Electrical Transport Mechanisms and Photoconduction in Undoped Crystalline Flash-Evaporated Lead Iodide Thin Films

Abstract

The flash-evaporation technique was utilized to fabricate undoped 1.35-μm and 1.2-μm thick lead iodide films at substrate temperatures $T_{\rm{s}} = 150$°C and 200°C, respectively. The films were deposited onto a coplanar comb-like copper (Cu-) electrode pattern, previously coated on glass substrates to form lateral metal–semiconductor–metal (MSM-) structures. The as-measured constant-temperature direct-current (dc)-voltage ($I\left( {V;T} \right) - V$) curves of the obtained lateral coplanar Cu-PbI₂-Cu samples (film plus electrode) displayed remarkable ohmic behavior at all temperatures ($T = 18 - 90\,^\circ {\hbox{C}}$). Their dc electrical resistance $R_{\rm{dc}} (T$) revealed a single thermally-activated conduction mechanism over the temperature range with activation energy $E_{\rm{act}} \approx 0.90 - 0.98 \,{\hbox{eV}}$, slightly less than half of room-temperature bandgap energy $E_{\rm{g}}$ ($\approx \,2.3\, {\hbox{eV}}$) of undoped 2H-polytype PbI₂ single crystals. The undoped flash-evaporated ${\hbox{PbI}}_{\rm{x}}$ thin films were homogeneous and almost stoichiometric ($x \approx 1.87$), in contrast to findings on lead iodide films prepared by other methods, and were highly crystalline hexagonal 2H-polytypic structure with c-axis perpendicular to the surface of substrates maintained at $T_{\rm{s}} { \gtrsim }150^\circ {\hbox{C}}$. Photoconductivity measurements made on these lateral Cu-PbI₂-Cu-structures under on–off visible-light illumination reveal a feeble photoresponse for long wavelengths ($\lambda > 570\,{\hbox{nm}}$), but a strong response to blue light of photon energy $E_{\rm{ph}}$ $\approx \,2.73 \, {\hbox{eV}}$ ($> E_{\rm{g}}$), due to photogenerated electron–hole (e–h) pairs via direct band-to-band electronic transitions. The constant-temperature/dc voltage current–time $I\left( {T,V} \right) - t$ curves of the studied lateral PbI₂ MSM-structures at low ambient temperatures ($T < 50^\circ {\hbox{C}}$), after cutting off the blue-light illumination, exhibit two trapping mechanisms with different relaxation times. These strongly depend on $V$ and $T$, with thermally generated charge carriers in the PbI₂ mask photogenerated (e–h) pairs at higher temperatures.