Structural, optical and electrical properties of Nd-doped SnO2 thin films fabricated by reactive magnetron sputtering for solar cell devices

• Nd-doped SnO2 thin films as TCO and down shifting converter layers are produced.
• Efficient energy transfer from the SnO2 host matrix to Nd3+ ions is demonstrated.
• Enhancement of the CIGS solar cells characteristics by adding SnO2:Nd layers

The use of photon conversion layers is an interesting way to improve the overall efficiency of solar cells. Herein we report on Nd-doped SnO2 thin films with photon management property inserted further into CIGS based solar cells. The functionalized layers were deposited by reactive magnetron sputtering whose structural, optical and electrical properties were tuned by varying the deposition temperature. Careful analysis of the structure using XRD and XPS showed that the tetragonal rutile SnO2 phase can be obtained at a deposition temperature as low as 100 °C. Transparency was found to be as high as 90% for all layers while the absorption edge is found to increase when increasing the deposition temperature up to 300 °C. The photoluminescence measurements under 325 nm UV laser excitation showed that 100 °C is needed for the optical activation of the rare earth. Despite the small amounts of Nd (around 0.62 at%), intense and narrow emission bands have been collected in the Near Infrared Region (NIR) which are characteristics of Nd3+ ions whose the ionic state was confirmed by the 3d XPS core levels. Thus, the emission spectra cover a good part of the spectrum useful to the solar cell. Photoluminescence excitation spectroscopy experiments were also carried out on Nd:SnO2 samples to get insights on the energy transfer. By exciting in the deep UV from 250 to 400 nm intense Nd emission was collected giving an experimental evidence of the down-shifting process through a resonant energy transfer from the SnO2 host matrix to the Nd3+ ions. Hall Effect measurements showed that the n-type character and good conductivity of the Nd doped SnO2 films can be correlated to the highest optical activity of Nd in the matrix. An optimal condition is found for the Nd-doped SnO2 film grown at 300 °C for which the highest PL and the best electrical data were measured. Finally, we show that the implementation of such optimized Nd–SnO2 films on CIGS based solar cells serving as a transparent conducting oxide and a down shifting converter results in the best power conversion efficiency.

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