Open-Circuit Photovoltage Exceeding 950 mV with an 840 mV Average at Sb₂S₃-Thianthrene^+/0 Junctions Enabled by...

Covalently attached perylene monolayers serve as back contacts for Sb₂S₃ photoelectrochemical cells with a thianthrene^+/0 front, rectifying contact. Covalent attachment of perylenetetracarboxylic dianhydride, PTCDA, to Si(111) utilizes an anhydride-to-imide conversion at surface-attached amines. For Sb₂S₃ solar absorbers, we hypothesized that a terminal thioperylene anhydride, i.e., S=C-O-C=S, formed from thionation of the terminal perylene anhydride would serve as a soft, electron-selective and hole-blocking back contact. We explored several routes to convert carbonyls to thiocarbonyls on surface-attached perylene anhydrides including Lawesson's reagent, P₄S₁₀, and a P₄S₁₀-pyridine complex. Here, P₄S₁₀ in toluene yielded the highest conversion as quantified by thioperylene-anhydride-S-to-imide-N ratios in X-ray photoelectron spectroscopy (XPS). Spectra demonstrated minimal residual reagent as determined by the absence of quantifiable phosphorus following sonication and rinsing. Photoelectrochemistry yielded an average |V _oc| = 840 ± 90 mV with the highest value of 952 mV under ELH-simulated AM1.5G illumination for chemical-bath-deposited Sb₂S₃ in the strongly oxidizing thianthrene^+/0 redox couple when thioperylene-anhydride-tethered surfaces formed the back contact. Sb₂S₃ absorbers in which perylene anhydride, esters, thionoesters, and thiols form the back contact yielded significantly decreased |V _oc| magnitudes vs Sb₂S₃ on perylene-thioanhydride-terminated surfaces. We attribute the large V _oc to the combination of favorable sulfur-functionalized surfaces for deposition, charge transfer properties of the perylene layer, and use of the thianthrene^+/0 redox couple.