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978-3-8439-0840-5, Reihe Ingenieurwissenschaften
Fabrication, Analysis and Modelling of Emitter Wrap-Through Silicon Solar Cells
206 Seiten, Dissertation Albert-Ludwigs-Universität Freiburg im Breisgau (2012), Softcover, A5
The present work is dealing with an advanced cell concept, namely the emitter wrap-through solar cell. This concept belongs to the group of back-contacted solar cells which feature reduced shading losses. Due to that these cells are able to generate higher currents. Additionally the EWT solar cell features a region of double-sided collection, which makes this concept attractive for low quality material.
Network simulation is used to determine the series resistance losses, which are known to have a large impact on cell performance. The simulation results are compared to analytical models from literature. A new model is developed which descibes the series resistance losses more accurately than existing models from literature.
Since the phosphorus emitter exerts a great influence on the recombination losses, a special focus is put on diffusion processes. Several diffused emitter profiles are characterized with respect to their recombination properties. Additionally a co-diffusion process is developed which enables a simultaneous diffusion of a moderately and a heavily doped emitter area in a single high temperature step. Therefore a finite diffusion source consisting of a phosphosilicate glass is unilaterally deposited prior to the diffusion process. The impact of subsequent oxidation processes on the emitter profiles is investigated.
All surfaces present in an EWT cell are analyzed concerning their recombination properties. The recombination properties are used to determine the maximum Voc of a cell with certain geometerical paramters. To estimate the influence of the emitter recombination and surface passivation on the Jsc one dimensional simulation is performed. Taking into account the resistance losses by using the new analytical model enables the calculation of an upper efficiency limit.
In the last part of this work EWT solar cells are manufactured applying different process sequences. It is shown that the application of a co-diffusion process results in an increased Jsc of the cells. A final annealing step leads to a decrease in the solar cell efficiency. This effect is attributed to thermal activated spiking of the Al metallization contacting directly the emitter. A screen-printed Ag finger, which is also used for contacting the emitter, acts as a barrier and prevents Al spiking. Using FZ-Si, peak conversion efficiencies of 18.8% on an active cell area of 16.63cm^2 are achieved.