Si-Doped Cu(In,Ga)Se₂ Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

In this communication, novel and simplified structure Cu(In,Ga)Se₂ (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si-doped CIGS films grown by the three-stage coevaporation method, B-doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si-doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer-free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer-based CIGS device performance, are found to be also effective in enhancing buffer-free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer-free CIGS photovoltaic devices.