Novel Cu2ZnSnS4 nanocrystals: a promising approach for high-performance and low-cost Cu2ZnSnS4 photovoltaics

Abstract

Cu2ZnSnS4 (CZTS) nanocrystals can be prepared as the ink for fabrication of CZTS thin film and corresponding solar cells. Nanocrystal ink allows for the preparation of low-cost thin film semiconductors at low temperatures and at high production rates and yields. However, great difficulties in achieving nearly micron-sized large grains from nano-sized crystals using sulfurization treatment challenge the use of CZTS nanocrystals for pure-sulfide CZTS solar cells. This thesis aims at overcoming this grain growth problem, achieving high quality CZTS absorber layers for solar cells, and revealing the grain growth mechanism during sulfurization.

In this work, CZTS large grains are successfully obtained by an efficient phase-transition-driven grain growth strategy. Metastable wurtzite CZTS nanocrystals are chose and synthesized to make precursor coatings instead of stable kesterite CZTS nanocrystals. Three necessary factors containing precursor’s composition, sulfurization annealing condition, and incorporation of extrinsic dopant are firstly exploited. The annealed CZTS absorber layer has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. The preliminary photovoltaic cells demonstrate 4.83% efficiency without anti-reflection coatings.

To enhance phase and compositional controllability as well as reduce the thickness of fine-grained layers, a solution-based-doping method is devised for tuning Na content and SnS powder in introduced into the sulfurization treatment. The results demonstrate that Na amount and the use of SnS powder provide leverage with which to improve the microstructure and compositional distribution of the final absorber. By employing this leverage to optimize CZTS absorbers prepared from wurtzite nanocrystals, the thickness of the notorious fine-grained layer is significantly reduced and the energy conversion efficiency of solar cells is increased to 6.0% in the absence of an antireflection coating layer due to the improvement of absorbers’ quality.

By classifying samples in terms of temperature and time series, we systematically study the formation pathway from nanometer-sized crystals in a wurtzite phase to micrometer-sized grains in a kesterite phase. An in-depth understanding of the spatial grain growth and composition and microstructure evolution during the transformation course is clearly revealed, paving the way of delivering better device performance by this solution-based nanocrystal method in future.