Amanda Youssef

Identification of lifetime limiting defects by temperature- and injection-dependent photoluminescence imaging

Identification of the lifetime limiting defects in silicon plays a key role in systematically optimizing the efficiency potential of material for solar cells. We present a technique based on temperature and injection dependent photoluminescence imaging to determine the energy levels and capture cross section ratios of Shockley–Read–Hall defects. This allows us to identify homogeneously and inhomogeneously distributed defects limiting the charge carrier lifetime in any silicon wafer. The technique is demonstrated on an n-type wafer grown with the non-contact crucible (NOC) method and an industrial Czochralski (Cz) wafer prone to defect formation during high temperature processing. We find that the energy levels for the circular distributed defects in the Cz wafer are in good agreement with literature data for homogeneously grown oxide precipitates. In contrast, the circular distributed defects found in NOC Si have significantly deeper trap levels, despite their similar appearance.

Utilization of Tabula Rasa to stabilize bulk lifetimes in n-Cz silicon for high-performance solar cell processing

We investigate a high temperature, high cooling-rate anneal Tabula Rasa (TR) and report its implications on n-type Czochralski-grown silicon (n-Cz Si) for photovoltaic fabrication. Tabula Rasa aims at dissolving and homogenizing oxygen precipitate nuclei that can grow during the cell process steps and degrade the cell performance due to their high internal gettering and recombination activity. The Tabula Rasa thermal treatment is performed in a clean tube furnace with cooling rates >100°C/s. We characterize the bulk lifetime by Sinton lifetime and photoluminescence mapping just after Tabula Rasa, and after the subsequent cell processing. After TR, the bulk lifetime surprisingly degrades to <; 0.1ms, only to recover to values equal or higher than the initial non-treated wafer (several ms), after typical high temperature cell process steps. Those include boron diffusion and oxidation; phosphorus diffusion/oxidation; ambient annealing at 850°C; and crystallization annealing of tunneling-passivating contacts (doped polycrystalline silicon on 1.5 nm thermal oxide). The drastic lifetime improvement during high temperature cell processing is attributed to improved external gettering of metal impurities and annealing of intrinsic point defects. Time and injection dependent lifetime spectroscopy further reveals the mechanisms of lifetime improvement after Tabula Rasa treatment. Additionally, we report the efficacy of Tabula Rasa on n-type Cz-Si wafers and its dependence on oxygen concentration, correlated to position within the ingot.

Electrically-inactive phosphorus re-distribution during low temperature annealing

An increased total dose of phosphorus (P dose) in the first 40 nm of a phosphorus diffused emitter has been measured after Low Temperature Annealing (LTA) at 700 °C using the Glow Discharge Optical Emission Spectrometry technique. This evidence has been observed in three versions of the same emitter containing different amounts of initial phosphorus. A stepwise chemical etching of a diffused phosphorus emitter has been carried out to prepare the three types of samples. The total P dose in the first 40 nm increases during annealing by 1.4 × 1015 cm–2 for the sample with the highly doped emitter, by 0.8 × 1015 cm–2 in the middle-doped emitter, and by 0.5 × 1015 cm–2 in the lowest-doped emitter. The presence of surface dislocations in the first few nanometers of the phosphorus emitter might play a role as preferential sites of local phosphorus gettering in phosphorus re-distribution, because the phosphorus gettering to the first 40 nm is lower when this region is etched stepwise. This total increase in phosp...

Swirl defect investigation using temperature- and injection-dependent photoluminescence imaging

The material class comprising of CU2ZnSn(S,Se)4 (CZTS) is non-toxic and comprises of abundant elements, which makes it to be very interesting for the application in solar cells. Recent progress on the understanding of the materials and devices resulted in increasing efficiencies and it is expected that these will increase further. For a commercial success of CZTS solar cells an industrial low-cost production is required. The CZTS monograin solar cell technology allows developing a solar cell production process independently from material and device research. Challenges and latest results on the roll-to-roll production of monograin CZTS solar cells are discussed and presented.

Using Atom-Probe Tomography to Understand Zn O ∶ Al / Si O 2 / Si Schottky Diodes

We use electronic transport and atom probe tomography to study ZnO:Al / SiO2 / Si Schottky junctions on lightly-doped nand p-type Si. We vary the carrier concentration in the the ZnO:Al films by two orders of magnitude but the Schottky barrier height remains constant, consistent with Fermi level pinning seen in metal / Si junctions. Atom probe tomography shows that Al segregates to the interface, so that the ZnO:Al at the junction is likely to be metallic even when the bulk of the ZnO:Al film is semiconducting. We hypothesize that Fermi level pinning is connected to the insulator-metal transition in doped ZnO, and that controlling this transition may be key to un-pinning the Fermi level in oxide / Si Schottky junctions.

Oxygen migration enthalpy likely limits oxide precipitate dissolution during tabula rasa

In industrial silicon solar cells, oxygen-related defects lower device efficiencies by up to 20% (rel.). In order to mitigate these defects, a high-temperature homogenization anneal called tabula rasa (TR) that has been used in the electronics industry is now proposed for use in solar-grade wafers. This work addresses the kinetics of tabula rasa by elucidating the activation energy governing oxide precipitate dissolution, which is found to be 2.6 ± 0.5 eV. This value is consistent within uncertainty to the migration enthalpy of oxygen interstitials in silicon, implying TR to be kinetically limited by oxygen point-defect diffusion. This large activation energy is observed to limit oxygen precipitate dissolution during standard TR conditions, suggesting that more aggressive annealing conditions than conventionally used are required for complete bulk microdefect mitigation.

Improvement of minority-carrier lifetime in tin monosulfide via substrate engineering

Tin (II) monosulfide (SnS) is a promising Earthabundant, non-toxic thin-film absorber due to its near-ideal optoelectronic properties and manufacturability, but low minority-carrier lifetimes limit SnS device efficiencies to below 5%. We employ electron beam-induced current measurements to deduce the effect of structural defects on bulk recombination in our champion thermally evaporated devices, and then use graphene substrates as a means to reduce detrimental structural defects. We demonstrate an improvement in the effective minority-carrier lifetime of SnS using a graphene substrate, suggesting that the use of substrates with van der Waals surface termination may provide a route toward higher-efficiency SnS-based devices.