Inhibiting the absorber/Mo-back contact decomposition reaction in Cu2ZnSnSe4 solar cells: the role of a ZnO intermediate nanolayer

Abstract

This work reports a process based on the use of an ultrathin (10 nm) ZnO intermediate layer for the improvement of the absorber/back contact interface region in Cu 2 ZnSnSe 4 (CZTSe) kesterite solar cells.Raman microprobe measurements performed directly on the substrate surface after mechanical removal of the absorber layer indicate the occurrence of a decomposition reaction of Cu 2 ZnSnSe 4 in contact with the Mo substrate. This leads to a significant degradation of the quality of the absorber/back contact interface, with the formation of a high density of voids. The presence of an intermediate ZnO layer on the Mo coated substrates inhibits the decomposition reaction, because it prevents interaction between the CZTSe and Mo layers during the annealing process. This leads to a significant improvement in the interface morphology as observed by detailed cross-section scanning electron microscopy. It also correlates with the observed increase of the device conversion efficiency from 2.5% up to 6.0%. The improvement in the optoelectronic characteristics of the cells could be related to a significant decrease of the device series resistance due to the formation of a smoother interface with low density of voids, resulting from the effective inhibition of the CZTSe decomposition reaction at the Mo back contact layer.

High efficiency bifacial Cu2ZnSnSe4 thin-film solar cells on transparent conducting oxide glass substrates

“…4(a) and 4(b) suggests a back contact problem, further supporting a decrease in V oc under bifacial illumination. To minimize the damage and loss of electrical conductivity in ITO, annealing temperatures should be kept below 500 • C. 11 At the same time, In and Sn diffusion in CZTSe can be mitigated by implementing a thin barrier layer such as Mo, 8 ZnO, 23 TiN, 27 and TiB 2 28 between ITO and the precursor. The optical properties and external quantum efficiency (EQE) curves of CZTSe on FTO and ITO were also measured (Figs.…”

“…To enhance the quality of the CZTSe/FTO interface in Fig. 1(c), either additional elements such as ZnO, 23 Ag, 24 and carbon 25 could be used or rapid thermal annealing 26 could be employed. CZTSe devices on ITO had efficiencies of 4.12% and 0.57% under front and rear illumination, respectively.…”

Influence of Amorphous Silicon Carbide Intermediate Layer in the Back-Contact Structure of Cu2ZnSnSe4Solar Cells

“…In this sense, different materials such as Ag [12][13], ZnO [10][11], TiB 2 [14], TiN [9], Carbon [15] and MoO 2 [16] were successfully tested leading to an enhancement of the solar cell electrical performance. The effects on the structural and electrical properties of the cells strongly depend on the nature of the material selected as intermediate layer.…”

“…and is expected to be quite resistant to selenium incorporation due to its compactness. The methodology of preparation and a deeper explanation of the Mo structure can be found elsewhere [10][16] [23]. Secondly, an intermediate layer of a-SiC was included on the top of the Mo tri-layer structure as shown in figure 1.…”

“…In order to overcome this decomposition reaction and to inhibit the formation of a thick MoS 2 layer, multiple studies based on the addition of intermediate layers between the absorber and the molybdenum back contact have been proposed [10][11] [12]. In this sense, different materials such as Ag [12][13], ZnO [10][11], TiB 2 [14], TiN [9], Carbon [15] and MoO 2 [16] were successfully tested leading to an enhancement of the solar cell electrical performance.…”