Transparent conductive oxide bi-layer as front contact for multijunction thin film silicon photovoltaic cells
P.B. Kalpoe (TU Delft - Mechanical Engineering)
Federica Saitta – Mentor (TU Delft - Photovoltaic Materials and Devices)
Gianluca Limodio – Graduation committee member
Arno Hendrikus Marie Smets – Coach (TU Delft - Photovoltaic Materials and Devices)
AJ Böttger – Coach (TU Delft - Team Amarante Bottger)
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Abstract
The journey towards increasing thin film solar cell efficiency is a continuously ongoing one, where each layer in the cell adds its own contributions and limitations. The transparent conductive oxide (TCO) is the first layer to encounter incident light on these cells and therefore needs to fulfil the requirement of high transparency. Carriers generated in the absorber layers of a thin film solar cell are transported to a metal electrode through the TCO, laying a conductivity requirement as well. A trade-off exists between transparency and conductivity, where one cannot be enhanced without sacrificing the other.
Indium tin oxide (ITO) currently delivers the best trade off, thus is the most commonly applied TCO.
In this thesis study, candidate TCO materials were deposited and analysed in order to surpass the opto-electrical properties of ITO. In addition to depositing ITO, hydrogen doped indium oxide (IOH) and intrinsic zinc oxide (i-ZnO) thin films were deposited using RF magnetron sputtering. Substrate temperature, RF power, deposition time and H2O partial pressure (only for IOH) were the varied parameters during depositions. IOH was found to surpass ITO in terms of conductivity, while i-ZnO surpassed ITO in terms of transparency. The best performing IOH and i-ZnO samples with regard to their respective superior parameters were chosen to be combined.
A TCO bi-layer was constructed by stacking a i-ZnO layer on top of a IOH layer. The IOH layer ensures good lateral conductivity, while the i-ZnO layer secures minimized parasitic absorption in the near infrared region. After being subjected to post deposition annealing, the bi-layer displayed opto-electrical properties superior to that of the individual i-ZnO and IOH layers. The highest electron mobility achieved for the bi-layer was 103,70 cm^2 /Vs with a carrier density of 0,3*1020 carriers/cm^3. The working principle is the capping effect which i-ZnO has on IOH, keeping hydrogen contained within the bilayer during annealing. Further investigation will lead to additional information on the behaviour of hydrogen within the as deposited bi-layer in comparison to the annealed
one.