Efficiency enhancement of bi-layer solar cells utilizing graded bandgap active layer
The graded bandgap light absorption layers were inserted as an active layer in the bi-layer solar cell of ITO/PEDOT:PSS/CuPc/C60/BCP/Al structure. The effect of variation of relative thicknesses of the graded bandgap absorption layers were elucidated and optimized for the improvement in photovoltaic efficiency. The organic bi-layer solar cells were fabricated by using thermally evaporated graded bandgap layers, while maintaining the total active layer thickness constant. It was found that the thickness of lower bandgap layer beneath the higher bandgap layer predominantly controls the overall efficiency of the bi-layer cell, converting most of the light into the photocurrent. It was also found that the deposition rate control is critical in obtaining the enhancement of device efficiency, in that slower deposition rate provides better cell performance. The solar cell efficiency was measured from a solar simulator at AM 1.5 100 mW/cm2.