Daniel Macdonald

Recombination activity of interstitial iron and other transition metal point defects in p- and n-type crystalline silicon

This work has been supported by the Australian Research Council and The Netherlands Agency for Energy and the Environment.

Trapping of minority carriers in multicrystalline silicon

Abnormally high effective carrier lifetimes have been observed in multicrystalline silicon wafers using both transient and steady-state photoconductance techniques. A simple model based on the presence of trapping centers explains this phenomenon both qualitatively and quantitatively. By fitting this model to experimental data acquired with a quasi-steady-state photoconductance technique, it is possible to determine the trap density, trap energy, and the ratio between the mean-trapping time and mean-escape time. A correlation between trap density and dislocation density in the material has been found.

Iron detection in crystalline silicon by carrier lifetime measurements for arbitrary injection and doping

This work has been supported by NOVEM (The Netherlands Agency for Energy and the Environment) under contract no. 2020.01.13.11.2002.

Transition-metal profiles in a multicrystalline silicon ingot

The concentrations of transition-metal impurities in a photovoltaic-grade multicrystalline silicon ingot have been measured by neutron activation analysis. The results show that the concentrations of Fe, Co, and Cu are determined by segregation from the liquid-to-solid phase in the central regions of the ingot. This produces high concentrations near the top of the ingot, which subsequently diffuse back into the ingot during cooling. The extent of this back diffusion is shown to correlate to the diffusivity of the impurities. Near the bottom, the concentrations are higher again due to solid-state diffusion from the crucible after crystallization has occurred. Measurement of the interstitial Fe concentration along the ingot shows that the vast majority of the Fe is precipitated during ingot growth. Further analysis suggests that this precipitation occurs mostly through segregation to extrinsic defects at high temperature rather than through solubility-limit-driven precipitation during ingot cooling.

Imaging interstitial iron concentrations in boron-doped crystalline silicon using photoluminescence

D.M. is supported by an Australian Research Council QEII Fellowship. The Centre of Excellence for Advanced Silicon Photovoltaics and Photonics at UNSW is funded by the Australian Research Council.

Light-induced boron-oxygen defect generation in compensated p-type Czochralski silicon

D.M. is supported by an Australian Research Council QEII Fellowship, L.J.G. acknowledges SenterNovem for support, and B.L. and J.S. acknowledge the support of the German Academic Exchange Service.

Reduced fill factors in multicrystalline silicon solar cells due to injection-level dependent bulk recombination lifetimes

Recombination lifetimes of multicrystalline silicon solar cell precursors have been measured experimentally as a function of injection-level, and modeled using Shock-ley-Read-Hall statistics. The expressions for the variable lifetimes are then used to predict the final cell open-circuit voltages and fill factors using a simple analytic method. When accurate recombination lifetimes measurements are possible, the predicted parameters match well with the measured values on finished cells. The cells are shown to be limited by the presence of bulk recombination, which not only limits the open-circuit voltage through lower lifetimes, but also reduces the fill factor due to a strong injection-level dependence around one-sun maximum-power conditions. It is shown that such non-ideal behaviour cannot be adequately explained by junction recombination. The specific effect of interstitial iron, an important impurity in silicon, on voltages and fill factors is modeled numerically and discussed.

Generation and annihilation of boron–oxygen-related recombination centers in compensated p- and n-type silicon

Funding was provided by the State of Lower Saxony. D.M. is supported by an Australian Research Council QEII Fellowship.

Capture cross sections of the acceptor level of iron–boron pairs in p-type silicon by injection-level dependent lifetime measurements

Injection-level dependent recombination lifetime measurements of iron-diffused, boron-doped silicon wafers of different resistivities are used to determine the electron and hole capture cross sections of the acceptor level of iron–boron pairs in silicon. The relative populations of iron–boron pairs and interstitial iron were varied by exposing the samples to different levels of illumination prior to lifetime measurements. The components of the effective lifetime due to interstitial iron and iron–boron pairs were then modeled with Shockley–Read–Hall statistics. By forcing the sum of the modeled iron–boron and interstitial iron concentrations to equal the implanted iron dose, in conjunction with the strong dependence of the shape of the lifetime curves on dopant density, the electron and hole capture cross sections of the acceptor level of iron–boron pairs have been determined as (3±2)×10−14 cm−2 and (2±1)×10−15 cm−2.

Doping dependence of the carrier lifetime crossover point upon dissociation of iron- boron pairs in crystalline silicon

The excess carrier density at which the carrier lifetime in crystalline silicon remains unchanged after dissociating iron-boron pairs, known as the crossover point, is reported as a function of the boron dopant concentration. Modeling this doping dependence with the Shockley-Read-Hall model does not require knowledge of the iron concentration and suggests a possible refinement of reported values of the capture cross sections for electrons and holes of the acceptor level of iron-boron pairs. In addition, photoluminescence-based measurements were found to offer some distinct advantages over traditional photoconductance-based techniques in determining recombination parameters from low-injection carrier lifetimes.