Positional irradiance measurement: characterization of spectrum-splitting and concentrating optics for photovoltaics

Abstract

Multijunction photovoltaics enable significantly improved efficiency over their single junction analogues by mitigating unabsorbed sub-bandgap photons and voltage loss to carrier thermalization. Lateral spectrum-splitting configurations promise further increased efficiency through relaxation of the lattice-and current-matching requirements of monolithic stacks, albeit at the cost of increased optical and electrical complexity. Consequently, in order to achieve an effective spectrum-splitting photovoltaic configuration it is essential that all optical losses and photon misallocation be characterized and subsequently minimized.We have developed a characterization system that enables us to map the spatial, spectral, and angular distribution of illumination incident on the subcell reception plane or emerging from any subset of the concentrating and splitting optics. This positional irradiance measurement system (PIMS) comprises four motorized stages assembled in an X-Z-R-Y configuration with three linear degrees of freedom and one rotational degree of freedom, on which we mount an optical fiber connected to a set of spectrometers covering the solar spectrum from 280-1700 nm. In combination with a xenon arc lamp solar simulator with a divergence half angle of 1.3 degrees, we are able to characterize our optics across the full spectrum of our photovoltaic subcells with close agreement to outdoor conditions. We have used this tool to spectrally characterize holographic diffraction efficiency versus diffraction angle; multilayer dielectric filter transmission and reflection efficiency versus filter incidence angle; and aspheric lens chromatic aberration versus optic-to-receiver separation distance. These examples illustrate the versatility of the PIMS in characterizing optical performance relevant to both spectrum-splitting and traditional multijunction photovoltaics.

A compact spectrum splitting concentrator for high concentration photovoltaics based on the dispersion of a lens

“…The optimum placement of the four-terminal two-junction photovoltaic collector relative to the lens is determined by scanning the device along the optical axis until maximum power output is read. This is accomplished by illuminating the device with a simulated AM 1.5 D spectrum on a custommade PIMS [39] which is comprised of four motorized stages to provide precise control of the displacement of the device, defined as the distance between the InGaP cell and a fixed position along the optical axis. (A larger displacement corresponds to a greater distance of the device from the lens.)…”