Tae Gun Kim - Academia.edu (original) (raw)

Papers by Tae Gun Kim

Organic Electronics, May 1, 2012

Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO 2) electron... more Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO 2) electron injection layer (EIL) in Alq 3-based organic light-emitting diodes. Insertion of the MnO 2 EIL between the Al cathode and Alq 3 results in highly improved device characteristics. In situ photoelectron spectroscopy shows remarkable reduction of the electron injection barrier without significant chemical reactions between Alq 3 and MnO 2 , which could induce Alq 3 destruction. The reduction of the electron injection barrier is due to the n-type doping effect, and the lack of strong interfacial reaction is advantageous with regards to more efficient electron injection than a conventional LiF EIL. These properties render the MnO 2 , a potential EIL.

Applied Physics Letters, May 14, 2012

Journal of Physical Chemistry C, Nov 21, 2014

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical liga... more The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances made thereof. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2-ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, respectively which gives the trace of band bending and its saturation. Consequently the energy level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO 2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.

ACS Applied Materials & Interfaces, Mar 12, 2013

We report on the identification of molecular orientation and its order-disorder transition during... more We report on the identification of molecular orientation and its order-disorder transition during the initial growth of 1,3-bis(N-carbazolyl)benzene (mCP) thin films on a highly ordered pyrolytic graphite (HOPG) surface by using photoelectron spectroscopy (PES). Theoretical PES amplitudes using a quantum mechanical calculation that adapts independent atomic center approximation (IAC) were calculated to compare with experimental observations. At low coverage, an equilibrium orientation of isolated adsorbate was estimated. As the coverage increases, the interaction between adsorbates becomes dominant and raises the disorder, which results in changes in the PES shapes as well as the line broadening of each peak.

Organic Electronics, Nov 1, 2012

ABSTRACT A simple method based on capacitance–voltage (C–V) measurements is reported to determine... more ABSTRACT A simple method based on capacitance–voltage (C–V) measurements is reported to determine the interface energy level alignment at the junction of 15 mol% Cs2CO3 doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) fabricated under high vacuum. The junction properties, such as the depletion layer thickness, built-in potentials and vacuum level shift were calculated with simple Mott–Schottky and Poisson’s equations with the boundary condition of a continuous electric flux density using the information from the C–V data. The interface energy level alignment determined by this method is well matched with the one determined using the in situ ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) experiments performed under ultra-high vacuum. This method can be applied to other semiconductor junctions such as the organic p–n homojunctions and heterojunctions with known energy levels, as long as the metal/semiconductor contact is Ohmic without referring to the photoemission spectroscopies. Moreover, the energy level alignment determined by the C–V measurement gives a more realistic result since the films for the measurements are formed under high vacuum which is a normal device fabrication environment rather than under ultra high vacuum.

Journal of The Korean Chemical Society, Dec 20, 2016

Thin films of lead sulfide colloidal quantum dots (CQDs) of 2.8 nm in diameter are fabricated and... more Thin films of lead sulfide colloidal quantum dots (CQDs) of 2.8 nm in diameter are fabricated and their surfaces are passivated by 3-mercaptopropionic acid (MPA) ligand or hybrid type (MPA+CdCl2) ligand, respectively. The changes in valence band electronic structure and atomic composition of each PbS CQD film upon post-treatment such as air, N2 annealing or UV/Ozone have been studied by photoelectron spectroscopy. The air annealing makes the CQD fermi level to move toward the valence band leading to "p-type doping" regardless of ligand type. The UV/Ozone post-treatment generates Pb(OH)2, PbSOx and PbO on both CQD surfaces. But the amount of the PbO has been reduced in hybrid type ligand case, especially. That is probably because the extra Pb cations in (111) surface are additionally passivated by Cl2 ligand, which limits the reaction between the Pb cation and ozone.

Etri Journal, Apr 1, 2016

Nano Energy, 2018

Na has been believed to improve the device parameters of open circuit voltage (V OC) and fill fac... more Na has been believed to improve the device parameters of open circuit voltage (V OC) and fill factor (FF) presumably by increasing the carrier concentration (N A) of vacuum-processed Cu(In,Ga)Se 2 films. In solution-processed CI(G)Se devices as well, Na reportedy increases V OC and FF but this improvement is not correlated with the increase in N A, suggesting a different physical mechanism associated with Na in solution-based routes. In this contribution, experimental results on the role of Na addition in solution-processed CISe films and devices were reported, in which Na addition had no influence on N A nor film composition in spite of the notable increase in the device efficiency. On the contrary, Na was found to mitigate the interfacial recombination by reducing the undesirable surface defects. Along with this understanding, Na addition in our air-processable route resulted in a CISe device with 12.83 % efficiency, which is comparable to the current world record efficiency of solutionprocessed CISe devices.

ACS applied materials & interfaces, Jan 20, 2015

We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) so... more We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Post-annealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300 to 500 °C, to examine its thermal stability. Serious device degradation particularly in VOC was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after post-annealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency.

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical liga... more The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, which give the trace of band bending and its saturation. Consequently, the energy-level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO 2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.

