基于微腔-抗反射谐振杂化模式的吸收增强型有机太阳能电池的理论研究

Theoretical study of microcavity-antire?ection resonance hybrid modes enhanced absorption of organic solar cells

有机太阳能电池中的微腔模式可以在谐振波长附近增强光与物质的相互作用,提高有源层的光吸收,但是其内禀的窄带宽特性限制了器件的宽频谱吸收性能.本文提出一种模式杂化机制,通过在器件外部引入盖层,激发盖层内支持的抗反射谐振模式,使其与器件内在的微腔模式发生耦合作用,形成两个新的杂化模式.杂化模式可以拓宽模式谐振带宽,有利于增强太阳能电池的宽频谱光吸收.理论计算表明,通过设计杂化模式的谐振位置,基于模式杂化机制的平板器件的最优化总吸收率相比传统的微腔器件的最优化性能提高了37%,并同样优越于广泛研究的基于表面等离激元的光栅机制,这证明提出的模式杂化机制是一种简单高效的光束缚机制.

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.