IN-SITU KELVIN PROBE AND ELLIPSOMETRY STUDY OF THE DOPING OF A-SI-H AND A-SIC-H LAYERS - CORRELATION WITH SOLAR-CELL PARAMETERS

To underscore the effects of boron doping on the photovoltaic characte ristics of a-Si:H based pi-n solar cells we used the in-situ Kelvin pr obe technique to determine the evolution of the contact potential at t he SnO2/p interface in two series of solar cells: one with a p(a-Si:H) layer and another with a p(a-SiC:H) layer plus a buffer a-SiC:H layer . In each series, the flow of diborane was varied between 0.5 and 7 se em. The in-situ Kelvin probe measurements on thick layers indicate tha t the doping efficiency is higher in a-Si:H than in a-SiC:H layers. In the case of cells with a p(a-Si:H) layer we observe an optimum in V-o c as a function of the diborane flow rate. Below this optimum the incr ease in V-oc is correlated to the increase of the contact potential me asured by the Kelvin probe. Above the optimum value of the diborane fl ow, the further increase of the contact potential contrasts with the d ecrease of V-oc. Moreover, we found that V-oc of the cells with a p(a- SiC:H) is independent of the diborane how rate. This unexpected behavi our is interpreted in terms of the diffusion of impurities and changes in the p-layer induced by boron atoms; it is supported by secondary-i on mass spectrometry and in-situ spectroscopic ellipsometry measuremen ts. (C) 1998 Elsevier Science B.V. All rights reserved.