Ultrafast charge-transfer processes at an oriented phthalocyanine/C60interface

Abstract

The ultrafast dynamics at a well-characterized CuPc/ C 60 organic photovoltaic heterojunction have been directly measured with time-resolved two-photon photoemission ͑TR-2PPE͒. Phthalocyanine/C 60 donoracceptor interfaces, characterized in detail via scanning tunneling microscopy, provide model systems for studies of critical charge separation and recombination processes that determine device performance. TR-2PPE studies of copper phthalocyanine ͑CuPc͒ / C 60 interfaces reveal ultrafast charge separation and provide evidence for recombination via low-lying CuPc triplet levels.Photocurrent production in organic photovoltaic structures differs markedly from inorganic solar cells. Dissociation of excitons to produce free carriers in organic devices requires interfaces between a donor and acceptor to provide the driving force for charge separation. 1 Recent theoretical work has highlighted the impact of interfacial molecular structure on the competition between charge pair dissociation and recombination. 2 Improvements in device efficiency can be hastened by unraveling the dynamics of charge transfer and recombination and establishing correlations with interfacial molecular and electronic structure.Ultrafast optical and terahertz studies of polymer blend and multilayer photovoltaic systems have probed charge separation processes into the subpicosecond regime, 3-5 producing much useful insight. For instance, recent work has highlighted the potential impact of bound interfacial chargetransfer ͑ICT͒ states on charge separation. 6,7 However, it is difficult to correlate information from these techniques directly with characteristics of the interfaces. Time-resolved two-photon photoemission ͑TR-2PPE͒ provides the necessary sensitivity to the donor-acceptor interfacial region. 8 Previous studies have demonstrated the utility of TR-2PPE to investigate image state dynamics at organic surfaces 9 and electron transfer at molecule-inorganic semiconductor interfaces. 10,11 When coupled with molecular structure information from scanning tunneling microscopy, this allows correlation of interfacial molecular and electronic structure with the dynamics of charge separation.In this Brief Report, we provide results from TR-2PPE studies of exciton dynamics at a well-characterized organic donor-acceptor interface. The donor-acceptor system chosen for this work, copper phthalocyanine ͑CuPc͒ and C 60 , has been studied extensively for small molecule solar cells, producing promising efficiencies. 12 When CuPc or ZnPc are deposited on well-ordered, hexagonal C 60 surfaces, scanning tunneling microscopy ͑STM͒ ͑Refs. 13 and 14͒ reveals ordered phthalocyanine step heights Ͼ1 nm, indicating that the molecules adopt an "upright" orientation, with the molecular plane nearly perpendicular to the interface with the underlying C 60 molecules, as illustrated schematically at the bottom of Fig. 1. This provides an oriented model organic photovoltaic interface for study of the dominant relaxation processes using TR-2PPE.Photoemission, both one and two p...

Unoccupied states in Cu and Zn octaethyl-porphyrin and phthalocyanine

“…Future avenues to explore are time-resolved measurements of the LUMO and its lifetime via two-photon photoemission, 50,51 and custom-designed variations of porphyrins and phthalocyanines. 48,52…”

Quantum beats at the metal/organic interface

“…graphene[4] and 2D topological insulators [5]), and the electron dynamics 10 of semiconductor surfaces [6] have been successfully studied using TPPE (to only name a few). Recently, a significant focus of study has been on the metal/organic [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 15 31,32,33,34,35,36,37,38,39,40,41,42] and organic/organic [43,44,45,46,47,48,49] interfaces in an effort to understand charge generation and transport in organic electronic materials.…”

Effect of Interlayer Coupling on Ultrafast Charge Transfer from Semiconducting Molecules to Mono- and Bilayer Graphene

“…The initial intensity drop is fitted with an exponential decay convoluted with the finite width of the laser pulses to metals is reported to have a transfer time in the range of 10-50 fs [40,60], which is comparable to our CT rate for Au. For interfaces dominated by π-orbital coupling, e.g., CT at organic-organic interfaces [49,50,54,55,61], the CT times are around 100 fs, which can be compared to our CT rate for graphene. After the first few hundred femtoseconds, CT becomes progressively slower.…”