Hyung-Jun Song

Plasmonic Organic Solar Cells Employing Nanobump Assembly via Aerosol-Derived Nanoparticles

We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO3) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode via room temperature aerosol method. NBA structure is made by enclosing NPs with MoO3 layer via vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO3 without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells.

High performance inverted organic solar cells with solution processed Ga-doped ZnO as an interfacial electron transport layer

We demonstrate solution-processed Ga-doped ZnO incorporated as an interfacial electron transport layer into inverted organic solar cells with active layers comprising either PCDTBT or PTB7 mixed with PC71BM. The 5.03 at% Ga-doped ZnO showed the best efficiencies of 5.56% and 7.34% for PCDTBT and PTB7 polymers respectively.

Toward Mass Producible Ordered Bulk Heterojunction Organic Photovoltaic Devices

A strategy to fabricate nanostructured poly(3-hexylthiophene) (P3HT) films for organic photovoltaic (OPV) cells by a direct transfer method from a reusable soft replica mold is presented. The flexible polyfluoropolyether (PFPE) replica mold allows low-pressure and low- temperature process condition for the successful transfer of nanostructured P3HT films onto PEDOT/PSS-coated ITO substrates. To reduce the fabrication cost of masters in large area, we employed well-ordered anodic aluminum oxide (AAO) as a template. Also, we provide a method to fabricate reversed nanostructures by exploiting the self-replication of replica molds. The concept of the transfer method in low temperature with a flexible and reusable replica mold obtained from an AAO template will be a firm foundation for a low-cost fabrication process of ordered OPVs.

Hydroiodic acid treated PEDOT:PSS thin film as transparent electrode: an approach towards ITO free organic photovoltaics†

In this article, we introduce a method for fabricating highly conductive transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin nano-structured film by treating the pristine PEDOT:PSS film with concentrated (55%) hydroiodic acid (HI). HI treatment on the pristine PEDOT:PSS film could significantly reduce the film sheet resistance without sacrificing its transparency and other electronic properties, which are highly desirable for transparent conductors. A thin layer of HI treated PEDOT:PSS film (74 nm) has very high transmittance of 90% and a low sheet resistance of 95 ohm sq−1. The sheet resistance of the film can be reduced to 37 ohm sq−1 by increasing the film thickness but at the expense of its transparency. The low sheet resistance of the film can be attributed to the increase of polarons density, conformational changes and formation of clusters due to removal of some PSS from PEDOT:PSS particles. The results were confirmed by the UV/VIS/near-IR absorption spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. Given the high electrical conductivity of the HI treated PEDOT:PSS film, it was then tested as a transparent organic cathode in a polymer solar cell. This ITO-free solar cell showed superior current generation and charge collection with an efficiency of 5.83%, which is 18 times higher than the cell that used untreated PEDOT:PSS film. We, therefore, envisage this potential fabrication method, which uses HI acid, to be applied on other conjugated polymers to enhance their conducting properties for practical applications.

Improved photovoltaic performance of inverted polymer solar cells through a sol-gel processed Al-doped ZnO electron extraction layer

We demonstrate that nanocrystalline Al-doped zinc oxide (n-AZO) thin film used as an electron-extraction layer can significantly enhance the performance of inverted polymer solar cells based on the bulk heterojunction of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl C(71)-butyric acid methyl ester (PC(70)BM). A synergistic study with both simulation and experiment on n-AZO was carried out to offer a rational guidance for the efficiency improvement. As a result, An n-AZO film with an average grain size of 13 to 22 nm was prepared by a sol-gel spin-coating method, and a minimum resistivity of 2.1 × 10(-3) Ω·cm was obtained for an Al-doping concentration of 5.83 at.%. When an n-AZO film with a 5.83 at.% Al concentration was inserted between the ITO electrode and the active layer (PCDTBT:PC(70)BM), the power conversion efficiency increased from 3.7 to 5.6%.

Pretilt Control of a Nematic Liquid Crystal on Polymer Layers by Atmospheric Plasma Irradiation

We present the pretilt control of a nematic liquid crystal (LC) on polymer layers exposed to a plasma beam under atmospheric pressure. Depending on the exposure parameters of the atmospheric pressure plasma (APP) in a sheetlike shape, the vertical or the planar alignment with a variable pretilt can be produced on a homeotropic polyimide (PI) layer. The polar anchoring energy of the APP-exposed PI layer is of the order of 10-4 J/m2, comparable to the rubbed PI layer. Our approach to the plasma-assisted LC alignment under ambient pressure would be applicable for the in-line process of producing wideviewing LC displays with multidomains.

