Controllable Synthesis And Optoelectronic Properties Of Water-Soluble Cu₂ZnSn(S_xSe_1-x)₄ Nanocrystal Inks

Cu2ZnSnS4 (CZTS) is a promising p-type photovoltaic material with advantages of suitable direct energy bandgap (～1.5 eV), high absorption coefficient (～104 cm-1), abundance of its constituting elements, and low toxicity. Se is typically incorporated into CZTS to form Cu2ZnSn(SxSe1-x)4 (CZTSSe) for expanding its light absorption range. CZTSSe films are typically prepared by high-vacuum based deposition techniques (magnetron sputtering, multitage co-evaporation), which require expensive equipments. Solution-proceed CZTSSe nanocrystal thin-film is considered as a promising approach for manufacturing low-cost thin-film solar cells. However, CZTSSe nanocrystals are usually prepared by solvothermal methods using long chain organic ligands at higher temperature of 200-300 ℃. The long chain organic ligands capped on the surfaces of the CZTSSe nanocrystals are hard to be removed, which often leads to poor photovoltaic performance of the solar devices.In this work, we develop a room-temperature solution method to synthesize water-soluable CZTSSe nanocrystals by designing novel complex precursor solutions. No heating, no toxic solvent and no long chain organic ligands are required in this method, which has merits of low cost, environmental benefit, and large-scale synthesis. The composition, morphology, size, phase, and crystallinity of the CZTSSe nanocrystals were investigated by X-ray diffraction (XRD), scaning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer (EDS) and Raman analysises. The CZTSSe nanocrystals with tunable chalcogen composition (0.35≤ x≤ 1.0) are prepared and exhibit a tetragonal kesterite structure. The size of the CZTSSe nanocrystals is about 5 nm. The CZTSSe nanocrystals are water-soluable and can form stable nanocrystal ink in water, which benefits for low-cost photovoltaic devices. The bandgaps of the CZTS and CZTSSe (x=0.35) nanocrystals are estimated to be 1.96 and 1.83 eV, respectively, which are much larger than that (1.5 eV) of bulk CZTS. The absorption edge of the CZTSSe (x=0.35) nanocrystals expands with increasing annealing temperature (60-300 ℃), and shifts from 650 nm (CZTS) to 950 nm (CZTSSe, x=0.35), covering the whole visible light range. Distinct photoresponse performance of the CZTSSe nanocrystal film is revealed. Photovoltaic performance of the CZTSSe solar cells will be further studied.