Having an absorber layer thickness below 1 micrometre for a regular copper indium gallium di-selenide (CIGS) solar cell reduces the pathways for electron collection which lead to reduced recombination in the bulk absorber layer. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly, mainly due to incomplete light absorption and high Mo/CIGS rear-surface recombination . The aim of this research is to implement some innovative rear surface modifications for a <500 nm thick CIGS absorber layer to reduce these effects: a passivation layer to reduce the back-surface recombination and point contact openings using nano-particles (NPs) (CdS, Ag, etc.) to create electrical contacts, with some NPs also improving the optical confinement. For example, one approach is to use a layer of thermally evaporated and uniformly deposited Ag, which is then annealed to create Ag NPs with typical diameters of 300 to 450 nm. An Al2O3 layer is used to passivate the Mo/CIGS rear surface which also prevents the NPs from completely dissolving into the absorber during the absorber deposition. The implementation of all these rear-surface modifications and their impact on the electrical performance of the CIGS solar cell will be discussed and analyzed in this paper. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027.

In the previous decade, Cu2ZnSn(S,Se)4 (kesterite) compounds have attracted the interest of several research groups across the world. Their efforts have improved this technology from 5 % power conversion efficiency in 2004 to above 13 % in 2017 (communicated but not published) [1]. This thesis begins with an assessment of the current status of this technology which formed the motivation for the reported experiments. In particular, the open-circuit voltage (VOC) has been identified as one of the biggest challenges of this technology. To address this issue, a simple, non-vacuum, spin coating technique was developed for the introduction of alkali ions (Li+, K+, Rb+) into a pure selenide Cu2ZnSnSe4 absorber. The highest VOC achieved with this method was 454 mV, which is approximately 50 mV above the baseline level and on par with the highest known values for Cu2ZnSnSe4 solar cells [2]. The presence of Rb+ and Li+ ions in particular, was observed to reduce recombination in the bulk material and the quantity of shallow defects within the bandgap. Bandgap variations and electrostatic potential fluctuations in conduction and valence bands were investigated as aspects likely affected by alkali ions. In order to exploit the potential for bandgap engineering of this material, Cu2ZnGeSe4 based solar cells were developed with experiments for the selection of appropriate substrates, absorber thickness and chemical etching of unwanted compounds. The highest efficiency achieved was 5.9 %, which is close to the state of the art for this material at the time of the writing of this thesis. Experiments were also conducted to modify the solar cell architecture by introducing a Al2O3 passivation layer deposited by atomic layer deposition. A maximum short-circuit current density of 36 mA/cm2 was achieved but important questions were raised about the dominance of the bulk material in this improvement.