RF PLANAR MAGNETRON SPUTTERING OF POLYCRYSTALLINE CdTe THIN-FILM SOLAR CELLS

Abstract

Polycrystalline CdTe thin films have been prepared (using radio frequency (rf) planar magnetron sputtering), characterized, and used to fabricate CdS/CdTe solar cells. The performance characteristics of the best device (as measured under standard reporting conditions) are a conversion efficiency of 8.2% and 0.68 V and 21.7mA/cm2 for open-circuit voltage (Voc) and short-circuit: current density (J), respectively. Photoluminescence emission from the polycrystalline CdTe films (after the high-temperature heat treatment) showed that this material is dominated by four defects, including two donor levels Dl and D2 (at 120 and 180 meV below the conduction band) and two acceptor levels Al and A2 (at 30 and 65 meV above the valence band [VB]). Capacitance-voltage (C-V) measurements have indicated the formation of an n-i-p structure dominated by a high density of interface states. Deep level transient spectroscopy (DLTS) data also confirmed a minority electron trap at about 1.212 meV below the conduction band (CB) dominating the CdTe device. The trap level detected in a Schottky barrier, fabricated on a CdTe film without heating at high temperature, is deeper and higher in density than that in the heterojunction. This explains the enhancement of the Voc of the heterojunction devices after annealing at 400°C or higher. Although the device performance is modest by comparison with state-of-the-art devices, these results support the potential of sputter depositing CdTe films as a production technique. Considerable work must still be done to improve the material quality and reduce the density of defect levels.

KEY WORDS:

• Solar cells
• Sputtering
• Thin films
• CDTe