Rosa Chierchia

Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy

The valence band offset (VBO) at the interface CdS/Cu2ZnSnS4 was investigated by x-ray photoelectron spectroscopy (XPS). The VBO was measured by two different procedures: an indirect method involving the measurements of the core levels together with the XPS bulk valence band (VB) spectra and a direct method involving the analysis of XPS VB spectra at the interface. The indirect method resulted in a VBO value of (?1.20???0.14)?eV while the direct method returned a similar value of (?1.24???0.06)?eV but affected by a lower uncertainty. The conduction band offset (CBO) was calculated from the measured VBO values. These two measured values of the VBO allowed us to calculate the CBO, giving (?0.30???0.14)?eV and (?0.34???0.06)?eV, respectively. These values show that the CBO has a cliff-like behaviour which could be one of the reasons for the Voc limitation in the CdS/CZTS solar cells.

Advances in screen printing metallization for a-Si:H/c-Si heterojunction solar cells

Amorphous / crystalline silicon heterojunction is the most attractive technique to obtain high efficiency solar cells. Usually such a kind of cells is produced starting from n-type silicon wafers, because of several advantages, like the high bulk lifetime and the possibility to easily contact the n-type base with i-n amorphous layers. The emitter is usually covered by Transparent Conductive Oxides (TCO) which works as high conductive layer and Anti Reflection Coating (ARC). The device is completed by a metal grid, made by screen printed silver, sintered at low temperature. Both the TCO and the grid strongly influence the final cell series resistance, and consequently the cell efficiency. When p-type wafer is adopted as substrate for heterojunction cell, the base contact is more difficult to obtain because of the energy bands alignment between the c-Si and the p-type a-Si:H layer. Recently n-type doped SiOx layer has attracted interest as emitter layer in heterojunction device, therefore in this work we show the results obtained on the metallization of n-type SiOx/ptype c-Si heterojunction solar cells by means of low temperature screen printing technique. In particular a new kind of low temperature sintering (<; 200°C) screen printable silver paste has been developed able to ensure high linear conductivity, low specific contact resistivity and strong adhesion to TCOs. We present electrical characterization using Transfer Length Method (TLM) technique. Since the base contact of SiOx/c-Si heterojunction is ensured by laser doping technique starting from p-type a-Si:H layer, we also show how the screen printed Ag paste can enhance the base contact of this solar cell.