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978-3-86853-935-6, Reihe Ingenieurwissenschaften
High-Efficiency n-Type Solar Cells with a Front Side Boron Emitter
194 Seiten, Dissertation Albert-Ludwigs-Universität Freiburg im Breisgau (2010), Softcover, A5
n-type silicon has an enormous potential for wide-scale application in the photovoltaics industry. Its relative tolerance to common impurities (e.g. Fe) potentially results in higher minority carrier diffusion lengths compared to p-type c-Si substrates with a similar impurity concentration. Furthermore, n-type c-Si does not suffer from the boron-oxygen related light-induced degradation (LID), which is known to cause the light-induced degradation for c-Si solar cells based on p-type Cz c-Si.
The objective of this work, therefore, was the realization of highly efficient and stable n-type solar cells with a front side boron emitter. Based on the existing cell technology for the processing of high-efficiency p-type solar cells, all process steps (metallization, emitter diffusion, BSF diffusion, passivation, ...) as well as the process sequence were adapted to the utilization of n-type silicon as the base material. The main focus, however was in the diffusion as well as, in particular, the passivation of the front side boron emitter.
For the passivation of the boron emitter, different approaches have been applied. Thermally grown SiO2 as well as layer stack systems of a thin thermal grown SiO2 covered by PECVD SiNx do not reach the same exceptionally high level of surface passivation that can be reached on n-type surfaces. The saturation current density of all samples did not reach values below 100 fA/cm2. A maximum efficiency of 20.4% could be achieved for n-type PERL solar cells passivated by a SiO2/SiNx stack system and 19.5% for solar cells passivated by a 105 nm thick SiO2. In both cases the open- circuit voltage did not exceed 640mV. After one year storage in the dark, the open-circuit voltage of the solar cells passivated by the single layer SiO2 degraded 30 mV, from 638 mV to 607 mV, whereas the Voc of the solar cells passivated by the SiO2/SiNx stack remained relatively stable (638 mV → 633 mV).