Paper EM-ThP12
Electrical Characterization of the Reduced Effective Schottky Barrier Height by Nanoscale Ge bi-layer of CZTSe Solar Cells

Thursday, October 25, 2018, 6:00 pm, Room Hall B

In the past ten years, there have been constant attempts to develop high efficient thin film solar cells, which are cost-effective, environmentally benign, and reliable. The most strongest candidate of emerging alternatives is Cu₂ZnSn(S,Se)₄ (or kesterite) solar cells, herein CZT(S,Se). With abundant elements in earth’s crust, CZT(S,Se) shows high absorptions coefficient (>10⁴ cm^-1) and a tunable direct band gap energy, ranging from 1 to 1.4 eV, which makes it an ideal platform for future renewable energy devices . However, the efficiency improvement and understanding of emerging CZT(S,Se) is still in early stage compared to the counterparts such as Cu(In,Ga)Se₂ (CIGS) and CdTe. In recent progress, Germanium (Ge) incorporation into CZT(S,Se) solar cells has received extensive attention to deepen the understanding of this types of devices as Ge-alloyed CZT(S,Se) solar cells have demonstrated improvements in device performance. However, several challenges still remain such as a large Voc-deficit, sever heterojunction interface recombination, and a Schottky-type back contact barrier.

The presence of Schottky barrier near back contact limits the hole movement which influences the current-voltage characteristics and deteriorates the Voc-deficit as well. To investigate the impact of this back contact barrier height, we fabricated and characterized a set of CZTSe solar cells by utilizing DC magnetron sputtering by applying ultra thin Ge nanolayers. We investigated the back contact interface between CZTSe/MoSe₂ and Mo metal contact in an effort to improve a back contact barrier. By incorporating nanoscale Ge bi-layers below and below the absorber, a barrier height is considerably improved. The results indicate that nanoscale Ge bi-layers improves the back contact barrier height by 27 % as compared to CZTSe:Ge monolayer devices (see a supporting document device A). The back contact improvement is possibly caused underlying Ge nanolayer (<2.5 nm) between the absorber and Mo metal contact. The improvement of the efficiency loss caused by the series resistance component is reduced by 50%, which is attributed to the improved Schottky-type back contact barrier. This allows the improved efficiency up to 8.3% by incorporating nanoscale CZTSe: Ge bi-layers.