Electrochemical Characterization of CuSCN Hole-Extracting Thin Films for Perovskite Photovoltaics

Abstract

CuSCN thin films (optimized previously for perovskite photovoltaics) are deposited on glass, F:SnO 2 (FTO), Au, glass-like carbon (GC), and reduced graphene oxide (rGO). They exhibit capacitive charging in an electrochemical window from ca. −0.3 to 0.2 V vs Ag/AgCl. Outside this window, CuSCN film is prone to chemical and structural changes. Anodic breakdown (at ca. 0.5 V) causes restructuring into submicrometer particles and denuding of the substrate. The natural p-doping is demonstrated by both the Hall effect and Mott−Schottky plots from electrochemical impedance. The corresponding flatband potentials (in V vs Ag/AgCl) varied with the substrate type as follows: 0.12 V (CuSCN@FTO), 0.08 V (CuSCN@Au), −0.02 V (CuSCN@GC), 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 the Hall effect, the latter being regarded correct. 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 is tested by a newly designed redox probe, Ru(NH 3 ) 6 3+/2+. It not only has the appropriate redox potential for testing of the CuSCN films but also avoids complications of the standard "ferrocyanide test" which is normally used for this purpose. The perovskite solar cells exhibit better solar conversion efficiency, fill factor, and open-circuit voltage for the rGO-containing devices, which is ascribed to a larger driving force for the hole injection from CuSCN into rGO.

Selected Electrochemical Properties of 4,4’-((1E,1’E)-((1,2,4-Thiadiazole-3,5-diyl)bis(azaneylylidene))bis(methaneylylidene))bis(N,N-di-p-tolylaniline) towards Perovskite Solar Cells with 14.4% Efficiency

“…The conductivity of naturally doped CuSCN (which is another promising hole-conductor for perovskite photovoltaics) is ca. 10 −4 S/cm but the experimental values significantly depend on the sample environment [5]. The two-point conductivity measured across the film (Au/bTAThDaz/Au) was by several orders of magnitude better (see also below), but again poorly reproducible, perhaps due to inhomogeneity in the film (see Figure 6 and Figure S2).…”

“…The development of novel electron/hole conducting materials for perovskite photovoltaics [4,5,27] including molecular conductors to replace spiro-OMeTAD is surely one of the leading challenges in the field. The UV-Vis absorption spectra of the investigated imine were measured in chloroform and chlorobenzene solutions and in thin film.…”

“…In the regular (and so far the most efficient) n-i-p architecture of PSC, the oxide-based electron-selective layer (usually TiO 2 ) provides a negative electrical contact to the perovskite photo-absorber. The second (positive) electrical contact is fabricated from hole transporting material (HTM), like CuSCN [5] or spiro-OMeTAD, the latter being ubiquitously used in almost all highly-efficient PSCs [1].…”

New Strategies for Defect Passivation in High‐Efficiency Perovskite Solar Cells

“…Among the various inorganic‐based HTLs, copper thiocyanate (CuSCN) has been actively pursued by researchers. Improvement of the photovoltaic performance from 12.4% to the very recent achievement over 20% was reported for CuSCN‐based PSCs …”

Atomic layer deposited films of Al2O3 on fluorine-doped tin oxide electrodes: stability and barrier properties

“…The voltammogram of bare FTO ( Figure 6, black curve) exibited additional waves near 0.8-0.9 V assigned to a Prussian blue deposit, which is known to sometimes interfere with the blocking tests with ferrocyanide/ferricyanide [23]. Interestingly, a clean surface (free of Prussian blue) was observed when the Al 2 O 3 -protected electrode was denuded by the dissolution of its coating, both in acidic ( Figure 6) and alkaline (Figure 4) media.…”