title: Ligand-dependent exciton dynamics and photovoltaic properties of PbS quantum dot heterojunction solar cells.
abstract: The surface chemistry of colloidal quantum dots (QDs) plays an important role in determining the photoelectric properties of QD films and the corresponding quantum dot heterojunction solar cells (QDHSCs). To investigate the effects of the ligand structure on the photovoltaic performance and exciton dynamics of QDHSCs, PbS QDHSCs were fabricated by the solid state ligand exchange method with mercaptoalkanoic acid as the cross-linking ligand. Temperature-dependent photoluminescence and ultrafast transient absorption spectra show that the electronic coupling and charge transfer rate within QD ensembles were monotonically enhanced as the ligand length decreased. However, in practical QDHSCs, the second shortest ligand 3-mercaptopropionic acid (MPA) showed higher power conversion efficiency than the shortest ligand thioglycolic acid (TGA). This could be attributed to the difference in their surface trap states, supported by thermally stimulated current measurements. Moreover, compared with the non-conjugated ligand MPA, the conjugated ligand 4-mercaptobenzoic acid (MBA) introduces less trap states and has a similar charge transfer rate in QD ensembles, but has poor photovoltaic properties. This unexpected result could be contributed by the QD-ligand orbital mixing, leading to the charge transfer from QDs to ligands instead of charge transfer between adjacent QDs. This work highlights the significant effects of ligand structures on the photovoltaic properties and exciton dynamics of QDHSCs, which would shed light on the further development of QD-based photoelectric devices.
authors.affiliation: Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China and Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. shen@pc.uec.ac.jp.
authors.forename: Jin
authors.identifier: http://orcid.org/0000-0001-7449-4080
authors.initials: J
authors.lastname: Chang
authors.affiliation: Faculty of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Yuhei
authors.initials: Y
authors.lastname: Ogomi
authors.affiliation: Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. shen@pc.uec.ac.jp.
authors.forename: Chao
authors.initials: C
authors.lastname: Ding
authors.affiliation: Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. shen@pc.uec.ac.jp.
authors.forename: Yao Hong
authors.initials: YH
authors.lastname: Zhang
authors.affiliation: Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. shen@pc.uec.ac.jp and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Taro
authors.initials: T
authors.lastname: Toyoda
authors.affiliation: Faculty of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Shuzi
authors.initials: S
authors.lastname: Hayase
authors.affiliation: Department of Applied Chemistry, Chuo University, Tokyo, 112-8551, Japan.
authors.forename: Kenji
authors.identifier: http://orcid.org/0000-0003-3278-6485
authors.initials: K
authors.lastname: Katayama
authors.affiliation: Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. shen@pc.uec.ac.jp and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Qing
authors.initials: Q
authors.lastname: Shen