Tine Uberg Nærland

Studying Light-Induced Degradation by Lifetime Decay Analysis: Excellent Fit to Solution of Simple Second-Order Rate Equation

Twenty different boron-doped Czochralski silicon materials have been analyzed for light-induced degradation. The carrier lifetime degradation was monitored by an automated quasi-steady-state photoconductance setup with an externally controlled bias lamp for in-situ illumination between measurements. Logarithmic plots of the time-resolved lifetime decays clearly displayed the previously reported rapid and slow decays, but a satisfactory fit to a single exponential function could not be achieved. We found, however, that both decay curves, for all the investigated samples, can be fitted very well to the solution of a simple second-order rate equation. This indicates that the defect generation process can be described by second-order reaction kinetics. The new information is used to discuss the role of holes in the defect reaction and the rate-determining steps of the rapid and slow defect reactions.

Direct monitoring of minority carrier density during light induced degradation in Czochralski silicon by photoluminescence imaging

In this paper, we present a new method for studying the light induced degradation process, in which the minority carrier density is monitored directly during light soaking by photoluminescence imaging. We show experimentally that above a certain minority carrier concentration limit, Δnlim, the boron oxygen (B-O) defect generation rate is fully independent of the injected carrier concentration. By simulation, we determine Δnlim for a range of p-type Czochralski silicon samples with different boron concentrations. The normalized defect concentrations, Nt*, are determined for the same samples by time-resolved Quasi Steady State Photoconductance measurements. After 10 min of light degradation, no correlation between Δnlim, and Nt* is observed. These results indicate that the role of the excess carriers during the rapid decay is to first change the charge state of the defects by shifting the electron quasi-Fermi level across the energy level of the defect centre in its passive state (Elat = EV + (635 ± 18) meV...

The role of excess minority carriers in light induced degradation examined by photoluminescence imaging

A new approach to investigate light induced degradation (LID) effects in boron-doped silicon has been developed. By studying spatial variations in LID resulting from localized carrier excitation (spot-LID), it is verified that the generation of the boron-oxygen complexes responsible for the degradation is directly related to the presence of excess minority carriers. Through the examination of the diffused minority carrier density distribution (during light exposure), from an exposed into an unexposed wafer area compared to the observed defect generation, we are able to monitor the generation of excess carrier induced defects over a range of carrier concentrations. The results show that very low concentrations of minority excess carrier densities are sufficient to generate the defects. For the investigated material carrier concentrations down to 1.7 ± 0.2 × 109 cm−3 are observed to cause lifetime degradation.

Nano-XRF Analysis of Metal Impurities Distribution at PL Active Grain Boundaries During mc-Silicon Solar Cell Processing

Metal impurities are known to hinder the performance of commercial Si-based solar cells by inducing bulk recombination, increasing leakage current, and causing direct shunting. Recently, a set of photoluminescence (PL) images of neighboring multicrystalline silicon wafers taken from a cell production line at different processing stages has been acquired. Both band-to-band PL and sub-bandgap PL (subPL) images showed various regions with different PL signal intensity. Interestingly, in several of these regions a reversal of the subPL intensity was observed right after the deposition of the antireflective coating. In this paper, we present the results of the synchrotron-based nano-X-ray fluorescence imaging performed in areas characterized by the subPL reversal to evaluate the possible role of metal decoration in this uncommon behavior. Furthermore, the acquisition of a statistically meaningful set of data for samples taken at different stages of the solar cell manufacturing allows us to shine a light on the precipitation and rediffusion mechanisms of metal impurities at these grain boundaries.

On the recombination centers of iron-gallium pairs in Ga-doped silicon

Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determined to be 220.Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determin...

Nano-XRF analysis of metal impurities distribution at grain boundaries during mc-silicon solar cell processing

Photoluminescence (PL) imaging is a widely accepted tool to characterize the quality of multicrystalline and monocrystalline silicon cells. Recently a set of neighboring multicrystalline silicon wafers taken from a cell production line at different processing stages have shown an unexpected PL trend. Band-to-band PL (BPL) and sub-bandgap PL (subPL), were collected for the entire silicon wafers. Interestingly, in various regions of the wafer a reversal of the subPL intensity is observed right after the deposition of the anti-reflective coating (ARC). In this work we present the results of the nanoscale X-ray fluorescence imaging at the points of subPL reversal to evaluate the role of metal decoration on this uncommon behavior and we complement it with our previous findings on the distribution of impurities during cell processing.

Studying Light Soaking of Solar Cells by the Use of Solar Simulator

A review of light soaking of solar cells by the use of commercial IV-characterization instruments is presented. The paper addresses the challenges of studying light induced degradation (LID) using a high intensity light source. Issues related to heating of the cell, temporal intensity instability and the impact of the irradiance spectrum are discussed. The main focus of the paper is devoted to the degradation of boron-doped Czochralski silicon (Cz-Si) where boron-oxygen related complexes are responsible for a metastable defect formation. Some advantages and limitations concerning the use of IV characteristics to reveal the degradation properties of boron-doped Cz-Si compared to applying minority carrier lifetime techniques are also presented.

Electrical Properties of Silicon with Bistable Impurity Complexes

Many impurity complexes in silicon such as boron-oxygen and iron-aluminium complexes are found to be bistable. Such defect complexes can be in two different configurations separated by a potential barrier. Commonly bistable recombinative complexes in silicon are studied through carrier lifetime experiments and are analysed by use of Shockley-Read-Hall (SRH) recombination theory. SRH recombination theory is valid for stable defects with one configuration and one energy level in the band gap. However, the theory might fail when applied to the recombination centers formed by bistable defects. This work presents a theoretical study of electrical properties of silicon with bistable impurity complexes. The analysis has been performed for statistics of free electrons and holes, their recombination rate and lifetime. The results have been compared with those obtained from the SRH recombination theory.