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To improve the quality of CuInSe2 (CIS) thin films fabricated by a non-vacuum, nanoparticle-based approach, in this study, two categories of nanoparticles, Cu (InGa) Se2 (CIGS) nanoparticles and copper selenide (Cu–Se) with indium selenide (In–Se) nanoparticles are investigated. It is found that the Cu–Se with In–Se nanoparticles show a higher crystallization velocity than CIGS nanoparticles. The films obtained from Cu–Se with In–Se nanoparticles exhibit higher crystallinity with a larger grain size. Thiourea is applied as a sintering additive during the selenization process. It is clarified that the addition of thiourea is very effective for grain growth and the fabrication of a dense CIS layer. The cell performance is measured under Air Mass 1.5 irradiation. The efficiency of the solar cell, fabricated using Cu–Se, In–Se nanoparticles with thiourea, is 2.15%, higher than that of the solar cell fabricated using CIGS nanoparticles, which is 0.28%.

The equilibrium shape of particles embedded in a matrix is usually dominated by the elastic strain energy at large size while the interface energy becomes important at small size. Therefore, a systematic change in shape with increasing particle size is attributed to the increasing influence of elastic strain energy. Using conventional electron microscopy, we conduct experimental observations of coherent Co-Cr particles in a Cu matrix and demonstrate the evolution of the equilibrium shape of particles as a function of size ranging from 5nm to 50nm. Then the equilibrium shape change from sphere to cuboid is explained by considering anisotropic elasticity of the particles. Typical distribution of Co-Cr particles are shown in FIG. 1a. It is found that the Co-Cr particles have intermediate shapes between sphere and cuboid. The observed intermediate shapes can be described as superspheres schematically illustrated in FIG . 1b where the x i axes are parallel to the 〈100〉 direction of the particles. We then defined the shape parameter η as a ratio d/a in FIG. 1b and the characteristic radius r as a radius of an equivalent sphere of the same volume, respectively. A plot of the shape parameter η versus the characteristic radius r for more than 700 particles is shown in FIG. 2. It is apparent that the shape of particles changes from sphere to cuboid with increasing size.Considering the sum of the elastic strain energy and the interface energy associated with the superspherical particles, the explanation of the equilibrium shape transition from sphere to cuboid is attempted. To accomplish this object, the simplified energy analysis proposed by Onaka et al. [1,2] is adopted. Applying the energy minimizing condition, the parameter η representing the equilibrium shape can be formulated byas a linear function of the characteristic radius r. Here, ε is the misfit strain, γ I the isotropic interface energy, and the dimensionless parameter k is defined as the function of the elastic stiffness constants C 11 , C 12 and C 44 . The least-square fit of the experimental data is shown as a solid line in FIG. 2. Using this result and substituting the appropriate values k = 4.67x10 -2 , C 44 = 75.4GPa, and ε = 0.018 (experimentally obtained), we can evaluate the Co-Cr/Cu interface energy as 290mJ/m 2 . This value is about 1.5 times greater than the interface energy of Co/Cu, and is reasonably understood as the effect of Cr addition into the Co particles. It is concluded that the simplified energy analysis is well applicable to explain the experimental results on the systematic shape change of the Co-Cr particles in Cu.

A nanoparticle-coating approach is applied to prepare In2S3 buffer layers in Cu(InGa)Se2 solar cells for the first time. In this research, the electric and optical characteristics of the In2S3 films at different annealing temperatures are investigated. It is found that 275 °C is the optimum annealing temperature in our case and the highest efficiency of 10.0% solar cell is achieved at this temperature. The highest efficiency solar cell with the In2S3 buffer layer exhibits a higher light absorption in the region of 350–500 nm than the reference cell with a CdS layer.

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