Bin Wei

High-efficiency, broad-band and wide-angle optical absorption in ultra-thin organic photovoltaic devices

Metal nanogratings as one of the promising architectures for effective light trapping in organic photovoltaics (OPVs) have been actively studied over the past decade. Here we designed a novel metal nanowall grating with ultra-small period and ultra-high aspect-ratio as the back electrode of the OPV device. Such grating results in the strong hot spot effect in-between the neighboring nanowalls and the localized surface plasmon effect at the corners of nanowalls. These combined effects make the integrated absorption efficiency of light over the wavelength range from 400 to 650 nm in the active layer for the proposed structure, with respect to the equivalent planar structure, increases by 102% at TM polarization and by 36.5% at the TM/TE hybrid polarization, respectively. Moreover, it is noted that the hot spot effect in the proposed structure is more effective for ultra-thin active layers, which is very favorable for the exciton dissociation and charge collection. Therefore such a nanowall grating is expected to improve the overall performance of OPV devices.

Flexible organic light-emitting diodes with enhanced light out-coupling efficiency fabricated on a double-sided nanotextured substrate.

High-efficiency organic light-emitting diodes (OLEDs) have generated tremendous research interest. One of the exciting possibilities of OLEDs is the use of flexible plastic substrates, which unfortunately have a mismatching refractive index compared with the conventional ITO anode and the air. To unlock the light loss on flexible plastic, we report a high-efficiency flexible OLED directly fabricated on a double-sided nanotextured polycarbonate substrate by thermal nanoimprint lithography. The template for the nanoimprint process is a replicate from a silica arrayed with nanopillars and fabricated by ICP etching through a SiO2 colloidal spheres mask. It has been shown that with the internal quasi-periodical scattering gratings the efficiency enhancement can reach 50% for a green light OLED, and with an external antireflection structure, the normal transmittance is increased from 89% to 94% for paraboloid-like pillars. The OLED directly fabricated on the double-sided nanotextured polycarbonate substrate has reached an enhancing factor of ∼2.8 for the current efficiency.

Visibly transparent organic photovoltaic with improved transparency and absorption based on tandem photonic crystal for greenhouse application

We demonstrate a visible transparent organic photovoltaic (OPV) with improved transmission and absorption based on tandem photonic crystals (TPCs) for greenhouse applications. The proposed device has an average transmittance of 40.3% in the visible range of 400-700 nm and a high quality transparency spectrum for plant growth with a crop growth factor of 41.9%, considering the weight of the AM 1.5G solar spectrum. Compared with the corresponding transparent OPV without photonic crystals, an enhancement of 20.7% in the average transmittance and of 24.5% in the crop growth factor are achieved. Detailed investigations reveal that the improved transmittance is attributed to the excitation of the optical Tamm state and the light interference effect in TPC. Concomitantly, the total absorption efficiency in the active layer of the designed TPC based transparent OPV reaches 51.5%, being 1.78% higher than that of the transparent OPV without PC and 76% of that of the opaque counterpart. The improved absorption originates from the Bragg forbidden reflectance of TPC. Overall, our proposal achieves the optimized utilization of sunlight by light manipulation of TPC.

Omnidirectional and polarization-insensitive light absorption enhancement in an organic photovoltaic device using a one-dimensional nanograting

A novel poly-3-hexylthiophene (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) solar cell was designed with both the silver cathode and the organic layers structured into one-dimensional periodic patterns. Its optical behavior was investigated systematically by the finite element method. The numerical results indicate that a broadband, omnidirectional, and polarization-insensitive light absorption enhancement is realized by utilizing this structure. This is attributed to the simultaneous excitation of both plasmonic and photonic modes by particular structural designs. Moreover, the increased absorption allows for the use of very thin active layers, resulting in decreased losses of charge carriers transporting to the electrodes. Therefore, this proposal is expected to find potential applications in organic solar cells for improving the overall device performance.

Enhancement of Efficiency and Lifetime of Blue Organic Light-Emitting Diodes Using Two Dopants in Single Emitting Layer

We have demonstrated efficient blue organic light-emitting diode with the structure of indium tin oxide/4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine/1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine/9,10-di(2-naphthyl)anthracene (ADN): 1-4-di-[4-(N,N-di-phenyl)amino]styryl-benzene (DSA-ph) 3u2009wt%/tris-(8-hydroxyquinoline)aluminum/LiF/Al. Improved efficiencies and longer operational lifetime were obtained by codoping a styrylamine-based dopant BD-3 (0.1u2009wt%) into the emitting layer of ADN doped with DSA-ph compared to the case of non-codoping. This was due to the improved charge balance and expansion of exciton recombination zone. The better charge balance was obtained by reducing the electron mobility of ADN which was higher than the hole mobility in the case of non-codoping.

Ultra-Thin Organic Solar Cells Incorporating Dielectric-Coated Comb Silver Nanogratings

Although coating engineering on metallic nanostructures has been frequently applied in organic solar cells (OSCs) for improving the device performance by preventing the possible exciton quenching and charge recombination at the metal surface, there have been few theoretical researches addressing how the dielectric-coating of metal nanostructures influences the optical performance of OSCs. Here, we theoretically investigate the effect of coating a comb silver nanograting by a dielectric film on the absorption performance of OSCs. It is found that, under transverse-magnetic (TM) polarization, when the refractive index or thickness of the dielectric-coating is tuned, different orders of plasmonic waveguide modes are excited by the P3HT:PC61BM/coating/Ag triple-layered system. The dispersion relationship of such a plamsonic waveguide mode is solved and analyzed for clarifying the physical mechanism of the induced light trapping effect. It shows that for optimizing the light absorption in active layer under TM polarization, it is more favorable to coat a thin dielectric film (of 1xa0nm thick) with a suitable refractive index. While under transverse-electric (TE) polarization, the higher refractive index of dielectric-coating, the better the effect of light trapping.

Semitransparent inverted organic solar cell with improved absorption and reasonable transparency perception based on the nanopatterned MoO3/Ag/MoO3 anode

Abstract. We demonstrate an inverted low bandgap semitransparent organic solar cell with improved absorption as well as reasonable transparency perception based on a nanopatterned MoO3/Ag/MoO3 (MAM) multilayer film as the transparent anode under illumination from the MAM side. The integrated absorption efficiency of the active layer at normal hybrid-polarized incidence considering an AM 1.5G solar spectrum is up to 51.69%, increased by 18.53% as compared to that of the equivalent planar device (43.61%) and reaching 77.3% of that of the corresponding opaque nanopatterned device (66.90%). Detailed investigations reveal that the excitation of plasmonic waveguide modes (at transverse magnetic polarization) and photonic modes (at transverse electric polarization) are responsible for the observed enhancement in absorption. Importantly, the proposed device exhibits an average transmittance of up to 28.4% and an average transparency perception of 26.3% for the human eyes under hybrid-polarized light illumination along with a good color rendering property. Additionally, our proposal works very well over a fairly wide angular range.

High-Performance Thin Film Encapsulation for the Application of White Organic Light-Emitting Diodes

Color conversion method has been used to fabricate chromatic-stability white organic light-emitting diodes (OLEDs). Experimental results found little CIE coordinate migration while changing operation voltage from 5 V to 12 V. A simple thin-film encapsulation (TFE) structure has been developed through vacuum thermal deposition in combination with plasma enhanced chemical vapor deposition. The luminance of the encapsulated white OLED remain unchanged during the test time. The novel approach is being expected to lower the cost and achieve high-performance TFE.