Probing Transient Electric Fields in Photoexcited Organic Semiconductor Thin Films and Interfaces by Time-Resolved Second Harmonic Generation

Abstract

We have probed photoinduced charge separation dynamics in organic semiconductor thin films and at their interfaces by femtosecond time-resolved second harmonic generation (TR-SHG), using the model systems of fullerene (C 70 ) thin films and copper phthalocyanine (CuPc)/C 70 interfaces. In neat C 70 thin films, the formation of an internal electric field on a ∼10-ps time scale following photoexcitation is attributed to the photo-Dember effect, namely, charge separation resulting from a gradient in excitation density and the differential electron/hole mobility. When an ultrathin film of the electron donor CuPc was deposited on top of the C 70 film, an additional interfacial charge separation channel occurring on the ultrafast time scale of ∼0.1 ps was observed. We discuss how SHG fields from different origins can interfere to give the overall transient response.

Charge carrier mobility dynamics in organic semiconductors and solar cells

“…Photocurrent causing the discharge of the plate capacitor-like sample may be calculated as a derivative of the electric field kinetics. A similar ultrafast time-resolved EFISH probing technique, only without the external electric field, has been also applied by Zhu and co-workers to probe electron transfer at the interface between two molecular layers [34,35]. An EFISH as a probe was also used to monitor slow electric field dynamics in organic solar cells [36].…”

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High‐Performance Solar Cells

“…These problems can be solved by either stacking many PHJs in the film samples or by using multiple passes of the incident light . Non‐linear optical measurements, such as second‐harmonic generation and sum frequency generation, can exploit the well‐defined interface of PHJs to obtain information about the interface selectively. Further development of spectroscopic analysis for obtaining PHJs with high sensitivity and selectivity is anticipated.…”

Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers

“…However, due to the low film thickness we expect only a small phase shift, which does not lead to destructive interference. 47 Considering the computational results, influences of the substrate and of the aggregation between adjacent molecules need to be discussed. As recently demonstrated by Ne ´non et al in an extensive computational approach, the presence of the surface potentially has a non-neglectable influence on the absolute values of the calculated hyperpolarizabilities, depending primarily on the chromophore and the applied linker chain.…”