Revealing the beneficial role of K in grain interiors, grain boundaries, and at the buffer interface for highly efficient CuInSe<sub>2</sub> solar cells

Muzzillo, Christopher P.; Poplawsky, Jonathan D.; Tong, Ho Ming; Guo, Wei; Anderson, Timothy J.

doi:10.1002/pip.3022

Cited by 22 publications

(25 citation statements)

References 38 publications

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“…The increases in V OC and FF are attributed to a decrease of recombination in the bulk as well as at the interface. We propose that the presence of an alkali–indium–selenide phase between buffer and absorber layer, suggested by multiple laboratories, possibly reduces the front interface recombination.…”

Section: Discussionmentioning

confidence: 92%

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Feurer

Carron

Sevilla

et al. 2019

Advanced Energy Materials

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the formation of defect complexes and possibly even phases of ordered defect compounds. [9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes. It has been shown that stoichiometric Cu concentrations can be beneficial for absorber crystallinity, [13] defect density, [7,13] mobility, [14,15] and doping density. [14][15][16] Despite this, solar cells based on Cu stoichiometric CuInSe 2 (CIS) have never reached efficiencies above 13.5%, [7,[17][18][19] while Cudeficient CIS yielded efficiencies only up to 15.0%. [20] CIS cells with high CGI are mainly limited by a low open circuit voltage (V OC ), and to a lesser extent, by a reduced fill factor (FF) and current density. However, the implementation of a Cudeficient surface layer has shown to recover the V OC loss in those high CGI cells, while a decrease in current density remains attributed to tun neling recombination. [17,21] Similar improvements of the absorber surface have been achieved with alkali treatments after etching of the secondary phases in Curich samples, [22,23] although most recent results indicate that this treatment may passivates defects that were at least partly generated by the etching in the first place. [24] This suggests that the front absorber surface limits the efficiency of Cu stoichiometric devices. Therefore, it is important to reduce the front interface recombination, besides any other, in such devices.In an earlier publication, we have shown that ungraded CIS solar cells are constrained by recombination at the back interface, [25] which limits the solar cell efficiency. This back sur face recombination can be effectively suppressed by the imple mentation of a single bandgap grading achieved by adding Ga close to the Mo back contact, leading to improved efficiencies up to 16.1% with a bandgap of 1.0 eV absorber [25] and later to 18.0% with application of a RbF postdeposition treatment (PDT). [26] Those solar cells were processed with Cudeficient absorbers (CGI: 0.85-0.90). CIS solar cells processed with high Cu content show lower V OC , and therefore lower efficiency. Believing in recombination as the root cause, we hypothesize that the application of heavy alkali PDT, for example with RbF, may suppress the recombination observed for absorbers with stoichiometric Cu concentrations. To investigate this, we processed and analyzed CISbased solar cells with various Cu concentrations and amounts of RbF during PDT treatment.Here we report a large improvement in efficiency of solar cells processed with Cu composition close to stoichiometry. In State-of-the-art Cu(In,Ga)Se 2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe 2 sola...

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Section: Discussionmentioning

confidence: 92%

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Feurer

Carron

Sevilla

et al. 2019

Advanced Energy Materials

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“…Since EBIC is sensitive to topography, polishing methods need to be employed to avoid artifacts arising from the rough topography of CIGSe. Both mechanical polishing [ 12,150,151 ] and ion polishing [ 23,42,152,153 ] methods have been implemented to polish the surface in order to reveal the EBIC at GBs. Contrary to the mechanically polished samples where the EBIC results are sometimes contradictory, [ 12,150,151 ] the GBs observed in ion‐polished samples demonstrate all three possible traits, i.e., brighter/darker/neutral (corresponding to higher/lower/same EBIC currents) as compared to their neighboring grains irrespective of the high and low injection conditions used.…”

