Gradient doping of sulfur in Sb2Se3 nanowire arrays as photoelectrochemical photocathode with a 2% half-cell solar-to-hydrogen conversion efficiency

Abstract

Exploring Sb2Se3 as a photoelectrochemical (PEC) photocathode for water reduction has recently attracted much attention, mainly due to its excellent photophysical properties and perfect band structure matching with water reduction potential. Whereas significant achievements have been made in improving its photocurrent density, the PEC performance remains poor mostly due to the low onset potential or low fill factor. Here, we fabricated S doped Sb2Se3 nanowire arrays with a gradient S concentration (grad-S:Sb2Se3). Gradient S doping creating a cascade valence band structure in Sb2Se3 simultaneously enhances its onset potential and fill factor. Light-trapping effects in nanowire geometry improve the light harvesting efficiency. A further deposition of a thin TiO2 layer is utilized to passivate the detrimental surface defects and form a buried junction with grad-S:Sb2Se3. By employing the prepared grad-S:Sb2Se3 nanowire arrays as the photocathode for water reduction, we achieved a high onset potential of 0.42 V vs RHE, a fill factor of 34%, and a record high half-cell solar-to-hydrogen conversion efficiency of 2% from Sb2Se3-based photocathodes.

A Novel Thermochemical Metal Halide Treatment for High-Performance Sb2Se3 Photocathodes

“…The open-circuit voltage ( V OC ) of the pristine film was found to be −0.13 V vs. SCE (0.49 V vs. RHE, Supplementary Equations (S1) and (S2) ), while the V OC values of treated Sb 2 Se 3 films shifted toward more positive potentials of about −0.05 V vs. SCE (0.57 V vs. RHE, Supplementary Equation (S3) ). For most Sb 2 Se 3 -based photoelectrodes, V OC values less than 0.5 V vs. RHE have been typically reported [ 20 , 21 , 22 , 23 ]. This V OC deficiency could be related to different material aspects such as (a) low quality of the single-phase Sb 2 Se 3 , a high concentration of defects and grain boundaries along with the presence of secondary crystalline or amorphous phases, which are harmful to the device performance [ 8 , 20 , 21 , 22 , 23 ]; (b) insufficient structure optimization, e.g., lattice mismatch, and an inauspicious band offset between p-type Sb 2 Se 3 and n-type CdS grown using a chemical bath [ 22 ].…”

“…For most Sb 2 Se 3 -based photoelectrodes, V OC values less than 0.5 V vs. RHE have been typically reported [ 20 , 21 , 22 , 23 ]. This V OC deficiency could be related to different material aspects such as (a) low quality of the single-phase Sb 2 Se 3 , a high concentration of defects and grain boundaries along with the presence of secondary crystalline or amorphous phases, which are harmful to the device performance [ 8 , 20 , 21 , 22 , 23 ]; (b) insufficient structure optimization, e.g., lattice mismatch, and an inauspicious band offset between p-type Sb 2 Se 3 and n-type CdS grown using a chemical bath [ 22 ]. Several authors reported the V OC values of about 0.6 V vs. RHE measured in Na 2 SO 4 electrolytes with pH values lying between 5 and 6.5 [ 39 , 40 ].…”

“…Antimony (III) selenide (Sb 2 Se 3 ) has recently emerged as an eco-compatible and low-cost absorber material for various photo-assisted applications due to its advantageous optoelectronic properties ( E g ~1.2 eV, absorption coefficient > 10 5 cm −1 , high mobility) [ 7 , 17 , 18 , 19 , 20 , 21 ]. Besides, Sb 2 Se 3 is internally stable towards photocorrosion, and its elemental composition suggests less-complicated chemistry within the device processing compared to the multielement compounds such as CIGS and CZTS [ 7 , 10 , 22 ].…”

“…Despite significant efforts for improving the properties of typical absorber layers by various post-depositional treatments (PDTs) experienced for TFSCs [ 8 , 24 , 25 , 26 , 27 , 28 ], the procedures and their productivities of stable absorber/electrolyte interface are limited [ 29 ]. To date, most devices utilizing Sb 2 Se 3 layers exhibited parameters significantly below expectations [ 17 , 18 , 19 , 20 , 21 , 22 , 30 , 31 ]. The current record photocurrent density for compact Sb 2 Se 3 photoelectrodes has been predominantly attained by improvements in deposition methods in preference to realizing any suitable PDTs [ 22 ].…”

Unravelling Defect Passivation Mechanisms in Sulfur-treated Sb2Se3

“…Several strategies have been presented in the literature such as Te incorporation for defect passivation, [5] sulfurization treatment for improved photovoltage [6] and gradient sulfur doping in Sb2Se3. [7] More fundamental insights into the basic optoelectronic properties for defect passivation/suppression for Sb2Se3, however, have not yet been reported.…”