Electronic structure of Ba(OH)                         2                          interface in inverted organic photovoltaics: Improved electron transport by charged state of C                         60

Jisu Yoo; Junkyeong Jeong; Kwanwook Jung; Gyeongho Hyun; Minju Kim; Hyunbok Lee; Yeonjin Yi

doi:10.1016/j.apsusc.2019.01.093

Electronic structure of Ba(OH) ₂ interface in inverted organic photovoltaics: Improved electron transport by charged state of C ₆₀

Jisu Yoo, Junkyeong Jeong, Kwanwook Jung, Gyeongho Hyun, Minju Kim, Hyunbok Lee, Yeonjin Yi

Department of Physics

Research output: Contribution to journal › Article › peer-review

4 Citations (Scopus)

Abstract

Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH) ₂ is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH) ₂ layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH) ₂ interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH) ₂ layer. On implementing the optimum thickness of the Ba(OH) ₂ layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C ₆₀ /indium tin oxide (ITO) and C ₆₀ /Ba(OH) ₂ /ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH) ₂ was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH) ₂ , the reduced WF of the ITO is lower than the electron affinity of C ₆₀ . Thus, a charge transfer is induced from Ba(OH) ₂ /ITO to C ₆₀ , and the charged states of C ₆₀ are observed within the monolayer. These charged states result in significant band bending in the C ₆₀ layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (E _F ) of the Ba(OH) ₂ /ITO. As a result, the energy offset between the C ₆₀ LUMO level and the cathode E _F is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH) ₂ layer. This is the origin of the enhanced device performance of OPVs.

Original language	English
Pages (from-to)	435-441
Number of pages	7
Journal	Applied Surface Science
Volume	476
DOIs	https://doi.org/10.1016/j.apsusc.2019.01.093
Publication status	Published - 2019 May 15

Bibliographical note

Funding Information:
This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026, 2017R1A2B4002442, and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University.

Funding Information:
This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026 , 2017R1A2B4002442 , and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University.

Publisher Copyright:
© 2019 Elsevier B.V.

All Science Journal Classification (ASJC) codes

Chemistry(all)
Condensed Matter Physics
Physics and Astronomy(all)
Surfaces and Interfaces
Surfaces, Coatings and Films

Access to Document

10.1016/j.apsusc.2019.01.093

Cite this

@article{2e83a724519e4e03be46900bdcb91243,

title = "Electronic structure of Ba(OH) 2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C 60",

abstract = " Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH) 2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH) 2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH) 2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH) 2 layer. On implementing the optimum thickness of the Ba(OH) 2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C 60 /indium tin oxide (ITO) and C 60 /Ba(OH) 2 /ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH) 2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH) 2 , the reduced WF of the ITO is lower than the electron affinity of C 60 . Thus, a charge transfer is induced from Ba(OH) 2 /ITO to C 60 , and the charged states of C 60 are observed within the monolayer. These charged states result in significant band bending in the C 60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (E F ) of the Ba(OH) 2 /ITO. As a result, the energy offset between the C 60 LUMO level and the cathode E F is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH) 2 layer. This is the origin of the enhanced device performance of OPVs.",

author = "Jisu Yoo and Junkyeong Jeong and Kwanwook Jung and Gyeongho Hyun and Minju Kim and Hyunbok Lee and Yeonjin Yi",

note = "Funding Information: This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026, 2017R1A2B4002442, and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University. Funding Information: This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026 , 2017R1A2B4002442 , and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University. Publisher Copyright: {\textcopyright} 2019 Elsevier B.V.",

year = "2019",

month = may,

day = "15",

doi = "10.1016/j.apsusc.2019.01.093",

language = "English",

volume = "476",

pages = "435--441",

journal = "Applied Surface Science",

issn = "0169-4332",

publisher = "Elsevier",

}

Electronic structure of Ba(OH) ₂ interface in inverted organic photovoltaics: Improved electron transport by charged state of C ₆₀. / Yoo, Jisu; Jeong, Junkyeong; Jung, Kwanwook et al.
In: Applied Surface Science, Vol. 476, 15.05.2019, p. 435-441.

Research output: Contribution to journal › Article › peer-review

TY - JOUR

T1 - Electronic structure of Ba(OH) 2 interface in inverted organic photovoltaics

T2 - Improved electron transport by charged state of C 60

AU - Yoo, Jisu

AU - Jeong, Junkyeong

AU - Jung, Kwanwook

AU - Hyun, Gyeongho

AU - Kim, Minju

AU - Lee, Hyunbok

AU - Yi, Yeonjin

N1 - Funding Information: This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026, 2017R1A2B4002442, and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University. Funding Information: This work was supported by the National Research Foundation of Korea [Grant Nos. NRF-2015R1C1A1A01055026 , 2017R1A2B4002442 , and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry–Academy joint research program between Samsung Electronics and Yonsei University. Publisher Copyright: © 2019 Elsevier B.V.

PY - 2019/5/15

Y1 - 2019/5/15

N2 - Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH) 2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH) 2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH) 2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH) 2 layer. On implementing the optimum thickness of the Ba(OH) 2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C 60 /indium tin oxide (ITO) and C 60 /Ba(OH) 2 /ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH) 2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH) 2 , the reduced WF of the ITO is lower than the electron affinity of C 60 . Thus, a charge transfer is induced from Ba(OH) 2 /ITO to C 60 , and the charged states of C 60 are observed within the monolayer. These charged states result in significant band bending in the C 60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (E F ) of the Ba(OH) 2 /ITO. As a result, the energy offset between the C 60 LUMO level and the cathode E F is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH) 2 layer. This is the origin of the enhanced device performance of OPVs.

AB - Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH) 2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH) 2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH) 2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH) 2 layer. On implementing the optimum thickness of the Ba(OH) 2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C 60 /indium tin oxide (ITO) and C 60 /Ba(OH) 2 /ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH) 2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH) 2 , the reduced WF of the ITO is lower than the electron affinity of C 60 . Thus, a charge transfer is induced from Ba(OH) 2 /ITO to C 60 , and the charged states of C 60 are observed within the monolayer. These charged states result in significant band bending in the C 60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (E F ) of the Ba(OH) 2 /ITO. As a result, the energy offset between the C 60 LUMO level and the cathode E F is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH) 2 layer. This is the origin of the enhanced device performance of OPVs.

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