• Bose, Sourav
• Pierson, Jean-François
• Boulet, Pascal
• Horwat, David
• Hamady, Sidi Ould Saad
• Gries, Thomas
• Aubert, Thierry
• Chevallier, Christyves
• Fressengeas, Nicolas

Abstract

All-oxide photovoltaics is a new and very promising technology for low cost and efficient solar cells using earth-abundant and environmentally compatible materials. To match the requirements of this technology, beyond the optimization of the absorber layer, it is crucial to elaborate window and transport layers with excellent optical and structural properties in a wide range of thickness. Among the oxide materials, zinc oxide is a key material for the realization of these layers in the targeted all-oxide solar cells. Beside the low toxicity and abundance of ZnO, the reduction of the environmental footprint of photovoltaic technology could benefit from the low energy and low precursor consumption of the ultrasonic spray pyrolysis (USP) process. In addition to the environmental aspects, the USP process combines many advantages compared to conventional methods such as low-cost and large-scale production capability. The potential for elaboration of thin films of suitable structural and optical properties of this process combined with the previously mentioned advantages makes it of high interest for the future of photovoltaics. To date, the optical and structural properties of ZnO elaborated by USP have not been fully investigated with respect to the precursor solution concentration, which could have a strong impact on these properties and thus on the performances of the solar cell. ZnO thin films were deposited using the USP process after a precise optimization of the main growth conditions, namely the substrate temperature, spray pressure, nozzle speed and precursor solution flux, according to a specific design of experiment. ZnO films were confirmed to be conductive by showing resistivity values in the order of 1 Ω.cm. For this study, we used X-ray diffraction (XRD) and Raman spectroscopy to analyze the crystal structure, orientation and texture coefficient. Our results show that the preferential orientation can be controlled to be either (002) or (100) by adjusting the precursor solution concentration. On the one hand, the precise control of the orientation allows to better control the optical properties, that are crucial in our application, and, on the other hand, limit the defects at the heterointerface. The morphological properties were studied using atomic force microscopy and were correlated to the structural properties of the ZnO thin films. For instance, high consistency was evidenced between the AFM measured grain size and the XRD measured crystallite size. Finally, the optical properties were studied from UV/visible transmittance spectroscopy. The deposited ZnO films exhibit high transmittance above 90 % for the whole concentration range of the precursor solution with a maximum as high as 95 %. The resulting optical and structural properties could benefit to the performances of all-oxide solar cells deposited by USP.

Topics

• impedance spectroscopy
• grain
• resistivity
• grain size
• x-ray diffraction
• experiment
• thin film
• atomic force microscopy
• zinc
• texture
• defect
• ultrasonic
• toxicity
• Raman spectroscopy
• spray pyrolysis