METHOD OF THE SURFACE PHOTOVOLTAGE EXTENDED TO SILICON WAFERS AND SOLAR CELLS OF ARBITRARY THICKNESS

摘要

The photovoltage spectrum measured on back illuminated solar cells of the PESC type (passivated emi-tor solar cell) without original bottom ohmic electrode is evaluated with the aim to find the diffusion length of minority carriers in bulk of the absorber (L). Two junctions, namely pn~+ junction of the cell and that spontaneously created on the free surface generally exist in such samples. They give rise to two signals of opposite sign. Six parameters (including L) are needed to characterize the spectrum. Special simple arrangement removes influence of spontaneously created junction on the free surface, which, in this way, reduces the number of parameters needed for fitting to three and enhances reliability of the measurement. Silicon wafers were examined in an electrochemical cell and signal from the junctions existing on the top and the bottom surfaces was analyzed using the well known SPV technique and our modified method usable for arbitrary thickness of the samples. It is shown that the "classical" technique gives different diffusion lengths (L) depending on the level of illumination but at low light levels practically the same values of L from both methods were evaluated.