Characterization and loss analyses of passivated emitter and rear cells

Abstract

This work presents characterization methods and simulation-based loss analyses for passivated emitter and rear (PERC) solar cells. Furthermore, it discusses possible ways of introducing poly-Si on thin inter-facial oxides (POLO) junctions into industrial solar cells.

Achieving a further efficiency increase of industrial \gls{PERC} cells is becoming more and more difficult because the margin to the theoretical limit is reduced step by step. Identifying the major loss channel in terms of a potential efficiency gain, thus, plays an increasingly important role in solar cell optimization. The free energy loss analysis (FELA) and the synergistic efficiency gain analysis (SEGA) as simulation-based loss analyses can address this task. The basis for both the FELA and the SEGA are numerical device simulations based on experimentally determined input parameters. The determination of most of these input parameters can be achieved with measurement and data analysis tools, which are commonly used in PV-research. The recombination at local metal contacts, however, has not been studied to the same extent and standard techniques do not apply.

In this work, we study the determination of contact recombination parameters. We first analyze the required sample structures and develop an analytical model to calculate the length scale on which regions of different charge carrier lifetimes affect each other. We find that a metallization pattern with three metallized and one non-metallized quarters fits our requirements best. In the analysis of suitable measurement setups we find that photoconductance-calibrated photoluminescence imaging is best suited because of the low uncertainty. For the extraction of contact recombination parameters we study the analytical model by Fischer and find an excellent agreement of better than 5% deviation with numerical device simulations, provided the assumptions of low level injection and either full line or periodic point contacts are fulfilled. For arbitrary contact layouts and for the full injection dependence we introduce an approach based on numerical device simulations. In this context, we develop a new model for injection dependent contact recombination currents. The model is based on the superposition of recombination at the Si-metal interface and within the highly doped layer underneath.

We use standard measurement and evaluation techniques and determine contact recombination parameters to perform a complete characterization of a PERC cell batch. In this characterization we determine all input parameters required for a SEGA along with the respective uncertainties. From the uncertainties of the input parameters we determine the uncertainties of the SEGA results using a Monte-Carlo simulation. We also analyze the differences between SEGA and FELA and introduce a graphical user interface for automatic SEGA simulations.

Finally we discuss different cell structures for integrating POLO junctions into industrial solar cells by means of SEGA simulations and hypothetical process flows. We identify cells featuring conventional screen-printed Al base contacts and n-type POLO (n-POLO) junctions as promising candidates for industrial integration in the near future. A further development step are solar cells with POLO junctions for both polarities, which show an absolute efficiency benefit between 0.3% and 0.4% compared to similar cells with Al base contacts. However, further research in structuring POLO layers and screen-printed contacting of p-type POLO (p-POLO) is required. From the SEGA simulations and the hypothetical process flows a cell development roadmap was derived, in order to focus research activities on the most promising cell concepts.