We theoretically and experimentally determined the optimized film properties of luminescent down‐shifting (LDS) layers for thin film solar cells. Based on the predictions from an adapted optical model, we coated thick (300–500 μm) and efficient luminescent down‐shifting layers from environmentally friendly solvents and industrially scalable inks. LDS layers consisted of polyvinyl butyral (PVB) as binder and organic luminescent dyes as UV‐converters. The luminescence quantum yields of the dyes were studied in solution (benzyl alcohol) and for solid thick films. Our data shows that the studied dyes retain luminescent efficiencies of approximately 90 % in the solid state when processed from solution. We further apply the produced layers onto copper indium gallium diselenide (CIGS) solar cells to verify the theoretical predictions for enhancing the external quantum efficiency (EQE) in the UV region. For the best converters a remarkable enhancement of the EQE from 9 % to 52 % was recorded at 380 nm. These findings underline that printed LDS layers indeed have the potential to enhance the efficiency and the light harvesting capabilities of industrially relevant photovoltaic modules.
Abstract:We study the optical properties of polymer layers filled with phosphor particles in two aspects. First, we used two different polymer binders with refractive indices n = 1.46 and n = 1.61 (λ = 600 nm) to study the influence of Δn with the phosphor particles (n = 1.81). Second, we prepared two particle size distributions D 50 = 12 µm and D 50 = 19 µm. The particles were dispersed in both polymer binders in several volume concentrations and coated with thicknesses of 150-600 µm onto glass substrates. Experimental results and numerical simulations show that the layers of the higher refractive index binder with larger particles result in the highest optical transmittance in the visible light spectrum. Finally, we used numerical simulations to determine optimal layer composition for application in realistic photovoltaic devices.
Surface structures in micrometer scale were prepared on different substrates by a coating process using particle‐filled slurries based on a blend of preceramic polymers. Parameters like solvent fraction, substrate material, filler particle size, and layer thickness were varied. Demixing reactions of solvent and polymers led to surface structures with pores of 1–60 μm in diameter. The solubility parameters δt of the polysiloxanes used are about 18.7 MPa1/2 for poly(methyl phenyl vinyl siloxane) and 19.5 MPa1/2 for poly(methyl siloxane), respectively. By pyrolysis of the green layers, ceramic coatings were obtained by keeping the particular surface structures.
We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.
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