Time-Resolved Photoluminescence and Light‑Induced Electron Spin Resonance Studies of Photo-Induced Charge Transfer in New Polyazomethines:Fullerene for Organic Solar Cells

The organic solar cells (OSCs) are considered as promising, cost-effective way to harness solar energy. They are built of two organic materials, mostly: conjugated polymers and fullerene derivatives, e.g. [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), which are electron donor and electron acceptor, respectively. Upon illumination, electron-hole pairs called excitons are generated on a donor site. Through dissociation at the donor/acceptor interface, excitons separate to free charges: electrons pass into an acceptor site, holes stay on the donor site. This phenomenon is called a photo-induced charge transfer (CT) and is a crucial step in the OSCs operations.

Techniques, which directly detect this phenomenon are: Time-Resolved Photoluminescence (TRPL) and Light-Induced Electron Spin Resonance (LESR) Spectroscopies. The idea of the TRPL experiment is to excite the sample by pulse laser and observe decay of its luminescence in time. In the case of effective photo-induced CT, the luminescence quenching is observed. The ESR technique is based on a magnetic resonance absorption, which only occurs for samples having unpaired electrons. As was mentioned above, the photo-induced CT generates free charges – holes and electrons, which appear in a LESR spectrum as two lines from positive polaron (P⁺) on a donor site and negative polaron (F^-) on an acceptor site (1, 2).

This article presents studies of the photo-induced CT in mixtures made of PCBM and two new polyazomethines: 25Th-cardo and 2252Th-DMB. Polymer 25Th-cardo was obtained from 2,5-thiophenedicarboxaldehyde and 4-[9-(4-aminophenyl)-9H-fluoren-9-yl]aniline, while 2252Th-DMB from 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde and 3,3′-dimethoxybenzidine (3). P3HT:PCBM mixture was used as reference.

All measurements were done for pure components and polymer:PCBM mixtures. The TRPL was measured using a spectrometer (0.05 nm resolution) combined with a Hamamatsu Streak Camera (2.5 ps resolution) for samples excitated by titanium-sapphire laser pulses. The ESR was carried out using a Bruker ELEXSYS E580 X-band spectrometer (9.5 GHz/ 0.33T) equipped with a continuous-flow cryostat, allowing decrease of temperature down to 2.5 K. For all the samples dark, light on (LESR) and light off spectra were recorded. An illumination of the samples was performed directly in the microwave cavity of the ESR spectrometer by DH-2000 Deuterium-Tungsten Halogen Light Sources.

For pure P3HT we observed a strong luminescence having maximum around 1.8 eV. This luminescence was quenched after adding a strong electron acceptor - PCBM. This behaviour is assigned to the effective photo-induced CT. Both polyazomethines has a luminescence with max. around 2 eV. For their mixtures with PCBM, the luminescence quenching is observed, but it is not so significant as in the case of P3HT:PCBM system. Moreover for 25Th-cardo:PCBM composite an additional luminescence appear in the range from 1.3 to 1.8 eV, which turned out being a PCBM luminescence.

No dark ESR signal from all polymers:PCBM composite was detected. However, as the P3HT:PCBM and 2252Th-DMB:PCBM mixtures were illuminated, two overlaping lines appeared from P⁺ and F^- polarons - confirming effective photo-induced CT. In the case of 25Th-cardo:PCBM mixture no LESR signal was recorded.

We can conclude that photo-induced CT occurs in both polyazomethines:PCBM mixtures, however for 2252Th-DMB:PCBM it is much more effective than for 25Th-cardo:PCBM.

This work shows that the TRPL and the LESR can be used as universal methods to detect the photo-induced CT in the donor:acceptor mixtures, thereby, can be helpfull tools for preliminary investigation of new materials for OSCs.

This work was supported by the Foundation for Polish Science International PhD Projects Programme co-financed by the EU European Regional Development Fund and the NCBiR project PBS1/A5/27/2012.

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