{"id":286,"date":"2015-07-16T19:18:31","date_gmt":"2015-07-16T23:18:31","guid":{"rendered":"http:\/\/research.mse.ncsu.edu\/oemdlab\/?page_id=5"},"modified":"2025-08-26T19:11:09","modified_gmt":"2025-08-26T23:11:09","slug":"research","status":"publish","type":"page","link":"https:\/\/mse.ncsu.edu\/oemdlab\/research\/","title":{"rendered":"Research"},"content":{"rendered":"\n\n\n\n
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OLEDs<\/h2>\n

\n Organic Light-Emitting Diodes\n <\/p>\n <\/div>\n <\/div>\n \n\n <\/div>\n\n\n

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We develop new materials, processing methods, new light extraction schemes and device structures for OLED applications. Along with the standard LED characterization techniques (L-I-V, QE, EL spectra, angular emission profiles) we also investigate the fundamental physics of OLED operation such as carrier transport and recombination as well as the mechanisms of degradation and efficiency roll-off. These studies are carried out using additional characterization techniques such as photoluminescence (PL), transient PL and transient EL.<\/p>\n\n\n <\/div>\n\n\n <\/div>\n \n\n\n

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OLEDs: Recent representative results<\/h2>\n <\/summary>\n

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Figure 2. Current efficiency (cd\/A) and external quantum efficiency (%) for the 260 nm periodicity grating (red) and reference (black) devices.<\/figcaption><\/figure>\n\n\n\n

In an organic light-emitting diode (OLED), only about 20\u201330% of the generated light can be extracted because of losses to substrate, film-guided and surface plasmon modes. Using corrugated high-index-refractive substrates, the film guided modes can be effectively out-coupled from the device and the loss to surface plasmon modes is effectively suppressed. Our research group has recently demonstrated a corrugated green phosphorescent OLED with an extremely high external quantum efficiency of 63% using a macro-lens to extract the substrate modes. (Fig 2) The light extraction enhancement is independent of wavelength and the emission profile is similar to a Lambertian emitter, demonstrating that this device is promising for OLED lighting applications.<\/p>\n\n\n <\/p>\n <\/details>\n\n\n <\/div>\n <\/div>\n\n\n

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OLEDs<\/h2>\n

OLEDs are used to create digital displays in devices such as television screens, computer monitors, and portable systems such as smartphones and handheld game consoles. A major area of research is the development of white OLED devices for use in solid-state lighting applications.<\/p>\n <\/div>\n\n\n

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Solar Cells<\/h2>\n

\n Photovoltaic cell (PV cell)\n <\/p>\n <\/div>\n <\/div>\n \n\n <\/div>\n\n\n

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Our group investigates the processing-structure-property relationships in emerging solution-processed solar cells, especially polymer-based and hybrid organic-inorganic perovskite-based devices. Electrical and photophysical measurements, such as transient photovoltage (TPV), space-charge limited current (SCLC) and charge-modulated electroabsorption spectroscopy (CMEAS), are used to understand fundamental material properties and how they relate to the performance of completed devices. This understanding guides our optimization of photovoltaic devices toward industrially relevant efficiencies.<\/p>\n\n\n <\/div>\n\n\n <\/div>\n \n\n\n

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Solar Cells: Recent representative results<\/h2>\n <\/summary>\n

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Figure 3. (a) Sub-bandgap CMEAS spectra of polymer: PC71BM blends showing the cutoff that is used to define the effective bandgap (Eeff). (b) ESC (= Eeff \u2013 eV +kT ln(np\/NcNv)) plotted versus the inverse of the electric constant of the various polymer blends examined<\/figcaption><\/figure>\n\n\n\n

Our group utilizes charge-modulated electroabsorption spectroscopy (CMEAS) to determine the effective bandgap of polymer bulk heterojunction devices. CMEAS is a sensitive absorption technique that can directly probe sub-bandgap charge generation in completed solar cells. By examining the onset of this charge generation, we determine the effective transport bandgap of the device (Fig. 3a). The effective bandgap determined this way is better correlated to the open-circuit voltage (VOC) observed than previous measurements. Combining the CMEAS results with TPV allows for a complete understanding of the VOC observed in polymer photovoltaic devices. The difference between the effective bandgap and the VOC (ESC) was shown to be proportional to the inverse of the dielectric constant (Fig. 3b). Therefore, we were able to precisely demonstrate the relationship between dielectric constant and the observed VOC in polymer bulk heterojunction devices.<\/p>\n\n\n <\/p>\n <\/details>\n\n\n <\/div>\n <\/div>\n\n\n

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Solar Cells<\/h2>\n

A solar cell is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. It is a type of photoelectric cell, a device whose electrical characteristics (such as current, voltage, or resistance) vary when it is exposed to light.<\/p>\n <\/div>\n\n\n

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Vertical Field Effect Transistors <\/h2>\n

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Our group is developing novel organic field-effect transistors (OFETs) and specifically focusing on high-performance vertical organic field-effect transistors (VOFETs). We are investigating the switching mechanisms of these novel organic transistors based on an understanding of charge transport\/injection behaviors in organic materials using various electrical measurements complemented by device simulations. By demonstrating record-high performance VOFETs, our goal is to enable their commercial use as low-cost drivers for applications such as displays, sensors, and memory.<\/p>\n\n\n <\/div>\n\n\n <\/div>\n \n\n\n

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VFETs: Recent representative results<\/h2>\n <\/summary>\n

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Figure 4. The structure and performance of PMBT with a porous Al base electrode<\/figcaption><\/figure>\n\n\n\n

Our group demonstrated high-gain organic and inorganic\/organic hybrid permeable metal-base (PMBTs). Vertical current flow in PMBTs is modulated by the potential screening effect of the permeable metal-base electrode sandwiched between two semiconductors. By controlling the pore size of a permeable metal-based material at the nanoscale, we achieved a record high current gain of 476. In spite of the short vertical organic channel, our PMBTs exhibited output current saturation, unlike previously reported PMBTs, due to a potential pinning effect at the permeable-base electrode with a small pore size of 20 nm (Fig. 4).<\/p>\n\n\n <\/p>\n <\/details>\n\n\n <\/div>\n <\/div>\n\n\n

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VFETs<\/h2>\n

Vertical Field Effect Transistors (VFETs) were once limited to costly high-end audio equipment, but today they\u2019re being developed for next-generation computing, smartphones, and wearables. This cutting-edge technology is moving from specialized R&D toward broader applications.<\/p>\n <\/div>\n\n\n

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About Our Lab<\/h2>\n

The OEMD Research Lab at NC State is developing new materials, device structures, and light-extraction schemes to dramatically boost efficiency, exemplified by a corrugated green phosphorescent OLED achieving 63% external quantum efficiency. The team also explores emerging solar cell materials and optimizes high-performance vertical organic transistors (VOFETs), achieving record-high current gains through innovative permeable metal-base structures. <\/a><\/p>\n \n\n\n <\/div>\n\n\n <\/div>\n <\/div>\n <\/div>\n\n\n <\/div>\n <\/div>\n <\/div>\n\n\n\n

Contact<\/h2>\n\n\n\n
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FS<\/div>\n \"\"\n <\/div>\n <\/a>\n <\/div>\n \n
\n \n Franky So \n <\/a>\n \n \n

Associate Department Head and Freeman Distinguished Professor<\/p>\n \n \n \n \n \n fso@ncsu.edu<\/a>\n \n <\/div>\n <\/div>\n<\/div>\n<\/div><\/div><\/div>\n","protected":false},"excerpt":{"rendered":"

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