors report on highly flexible and transparent Ag nanowire (NW) coated graphene hybrid electrodes for flexible organic solar cells (FOSCs). Brush painted Ag NW percolating network on the transparent graphene sheet led to an invisible Ag NW/graphene hybrid electrode with a sheet resistance of 15.25 Ω/sq. and a high optical transmittance of 77.4% as well as superior flexibility. In particular, similar bending radius of Ag NW/graphene hybrid electrode to graphene bilayer electrode during outer bending test demonstrated the superior mechanical flexibility of the Ag NW/graphene hybrid electrodes. FOSCs fabricated on Ag NW/graphene hybrid electrode showed higher power conversion efficiency (2.681%) than that (1.681%) of FOSC with graphene bilayer electrode due to lower sheet resistance and improved wettability for hole extracting layer. This indicates that brush painting of conductive Ag NW is a critical solution to solve the problem of high resistance and hydrophobic graphene electrode for use in FOSC.

An approach for an advanced anode interfacial layer with electron-blocking ability to achieve high-efficiency organic photovoltaics.

The interfacial properties of PEDOT:PSS, pristine r-GO, and r-GO with sulfonic acid (SR-GO) in organic photovoltaic are investigated to elucidate electron-blocking property of PEDOT:PSS anode interfacial layer (AIL), and to explore the possibility of r-GO as electron-blocking layers. The SR-GO results in an optimized power conversion efficiency of 7.54% for PTB7-th:PC71BM and 5.64% for P3HT:IC61BA systems. By combining analyses of capacitance-voltage and photovoltaic-parameters dependence on light intensity, it is found that recombination process at SR-GO/active film is minimized. In contrast, the devices using r-GO without sulfonic acid show trap-assisted recombination. The enhanced electron-blocking properties in PEDOT:PSS and SR-GO AILs can be attributed to surface dipoles at AIL/acceptor. Thus, for electron-blocking, the AIL/acceptor interface should be importantly considered in OPVs. Also, by simply introducing sulfonic acid unit on r-GO, excellent contact selectivity can be realized in OPVs.