All-carbon photovoltaic devices have attracted attention in terms of resources and environment. However, the device application is very limited because of poor performance. In this work, we studied the solar cell characteristics of amorphous carbon (a–C)/fullerene (C₆₀) junction when the thickness of the a–C layer was varied. When the thickness of the a–C layer was varied, the short-circuit current density and open-circuit voltage increased with increasing film thickness and then decreased after a certain value. Also, the spectral response measurement results suggest that most of the power generation is due to the light absorbed by the C₆₀ layer, and that the light absorbed by the a–C layer may contribute little to power generation. This study suggests that the improvement in the electronic properties of a–C is necessary to make a photovoltaic device with high performance.

To date, the lead-free perovskite has shown remarkable progress in solar cell development owing to its outstanding properties. Here, we report on the structural, optical, and photovoltaic properties of methylammonium bismuth iodide ((CH₃NH₃)₃Bi₂I₉, MBI) and cesium bismuth iodide (Cs₃Bi₂I₉, CBI) hybrid perovskite solar cells (HPeSCs) fabricated using the hot immersion method (HIM) with changing the composition x, ((CH₃NH₃)_1-xCs_x)₃Bi₂I₉; from x = 0 to x = 1.0. The compact MBI and CBI films were successfully fabricated on FTO glass substrates at x = 0 and x = 1, respectively. On the other hand, the CBI/MBI mixed structure with a rough surface was obtained in the range from x = 0.2 to x = 0.8. An incorporation of CBI in MBI showed improvement, especially in optical properties, indicating that the absorption region was extended toward a longer wavelength region with increasing x. Interestingly, the open-circuit voltage of the hybrid cell was higher than that of the MBI or CBI cell, whereas the short-circuit current was lower than that of the MBI or CBI cell. This work provides alternative ways to fabricate lead-free PeSCs using a simple and low-cost method in the future.