Joule heating-assisted growth of Cu(In,Ga)Se<sub>2</sub> solar cells: Fid-Bau Portal

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

Rampino, S. / Annoni, F. / Bronzoni, M. / Calicchio, M. / Gombia, E. / Mazzer, M. / Pattini, F. / Gilioli, E.

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 V_OC 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.