Hyperfine interaction in spin response of organic devices

Abstract

Recent advances in spin response of organic semiconductors include long polaron spin coherence time measured by optically detected magnetic resonance (ODMR); substantive room-temperature magneto-electroluminescence and magneto-conductance obtained in organic light emitting diodes (OLED); and spin-polarized carrier injection from ferromagnetic electrodes in organic spin valves (OSV). Although the hyperfine interaction (HFI) has been foreseen to play an important role in organic spin response, clear experimental evidence has been lacking. Using the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and OSV devices based on π-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI plays a crucial role in all three spin responses. OLEDs and films based on the D-polymers show substantial narrower magneto-electroluminescence, magneto-conductivity and ODMR responses; whereas due to the longer spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance that reaches ~330% at small bias voltage and low temperatures.