Atomic layer deposition of metal oxides for photovoltaic applications

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Ryan, Louise P.
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University College Cork
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Organic-inorganic halide perovskite solar cells have gained a lot of traction in recent years due to their high power conversion efficiencies (PCE), >20%, tunable band gap and low fabrication costs. However, as the maximum theoretical efficiency outlined by the Shockley-Quiesser model for a single-junction perovskite solar cell is close to being reached, researchers have begun fabricating tandem solar cells to ensure the continued improvements in PCEs. Tandem solar cells require the stacking of two solar devices such as c-Si and perovskite solar cells, the PCEs of each device is additive thus exceeding the theoretical limit. Transferring from a single junction perovskite device to a tandem device comes with its own challenges. The sensitive nature of the perovskite material requires changes to be made in the fabrication process of the cell to ensure that all layers are deposited using a low-energy process at low temperatures to prevent the degradation of the underlying layers. The changes in the fabrication processes must also not affect the electrical, optical and morphological properties of the materials which in turn could reduce the PCE of the device. To date, indium-tin oxide (ITO) is the commercial standard for use as the transparent conducting oxide (TCO) in perovskite solar cells. Although ITO has low sheet resistance values (8-100 Ω/□) the material is deposited using sputtering methods and undergoes high temperature (>400⁰C) heat treatments. When incorporating this material into a tandem solar device such a high temperature process is not viable as it can affect the integrity of the device. As a result an alternative material of similar properties is required to replace ITO in tandem solar cells. This thesis focuses on the deposition of ZnO and doped ZnO by atomic layer deposition (ALD), a scalable industry relevant technology, to replace ITO in tandem solar cells. To begin, a fundamental study of ZnO and its doped counterparts were deposited at the optimum temperature, 200⁰C and characterized. Ti-doped ZnO (TZO) was shown to have the most potential for replacing ITO in a single-junction perovskite solar cell. The optimized TZO was then incorporated into a single-junction perovskite solar cell in the p-i-n configuration and compared to the state-of-the-art perovskite solar devices as proof of concept. TZO was then deposited at low temperatures by the same methodology to evaluate its potential for incorporation into a tandem solar device. Low temperature deposition of TZO resulted in significantly higher sheet resistance values compared to values of the material deposited at the optimum temperature. Although the material maintained good optical properties the electrical properties could not compete with those of commercial ITO. The hole transport layer (HTL) in perovskite solar cells has, to date, focused on organic thin films, however due to instabilities and parasitic optical absorptions there has been a drive for an inorganic alternative. In this thesis, as a proof of concept, vanadium oxide is investigated as a possible hole transport medium. ALD grown VOx was directly incorporated into a perovskite solar device in the p-i-n configuration that was demonstrated to generate a photocurrent. However, further work is required to develop this device and determine the viability of a vanadium based HTL.
Doped ZnO , Perovskite , Solar Cells , Atomic layer deposition , Metal oxides , Transparent conducting oxides , Zinc oxide
Ryan, L. P. 2020. Atomic layer deposition of metal oxides for photovoltaic applications. PhD Thesis, University College Cork.
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