Advanced Stacked Elemental Layer Process for Cu(InGa)Se₂ Thin Film Photovoltaic Devices

Abstract

Targeting large area and low cost processing of highly efficient thin film solar modules an advanced stacked elemental layer process for Cu(InGa)Se2 (CIGS) thin films is presented. Key process steps are i) barrier coating of the soda lime glass substrate combined with the addition of a sodium compound to the elemental Cu/In/Ga/Se-precursor stack and ii) rapid thermal processing (RTP) to form the CIGS compound.By this strategy exact impurity control is achieved and the advantageous influence of sodium on device performance and on CIGS film formation is demonstrated unambiguously by means of electrical characterisation, XRD, SEM, TEM and SIMS. Sodium enriched and sodium free precursor stacks were heated to intermediate states (300°C–500°C) of the RTPreaction process. The experiment clearly reveals that on the reaction pathway to the chalcopyrite semiconductor increased amounts of copper-selenide are formed, if sodium is added to the precursor films. TEM-electron diffraction unambiguously identifies the CuSe-phase which is localised at the surface of the forming CIGS-film. These experimental findings propose a sodium assisted quasi liquid growth model for the CIS formation taking into account that sodium promotes the existence of CuSe at higher temperatures and its effect as a flux agent. The model contributes to a better understanding of the observed superior crystal qualitiy for sodium enriched in contrast to sodium free CIGS films.Application of these experimental findings in the technique of the optimized and controlled sodium incorporation significantly improves process reproducibility, CIGS film homogenity over larger substrate areas and shifts the average efficiency of cells and modules to a significantly higher level. This is demonstrated by a 12-cell integrated series connected minimodule with an aperture area of 51 cm2 and a confirmed efficiency of 11.75 %.

Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance

“…[29,33] A thin film of SiO x , Al 2 O 3 , or Si x N y acts as an efficient barrier against Na diffusion. [25,27] This provides further support for linking Na to out-diffusion from the SLG. We also note that high Na concentrations, similar to those obtained from growth on SLG, may be ªartificiallyº introduced into the CIGS films by ion implantation [26] or by using a Na-containing precursor.…”

“…[25,26] Na was found to reside primarily at grain boundaries by comparing a secondary ion mass spectroscopy (SIMS) map with a scanning electron microscopy (SEM) picture [25] and the Na profile to the grain boundary density. [27] If spherical grains (or cylindrical grains of any aspect ratio) of~1 mm diameter are assumed, a concentration of 10 19 ±10 20 cm ±3 distributed on grain boundaries implies at least 0.1 monolayer coverage of Na on each grain. .…”

“…We also note that high Na concentrations, similar to those obtained from growth on SLG, may be ªartificiallyº introduced into the CIGS films by ion implantation [26] or by using a Na-containing precursor. [27] Use of the latter in combination with a Na diffusionbarrier, which prevents Na incorporation from the SLG, has been reported to increase greatly the lateral homogeneity of Na incorporation. [27] Na and O: A point of importance is the relation between the efficiency of Na diffusion and the presence of oxygen.…”

“…[27] Use of the latter in combination with a Na diffusionbarrier, which prevents Na incorporation from the SLG, has been reported to increase greatly the lateral homogeneity of Na incorporation. [27] Na and O: A point of importance is the relation between the efficiency of Na diffusion and the presence of oxygen. As much as 10 % of oxygen is typically found in the Mo layer.…”

“…[48] Other Na Effects: i) Na-related morphological effects have also been found, which suggests that catalysis of air oxidation is not the only role that Na plays: Na was found to induce a preferred á112ñ orientation in the co-evaporation process [25,28] and to increase grain size significantly when using rapid thermal processing. [27,33] ii) The thermomechanical properties of the complete cell are also affected by using SLG, regardless of Na effects: Compressive, rather than tensile, stress is induced in the CIGS film upon cooling, [25,45] as originally suggested. [9] Some of the observed improvement in photovoltaic properties may certainly be attributed to increased grain size and thermomechanics, rather than to catalytic effects.…”