Light management for high efficiency silicon solar cells

In this thesis, light trapping behaviour in silicon solar cells with textured front surfaces and rear reflectors has been characterized experimentally via two optical setups. Various types of novel rear reflectors have been applied on the rear of front-planar PERT (Passivated Emitter and Rear Totally-Diffused) cells with their optical and electrical properties extensively investigated. Reflected light from textured front surfaces of a solar cell contains useful information about the surface geometry as well as the optical properties of the cell. The measured 2-D reflected light distributions from front surfaces of silicon cells textured in various ways are compared to those from conventional ray tracing models and are used to extract details of the surface morphologies. The rear surface reflection of a solar cell is angularly dependent if a textured front surface is applied. The use of hemispherical silicon as a test substrate has been successfully implemented enabling the analysis of the angular reflection properties of the back surface reflector over all incident angles without the restriction caused by refraction at the Si-air interface. Results show that a dielectrically displaced rear reflector scheme using 200 to 300 nm of SiO2 and an Ag mirror provides best angular reflection. The novel planar rear structures with dielectric stacks have been experimentally demonstrated to increase the reflected light intensity by 2.5% absolute at 1200 nm and the internal quantum efficiency (IQE) by 30% relatively at 1150 nm with similar surface passivation quality, compared to a conventional reflector. The best performing scattering reflector using Ag nanoparticles to create localised surface plasmons on the rear of the solar cell enhances the measured external quantum efficiency (EQE) by more than 4-fold at 1160 nm, corresponding to a 16% photocurrent increase (calculated from 900 nm to 1200 nm), compared to a cell with a conventional Al reflector. Thicknesses of the rear surface passivation SiO2 layer and the precursor evaporated Ag film are optimised to achieve maximum optical enhancement with minimum electrical losses. An optical enhancement of 6-fold is achieved at 1200 nm. Finally, an improved double-layer reflector is developed and optimised achieving a further current enhancement of 4.9 % compared to the single layer Ag nanoparticles scheme