title: Tailor-made hole-conducting coadsorbents for highly efficient organic dye-sensitized solar cells.
abstract: The Y-shaped, low molecular mass, hole-conductor (HC), acidic coadsorbents 4-{3,7-bis[4-(2-ethylhexyloxy)phenyl]-10H-phenothiazin-10-yl}benzoic acid (PTZ1) and 4-{3,7-bis[4-(2-ethylhexyloxy)phenyl]-10H-phenothiazin-10-yl}biphenyl-4-carboxylic acid (PTZ2) were developed. Owing to their tuned and negative-shifted HOMO levels (vs. NHE), they were used as HC coadsorbents in dye-sensitized solar cells (DSSCs) to improve cell performance through desired cascade-type hole-transfer processes. Their detailed functions as HC coadsorbents in DSSCs were investigated to obtain evidence for the desired cascade-type hole-transfer processes. They have multiple functions, such as preventing π-π stacking of dye molecules, harvesting light of shorter wavelengths, and faster dye regeneration. By using PTZ2 as the tailor-made HC coadsorbent on the TiO2 surface with the organic dye NKX2677, an extremely high conversion efficiency of 8.95 % was achieved under 100 mW cm(-2) AM 1.5G simulated light (short-circuit current JSC =16.56 mA cm(-2) , open-circuit voltage VOC =740 mV, and fill factor of 73 %). Moreover, JSC was increased by 13 %, VOC by 27 % and power-conversion efficiency by 49 % in comparison to an NKX2677-based DSSC without an HC coadsorbent. This is due to the HC coadsorbent having a HOMO energy level well matched to that of the NKX-2677 dye to induce the desired cascade-type hole-transfer processes, which are associated with a slower charge recombination, fast dye regeneration, effective screening of liquid electrolytes, and an induced negative shift of the quasi-Fermi level of the electrode. Thus, this new class of Y-shaped, low molecular weight, organic, HC coadsorbents based on phenothiazine carboxylic acid derivatives hold promise for highly efficient organic DSSCs.
authors.affiliation: Global GET-Future Laboratory and Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Jochiwon, Sejong 339-700 (Korea), Fax: (+82) 44-860-1739.
authors.forename: In Taek
authors.initials: IT
authors.lastname: Choi
authors.forename: Myung Jong
authors.initials: MJ
authors.lastname: Ju
authors.forename: Sang Hyun
authors.initials: SH
authors.lastname: Song
authors.forename: Sang Gyun
authors.initials: SG
authors.lastname: Kim
authors.forename: Dae Won
authors.initials: DW
authors.lastname: Cho
authors.forename: Chan
authors.initials: C
authors.lastname: Im
authors.forename: Hwan Kyu
authors.initials: HK
authors.lastname: Kim
keywords: charge transfer; dyes/pigments; electrochemistry; photophysics; solar cells