Interface modification by ion implantation and optical characterization of high-efficiency Cu(In,Ga)Se2 solar cells
Three different issues are discussed in this thesis, which aim for a deeper understanding of Cu(In,Ga)Se2 material properties and solar cell functionality: (1) Near-surface ion implantation in Cu(In,Ga)Se2 absorber layers is studied as a method for the fabrication of buffer-free solar cells. Simulations show the beneficial effect of an n-type surface layer (buried junction). To obtain an inversion by means of ion implantation, an annealing procedure is developed that avoids annealing-induced degradation of the cell, minimizes the diffusion of the ions, and recovers the implantation damage. Buffer-free solar cells made from implanted absorbers show strongly improved diode characteristics comparable to the ones of cells with a CdS buffer and a maximum efficiency of 10.2% (11.2% active area). (2) A model is developed for the application of luminescence techniques on band-gap graded semiconductor thin-films. Several DA-emissions are detected in the cathodoluminescence (CL) and photoluminescence (PL) spectra of Ga-graded in Cu(In,Ga)Se2 and related to the local Ga-contents in the layer. The depth-distribution of the luminescence signal is correlated to the band-gap profile. Drift of excited charge carriers towards the band-gap minimum is caused by the grading-induced quasi-electric field, which must be considered when explaining the spectra obtained in plan-view measurements. Monochromatic CL imaging on cross-sections is capable to determine the minority carrier mobility. (3) An increase of the substrate temperature from the standard value of 530°C to 610°C, which was possible only using specially developed Na-containing high-temperature resistant glass, is shown to strongly influence the formation of the Cu(In,Ga)Se2 absorber layers. The higher temperature leads to an increased vertical and lateral homogeneity, an increase of the grain size, a decrease of the losses at the p-n-junction and, as a consequence, a considerable increase of the solar cell efficiency up to 19.4%.