Glass-ceramic covers for highly efficient solar cells

A conventional mono-bandgap solar cell cannot take advantage of the entire solar spectrum. The UV- and IR-wavelength regions are particularly problematic: the energy of IR light is lower than the bandgap energy of typical semiconductor solar-cell material, while a part of the photon energy in the UV spectral range is ‘wasted’ by thermalization and surface recombination of charge carriers. However, frequency ‘downshifters,’ such as fluorescent glasses and glass ceramics doped with rare-earth ions (for example, divalent europium, Eu²⁺), can convert the high-energy part of the solar spectrum into photons in the visible spectral range.

Downshifting describes the absorption of a high-energy photon and the subsequent emission of a photon with lower energy that can be absorbed more efficiently by a solar cell. Eu²⁺-doped fluorozirconate (FZ)-based glass ceramics have suitable optical properties for this purpose. They can be excited in the UV spectral range, leading to emission in the blue spectral regime. Here, the conversion efficiency depends mainly on the host material's phonon energy (lattice vibrations). FZ-based glasses are well known for their low phonon energies and, therefore, are desirable hosts as downshifters. They are based on a modified glass system, so-called ZBLAN, which is an abbreviation for glasses made of a mixture of zirconium (Zr), barium (Ba), lanthanum (La), aluminum (Al), and sodium (Na) fluorides. In our case, they are additionally doped with chlorine ions to enable the growth of barium chloride (BaCl₂) nanoparticles upon thermal treatment (annealing) in which the Eu²⁺ is incorporated.

Annealing for 20min at temperatures between 260 and 280°C leads to a phase transformation from hexagonal to orthorhombic BaCl₂. (The latter describes a system of crystallization characterized by three unequal axes at right angles to each other.) At a temperature of 290°C only orthorhombic-phase BaCl₂ is observed. The size of the BaCl₂ nanoparticles is between 10 and 90nm (see Figure 1).

Figure 1. Barium chloride (BaCl₂) particle size versus annealing temperature for a 5mol% (mole percent) europium (Eu)-doped glass ceramic. The inset shows Eu²⁺ fluorescence spectra for glass ceramics containing hexagonal- (solid) and orthorhombic-phase (dashed) BaCl₂ nanocrystals under UV excitation (280nm).

Europium can be incorporated in the glass in its divalent and trivalent states. Interestingly, Eu²⁺ does not fluoresce in the FZ base glass, although it is clear from electron paramagnetic resonance¹ and Mössbauer spectroscopy² that it is present in the glass. However, Eu³⁺ shows its typical emissions in the red spectral range. On annealing, some of the Eu²⁺ ions are incorporated into the BaCl₂ nanocrystals, leading to intense Eu²⁺-related and BaCl₂ phase-dependent fluorescence under UV excitation (see Figure 1, inset).

For fluorescent (and, in particular, photovoltaic) applications, the metastable hexagonal phase is preferred because of its larger integral fluorescence intensity. The orthorhombic phase is attractive for medical x-ray imaging because of its storage effect. This means that electron-hole (charge-carrier) pairs are ‘stored’ in the glass plate until they are released by a scanning laser beam.

Placing an annealed glass ceramic on top of a conventional silicon solar cell and comparing the corresponding short-circuit current to that of a cell without a cover glass gives a higher short-circuit current, especially for the UV spectral region. That is, the downshifting glass ceramic leads to enhancement of solar-cell efficiency in the UV range. Figure 2 illustrates the experimental setup. The increase at approximately 360nm (see Figure 3) is due to a Eu²⁺ excitation band. For wavelengths longer than 400nm, the short-circuit ratio drops to below ‘1,’ which implies a decrease in the solar-cell efficiency in this spectral range. This decrease is caused by scattering effects of the incident light at the glass-ceramic surface.

Figure 2. Fluorescent Eu-doped glass ceramic as a cover for a solar cell under UV excitation.

Figure 3. Short-circuit current ratio between a conventional silicon solar cell with a glass-ceramic cover and one without a cover. The Eu-doping level of the glass ceramic is 5mol%. The sample was annealed at 280°C.

In summary, on excitation in the UV spectral region, Eu²⁺-doped FZ-based glass ceramics show strong downshifted fluorescence in the visible spectral region. Thermal annealing of the glasses leads to precipitation of fluorescent BaCl₂ nanocrystals. Initial experiments have shown that using these Eu²⁺-doped glass ceramics as a downshifting top layer leads to an increased short-circuit current in the UV spectral range. Our next steps will be to create completely transparent glass ceramics to avoid scattering losses in the visible and near-IR spectral ranges and to create glass ceramics predominantly containing hexagonal BaCl₂ nanocrystals for high light output.