The need to resolve the energy shortage and environmental pollution leads to improving and exploiting thin films for photovoltaic (PV) applications. The current promising PV technologies are CdTe and CuInGaSe2 (CIGS), which have achieved high efficiencies and already reached the commercialisation stage. However, the scarcity of elements like indium and tellurium has limited the deployment of these technologies on a terawatt scale. A search for alternative materials has become crucial to replace and overcome current technology limitations. Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) has attracted a lot of attention as a potential alternative light-absorbing material that consists of abundant elements, non-toxic and inexpensive. Furthermore, CZTS has a direct band gap of 1.4-1.6 eV and high light absorption coefficient of 104 cm-1, which favourably matches the solar spectrum. CZTS material's have reached efficiencies up to 12.6%, as prepared by a hydrazine-based solution method. The danger of this reaction due to hydrazine auto-ignition temperature of 24°C and flash point of 38 °C makes this method unreliable for large-scale production. However, the efficiency gap between CZTS and CIGS is still large, with a conversion efficiency of around >22% for CIGS solar cells. CZTS shows lower open-circuit voltage, Voc, lower short-circuit current density, Jsc, and smaller minority-carrier lifetimes. These deficiencies could be related to the formation of defects in nanocrystals that cause trapping or recombination of carriers. This thesis aims to study the structure and defects in CZTS nanocrystals using transmission electron microscopy (TEM)-based techniques. The hot-injection method was used to synthesize CZTS due to the ability to produce large-scale and high-quality nanocrystals. In addition, CZTS nanoparticles growth was investigated after deposition on Molybdenum on glass substrate, providing annealing conditions that significantly improved grain growth to be suitable for PV applications. A detailed analysis of the CZTS crystal structure was undertaken, confirming a kesterite (tetragonal) structure of annealed CZTS nanocrystals. Furthermore, a fingerprint map for CZTS was obtained using selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED). These techniques provide an approach enabling to distinguish CZTS from secondary phases such as ZnS that have a negative impact on the solar cell performance. Bright-field and dark-field were used to visualize the extended defects exhibited in nanocrystals. Nanocrystals showed that growth of defects in the form of lamellar twinning and dislocations occurred in the {112} planes, which are the preferential growth direction of annealed CZTS. The presence of these defects results in a local change to hexagonal phases in lamellar twinning boundaries. Moreover, high-angle annular dark field (HAADF) imaging was used to obtain high-resolution images of CZTS nanocrystals at a sub-O.l nm resolution that visualized the CZTS crystal unit cell, showing for the first time all atoms of Cu, Zn, Sn, and S are presented. These images allow to investigate the formation of antisite defects that have a significant impact on CZTS performance. These defects formed antisite domain boundaries that lie in different planes, causing disorder on the Cu, Zn, or Sn sites with some of the boundaries affecting local changes in stoichiometry. These studies can provide key information on the defects occurring at the atomic scale that have important consequences on CZTS devices' performance. The growth of nanocrystals 'on molybdenum substrates was also investigated to improve the grain size and the electronic properties of the material. An annealing condition is established that achieved a significant improvement in nanocrystals grain size from the initial average size of as-grown nanoparticles of ~7-12 nm up to 1 μm grain size. Annealing under hydrogen atmosphere with additional to SnS and S in a powder form was used to improve the nanoparticles growth. In addition to present a comparison of nanoparticles growth under other annealing conditions including nitrogen atmospheres and additional elements and binaries such as Na2S, SnS and S. The presence of hydrogen demonstrated an annealing atmosphere produces a significant improvement in nanocrystals growth compared with other annealing atmospheres. A promising efficiency is achieved for the CZTS solar cell of (0.8 %), Voc (253 mV), Jsc (6.84 mA/cm2), fill factor (FF) (45.9%), Rs (44.9 Ώ cm2), and Rsh (169.7 Ώ cm2) with a cell configuration (glass/Molybdenum/CZTS/CdS/intrinsic-ZnO)/Aluminum doped ZnO (AZO)/NiAl).