Introduction
Organic solar cells (OSCs) are attractive for low-cost, lightweight, and mechanically flexible photovoltaic technologies. However, their efficiency is limited by suboptimal light absorption and charge extraction. Incorporating an optical spacer, such as zinc oxide (ZnO), has been proposed to improve device performance by enhancing both optical and electrical responses.
Methods
Finite element method (FEM)-based simulations were carried out to examine the effect of a ZnO optical spacer in OSCs. The active layer consisted of a PTB7 donor blended with a PCBM acceptor. Two device structures were analyzed: a reference design (Glass/SiO₂/ITO/PEDOT:PSS/PTB7:PCBM/Al) and a modified configuration with a ZnO spacer inserted between the PEDOT:PSS layer and the active blend (Glass/SiO₂/ITO/PEDOT:PSS/ZnO/PTB7:PCBM/Al). Optical field distribution, exciton generation rate (G), and short-circuit current density (JSC) were evaluated under monochromatic illumination (450–850 nm) and standard AM1.5 solar conditions at 100 mW/cm².
Results
The inclusion of ZnO significantly enhanced the internal electric field distribution and increased exciton generation across the active layer. This improvement is attributed to the dual role of ZnO: (i) its optical spacer effect, which reduces reflection losses at the PEDOT:PSS/active interface and redistributes light more effectively within the absorber, and (ii) its favorable electronic properties, which facilitate electron transport and mitigate interfacial recombination. Under AM1.5 illumination, the ZnO-modified structure exhibited nearly 30% higher light absorption and a notable increase in JSC compared to the reference cell.
Conclusion
This numerical study demonstrates that integrating a ZnO spacer layer into OSCs simultaneously improves light harvesting and charge extraction. The findings, in agreement with experimental reports, confirm the multifunctional role of ZnO as both an optical spacer and an electron-transporting interfacial layer, providing a practical strategy for enhancing the efficiency of organic photovoltaics.
