From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Andrés, Christian; Cabas-Vidani, A.; Tiwari, Ayodhya N.; Romanyuk, Yaroslav E.

doi:10.1016/j.tsf.2018.09.022

Cited by 9 publications

(11 citation statements)

References 21 publications

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“…On the other hand, alloying approaches have been also investigated recently. As in the case of doping, Ge alloying of kesterite has immediately demonstrated a great potential, with device efficiencies ranging from 6.8 to 12.3% [114][115][116][117][118][119][120][121][122]. Among them, it is interesting to note that the best efficiency was achieved with a relative Ge content of 39% (i.e.…”

Section: Germanium (Ge)mentioning

confidence: 99%

Doping and alloying of kesterites

et al. 2019

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Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu 2 ZnSn(S,Se) 4 matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

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Section: Germanium (Ge)mentioning

confidence: 99%

Doping and alloying of kesterites

et al. 2019

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“…Ge has been revealed to naturally accumulate at the back region of the CZTGSe absorber with a demonstrated increased band gap, in a similar fashion to Ga in CIGS. 54 The accumulation of Ge at the back interface of kesterite films has previously been reported, 45 and no promising steps such as solar cell improvement attributable to the presence of the back surface field (BSF) was reported. In that regard, this work demonstrates for the first time that deliberate band engineering can lead to improved photovoltaic performances.…”

Section: ■ Introductionmentioning

confidence: 99%

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

Andrade‐Arvizu

Fonoll‐Rubio

Sánchez

et al. 2020

ACS Appl. Energy Mater.

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Kesterite solar cells are at a crossroads, and a significant breakthrough in performance is needed for this technology to stay relevant in the upcoming years. In this work, we propose to follow the proven strategy of band engineering to assist charge carrier collection taking inspiration from chalcopyrite solar cells. Using a process based on a combination of metallic precursor sputtering and chalcogen-reactive annealing, we achieve controlled cationic substitutions by partly replacing Sn by Ge, hence tailoring several rear band gap grading profiles along the absorber thickness. A complete set of results is presented, with samples ranging from pure Sn to pure Ge compounds. The formation of a rear band gap grading is determined through different characterization techniques, specifically through a combination of glow discharge optical emission and Auger spectroscopies with an advanced multiwavelength Raman spectroscopy analysis carried out at the front and back (rear) sides of the films using a lift-off process. As such, a preferential Ge enrichment toward the back of the absorber is unequivocally demonstrated in kesterite absorbers and further applied to complete devices for deliberately generating distinct rear band gap profiles, leading to an efficient back surface field that potentially enhances the carrier selectivity of the back interface. The electrical analysis of the complete devices shows a complex interplay between the benefits of band gap grading and possible Ge-related defects in the absorber. Using optimized synthesis conditions, an absolute increase in efficiency (compared to the Ge-free reference) is obtained for the record device (η > 9%) without any antireflective coating or metallic grid. This performance enhancement is mostly ascribed to the presence of a drift electric field assisting in the carrier collection while preventing back side recombination. These results confirm the possibility of generating back band gap grading in kesterite solar cells and open the way to further development of the kesterite photovoltaic technology toward higher efficiencies through tailored band gap engineering.

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“…A 50 nm thick Ge layer was deposited on top of the metal precursor, but preferential Ge diffusion towards the back interface was observed after selenization. In a recent study, elemental targets were co‐sputtered to fabricate CZGTSe devices with shallow Ge grading from both graded and stacked precursors . Unlike in the work by Márquez et al, no preferential accumulation of Ge near the back contact was observed.…”

Section: Introductionmentioning

confidence: 99%

Germanium Incorporation in Cu₂ZnSnS₄ and Formation of a Sn–Ge Gradient

Saini

Larsen

Sopiha

et al. 2019

Physica Status Solidi (a)

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Alloying of Cu2ZnSnS4 (CZTS) with Ge can potentially promote grain growth and suppress the formation of Sn‐related defects. Herein, a two‐step fabrication route based on compound co‐sputtering and sulfurization at a high temperature is used to prepare Ge‐incorporated CZTS (Cu2ZnGexSn1 − xS4 [CZGTS]). For Cu2ZnGeS4 (CZGS), films deposited using elemental Ge and binary GeS targets are compared. The recrystallization is shown to be promoted for the absorbers deposited using Ge target, possibly due to lower sulfur content in the precursor suppressing the formation of wurtzite‐like phases during sputtering. The grain growth and crystallinity in CZGTS are slightly improved for x = 0.2 but not for higher concentration of the incorporated Ge. Owing to the composition‐dependent electronic properties, compositionally graded CZGTS films may be beneficial for reducing recombination towards the back contact. Hence, herein, the successful formation of a steep concentration gradient with Ge and Sn is demonstrated by the deposition of a CZGS/CZTS precursor stack followed by sulfurization with varying time periods.

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From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Cited by 9 publications

References 21 publications

Doping and alloying of kesterites

Doping and alloying of kesterites

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

Germanium Incorporation in Cu₂ZnSnS₄ and Formation of a Sn–Ge Gradient

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From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Cited by 9 publications

References 21 publications

Doping and alloying of kesterites

Doping and alloying of kesterites

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

Germanium Incorporation in Cu2ZnSnS4 and Formation of a Sn–Ge Gradient

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Germanium Incorporation in Cu₂ZnSnS₄ and Formation of a Sn–Ge Gradient