Enhancing light capture by silicon - with the help of molecules

Efficient capture of sunlight remains one of the great challenges to photovoltaics today. This is particularly so for the dominant photovoltaic material - crystalline silicon - which, as an indirect gap semiconductor, needs several hundred micrometers thickness for efficient operation. This paper gives an overview of the principal concepts that are currently being considered to enhance light capture by the solar cell. We shall, in particular, compare and contrast two main ideas of thought that underpin the current status of the field. The first, based on thermodynamics, makes use of light trapping where photon path within a structure is extended by virtue of a stochastic photon distribution inside a dielectric / weakly absorbing semiconductor. The second approach rests on the use of sub-wavelength or nano-scale structures which allow the possibility of electromagnetic energy injection into very thin semiconductor layers, by direct interaction with the trapped modes or via the near field of an intermediate dipole absorber or scatterer. We review a range of techniques which are available to reducing the thickness of crystalline silicon solar cells to below 1μm with the use of molecular layers deposited on thin crystalline silicon layers by spin coating, as Langmuir-Blodgett films, or directly anchored to silicon by covalent bonding.