PbI₂ has become a highly prominent material in the field of photovoltaics as the key precursor in the fabrication of organo-halide perovskites films [1]. Despite intensive research in this area, the role of unconverted PbI₂ in perovskite cells remains controversial. In fact, very little is known about the behaviour of PbI₂ as a photocathode. In this contribution, we examine the photoelectrochemical properties of high quality PbI₂ thin films obtained by gas phase anion replacement [2].

The preparation of PbI₂ films is initiated by growing a thin PbS film on F:SnO₂ electrodes, followed by spontaneous gas phase iodination at 200 ˚C. The structure and optical properties of the thin films were investigated by XRD, SEM, Raman Spectroscopy, diffuse reflectance and room temperature photoluminescence. The structural and spectroscopic information, supported by DFT calculations (PBESOL functionals), demonstrated the formation of the 2H- phase of PbI₂ with a high degree of purity and a band gap of 2.4 eV. Electrochemical impedance spectroscopy in KI containing solutions showed a clear p-type behaviour with a flat band potential located 0.175 V vs Ag/AgCl and an effective majority carrier density of 1.6×10¹⁵ cm^-3.

Photoelectrochemical studies in Ar-saturated KI solutions were characterized by a nearly ideal semiconductor-electrolyte junction at low illumination levels. Incident-photon-to-current efficiency exhibits values close to 50%, which can be quantitatively rationalized in terms of reflection losses, the width of the space charge region and a minority carrier diffusion length of 80 nm. The remarkable feature of this photocathode is the absence of surface recombination even at potentials close to the flat band value. The efficient hydrogen photogeneration is achieved at neutral pH and in the absence of co-catalysts. We shall briefly discuss the origin of the nearly ideal semiconductor-electrolyte junction behaviour and potential routes for stabilising the PbI₂ surface for solar fuel applications.

References

1. Yang and J. You, Nature 544 (2017) 155
2. Tiwari and D.J. Fermin, Electrochim. Acta 254 (2017) 223