PhD Defense by Alireza Hajijafarassar

On Friday, 25 September 2020, Alireza Hajijafarassar will defend his PhD thesis:

“Technology for CZTS-Silicon Tandem Solar Cells”

Time and place: 13:00. Due to Covid-19 the defense will be held on-line via Zoom.

Zoom registration link: https://dtudk.zoom.us/j/69450519983

Principal supervisor: Director Jörg Hübner, DTU Nanolab

Co-supervisor:  Professor emeritus Ole Hansen, DTU Nanolab

Co-supervisor: Deputy director Anders Michael Jørgensen, DTU Nanolab

 

Examiners:

Professor Erik Vilain Thomsen – DTU Health

Associate Professor Hele Irene Savin - Aalto University, Finland

Dr. Jonathan J. S. Scragg - Uppsala University, Sweden

 

 

Chairperson at defense:
Associate Professor Christian Danvad Damsgaard

 

Abstract:

Silicon-based tandem solar cells, which combine the well-established and market-proven silicon (Si) solar cells with another high bandgap semiconductor into a single device, can enable highly efficient devices (> 30%) with small additional costs. Such next-generation technology demands earth-abundant, stable, and non-toxic materials with costs comparable to the silicon bottom cell. Thin-film chalcogenides, in particular Cu2ZnSnS4 (CZTS), may emerge as excellent top cell candidates to partner with silicon as they comply with all essential requirements. However, the integration of such materials on Si is a formidable task due to their harsh fabrication process. The fabrication process of chalcogenides generally constitutes high-temperature annealing of the metallic precursors (e.g., Cu) in reactive atmospheres (e.g., S or Se), which could result in severe degradation of the silicon cell. Thus, research is ongoing to develop new technology designs at the interface of the two cells, which not only provide sufficient protection to the silicon but also ensures an efficient electrical and optical transport. In this thesis, we proposed a new monolithic tandem cell architecture, which features a thermally resilient silicon bottom cell with electron and hole selective polysilicon contacts on both sides (the so-called Tunnel Oxide Passivated Contacts (TOPCon) structure). At the interface of the two cells, we used an ultrathin TiN-based layer to double as the diffusion barrier and recombination layer.

 

Firstly, we developed and optimized the bottom cell structure inhouse in our cleanroom facility. By optimizing the passivation quality of both n-type and p-type polySi contacts, we managed to obtain an excellent effective lifetime above 3 ms and iVoc above 700 mV for device precursor samples. We demonstrated a high external Voc of 681 mV and identified a moderate FF of 70% as the main limitation of our silicon single-junction cells. By performing detailed electrical analysis, we attributed the low FF to a high series resistance problem at the p-n junction.

 

Once the bottom cell was developed, we systematically investigated the monolithic integration of CZTS, as a promising representative from chalcogenides, on the silicon bottom cell using TiN as the barrier layer. A series of contamination studies revealed that the TiN barrier layer successfully mitigates the indiffusion of elements into the bottom cell. As a result, a high millisecond range lifetime in Si could be achieved after CZTS processing. Based on these findings, the first proof-of-concept working monolithic CZTS-Si device was fabricated with a tandem Voc of 900 mV. However, ii the tandem efficiency was severely limited by a very low FF, which we ascribed to inefficient carrier transport across the TiOxNy barrier due to the high oxygen content. Despite the weak performance, we successfully showed that full preservation of the silicon bottom cell is feasible using a 10 nm TiOxNy barrier layer.

 

Next, we improved the performance of our CZTS-Si tandem solar cell by further engineering of the barrier layer. We performed a comparative study among three TiN-based barrier layers, namely TiOxNy, TiN-Al-TiN, and TiN, with different electrical, optical, and barrier protection properties. We showed that lowering the oxygen content in the TiN layer could eliminate the roll-over of the IV curve and thereby significantly improve the FF of our tandem devices. However, we noticed that decreasing the oxygen content diminishes the barrier protection, which led to severe degradation of the silicon bottom cell Jsc after CZTS fabrication.

 

Subsequently, we introduced a second protection mechanism in the silicon bottom cell based on impurity gettering within the highly doped polySi contacts. In this regard, we increased the front n+polySi thickness from 40 to 200 nm, which not only improved the initial surface passivation but also increased the bottom cell resilience significantly. The increased protection helped to reduce the necessary TiN thickness to less than 5 nm. As a result, a new tandem record efficiency of 4.1% was achieved in a CZTS-Si tandem device, i.e., the best value reported so far for this type of structure. Ultimately, we successfully generalized our findings to other similar high bandgap chalcogenides using CGSe and AIGSe materials, where we achieved a tandem efficiency of 5.5% and a promising Voc around 1.3 V for the CGSe-Si tandems.

Tidspunkt

fre 25 sep 20
13:00 - 17:00

Arrangør

DTU Nanolab

Hvor

Time and place: 13:00. Due to Covid-19 the defense will be held on-line via Zoom.

Zoom registration link: https://dtudk.zoom.us/j/69450519983