Giso Hahn

Influence of hydrogen on the regeneration of boron-oxygen related defects in crystalline silicon

When exposed to light, boron doped monocrystalline Czochralski grown silicon suffers from degradation of the minority carrier lifetime due to the formation of recombination active boron-oxygen related defects. The so called regeneration procedure is able to convert these recombination active defects into a new less recombination active state characterized by a higher minority charge carrier lifetime and stability under illumination. However, the exact working principle on microscopic scale is still unknown even though some influencing factors were identified. The role of hydrogen in the regeneration process is investigated in this work. We find that the characteristic regeneration time constant is subject to variation depending on the process parameters of a Plasma Enhanced Chemical Vapor Deposition a-SiNx:H deposition, namely the applied gas flows, as well as on the thermal history of the sample prior to applying the regeneration procedure. The positive effect of a short high temperature (800–900 °C) ste...

New crystalline silicon ribbon materials for photovoltaics

The objective of this chapter is to review, for photovoltaic application, the current status of crystalline silicon ribbon technologies as an alternative to wafers originating from ingots. Increased wafer demand, the current silicon feedstock shortage and the need of a substantial module cost reduction are the main issues that must be faced in the booming photovoltaic market. Ribbon technologies make excellent use of the silicon, as wafers are crystallised directly from the melt in the desired thickness and no kerf losses occur. Therefore, they offer a high potential to significantly reduce photovoltaic electricity costs when compared to wafers cut from ingots. Nevertheless, the defect structure present in the ribbon silicon wafers can limit material quality and cell efficiency.

Kinetics of the boron-oxygen related defect in theory and experiment

The formation of boron-oxygen complexes in boron-doped crystalline silicon can lead to a severe reduction in the minority charge carrier lifetime. This strongly influences, e.g., solar cell efficiencies if the material is used for photovoltaic application. Recent investigations have shown that a recovery of the carrier lifetime can be achieved by a subsequent thermally enhanced reaction induced by charge carriers. A model of the reaction dynamics of the boron-oxygen complex by means of rate equations is presented in this paper. Following a mathematical description of the reactions involved, the consequences based on the calculations are presented and allow a prediction of the observable electrical parameters. The fundamental agreement with measured data is proven experimentally for different phenomena.

A New Approach to Prevent the Negative Impact of the Metastable Defect in Boron Doped CZ Silicon Solar Cells

A new reaction model concerning the boron-oxygen related degradation is presented, introducing a third recombination inactive state, that stabilizes the electrical parameters of Cz-Si solar cells, and the transition to this new inactive state is proven by experimental data. Furthermore, the stability under solar cell working conditions and the formation kinetics of this additional state are discussed

Influence of domain size on optical properties of ordered GaInP2

Using dark‐field transmission electron microscopy images of ordered GaInP samples, we show how the ordering domain size depends on the growth temperature. Samples with different average domain sizes are compared with regard to their photoluminescence (PL) and excitation spectra. We find a close correlation between the size of the ordered domains and the relative intensity of the PL peak from band–band recombination compared with the rapidly shifting, below‐band‐gap luminescence emission.

Observation of transition metals at shunt locations in multicrystalline silicon solar cells

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescencemicroscopy,transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cellmaterials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Improved iron gettering of contaminated multicrystalline silicon by high-temperature phosphorus diffusion

The efficacy of higher-temperature gettering processes in reducing precipitated iron concentrations is assessed by synchrotron-based micro-X-ray fluorescence. By measuring the same grain boundary before and after phosphorus diffusion in a set of wafers from adjacent ingot heights, the reduction in size of individual precipitates is measured as a function of gettering temperature in samples from the top of an ingot intentionally contaminated with iron in the melt. Compared to a baseline 820 °C phosphorus diffusion, 870 °C and 920 °C diffusions result in a larger reduction in iron-silicide precipitate size. Minority carrier lifetimes measured on wafers from the same ingot heights processed with the same treatments show that the greater reduction in precipitated metals is associated with a strong increase in lifetime. In a sample contaminated with both copper and iron in the melt, significant iron gettering and complete dissolution of detectable copper precipitates is observed despite the higher total metal concentration. Finally, a homogenization pre-anneal in N2 at 920 °C followed by an 820 °C phosphorus diffusion produces precipitate size reductions and lifetimes similar to an 870 °C phosphorus diffusion without lowering the emitter sheet resistance.

Infrared birefringence imaging of residual stress and bulk defects in multicrystalline silicon

This manuscript concerns the application of infrared birefringence imaging (IBI) to quantify macroscopic and microscopic internal stresses in multicrystalline silicon (mc-Si) solar cell materials. We review progress to date, and advance four closely related topics. (1) We present a method to decouple macroscopic thermally-induced residual stresses and microscopic bulk defect related stresses. In contrast to previous reports, thermally-induced residual stresses in wafer-sized samples are generally found to be less than 5 MPa, while defect-related stresses can be several times larger. (2) We describe the unique IR birefringence signatures, including stress magnitudes and directions, of common microdefects in mc-Si solar cell materials including: β-SiC and β-Si3N4 microdefects, twin bands, nontwin grain boundaries, and dislocation bands. In certain defects, local stresses up to 40 MPa can be present. (3) We relate observed stresses to other topics of interest in solar cell manufacturing, including transition...

Minority charge carrier lifetime mapping of crystalline silicon wafers by time-resolved photoluminescence imaging

A camera-based method to record spatially and time-resolved photoluminescence images of crystalline silicon wafers was developed. The camera signal is modulated by a rotating shutter wheel, allowing for a wide range of camera types to be used for the measurement and easy integration into existing photoluminescence setups. The temporal resolution is sufficient to record the decay curve of photoexcited charge carriers in surface-passivated silicon wafers. A transient measurement of minority carrier lifetimes down to less than 10 μs can be obtained for each pixel individually, without the need for any external calibration.

Minimizing the electrical losses on the front side: Development of a selective emitter process from a single diffusion

In this paper we present latest results in the development of a process for the fabrication of a selective emitter structure on mono- and multicrystalline silicon solar cells. The process is based on an approach that was first introduced by Zerga et al. [1]. We have chosen a wet chemical route for an emitter etch back where the areas of the wafer that are intended for emitter metallization are shielded from etching by a screen printable etch barrier. The etch barrier is later removed by wet chemical etching. The process has yielded a gain in open circuit voltage of more than 1% and a gain in short circuit current of more than 2%. The overall efficiency gain was more than 0.3%abs due to slightly lower fill factor of the cells.