Nicolas Enjalbert

Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells

Compensation effects are intensively studied on two highly doped ingots grown from solar-grade silicon feedstocks purified using metallurgical routes, through a comparison of the electrical properties at iso-carrier densities. Working at given carrier densities enables a clearer extraction of the compensation effects, at the wafer and solar cell levels. At the wafer level, the majority carrier mobility and the carrier lifetime are investigated. Regarding the mobilities, it was found that current models may underestimate the amount of incomplete ionization of boron leading to underestimated mobilities. In addition, the majority carrier mobility was found to be strongly affected at high compensation level. Regarding the carrier lifetimes, our results show that after a phosphorus diffusion step, dopants alone — and especially boron — can limit the lifetime in highly doped solar-grade silicon. At the cell level, I-V characteristics under standard illumination were studied. In particular, the observed reductio...

Oxygen-Related Thermal Donor Formation in Dopant-Rich Compensated Czochralski Silicon

The thermal donor (TD) generation in dopant-rich compensated Czochralski silicon was studied by pulling an ingot from a feedstock containing large amounts of donors and acceptors. In a wafer located in the vicinity of the change of conductivity type, thermal donors were formed and the evolution of their concentration was similar to that in noncompensated lowly boron-doped silicon. Thus, simultaneous high densities of both boron and phosphorus do not have a significant impact on the TD formation. This brings the experimental evidence that for a given oxygen concentration and annealing temperature, the TD formation is controlled by the electron density.

Mapping of the dopant compensation effects on the reverse and forward characteristics of solar cells

The effect of doping compensation on the electrical and photovoltaic properties of Silicon (Si) wafers and solar cells are mapped on peculiar wafers taken from a highly doped and compensated ingot. Extensive mapping characterizations were performed at the material (carrier density and mobility) and at the solar cell levels (I-V parameters under illumination, breakdown voltages). One of the striking features of this work is that, despite the high concentrations of doping impurities in the substrates, most solar cells feature breakdown voltages high enough to allow standard module architectures to be used.

Influence of carrier density spatial heterogeneities on the electrical breakdown of crystalline silicon solar cells: Experiment and simulation

In this work we investigate how spatial heterogeneities of the substrate hole density influence the reverse characteristics of UMG solar cells. Electroluminescence and current-voltage data on highly heterogeneous cells are compared with the predictions of TCAD simulations. Reverse-bias electroluminescence images reveal that the distribution of breakdown sites follows the general pattern of the carrier density distribution, but are highly localized at some specific defects. Measurements show that the overall breakdown voltage is governed by the highest carrier density across the wafer, suggesting that this maximum density should be controlled with care. These results are in good agreement with simulation showing that TCAD tools can successfully include the effect of spatial heterogeneities.