Hyesung Park

Enhancing the conductivity of transparent graphene films via doping

We report chemical doping (p-type) to reduce the sheet resistance of graphene films for the application of high-performance transparent conducting films. The graphene film synthesized by chemical vapor deposition was transferred to silicon oxide and quartz substrates using poly(methyl methacrylate). AuCl(3) in nitromethane was used to dope the graphene films and the sheet resistance was reduced by up to 77% depending on the doping concentration. The p-type doping behavior was confirmed by characterizing the Raman G-band of the doped graphene film. Atomic force microscope and scanning electron microscope images reveal the deposition of Au particles on the film. The sizes of the Au particles are 10-100 nm. The effect of doping was also investigated by transferring the graphene films onto quartz and poly(ethylene terephthalate) substrates. The sheet resistance reached 150 Omega/sq at 87% transmittance, which is comparable to those of indium tin oxide conducting film. The doping effect was manifested only with 1-2 layer graphene but not with multi-layer graphene. This approach advances the numerous applications of graphene films as transparent conducting electrodes.

Doped graphene electrodes for organic solar cells

In this work graphene sheets grown by chemical vapor deposition (CVD) with controlled numbers of layers were used as transparent electrodes in organic photovoltaic (OPV) devices. It was found that for devices with pristine graphene electrodes, the power conversion efficiency (PCE) is comparable to their counterparts with indium tin oxide (ITO) electrodes. Nevertheless, the chances for failure in OPVs with pristine graphene electrodes are higher than for those with ITO electrodes, due to the surface wetting challenge between the hole-transporting layer and the graphene electrodes. Various alternative routes were investigated and it was found that AuCl(3) doping on graphene can alter the graphene surface wetting properties such that a uniform coating of the hole-transporting layer can be achieved and device success rate can be increased. Furthermore, the doping both improves the conductivity and shifts the work function of the graphene electrode, resulting in improved overall PCE performance of the OPV devices. This work brings us one step further toward the future use of graphene transparent electrodes as a replacement for ITO.

Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells

Growth of semiconducting nanostructures on graphene would open up opportunities for the development of flexible optoelectronic devices, but challenges remain in preserving the structural and electrical properties of graphene during this process. We demonstrate growth of highly uniform and well-aligned ZnO nanowire arrays on graphene by modifying the graphene surface with conductive polymer interlayers. On the basis of this structure, we then demonstrate graphene cathode-based hybrid solar cells using two different photoactive materials, PbS quantum dots and the conjugated polymer P3HT, with AM 1.5G power conversion efficiencies of 4.2% and 0.5%, respectively, approaching the performance of ITO-based devices with similar architectures. Our method preserves beneficial properties of graphene and demonstrates that it can serve as a viable replacement for ITO in various photovoltaic device configurations.

Flexible graphene electrode-based organic photovoltaics with record-high efficiency.

Advancements in the field of flexible high-efficiency solar cells and other optoelectronic devices will strongly depend on the development of electrode materials with good conductivity and flexibility. To address chemical and mechanical instability of currently used indium tin oxide (ITO), graphene has been suggested as a promising flexible transparent electrode but challenges remain in achieving high efficiency of graphene-based polymer solar cells (PSCs) compared to their ITO-based counterparts. Here we demonstrate graphene anode- and cathode-based flexible PSCs with record-high power conversion efficiencies of 6.1 and 7.1%, respectively. The high efficiencies were achieved via thermal treatment of MoO3 electron blocking layer and direct deposition of ZnO electron transporting layer on graphene. We also demonstrate graphene-based flexible PSCs on polyethylene naphthalate substrates and show the device stability under different bending conditions. Our work paves a way to fully graphene electrode-based flexible solar cells using a simple and reproducible process.

Ion doping of graphene for high-efficiency heterojunction solar cells

We demonstrated the p-type chemical doping by chlorine and nitrate anions to enhance the Schottky junction in the solar cell. Nitrate ions were found to be more effective for reducing the sheet resistance and enlarging the work function of graphene for effective charge separation and transport, and the efficiency was increased to 9.2% by a factor of 1.68 under AM 1.5 illumination.

