Chemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu 2 ZnSnS 4 . Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor-acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of Zn Cu + antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are 'potential wells' for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes 'trapped' within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu 2 ZnSnS 4 crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices.

Cathodoluminescence is used to measure the recombination velocity of the heterointerfaces between Cu2ZnSnS4 (CZTS) and CuxSnySz, SnS secondary phases precipitated along the grain boundaries as well as ZnS precipitated within the CZTS grain interiors. The CZTS/CuxSnySz and CZTS/ZnS heterointerfaces had recombination velocities smaller than the bulk carrier diffusion velocity while the opposite is true for the CZTS/SnS heterointerface. Secondary phases having crystal structures compatible with CZTS (e.g., ZnS, Cu2SnS3) are likely to form heterointerfaces with small misfit strain and hence low interfacial recombination velocity. The precipitation of such secondary phases along grain boundaries in CZTS provides a novel mechanism for grain boundary passivation. However, it is not known if grain boundary passivating secondary phases would necessarily increase the overall photovoltaic device efficiency since other factors, such as the band gap of the secondary phase compared to the Shockley-Queisser ideal value and the nature of the heterointerface between CZTS (i.e., type-I vs type-II), also affect device operation and must therefore be taken into consideration.

The growth of Cu2ZnSnS4 (CZTS) polycrystals from solid state reaction over a range of compositions, including the regions which produce the highest efficiency photovoltaic devices, is reported. X-ray measurements confirm the growth of crystalline CZTS. Temperature and intensity dependent photoluminescence (PL) measurements show an increase in the energy of the main CZTS luminescence peak with both increasing laser power and increasing temperature. Analysis of the PL peak positions and intensity behavior demonstrates that the results are consistent with the model of fluctuating potentials. This confirms that the polycrystals are heavily doped with the presence of a large concentration of intrinsic defects. The behavior of the main luminescence feature is shown to be qualitatively similar over a broad range of compositions although the nature and amount of secondary phases vary significantly. The implications for thin-film photovoltaic devices are discussed.

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