Proceedings Article | 23 May 2018
KEYWORDS: Waveguides, Solar cells, Tandem solar cells, Solar energy, Gallium arsenide, Silicon, Absorption, Energy efficiency, Quantum wells, Electrodes
Thermalization and lack of absorption inherently form the main energy losses of any single-junction solar cell, limiting the power conversion efficiency. As is well known, this ‘quantum defect’ problem can be partially solved when different bands of the solar spectrum are distributed over multiple different absorber layers with corresponding bandgaps. In this fashion, conventional vertically-stacked, series-connected tandems have shown to reach higher efficiencies than single-junction solar cells. However, the inherent problems of this geometry are the need for current-matching, transparent intermediate (electrode) layers and, in some geometries, epitaxial growth constraints of the subsequent layers. In addition, fabrication of (epitaxial) layer stacks is expensive and not always compatible with large-scale processing. A parallel-stacked geometry, in which individual solar cells are placed next to each other, could overcome these problems. Such a geometry requires a spectrum-splitting mechanism to guide different spectral bands of the incoming sunlight to the cells with corresponding bandgap [1].
Here, we introduce a tapered dielectric waveguide geometry in which light is coupled out when the waveguide thickness reaches the cut-off condition at a given wavelength. Numerical finite-difference time domain (FDTD) simulations show that the light that is coupled out of the tapered waveguide is spatially separated as a function of wavelength. Light in the 850-1150 nm spectral band is mostly coupled out of the first section of the tapered waveguide, while the 450-850 nm spectral band is coupled out of the last section of the waveguide. Choosing readily available (thin film) Si and GaAs solar cells underneath such a tapered waveguide, only light with energy lower than the GaAs bandgap (~892 nm) reaches the Si subcell, while most of the light with higher energy reaches only the GaAs subcell. This geometry benefits from both a reduction of the ‘quantum defect’ losses and a concentration factor for both subcells.
We fabricate tapered waveguides by e-beam physical vapor deposition (EB PVD) of TiO2 onto a silica glass substrate using a moving shutter during evaporation. Light is coupled into the waveguide using a confocal microscope and optical transmission spectroscopy is used to measure the outcoupling spectrum as a function of position along the tapered waveguide. In addition, Fourier spectroscopy is used to determine the outcoupling angle as a function of wavelength at each position. The tapered geometry is then placed on top of Si and GaAs solar cells and the power coupled into each cell is measured as a function of wavelength. The distribution of light over the subcells can be optimized by controlling the precise tapering geometry. In a further advanced design, the tapered waveguides are integrated with light concentrating optical troughs that couple light into the waveguide taper.
[1] A. Polman and H. A. Atwater, “Photonic design principles for ultrahigh-efficiency photovoltaics,” Nat. Mater., vol. 11, no. 3, pp. 174–177, 2012.
