High-efficiency Cu2ZnSn(S,Se)4 solar cells are reported by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in a substantial enhancement in open-circuit voltage. The 12.4% device is obtained with a record open-circuit voltage deficit of 593 mV.

narrows the CZTSSe band gap. The low energy barrier to Cu/Zn antisite formation is related to the similarity between the covalent radii of Cu and Zn. [ 4 ] The elevated processing temperatures (550-600 °C) needed to form large-grained CZTSSe fi lms (and peak device effi ciencies) provide the required thermal energy to randomize Cu and Zn in the unit cell, leading to a high density of antisite defects. [ 5 ] If disorder is the primary cause of performance loss in CZTSSe, then suppressing or eliminating it might offer a path to effi ciencies that compete with Cu(In,Ga)Se 2 (CIGS) technology.Ag is an interesting candidate for replacement of Cu since, in addition to belonging to the same chemical group as Cu, it possesses an atomic radius roughly 16% larger. This leads to the intriguing possibility that antisites can be suppressed by increasing the strain required to accommodate each defect (due to larger dissimilarity in radius). Ab initio calculations predict that substitution of Cu with Ag more than doubles the formation energy for antisites, which should result in an order of magnitude lower density of defects for equivalent processing. [ 6 ] Previous studies have examined the optical and crystallographic properties of the mixed Cu-Ag kesterite [ 7 ] and found that introducing 10% or 5% Ag into the CZTS(Se) layer gave 4.4% [ 8 ] or 7.1% [ 9 ] effi ciencies, respectively. These effi ciencies were shown to be an improvement over the baseline pure-Cu material; however, few direct measurements have been made to demonstrate how this substitution impacts the fundamental properties of the material.In this study, we have prepared thin fi lms of the mixed alloy (Ag x ,Cu 1x ) 2 ZnSnSe 4 (ACZTSe) across the full range of Ag/(Ag + Cu) ratios. We show, using Hall effect measurements, that while the pure-Cu kesterite compound is p-type, the carrier density decreases with increasing Ag content. For the highest values of Ag content (>50%), the material inverts to n-type. We also show, using femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) measurements, that unlike in CZTSSe the Fermi level of AZTSe is not pinned near the center of the band gap, indicating that AZTSe does not suffer from the same degree of heavy compensation. Additionally, the energetic difference between the measured band gap and the photoluminescence (PL) peak position approaches zero for the pure-Ag compound. These results imply that the magnitude The photovoltaic absorber Cu 2 ZnSn(S x Se 1-x ) 4 (CZTSSe) has attracted interest in recent years due to the earth-abundance of its constituents and the realization of high performance (12.6% effi ciency). The open-circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSSe with Ag, whose covalent radius is ≈15% larger than that of Cu and Zn, the density of I-II antisite defects is predicted to drop. The fundamental properties of the mixed Ag-Cu kesterite compound are reported as a function of ...

It is well‐known that sodium improves the performance of Cu2ZnSnS4 (CZTS) devices, yet the mechanism of the enhancement is still not fully understood. This work aims to present a unified account of the relationships between grain boundaries in CZTS, sodium content at these boundaries, non‐radiative recombination, and surfactant effects that produce large microstructural changes. Using temperature‐dependent photoluminescence measurements, it is demonstrated that samples containing dramatically different grain sizes display identical radiative and non‐radiative decay characteristics when sufficient sodium is present in the film. It is also shown that the sodium concentration needed to efficiently passivate non‐radiative defects is significantly less that the quantity needed to obtain micrometer‐sized CZTS grains. Finally, the high densities of donor‐acceptor pairs that are observed in CZTS films appear to reside within the grains themselves, rather than at grain boundaries.

Energy band alignments between CdS and Cu2ZnSn(SxSe1−x)4 (CZTSSe) grown via solution-based and vacuum-based deposition routes were studied as a function of the [S]/[S+Se] ratio with femtosecond laser ultraviolet photoelectron spectroscopy, photoluminescence, medium energy ion scattering, and secondary ion mass spectrometry. Band bending in the underlying CZTSSe layer was measured via pump/probe photovoltage shifts of the photoelectron spectra and offsets were determined with photoemission under flat band conditions. Increasing the S content of the CZTSSe films produces a valence edge shift to higher binding energy and increases the CZTSSe band gap. In all cases, the CdS conduction band offsets were spikes.

Temperature-programmed reaction spectroscopy has been used to study the surface reaction between CO and O-atoms on Rh͑100͒ and Rh͑111͒ at a range of different adsorbate coverages. Comparison of the reaction on both surfaces in the low coverage regime, where the kinetics can be described by a straightforward Langmuir-Hinshelwood mechanism reveals that the CO oxidation is structure sensitive, with the rate constant being an order of magnitude higher on the Rh͑100͒ than on the Rh͑111͒ surface. As a consequence, the selectivity of the COϩO reaction to CO 2 is about 100% on Rh͑100͒, whereas on Rh͑111͒ the oxidation reaction competes with CO desorption. At low CO coverage, CO oxidation is an elementary step on Rh͑100͒ for a broad range of oxygen coverages. We report kinetic parameters E a ϭ103Ϯ5 kJ/mol and ϭ10 12.7Ϯ0.7 for O ϭ CO →0 on Rh͑100͒. The activation energy for CO oxidation on Rh͑100͒ decreases continuously with increasing O-coverage. At low coverage ( O Ͻ0.25 ML͒ we attribute this to destabilization of CO, leading to an increase in the CO 2 formation rate. At higher coverage ( O Ͼ0.25 ML͒ O-atoms become destabilized as well, as lateral interactions between O-atoms come into play at these coverages. The interactions result in a greatly enhanced rate of reaction at higher coverages.

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High resolution electron energy loss spectroscopy ͑HREELS͒, low-energy electron diffraction ͑LEED͒, and thermal desorption spectroscopy ͑TDS͒ were used to study lateral interactions in the adsorbate layer of the CO/Rh͑111͒ system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C-O vibrations is observed over the whole coverage range. The frequency of the metal CO ͑M-CO͒ vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M-CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of 12 CO and 13 CO were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C-O stretching vibration at total CO coverage of 0.33 ML in the (ͱ3ϫͱ3)R30°structure arise purely from dipole-dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole-dipole coupling. Therefore, experiments with a small amount of 13 CO as a tracer to monitor the frequency shifts in the 12 CO adlayer were carried out over the entire coverage range of 12 CO. The results demonstrate that dipole-dipole coupling causes the frequency shifts at low coverage ͑Ͻ0.5 ML͒, whereas chemical effects set in at higher coverage ͑0.5-0.75 ML͒, connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules.

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