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Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.ORCID iD: 0000-0003-0741-5068
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
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2018 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 187, no 1, p. 160-169Article in journal (Refereed) Published
Abstract [en]

In this work, rear-contact passivated Cu(In,Ga)Se2 (CIGS) solar cells were fabricated without any intentional contact openings between the CIGS and Mo layers. The investigated samples were either Na free or one of two Na supply methods was used, i) a NaF precursor on top of the Al2O3 rear passivation layer or ii) an in situ post- deposition treatment with NaF after co-evaporation of the CIGS layer. The thickness of the ALD-Al2O3 passi- vation layer was also varied in order to find an optimal combination of Na supply and passivation layer thickness. Our results from electrical characterization show remarkably different solar cell behavior for different Na supplies. For up to 1nm thick Al2O3 layers an electronically good contact could be confirmed independently of Na deposition method and content. When the Al2O3 thickness exceeded 1 nm, the current was blocked on all samples except on the samples with the NaF precursor. On these samples the current was not blocked up to an Al2O3 layer thickness of about 6 nm, the maximum thickness we could achieve without the CIGS peeling off the Al2O3 layer. Transmission electron microscopy reveals a porous passivation layer for the samples with a NaF precursor. An analysis of the dependence of the open circuit voltage on temperature (JVT) indicates that a thicker NaF precursor layer lowers the height of the hole barrier at the rear contact for the passivated cells. This energy barrier is also lower for the passivated sample, compared to an unpassivated sample, when both samples have been post-deposition treated.

Place, publisher, year, edition, pages
2018. Vol. 187, no 1, p. 160-169
Keywords [en]
Alkali, Back contact, CIGS, Passivation, Thin films, Rear contact, Tunneling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Energy Systems Other Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-357123DOI: 10.1016/j.solmat.2018.07.017ISI: 000445308300019OAI: oai:DiVA.org:uu-357123DiVA, id: diva2:1238097
Part of project
ARCIGS-M
Funder
Swedish Research Council, 43523-1StandUpEU, Horizon 2020, 720887Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2019-08-08Bibliographically approved
In thesis
1. The Multiple Faces of Interfaces: Electron microscopy analysis of CuInSe2 thin-film solar cells
Open this publication in new window or tab >>The Multiple Faces of Interfaces: Electron microscopy analysis of CuInSe2 thin-film solar cells
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The CIS solar cell family features both a high stability and world-class performances. They can be deposited on a wide variety of substrates and absorb the entire solar spectrum only using a thickness of a few micrometers. These particularities allow them to feature the most positive Energy returned on energy invested (EROI) values and the shortest Energy payback times (EPBT) of all the main photovoltaic solar cells. Using mainly electron microscopy characterization techniques, this thesis has explored the questions related to the interface control in thin-film photovoltaic solar cells based on CuInSe2 (CIS) absorber materials. Indeed, a better understanding of the interfaces is essential to further improve the solar cell conversion efficiency (currently around 23%), but also to introduce alternative substrates, to implement various alloying (Ga-CIS (CIGS), Ag-CIGS (ACIGS)…) or even to assess alternative buffer layers.

The thread of this work is the understanding and the improvement of the interface control. To do so, the passivation potential of Al2O3 interlayers has been studied in one part of the thesis. While positive changes were generally measured, a subsequent analysis has revealed that a detrimental interaction could occur between the NaF precursor layer and the rear Al2O3 passivation layer. Still within the passivation research field, incorporation of various alkali-metals to the CIS absorber layer has been developed and analyzed. Large beneficial effects were ordinarily reported. However, similar KF-post deposition treatments were shown to be potentially detrimental for the silver-alloyed CIGS absorber layer. Finally, part of this work dealt with the limitations of the thin-barrier layers usually employed when using steel substrates instead of soda-lime glass ones. The defects and their origin could have been related to the steel manufacturing process, which offered solutions to erase them.

Electron microscopy, especially Transmission electron microscopy (TEM), was essential to scrutinize the local changes occurring at the different interfaces within a few nanometers. The composition variation was measured with both Electron energy loss spectroscopy (EELS) and Energy dispersive X-ray spectroscopy (EDS) techniques. Finally, efforts have been invested in controlling and improving the FIB sample preparation, which was required for the TEM observations in our case.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1701
Keywords
Electron microscopy, TEM, STEM, EELS, EDS, solar cells, CIGS, ACIGS, CZTS, post deposition treatment, KF, RbF, buffer layers, interfaces, inter layers, barrier layers, passivation layers
National Category
Energy Systems Other Electrical Engineering, Electronic Engineering, Information Engineering Other Materials Engineering
Research subject
Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-357127 (URN)978-91-513-0402-1 (ISBN)
Public defence
2018-09-28, Polhemssalen, The Angstrom laboratory, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2018-09-07 Created: 2018-08-12 Last updated: 2018-10-02
2. In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers
Open this publication in new window or tab >>In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The material supply to build renewable energy conversion systems needs to be considered from both a cost and an energy security perspective. For Cu(In,Ga)Se2 (CIGS) thin film solar cells the use of indium in the absorber layer is most problematic. The material input per service unit can be reduced, if the absorber layers are thinned down without a loss in power conversion efficiency.

Thinning down absorber layers can increase the conversion efficiency. However, for real CIGS solar cells absorption losses and recombination rates at the rear surface between the CIGS absorber and the Mo rear contact as well as shunt-like behavior increase. Thus, both rear surface passivation and optical management are essential for maintaining high power conversion efficiencies.

In this work, thin oxide layers, so-called passivation layers, are introduced between the CIGS absorber layer and the Mo contact. They can passivate the CIGS surface, if the CIGS-oxide interface has a lower defect density than the CIGS-Mo interface and/or if they contain a negative fixed oxide charge, which increases the hole concentration and reduces the electron concentration in the CIGS in the vicinity of the oxide.

As these oxides are insulators, electrical conduction through the passivation layer has to be ensured. In this work, nanopoint contacts were etched into ALD-Al2O3 passivation layers in CIGS solar cells. These solar cells had 0.5 -1.5 µm thin absorber layers with a low In content and a high band gap. Ga grading was not used. Although absorber layers with a high Ga content have a short minority carrier diffusion length, a passivation effect could be discerned with the help of external quantum efficiency measurements and current-voltage measurements under varying temperatures in combination with optical and electrical modeling with a two-diode model. Moreover, the possibility of leaving out the additional fabrication step has been explored for ALD-Al2O3 and HfO2 as passivation layers. The results suggest that the passivation layer does not necessarily need to be opened for electrical conduction in an additional fabrication step, if sodium fluoride (NaF) is deposited onto Al2O3 layers prior to CIGS evaporation. In this case solar cells with 215 nm absorber layers and 6 nm thin passivation layers have a power conversion efficiency of 8.6 %, which is 3 % (absolute) higher than the conversion efficiency on a reference. Shunt-like behavior is additionally reduced. For the HfO2 layers photoluminescence data indicate a good passivation effect, but the layers need to be opened up to ensure conduction.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 118
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1834
Keywords
Alkali, Alumina, Back contact, CIGS, CIGSe, Hafnia, Passivation, Rear contact, Sodium fluoride, Ultra-thin
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-390314 (URN)978-91-513-0713-8 (ISBN)
Public defence
2019-09-30, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2019-09-09 Created: 2019-08-08 Last updated: 2019-09-17

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