Bandgap Tunable Ternary Cd x Sb2-y S3-δ Nanocrystals for Solar Cell Applications.

We report the synthesis and photovoltaic performance of a new nonstoichiometric ternary metal sulfide alloyed semiconductor-Cd x Sb2-y S3-δ nanocrystals prepared by the two-stage sequential ionic layer adsorption reaction technique. The synthesized Cd x Sb2-y S3-δ nanocrystals retain the orthorhombic structure of the host Sb2S3 with Cd substituting a fraction (x = 0-0.15) of the cationic element Sb. The Cd x Sb2-y S3-δ lattice expands relative to the host, Sb2S3, with its lattice constant a increasing linearly with Cd content x. Optical and external quantum efficiency (EQE) spectra revealed that the bandgap E g of Cd x Sb2-y S3-δ decreased from 1.99 to 1.69 eV (i.e., 625-737 nm) as x increased from 0 to 0.15. Liquid-junction Cd x Sb2-y S3-δ quantum dot-sensitized solar cells were fabricated using the polyiodide electrolyte. The best cell yielded a power conversion efficiency (PCE) of 3.72% with the photovoltaic parameters of J sc = 15.97 mA/cm2, V oc = 0.50 V, and FF = 46.6% under 1 sun. The PCE further increased to 4.86%, a respectable value for a new solar material, under a reduced light intensity of 10% sun. The PCE (4.86%) and J sc (15.97 mA/cm2) are significantly larger than that (PCE = 1.8%, J sc = 8.55 mA/cm2) of the Sb2S3 host. Electrochemical impedance spectroscopy showed that the ZnSe passivation coating increased the electron lifetime by three times. The EQE spectrum of Cd x Sb2-y S3-δ has a maximal EQE of 82% at λ = 350 nm and covers the spectral range of 300-750 nm, which is significantly broader than that (300-625 nm) of the Sb2S3 host. The EQE-integrated current density yields a J ph of 11.76 mA/cm2. The tunable bandgap and a respectable PCE near 5% suggest that Cd x Sb2-y S3-δ could be a potential candidate for a solar material.

Dielectric Behavior as a Screen in Rational Searches for Electronic Materials: Metal Pnictide Sulfosalts.

Dielectric screening plays an important role in reducing the strength of carrier scattering and trapping by point defects for many semiconductors such as the halide perovskite solar materials. However, it was rarely considered as a screen to find new electronic semiconductors. We performed a material search study using the dielectric properties as a screen to identify potential electronic materials in the class of metal-pnictide ternary sulfosalts, containing Bi or Sb. These salts are basically ionic due to the electronegativity difference between the S and both the metal and pnictogen elements. However, we do find significant cross-gap hybridization between the S p-derived valence bands and pnictogen p-derived conduction bands in many of the materials. This leads to enhanced Born effective charges and, in several cases, highly enhanced dielectric constants. We find a chemical rule for high dielectric constants in terms of the bond connectivity of the pnictogen-chalcogen part of the crystal structure. We additionally find a series of compounds with low effective mass, high dielectric constant, and other properties that suggest good performance as electronic materials and also several potential thermoelectric compounds. Experimental optical data and solar conversion efficiency are reported for Sn-Sb-S samples, and results in accord with predicted good performance are found. The results illustrate the utility of dielectric properties as a screen for identifying complex semiconductors.

Ag8SnS6: a new IR solar absorber material with a near optimal bandgap.

We report the synthesis and photovoltaic properties of a new ternary solar absorber - Ag8SnS6 nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique. The synthesized Ag8SnS6 nanocrystals have a bandgap E g of 1.24-1.41 eV as revealed from UV-Vis and external quantum efficiency (EQE) measurements. Its photovoltaic properties were characterized by assembling a liquid-junction Ag8SnS6 sensitized solar cell for the first time. The best cell yielded a J sc of 9.29 mA cm-2, a V oc of 0.23 V, an FF of 31.3% and a power conversion efficiency (PCE) of 0.64% under 100% incident light illumination using polysulfide electrolyte and Au counter electrode. The efficiency improved to 1.43% at a reduced light intensity of 10% sun. When the polysulfide was replaced by a cobalt electrolyte with a lower redox level, the V oc increased to 0.54 V and PCE increased to 2.29% under 0.1 sun, a respectable efficiency for a new solar material. The EQE spectrum covers the spectral range of 300-1000 nm with a maximum EQE of 77% at λ = 600 nm. The near optimal E g and the respectable photovoltaic performance suggest that Ag8SnS6 nanocrystals have potential to be an efficient IR solar absorber.