Time-resolved photoelectron spectroscopy of solar cell materials using high harmonic generation
Solar cells have a great potential in replacing fossil fuels in electricity generation, if requirements of low production costs can be met. In the last years, much research has been focused on developing new solar cells made from organic or hybrid materials, which can be fabricated by cheap methods. The success of this development crucially depends on understanding the energetics of the interfaces of different materials in a solar cell as well as the transfer kinetics of excited electrons across these interfaces. X-ray based techniques such as photoelectron spectroscopy are powerful for obtaining such information. In this talk, I will present a new approach for studying the time-dependence of the electronic structure by direct pump-probe measurements of the excited electrons in materials for solar energy conversion.
Specifically, I will show results from extreme ultraviolet (XUV) based time-resolved photoelectron spectroscopy in the HELIOS laboratory at Uppsala University [1,2]. In these experiments, the fundamental of a femtosecond Ti:sapphire laser was used to generate XUV pulses in a high harmonic generation 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 . 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, I 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.
 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.
 Cappel, U. B.; Plogmaker, S.; Terschlüsen, J. A.; Leitner, T.; Johansson, E. M. J.; Edvinsson, T.; Sandell, A.; Karis, O.; Siegbahn, H.; Svensson, S.; Mårtensson, N.; Rensmo, H.; Söderström, J. Phys. Chem. Chem. Phys. 2016, 18, 21921-21929.
 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.