Quinta sesión

2.4.2.3 Application of diffuse reflector on other types of solar cells

Non-Si solar cells such as thin film chalcogenide [83, 84] and organic solar cells [85] have also received considerable interest from researchers. These types of cells are kept as thin as possible, due variously to slow deposition rates [86], limited abun- dance of the mineral materials [86] and/or relatively short carrier diffusion length [87]. As the result, the optical losses for these types of solar cells are significant. An excellent scattering reflector such as DR becomes especially important for these cells to reach high conversion efficiency.

Dahan et al. use white paint as the back reflector on a CIGS solar cell with TCO used as the rear contact [77]. The Jscachieved with the cell with white paint is higher

than with Au or Ag reflector and is double of the cell with the standard Mo back contact.

TiO2based DR has also been applied on organic semitransparent solar cells (OSC)

to enhance the photo-current [78]. The Jscof the organic solar cell is improved by 79%

(from 5.5 mA/cm2 _{to 9.9 mA/cm}2_{) with a DR at the back as compared to the 47%}

increase with Ag reflector. Tang et al. extend the work to the application of DR on parallel tandem organic solar cells (TOSC) which are made by stacking two identical organic solar cells on top of each other with a thin layer of PDMS in between. The Ag reflector can improve the Jsc of TOSC by 9.5% while the DR can reach up to

32% on the same cells. Interestingly, by modifying the reflectance and transmittance parameters of the DR, it can also be used as an intermediate scatterer between two subcells with different band-gap to balance the photo-current of the subcells [78], which is crucial to achieve high overall efficiency in tandem devices [88].

2.5

Plasmonic light trapping

Plasmonic light trapping is a technique that applies the interaction of free electrons in the metal with incident photons to increase the optical absorption in photovoltaic devices and light emitting diodes [32, 59, 62]. The free electrons at the metal surface can be excited by electromagnetic wave to form surface plasmons which is the col- lective oscillation of electrons in a conductor [47, 89, 90]. The frequency at which the free electrons response strongly to the incident light is called the surface plasmon res- onance [91]. Surface plasmon polaritons (SPP) are the electromagnetic eigenmodes combined excitations of the electrons and photons that propagate along the interface of the metal and dielectric with wave-vectorK||.

k_|| = ω c

s

em(ω)eD(ω)

em(ω) +eD(ω) (2.12) whereωis the angular frequency of the photon, andcis the speed of the light in the medium. em(ω)andem(ω)denote the dielectric function of the dielectric and metal respectively.

lated metal particles with dimensions smaller than the wavelength of light, they are also called optically driven localized surface plasmon polaritons (LSPP) [92]. Prop- erly designed metal nanoparticles can effectively scatter light at their resonance wave- length. If the nanoparticles are fabricated on the material with high refractive index such as Si, the scattered light will be preferably coupled into optical modes in the material thus increasing the light absorption. The optical behavior of metal nanopar- ticles can be explained to a good approximation by a dipole model [92]. Treated as a dipole, the metal nanoparticle can scatter as well as absorb the light at its plas- mon resonance. The scattering efficiency Qsca and absorption efficiency Qabs can be

defined as: Qsca= Ps IoANP Qabs = Pa IoANP (2.13)

WherePsis the power of the incident light scattered by nanoparticles, Io is the inten-

sity of the incident light, ANP is the cross section area normal to the incident light,

Pa is the power of the light absorbed by nanoparticles.

The special optical characteristics of metal nanoparticles have driven enormous attention and study in the field of solar cell application. Therefore, here we only summarize the important works in the early development of plasmonic light trap- ping. Stuart and Hall first discovered a metal island film can be used to increase the light absorption in a thin semiconductor film. When illuminated by the light with sufficient energy, the metal particles behave as dipoles and oscillate at their resonance frequency, and couple the light into the waveguide modes of the semi- conductor film [93]. Several years later, Schaadt et al. applied Au nanoparticles on highly doped wafer-based solar cells, achieving 80% photo-current enhancement at a single wavelength of 500 nm [94]. Similarly, Derkacs et al. reported an op- tical absorption and photo-current enhancement in a thin-film a-Si:H solar cell by exciting surface plasmon resonance on spherical Au nanoparticles, obtaining an 8% of total efficiency increase [95]. Overall photo-current enhancement of 33% on 1.25 µm silicon-on-insulator cells and 19% on wafer-based silicon solar cells have been reported by Pillai et al. [96]. Catchpole and Polman have developed a fundamen- tal design principle for increasing light trapping probability in solar cells by using plasmonic structures and proved that the particle shape is an essential parameter for efficient light trapping, and a factor of 30 path length enhancement can be achieved in thin-film cells [32]. In the spectral region with wavelength longer than the surface polarization resonance wavelength of nanoparticles, the presence of nanoparticles leads to constructive interference between transmitted and scattered light, thereby increasing the photo-current response in solar cells. But at wavelength shorter than the particle resonance, a reduction in photo-current response presents due to the de- structive interference between transmitted and scattered field components [97]. This phenomenon can be prevented by applying the nanoparticles on the rear side of so-