Section: Electronic Properties Of Bulk and Grain Boundaries In Cigsementioning

confidence: 99%

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Cojocaru-Mirédin

Raghuwanshi

Wuerz

et al. 2021

Adv Funct Materials

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Cu(In,Ga)Se 2 thin-film solar cells have attracted significant research interest in recent decades due to their high efficiency in converting solar energy into electricity for enabling a sustainable future. Although the Cu(In,Ga)Se 2 absorber can be grown as a single crystal, its polycrystalline form is dominating the market not only due to its lower costs, but also due to its unexpectedly higher cell efficiency. However, this absorber contains a high fraction of grain boundaries. These are structural defects where deep-trap states can be localized leading to an increase in recombination activity. This controversy is mirrored in the existing literature studies where two main contradictory believes exist: 1) to be crucial grain boundaries in Cu(In,Ga)Se 2 absorber are anomalous, being benign in terms of cell performance, and 2) grain boundaries are regions characterized by an increased recombination activity leading to deteriorated cell performance. Therefore, the present review tackles this issue from a novel perspective unraveling correlations between chemical composition of grain boundaries and their corresponding electronic properties. It is shown that features such as Cu depletion/In enrichment, segregation of 1-2at.% of alkali dopants, and passivation by a wide-bandgap or type inversion at grain boundaries are crucial ingredients for low open-circuit voltage loss and, hence, for superior cell performance.

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“… 16 , 17 Alkali elements have limited solubility in CIGSe grain interiors (GIs), 18 with preferential segregation to lattice defects and phase boundaries in the polycrystalline film. 19 This is especially apparent for heavy alkali elements (referring to K, Rb, or Cs) because of the higher defect formation energies and increased diffusion barriers in bulk CIGSe. 20 Nonetheless, because the volume fraction of GBs is small compared with that of the GIs, the integral Na amounts can be comparable.…”

Section: Introductionmentioning

confidence: 99%

Alkali Dispersion in (Ag,Cu)(In,Ga)Se₂ Thin Film Solar Cells—Insight from Theory and Experiment

Aboulfadl

Sopiha

Keller

et al. 2021

ACS Appl. Mater. Interfaces

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Silver alloying of Cu(In,Ga)Se 2 absorbers for thin film photovoltaics offers improvements in open-circuit voltage, especially when combined with optimal alkali-treatments and certain Ga concentrations. The relationship between alkali distribution in the absorber and Ag alloying is investigated here, combining experimental and theoretical studies. Atom probe tomography analysis is implemented to quantify the local composition in grain interiors and at grain boundaries. The Na concentration in the bulk increases up to ∼60 ppm for [Ag]/([Ag] + [Cu]) = 0.2 compared to ∼20 ppm for films without Ag and up to ∼200 ppm for [Ag]/([Ag] + [Cu]) = 1.0. First-principles calculations were employed to evaluate the formation energies of alkali-on-group-I defects (where group-I refers to Ag and Cu) in (Ag,Cu)(In,Ga)Se 2 as a function of the Ag and Ga contents. The computational results demonstrate strong agreement with the nanoscale analysis results, revealing a clear trend of increased alkali bulk solubility with the Ag concentration. The present study, therefore, provides a more nuanced understanding of the role of Ag in the enhanced performance of the respective photovoltaic devices.

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Revealing the beneficial role of K in grain interiors, grain boundaries, and at the buffer interface for highly efficient CuInSe₂ solar cells

Cited by 22 publications

References 38 publications

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Alkali Dispersion in (Ag,Cu)(In,Ga)Se₂ Thin Film Solar Cells—Insight from Theory and Experiment

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Revealing the beneficial role of K in grain interiors, grain boundaries, and at the buffer interface for highly efficient CuInSe2 solar cells

Cited by 22 publications

References 38 publications

Efficiency Improvement of Near‐Stoichiometric CuInSe2 Solar Cells for Application in Tandem Devices

Efficiency Improvement of Near‐Stoichiometric CuInSe2 Solar Cells for Application in Tandem Devices

Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells—Insight from Theory and Experiment

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Revealing the beneficial role of K in grain interiors, grain boundaries, and at the buffer interface for highly efficient CuInSe₂ solar cells

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Alkali Dispersion in (Ag,Cu)(In,Ga)Se₂ Thin Film Solar Cells—Insight from Theory and Experiment