Jin-Mun Yun

Solution‐Processable Reduced Graphene Oxide as a Novel Alternative to PEDOT:PSS Hole Transport Layers for Highly Efficient and Stable Polymer Solar Cells

The potential of a fl exible, roll-to-roll manufacturing process has made bulk-heterojunction (BHJ) organic solar cells (OSCs) very attractive as a promising solution to energy and environmental issues. [ 1–8 ] In polymer solar cells, device characteristics such as fi ll factor (FF), short-circuit current density (J sc ) and open-circuit voltage (V oc ) as well as the cell life-time all are highly dependent on the interface properties between the electrodes and the active layers and on the bulk properties of the materials. [ 9 ] For these reasons, numerous modifi cations of electrodes by introduction of an interfacial layer have been studied intensively for high-performance and stable OSCs, [ 10–15 ] and several key factors such as transparency, conductivity, passivation property, fi lm morphology, stability, and solution-processability have been considered for uses of these promising interfacial layers. [ 9–15 ]

Planar heterojunction perovskite solar cells with superior reproducibility

Perovskite solar cells (PeSCs) have been considered one of the competitive next generation power sources. To date, light-to-electric conversion efficiencies have rapidly increased to over 10%, and further improvements are expected. However, the poor device reproducibility of PeSCs ascribed to their inhomogeneously covered film morphology has hindered their practical application. Here, we demonstrate high-performance PeSCs with superior reproducibility by introducing small amounts of N-cyclohexyl-2-pyrrolidone (CHP) as a morphology controller into N,N-dimethylformamide (DMF). As a result, highly homogeneous film morphology, similar to that achieved by vacuum-deposition methods, as well as a high PCE of 10% and an extremely small performance deviation within 0.14% were achieved. This study represents a method for realizing efficient and reproducible planar heterojunction (PHJ) PeSCs through morphology control, taking a major step forward in the low-cost and rapid production of PeSCs by solving one of the biggest problems of PHJ perovskite photovoltaic technology through a facile method.

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.

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.

Variations of cell performance in ITO-free organic solar cells with increasing cell areas

This study examined the effects of a cell area on the cell performances in ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with two different sheet resistances (Rsh) were used as polymeric transparent anodes for cost-effective ITO-free OSCs. Changes in the power conversion efficiency (PCE), the fill factor (FF), the short-circuit current (Jsc), and the open-circuit voltage (Voc) that resulted from changing the cell area or sheet resistance of transparent electrodes were systematically investigated. With increasing cell area from 4.5 to 49.5 mm2, the device performance of ITO-free OSCs was continuously decreased mainly due to the decrease in the FF and the series resistance (Rs). In addition, the performance of OSCs was critically dependent on Rsh of the PEDOT:PSS electrode. Upon reducing Rsh of the polymer anode from ~200 to ~90 Ω/, the FF and PCE showed better values at an identical large cell area and exhibited a relieved cell performance degradation with increasing cell area, suggesting that the sheet resistance of transparent electrodes is a dominant factor to limit cell efficiencies in practical large-area solar cells.

Synthesis of novel arylamine containing perfluorocyclobutane and its electrochromic properties

A new electrochromic oligomer based on an arylamine compound containing a perfluorocyclobutane (PFCB) ring has been synthesized via 2 + 2 cyclodimerization. A new N,N,N′,N′-tetraphenyl-biphenyl-4,4′-diamine (TPD) with PFCB (TPD-PFCB) showed high thermal stability with a decomposition temperature (Td) of 435 °C. Cyclic voltammograms of TPD-PFCB films coated onto an indium–tin oxide substrate showed two reversible redox steps during a potential scan between 0.7 and 1.2 V. The spectroelectrochemical series of TPD-PFCB films exhibited color change from colorless to yellow and then to greenish blue during the oxidation reaction of TPD-PFCB. This oligomer exhibited a high electrochromic coloration efficiency (602 cm2/C) and a comparable response speed (coloring and bleaching time at 700 nm were 0.71 and 0.12 s, respectively).

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.

Efficient polymer solar cells with a solution-processed gold chloride as an anode interfacial modifier

The use of a solution-processed gold chloride (AuCl3) as an anode interfacial modifier was investigated for high-performance polymer solar cells (PSCs). Kelvin probe, 4-point probe, and X-ray photoelectron spectroscopy studies demonstrated that AuCl3 increases the indium-tin-oxide (ITO) work-function and decreases the ITO sheet resistance, because of Au nanoparticle formation and Cl adsorption by the AuCl3 treatment to induce a p-doping effect, thereby improving the built-in potential and interface resistance. As a result, the introduction of AuCl3 by simple solution processing remarkably improved cell-performances, indicating that AuCl3 is an efficient anode interfacial modifier for enhancing PSC-performance.

Bi-axial grown amorphous MoS x bridged with oxygen on r-GO as a superior stable and efficient nonprecious catalyst for hydrogen evolution

Amorphous molybdenum sulfide (MoSx) is covalently anchored to reduced graphene oxide (r-GO) via a simple one-pot reaction, thereby inducing the reduction of GO and simultaneous doping of heteroatoms on the GO. The oxygen atoms form a bridged between MoSx and GO and play a crucial role in the fine dispersion of the MoSx particles, control of planar MoSx growth, and increase of exposed active sulfur sites. This bridging leads to highly efficient (−157 mV overpotential and 41 mV/decade Tafel slope) and stable (95% versus initial activity after 1000 cycles) electrocatalyst for hydrogen evolution.