Back Shallow Ge Gradient Enhanced Carrier Separation for CZTSe Solar Cells through a Coselenization Process

Abstract

Given the prominent success of the Ga gradient in CuIn 1−x Ga x Se 2 (CIGSe) solar cells, Ge gradient implementation is a promising way to boost Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. However, Ge-graded CZTSSe solar cells only possess a low efficiency of 9.2%, far from that of Geincorporated CZTSSe without a gradient (12.3%). Herein, we demonstrated a shallow Ge gradient CZTSe solar cell with an improved efficiency over 10%. The Ge gradient was achieved through a GeSe 2 −Se coselenization process, where GeSe 2 acts as a low-temperature fluxing agent to assist crystallization and induce Ge transport toward the back interface. The relieved band tails and improved junction quality, leading to a better carrier separation, were found to take a primary responsibility for device improvement. These results highlight a remarkable breakthrough for Ge-graded CZTSe solar cells and offer a promising way to develop Ge-involved solar cells.

Ge-alloyed kesterite thin-film solar cells: previous investigations and current status – a comprehensive review

“…Other groups reported similar observations, 30,72,83,95,96,109,112 sometimes with even higher solar cell performance possibly explained to some extent by the alternative Ge doping or absorber deposition approaches used, i.e. either solution-based 30,95,96,112 and/or relying on Ge-atmosphere annealing 72,109 which respectively mitigate secondary phase formation and Ge-loss, 104 as discussed in Section 2. It is quite remarkable to note that the natural Sn–Ge segregation appearing in these materials may be either an asset or a disadvantage.…”

“…Besides that, slightly relieved band tails were also observed after Ge doping. 30,72,109 Tackling most of the loss mechanisms discussed herein led to the all-time record efficiency of 13.1% for Ge-alloyed kesterite solar cells, 30 based on a CZTSSe absorber deposited on a GeO 2 bottom nanolayer, both solution-processed (Fig. 7(e)).…”

“…The former focused on Ge doping ( x < 0.02) and highlighted the widely seen bi-layered absorber with a Ge gradient towards the back promoting alkali diffusion, no secondary phases and reduced band tails. 72 The latter discovered two dependencies on the value of x ranging from 0 to 1: 73 first, an enlarging spread of the Sn–Se binary phase in both the bulk and at the surface, the evaporation of which may likely explain the voids observed in between the Ge-enlarged grains (Fig. 5(b)).…”

“…Many references report investigations about CZTGSe absorbers deposited by two-step processes under vacuum, most of them attributed to one research group from IREC and close collaborations, 60–70 and a few other investigations carried out by other teams. 71–74…”

Efficient Cd-Free Flexible CZTSSe Solar Cells with Quality Interfaces by Using the Zn1–xSnxO Buffer Layer

“…Environmentally friendly and portable solar cells with high efficiency and low cost have been the research focus for scientists . Flexible CZTSSe solar cells have attracted great attention in the flexible photovoltaic market because of their light weight, bending resistance, and abundant sources of constituent elements. − At present, the flexible CZTSSe solar cell usually adopts a CZTSSe/CdS heterojunction structure. − However, the toxicity of Cd and the toxic waste liquid produced by the deposition process are extremely harmful to the environment.…”