Role of exciton blocking layers as optical spacer in CuPc/C60 based organic solar cells

The role of exciton blocking layer (EBL) in organic solar cells have been under intense investigation in the recent past. Although BCP (bathocuproine) has been commonly used as an EBL in CuPc/C60 based devices, its role as an effective optical spacer to increase the optical electric field intensity at the active layers, needs further investigation. In this work, using numerical techniques, we study the role of BCP as an optical spacer in CuPc/C60 based devices. For comparison, a higher refractive index material (TiOx; titanium suboxide) was also used. The optical constants of the BCP layer was extracted using spectroscopic ellipsometry using Tauc-Lorentz model dielectric function. The maximum device photocurrent density (Jsc-max) was simulated using transfer matrix formalism and exciton diffusion dynamics. The results with BCP as an optical spacer indicate that although a high gain in photocurrent can be obtained for devices with low active layer thicknesses, the enhancement in photocurrent from an already optimized device can at best be from 93.7 A/m2 to 98.7 A/m2, corresponding to a gain of only 5.3 %. Using a higher refractive index material such as TiOx, the current density for an already optimized device can at best be enhanced from 93.7 A/m2 to 97.5 A/m2, a gain of only 4 %. Overall, our results reveal that although the EBL acts as an optical spacer, the improvement in device absorption due to the optical effect is limited for an already optimized device. This indicates that the well known improvement in device performance by incorporating the buffer layer should be primarily related to other properties such as exciton blocking, electron transport, and avoiding acceptor damage during cathode deposition.