Electron/Hole-Selective Interfaces in Perovskite Photovoltaics: Electrochemical Studies
Authors: Kavan Ladislav 
Published: February 2020

Though the first perovskite solar cell (PSC) reported in 2009, was an electrochemical (liquid-junction) device, there were only few electrochemical studies during the next decade1. This paper will show, that electrochemistry can, nevertheless, provide inputs for the development of state-of-art PSCs’ components. The vectorial charge-transfer over several interfaces in PSCs requires right alignment of conduction band minima (CBM) at the electron selective layer terminal and valence band maxima (VBM) at the hole-selective terminal, respectively. Furthermore the electron/hole selective layers should be compact (pinhole-free) to avoid recombination. All these properties can be characterized electrochemically. The electron-selective layer is usually made from TiO2 or SnO2
2. Amorphous ALD-made SnO2 or TiO2 films are pinhole-free for thicknesses down to several nm, but amorphous and crystalline ALD SnO2 films substantially differ in their conduction band positions. The energy of CBM is usually measured by optical spectra (giving the band gap) and photoelectron spectra (XPS, UPS) providing VBM. Electrochemical alternative is the measurement of the flatband potential, which yields CBM, too. However, there is a considerable controversy between the electrochemical and vacuum (XPS, UPS) techniques concerning the position of CBM in TiO2 (anatase, rutile, including the crystals with distinguished facets). A refined electrochemical analysis of single crystal TiO2 electrodes (anatase, rutile, brookite) together with vacuum and near-ambient pressure XPS studies as well as theoretical (DFT) modelling points at the effect of interface influencing the CBM positions. Even 1-2 monolayers of water cause the relevant CBM shifts, which could explain these conflicts. Similar tasks are addressed at the positive (hole-selective) terminal of PSC. A promising hole-conductor to replace spiro-OMeTAD is CuSCN, particularly if it is interfaced to reduced graphene oxide (rGO). The natural p-doping of CuSCN is demonstrated by both Hall-effect and by Mott-Schottky plots in aqueous electrolyte solution. The corresponding flatband potentials (in V vs. Ag/AgCl) varied with the substrate type as follows: 0.12 (CuSCN@FTO), 0.08 (CuSCN@Au), -0.02 (CuSCN@glass-like-carbon) and 0.00 V (CuSCN@rGO). The acceptor concentrations determined from electrochemical impedance spectroscopy are by orders of magnitude larger than those from electrical conductivity and Hall effect. Raman spectra confirm that thiocyanate is the dominating structural motif over the isomeric isothiocyanate. In-situ Raman spectroelectrochemistry discloses substrate-specific intensity changes upon electrochemical charging. The blocking function of CuSCN is tested by a newly designed redox-probe, Ru(NH3)6
3+/2+. It has not only the appropriate redox potential for investigation of the CuSCN films, but also avoids complications of the standard ‘ferrocyanide test’ which is normally used for this purpose.

L. Kavan, Curr. Opinion Electrochem.,
11, 122 (2018).

L. Kavan, Catal. Today,
DOI: 10.1016/j.cattod.2018.10.065.