Maurilio Meschia

Fabricating Cu(In,Ga)Se2 solar cells on flexible substrates by a new roll-to-roll deposition system suitable for industrial applications

In this work, a new hybrid sputtering–evaporation system providing a scalable process for deposition of Cu(In,Ga)Se2 (CIGS) layers is presented. The growth apparatus has been designed and realized to fit a size suitable for direct industrial transfer. In this process the metal precursors are first of all sputtered on rotating transfer devices, then evaporated on the substrate by local heating in a Se atmosphere. The desired thickness and composition of the CIGS film are obtained by repeated sputtering–evaporation cycles. The cylindrical geometry of the deposition chamber has been designed to accommodate different types of flexible substrates with a maximum size of 20 × 120 cm2 in a roll-to-roll configuration. Several techniques, including secondary ion mass spectrometry, Raman and photoluminescence spectroscopies, x-ray diffraction, scanning electron microscopy, external quantum efficiency, and I–V under 1 Sun illumination, have been used to test both the as-grown CIGS layers and the solar cell devices based on them. A significant performance and good control of Ga grading and Na content were obtained for solar cells grown at 450 °C on polyimide substrates with high deposition rates. In spite of the fact that the present efficiency record for CIGS solar cells on polyimide substrates is 20.4%, the 10.1% obtained using the hybrid method presented in this work is significant because the growth apparatus meets the requirements for direct industrial transfer. In fact, this process is being transferred in a 1 MW production line, where standard CIGS layers are deposited at low temperature on flexible substrates in a single-step process with a 1 mm sec−1 substrate velocity.

Cu(In,Ga)Se2 hybrid sputtering/evaporation deposition for thin film solar cells application

CuInSe2-based solar cells showed long-term stability and the highest conversion efficiencies among thin film solar cells, overcoming 20%. The optoelectronic properties of the absorber layers and interfaces are strongly dependent upon the deposition method and strictly related to the solar cell efficiency. In this work, an alternative approach for CIGS thin film growth has been developed and tested. Such approach consists of sputtering deposition of the metal elements combined with selenium co-evaporation, which allows deposition time of the CIGS layer lower than 30 min, matching industrial application requirements. The relationships between the growth parameters of such hybrid sputtering/evaporation method and the chemical-physical properties of the CIGS films have been studied. The CIGS cell performances have been monitored by External Quantum Efficiency measurements and I-V measurements. Test solar cells of 0.5 cm2 have shown an efficiency of 14 % on glass substrates.