Proton density monitoring at the interface of proton-donor and proton-acceptor regions in a protonic p-n junction with bias voltage

The innovative method applied leads to direct insight into the protonic p-n junction, including changes in proton density with bias voltage. The concentration of protons in the p-n junction area increases with forward polarization (positive at the proton-donor doped site) and decreases with the blocking polarization (in the reverse direction). Due to the dependence of electrical conductivity and spin density from proton concentration (affecting the protonation degree of polyaniline molecules), polyaniline in the emeraldine base form (PANI EB) is an effective proton-sensitive spin probe, which has been used in studying the protonic p-n junction with L-band EPR. The results are the first direct evidence of the key role of protons in the protonic p-n junction. The proton density in the p-n junction area increases with forward polarization (positive on the proton donor-doped side) and decreases with blocking polarization (the reverse direction), which affects the amplitude of the recorded EPR signal.