Zinctelluride as an alternative back contact for CdTe/CdS solar cells

摘要

Electrodeposition is chosen for preparation of the back contact for CdS/CdTe thin film solar cells because it is one of the most inexpensive ways to form semiconductors. Additionally, it offers the possibility for in-situ doping by adding very few amounts of the dopant into the deposition bath. Common CdTe solar cells have the following superstrate configuration: TCO/CdS/CdTe/metal. The CdTe/metal back contact leads to losses of the fill-factor and the efficiency due to the formation of Schottky contact. This is caused by a high work function of CdTe and the difficulty to get p{sup}(++)-CdTe. In order to create an ohmic contact, a thin layer of ZnTe is formed electrochemically. Low long term stability is caused by diffusion of metal atoms from the back contact. Both problems shall be solved by deposition of an intermediate layer between CdTe and the metal back contact. ZnTe is supposed to lower the barrier height and act as a diffusion barrier leading to a decreased degradation of the cell. The deposition parameters pH-value, temperature, bath-composition and the deposition potential are varied. In order to find the correct parameters ZnTe is deposited on ITO-substrates first. These ZnTe layers are characterized by transmission spectra, energy dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM) and x-ray diffraction (XRD). On CdTe substrates the deposition potential is altered slightly. Uniform ZnTe layers are obtained, if the CdTe surface is etched with lithium hydroxide solution. After electrodeposition of ZnTe the cells are completed with Ni and Au by physical vapour deposition. Cells are investigated by means of I-U measurements and compared to those without intermediate ZnTe-layer.