PhD defense of Jérôme MICHALLON

The photovoltaic conversion is a very attractive process for the renewable energy supply. In this field, many works are focusing on second generation solar cells that aim to reduce the fabrication prices with the use of thin film technology. In order to efficiently absorb light in the thin films, direct band gap semi-conductors such as CIGS, CdTe, a-Si are usually used. The small thickness of these solar cells allows for instance the fabrication of flexible solar cells that open new fields for nomad applications (camping-car, tent, backpack, …).
However, one of the drawbacks of such thin film solar cells is the use of non-abundant material on earth such as indium or tellurium.
It would therefore be useful to further decrease the volume of semi-conductor used while maintaining high light absorption and efficient charge carrier collection. Extremely thin absorber (ETA) solar cells based on ZnO nanowire arrays coated with CdTe are thus promising. Indeed, 12.3 % power conversion efficiency has been demonstrated for ZnO/CdTe planar solar cells while ZnO nanowire arrays coated with CdSe or CdS have shown power conversion efficiency of 4.7 % and 3.5 %, respectively.
Despite the use of efficient materials and the promising geometry for solar cells applications, the power conversion efficiency of ZnO/CdTe nanowire arrays remains low. In order to improve the power conversion efficiency of these type of heterostructures, light absorption and electrical charge transport mechanisms are thoroughly studied in this work.
We have firstly optimized the geometrical dimensions of the ZnO/CdTe nanowire arrays in order to maximize the light absorption with a RCWA software developed at the IMEP-LAHC laboratory. We have thus shown that the optimized ZnO/CdTe nanowire array is arranged in square array, grown on reflective substrate with array period of about 400 nm, nanowire diameter and height of 200 nm and 1 to 3 µm respectively, while the optimal CdTe thickness is of 40 to 60 nm. The square nanowire arrangement is optimum since the light absorption is efficient with small absorber volume. The use of the reflective substrate increases the Fabry-Perot resonances (round trip of light) in the nanowires and thus the light absorption. The optimal period of 400 nm is related to efficient diffraction properties of the nanowire arrays especially for low wavelengths. Finally, the light is efficiently confined and guided in the individual nanowires for long wavelengths with the optimal diameter of 200 nm.
Secondly, ETA solar cells based on ZnO nanowire arrays coated with CdTe were fabricated with low cost sol-gel and vapor phase techniques. The fabricated solar cells show efficient light absorption but low power conversion efficiencies. In order to analyze the reason of such low power conversion efficiencies, the charge carrier transport mechanisms in nanowires were studied with dark electrical characterizations associated with numerical and analytical models. It is shown that the charge carrier transport originates from trap assisted tunneling effect with a trap located at energy within the band gap of 0.4 eV with respect of the valence band. Furthermore, the double ZnO/CdTe/CuSCN heterojunction probably plays a role in the charge transport with large ideality factors.
Finally, opto-electronic modeling have shown that ETA solar cells based on ZnO nanowire arrays coated with CdTe can theoretically achieve power conversion efficiency of about 20 %, which shows the potentiality of this type of heterostructure.

Members of the jury :
• Anne KAMINSKI·CACHOPO - Supervisor
• Abdelilah SLAOUI - Examiner
• Christian SEASSAL - Rapporteur
• Denis MENCARAGLIA - Rapporteur
• Vincent CONSONNI - Co-supervisor
• Yves JOUR LIN - Examinateur