Growth, etching, and stability of sputtered ZnO:Al for thin-film silicon solar cells

Aluminum doped zinc oxide (ZnO:Al) can fulfill many requirements in thin film solar cells, acting as (1) a transparent contact through which the incident light is transmitted, (2) part of the back reflector, and (3) a source of light scattering. Magnetron sputtered ZnO:Al thin films are highly transparent, conductive, and are typically texturized by post deposition etching in a dilute hydrochloric acid (HCl) solution to achieve light scattering. The ZnO:Al thin film electronic and optical properties, as well as the surface texture after etching, depend on the deposition conditions and the post deposition treatments. Despite having been used in thin film solar cells for more than a decade, many aspects regarding the growth, effects of heat treatments, environmental stability, and etching of sputtered ZnO:Al are not fully understood. This work endeavors to further the understanding of ZnO:Al for the purpose improving silicon thin film solar cell efficiency and reducing ZnO:Al production costs. With regard to the growth of ZnO:Al, the influence of various deposition conditions on the resultant electrical and structural properties and their evolution with film thickness were studied. The surface electrical properties extracted from a multilayer model show that while carrier concentration of the surface layer saturates already at film thickness of 100 nm, the surface mobility continues to increases with film thickness, and it is concluded that electronic transport across grain boundaries limits mobility in ZnO:Al thin films. ZnO:Al deposited onto a previously etched ZnO:Al surface grows epitaxially, preserving both the original orientation and grain structure. Further, it is determined that a typical ZnO:Al used in thin film silicon solar cells grows Zn terminated on glass substrates. Concerning the affects of heat treatments and stability, it is demonstrated that a layer of amorphous silicon can protect ZnO:Al from degradation during annealing, and the mobility of ZnO:Al films can be increased up to 76 cm²/Vs by annealing at high temperatures. Further, it is observed that this annealing process does not affect the morphology of the film and increases the total transmission in the solar cell relevant portion of the spectrum. Accelerated aging in a damp heat is shown to affect the mobility primarily, and is thus likely related to poor transport across grain boundaries. Using deuterium as an isotopic marker, it is observed that the deuterium uptake was fairly linear with time and penetrated the whole film within 24 hours. Additionally, the deuterium is quickly replaced by hydrogen upon damp heat treatment in water. It is concluded that the electrical degradation is related to the penetration of water via grain boundaries, and that the bulk of the reactions causing this degradation are fairly weak, such as the formation of zinc hydroxide. Additionally, the annealed high-mobility ZnO:Al films prove to be very stable in the damp heat environment, exhibiting electron mobilites above 70 cm2/Vs after 1000 hours of exposure. The initial points of attack during HCl etching of ZnO:Al, and the long term etch evolution are shown to be related to certain grain boundaries built into the film during growth. The density of craters from an HCl etch is controllable by altering the temperature and concentration of the etching solution. It is further demonstrated that most acids exhibit etching behavior similar to that of HCl. Hydrofluoric acid (HF), however, exhibits unique etching characteristics with higher crater densities and sharper features. The crater density and shape are also shown to be controllable by etching in various concentrations of HF and in various mixtures of HF and HCl. Additional etching experiments are made to develop a polycrystalline ZnO:Al etching model. It is shown that basic and acidic solutions attack the same points on a polycrystalline ZnO:Al sample. The dependence of crater density on the acid concentration, temperature, and type holds true for single crystal ZnO. Planar and cross sectional transmission electron microscopy (TEM) measurements confirmed grain boundaries as the most prevalent catalysts for etching. A three postulate etching model is proposed, the basics of which are: (1) every grain boundary has certain potential to etch, and this potential depends on the compactness of the grain boundary;(2) the etchant solution and conditions define an etching threshold, and grain boundaries with etching potentials above the threshold are actively etched; and (3) the vertical and horizontal etch rates are defined by the solution, and vertically limited etching leads to the characteristic crater opening angles while a horizontal limited etch yields sharper structures. Examples on how the better understanding of ZnO:Al can lead to more efficient solar cells and more reliable ZnO:Al film preparation are also given. It is shown that the efficiency of amorphous silicon solar cells can be increased by optimizing the ZnO:Al crater size, and that the light trapping and efficiency in tandem silicon solar cells can be improved by fabricating modulated ZnO:Al surfaces. Finally, it is shown that by developing a better method for sepa-rating electro optical optimization from texturization, industrially relevant low cost ZnO:Al films can be textured effectively, such that the efficiency of microcrystalline silicon solar cell on these substrates rival those of the relatively expensive laboratory ZnO:Al films