D. Rudmann

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.

Comparison of structural and electrical properties of Cu(In, Ga)Se2 for substrate and superstrate solar cells

Abstract Cu(In, Ga)Se 2 absorber layers were deposited on glass coated with a ZnO/ZnO:Al double layer for the fabrication of superstrate solar cells. Their photovoltaic performance and material properties were compared with cells in the substrate configuration which were grown on Mo-coated glass. Measurements of the capacitance–voltage characteristics were used to study the carrier density of the absorber material. Compared to substrate cells, superstrate cells revealed a low acceptor concentration. Profiling with secondary mass spectroscopy revealed a low concentration of Na, which is an effective acceptor for Cu(In, Ga)Se 2 . In contrast to the Mo layer in substrate cells, the ZnO/ZnO:Al bilayer acts as barrier against the diffusion of Na from the glass substrate. Furthermore, the carrier density analysis revealed a high concentration of trap states close to the interface in the absorber layer of superstrate solar cells. Light soaking saturates such trap states and increases the net carrier density, which improves the open circuit voltage and the efficiency of the solar cells.

Towards the development of flexible CIGS solar cells on polymer films with efficiency exceeding 15

Development of Cu(In,Ga)Se/sub 2/ (called CIGS) solar cells on polymers is challenging because of the thermo-physical properties of layers and substrates. CIGS layers of suitable structural and opto-electronic properties should be grown at low temperature (< 500/spl deg/C) as polyimides tend to degrade at higher deposition temperatures. Additionally, a method for controlled incorporation of an optimum amount of Na in CIGS is needed for high-efficiency cells since polyimides do not contain Na. Solar cells were developed on commercially available Upilex foils. CIGS layers were grown by evaporation of elemental Cu, In, Ga and Se at different substrate temperatures. Na from a NaF film was incorporated into CIGS layers with a post-deposition diffusion method that is suitable for in-line production of solar cells. Independent measurements have confirmed 14.1% efficiency under simulated AM1.5 standard test conditions. This is the highest efficiency reported to date for any kind of solar cell grown on polymer films. An average reflectance loss of about 13% was measured for these cells. Application of a commonly used anti-reflection coating would enable more than 15% efficiency flexible CIGS solar cells on polyimide foils.

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.

Defects and interfaces in Cu(In,Ga)Se2–based thin-film solar cells with and without Na diffusion barrier

To characterize the influence of sodium diffusion on the Cu(In,Ga)Se 2 (CIGS) layer, samples with and without an A1 2 O 3 diffusion barrier were investigated by transmission electron microscopy. The soda lime glass acts as a sodium source for samples without diffusion barrier while a post deposition treatment with NaF adds sodium to the CIGS layer for samples with diffusion barrier. For low deposition temperatures in the three-stage growth process for CIGS slightly columnar CIGS grains near the Mo back contact are found and larger grains near the top surface. Samples subject to high-temperature post deposition treatments show slightly larger grain sizes in the CIGS layer than samples prepared at the same temperature but with sodium from the soda lime glass. The surface smoothness of the CIGS layer and therefore the continuity of the following layers is improved for samples with higher deposition temperature. Occasional defects are found on the surface of the ZnO layer which can locally alter reflectivity and absorption properties of these solar cells. Despite the high CIGS surface roughness, the CdS layer with an average thickness of only 50 nm appears continuous and even fills trenches between CIGS grains near the interface.