Improved modeling of grain boundary recombination in bulk and p-n junction regions of polycrystalline silicon solar cells

Abstract

This article provides a theoretical investigation of recombination at grain boundaries in both bulk and p-n junction regions of silicon solar cells. Previous models of grain boundaries and grain boundary properties are reviewed. A two dimensional numerical model of grain boundary recombination is presented. This numerical model is compared to existing analytical models of grain boundary recombination within both bulk and p-n junction regions of silicon solar cells. This analysis shows that, under some conditions, existing models poorly predict the recombination current at grain boundaries. Within bulk regions of a device, the effective surface recombination velocity at grain boundaries is overestimated in cases where the region around the grain boundary is not fully depleted of majority carriers. For vertical grain boundaries (columnar grains), existing models are shown to underestimate the recombination current within p-n junction depletion regions. This current has an ideality factor of about 1.8. An improved analytical model for grain boundary recombination within the p-n junction depletion region is presented. This model considers the effect of the grain boundary charge on the electric field within the p-n junction depletion region. The grain boundary charge reduces the p-n junction electric field, at the grain boundary, enhancing recombination in this region. This model is in agreement with the numerical results over a wide range of grain boundary recombination rates. In extreme cases, however, the region of enhanced, high ideality factor recombination can extend well outside the p-n junction depletion region. This leads to a breakdown of analytical models for both bulk and p-n junction recombination, necessitating the use of the numerical model.

Charged grain boundaries reduce the open-circuit voltage of polycrystalline solar cells—An analytical description

“…The J ( V ) relations for these cases are summarized in Table I. Similar results have been obtained in previous works on this problem 20,22 , however some of the relations we present are new. In Sec.…”

Study of the Microstructures and Lifetime of Spin-Cast Silicon Sheets for Photovoltaics

“…Consequently, the possibility of carbon and oxygen contamination is low because the duration for which the sheet is in contact with the mold is short. The vertically aligned grains produced via spin casting differ from the horizontal grains or random microstructures formed by other methods, with such differences having been studied by Edmiston et al 9 In present study, the bulk lifetime of the spin-cast sheets were measured by the microwave photoconductance decay (MW-PCD) techinique using a SEMILAB WTN-2000 instrument. The microstructure of the sheets was investigated thorough analysis of backscattered electron (BSE) images obtained using an electron probe microanalyzer (EMPA), while the character of the grain boundaries was determined by a setup incorporating a scanning electron microscope (SEM)/electron backscatter diffraction (EBSD)/orientation image micrograph (OIM).…”

2D modelling of polycrystalline silicon thin film solar cells

“…on J-V characteristics) is device simulation. A number of studies on the influence of grain boundaries in silicon-based devices can be found in references [8][9][10][11][12][13]. In general, the GB is modeled as an interface layer with a specific trap density and interface recombination velocity.…”