Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C60

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

Research output: Contribution to journalArticle

2 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 C60/indium tin oxide (ITO) and C60/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 C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF 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 languageEnglish
Pages (from-to)435-441
Number of pages7
JournalApplied Surface Science
Volume476
DOIs
Publication statusPublished - 2019 May 15

Fingerprint

Tin oxides
Indium
Electronic structure
Cathodes
Conversion efficiency
Molecular orbitals
Electron affinity
Functional materials
Buffer layers
Photoelectron spectroscopy
Fermi level
Optoelectronic devices
Electron Transport
indium tin oxide
Charge transfer
Monolayers
Molecules

All Science Journal Classification (ASJC) codes

  • Surfaces, Coatings and Films

Cite this

Yoo, Jisu ; Jeong, Junkyeong ; Jung, Kwanwook ; Hyun, Gyeongho ; Kim, Minju ; Lee, Hyunbok ; Yi, Yeonjin. / Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics : Improved electron transport by charged state of C60. In: Applied Surface Science. 2019 ; Vol. 476. pp. 435-441.
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title = "Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C60",
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 C60/indium tin oxide (ITO) and C60/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 C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF 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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Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics : Improved electron transport by charged state of C60. / Yoo, Jisu; Jeong, Junkyeong; Jung, Kwanwook; Hyun, Gyeongho; Kim, Minju; Lee, Hyunbok; Yi, Yeonjin.

In: Applied Surface Science, Vol. 476, 15.05.2019, p. 435-441.

Research output: Contribution to journalArticle

TY - JOUR

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

T2 - Improved electron transport by charged state of C60

AU - Yoo, Jisu

AU - Jeong, Junkyeong

AU - Jung, Kwanwook

AU - Hyun, Gyeongho

AU - Kim, Minju

AU - Lee, Hyunbok

AU - Yi, Yeonjin

PY - 2019/5/15

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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 C60/indium tin oxide (ITO) and C60/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 C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF 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 C60/indium tin oxide (ITO) and C60/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 C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF 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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