STUDIES ON THE INTERFACE BETWEEN NOVEL TYPE ZNO AND P-A-SIC-H IN 1.5 EV A-SIGE-H PIN DIODES IN COMPARISON TO SNOX AND ITO

1.5 eV a-SiGe: H Pin diodes with p-a-SiC: H window layers (E(g) = 1.95 eV) have been deposited on conventional ITO, SnOx and novel Al-doped ZnO (ZnO:Al) transparent conductive oxides (TCO) in order to compare the influence of TCO/p-a-SiC:H interfaces on diode parameters. Characterization by transmission, sheet resistance and SEM photographs reveals comparable features for the three types of TCOs, ZnO:Al additionally shows a very high chemical stability in hydrogen containing plasmas. Transmission and absorption experiments also were performed on the layer-system TCO + thin p-a-SiC: H indicating different growth rates of p-a-SiC:H on ZnO:Al and SnOx. SCLC experiments on sandwich structures consisting of TCO/n+ in+/Cr demonstrate an increase in the midgap defect density of states N(t)(E) for a-SiGe: H layers on ITO but no dependence of N(t)(E) on the crystallite size of the substrates. In the case of ZnO:Al the analysis of I-V characteristics under AM1 illumination (100 mW/cm2) displays a clear improvement of open-circuit voltage V(oc) and short-circuit current density I(sc). This is due to a higher work function of ZnO:Al in comparison to SnOx and, therefore, increased built-in potential V(bi). Diffusion of Zn and In into the p-a-SiC:H-layer, however, causes a rise in series resistance R(S) and lowering of the fillfactor FF for ZnO:Al and ITO as front contacts. With respect to extended optimization, in particular avoidance of the high series resistance, the very stable, high temperature deposited ZnO:Al seems to be a promising material for a-Si:H based solar cells.