From thin film growth in plasma-enhanced chemical vapor deposition to interface engineering in (plasma-assisted) atomic layer deposition
Dr. Mariadriana Creatore
TU Eindhoven, Department of Applied Physics, Plasma and Materials Processing group
17:15 - 18:15 Tuesday 01 December 2015 TUG P2 This colloquium will review the main features of plasma-enhanced and plasma–assisted deposition methods in terms of gas phase production of reactive species at low thermal budget and control in thin film growth and interface engineering. Two case-studies will be addressed, both in the field of thin film photovoltaics.
The first case study will address the expanding thermal plasma – chemical vapor (ETP-CVD) deposition of Al-doped ZnO (ZnO:Al) transparent conducting oxides. Specifically, insights into the role of the plasma chemistry in affecting the microstructure of these poly-crystalline layers will be presented. A significant limitation to ZnO:Al grown by CVD processes is that its bulk resistivity at low film thicknesses typically exceeds 10-3 Ω.cm, which is too high for most opto-electronic device applications, such as solar cells. The high resistivity is a result of high grain and high void densities, and only at high film thickness, grains develop sufficiently to yield conductivities (~ 10-4 Ω.cm) that render the film suitable as a transparent conducting oxide (TCO). Here, we review this behaviour in detail, and demonstrate that the underlying cause is a high grain density at the early growth stage, accompanied by a strong <0002>-texture which drives an unfavourable growth mode. Since the role of the precursor feed composition, i.e. the diethylzinc-to-O2 flow rate ratio, has previously been identified to be highly influential on the ZnO microstructure, here we show that the diethylzinc-to-O2 ratio allows tuning of the atomic oxygen flux arriving at the surface, thereby influencing the development of specific crystal orientations, and hence the early growth mechanism.
The second case study will address the application of (plasma-assisted) atomic layer deposited (ALD) of functional layers for mixed halide organo-lead perovskite solar cells. Atomic layer deposition will be shown to offer accurate control in terms of film thickness, chemical and opto-electrical properties, which are all appealing characteristics for mesoscopic solar cells. These latter exhibit complex 3D interfaces between the absorber, the charge carrier transport layers and the electrical contacts, which inevitably require an accurate control in terms of charge transfer and recombination processes. Specifically, it will be discussed how plasma-assisted ALD TiO2 deposited on ITO/PET substrates serve as excellent blocking layer for suppressing charge recombination processes at the interface ITO/ CH3NH3PbI3-xClx perovskite/hole transport layer.
References
Williams, B.L., Smit, S., Kniknie, B.J., Bakker, K., Keuning, W., Schropp, R.E.I., Kessels W.M.M. and Creatore, M. (2015). Identifying parasitic current pathways in CIGS solar cells by modelling dark J-V response. Progress in Photovoltaics: Research and Applications, 23(11), 1516-1525
Ponomarev, M., Verheijen, M.A., Keuning, W., Sanden, M.C.M. van de, and Creatore, M. (2012). Controlling the resistivity gradient in aluminum-doped zinc oxide grown by plasma-enhanced chemical vapor deposition. Journal of Applied Physics, 112(4):043708
Knoops, H.C.M., Loo, B.W.H. van de, Smit, S., Ponomarev, M., Weber, J.W., Sharma, K., Kessels, W.M.M. and Creatore, M. (2015). Optical modeling of plasma-deposited ZnO films : electron scattering at different length scales. Journal of Vacuum Science and Technology, A: Vacuum, Surfaces, and Films,33, 021509-1/13.
Di Giacomo, F., Zardetto, V., D'Epifanio, A., Pescetelli, S., Matteocci, F., Razza, S., Di Carlo, A., Licoccia, S., Kessels, W.M.M., Creatore, M. and Brown, T.M. (2015). Flexible perovskite photovoltaic modules and cells based on atomic layer deposited compact layers and UV-irradiated TiO2 scaffolds on plastic substrates. Advanced Energy Materials, 5(8):1401808
Zardetto, V., Di Giacomo, F., Mohammed, M.A., Lucarelli, G., Razza, S., D’Epifanio, A., Licoccia, S., Kessels, W.M.M., Di Carlo, A., Brown. T.M., and Creatore, M. (2015). Opportunities of atomic layer deposition for perovskite solar cells, ECS Transactions, 69(7):15.
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