Jong-Am Hong

Energy level alignment in polymer organic solar cells at donor-acceptor planar junction formed by electrospray vacuum deposition

Using ultraviolet photoelectron spectroscopy (UPS), we have measured the energy level offset at the planar interface between poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM). Gradual deposition of PCBM onto spin-coated P3HT in high vacuum was made possible by using electrospray vacuum deposition (EVD). The UPS measurement of EVD-prepared planar interface resulted in the energy level offset of 0.91 eV between P3HT HOMO and PCBM LUMO, which is considered as the upper limit of Voc of the organic photovoltaic cells.Using ultraviolet photoelectron spectroscopy (UPS), we have measured the energy level offset at the planar interface between poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM). Gradual deposition of PCBM onto spin-coated P3HT in high vacuum was made possible by using electrospray vacuum deposition (EVD). The UPS measurement of EVD-prepared planar interface resulted in the energy level offset of 0.91 eV between P3HT HOMO and PCBM LUMO, which is considered as the upper limit of Voc of the organic photovoltaic cells.

Mechanism for the direct electron injection from Al cathode to the phosphine oxide type electron transport layer

A high efficiency blue fluorescent organic light-emitting diode without LiF electron injection layer was developed. Aluminum electrode was directly deposited on a phosphine oxide type electron transport layer and the observed quantum efficiency was as high as 6.13%. The ultraviolet photoemission spectroscopy data clearly indicated that the electron injection barrier (the offset between Al Fermi level and the lowest unoccupied molecular orbital of the organic layer) is less than 0.1 eV, which led us to believe that more efficient electron injection through the lower barrier is mainly responsible for the high efficiency.

Highly efficient green, blue, and white phosphorescent inverted organic light-emitting diodes by improving charge injection and balance

To improve the performance of inverted organic light-emitting diodes (OLEDs), we investigated the electrical, optical, and interfacial properties of three different lithium (Li)-doped electron transport materials (ETMs): tris(3-(3pyridyl)mesityl)borane (3TPYMB), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB). The electron injection barriers (EIBs) between indium-tin-oxide and the ETMs were deduced for both pristine and Li-doped cases from ultraviolet photoelectron spectroscopy measurements and optical band gap values. The Li-doped ETMs showed EIB values of approximately 0.03 eV, 0.77 eV, and 0.81 eV for 3TPYMB, TmPyPB, and BmPyPB, respectively, which are much lower than those of their pristine counterparts of 0.94 eV, 1.14 eV, and 1.48 eV, respectively. The Li-doped ETMs were employed as electron injection layers (EILs) of inverted bottom-emission OLEDs (IBE-OLEDs) with green phosphorescence. IBE-OLEDs with 3TPYMB, TmPyPB, and BmPyPB EILs exhibited driving voltages of 3.6 V, 4.0 V, and 4.5 V at 1000 cd m−2 and maximum external quantum efficiencies (EQEs) of 20.3%, 19.7%, and 16.5%, respectively. From the low EIB of Li-doped 3TPYMB, we also demonstrated highly efficient blue and white phosphorescent IBE-OLEDs. We optimized the device structure to improve the charge balance and out-coupling efficiency by changing the hole injection layer and the thickness of the hole and electron transport layers with optical simulation. The blue device showed a maximum EQE and luminous current efficiency of 22.9% and 43.1 cd A−1, respectively. In addition, the white device exhibited a high EQE and luminous efficacy of 19.3% and 37.8 lm W−1 at 3 mA cm−2 (∼1000 cd m−2), respectively. To the best of our knowledge, the efficiencies of these green, blue, and white devices are the highest values obtained to date with a low driving voltage for IBE-OLEDs without any additional light-extraction structure. Since the Li-doped 3TPYMB has an extremely low EIB and shows good device performance, it can be utilized as an effective EIL in inverted-structure devices.