We propose that the carbon dimer defect CBCN in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 3.8 eV is close to the experimental value, and the deduced Huang-Rhys factor of S ≈ 2.0, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized π-type defects states, with a very short radiative lifetime of 1.7 nanoseconds, in very good accord with experiments.

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Cu(In,Ga)Se2-based solar cells often exhibit fill factor losses at low temperature, in particular after red illumination or reverse biasing. A narrow p+ layer between the absorber and buffer layers is commonly assumed to explain these effects. In this contribution, we analyze by means of analytical and numeric models the influence of the parameters of the p+ layer on current-voltage characteristics. Specifically, we find expressions linking the voltage at which the deterioration of the current-voltage curve begins with the acceptor density and the width of the p+ layer. Moreover, we derive formulas describing the height of the barrier in the conduction band caused by the p+ layer. Examples of the analysis of simulated current-voltage characteristics using our approach are presented. Based on the voltage at which the decrease of the current starts, it is possible to estimate the upper and lower limits of the density of the acceptors in the p+ layer. Furthermore, we analyze the relationship between the fill factor and the height of the barrier in the conduction band, which is determined by the density of acceptors in the p+ layer.

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The persistent increase of conductivity after illumination in CuInGaSe 2 thin films was investigated as a function of the temperature and light intensity at its creation. Experimentally observed dependences were compared with the results of calculations based on the Lany-Zunger (L-Z) model relating the phenomenon to configurational changes of V S e -V C u divacancy. The calculations showed that the description of all results within this model would require, in some cases, unrealistic values of cross section for electron capture by the center in donor configuration. We concluded that the model does not fully account for the observed phenomena, and that additional mechanisms, in particular to explain the temperature effect, are required.Index Terms-Cu(In, Ga)Se 2 (CIGS) thin-film solar cells, defects, photoconductivity.