Thin film CDTE solar cells deposited by pulsed DC magnetron sputtering

Thin film cadmium telluride (CdTe) technology is the most important competitor for silicon (Si) based solar cells. Pulsed direct current (DC) magnetron sputtering is a new technique has been developed for thin film CdTe deposition. This technique is industrially scalable and provides uniform coating. It is also possible to deposit thin films at low substrate temperatures. A series of experiments are presented for the optimisation of the cadmium chloride (CdCl2) activation process. Thin film CdTe solar cells require CdCl2 activation process to improve conversion efficiencies. The role of this activation process is to increase the grain size by recrystallisation and to remove stacking faults. Compaan and Bohn [1] used the radio-frequency (RF) sputtering technique for CdTe solar cell deposition and they observed small blisters on CdTe layer surface. They reported that blistering occurred after the CdCl2 treatment during the annealing process. Moreover, void formation was observed in the CdTe layer after the CdCl2 activation process. Voids at the cadmium sulphide (CdS)/CdTe junction caused delamination hence quality of the junction is poor. This issue has been known for more than two decades but the mechanisms of the blister formation have not been understood. One reason may be the stress formation during CdTe solar cells deposition or during the CdCl2 treatment. Therefore, the stress analysis was performed to remove the defects observed after the CdCl2 treatment. This was followed by the rapid thermal annealing to isolate the CdCl2 effect by simply annealing. Small bubbles observed in the CdTe layer which is the first step of the blister formation. Using high resolution transmission electron microscopy (HR-TEM), it has been discovered that argon (Ar) working gas trapped during the deposition process diffuses in the lattice which merge and form the bubbles during the annealing process and grow agglomeration mainly at interfaces and grain boundaries (GBs). Blister and void formation were observed in the CdTe devices after the CdCl2 treatment. Therefore, krypton (Kr), neon (Ne) gases were used as the magnetron working gas during the deposition of CdTe layer. The results presented in this thesis indicated that blister and void formation were still existing with the use of Kr an Ne. Xe, which has a higher atomic mass than Kr, Ne, Ar, Cd and Te, was used as the magnetron working gas and it resulted in surface blister and void free devices.