Strategies for back contact engineering in high-performance flexible kesterite solar cells†

Kesterite solar cells are pivotal in advancing flexible photovoltaic devices integrated into buildings and products. High-purity Mo foil is one of the most promising flexible substrates, thanks to its outstanding properties. However, the kesterite/Mo foil interface is extremely reactive and chemically unstable during the high-temperature selenization process required to obtain the kesterite crystalline phase, forming a thick MoSe₂ layer. The role of MoSe₂ in kesterite solar cells is still under discussion, as it can affect the charge extraction at the back contact and the kesterite grain growth. This work reports on the functionalisation of Mo foil to fabricate flexible kesterite solar cells based on Li-doped and Ag-alloyed Cu₂ZnSn(S,Se)₄ (Li-ACZTSSe) films grown using the molecular ink method. MoS₂, Al₂O₃, MoO₂, and MoO₃ were inserted between the precursor layer and the substrate to investigate their impact on the MoSe₂ thickness, the morphology and composition of the absorber, interface chemistry, carrier collection at the back contact, and the related photovoltaic parameters. It is demonstrated that MoO₃ as an interlayer significant enhances device performance by improving the absorber quality and back contact, achieving an efficiency of 11.2% with a 15 μm thick MoSe₂ layer. To the best of our knowledge, this is the first report demonstrating that an over-thick MoSe₂ layer is not significantly detrimental to the performance of flexible kesterite-based devices when the Li-ACZTSSe crystallinity and grain growth are improved and its decomposition at the back is prevented.