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.

To make silicon thin film solar cells attractive to the market, their efficiencies should reach the efficiencies of the dominant crystalline silicon solar cell. Therefore the search for ways to improve the efficiency of silicon thin film solar cells continues. TCO front contact layers and back reflector layers play a significant role in the increase of thin film solar cell efficiencies. For the optical characterization of these TCO layers, commonly used methods are found to differ significantly in results regarding the extinction coefficient which makes their accuracy questionable.

This research consists of two main parts. For the first part, the commonly used optical characterization methods spectroscopic ellipsometry and spectrophotometry + data analysis in SCOUT are analyzed in accuracy and compared to newly introduced methods using spectrophotometry + data analysis in GenPro4 and photothermal deflection spectroscopy + data analysis in GenPro4 to build a guide on optical characterization of TCO materials. For the second part, the optical response determined using the software GenPro4 of a double junction silicon thin film solar cell for a novel bi-layer front contact design consisting of IOH and i-ZnO will be compared to the optical response for standard used AZO, and ITO single layers to find the best front contact design. Furthermore, the optical response of a double junction silicon thin film solar cell for a back reflector containing an i-ZnO layer on top of the silver back contact will be compared to the optical response for a back reflector containing an AZO layer on top of the silver back contact to find the best back reflector design.

From the results of the optical characterization methods, it is concluded that photothermal deflection spectroscopy + data analysis in GenPro4 is the most accurate method for determining the extinction coefficient of a TCO material. The results of the optical simulations for front contact TCO and back reflector TCO designs showed that the bi-layer can enhance the optical response of a double junction silicon thin film solar cell significantly compared to the AZO and ITO single layers. The i-ZnO TCO back reflector layer was found to induce less parasitic absorption and therefore a better optical response of the double junction silicon thin film solar cell.