Development of a ZnO-based transparent conductive adhesive for the application in III-V/silicon tandem solar cells

Media type: E-Book

Title: Development of a ZnO-based transparent conductive adhesive for the application in III-V/silicon tandem solar cells

Contributor: Heitmann, Ulrike [Verfasser]; Glunz, Stefan [Akademischer Betreuer]; Fiederle, Michael [Sonstige]

Corporation: Albert-Ludwigs-Universität Freiburg, Fakultät für Angewandte Wissenschaften

imprint: Freiburg: Universität, 2022

Extent: Online-Ressource

Language: English

DOI: 10.6094/UNIFR/226684

Identifier:

Keywords: Solar cells ; Silicon ; Mehrfach-Solarzelle ; Transparent-leitendes Oxid ; Klebeverbindung ; (local)doctoralThesis

Origination:

University thesis: Dissertation, Universität Freiburg, 2021

Footnote:

Description: Abstract: This work describes the development of a new ZnO-based adhesive for the application as interconnection layer for III V/silicon tandem solar cells. Tandem solar cells are a new generation of solar cells that allow photoconversion efficiencies beyond the theoretical limit of single junction solar cells. Within this work, III V/silicon tandem solar cells interconnected in series (resulting in a two-terminal device) are investigated. Options for the interconnection of the III V and silicon solar cells include direct wafer bonding and the direct growth, with both requiring sophisticated processing conditions as particle-free and smooth surfaces. Another option for the interconnection of III V top cells and silicon bottom solar cell is the implementation of a transparent conductive adhesive. All approaches based on transparent conductive adhesives for the interconnection of tandem solar cells published so far focus on the combination of non-conductive, polymer-based adhesives with embedded conductive particles. This work, however, describes the development of a ZnO-based adhesive (precursor: Zn acetylacetonate dissolved in methanol) where the transparent adhesive matrix itself becomes electrically conductive upon thermal annealing at a max. temperature of 290°C. By adding 3 at% of indium, the ZnO is doped to form the transparent conductive oxide (TCO) ZnO:In. <br>The first attempts at interconnecting test structures showed that TCO contact layers deposited at the adhesive/subcell interfaces prior to the deposition of the adhesive significantly improved the conductivity of the formed interconnection. Both sprayed (ZnO:In) and sputtered (In2O3:Sn) TCOs were investigated and the differences in contact formation to the utilized semiconductor materials were analyzed with regard to the deposition process by analyzing the TCO/semiconductor interface with Transmission Electron Microscopy. The sprayed ZnO:In layer was further characterized for it’s crystalline structure by Transmission Kikuchi Diffraction to characterize the deposition process. The conductivity of the ZnO-based adhesive interconnection was analyzed in dependence of the process parameters (applied pressure/temperature) and a correlation between the bond‘s homogeneity and the connecting resistivity was found. The structure and homogeneity of the resulting bond was analyzed by Scanning Acoustic Microscopy (SAM). For a more detailed characterization of the bond layer, cross sections were analyzed by Scanning Electron Microscopy (SEM). The homogeneity of the bond was found to be mainly compromised by the outgassing from the adhesive layer during the annealing, as this leads to embedded gas within the bond layer and thus non-bonded areas. To improve the bond‘s homogeneity, and thereby the conductivity, the pressing program was adapted to match the stages of thermal decomposition of the precursors, resulting in a significantly improved homogeneity in bonded test structures. The lowest measured connecting resistivity of the developed ZnO-based adhesive (measured in planar silicon-silicon test structures) was 0.073 Ωcm². Such a low connecting resistivity will not limit a glued tandem solar cell (dual-junction or triple-junction, under 1 sun illumination). The optical effect of the ZnO-based adhesive interconnection on a glued III V/silicon tandem solar cell was investigated by both bonded test structures and optical simulations. An optical simulation of the bond showed a weighted reflectance at the bond layer of 16.5 % if sputtered In2O3:Sn contact layers are implemented. It was found that the anti-reflective effect of the contact layers is highest if the refractive index of the contact layer is n ≈ 2.5, which can be achieved by TiO2-based contact layers. Another option for reducing the reflectance at the bond is a textured silicon surface (random pyramid texture) as the optical simulation showed a further reduction of the weighted reflectance down to 10.6 %. A bonded test structure featuring one textured surface resulted in a connecting resistivity of 0.083 Ωcm², which is in the same range as the bonded planar reference. <br>For the integration of the developed ZnO-based adhesive into III V/silicon tandem solar cells, it was found that the gluing process needed to be adjusted because of the difference in coefficient of thermal expansion between Si and GaAs. A first connecting step at a low temperature was complemented with a second step at high temperature, which was only possible at lower pressure without risking a breakage of the III-V material. The adjusted process resulted in a first working tandem solar cell which demonstrates the proof-of-concept for the developed ZnO-based adhesive. The glued tandem device was mainly limited by a low short-circuit current (non-optimized optics) and low fill factor. The latter is limited by the connecting resistivity of the bond, as the optimized pressing process could not be used in the cells. The voltage of the obtained device is close to the one reported for record wafer bonded devices, close to 3V, which indicates that the developed gluing process does not degrade the individual solar cells. The reflectance at the bond layer, even for In2O3:Sn-based contact layers and a textured silicon surface, would still limit tandem solar cells interconnected by the developed ZnO-based adhesive. To reduce this optical loss, a contact layer with a high refractive index (TiO2:Nb, n ≈ 2.5) was investigated for it’s application in III V/silicon tandem solar cells and found to be suitable. A simulation of the optical effect in a glued device showed a reduction in reflectance caused at the bond layer down to < 2 %, which should no longer limit the potential of a glued tandem solar cell

Access State: Open Access

Rights information: Attribution (CC BY)

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