Influence of Copper Composition on Cu2BaSn(S,Se)4Solution-Deposited Films and Photovoltaic Devices with over 5% Efficiency
Cu2BaSn(S,Se)4is currently in the spotlight for prospective environmentally friendly, stable, thin-film solar cell application, with demonstrated device power conversion efficiency (PCE) exceeding 5% for vacuum-deposited absorbers. As suggested by first-principles calculations, experimental studies involving related Cu2ZnSn(S,Se)4and Cu(In,Ga)(S,Se)2absorbers prove that the detailed chemical composition typically plays a sensitive role in altering defects and electronic properties of these complicated compound semiconductors. Herein, the copper composition of Cu2BaSn(S,Se)4has been systematically modified, employing a solution-based deposition approach, to provide a more complete picture of the phase stability and optoelectronic property sensitivity for this material. X-ray diffraction and scanning electron microscopy show that phase purity is preserved over a film Cu content range of nominally 0.94 ≤ [Cu]/[Ba + Sn] ≤ 1.01. Terahertz spectroscopy and Hall effect measurements reveal that the majority carrier hole density of ∼1013cm-3and mobility (∼5 cm2/V s), as well as the minority carrier lifetime (a bulk lifetime of 180 ps and a surface recombination velocity >106cm/s), are nominally independent of Cu content. The champion PCEs exceed 4.7% for all copper compositions in the phase-pure region, with a record value of 5.1%, similar to the reported values for record vacuum-deposited devices. These results suggest that Cu2BaSn(S,Se)4films and solar cells (at the current performance level) may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore may provide a more stable material platform to prepare thin-film solar cells.