Nanostructured Electron-Selective Interlayer for Efficient Inverted Organic Solar Cells.

We report a unique nanostructured electron-selective interlayer comprising of In-doped ZnO (ZnO:In) and vertically aligned CdSe tetrapods (TPs) for inverted polymer:fullerene bulkheterojunction (BHJ) solar cells. With dimension-controlled CdSe TPs, the direct inorganic electron transport pathway is provided, resulting in the improvement of the short circuit current and fill factor of devices. We demonstrate that the enhancement is attributed to the roles of CdSe TPs that reduce the recombination losses between the active layer and buffer layer, improve the hole-blocking as well as electron-transporting properties, and simultaneously improve charge collection characteristics. As a result, the power conversion efficiency of PTB7:PC70BM based solar cell with nanostructured CdSe TPs increases to 7.55%. We expect this approach can be extended to a general platform for improving charge extraction in organic solar cells.

A Study on the Optimization of CP Based Low-temperature Tabbing Process for Fabrication of Thin c-Si Solar Cell Module

Thin crystalline silicon (C-Si) solar cell is expected to be a low price energy source by decreasing the consumption of Si. However, thin c-Si solar cell entails the bowing and crack issues in high temperature manufacturing process. Thus, the conventional tabbing process, based on high temperature soldering (> 250°C), has difficulties for applying to thin c-Si solar cell modules. In this paper, a conductive paste (CP) based interconnection process has been proposed to fabricate thin c-Si solar cell modules with high production yield, instead of existing soldering materials. To optimize the process condition for CP based interconnection, we compared the performance and stability of modules fabricated under various lamination temperature (120, 150, and 175°C). The power from CP based module is similar to that with conventional tabbing process, as modules are fabricated. However, the output of CP based module laminated at 120°C decreases significantly (14.1% for Damp heat and 6.1% for thermal cycle) in harsh condition, while the output drops only in 3% in the samples process at 150°C, 175°C. The peel test indicates that the unstable performance of sample laminated at 120°C is attributed to weak adhesion strength (1.7 N) between cell and ribbon compared to other cases (2.7 N). As a result, optimized lamination temperature for CP based module process is 150°C, considering stability and energy consumption during the fabrication.

Simultaneous Engineering of the Substrate Temperature and Mixing Ratio to Improve the Performance of Organic Photovoltaic Cells.

In this study, we investigated the effect of the donor/acceptor mixing ratio and the substrate temperature (T(SUB)) during the co-deposition process on the performance of bulk heterojunction organic photovoltaic cells. We found that the ratio of dispersed donor islands (less than 10 nm), which hinders charge carrier transport, increased as the donor concentration (C(D)) increased in the film processed at room temperature. By contrast, the donor cluster (larger than 10 nm), providing percolation paths for the carriers, was enlarged in the film containing a high C(D) fabricated at high T(SUB) (70 degrees C). This enhanced phase separation in the mixed layer led to an improved fill factor and a decreased activation energy of the short-circuit current (J(SC)). Therefore, we demonstrated a 23% improvement in the device performance by employing an elevated T(SUB) and optimized mixing ratio in comparison with the device fabricated at room temperature.

Nanobump assembly for plasmonic organic solar cells

We demonstrate novel plasmonic organic solar cells (OSCs) by embedding an easy processible nanobump assembly (NBA) for harnessing more light. The NBA is consisted of precisely size-controlled Ag nanoparticles (NPs) generated by an aerosol process at atmospheric pressure and thermally deposited molybdenum oxide (MoO3) layer which follows the underlying nano structure of NPs. The active layer, spin-casted polymer blend solution, has an undulated structure conformably covering the NBA structure. To find the optimal condition of the NBA structure for enhancing light harvest as well as carrier transfer, we systematically investigate the effect of the size of Ag NPs and the MoO3 coverage on the device performance. It is observed that the photocurrent of device increases as the size of Ag NP increases owing to enhanced plasmonic and scattering effect. In addition, the increased light absorption is effectively transferred to the photocurrent with small carrier losses, when the Ag NPs are fully covered by the MoO3 layer. As a result, the NBA structure consisted of 40 nm Ag NPs enclosed by 20 nm MoO3 layer leads to 18% improvement in the power conversion efficiency compared to the device without the NBA structure. Therefore, the NBA plasmonic structure provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of organic solar cells.