Effects of Back Contact Instability on Cu₂ZnSnS₄ Devices and Processes

Abstract

Cu 2 ZnSnS 4 (CZTS) is a promising material for thin film solar cells based on sustainable resources. This paper explores some consequences of the chemical instability between CZTS and the standard Mo "back contact" layer used in the solar cell. Chemical passivation of the back contact interface using titanium nitride (TiN) diffusion barriers, combined with variations in the CZTS annealing process, enables us to isolate the effects of back contact chemistry on the electrical properties of the CZTS layer that result from the synthesis, as determined by measurements on completed solar cells. It is found that instability in the back contact is responsible for large current losses in the finished solar cell, which can be distinguished from other losses that arise from instabilities in the surface of the CZTS layer during annealing. The TiN-passivated back contact is an effective barrier to sulfur atoms and therefore prevents reactions between CZTS and Mo. However, it also results in a high series resistance and thus a reduced fill factor in the solar cell. The need for high chalcogen pressure during CZTS annealing can be linked to suppression of the back contact reactions and could potentially be avoided if better inert back contacts were to be developed.

Growth of Cu2ZnSnS4 absorber layer on flexible metallic substrates for thin film solar cell applications

“…In the case of Mo back contact, high temperature sulfurization process leads to the decomposition of the CZTS absorber layer, resulting in MoS 2 phase formation [22,23], which restricts the hole transport from CZTS to Mo layer [24]. The interfacial MoS 2 layer may affect open circuit voltage (V oc ) and change the band alignment between the CZTS absorber layer and Mo substrate.…”

A low-temperature order-disorder transition in Cu2ZnSnS4 thin films

“…8 Next, we analysed CZTS thin films made by reactive sputtering of CuSn alloy and Zn in H 2 S, followed by annealing in sulfur vapour at 833 K, as described elsewhere. 20 The sample composition was Zn rich (Cu/Sn ¼ 1.9, Zn/Sn ¼ 1.2), comparable to that used for efficient solar cells. 1 The films are highly uniform and Raman spectra recorded at different positions are reproducible.…”

The synthesis and characterization of Cu2ZnSnS4 thin films from melt reactions using xanthate precursors

“…The photoelectrical performance of CZTS is highly dependent on good stoichiometric control, and such evaporation can make it difficult to control the composition of the target phase [18][19][20][21][22]. The second issue is the stability of the Mo electrode that the CZTS is often deposited on; this can react with sulfur to form a MoS 2 layer between the electrode and the CZTS, resulting in a dramatic decrease in performance of the photovoltaic device [23,24]. These two problems indicate a requirement for a new synthetic route to CZTS that avoids higher temperatures.…”