The Role of Transient Heat and Mass Transfer in Controlling the Photovoltaic Properties of Solution-Processed Cu(In,Ga)Se₂

Cu(In,Ga)Se₂ (CIGS) has been extensively studied as an absorber material in thin film photovoltaic (PV) devices. Lab scale efficiencies have reached 23.4%, making it a promising material that is already being deployed commercially. Further market penetration requires the development of low capex fabrication routes such as solution-processing to reduce manufacturing costs. However, the efficiencies of solution-processed devices continue to lag behind those of their vacuum-processed counterparts.<br/>This efficiency gap is further exacerbated by difficulties in reproducing results from batch-to-batch and from lab-to-lab. This inhibits globally cohesive improvement of overall device properties and potentially obscures the true impact of intentional modifications made during experiments.<br/>In this work we address the uncontrolled variability and provide access to previously unexplored methods of PV property manipulation through the careful study of transient heat and mass transfer during the reactive selenization step of blade-coated CIGS absorber processing. We demonstrate that, even under furnace conditions that are nominally the same (same apparatus, setpoint temperature, and duration), subtle changes in the resistances to heat and mass transfer within the system result in significant changes to all of the relevant PV parameters of the completed device. A common starting precursor film leads to devices that range in power conversion efficiency (PCE) from &lt;5% to &gt;12% based on these small manipulations of heat and mass transfer properties.<br/>Results of a suite of optoelectronic characterization techniques will be presented that inform the nature of these changes to PV properties. We have further coupled these experiments with modelling based on fundamental theories of heat and mass transfer to understand and identify the relevant parameters controlling absorber quality during the selenization step. This understanding makes the control of these transient heat and mass transfer properties a powerful tool to control the final device properties.<br/>The understanding of these heat and mass transfer properties can then be used to reduce variability during CIGS processing and rationally design innovative heat treatments to improve the PV performance of solution-processed CIGS devices. This reduction in variability will also then allow for rapid advancement in the properties of solution-processed photovoltaics, as proven strategies in the traditional processing space can now be studied and optimized without the interfering effects of this previously underestimated variable. The ideas presented here should have impacts beyond the CIGS material system, as these transient heat and mass transfer variables may also be important in the processing of other thin film technologies such as Cu₂ZnSnS₄.