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Nanostructural Properties of Thin Film Solar Cells Materials by AFM

The typical thin film silicon solar cell consist of some hundred nano-meters p-i-n junction deposited on glass substrate coated by thin SnOx film that is used as transparent conductive substrate (Fig. 1). The back electrical contact is usually evaporated Al layer. For high cell efficiency, the light passing trough SnOx layer should be scattered in order to increase the path trough the cell and in that way the absorption. This demand is satisfied by rough surface e.g. when the layer grows under the condition that prefer formation of larger “ particles” , presumably with polycrystalline structure. The next layer, nano-crystalline silicon, when deposited by typical growing condition, follows the substrate morphology (Fig.4). However, when the roughness exceeds certain level, the influence on cells performances become negative – the output voltage drops substantially. In praxis the problem can be lowered by exposing SnOx layer to Ar plasma for some time, before deposition of silicon layer. In order to found out what is the origin of beneficial effects of above mentioned process, the influence of plasma treatment on surface morphology was studied by AFM microscopy. The SnOx thin films were deposited by Atmospheric Pressure Chemical Vapor Deposition method (APCVD) on glass substrate and after deposition exposed to the RF discharge in Ar gas. The surface morphology of as deposited and plasma treated samples was examined by Atomic Force Microscope (AFM). The AFM images have been collected using Multimode AFM (Rudjer Boskovic Institute – Division for Marine and Environmental Research) with Nanoscope IIIa controller (Veeco Instruments, Santa Barbara, CA) with a vertical engagement (EV) 10 &igrave ; ; m x 10 &igrave ; ; m scanner in contact imaging mode. For all of measurements the commercially available sharpened silicon-nitride tips (NP-20, Veeco) were used (k=0, 32 N/m). Typical results of AFM spectroscopy are shown in Fig 2. for SnOx samples before (2a) and after 10 minutes Ar plasma treatment (2b). The z-axe, the direction perpendicular to the sample surface, is in 10 times smaller units than x and y axes. Apparently, the shape of the conical surface structures is less sharp after plasma treatment, which is more evident from Fig.3 where the typical cross-sections of roughness for as deposited (full line) and plasma treated (dashed line) are plotted. Furthermore, the calculated average values of root mean square (RMS) of height deviations over the whole measured area, given in Tab. I, confirms the lowering of roughness by plasma treatment. It is most probably that conical shape of structure causes, during the plasma treatment, the local field enhancement that is more pronounced near the “ sharper” spikes which results in local increase of plasma-surface interactions and changes of theirs shape, e.g. lowers the surface roughness. The beneficial effect of removal of “ spikes” must be connected with solar cell structure. The whole p-i-n junction is several hundred nanometers thick while the first, “ p” layer is lower thickness than the surface roughness and the cell could be broken on the too sharp “ spikes” following by short-circuits between front electrode, SnOx layer, and back electrode, thin Al film.

AFM; Tin-oxide; Solar Cell; Thin Film; Surface Roughness

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