The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se<sub>2</sub> solar cells

Colombara, Diego; Conley, Kevin; Malitckaya, Maria; Komsa, Hannu Pekka; Puska, Martti J.

doi:10.1039/d0ta01103g

Cited by 30 publications

(36 citation statements)

References 84 publications

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“…Grain growth is closely related to atomic diffusions which are triggered by sodium dopant addition. Higher Na doping concentrations were seen to increase Ga diffusion; however, Ga diffusivity decreased when CIGS had low Cu deficiency because the diffusion occurs through Cu vacancies 31,32 . This is consistent with the lower CGI measured for the 150 nm NaCl sample (Table 1).…”

Section: Resultssupporting

confidence: 82%

“…This last sample has relatively improved PCE s reaching up to 9%, with the narrowest distribution of V OC among the samples, at ~580 mV. The existence of two PL peaks confirms the theories of atomic interdiffusion discussed earlier, where larger amounts of Na introduced to the CIGS matrix cause higher Ga diffusivities resulting in changes in the bandgap across the absorber depth 19,31,32 . In the 100 nm NaCl, sample, the less intense low bandgap peak at ~1.14 eV corresponds to a Ga‐poor phase whilst the more pronounced high bandgap peak at ~1.19 eV to a Ga‐rich phase.…”

Section: Resultssupporting

confidence: 76%

“…It is suggested that high concentration of Na accumulates along CIGS GBs 19 . Na can also be found in grain interior; however, the concentration is in order of magnitude lower than at GBs due to slower volume diffusion 32,40 . A study performed by Forest et al regarding the reversibility of the Na incorporation can be used to infer the presence of interface states 41 .…”

Section: Resultsmentioning

confidence: 99%

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Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Uličná

Welch

Abbas

et al. 2021

Progress in Photovoltaics

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Poor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution‐processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum‐based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution‐based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto‐electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda‐lime glass substrate. NaCl is non‐toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution‐processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter‐grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na‐doping strategy for CIGS absorbers and reveals the current limitations of the Na‐doping in solution‐processed atmospherically deposited cells.

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Section: Resultssupporting

confidence: 82%

Section: Resultssupporting

confidence: 76%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Uličná

Welch

Abbas

et al. 2021

Progress in Photovoltaics

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“…Realizing today’s highly efficient CIGS, solar cells require a carefully conceived fabrication process, one that derives from a decades-long research endeavour 12 . One key innovation in CIGS fabrication was enabled in the 1990s by crossing the phase homogeneity boundary during growth from ‘Cu-poor’ to ‘Cu-rich’, then back again to Cu-poor compositions (known as the three-stage process) 13 .…”

Section: Introductionmentioning

confidence: 99%

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

et al. 2020

Self Cite

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The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe 2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe 2 solar cells.

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“…[ 5 ] Consequently, the steeper Ga grading is generally observed after selenization. Various factors were investigated to manipulate the Ga grading during the selenization process, such as the amount of alkali metals existing, [ 6,7 ] the selenization temperature, [ 8 ] the modified precursor structure, [ 9 ] and Cu/(Ga + In) (CGI) ratio in the bulk. [ 10 ] However, the modification of Ga profile is still quite limited.…”

Section: Introductionmentioning

confidence: 99%

Tuning Ga Grading in Selenized Cu(In,Ga)Se₂ Solar Cells by Formation of Ordered Vacancy Compound

Cai

Lai

2020

Solar RRL

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Severe Ga accumulation at the back contact is regarded as one of the major limiting factors for high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells fabricated by the sequential process. The Ga deficiency near the front surface of absorber leads to the reduction of open‐circuit voltage; however, controlling the Ga grading is quite challenging during the selenization process. Herein, the tuning of Ga profile is demonstrated by forming the ordered vacancy compound (OVC) phase at the beginning of selenization process under high Se partial pressure (PSe) with the precursor composed of a relatively high In content. The OVC phase formed with a columnar structure promotes Ga diffusion through Cu vacancies, resulting in Ga increase at the front side of CIGSe. However, high PSe also increases the thickness of MoSe2, significantly raising the series resistance. Thereby, a 5 nm TiN layer is deposited on top of Mo to suppress the formation of MoSe2. In addition, the extra Na‐contained precursor is added to increase the carrier concentration because the TiN layer also blocks the Na outdiffusion from the substrate. By controlling PSe, interlayer TiN thickness, and carrier concentration, a 16.04% record high efficiency of CIGSe solar cells (sulfur‐free) via elemental Se source is achieved.

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The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se₂ solar cells

Abstract: Highly efficient chalcopyrite photovoltaic cells display complex distributions of sodium dopant and gallium: how are these distributions related to each other?

Cited by 30 publications

References 84 publications

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

Tuning Ga Grading in Selenized Cu(In,Ga)Se₂ Solar Cells by Formation of Ordered Vacancy Compound

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The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se2 solar cells

Abstract: Highly efficient chalcopyrite photovoltaic cells display complex distributions of sodium dopant and gallium: how are these distributions related to each other?

Cited by 30 publications

References 84 publications

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

Tuning Ga Grading in Selenized Cu(In,Ga)Se2 Solar Cells by Formation of Ordered Vacancy Compound

Contact Info

Product

Resources

About

The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se₂ solar cells

Tuning Ga Grading in Selenized Cu(In,Ga)Se₂ Solar Cells by Formation of Ordered Vacancy Compound