Site-specific probing of charge transfer dynamics in organic photovoltaics

TL;DR

This study uses picosecond X-ray photoelectron spectroscopy to observe charge transfer and recombination dynamics in organic photovoltaic materials at specific sites, providing insights for optimizing device performance.

Contribution

It introduces a novel site-specific, time-resolved X-ray spectroscopy method to directly monitor charge transfer in organic photovoltaics.

Findings

Charge transfer from CuPC to C60 observed as a reversible core level shift.

C60 core level shifts while CuPC remains unchanged during charge dynamics.

Method enables direct observation of charge processes in OPV materials.

Abstract

We report the site-specific probing of charge-transfer dynamics in a prototype system for organic photovoltaics (OPV) by picosecond time-resolved X-ray photoelectron spectroscopy. A layered system consisting of approximately two monolayers of C$_{60}$ deposited on top of a thin film of Copper-Phthalocyanine (CuPC) is excited by an optical pump pulse and the induced electronic dynamics are probed with 590 eV X-ray pulses. Charge transfer from the electron donor (CuPC) to the acceptor (C$_{60}$) and subsequent charge carrier dynamics are monitored by recording the time-dependent C 1$s$ core level photoemission spectrum of the system. The arrival of electrons in the C$_{60}$ layer is readily observed as a completely reversible, transient shift of the C$_{60}$ associated C 1$s$ core level, while the C 1$s$ level of the CuPC remains unchanged. The capability to probe charge transfer and recombination dynamics in OPV assemblies directly in the time domain and from the perspective of well-defined domains is expected to open additional pathways to better understand and optimize the performance of this emerging technology.