Joule heating-assisted growth of Cu(In,Ga)Se2 solar cells

We report on the development of an unconventional method for heating a Mo-coated substrate during the deposition of a Cu(In,Ga)Se2 (CIGS) layer by the pulsed electron deposition technique, to be used as absorber in thin film solar cells. This method is based on the application of a DC electrical power directly through the Mo back contact of the cell, converting electrical energy into heat by Joule effect. Since the current flows only on the superficial metal-coated region of the substrate, a localized heating of the surface can be achieved, thus limiting the heat losses. Due to the very efficient heat transfer to the thin Mo layer, a very little electrical power density (few W/cm2) is enough to achieve the required deposition temperature on the Mo surface, much lower compared to the traditional resistor- or lamp-based external heaters. The morphological and electrical properties of Joule-heated samples have been compared to those of CIGS films heated by a conventional external heater. As far as the structure concerns, a remarkable difference is revealed by Scanning Electron Microscopy analysis, indicating a significant enlargement of the CIGS grains size on Joule-heated samples. On the contrary, Capacitance-Voltage and Current-Voltage measurements evidence similar electrical features: both types of heated samples have a net free carrier concentration ≈5 × 1015 cm−3, resulting in a similar photovoltaic conversion efficiency (≈15%). The main recombination path, deduced from the dependence of VOC on the temperature, results to be the Shockley-Read-Hall mechanism in both types of the absorber layer. These results indicate that the Joule effect could be adopted as a feasible, low cost alternative heating method for growing high quality CIGS layers.