Investigating the origin of dielectric responses in semiconductor devices using equilibrium photocapacitance measurements

A new dielectric spectroscopic technique, termed equilibrium photocapacitance measurement (EPCM), using the Lorentz oscillator model, has been developed to understand the photocapacitance of semiconductor devices. This new technique reveals that the density of dipoles and the dipole moment of each dipole in a semiconductor change under illumination. These changes counteract and lead to an equilibrium at a specific applied AC frequency ${\ensuremath{\omega}}_{e}$, at which the photocapacitance response (PCR), a frequency-dependent phenomenon, vanishes. The measured ${\ensuremath{\omega}}_{e}$ is found to be equivalent to a fundamental parameter governing charge dynamics, the effective Lorentz resonant frequency (${\ensuremath{\omega}}_{0}$), that cannot be measured directly. Here, we use EPCM to probe ${\ensuremath{\omega}}_{0}$ indirectly via measuring ${\ensuremath{\omega}}_{e}$ to investigate how external perturbations (light, DC bias, etc.) influence the dielectric properties of a semiconductor device. Using ITO/PTB7/Ag and $\mathrm{ITO}/\mathrm{ZnO}/\mathrm{PTB}7/{\mathrm{MoO}}_{3}/\mathrm{Ag}$ devices, we demonstrate that EPCM allows probing both free and bound charge carriers, unlike conventional current measurements, which only account for the mobile charges. Additionally, EPCM enables precise identification of bound and free charge dominated regimes in the C-V profile and provides a robust method to determine the intrinsic dielectric constant of materials, which is challenging to ascertain due to its sensitivity to measurement conditions.