Revision as of 13:16, 4 December 2020

Fabrication of surfaces for the promotion of bacterial biofilms

Project type: PhD project
Project responsible: Alireza Hajijafarassar
Supervisors: Ole Hansen, Jörg Hübner, Anders Michael Jørgensen
Partners involved: DTU Fotonik
Project start: 2017-06-01
Thesis: https://orbit.dtu.dk/en/projects/technology-for-czts-silicon-tandem-solar-cells

Project description

Thanks to the drastic cost reductions over the last few decades, photovoltaic (PV) technology, which directly converts the sunlight into usable electricity, has become one of the most prominent sources of renewable energy. As the balance of system costs now dominates the cost of PV systems, the demand to increase the efficiency of PV modules has surged. However, the efficiency improvements in most mature PV technologies, in particular the Si solar cells as the most dominant technology, are only marginal as they are approaching their fundamental limits. In this regard, multijunction or tandem solar cells have emerged as the most compelling solution to surpass the so-called Shockley–Queisser limit and realize efficiency beyond 30%. In a tandem solar cell, two (or more) sub-cells with complementary bandgaps are combined in a vertical stack to harvest the solar spectrum more efficiently with lower thermalization (heat) losses. Silicon-based tandem solar cells, which couple the conventional silicon technology with a high bandgap material, are practically and economically the most favorable configuration since the enhanced efficiency can be realized by adding a few additional steps to the existing well-developed technology. Thin-film chalcogenides are enticing candidates for the top cell material as they have exhibited cost, performance, and stability comparable to the silicon technology. Despite their promise, the growth of such materials, which are usually produced at high temperatures (500-600 oC) under harsh reactive atmospheres (e.g., S or Se), on silicon is exceptionally challenging. During the high-temperature process, harmful metallic elements (e.g., Cu), which chalcogenides contain in abundance, can diffuse to the silicon bottom cell and severely damage its photovoltaic performance.

In this thesis, we chose CuZnSnS4, a quaternary compound semiconductor with a bandgap of 1.5 eV, as a promising non-toxic, earth-abundant, and cheap representative candidate from the chalcogenide family, and systematically studied the integration challenges with silicon. For this purpose, we developed and optimized a thermally resilient silicon cell structure with polysilicon carrier selective contacts, and used an ultrathin (< 10 nm) titanium nitride-based diffusion barrier at the interface of the two cells (called the “barrier layer”) to protect the silicon cell against contamination. Throughout the thesis, we showed that the performance of the CZTS-Si tandem devices heavily relies on the electrical, optical, and protection behavior of the barrier layer. By proper engineering of the TiN and polysilicon interfacial layers, we managed to keep the silicon cell almost intact during the full fabrication of CZTS, and demonstrated a world-record efficiency of 4.1% for this type structure. Our findings implicate that the growth of new materials, with a wide range of thermal budgets and compositions, is technically feasible on silicon. Moreover, we believe that our proposed tandem structure may provide new insights for the Si community in terms of device architecture engineering for future silicon-based tandem cells.

Publications

Name of publication 1 made in this project

Reference and link to the publication

Process flow(s) relevant to this publication including links to Process development made in connection to this publication this is described in either LabAdviser or Process2Share

Name of publication2 made in this project

Reference and link to the publication

Process flow(s) relevant to this publication including links to Process development made in connection to this publication this is described in either LabAdviser or Process2Share

Name of publication3 made in this project

Reference and link to the publication

Process flow(s) relevant to this publication including links to Process development made in connection to this publication this is described in either LabAdviser or Process2Share

Fabrication: Process flows

This fabrication process flow is used for the fabrication of the silicon bottom cell.

Process flow (word format):

