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Jeong, Junkyeong,Kang, Donghee,Chun, Do Hyung,Shin, Dongguen,Park, Jong Hyeok,Cho, Sang Wan,Jeong, Kwangho,Lee, Hyunbok,Yi, Yeonjin Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.495 No.-
<P><B>Abstract</B></P> <P>Direct evidence of chemical interaction and origin of electron accumulation at a “buried” methylammonium lead triiodide (CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>, hereafter “MAPI”)/TiO<SUB>2</SUB> interface is presented in this study for the first time. Despite the high power conversion efficiency of perovskite solar cells (PSCs) using a TiO<SUB>2</SUB> electron transport layer, the MAPI/TiO<SUB>2</SUB> interface is believed as an electron accumulation position during device operation. To elucidate the cause of the electron accumulation, the energy level alignment at the MAPI/TiO<SUB>2</SUB> interface should be understood. However, a buried MAPI/TiO<SUB>2</SUB> interface forms after a thick MAPI layer deposition; thus, the electronic structure of the MAPI/TiO<SUB>2</SUB> interface cannot be measured using surface-sensitive photoelectron spectroscopy in a conventional stack-up manner. In this study, we investigated the electronic structure of a buried MAPI/TiO<SUB>2</SUB> interface by removing the MAPI and organic layers using solvent immersion. As a result, we reveal that a conduction band minimum (CBM) mismatch occurs owing to the TiOPb bonding on the TiO<SUB>2</SUB> surface. The TiOPb bonds form by the Pb ions penetrating during the spin coating of the MAPI solution. When a [6,6]-phenyl C<SUB>61</SUB> butyric acid methyl ester (PCBM) layer was inserted, the CBM mismatch was removed owing to the high work function of PCBM.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The electronic structure of a buried MAPI/TiO<SUB>2</SUB> interface was investigated. </LI> <LI> The TiOPb bonding on the TiO<SUB>2</SUB> surface occurred after MAPI deposition. </LI> <LI> The CBM mismatch was observed owing to the TiOPb bond formation. </LI> <LI> With PCBM, the cascaded CBM was formed owing to its high work function. </LI> <LI> The cascaded CBM significantly improved the solar cell performance. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Jeong, Junkyeong,Lee, Jiyeon,Lee, Hyunchan,Hyun, Gyeongho,Park, Soohyung,Yi, Yeonjin,Cho, Sang Wan,Lee, Hyunbok Elsevier 2018 Chemical physics letters Vol.706 No.-
<P>The energy level alignment at the interface between an organic layer and an electrode plays a critical role in the performance of organic electronic devices. To improve the charge injection efficiency, the energy barrier between the charge transport level of an organic layer and the Fermi level of an electrode should be reduced. Especially, poly(9-vinylcarbazole) (PVK) is a polymeric semiconductor that is widely used as a hole transport and electron blocking layer in various optoelectronic devices. Thus, an understanding of the energy level alignment between PVK and an electrode is of great importance. In this study, the energy level alignment of PVK and indium tin oxide (ITO) was determined with X-ray and ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy measurements. The effect of UV-ozone (UVO) treatment on the formation of a hole injection barrier (Phi(h)) in ITO was also investigated. In both cases of bare ITO and UVO ITO, only small interface dipole and band bending were observed, which indicates charge transfer between PVK and ITO is miniscule. The UVO treatment significantly increases the work function of ITO from 4.05 eV to 4.40 eV, which results in the reduction of Phi(h) from 1.70 eV to 1.50 eV. This reduced Phi(h) value dramatically improves the current density-voltage characteristics of PVK-based hole-only devices. (C) 2018 Elsevier B.V. All rights reserved.</P>
Direct p-doping of Li-TFSI for efficient hole injection: Role of polaronic level in molecular doping
Kim, Kiwoong,Jeong, Junkyeong,Kim, Minju,Kang, Donghee,Cho, Sang Wan,Lee, Hyunbok,Yi, Yeonjin Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.480 No.-
