Jordi Veirman

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...

Spatial characterization of interstitial oxygen and its related defects in Czochralski silicon wafers and ingots: A way to improve the material and device quality

In this paper a new characterization technique, the OXYMAP technology, allowing measurement of the interstitial oxygen concentration and the oxygen related defects in Czochralski grown silicon, is presented. We applied this technique to 8 inch industrial-like p-type ingots. Relevant information regarding the material quality (compositional and electrical properties) were extracted, and we demonstrated the ability of the developed technic to predict the impact of harmful oxygen-related defects on the solar cells performances.

Photoluminescence due to Impurity-cluster-bound Exciton in Highly Doped and Highly Compensated Si

We have investigated photoluminescence (PL) at 4.2 K in highly doped and highly compensated Si with donor and acceptor impurities in the intermediate concentration range from 1 × 1016 to 3 × 1018 cm−3. PL spectra were dominated by the radiative recombination of excitons bound by impurity clusters and the donor–acceptor pair emission. The peak position of the exciton emission shifts to the lower energy side monotonically with an increase in the sum of the donor and acceptor concentrations, where the relationship between the position and the concentration is universal regardless of the species of impurities and is valid also for uncompensated Si. This allowed us to suggest that the cluster consists of multiple species of donor and acceptor impurities and that the difference in the species does not cause a detectable variation in the binding energy of an exciton. A possible method for quantifying the donor and acceptor impurities is proposed.

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.

Effect of dopant compensation on the temperature dependence of the transport properties in p-type monocrystalline silicon

In this paper, we investigate the temperature variations of the hole transport properties in initially uncompensated boron-doped Czochralski silicon progressively compensated through thermal donors activation. After each donor generation anneal, the boron and thermal donor concentrations in the samples are determined using (1) the change in carrier concentration at room temperature and (2) the analysis of the temperature variation of the carrier concentration in the range 77–350 K. By comparing both methods with theory, evidence is brought that down to 77 K the Hall factor is unaffected by compensation up to high compensation levels. This is of great interest for researchers working on new solar-grade materials since it nicely suggests that Hall factor models previously established for non-compensated silicon can be applied to compensated samples, for example, when extracting the individual dopant concentrations from the temperature variations of the hole concentration. At very high compensation levels, a...

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.