Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (Ag_xCu_1–x)₂ZnSnSe₄

Abstract

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 ...

Silver-Doped Cu2SnS3Absorber Layers for Solar Cells Application

“…The apparent grain size increases from hundreds of nanometers to several microns. The effect of Ag on the intragrain defects and crystal growth is discussed for both ACZTS [22], [23], [25] and ACIGS [20]. Gershon et al [23] and Su et al [25] found largely increased grain size for ACZTS for a Ag/(Ag + Cu) ratio of ∼10%.…”

“…Alloying chalcopyrite materials with Ag has been shown to have several advantageous properties for kesterite and Cu(In,Ga)S 2 (CIGS) absorbers [19]- [26] [(Ag, Cu) 2 ZnSnS 4 (ACZTS) and (Ag,Cu)(In,Ga)S 2 (ACIGS)]. Ag replaces Cu in these materials, which increases the band gap [19], [23], [24], [26], decreases the p-type doping [23], as well as reduces intragrain defects and defect states [19]- [21], [23], [26]. All these effects may be beneficial for CTS as well.…”

Proposition of an Environment Friendly Triple Junction Solar Cell Based on Earth Abundant CBTSSe/CZTS/ACZTSe Materials

“…As a result, current density falls significantly with the higher thickness of CZTS (Figure 2(a)) because of the reduced optical power absorption in the bottom cell. Thus, higher thickness of [19] 7 [18] 8.5 [17] 7 [16] Band gap (eV) 1.9 [14] 1.45 [18] 0.97 [17] 0.9 [16] Electron affinity (eV) 3.6 [19] 4.1 [24] 4.05 [17,25] 4.05 [13] Electron effective mass (m e /m o ) 0.37 [14,15] 0.18 [13,16] 0.08 [17] 0.07 [13,16] Hole effective mass (m p /m o ) 1.68 [14] 2 [13,16] 0.3 [17] 0.2 [13,16] Electron mobility (cm 2 V À1 s À1 ) 3 0 [26] 40 [21,24] 75 [23] 145 [13,21] Hole mobility (cm 2 V À1 s À1 ) 1 0 [27] 25 [21] 1 [17,23] 35 [13,21] Acceptor concentration (cm…”

“…was previously achieved at 45 and 0% ratio of Ag/(Ag þ Cu) in ACZTSe material system, respectively. [17] The detailed design of the bottom cell can be found in Supporting Information (Section S2) . The thicknesses of ZnO, ZnS, and CZTSe in the bottom cell are 70, 100, and 200 nm, respectively.…”

“…Recently, Shin et al [14] demonstrated a solar cell of 5.2% PCE with Cu 2 BaSnS 4Àx Se x (CBTSSe) whose bandgap can be in the range of %1.6-2.0 eV [15] making it suitable for the top cell of a triple junction chalcogenide solar cell. Therefore, in this work, we propose a triple junction solar cell in which CBTSSe (bandgap %1.9 eV at x ¼ 1 [14] ), CZTS (bandgap %1.45 eV [16] ), and silver (Ag) mixed, CZTSe (ACZTSe, bandgap %0.97 eV [17] ) act as the primary absorbing layers of top, middle and bottom cells, respectively. We have also adjusted the thicknesses of the primary absorbers for matching the current densities of the three respective cells.…”