• Energy Research

AbstractIron-acceptor (FeAc) pair association has been studied in compensated n-type silicon. A dynamic approach, based on the charge carrier recombination rates over the Fei trap level, leads to an explanation of the observed FeAc pairing reaction in compensated n-type silicon and extends the understanding of FeAc pairing kinetics. Association kinetics was used to measure a height dependent acceptor concentration profile. Even in compensated n-type silicon good agreement with expected concentrations is found.

Excess-carrier recombination lifetime is a key parameter in silicon solar cell design and production. With the vast international use and recent standardization (SEMI PV13) of eddy-current wafer and brick silicon lifetime test instruments, it is important to quantify the inter- and intralaboratory repeatability. This paper presents the results of an international interlaboratory study conducted with 24 participants to determine the precision of the SEMI PV13 eddy-current carrier lifetime measurement test method. Overall, the carrier recombination lifetime between-laboratory reproducibility was found to be within ±11% for the quasi-steady-state mode and ±8% for transient mode for wafer samples, and within ±4% for bulk samples.

Abstract There is a strong need for low-cost silicon for solar cells and accordingly, there is an on-going debate about what are the acceptable contamination levels within the purified silicon feedstock to specify the material as solar grade silicon. Impurities in silicon can drastically modify the material properties relevant for solar cells. Transition metal impurities such as iron, copper and chromium are known to enhance the net recombination of minority carriers within the bulk material or negatively influence the emitter region and thus are detrimental for the cell efficiency. The knowledge about the level of metals and dopants, which are present in solar cell material, is of utmost interest for the use of alternative feedstock materials such as upgraded metallurgical silicon (UMG-Si) feedstock and for the improvement of actual industrial materials for solar cell production. Whereas most previous studies used artificial high contamination levels to investigate their influence on the solar cells efficiency, this work presents data using recent industrial feedstock material. For the first time, sensitive and direct chemical analyses of silicon before and after crystallisation were performed and correlated with final solar cell results.

AbstractIn this work the temperature of a silicon sample excited by a laser during a time dependent microwave-detected photoconductance decay (MWPCD) measurement was logged. 2°C temperature increase on the backside of the wafer was observed. Temperature dependent charge carrier lifetime calculations using the determined temperature characteristic are compared to time dependent MWPCD measurements of a boron-doped wafer with high oxygen content. The time duration to reach the final temperature fits well with the time scale of the fast forming recombination center of the boron-oxygen related defect. Charge carrier lifetime variation due to temperature increase is discussed in view of different defect parameters.

AbstractThe impact of the three main process steps during p-type solar cell processing, namely phosphorus diffusion, anti- reflection coating and contact formation, on the electrical quality of Cz silicon is investigated. Adjacent wafers from the middle and tail part of three Czochralski grown silicon ingots of varying quality were treated by the process steps as well as process step combinations. The impact of the thermal budget during the process steps without the application of phosphorus dopant, ammonia and silan gases and metal contact pastes on the Cz silicon was examined. The excess charge carrier lifetime and the interstitial iron content were measured separately for each step. Besides other effects, phosphorus diffusion gettering was found to be very efficient in removing interstitial iron from the Cz silicon. The antireflection coating step as well as the contact formation step were found to be most detrimental to the Cz silicon quality. Changes observed in the Cz silicon quality were discussed within the frame of recent models.

AbstractA laboratory type PID-test system was used to measure degradation curves of the shunt resistance during the stress test. It was found that these curves feature typically an initial plateau without significant changes and a mono-exponential decay, both having temperature depended time constants: The plateau length as well as the decay time constant behave Arrhenius-like. Performing these degradation measurements under various temperatures enable the identification of PID relevant activation energies. A solar module compound made of industrial-type crystalline silicon solar cells was investigated and an activation energy of the decay was determined to (0,95 ± 0,14) eV.

AbstractState of the art Silicon feedstock refined by the metallurgical route readily achieves impurity and dopant concentrations well below 1 ppmw. To avoid costly excessive feedstock refinement, relevant threshold concentrations need to be established for impurities known to have a deleterious effect on the cell performance and reliability. Since compensation may reduce the recombination activity of some impurities, this study evaluates the impact of two important contaminants, Aluminum and Iron, in compensated Silicon. For this, model materials based on electronic grade feedstock are produced and processed to solar cells. The feedstock is co-doped with B (0.25 ppmw) and P (0.43 ppmw) to simulate compensated material, the feedstock was additionally contaminated with Al and Fe and cast to p-type multicrystalline ingots. In cells processed from these materials, Fe has been effectively gettered and the tolerance to Al contamination is found to be much higher than expected from theoretical extrapolations in the literature: for 1 ppmw Al, the efficiency is reduced by about 3% relative. Further, Al doped ingots show weaker light induced degradation than the reference ingots. This is explained by a preferential formation of Al-O complexes in competition with the B-O complex. Breakdown behavior of the pn-junction is influenced by Al doping. No interaction is observed between Al and Fe. Based on the results a threshold concentration close to 0.5 ppmw Al in compensated silicon feedstock is concluded as a safe level in terms of cell performance and reliability.

AbstractLow-temperature FTIR spectroscopy is further developed to be applicable to measure the aluminum concentration in solar-grade silicon in concentrations up to 4 × 1016 atoms/cm3. Absorption spectra of multicrystalline silicon samples doped with varying aluminum content are measured at 10 K and correlated to the dopant density obtained by four point probe resistivity measurements. Calibration factors for absorption peaks of unpaired substitutional aluminum at 443, 472, 516+524 and 867 cm-1 as well as for a Fano anti-resonance at 962 cm-1 are reported.