title: The origin of high PCE in PTB7 based photovoltaics: proper charge neutrality level and free energy of charge separation at PTB7/PC71BM interface.
abstract: The energy level alignments at donor/acceptor interfaces in organic photovoltaics (OPVs) play a decisive role in device performance. However, little is known about the interfacial energetics in polymer OPVs due to technical issues of the solution process. Here, the frontier ortbial line-ups at the donor/acceptor interface in high performance polymer OPVs, PTB7/PC71BM, were investigated using in situ UPS, XPS and IPES. The evolution of energy levels during PTB7/PC71BM interface formation was investigated using vacuum electrospray deposition, and was compared with that of P3HT/PC61BM. At the PTB7/PC71BM interface, the interface dipole and the band bending were absent due to their identical charge neutrality levels. In contrast, a large interfacial dipole was observed at the P3HT/PC61BM interface. The measured photovoltaic energy gap (EPVG) was 1.10 eV for PTB7/PC71BM and 0.90 eV for P3HT/PC61BM. This difference in the EPVGleads to a larger open-circuit voltage of PTB7/PC71BM than that of P3HT/PC61BM.
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul, 03722, South Korea.
authors.forename: Soohyung
authors.initials: S
authors.lastname: Park
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul, 03722, South Korea.
authors.forename: Junkyeong
authors.initials: J
authors.lastname: Jeong
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul, 03722, South Korea.
authors.forename: Gyeongho
authors.initials: G
authors.lastname: Hyun
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul, 03722, South Korea.
authors.forename: Minju
authors.initials: M
authors.lastname: Kim
authors.affiliation: Department of Physics, Kangwon National University, 1 Gangwondaehak-gil, Chuncheon-si, Gangwon-do, 24341, South Korea.
authors.forename: Hyunbok
authors.initials: H
authors.lastname: Lee
authors.affiliation: Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul, 03722, South Korea.
authors.forename: Yeonjin
authors.initials: Y
authors.lastname: Yi