Diode laser annealing of CZTS thin film solar cells

Abstract

Continuous wave (CW) diode laser processing, in the form of millisecond annealing, is an interesting option for thermal treatment of the thin films. The localised nature of laser processing gives an opportunity to heat the film to a higher temperature over a much shorter time than rapid thermal annealing (RTA), while allowing the substrate to remain at low temperature with a low ramp-up rate. As a result, the chance of substrate damage and delamination between the interfaces are significantly reduced. Here, numerical simulation of the CW diode laser annealing of copper zinc tin sulfide (CZTS) thin film deposited on molybdenum (Mo) coated soda lime glass (SLG) substrate is carried out to reveal the thermal behaviour. The numerical simulations are performed using the open source C++ solver known as OpenFOAM [1]. A number of simulations has been conducted to investigate the influence of key laser annealing parameters on the spatial and temporal temperature distributions of the CZTS/Mo/glass structure. Our simulation results show that, CZTS and Mo are practically isothermal during the annealing process, and a sharp temperature gradient can be observed within the glass substrate. This sharp temperature gradient, which may lead to a delamination between the interfaces, can be reduced by using background substrate heating along with a slow laser beam scanning speed. Additionally, the controlled heating and cooling rate can be observed with the slow laser beam scanning speeds.