Stabilized Polymers with Novel Indenoindene Backbone against Photodegradation for LEDs and Solar Cells

Abstract

PPV derivatives, polymers with vinylene units, have their tendency to exhibit degradation, after irradiation with white light or operation of the device, resulting in the appearance of a shifted absorption and emission band in the short wavelength regions of the spectra. In this paper, we report the synthesis and properties of new polymers utilizing new backbone, poly(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl) (PININE). In order to reduce oxidation of the vinylene group, the vinylene group was cyclized using two 5-membered rings. While poly(p-phenylenevinylene) derivatives show significantly blue-shifted and decreased peaks of UV-vis, blue-shifted maximum peaks of photoluminescence (PL) after irradiation with white light in air, and blue-shifted maximum peaks of electroluminescence (EL) after operation of the device, PININE shows stable spectra of UV-vis, and PL, and EL under the same conditions. PININE copolymers with benzothiadiazole and thiophene units exhibit high power conversion efficiency (PCE) for polymer solar cells. Under white light illumination (AM 1.5 G, 100 mW/cm2), the cell based on PININEDTBT/PCBM as the active layer has a short circuit current density (ISC) of 5.93 mA/cm2, a fill factor (FF) of 43%, and PCE of 1.88%. These copolymers have not only good processability due to the indenoindene unit, in which four alkyl groups can be incorporated, but also strong and uniform absorbance in the whole visible region.