Organic wavelength-selective mirrors are used to reduce the loss of emitted photons through the surface of a luminescent solar concentrator (LSC). A theoretical calculation suggests that application of a 400 nm broad reflector on top of an LSC containing BASF Lumogen Red 305 as a luminophore can reflect 91% of all surface emitted photons back into the device. Used in this way, such broad reflectors could increase the edgeemission efficiency of the LSC by up to 66%. Similarly, 175 nm broad reflectors could increase efficiency up to 45%. Measurements demonstrate more limited effectiveness and dependency on the peak absorbance of the LSC. At higher absorbance, the increased number of internal re-absorption events reduces the effectiveness of the reflectors, leading to a maximum increase in LSC efficiency of ~5% for an LSC with a peak absorbance of 1. Reducing re-absorption by reducing dye concentration or the coverage of the luminophore coating results in an increase in LSC efficiency of up to 30% and 27%, respectively.
The efficiency of luminescent solar concentrators could be enhanced by use of wavelength-selective filters, reducing the amount of luminescent light lost. To accomplish this, polarization-independent filters with reflectivity >97% were made by combining layers of cholesteric liquid crystals, either a right- with a left-handed layer, or two right-handed layers with a half-lambda waveplate. Normal cholesteric filters have a reflection bandwidth which is narrower than the spectral and angular range of the luminescent emission. The reflection band is broadened from 80 to 200 nm by employing a pitch gradient in the cholesteric layer. The measured transmission bands compare well with calculations.
Azo copolymers are nonlinear-optical materials, in which polar orientation can be induced by optical poling or electrical poling. We report a new efficient approach to performing photo-assisted poling (PAP) by atomic force microscopy (AFM) for azo copolymer films containing disperse-red-1 chromophores, and to characterize the polar orientation by electrostatic force microscopy (EFM) at the submicrometre scale. Both PAP and contact electrification effects can be generated by the physical interaction between the probes and the films. We demonstrated that these two effects can be distinguished by the relationship between the signs of the charges (bound charges and transferred charges) and the probe bias. Finally, we achieve local PAP far below the glass transition temperature by AFM operated in the tapping mode, and the response of the polar chromophores to local PAP can be studied by EFM.
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