AVS 58th Annual International Symposium and Exhibition
    Energy Frontiers Focus Topic Wednesday Sessions
       Session EN+NS-WeM

Paper EN+NS-WeM9
Charge Separation and Relaxation in Phthalocyanine-C60 Photovoltaic Systems

Wednesday, November 2, 2011, 10:40 am, Room 209

Session: Organic Photovoltaics
Presenter: Steven Robey, National Institute of Standards and Technology
Authors: G.J. Dutton, University of Maryland -College Park
S.W. Robey, National Institute of Standards and Technology
Correspondent: Click to Email
Organic photovoltaic (OPV) structures depend on charge transfer processes occurring within 10’s of nanometers of donor-acceptor interfaces. Charge generation in these devices involves successive steps of (1) optical absorption to create excitons, (2) diffusion of the exciton population to the donor-acceptor interface, (3) exciton dissociation and charge transfer at the interface and (4) charge transport and collection at electrodes. The charge separation step depends critically on electronic level alignment between the donor and acceptor. Also, because exciton diffusion to the interface occurs on timescales of several to 10’s of picoseconds (ps), relaxation processes occurring on sub-ps and ps timescales as the exciton diffuses can impact the energy available for charge separation at the interface. We have investigated the exciton relaxation and charge separation processes close to phthalocyanine (Pc)-C60 interfaces by combining time-resolved two-photon photoemission (TR-2PPE) with organic MBE to form donor-acceptor interfaces layer-by-layer. Pc π→π* transitions are excited by a pump pulse to generate singlet (S1) excitons. The resulting population dynamics are then probed with a time-delayed UV pulse to follow the relaxation and charge separation as a function of energy. For CuPc /C60 interfaces, we compared the decay dynamics as a function of CuPc thickness, and thus distance from the C60 interface, to allow determination of the rate of charge transfer at the interface. We find a charge transfer rate of ≈ 8 x 10 12 sec-1 for the initial exciton population formed immediately after pumping. For CuPc, the exciton population also undergoes vibrational relaxation and intersystem crossing (ISC) on a timescale of ≈ 1-2 ps, ultimately resulting in the production a triplet exciton population at significantly lower energy. By again comparing results for thin and thick Pc layers we estimate that the charge transfer rate of this lower energy triplet population is reduced by at least a factor of ≈ 1000, giving charge transfer rates closer to 8 x 10 9 sec -1. We will also discuss measurements for the case of H2Pc and C60 where ISC of the Pc singlet excitons to triplet levels is negligible. Our results will be discussed within the context of the Marcus theory of charge transfer and connections will be made to calculated charge transfer rates for similar donor-acceptor interfaces.