Fabrication of high-efficiency Cu(In,Ga)Se2 solar cells by Pulsed Electron Deposition technique

In the past few years, a continuous rise of gas/oil prices promoted the exploitation of renewable energy especially from photovoltaic. This field is today monopolized by Silicon, either mono- or crystalline-, solar cells, although thin film technologies are gaining increasing interest for the possibility to reduce the material quantity and cost and to use light and flexible substrates. Among thin films, CuInGaSe2 (CIGS) is the material with the highest conversion efficiency, close to 22%. Despite the fact that the performances of CIGS-based solar cells are comparable to the silicon ones, their spread on the market has been limited by the high production cost; long and expensive multistage processes (thermal co-evaporation and sputtering/selenization) must be adopted to solve the problems related to the CIGS complex composition and incongruent melting. An innovative deposition technique (PED, Pulsed Electron Deposition) has been developed at IMEM-CNR, aiming to simplify the growth of complex materials such as CIGS. PED is based on a non-thermodynamic equilibrium process, consisting in the ablation of a target with the same composition of the film to be grown, leading to a simple and “single stage” CIGS deposition. The work is carried on in the frame of an industrial project “PED4PV” (Pulsed Electron Deposition for PhotoVoltaic), coordinated by IMEM-CNR and financed by the Italian Economic Development Ministry, with the purpose of optimizing the PED technique for depositing high efficiency (>15%) CIGS-based solar cells. The first part of the thesis is focused to exploit the PED peculiarities (specifically, the optimal stoichiometric transfer from the target to the substrate) in order to obtain CIGS thin films of high crystalline quality and, remarkably, at a much lower temperature compared to the alternative growth techniques. The optimization of the CIGS absorber layer and its doping, by Na addition, allowed to obtain solar cell efficiencies of 18.75% on active area. The low temperature CIGS deposition process has been successfully tested also on crystalline substrates (GaAs and Ge), onto which monocrystalline CIGS films have been epitaxially deposited; the absence of structural defects such as grain boundaries could furtherly increase the efficiencies (up to a theoretical value of 28%). The second part of the thesis is dedicated to a pre-industrial development of the PED process. In particular, the solutions to overcame the main problems typical to the high energy techniques have been studied: i) reduction of the micrometric particulate on the film surfaces, caused by the interaction between the high-energy electronic beam and the target, by applying an appropriate electric field between target and substrate. ii) increase of the deposition area, by designing and assemble a pre-industrial deposition chamber prototype equipped with different PED sources, suitable to fabricate photovoltaic cells with large area (16x16cm2, same as the conventional Silicon-based cells) with high thickness uniformity. iii) Stability of the electronic beam during long deposition time, limiting the PED sources heating. This has been achieved by designing realizing and testing a new type of heater based on the Joule effect (flowing a current through the solar cell metal back contact), enabling the growth of high quality CIGS on thermolabile and flexible materials such as polymers. This thesis work contributed to the rapid development of CIGS-based thin film solar cells with efficiencies comparable to the highest values at international level, with a simple and potentially scalable industrial process.