Research On Absorption Enhancement Of Ultrathin Organic Photovoltaic Devices By Metallic Coreshell Nanogratings

Organic photovoltaic devices (OPVs) have great potential as a renewableenergy source due to their relatively low-cost, ease of fabrication as well as theircompatibility with large area flexible substrates. However, their powerconversion efficiencies (PCE) are significantly low at present compared withtheir inorganic counterparts. To effectively increase absorption by active layer ofOPVs thereby improving their PCE, periodic metallic nanogratings have beenwidely adopted in OPVs. Nevertheless, the general one-dimensional (1D)nanogratings are strongly dependent on the light polarizations and the lighttrapping of1D nanogratings greatly relies on the angle of light incidence.Therefore, the polarization-sensitivity and angle-dependence OPVs areunfavorable to their practical applications. Concerning this, a class of metalliccoreshell nanogratings is designed, working as back reflector cathodes or toptransparent anodes of conventional OPVs. Simultaneously, the photoactive layeris also nanopatterned into periodic concavo-convex profiles. The impacts ofmetallic coreshell nanogratings on light absorption by active layer of OPVs aretheoretically investigated. Additionally,1D metallic coreshell nanogratings are assembled into transparent inverted OPVs and the influence of1D nanogratingson optical absorption active layer, light transmission properties of the device areexplored in theory. Major conclusions are as follows:1、 Research on absorption enhancement of OPVs by incorporating Agcoreshell nanograting as back reflector cathode. The difference from the existingworks is that, in addition to modifying the silver back reflector cathode into a1D nanograting, we also structured the active layer and the carrier transportlayer into periodical patterns. Investigations indicate that: the variations instructural parameters of coreshell nanograting affect on the types, orders,strength of resonant modes, specially, the nanograting period directly governsresonant modes. Compared to the equivalent planar architecture, the totalabsorption efficiency of the optimized structure with P=400nm, W=75nmand H=70nm is enhanced by11.3%,15.4%�'�13.3%upon TE, TM and hybridpolarizations, indicating a broadband, polarization-insensitivity absorptionenhancement, which is attributed to the collective excitations of thephotonic/photonic coupling modes and photonic/plasmonic coupling modes.Furthermore, the proposed configuration shows an omnidirectional absorptionenhancement.2、 Studies on absorption enhancement of CuPc/C60-based OPVs bycombining MoO3/Ag/MoO3(MAM) coreshell nanograting as top anode. Thisconfiguration is characterized by the nanopatterned MAM multilayer as topanode as well as the active layer structured into periodical patterns. By surveying, we conclude that: the variations in structural parameters of MAM coreshellnanograting have markedly influence on resonant modes and modes coupling.Relative to the planar control structure, the integrated absorption efficiency forthe device configuration with optimal structural parameters p=180nm, FF=0.25and h’=43nm is increased by178.88%,19.44%and99.16%(consideringthe weight of AM1.5G solar spectrum) upon TM, TE and hybrid polarizations,which is ascribed to the excitation of surface plasmon resonance, photonicmodes and strong modes coupling. Additionally, the proposed design also showsa broadband and omnidirectional absorption enhancement.3、Investigations on absorption of active layer and transmission behaviorsof transparent inverted OPVs based on MoO3/Ag/MoO3(MAM) coreshellnanograting. In this transparent inverted OPVs, MAM grating serves as thetransparent anode, ZnO/ITO acts as the transparent cathode and the active layeris patterned into periodic concavo-convex profiles. Systematical studies onabsorption demonstrate that: the periods of MAM nanograting directly regulateresonant modes and the heights show significant effects on the order of resonantmodes. Compared with the equivalent planar device, the proposed device with p=240nm, w=55nm and h’=90nm exhibits notable absorption enhancementswith the enhancement factors of25.27%,11.79%and18.53%at TM, TE andhybrid polarizations upon top illumination, indicating a broadband,polarization-independence absorption enhancement, which originates from theexcitation of SPR, plasmonic waveguide modes and Bloch modes. Moreover, an omnidirectional light absorption is obtained by the proposed device upon topillumination. Specially, the transparent architecture exhibits the similarbroadband, polarization-insensitivity and omnidirectional absorptionenhancement upon bottom illumination. Research on the transparent natureshows: compared to the unpatterned device, the transmittance of the proposedarchitecture degrades upon all polarizations. However, the total transmittancecan reach up to29.25%, higher than that of the benchmark for windowapplications,25%. Meanwhile, The color coordinate of the semitransparentOPVs Dx=(0.3110,0.3506) is particularly close to the standard daylightilluminant D65=(0.3128,0.3290), indicating good transparency color perceptionand rendering properties as well as the capability of providing extremely highquality illumination with achromatic sensation when looking through thedevices under AM1.5G illumination.