Photonics in photovoltaic systems

Abstract

This paper gives an overview on photonics for photovoltaic systems. Starting from the spectral and angular distribution of the electromagnetic radiation from the sun, many important optical approaches how to improve the efficiency of solar cells are presented and discussed. Topics include antireflec-1 Introduction The conversion of solar energy to electrical energy by using photovoltaic (PV) cells is one of the key elements of the future energy supply of mankind. Researchers in the field of photovoltaics are faced with the problem how to increase the efficiency of this conversion process for reasonable cost. The solution is related to optics to a large part. By optimizing the optics, the number of photons which are converted by the semiconductor material can be maximized. First of all, optical approaches are necessary to avoid losses by reflection or unwanted absorption which does not generate charge carriers. As an example, part of the solar radiation incident on a typical PV module made out of crystalline silicon solar cells can already be reflected at the air-glass interface or be absorbed in the glass pane or the lamination material before reaching the solar cell. Further, the light has to be trapped efficiently within the cell. And the photon energy is far away from being optimized. Due to the large range of wavelengths in the solar spectrum almost all photons have the wrong energy because the photon energy may exceed the bandgap energy of the semiconductor or fall below that. In the first case, the photon generates an electron-hole pair and the excess energy is thermalized, in the second case the photon is transmitted or absorbed without generating an electron-hole pair, e.g. by free carriers. Only photons with exactly the energy of the band gap of the semiconductor are generating electron-hole pairs with the maximum efficiency. Thus, the researcher is faced with the need for spectral control of the incident photons.

Optimization of large-size glass laminated luminescent solar concentrators

“…In an attempt to achieve optical concentrations suitable for practical applications, recent studies have focused on strategies to improve the optical efficiency of the luminescent matrix by limiting the loss factors occurring in the LSC device (Albers et al, 2013;Debije and Verbunt, 2012;Goldschmidt et al, 2009;Hernandez-Noyola et al, 2012;Tsoi et al, 2010). In particular, approaches including the use of liquid crystals to limit the re-absorption losses (Verbunt et al, 2009), photonic crystals and wavelength-selective mirrors to limit the surface losses (Debije et al, 2010;Gombert and Luque, 2008;Reisfeld, 2010) have been adopted. The increase of the emission quantum yield was also discussed by using plasmonic structures (Chandra et al, 2012;Giebink et al, 2011;Tummeltshammer et al, 2013).…”

Analysis of short circuit current gains by an anti‐reflective textured cover on silicon thin film solar cells

“…TCO, transparent conductive oxide. 1 Foil (with UV absorber) laminated to additional glass sheet. 2 Flexible foil (no UV absorber) directly attached to glass substrate.…”

“…The air-glass interface is the first interface causing optical reflection losses for the solar light impinging on a solar module or solar cell. For state-of-the-art glasses with low iron contents with a refractive index n glass % 1.5, the reflection r at the interface to air is r % 4% according to the simplification of the Fresnel equations for normal incidence r ¼ n 1 À n 2 n 1 þ n 2 2 (1) with n 1 = 1 and n 2 = n glass . For superstrate-type solar cells and modules, the second interface is that between the glass and the transparent front contact material, a transparent conductive oxide (TCO), typically aluminum-doped zinc oxide ZnO:Al or fluorine-doped tin oxide SnO 2 :F ( n ZnO=SnO2 % 1.6-1.9, depending on wavelength).…”

“…In general, reflection losses at interfaces can be reduced by the use of anti-reflective (AR) coatings. Such AR coatings may be based on (i) dielectric layers or layer stacks, (ii) graded index structures, or (iii) textured surfaces [1][2][3][4][5][6][7][8].…”

“…Dielectric layer stacks (i) are tunable to reduce the reflectance at a desired wavelength down to very low values. Inherent to the effect is a limitation in the spectral range [1] and a sensitivity to the angle of light incidence. Graded index structures (ii) are theoretically effective over a wide spectral range and even a wide range of angles of incidence.…”