High Efficiency CdTe/CdS Thin Film solar Cells by a Process Suitable for Large Scale Production

It has been demonstrated that CdTe/CdS thin film solar cells can exhibit an efficiency around 16.5%. However this efficiency has been obtained by adding some Cu at the back contact. Cu behaves as a shallow acceptor in CdTe and then it can increase the hole carrier density in CdTe. On the other hand, Cu is a fast diffusor in CdTe and at a long run it can segregates at the grain boundaries damaging the solar cell. In our process we did not use any copper but we developed a new ohmic contact which is very stable. This consists in a thin layer of 1000- 2000 Å of Sb2Te3 deposited by sputtering directly on top of the CdTe surface. With this new contact we were able to obtain an efficiency of 14% or more in CdTe/CdS solar cell whose area is 2 cm2. These cells kept under 20 suns for several days at 100°C not only do not degrade but they improve their efficiency. The process consist in a subsequent deposition of 5 different layers, 4 of which, namely TCO, CdS, Sb2Te3 and Mo are deposited by sputtering and CdTe by CSS. The treatment with CdCl2 is done by depositing 1500 Å of CdCl2 on top of CdTe and with an annealing at 400°C in 500 mbar of Ar. After the treatment, Ar is pumped away keeping the substrate at 400°C. In this way, CdCl2 is completely re-evaporated from the CdTe leaving a perfectly cleaned surface. Before the deposition of the contact, no etching is done on the CdTe surface. Due to the fact that no acids nor other kind of liquids such as Br-methanol are used in this process and considering that both sputtering and CSS are very scalable techniques, this process results to be very suitable for large scale production.