Preparation of Wide Bandgap Cu(InGa)(SeS)2 Solar Cells with Improved Fill Factor
Cu(InGa)(SeS)2 absorber films with a band gap Eg  ges 1.5 eV have been prepared using thermal co-evaporation with different sequences of Cu rich and Cu poor deposition. The composition through the film is, in some cases, non-uniform due to changes in relative S and Se incorporation. The compositional depth profile was characterized by grazing incidence X-ray diffraction and Auger electron spectroscopy with Ar sputtering. The composition ratio [S]/[S+Se] in the films depends on the process and the composition ratio [Cu]/[In+Ga] during deposition. Solar cells with a soda-lime glass/Mo/CIGSS/CdS/ZnO/ITO/(Ni-Al grid) structure were fabricated and a higher FF > 77 % than previous wide bandgap Cu(InGa)(SeS)2 devices has been demonstrated with these processes. The best Cu(InGa)(SeS)2 cell with Eg=1.5 eV is prepared by the two-layer process, and shows an efficiency of 11.2 %, with Voc =0.760 V, Jsc=19.2 mA/cm2, and FF=77.3 %. Quantum efficiency of the CIGSS cells shows relatively poor response for wavelength > 600 nm compared to that with CuInS2
Impact of roll-over-shaped current–voltage characteristics and device properties of Ag(In,Ga)Se2 solar cells
The roll-over shape often observed in the current–voltage curve of Ag(In,Ga)Se2 (AIGS) solar cells degrades the open circuit voltage (VOC) and particularly the fill factor (FF). The origin of the roll-over shape was investigated by experimental measurements and device simulation. By combining AC Hall measurement and the peel-off process, we estimated the AIGS hole concentration to be 2.2 × 1012 cm−3. Theoretical simulation revealed that the roll-over shape is attributed to this low hole concentration. Under an applied forward bias, the band bending near the back contact of the AIGS layer forms an intrinsic semiconductor owing to the injected electrons, leading to the formation of an inverted diode. To solve this issue, the addition of NaF by the postdeposition treatment of the AIGS layer was performed. As a result, the hole concentration of the AIGS layer increased, significantly improving its VOC, FF, and conversion efficiency.
Mechanism of Action of Antibacterial Agents
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