Enhanced light trapping for high efficiency crystalline solar cells by the application of rear surface plasmons

In this article, a novel rear structure using Ag nanoparticles to create surface plasmons to enhance light trapping is applied on the rear of planar high efficiency PERT (Passivated Emitter and Rear Totally Diffused) silicon wafer cells, targeting the spectrum range from 1000 nm to 1200 nm. Variations of this rear structure that combine Ag nanoparticles, dielectric layers and back metal reflectors were studied and analysed. Thickness of the rear surface passivation SiO2 spacer layer was optimised to achieve maximum optical enhancement using surface plasmons but with minimum electronic losses due to recombination effects. The effect of the precursor evaporated Ag film thickness was also studied as a means to vary the size/shape of the nanoparticles. The measured external quantum efficiency (EQE) of the best performing rear reflector shows a maximum enhancement of more than 4-fold at 1160 nm. This corresponds to a 16% photocurrent increase (calculated from 900 nm to 1200 nm) compared to the cell with conventional Al rear reflector. Moreover, from the measured spectral response and optical absorption data, we successfully separated and analysed the electrical and optical properties of the novel rear light trapping designs. Light trapping features were quantified using optical parameters characterised by an effective optical path length factor Z, while electrical parameters such as surface recombination velocity S (cm/s) and effective minority charge carrier lifetime τbulk (μs) were also extracted. Relative errors for these parameters were also calculated. For the cell with the best performing rear structure, we report a maximum Z factor enhancement of around 6-fold using Ag nanoparticles in conjunction with a detached Ag reflector, in comparison to the reference at 1200 nm.