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PEDOT:PSS/Si heterostructures are promising for silicon-based photovoltaics. The transparent polymer behaves as a $p$-type semiconductor and forms a $p$–$n$ heterojunction with $n$-Si, which separates electron–hole pairs photogenerated in silicon. In this work, a vertical PEDOT:PSS/$n$-Si structure was studied by temperature-dependent admittance spectroscopy.
The PEDOT:PSS layer was deposited onto an $n$-Si substrate ($4.5~\Omega\cdot\mathrm{cm}$) by drop-casting followed by spin-coating. A semitransparent Au film and InGa eutectic served as the top and bottom electrodes, respectively.
The structure was found to be rectifying, and the Mott-Schottky plot ($1/C^2$ versus $V$) showed a barrier height of 0.65 eV at the PEDOT:PSS/Si interface. A photovoltage signal appeared at photon energies above the Si band gap (1.1 eV). Frequency ($f$) spectra of the admittance $Y = G + j \cdot B$ ($G$ is conductance, $B$ is susceptance) were measured from 0.1 to 1000 kHz under dc biases of −0.5, 0 and +0.2 V and ac amplitude of 100 mV at temperatures from 10 to 300 K. From 150 to 300K, the frequency dependence of $G/f$ revealed a peak. The peak position $f_{\max}$ was found to shift towards higher temperatures $T$ when heating.
The Arrhenius plots of $f_{\max}$ versus $1/(kT)$ ($k$ is Boltzmann constant) in Fig. 1 showed two linear sections where the dependence followed function $\propto \exp[-E/(kT)]$. The dependence slope below 200 K allowed to extract the conductivity activation energy $E = 81 \pm 4$ meV, while the value of $222 \pm 6$ meV was obtained from the section from 200 to 280 K. The sign and magnification of the bias affected the results negligibly. The activation energies are attributed to the depths of acceptor-like states at the PEDOT:PSS/Si interface, which contribute to the conductivity via thermal activation.
This work was supported by the National Research Foundation of Ukraine (project 2023.03/0060).
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| Figure caption | Arrhenius plot of the resonant frequencies under different biases with estimated activation energies. |