Process flow

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 ==Project description==
-Satisfying the demand for renewable friendly energy is one of the urgent grand challenges. Photovoltaics (PV) is definitely one of the answers to this challenge. Since the efficiency of single material PV cells is approaching fundamental limits, a major challenge in photovoltaics today is to improve efficiency in order to reduce the levelized cost of electricity below that of fossil fuels. As a result, for production of PV modules with higher efficiencies, tandem (multijunction) approches have been introduced. The key benefit of a multijunction solar cell lies in exploiting the solar spectrum in a more efficient manner. In a double-junction cell architecture, the high energy photons are harvested in the top sub-cell with higher bandgap, while the low energy photos are harvested in the lower bandgap bottom sub-cell. Consequently, higher open circuit voltages (Voc) can be achieved due to lower thermalization losses of charge carriers.
+Thanks to the drastic cost reductions over the last few decades, photovoltaic (PV) technology, which directly converts the sunlight into usable electricity, has become one of the most prominent sources of renewable energy. As the balance of system costs now dominates the cost of PV systems, the demand to increase the efficiency of PV modules has surged. However, the efficiency improvements in most mature PV technologies, in particular the Si solar cells as the most dominant technology, are only marginal as they are approaching their fundamental limits. In this regard, multijunction or tandem solar cells have emerged as the most compelling solution to surpass the so-called Shockley–Queisser limit and realize efficiency beyond 30%. In a tandem solar cell, two (or more) sub-cells with complementary bandgaps are combined in a vertical stack to harvest the solar spectrum more efficiently with lower thermalization (heat) losses. Silicon-based tandem solar cells, which couple the conventional silicon technology with a high bandgap material, are practically and economically the most favorable configuration since the enhanced efficiency can be realized by adding a few additional steps to the existing well-developed technology. Thin-film chalcogenides are enticing candidates for the top cell material as they have exhibited cost, performance, and stability comparable to the silicon technology. Despite their promise, the growth of such materials, which are usually produced at high temperatures (500-600 oC) under harsh reactive atmospheres (e.g., S or Se), on silicon is exceptionally challenging. During the high-temperature process, harmful metallic elements (e.g., Cu), which chalcogenides contain in abundance, can diffuse to the silicon bottom cell and severely damage its photovoltaic performance.
-Silicon-based tandem structure can be exceptional candidates for further improving the efficiency while leveraging the well-established existing crystalline silicon manufacturing capacity. Ideally, the top sub-cell on the silicon (with band gap of 1.1 eV) should possess a bandgap of 1.6-1.9 eV. However, within this range, there are a limited number of materials that can exhibit high open circuit voltages. There has been efforts on utilizing methylammonium-lead-halide perovskite as the absorber layer of the top sub-cell. Even though high efficiencies have been reported in the literature3,4, the technology suffers from fast degradation of the perovskite material as well as incorporation of toxic elements such as lead (Pb). On the other hand, CZTS (Cu2ZnSnS4) material consisting of inexpensive, non-toxic elements with a band gap of 1.45 eV can also be a potential candidate for such a cell. The theoretical limit for a CZTS cell is close to 27 %. However, the current efficiency of CZTS is 9.5 % obtained by UNSW, but this value is expected to continue rising as a result of efforts in the solar cell community.
+In this thesis, we chose CuZnSnS4, a quaternary compound semiconductor with a bandgap of 1.5 eV, as a promising non-toxic, earth-abundant, and cheap representative candidate from the chalcogenide family, and systematically studied the integration challenges with silicon. For this purpose, we developed and optimized a thermally resilient silicon cell structure with polysilicon carrier selective contacts, and used an ultrathin (< 10 nm) titanium nitride-based diffusion barrier at the interface of the two cells (called the “barrier layer”) to protect the silicon cell against contamination. Throughout the thesis, we showed that the performance of the CZTS-Si tandem devices heavily relies on the electrical, optical, and protection behavior of the barrier layer. By proper engineering of the TiN and polysilicon interfacial layers, we managed to keep the silicon cell almost intact during the full fabrication of CZTS, and demonstrated a world-record efficiency of 4.1% for this type structure. Our findings implicate that the growth of new materials, with a wide range of thermal budgets and compositions, is technically feasible on silicon. Moreover, we believe that our proposed tandem structure may provide new insights for the Si community in terms of device architecture engineering for future silicon-based tandem cells.
-This PhD project is part of a larger effort at DTU Nanotech and DTU Fotonik, i.e. the ALTCELL project, which aims at developing the necessary technology to realize the potential for enhanced efficiency by integrating a Cu2ZnSnS4 (CZTS) solar cell on top of a silicon solar cell. The project is supported by the “Innovationsfonden” (PI, Jørgen Schou, DTU Fotonik) and is carried out with international (Nanyang Technological University, Singapore) and industrial (e.g. Haldor Topsøe A/S) partners.  This PhD project is partly funded by “Innovationsfonden” and partly by DTU Nanotech.
-The ultimate goal of the overall project is to develop technology for a CZTS-Silicon tandem solar cell. This entails an improvement of CZTS single cell technology to enhance the efficiency, development of a suitable silicon bottom cell, and solving issues related to integration of the two cells.
-This PhD project will be involved in all three main topics by:
-. Providing some of the front and back end films for the CZTS cell, participation in characterization of the CZTS films, whereas the actual synthesis of CZTS will mostly be done by other project partners.
-. Developing and characterizing the bottom silicon cell. The developed cell has to be suitable for the tandem structure and compatible with the CZTS layer.
-. Developing technology for tunnel/barrier layers between the two absorber layers will be the decisive innovation for the construction of such a cell. The purpose of these layers are twofold: 1) at the interface electron currents from the silicon cell must be transformed into hole currents in the CZTS cell. 2) The interface must have a diffusion barrier to protect the silicon cell from in-diffusion of metals (particularly Cu) from the CZTS. This is a particularly challenging task.
 ==Publications==