Abstract
A detailed analysis of the structure of Cu2ZnSnS4 (CZTS) nanocrystals synthesized by hot-injection in the presence of oleylamine is provided employing high resolution TEM, selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED). The nanostructures were investigated as-grown and after vacuum thermal treatment at 550°C. As-grown materials consisted of polycrystalline particles with an average size of 7±3nm, which grow an average size of 53±13nm after the vacuum annealing step. This thermal treatment allows investigating the initial stages of high quality film growth required in photovoltaic devices. Sets of SAED and CBED patterns, where individual crystals after annealing were viewed down different prominent zone axes, enabled us to reveal the presence of weak reflections due to cation ordering, and confirm a tetragonal unit cell consistent with either the kesterite or stannite structure. We demonstrate how these approaches enable to distinguish CZTS from secondary phases such as ZnS. Structure defects of partially annealed CZTS crystals were also investigated using bright and dark field images taken in 2-beam diffraction conditions as well as by high resolution lattice imaging. The material exhibited dislocations, along with lamellar twins and stacking faults characterized by local hexagonal structure on {112} planes. High resolution TEM images showed preferential growth on {112} planes during vacuum annealing, which is consistent with X-ray diffraction patterns. These studies provide key information on nanoscale crystal defects which may have important consequence on the performance of CZTS photovoltaic devices.
Highlights
Cu2ZnSnS4 (CZTS) is key material towards the development of scalable thin-film photovoltaic technology considering the abundancy and low-cost of the constituent elements as well as its direct bandgap (∼1.4 eV) and high optical absorption coefficient (∼104 cm−1) [1,2]
Kesterite type CZTS is derived from Chalcopyrite type CuInS2 (CIS) by substituting of In atoms with Zn and Sn while stannite and primitive-mixed CuAu (PMCA) are derived from a CuAu-like structure
The only difference between kesterite and stannite, which are tetragonal with c∼2a (Fig. 1b), is the distribution of Cu and Zn within the unit cell while Sn atoms occupy the same position in both structures
Summary
Cu2ZnSnS4 (CZTS) is key material towards the development of scalable thin-film photovoltaic technology considering the abundancy and low-cost of the constituent elements as well as its direct bandgap (∼1.4 eV) and high optical absorption coefficient (∼104 cm−1) [1,2]. Attention has been drawn towards solution processible methods, such as bulk inorganic and nanoparticle synthesis which have produced devices with efficiencies exceeding 9% [3,4] Nanoparticle precursors, such as those obtained by hot-injection methods, exhibit the advantage of controlling parameters such as phase, composition and particle size distribution prior to the deposition of CZTS ink [5,6]. The only difference between kesterite and stannite, which are tetragonal with c∼2a (Fig. 1b), is the distribution of Cu and Zn within the unit cell while Sn atoms occupy the same position in both structures. Based on density functional theory calculations, the kesterite type structure is the more stable energy structure with an energy difference of only ∼3 meV/atom between stannite and kesterite [7]
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