View source for LabAdviser/Technology Research/Technology for CZTS-Silicon Tandem Solar Cells

==Preface==
This PhD project was part of a larger effort at DTU Nanotech and DTU Fotonik, 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 was supported by the “Innovationsfonden” (PI, Jørgen Schou, DTU Fotonik) and was 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 Nanolab.

The ultimate goal of the overall project was 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. 

The PhD project was involved in all three main topics by:

# Providing some of the front and back end films for the CZTS cell, participation in the 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.
# Developing technology for tunnel/barrier layers on the silicon 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.

==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 Cu2ZnSnS4, 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 of 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 in peer-reviewed journal papers==

The results achieved during the Ph.D. project have been published in peer-reviewed journals, which are listed below:

===Publications as first author ===

'''[1] Hajijafarassar, A. ''', Martinho, F., Stulen, F., Grini, S., López-Mariño, S., Espíndola-Rodríguez, M., Döbeli, M., Canulescu, S., Stamate, E., Gansukh, M., Engberg, S., Crovetto, A., Vines, L., Schou, J., & Hansen, O. (2020). Monolithic thin-film chalcogenide–silicon tandem solar cells enabled by a diffusion barrier. Solar Energy Materials and Solar Cells, 207, 110334. https://doi.org/10.1016/j.solmat.2019.110334

'''[2]''' Martinho, F., '''Hajijafarassar, A.''', Lopez-Marino, S., Espíndola-Rodríguez, M., Engberg, S., Gansukh, M., Stulen, F., Grini, S., Canulescu, S., Stamate, E., Crovetto, A., Vines, L., Schou, J., & Hansen, O. (2020). Nitride-Based Interfacial Layers for Monolithic Tandem Integration of New Solar Energy Materials on Si: The Case of CZTS. ACS Applied Energy Materials, 3(5), 4600–4609. https://doi.org/10.1021/acsaem.0c00280 (Equal Contribution)

===Publications as co-author ===
'''[1]''' Crovetto, A., '''Hajijafarassar''', A., Hansen, O., Seger, B., Chorkendorff, I., & Vesborg, P. C. K. (2020). Parallel Evaluation of the BiI 3 , BiOI, and Ag 3 BiI 6 Layered Photoabsorbers. Chemistry of Materials, 32(8), 3385–3395. https://doi.org/10.1021/acs.chemmater.9b04925

'''[2]''' Crovetto, A., Børsting, K., Nielsen, R., '''Hajijafarassar, A.''', Hansen, O., Seger, B., Chorkendorff, I., & Vesborg, P. C. K. (2020). TaS 2 Back Contact Improving Oxide-Converted Cu2BaSnS4 Solar Cells. ACS Applied Energy Materials, 3(1), 1190–1198. https://doi.org/10.1021/acsaem.9b02251

'''[3]''' Martinho, F., Lopez-Marino, S., Espíndola-Rodríguez, M., '''Hajijafarassar, A.''', Stulen, F., Grini, S., Döbeli, M., Gansukh, M., Engberg, S., Stamate, E., Vines, L., Schou, J., Hansen, O., & Canulescu, S. (2020). Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review. ACS Applied Materials & Interfaces, 12(35), 39405–39424. https://doi.org/10.1021/acsami.0c10068

'''[4]''' Gansukh, M., López Mariño, S., Espindola Rodriguez, M., Engberg, S. L. J., Martinho, F. M. A., '''Hajijafarassar, A.''', Schjødt, N. C., Stamate, E., Hansen, O., Schou, J., & Canulescu, S. (2020). Oxide route for production of Cu2ZnSnS4 solar cells by pulsed laser deposition. Solar Energy Materials and Solar Cells, 215, 110605. https://doi.org/10.1016/j.solmat.2020.110605

==Fabrication: Process flows ==

The fabrication process flow of the silicon bottom cell can be found via the link below. Note that the process flow only contains fabrication steps of the device precursor wafer (asymmetrically passivated wafer with selective polySi-based contacts), and not include the backend processing (TCO deposition, metallization, tandem integration, etc.). 

Process flow (word format):
*[[media:Doubled-side TOPCon structure for Si-based tandems.docx|Silicon Bottom Cell Device Precursor - Process flow]]