High-Efficiency (LixCu1-x)(2)ZnSn(S,Se)(4) Kesterite Solar Cells with Lithium Alloying

The performance-boosting effect of alkali treatments is well known for chalcogenide thin-film solar cells based on Cu(In,Ga)Se-2 (CIGS) and Cu2ZnSn(S,Se)(4) (CZTSSe-kesterite) absorbers. In contrast to heavier alkali elements, lithium is expected to alloy with the kesterite phase leading to the solid solution (LixCu1-x)(2)ZnSn(S,Se)(4) (LCZTSSe), which offers a way of tuning the semiconductor bandgap by changing the ratio Li/(Li+Cu). Here is presented an experimental series of solution-processed LCZTSSe with lithium fraction Li/(Li+Cu) ranging from x = 0 to 0.12 in the selenized absorber as measured by means of inductively coupled plasma mass spectrometry. The proportional increase in lattice parameter a and bandgap from 1.05 to 1.18 eV confirms the lithium alloying in the kesterite phase. Increase in grain size is observed for x up to 0.07, whereas a higher lithium fraction leads to a porous absorber morphology due to an inhomogeneous distribution of Li-containing compounds in the kesterite layer. An increase of the photoluminescence quantum yield is observed as the Li fraction increases in the absorber layer. A champion device exhibits a remarkable efficiency of 11.6% (12.2% active area) for x = 0.06, close to the world record value of 12.6% demonstrating the effectiveness of lithium alloying.