Experimental Physics VI

Overview

To design an organic solar cell, adequate light absorbing materials need to be identified. The interest is in an efficient charge transfer from the (electron-) donor to the (electron-) acceptor. The first excitation takes place on the donor and leads to an exciton-state. This might recombine radiatively and thus can be detected as photoluminescence (PL). When an acceptor material is in the vicinity of the exciton, a charge transfer process can be triggered, generating two oppositely charged and spatially separated carriers, a hole on the donor, and an electron on the acceptor material. These are often called positive and negative polaron due to their strong coupling to their respective host molecule. Hence, an acceptor site can lead to PL quenching, which is the first indication of a charge transfer. However, the PL quenching might also occur because of a non-radiative decay mechanism, which cannot be distinguished by purely optical means. A way to directly measure the charge transfer at the donor:acceptor-interface can be achieved by electron spin resonance measurements, where non-radiative losses can be distinguished from the charge transfer process.

Two example spectra are shown below.

P3HT:PCBM

A free electron is a spin 1/2 particle. If a magnetic field is applied the zeeman-effect leads to an energy-splitting between the two allowed spin orientations. This energy-difference can be excited with microwaves of the matching energy to which results in spin-flips. So if microwave absorption is detected in a sample this is evidence of free charge carriers. If light induced ESR-signals are detected in a donor:acceptor-blend this is the confirmation of a charge transfer. Additional information can be gained from the spectrum. The g-factor, linewidth and its symmetry and signal-intensity give hints of the electronic environment of the charge carrier. Such as the next atomic neighbours or relaxation-paths, that lead to fast signal-degradation after the light excitation.
P3HT:PCBM shows distinct ESR-signals at different g-factors, but the two lines are overlapping. The signal of P3HT is due to a positive polaron on the polymer, generated by a charge transfer. The other signal at g=2.000 can be assigned to the negative charge on the PCBM-molecule. Even in the dark a signal of P3HT is present, which results from a ground state charge transfer process that is not applicable for the generation of free charge carriers: Only the light-induced part of the signal is relevant for solar applications. The inset in the Graph shows the energy-levels of the two materials.

P3HT-ZnO:Al

P3HT-ZnO:Al shows distinct ESR-signals at different g-factors, which show similarities if compared to the reference P3HT:PCBM spectrum. The negative charge is transfered from the polymer P3HT to the aluminium-doped nano-crystaline ZnO, where it can be detected at a g-factor of 1.96. The positive polaron stays on the polymer and causes a signal at g=2.002.

 

Experiments

We apply different experimental methods, such as

• Dark and Light-Induced Electron Spin Resonance
• Transient ESR
• Electron Nuclear Double Resonance (ENDOR)
• Optically Detected Magnetic Resonance (ODMR)

Contact

Prof. Dr. Vladimir Dyakonov, phone +49 931 888 3111

Andreas Sperlich, phone +49 931 888 3117