Effect of Thermal Annealing on the Photovoltaic Performance of D18:L8-BO Non-Fullerene Organic Solar Cells

Abstract

Organic solar cells (OSCs) represent a promising technology for third-generation photovoltaics due to their versatility, light weight, and solution-processability suitable for indoor energy harvesting and portable electronics. In recent years, Non-Fullerene Acceptors (NFAs) such as L8-BO have revolutionized organic solar cell performance by overcoming the limitations of traditional fullerene acceptors, offering higher absorption coefficients in the near-infrared region of the spectrum and adjustable energy levels when blended with high-performance donor polymers such as D18. However, optimizing the bulk heterojunction blend ratio and understanding the impact of post-deposition thermal annealing remain critical to controlling domain morphology and charge transport dynamics. In this work, we investigate the fabrication and photovoltaic performance of OSCs with the structure ITO/PDINO/D18:L8-BO/MoO3/Ag. The active layer was deposited by spin coating from chloroform solutions, evaluating two donor: acceptor weight ratios (1:1 and 1:2), both with and without thermal annealing at 80 °C for 10 minutes. The highest power conversion efficiency (PCE) of 7.58% was obtained for the as-cast devices with 1:2 donor: acceptor ratio. In contrast, thermal annealing at 80°C induced severe performance degradation for both formulations. For the 1:2 ratio, thermal annealing led to a reduction in short-circuit current density (JSC) and PCE, accompanied by a substantial increase in series resistance. This degradation could be associated with changes in the optical absorption of the active layer, potentially resulting from morphological reorganization of the organic phases. These findings demonstrate that processing of D18:L8-BO active layers do not require thermal annealing to achieve improved photovoltaic performance.

Keywords: Organic solar cells; Non fullerene acceptors; D18:L8-BO.