Abstract
Cadmium telluride (CdTe) is high-efficiency commercialised thin film photovoltaic technology. However, developing a stable low-resistivity back contact to the CdTe solar cells is still an issue. High work function and low level of doping of this material don't allow to create an ohmic contact with metals directly. Copper is commonly used to lower the back contact barrier in CdTe solar cells, but an excessive amount of copper diffusing through the cell is harmful for the device performance and stability. In this work a copper-doped ZnTe (ZnTe:Cu) buffer layer was incorporated in between CdTe and gold metal contact by high-rate pulsed DC magnetron sputtering. The back contact was then activated by rapid thermal processing (RTP) resulting in spectacular improvement in key device performance indicators, open circuit voltage (VOC) and fill factor (FF).
Highlights
Cadmium telluride is one of the leading and most promising materials for thin film solar cells
zinc telluride (ZnTe) deposition on glass devices were placed on an aluminium nitride (AlN) susceptor in the rapid thermal processing (RTP) furnace and received consecutive annealing steps starting at 260 C and ending at 400 C under flowing Ar
The bright field Transmission electron microscopy (TEM) image of the full Cadmium telluride (CdTe) device with asdeposited ZnTe:Cu layer is shown in Fig. 3 and Fig. 4 shows the detailed view of the back contact region
Summary
Cadmium telluride is one of the leading and most promising materials for thin film solar cells. The large electron affinity (c 1⁄4 4.5 eV) and low carrier density of the CdTe absorber contribute to the formation of a back contact Schottky barrier, which acts diode set up in opposition to the CdS/CdTe junction [5,6]. This barrier limits hole collection at the back contact, causes losses in open circuit voltage (VOC) and fill factor (FF) and can be observed through the occurrence of roll-over (current-limiting inflection point) in J-V curves [6,7]. Separation of the two can theoretically be achieved through deposition of ZnTe:Cu and the metal back contact layer at room temperature followed by either a simple thermal treatment in a furnace or rapid thermal processing (RTP) [4]
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