| AVS 57th International Symposium & Exhibition | |
| Energy Frontiers Topical Conference | Wednesday Sessions |
| Session EN+NS-WeM |
| Session: | Organic Photovoltaics |
| Presenter: | J.E. Reutt-Robey, University of Maryland |
| Authors: | G.J. Dutton, NIST W. Jin, University of California at San Diego D.B. Dougherty, North Carolina State University W.G. Cullen, University of Maryland S.W. Robey, NIST J.E. Reutt-Robey, University of Maryland |
| Correspondent: | Click to Email |
Photocurrent production in organic photovoltaic structures differs fundamentally from current generation in inorganic semiconductor solar cells. Dissociation of excitons formed by optical absorption in organic materials requires heterointerfaces between electron donor and acceptor components. The extent to which molecular architecture, particularly along the donor-acceptor interface, impacts electronic level alignment and charge separation is of fundamental interest. In this work, we prepare well-defined molecular interfaces by the physical vapor deposition of select donor (MPc, Pn) and acceptor (C60) components under UHV conditions. We determine the detailed structure of the donor-acceptor interface with Scanning Tunneling Microscopy and establish a correlation with electron band alignment (PES) and exciton dynamics (2PPES).
For technologically relevant interfaces between C60 and donors such as pentacene (Pn) or phthalocyanines (Pc), distinct structures/molecular orientations can be selectively engineered by organic MBE through deposition sequence and flux. For the case of C60 and Pn, “co-facial” C60-Pn interfaces are formed by C60 deposition on crystalline Pn bilayer films supported by Ag(111), whereas “edge-on” C60-Pn interfaces result from Pn deposition on hexagonal close-packed C60 monolayers supported by Ag(111). Such “edge-on” interfaces expand into large dendritic islands, as per reported “thin-film” phases, and support C60 cluster formation under subsequent C60 deposition. We show how electronic level alignments critical to Voc and charge separation efficiency are impacted by these structural changes, and extend this information to other small-molecule cases, ZnPc:C60 and perfluroinated ZnPc, as time permits.
Finally, for interfaces between CuPc and C60, we will present the first studies of charge separation at well-characterized organic donor-acceptor interfaces using TR-2PPE. By pumping the CuPc Q-band at 1.65eV, a time-delayed UV pulse then probes the excited state population. We identify dominant relaxation processes on timescales from 100fs to >100ps. By varying the CuPc film thickness, we observe significantly enhanced charge transfer of the singlet exciton at the interface with C60. Following the population dynamics as a function of energy also provides evidence for recombination from charge transfer states back to the low-lying CuPc triplet.
This work has been supported in part by the NSF under the UMD MRSEC (DMR0520471) and the Surface & Analytical Chemistry Program (CHE0750203).