Photovoltaics (PV) is the fastest growing electrical energy generation source in the world and has indeed a “bright” future. The cumulative installed capacity of PV in the world has already surpassed 600 GW by 2019. The main driving forces in the PV world have always been the reduction in module cost and the increase in the module efficiencies. Continued development of high-efficiency cell concepts for the future is of great interest in the R&D world.
The PV industry has been transitioning from Al-BSF solar cells to PERC solar cells in order to increase the module efficiencies. The focus of next-generation solar cell technology development has turned towards reducing contact recombination. For this, solar cells with passivated contacts are being widely investigated, with heterojunction contacts based on amorphous Si (a-Si) probably the best known example. With this technology and a back-contacted cell architecture, Kaneka broke the world record efficiency for single junction Si solar cells with its remarkable 26.7% cell , getting quite close to the theoretical and practical limit for single-junction Si solar cells.
To go beyond the fundamental single-junction limit, a tandem device architecture, which employs 2 or more absorber materials of complementary bandgaps in a stack, must be adopted to reduce thermalisation losses and use the solar spectrum more effectively. A wide bandgap perovskite top cell above a c-Si bottom cell is one of the emerging tandem solar cell configurations that has attracted plenty of recent interest, due to its potential of exceeding 30% while combining the attractive properties of perovskites with the well-understood technologies for c-Si. The best monolithic implementation of this material combination was very recently achieved this year by HZB with an efficiency of 29.15% .
Imec is also involved in the development of such 2 terminal tandem cells with a Si heterojunction (SHJ) bottom cell and a perovskite top cell. For the SHJ bottom cells, a thin film stack consisting of (1) intrinsic a-Si, (2) doped a-Si or nano-crystalline silicon (nc-Si) and (3) TCO forms the “skin” that not only passivates the solar wafer surfaces but is also responsible for efficient charge carrier transport, which are both crucial towards achieving high efficiencies. In addition, a recombination layer or tunnel junction is needed in between the two tandem cells. This is usually achieved by using a TCO layer or n+/p+ nc-Si junction. Thus, as part of this thesis, the student will focus on developing TCO layers (such as ITO, AZO, IZO) and n- and p-doped nc-Si films for use in the SHJ cell “skin” and as the intermediate recombination layer or the tunnel junction between the tandem cells. The student will characterise the developed layers structurally, optically and electrically using different characterisation methods such as ellipsometry, XRD, Raman, sheet resistance mapping and Hall measurements. The passivation properties of the developed films as part of the SHJ “skin” will also be assessed using carrier lifetime measurements. Finally, the charge transport through the developed material stack will be evaluated using contact resistivity measurements. These layers will subsequently be implemented in single-junction Si heterojunction solar cells to assess their performance at device level. The main location for this thesis is imec Leuven.
Type of work: 10% literature study + 10% modeling + 80% experimental
 K. Yamamoto, K. Yoshikawa, H. Uzu, and D. Adachi, “High-efficiency heterojunction crystalline Si solar cells,” Jpn. J. Appl. Phys., vol. 57, pp. 08RB20-1, 2018.
Type of project: Thesis, Internship, Combination of internship and thesis
Required degree: Master of Engineering Technology, Master of Engineering Science, Master of Science