Means to measure the electronic structure and its temporal evolution following a photo-excitation in conjugated polymers are a key for the understanding and optimisation of their function in applications such as organic solar cells. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful for obtaining such information. This talk will present a new approach for studying the time-dependence of the electronic structure by direct pump-probe measurements of the excited electrons in such materials. It represents a part of our larger effort to use photoelectron spectroscopy for the atomic level characterisation of the electronic states in solar cells materials and the talk will emphasise how new X-ray sources can be used. 

Specifically, we show results from extreme ultraviolet (XUV) based time-resolved photoelectron spectroscopy (TRPES) in the HELIOS laboratory at Uppsala University [1]. In these experiments, the fundamental of a femtosecond Ti:sapphire laser was used to generate XUV pulses in a high harmonic generation (HHG) process. These pulses with an energy of 39 eV were used to emit photoelectrons from a sample, which were then measured with an angular resolved time-of-flight spectrometer [2]. Another part of the laser was used as the pump pulse to electronically excite the sample. By varying the relative arrival times of the pump and the probe at the sample, the electronic structure could be measured as a function of pump-probe delay time. 

In this presentation, we will show the successful application of this technique to the polymer PCPDTBT. A clear signal  - directly detecting the energy of the excited electrons  - could be observed in the photoelectron spectra of this sample, when the pump arrived at the sample before the probe. We were then able to follow how the electronic structure redistributes and relaxes after excitation to either the 1st excited state or to a higher excited state as a function of electron energy and time. Furthermore, we will demonstrate the overall potential of measurements of ground and excited electronic structure by photoelectron spectroscopy for materials and interfaces in solar cells. 

[1]      Plogmaker, S.; Terschlüsen, J. A.; Krebs, N.; Svanqvist, M.; Forsberg, J.; Cappel, U. B.; Rubensson, J.-E.; Siegbahn, H.; Söderström, J. Rev. Sci. Instrum. 2015, 86 (12), 123107. 

[2]      Ovsyannikov, R.; Karlsson, P.; Lundqvist, M.; Lupulescu, C.; Eberhardt, W.; Föhlisch, A.; Svensson, S.; Mårtensson, N. J. Electron Spectros. Relat. Phenomena 2013, 191, 92–103.