Organic Solar Cells with Graphene Electrodes and Vapor Printed Poly(3,4-ethylenedioxythiophene) as the Hole Transporting Layers

For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphenes hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).

Interface engineering of graphene for universal applications as both anode and cathode in organic photovoltaics

The high transparency of graphene, together with its good electrical conductivity and mechanical robustness, enable its use as transparent electrodes in optoelectronic devices such as solar cells. While initial demonstrations of graphene-based organic photovoltaics (OPV) have been promising, realization of scalable technologies remains challenging due to their performance and, critically, poor device reproducibility and yield. In this work, we demonstrate by engineering the interface between graphene and organic layers, device performance and yield become close to devices using indium tin oxide. Our study confirms that the key issue leading to the poor performance or irreproducibility in graphene-based OPV originates from the graphene interface, and can be addressed by a simple interface modification method introduced in this work. We also show similar approach allows graphene to be used as cathode in inverted OPV geometry, thereby demonstrating the universal application of graphene as transparent conductors for both the anode and cathode.

Direct Observation of Wet Biological Samples by Graphene Liquid Cell Transmission Electron Microscopy

Recent development of liquid phase transmission electron microscopy (TEM) enables the study of specimens in wet ambient conditions within a liquid cell; however, direct structural observation of biological samples in their native solution using TEM is challenging since low-mass biomaterials embedded in a thick liquid layer of the host cell demonstrate low contrast. Furthermore, the integrity of delicate wet samples is easily compromised during typical sample preparation and TEM imaging. To overcome these limitations, we introduce a graphene liquid cell (GLC) using multilayer graphene sheets to reliably encapsulate and preserve biological samples in a liquid for TEM observation. We achieve nanometer scale spatial resolution with high contrast using low-dose TEM at room temperature, and we use the GLC to directly observe the structure of influenza viruses in their native buffer solution at room temperature. The GLC is further extended to investigate whole cells in wet conditions using TEM. We also demonstrate the potential of the GLC for correlative studies by TEM and fluorescence light microscopy imaging.

The use of an n-type macromolecular additive as a simple yet effective tool for improving and stabilizing the performance of organic solar cells

The discovery of an easy and powerful way to further improve and stabilize the performance of organic solar cells (OSCs) from the current levels would advance their commercialization. In this work, an unprecedented power conversion efficiency (PCE) of 11.6% with improved stability is demonstrated by using a high-quality n-type macromolecular additive P(NDI2OD-T2) via a simple route without additional processing steps, where the high-quality P(NDI2OD-T2) is isolated by a THF-soaking treatment. We attribute the improved performance to advantageous changes in the morphology of the photoactive materials induced by the macromolecular additive. In addition, using the ITO-free architecture on a flexible PET substrate, we obtain an impressive PCE of 5.66% in macromolecular additive-processed devices. Due to its great applicability and easy accessibility, the use of the macromolecular additive introduced in this study has great potential for broad applications with other OSC systems, which will accelerate the commercial viability of photovoltaic technology.

Application of solvent modified PEDOT:PSS to graphene electrodes in organic solar cells

Graphene has been proposed as a promising transparent conducting electrode material in organic photovoltaic (OPV) solar cells to substitute the widely used indium tin oxide (ITO). Various studies have reported OPV devices with graphene electrodes showing comparable performances to ITO-based OPV cells. However, the fabrication reliability or device yield has not been widely discussed. In our investigation it was found that graphene-based OPVs tend to have much lower device yield than the ITO-based ones if similar fabrication processes are used. One possible reason could be due to the challenge faced in obtaining a uniform coating of PEDOT:PSS hole injection layer (HIL) on graphene. In fact, several attempts have been made to overcome this challenge. In this work, we report a facile method of solvent modification of PEDOT:PSS to achieve a homogeneous coating on graphene. A significant enhancement in the device yield was observed as compared to devices using normal aqueous solution based PEDOT:PSS; nevertheless, it was still only ∼50%. Further improvement was made by adding an additional electron blocking layer (EBL), molybdenum trioxide, after the coating of PEDOT:PSS. The observation in this work calls for further investigation to understand the role played by the HIL or EBL in graphene-based OPV devices.