Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se<sub>2</sub> Solar Cells on Flexible Substrates

Carron, Romain; Nishiwaki, Shiro; Feurer, Thomas; Hertwig, Ramis; Avancini, Enrico; Löckinger, Johannes; Yang, Shih‐Chi; Buecheler, Stephan; Tiwari, Ayodhya N.

doi:10.1002/aenm.201900408

Cited by 189 publications

(169 citation statements)

References 47 publications

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“…In the investigated solar cells, the col lection of charge carriers generated deep within the absorber appears unaffected by the reduced SCR width; EQE measure ments even show a small increase in the nearinfrared region ( Figure S4, Supporting Information). [30] Narrowbandgap CIS cells are more sensi tive to free carrier absorption in the transparent conductive oxide (TCO) front contact than CIGS, due to the extended NIR response. Energy Mater.…”

Section: Resultsmentioning

confidence: 99%

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: Resultsmentioning

confidence: 99%

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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“…Recently, there have been several reports on the improvement of the efficiency of CIGS solar cells via alkali metal post‐treatments . Several mechanisms have been proposed for the improvement of efficiency including the reduction in Cu and Ga concentrations at the near‐surface region, the formation of a very dense shallow donor‐type Cd Cu during the chemical bath deposition (CBD)‐CdS process, a reduction in the thickness of the CdS buffer layer, a shift of the valence band to lower binding energies, surface band‐gap widening, the enhancement of hole concentration, the enhancement of minority carrier lifetime, the reduction in nanoscopic potential fluctuations, the formation of new secondary compounds at the surface, the mitigation of the shunting associated with high Cu content, and so on. Among different alkali metal‐treated solar cells, cesium fluoride (CsF)‐treated CIGS solar cells possess the highest conversion efficiency …”

Section: J–v and C–v Parameters Of Cigs Solar Cells Shown In Figurementioning

confidence: 99%

The Effect of Electron Irradiation on Cesium Fluoride‐Free and Cesium Fluoride‐Treated Cu(In_1−x,Ga_x)Se₂ Solar Cells

Khatri

Lin

Nakada

et al. 2019

Physica Rapid Research Ltrs

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Herein, the effects of electron irradiation on cesium fluoride (CsF)‐free and CsF‐treated Cu(In1−x,Gax)Se2 (CIGS) solar cells with fluence from 1 × 1013 to 1 × 1017 cm−2 are investigated. Both CsF‐free and CsF‐treated CIGS solar cells exposed to fluence up to 3 × 1015 cm−2 show self‐healing properties, due to the soaking effect from electron bombardment. This feature indicates the robustness of CIGS solar cells for long‐term space explorations. On the other hand, electron irradiation with fluence above 1 × 1016 cm−2 suppresses the solar cell performance of both CsF‐free and CsF‐treated samples, which is expected to be due to the formation of irradiation‐induced defects on the CIGS thin film. This result suggests that the formation of irradiation‐induced defects is independent of the incorporation of heavier alkali metals into the CIGS thin films. The defect passivation and/or introduction rates at different fluences are investigated.

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“…Thus, no Na diffuses from the substrates, and so these absorbers are subjected to a double PDT: first with NaF, and then with either KF or RbF. The best efficiency for low‐temperature absorbers is 20.4% with NaF+KF treatment and 20.8% with NaF+RbF treatment—both achieved on flexible substrates.…”

Section: Introductionmentioning

confidence: 99%

Heavy Alkali Treatment of Cu(In,Ga)Se₂Solar Cells: Surface versus Bulk Effects

Siebentritt

Avancini

Bär

et al. 2020

Advanced Energy Materials

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Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

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Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se₂ Solar Cells on Flexible Substrates

Cited by 189 publications

References 47 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

The Effect of Electron Irradiation on Cesium Fluoride‐Free and Cesium Fluoride‐Treated Cu(In_1−x,Ga_x)Se₂ Solar Cells

Heavy Alkali Treatment of Cu(In,Ga)Se₂Solar Cells: Surface versus Bulk Effects

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Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se2 Solar Cells on Flexible Substrates

Cited by 189 publications

References 47 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

The Effect of Electron Irradiation on Cesium Fluoride‐Free and Cesium Fluoride‐Treated Cu(In1−x,Gax)Se2 Solar Cells

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

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Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se₂ Solar Cells on Flexible Substrates

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

The Effect of Electron Irradiation on Cesium Fluoride‐Free and Cesium Fluoride‐Treated Cu(In_1−x,Ga_x)Se₂ Solar Cells

Heavy Alkali Treatment of Cu(In,Ga)Se₂Solar Cells: Surface versus Bulk Effects