Study and optimization of alternative MBE‐deposited metallic precursors for highly efficient kesterite CZTSe:Ge solar cells

Abstract

Nowadays, most of the best efficiencies of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are obtained from absorber layers fabricated using sequential processes, including the deposition of metallic stack precursors, typically by sputtering, and followed by reactive annealing under chalcogen atmosphere. The sputtering technique is widely known for the easy growth of metallic layers, although the deposition rates, growth morphology and nucleation, or the roughness can sometimes be an issue leading to inhomogeneities in the final layers. Nevertheless, MBE (molecular beam epitaxy) technique could have some advantages to obtain high‐quality metallic layers, with accurate control of the growth due to ultra‐high vacuum conditions and high purity. In this work, we study the use of advanced MBE systems to grow metallic stack precursors, alternatively to sputtering or thermal evaporation techniques, to obtain high‐quality CZTSe:Ge absorbers. Due to differences in the nature of each type of precursor, thermal annealing optimizations are presented by modifying some critical selenization parameters, such as the temperature or the selenium amount in order to obtain well‐crystallized absorbers. Detailed morphological, compositional, and structural characterizations show relevant features of each precursor, mainly related to the formation of MoSe2 at the back interface, and Se and Sn composition after selenization in different conditions. Regarding the solar cell devices, main efficiency limitations come from VOC and FF, which could be tentatively related to a noncontrolled selenization; different precursor reactivity, porosity, or composition; and different alkali diffusion during the reactive annealing. Finally, in the first optimization, a 9.2% efficiency device has been achieved with promising perspectives for future improvements.

Rear Band gap Grading Strategies on Sn–Ge-Alloyed Kesterite Solar Cells

“…Thin-film absorber layers were fabricated by the standardized sequential process developed in IREC based on the thermal reactive annealing of DC-magnetron sputtered metallic precursors. − In this work, Cu/Sn/Cu/Zn/Ge precursor structures were deposited (Alliance Ac450) on Mo-coated soda-lime glass (SLG) substrates. The sputtered Sn and Ge layer thicknesses were adjusted to produce the following Sn–Ge alloys: 0, 20, 40, and 100%.…”

Potential Role of Kesterites in Development of Earth‐Abundant Elements‐Based Next Generation Technology

“…To date, the highest reported efficiencies for CZTS, CZTSe, and CZTSSe are over 12.0%, [ 38 ] which are much lower than the predicted value of 32.0% [ 39 ] for the single‐junction Shockley–Queisser limit. The thin films can be deposited using vacuum‐based methods, i.e., sputtering, [ 40 ] thermal evaporation, [ 41 ] pulsed laser deposition (PLD), [ 42 ] molecular beam epitaxy (MBE), [ 43 ] and electron‐beam evaporation. [ 44 ] They can also be deposited using non‐vacuum‐based methods, i.e., CBD, [ 45 ] successive ionic layer adsorption and reaction (SILAR), [ 46 ] hydrothermal, [ 47 ] spin‐coating, [ 48 ] and electrodeposition.…”

Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review

“…1,2 Moreover, just in the last 3 years alone there have been significant improvements in device optimization and fundamental understanding. At the synthesis level, new methods have been successfully implemented for high-efficiency kesterite synthesis, such as Molecular Beam Epitaxy (MBE) 3 , and several optimizations have been achieved across traditionally reported methods, in particular sputtering, [4][5][6] including a recent record-tying certified 12.6%-efficient device, and an uncertified device with 13.0% efficiency 7 . A number of examples of strategies for back contact engineering and back surface passivation have been reported.…”

“…To this day, it remains the only solar cell technology, besides crystalline Si, that is composed of low-cost, earth-abundant, and environmentally friendly elements and that has surpassed 10% efficiency with long-term stability, without any encapsulation. , Moreover, just in the last 3 years alone, there have been significant improvements in device optimization and fundamental understanding. At the synthesis level, new methods have been successfully implemented for high-efficiency kesterite synthesis, such as Molecular Beam Epitaxy (MBE), and several optimizations have been achieved across traditionally reported methods, in particular sputtering, − including a recent record-tying certified 12.6%-efficient device and an uncertified device with 13.0% efficiency . A number of examples of strategies for back-contact engineering and back-surface passivation have been reported. − At the bulk level in kesterites, new insights have appeared on the coupling between O and Na, on deep donor–acceptor defect centers, and the origin of loss mechanisms, namely, the open-circuit voltage deficit ( V oc deficit), now widely regarded as the main bottleneck in kesterite solar cells. − Many efforts have been put recently on cationic substitution or alloying (loosely referred to as doping in this field), in particular with Ag, Ge, and alkali metals, − but also other elements such as Ba or Sr, , further revealing insights on bulk properties and performance limitations.…”