Wenwu Wang

The study of oxygen concentration in the CdTe thin film prepared by vapor transport deposition for CdTe photovoltaic devices

Polycrystalline CdTe thin films for CdTe solar cell devices are grown using vapor transport deposition (VTD) which is one of promising techniques to prepare CdTe thin film for high efficiency CdTe solar cells. However, few works report the study of the CdTe thin film growth by VTD so far. In this paper, the oxygen concentration is altered to be O2-free, 10, 20, 30, 40 and 50%. The morphology and crystallinity of as-deposited CdTe films are studied by X-ray diffraction and scanning electron microscopy. And the electrical properties for CdTe photovoltaic devices is investigated by current–voltage (J–V), capacitance voltage (C–V), and time-resolved photoluminescence spectroscopy (TRPL) measurements. The results show that a small amount of oxygen in the process has a positive impact on CdTe thin film quality because the oxygen incorporation could effectively change the grain size and make grain orientation more pronounced. The carrier concentration increases from 2.3 × 1013 to 7.5 × 1013 cm−3 when the oxygen concentration increases from 0 to 10%. The result of carrier lifetime also shows a positive correlation with oxygen concentration. A series of devices performance reveal the optimal technological conditions is under 10% oxygen partial pressure. The best small-area CdTe thin film solar cell is obtained with the conversion efficiency of 12.65% without back contact layer.

Grain boundary passivation by CdCl2 treatment in CdTe solar cells revealed by Kelvin probe force microscopy

The introduction of annealing treatment in CdCl2 atmosphere has been demonstrated to be an effective method in improving the efficiency of CdTe solar cells. Kelvin Probe Force Microscopy (KPFM) was used to study the effect of the CdCl2 treatment process by spatially resolved imaging of the surface contact potential difference (CPD). The results indicated that the grains were spatial depleted by grain boundaries (GBs). The quantitatively CPD distribution at GBs was analyzed by Gaussian fitting. During annealing process the carrier density was decreased and the electronic properties of the GBs were modified. The electronic structure changes resulted from the CdCl2 treatment at GBs were put forward to explain the efficiency enhancement in device performance. The results suggested that GB depletion regions were narrowed down and barrier height at the GBs was also significantly decreased. Therefore the carrier transport can be enhanced. A GB model including the electron traps was introduced. It was found that the barrier height at GBs can reach the maximum value at a specific hole density.

CuTe Nanoparticles/Carbon Nanotubes as Back Contactfor CdTe Solar Cells

The Schottky barrier between the CdTe layer and metal electrode has opposite polarity to the CdS/CdTe cell junction, which can greatly degrade cell performance. Adding a back contact (BC) layer can reduce the Schottky barrier at metal/p-CdTe interfaces. Paste including CuTe nanoparticles and carbon nanotubes (CuTe NPs/CNTs) was used as a BC in thin-film CdTe solar cells. The effect of the mass of carbon nanotubes (CNTs) in the paste and the BC annealing temperature on cell performance was explored. Cu film and paste including Cu nanoparticles and carbon nanotubes (Cu NPs/CNTs) were fabricated as the BC for CdTe solar cells. The performance of CdTe solar cells based on different kinds of Cu-containing BCs studied. The fill factor and open-circuit voltage (VOC) of devices with CuTe NPs/CNTs BC were greatly improved by optimizing the mass of CNTs in the paste and the annealing temperature. The carrier concentration in the CdTe layer was improved by one order of magnitude. The CuTe NPs/CNTs BC showed the best effect on cell efficiency for the Cu-containing BC.