The influence of Na on the growth of Cu(In,Ga)Se2 layers for thin film solar cells

Sammanfattning: This thesis consider aspects concerning the transport, distribution and incorporation of Na in Cu(In,Ga)Se2 absorber layers for thin film solar cells. The influence of Na on the growth of Cu(In,Ga)Se2 layers has been studied, as well as the influence on the device performance. In particular, the relationship between the oxygen content in the Mo back contact and the Na transport properties has been investigated.Na is supplied by out diffusion from the soda lime glass substrate through the Mo back contact, or by deposition of an Na precursor onto the Mo film prior to the Cu(In,Ga)Se2 growth. All investigated Na precursor compounds, Na2Se, Na2S and NaF, have resulted in high quality devices. The effect of Na on the microstructure and crystal quality of Cu(In,Ga)Se2 layers has been studied by Scanning Electron Microscopy and X-Ray Diffraction. The Na transport properties have been investigated by Secondary Ion Mass Spectrometry.It has been found that the presence of MO oxide phases in the MO back contact is necessary for the Na transport through the Mo film if Na is supplied from the soda lime glass. Na has beneficial effects on both the structural properties of the Cu(In,Ga)Se2 layer, yielding large grains and a flat surface, and on the electrical properties. The main influence of Na is a substantial increase in device efficiency compared to devices fabricated without Na. In order to obtain these beneficial effects a minimal quantity of Na is necessary, but the system is tolerant to excess Na. Both out-diffusion from the soda lime glass as well as Na precursors can supply the required Na concentrations.