Experimental Physics VI

Overview

The investigation and understanding of photophysical processes in organic materials are important in order to improve the efficiency of solar cells and other organic electronic devices. We apply different experimental setups for our optical and photophysical studies: absorption, photoluminescence (PL), and photoinduced absorption (PIA) measurements. In order to examine the dynamics of the excited states observed with PIA, we implemented a time-resolved experiment which allows us to measure lifetimes in the micro and millisecond range. Additionally, we can distinguish between different recombination types such as mono- or bimolecular recombination by our method of choice. Closely connected to our measurements are the Magnetic Resonance experiments, which are also performed in our research group.

In the following we will give three examples of our current research, photoluminescence quenching with respect to different P3HT:PCBM ratios, PIA measurements on new acceptor materials and time-resolved PIA.

Photoluminescence quenching with respect to different P3HT:PCBM ratios

A first insight into material features of organic semiconductors is given by photoluminescence (PL) measurements. We examine the PL of pure electron donor materials such as conjugated polymers, and study the decreasing PL with an increasing fraction of an acceptor material – e.g., a fullerene derivative – in a donor:acceptor blend. The photoluminescence is due to photoinduced singlet exciton states which undergo a radiative recombination. The measurement of this so-called PL quenching gives a strong indication of the potential of this material combination for a charge transfer, which is an important prerequisite for organic photovoltaic devices. The graph presented shows a series of different P3HT (poly(3-hexyltiophene)) : PCBM([6,6]-phenyl-C61-butyric acid methyl ester)) ratios for which we measured the PL quenching; it indicates a very efficient charge transfer from donor to acceptor. We also draw conclusions in view of a better understanding of morphology properties and quenching-radius.

PIA measurements on novel acceptor materials

A series of novel acceptor molecules – provided by the group of Prof. Martín, Universidad Complutense, Madrid, Spain – were measured with PIA. We the efficiency of charge carrier generation, and if the exciton dissociation probabilities is in a similar range as compared to the very efficient reference material PCBM. In the figure, the PIA curves of all three materials in blend with the electron donor P3HT are shown. The novel material blends show similar spectra as the reference blend P3HT:PCBM, with polaron peaks at 1.24eV and 0.3eV. Therefore, and due to our Electron Spin Resonance (ESR) results, we conclude that a charge transfer occurs. One interesting new feature is the small peak in JL44:P3HT at 0.9eV (see inset). To determine its origin, further spin-sensitive Magentic Resonance studies will be necessary. The next step in testing these molecules concerning their suitability fororganic photovoltaic cells will be charge carrier mobility measurements.

Time-resolved PIA

Our time-resolved PIA setup allows us to experimentally determine lifetimes of excited states detected with the above-mentioned steady state PIA. In order to do so, the frequency of the chopper modulating the laser beam is varied from ~60Hz up to 22kHz. The signal intensity of long-lived states is decreasing with increasing chopper frequency. Fitting the measured signals with appropriate functions – e.g., by solving the corresponding continuity equations – enables us to determine not only the lifetime but also the recombination mechanism. In the graph to the right the evolution of lifetime with respect to temperature is shown for two polaronic states in P3HT:PCBM blend. The inset shows the signal measured at one temperature and the overlaying fit function. The similar temperature behaviour of the two polaronic states supports the commonly made statement that they share their origin.

 

Experiments

We apply different experimental methods, such as

• Absorption
• Photoluminescence (PL)
• (time-resolved) Photoinduced Absorption (PIA)
• Electroabsorption (EA)

Contact

Dr. Carsten Deibel, phone +49 931 888 3119

Moritz Liedtke, phone +49 931 888 3116