M. Kaelin

Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se2 thin films

Abstract Cu(In,Ga)Se 2 (CIGS) layers have been deposited using the three-stage process on Mo coated soda-lime glass substrates with an alkali diffusion barrier. Sodium has been incorporated in such layers by NaF coevaporation, by deposition of NaF precursor layers and by diffusion of Na from a glass substrate without barrier. Scanning electron microscope pictures showed a reduction in grain size of the CIGS films when Na was available during growth. This effect depends on the Na concentration of the finished film, but not on the incorporation method. Furthermore, the [Ga]/[In] concentration ratio is decreased in Na containing CIGS layers at depths at around one-quarter of the absorber thickness. X-ray diffraction patterns indicate a preferred orientation change only in the case of NaF precursors, which is attributed to CIGS growth on a modified surface and/or to the high Na availability during the initial stages of film growth. Solar cells with an efficiency exceeding 15% have been processed from such absorber layers containing Na.

Electrosprayed and selenized Cu/In metal particle films

Abstract Nano-particulate copper and indium metal layers of 1–2 μm have been deposited by non-vacuum techniques such as doctor blade, screen printing and electrospray using alcoholic suspension pastes. Electrospray showed a high efficiency of material usage and yielded the most uniform morphology. The metal precursor layers were subjected to a thermal treatment (500–600 °C) in selenium vapor to convert the porous metal layers into CuInSe 2 compound layers. The chemical conversion, investigated by X-ray diffraction, showed the presence of the In 2 O 3 impurity phase in the precursor as well as in the selenized layers.

Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells

Low substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al 2 O 3 ), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm 2 area) has been achieved.

Characterization of CdS/CdTe Solar Cells Fabricated by Different Processes

This paper analyses the photovoltaic parameters of the most promising CdS/CdTe solar cells for large application prepared by close space sublimation (CSS) and high vacuum evaporation (HVE). CdS/CdTe solar cells that have an efficiency of~10 % have been studied by current-voltage, capacitance-voltage and quantum efficiency measurements. The current-voltage characteristics show that the addition of small amounts of Cu or Te to back contacts by thermal evaporation improves contact properties by p +-d oping the CdTe surface and creating a pseudo-ohmic contact. The cell deposited by CSS seriously suffers in FF compared to the cells prepared by HVE. For both types of the cells above mentioned measurements reveal that the efficiency of CdS/CdTe solar cells fabricated by CSS is limited by a light-dependent shunt resistance and by a high series resistance, but concerning of the cells fabricated by HVE - by the formation of a thick layer of CdS 1-x Te x at the interface.

Incorporation of copper into indium gallium selenide layers from solution

A chemical method for the incorporation of copper into indium gallium selenide (IGS) layers has been developed. The resulting copper-containing precursor layers have been annealed in the presence of selenium vapour with the goal of forming Cu(In, Ga)Se2 (CIGS) layers. It is found that copper ions in solution are incorporated into IGS layers during immersion, resulting in the formation of a precursor layer containing both copper selenides and IGS. When aqueous solutions are used for this process, corrosion of the molybdenum back contact occurs by reduction of copper ions in the solution. Use of an ethylene glycol solution prevents corrosion of the Mo and allows higher process temperatures, corresponding to higher reaction rates. During annealing, the precursor layers are converted into CIGS and the morphology of these layers is strongly affected by the availability of selenium whilst the substrate temperature is ramped up.