<P><B>Abstract</B></P> <P>Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) has been popularly employed as an efficient p-dopant that increases the conductivity of a hole transport layer (HTL) in perovskite solar cells and dye-sensitized solar cells. However, the working mechanism of the Li-TFSI dopant is a long-standing question. The hygroscopicity of Li-TFSI makes it difficult to isolate the exact doping mechanism. In this study, we unveil the role of Li-TFSI in the p-doping to the <I>N</I>,<I>N</I>′-di(1-naphthyl)-<I>N</I>,<I>N</I>′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) HTL. A series of systematic in situ measurements using ultraviolet and inverse photoelectron spectroscopy reveal that electron transfer from NPB to Li-TFSI occurs due to the lower-lying negative polaronic level of Li-TFSI rather than the positive polaronic level of NPB. The hole injection barrier between NPB and indium tin oxide is significantly reduced with Li-TFSI doping, enhancing the device performance of hole-only devices and organic light-emitting diodes dramatically. With excessive dopants, however, the agglomerative property of Li-TFSI became dominant, decreasing the doping efficiency. These results provide robust guidelines for developing an efficient doping method for a molecular system with high conductivity.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Electronic structure of Li-TFSI and NPB was investigated using in situ UPS and IPES. </LI> <LI> Electron transfer occurred from NPB to Li-TFSI through their polaronic levels. </LI> <LI> Hole injection barrier was reduced by 0.70 eV with Li-TFSI doping. </LI> <LI> Device performance of OLEDs was significantly enhanced with Li-TFSI doping. </LI> <LI> With excessive dopants, agglomeration of Li-TFSI decreased doping efficiency. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Energy level alignment at C<sub>60</sub>/DTDCTB/PEDOT:PSS interfaces in organic photovoltaics
Yoo, Jisu,Jung, Kwanwook,Jeong, Junkyeong,Hyun, Gyeongho,Lee, Hyunbok,Yi, Yeonjin Elsevier BV * North-Holland 2017 Applied Surface Science Vol.402 No.-
<P><B>Abstract</B></P> <P>The electronic structure of a narrow band gap small molecule ditolylaminothienyl–benzothiadiazole–dicyanovinylene (DTDCTB), possessing a donor-acceptor-acceptor configuration, was investigated with regard to its application as an efficient donor material in organic photovoltaics (OPVs). The interfacial orbital alignment of C<SUB>60</SUB>/DTDCTB/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was determined using in situ ultraviolet photoelectron and inverse photoelectron spectroscopic methods. The ionization energy and electron affinity values of DTDCTB were measured to be 5.27eV and 3.65eV, respectively, and thus a very small transport gap of 1.62eV was evaluated. Large band bending of DTDCTB on PEDOT:PSS was observed, resulting in a low hole extraction barrier. Additionally, the photovoltaic gap between the highest occupied molecular orbital level of the DTDCTB donor and the lowest unoccupied molecular orbital level of the C<SUB>60</SUB> acceptor was estimated to be 1.30eV, which is known to be the theoretical maximum open-circuit voltage in OPVs employing the C<SUB>60</SUB>/DTDCTB active layer. The unique electronic structures of DTDCTB contributed toward the recently reported excellent power conversion efficiencies of OPVs containing a DTDCTB donor material.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The interfacial energy level alignment of C<SUB>60</SUB>/DTDCTB/PEDOT:PSS was determined via in situ UPS and IPES measurements. </LI> <LI> A large photovoltaic gap of 1.30eV was evaluated between the DTDCTB donor and C<SUB>60</SUB> acceptor. </LI> <LI> A low hole extraction barrier of 0.42eV from DTDCTB to PEDOT:PSS was evaluated. </LI> <LI> The excellent electronic properties of DTDCTB with a narrow band gap were the source of its high OPV power conversion efficiencies. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>