The importance of back contact modification in Cu₂ZnSnSe₄ solar cells: The role of a thin MoO₂ layer

Highlights: • Several Mo configurations (mono-, bi- and tri- layers) tested in CZTSe solar cells. • Mo tri-layer avoids overselenization and effectively controls MoSe₂ thickness. • Thin MoO₂ prevents overselenization, increases CZTSe grain size and solar cell efficiency. • Efficiency improvement from 7.2% to 9.5% with large enhancement of V_OC, FF and R_SH. • One of the highest V_OC for CZTSe technology 459 mV is obtained using 20 nm of MoO₂. Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe) photovoltaic absorbers could be the earth-abundant and low toxicity replacement for the already commercialized CuIn_1−xGa_xSe₂ (CIGS) thin film technology. In order to make this possible, specific research efforts applied to the bulk, front and back interfaces must be performed with the aim of improving CZTSSe performance. In this paper the importance of back contact modification to obtain high efficiency Cu₂ZnSnSe₄ (CZTSe) solar cells and to increase a paramount and limiting parameter such as V_OC is highlighted. Several Mo configurations (monolayer, bi-layer and tri-layer) with different electrical and morphological properties are investigated in CZTSe solar cells. An optimum tri-layer configuration in order to minimize overselenization of the back contact during thermal annealing while keeping reasonable electrical features is defined. Additionally, a thin intermediate MoO₂ layer that results in a very effective barrier against selenization and innovative way to efficiently assist in the CZTSe absorber sintering is introduced. The use of this layer enhances grain growth and subsequently the efficiency of solar cells increases via major V_OC and FF improvement. An efficiency increase from 7.2% to 9.5% is obtained using a Mo tri-layer with a 20 nm intermediate MoO₂ layer.