Rune Søndenå

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.

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.

Recombination activity of grain boundaries in high-performance multicrystalline Si during solar cell processing

In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditio ...

Resistivity profiles in multicrystalline silicon ingots featuring gallium co-doping

Three ingots with different doping concentrations have been produced using Elkem Solar Silicon feedstock. Gallium dopants are added to the compensated Elkem Solar Silicon ingots in order to obtain ...

Identifying recombination parameters by injection-dependent lifetime spectroscopy on mc-silicon based on photoluminescence imaging

The minority carrier lifetime is a crucial material parameter in silicon (Si) wafers for use in solar cell applications, and precise measurements of carrier lifetime as a function of the excess carrier concentration (injection level) is of high importance. In this paper we present a method for extracting injection-dependent lifetime data with high spatial resolution, without the need for advanced time-resolved camera detection systems. This enables investigations of single grains, grain boundaries and structural defects in wafers with spatially non-uniform lifetime, such as high performance multicrystalline Si wafers. The local injection dependent lifetime curves are constructed from a series of photoluminescence images acquired using different steady state generation rates, carefully calibrated by a secondary quasi-steady state photoconductance measurement at a fixed light intensity. The local lifetime has been analyzed by linear parameterization of the Shockley-Read-Hall recombination model and solved for all combinations of defect parameters describing the observed recombination behavior. The recombination parameters found to dominate at high injection corresponds well with published recombination parameters due to Cri.

Temperature coefficients in compensated silicon solar cells investigated by temperature dependent lifetime measurements and numerical device simulation

Silicon solar modules typically operate at a higher temperature than the 25 °C used for standard testing, and the temperature coefficient (TC) therefore might have a significant impact on the field performance. In this paper the temperature dependent behavior of compensated Si solar cells has been simulated using PC1Dmod6.2, using a combination of physical models which include the effect of both temperature and compensation doping. The simulations were based on experimental input measured on two high performance multicrystalline ingots of similar resistivity of ∼1.3 Ωcm, as well as one ingot with a resistivity of 0.5 Ωcm. Two of the ingots are produced using Elkem Solar Silicon (ESS®), a compensated Si feedstock made using a metallurgical purification route, and the third is made from non-compensated reference material. Dopant concentrations as a function of height in the ingot were determined using a combination of experimental resistivity data, simulations and the Scheil equation. Temperature dependent lifetime images, measured on etched and passivated wafers after relevant solar cell processing steps were also acquired at different heights and used as input to the simulations. Taking all this into account, the simulated TC in the efficiency were found to be similar for the two 1.3 Ωcm ingots and slightly higher (less negative) in the 0.5 Ωcm ingot, mostly caused by differences in the TC of the short circuit current and fill factor. We find a reasonable agreement between the simulated and experimental TCs, with the main difference being a ∼0.02 %/K more negative TC in the open circuit voltage in the simulated values. This corresponds to only a 6-7% relative deviation from the experimental values, showing the validity of the PC1Dmod model.Silicon solar modules typically operate at a higher temperature than the 25 °C used for standard testing, and the temperature coefficient (TC) therefore might have a significant impact on the field performance. In this paper the temperature dependent behavior of compensated Si solar cells has been simulated using PC1Dmod6.2, using a combination of physical models which include the effect of both temperature and compensation doping. The simulations were based on experimental input measured on two high performance multicrystalline ingots of similar resistivity of ∼1.3 Ωcm, as well as one ingot with a resistivity of 0.5 Ωcm. Two of the ingots are produced using Elkem Solar Silicon (ESS®), a compensated Si feedstock made using a metallurgical purification route, and the third is made from non-compensated reference material. Dopant concentrations as a function of height in the ingot were determined using a combination of experimental resistivity data, simulations and the Scheil equation. Temperature dependent ...

Impact of thermal history on defects formation in the last solid fraction of Cz silicon ingots

:For the first time, the impact of the tail detachment on the quality of the last solid fraction of a Czochralski silicon ingot body is reported. Simulations of the thermal history were performed on CGSim software and showed that producing an ingot with a tail detached from the melt before the cone-end (the so called “popped-out” tail) changes the time that the last part of the ingot body remains at the 900-1200°C temperature range and could thus impact the growth of defects such as oxygen precipitates. In addition, ingots with tails completely grown were characterized and compared to ingots with popped-out tails. Lifetime measurements of the ingot last solid fraction were performed while voids and oxygen related defects were delineated with chemical etchants. These measurements were complemented with FTIR measurements performed at room and low temperature (30 K), before a two-step thermal oxidation took place. The results show no impact of the earlier detachment from the melt on the as-grown lifetime, as...

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.