Organic Electronics, May 1, 2012

Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO 2) electron... more Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO 2) electron injection layer (EIL) in Alq 3-based organic light-emitting diodes. Insertion of the MnO 2 EIL between the Al cathode and Alq 3 results in highly improved device characteristics. In situ photoelectron spectroscopy shows remarkable reduction of the electron injection barrier without significant chemical reactions between Alq 3 and MnO 2 , which could induce Alq 3 destruction. The reduction of the electron injection barrier is due to the n-type doping effect, and the lack of strong interfacial reaction is advantageous with regards to more efficient electron injection than a conventional LiF EIL. These properties render the MnO 2 , a potential EIL.

Applied Physics Letters, May 14, 2012

Journal of Physical Chemistry C, Nov 21, 2014

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical liga... more The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances made thereof. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2-ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, respectively which gives the trace of band bending and its saturation. Consequently the energy level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO 2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.

ACS Applied Materials & Interfaces, Mar 12, 2013

We report on the identification of molecular orientation and its order-disorder transition during... more We report on the identification of molecular orientation and its order-disorder transition during the initial growth of 1,3-bis(N-carbazolyl)benzene (mCP) thin films on a highly ordered pyrolytic graphite (HOPG) surface by using photoelectron spectroscopy (PES). Theoretical PES amplitudes using a quantum mechanical calculation that adapts independent atomic center approximation (IAC) were calculated to compare with experimental observations. At low coverage, an equilibrium orientation of isolated adsorbate was estimated. As the coverage increases, the interaction between adsorbates becomes dominant and raises the disorder, which results in changes in the PES shapes as well as the line broadening of each peak.

Organic Electronics, Nov 1, 2012

ABSTRACT A simple method based on capacitance–voltage (C–V) measurements is reported to determine... more ABSTRACT A simple method based on capacitance–voltage (C–V) measurements is reported to determine the interface energy level alignment at the junction of 15 mol% Cs2CO3 doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) fabricated under high vacuum. The junction properties, such as the depletion layer thickness, built-in potentials and vacuum level shift were calculated with simple Mott–Schottky and Poisson’s equations with the boundary condition of a continuous electric flux density using the information from the C–V data. The interface energy level alignment determined by this method is well matched with the one determined using the in situ ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) experiments performed under ultra-high vacuum. This method can be applied to other semiconductor junctions such as the organic p–n homojunctions and heterojunctions with known energy levels, as long as the metal/semiconductor contact is Ohmic without referring to the photoemission spectroscopies. Moreover, the energy level alignment determined by the C–V measurement gives a more realistic result since the films for the measurements are formed under high vacuum which is a normal device fabrication environment rather than under ultra high vacuum.

Journal of The Korean Chemical Society, Dec 20, 2016

Thin films of lead sulfide colloidal quantum dots (CQDs) of 2.8 nm in diameter are fabricated and... more Thin films of lead sulfide colloidal quantum dots (CQDs) of 2.8 nm in diameter are fabricated and their surfaces are passivated by 3-mercaptopropionic acid (MPA) ligand or hybrid type (MPA+CdCl2) ligand, respectively. The changes in valence band electronic structure and atomic composition of each PbS CQD film upon post-treatment such as air, N2 annealing or UV/Ozone have been studied by photoelectron spectroscopy. The air annealing makes the CQD fermi level to move toward the valence band leading to "p-type doping" regardless of ligand type. The UV/Ozone post-treatment generates Pb(OH)2, PbSOx and PbO on both CQD surfaces. But the amount of the PbO has been reduced in hybrid type ligand case, especially. That is probably because the extra Pb cations in (111) surface are additionally passivated by Cl2 ligand, which limits the reaction between the Pb cation and ozone.

Etri Journal, Apr 1, 2016

Nano Energy, 2018

Na has been believed to improve the device parameters of open circuit voltage (V OC) and fill fac... more Na has been believed to improve the device parameters of open circuit voltage (V OC) and fill factor (FF) presumably by increasing the carrier concentration (N A) of vacuum-processed Cu(In,Ga)Se 2 films. In solution-processed CI(G)Se devices as well, Na reportedy increases V OC and FF but this improvement is not correlated with the increase in N A, suggesting a different physical mechanism associated with Na in solution-based routes. In this contribution, experimental results on the role of Na addition in solution-processed CISe films and devices were reported, in which Na addition had no influence on N A nor film composition in spite of the notable increase in the device efficiency. On the contrary, Na was found to mitigate the interfacial recombination by reducing the undesirable surface defects. Along with this understanding, Na addition in our air-processable route resulted in a CISe device with 12.83 % efficiency, which is comparable to the current world record efficiency of solutionprocessed CISe devices.

ACS applied materials & interfaces, Jan 20, 2015

We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) so... more We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Post-annealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300 to 500 °C, to examine its thermal stability. Serious device degradation particularly in VOC was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after post-annealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency.

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical liga... more The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, which give the trace of band bending and its saturation. Consequently, the energy-level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO 2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy.