Absorber and interfaces of kesterite solar cells

Abstract

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.