Cu(In,Ga)Se2 (CIGS) thin film solar cells have long gained attention as a promising thin-film PV technology for achieving low-cost manufacturing, versatile applicabilities and the high efficiencies. Furthermore, based on the promise of CIGS-based materials yielding excellent chemical stability, high light absorption, and wide range of substrate options, this kind of thin film PV has been considered as a next generation urban solar energy source.
Recently, Ag incorporation into CIGS has widely been attempted to improve the performance of CIGS solar cells. The Ag incorporation helps to lower the melting temperature of CIGS and therefore, it may provide a way to reduce structural defects and electronic defects of CIGS. In this study, to improve the device performances of 3-stage co-evaporated CIGS cells, a small amount of Ag was incorporated during the 3rd stage and their materials/device characteristics were investigated. The amounts of the added Ag into CIGS were 0, 0.019, 0.038, 0.057, 0.076 at%.
The Ag-incorporated CIGS (i.e., ACIGS) thin films were found to exhibit a larger grain size than the CIGS without Ag based on SEM analyses. With temperature-dependent IV, capacitance-voltage, and admittance spectroscopy, well-controlled Ag incorporation (approximately, 0.057 at%)-turned out to lead increased fill factors (FFs) and reduced defect densities tin the (A)CIGS bulk. It appeared that favorable band alignment and charge transport across the CdS/CIGS junction have been established by the Ag incorporation. The CIGS film without Ag exhibited an efficiency of 15.1 % with FF = 68.42, V￢oc = 0.691 V, and Jsc = 31.92 mA/cm￢2 while the cell with well-controlled Ag incorporated CIGS (i.e., ACIGS) yielded an efficiency of 16.3 % with FF = 73.63%, Voc = 0.696 V, and Jsc = 31.80 mA/cm2, indicating that the improvement in FF was the major factor of the device performance improvements.