Abstract: Organic solar cells based on small molecules and conjugated polymers are attracting much attention due to their merits of low costs, simple fabrication processes, light weights, and mechanical flexibilities. Metals are usually considered as promising candidates for the semi-transparent electrodes. In such devices, a strong microcavity resonance can be supported between the two electrodes, resulting in a narrowed bandwidth of light absorption, which, unfortunately, will lower the performances of organic solar cells since broadband absorption is always highly desired. To overcome this obstacle, people have proposed many designs such as using ultra-thin electrodes or using dielectric-metal hybrid electrodes. Although the light absorption bandwidth can be improved considerably, the absorption efficiency would be lowered due to the weakened microcavity resonance. This is a tough problem that always bothers both researchers and engineers. To solve this problem, we propose a light trapping scheme based on broadband hybrid modes due to the hybridization between microcavity resonance and antireflection resonance. By introducing a capping layer outside the device structure, antireflection resonance can be excited inside the capping layer and can then couple with the intrinsic microcavity resonance, inducing dual microcavity-antireflection resonance hybrid modes. The hybrid modes are of broadband and their resonant wavelengths can be easily designed by tuning the capping layer thickness and cavity length, since the capping layer thickness would affect the antireflection resonance while the cavity length would affect the microcavity resonance. By matching the resonance with the high absorption region of the active layer, the overall absorptivity of the proposed device can be greatly enhanced by~37% compared to the conventional microcavity based device where only one mode, that is, the microcavity resonance can be supported. Moreover, we compare our light trapping scheme with the surface plasmon-polaritons based scheme where surface waves are excited to help improve the light absorption. We find that the overall absorptivity of the proposed device cannot be further improved when we introduce grating structure into the device in order to excite surface plasmon-polaritons. This is mainly because the light absorption based on our hybrid mode scheme is already thorough so that the introduction of grating structure can only improve the light loss dissipated in the metal electrodes due to scatterings and diffractions by the gratings. Therefore, the proposed hybrid mode based scheme can be considered as a simple and effective light trapping scheme for organic solar cells and may find applications in both polymer and small molecular based organic solar cells.

Keywords: 
• organic solar cells /
• microcavity resonance /
• antireflection resonance /
• hybrid mode