Technological processes for CIGS based solar cells

Abstract

The solar photovoltaics (PV) market has been booming since the late 1990’s with an impressive 40% average annual growth rate and rapid growth has continued even in the last four years despite the bottlenecks in silicon feedstock availability. , which have determined a slow down in the decline of PV module prices. As an economic alternative to silicon a great deal of attention has been devoted to Cu(InxGa1-x)Se2 (CIGS) thin film based solar cells, whose development lead to demonstrate in 2010 a record efficiency of 20.3% on a 0.5 cm2 laboratory cell. The activity discussed in this thesis was performed in the frame of PED4PV project (“Industria 2015” programme of the Italian Ministry of Economic Development) aiming at the low cost production of thin film CIGS-based solar cells on conventional glass substrates as well as on alternative substrates suitable for building integration (such as ceramic and cement tiles) by using the relatively new, highly efficient, but yet largely unexplored technology called “Pulsed electron deposition”. This thesis reports about the set up of the technological processes that complement PED growth of the multilayer structure in the fabrication of the solar cells and their characterization. In particular the optimization of the Mo back contact deposition by sputtering is detailed, and the activity devoted to support Marazzi S.p.a and CTG S.p.a. in the development of ceramic and cement substrates, respectively, is described. A relevant part of the work has been devoted to the characterization of the cells. Current-voltage, capacitance-voltage and infrared thermography measurements gave the PED group the feedbacks necessary to improve the multilayer structure properties up to achieve an efficiency of 15.5% on lab-scale solar cells completely prepared at IMEM by depositing CIGS from a stoichiometric quaternary target at low temperature (270 °C) and without any post-growth treatment.