Scanning infrared microscopy investigation of copper precipitation in cast multicrystalline silicon

The behavior of copper precipitation in cast multicrystalline silicon (mc-Si) annealed at different temperatures under air cooling (30 K/s) or slow cooling (0.3 K/s) was investigated by scanning infrared microscopy (SIRM). Comparing to Czochralski-grown silicon (Cz-Si), copper precipitated more easily in mc-Si, and the lowest temperature of copper precipitation in mc-Si was about 700 °C, lower than that in Cz-Si. It was also observed that copper preferably precipitated on grain boundaries so that near the grain boundaries the denuded zone formed. The results indicate that the defects including dislocations, grain boundaries and microdefects, as the heteronucleation sites, enhanced copper precipitation. Moreover, cooling rates had a great influence on the copper precipitation, especially at lower annealing temperatures. Generally air cooling led to the formation of high density of copper-precipitate colonies.     

Effect of contamination with iron on the electron-beam-induced current contrast of extended defects in multicrystalline silicon

The effect of contamination with iron on the recombination activity of extended defects in multicrystalline silicon has been studied by the electron-beam-induced current (EBIC) technique. It has been shown that this process does not lead to the appearance of EBIC contrast of the ??3 and ??9 grain boundaries. It has been revealed that iron diffusion results in a significant increase in the contrast of dislocations introduced by plastic deformation and of traces behind the dislocations in single-crystal silicon, while the dislocation contrast in multicrystalline silicon remains practically unchanged.     

Chemical etching to dissolve dislocation cores in multicrystalline silicon

Multicrystalline silicon wafers are used for approximately half of all solar cells produced at present. These wafers typically have dislocation densities of up to ∼10⁶ cm⁻². Dislocations and associated impurities act as strong recombination centres for electron–hole pairs and are one of the major limiting factors in multicrystalline silicon substrate performance. In this work we have explored the possibility of using chemical methods to etch out the cores of dislocations from mc-Si wafers. We aim to maximise the aspect ratio of the depth of the etched structure to its diameter. We first investigate the Secco etch (1K₂Cr₂O₇ (0.15 M): 2HF (49%)) as a function of time and temperature. This etch removes material from dislocation cores much faster than grain boundaries or the bulk, and produces tubular holes at dislocations. Aspect ratios of up to ∼7:1 are achieved for ∼15 μm deep tubes. The aspect ratio decreases with tube depth and for ∼40 μm deep tubes is just ∼2:1, which is not suitable for use in bulk multicrystalline silicon photovoltaics. We have also investigated a range of etches based on weaker oxidising agents. An etch comprising 1I₂ (0.01 M): 2HF (49%) attacked dislocation cores, but its etching behaviour was extremely slow (<0.1 μm/h) and the pits produced had a low aspect ratio (<2:1).     

Stress‐enhanced dislocation density reduction in multicrystalline silicon

Stress is generally perceived to be detrimental for multicrystalline silicon (mc‐Si), leading to dislocation multiplication during crystal growth and processing. Herein, we evaluate the role of stress as a driving force for dislocation density reduction in mc‐Si. At high temperatures, close to the melting point (>0.8T_m), we observe that the application of stress as well as the relief of residual stress, can modify the density of pre‐existing dislocations in as‐grown mc‐Si under certain conditions, leading to a net local reduction of dislocation density. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence activity of Yang and Secco etched multicrystalline silicon material

Ultraviolet and blue-green photoluminescence (PL) was investigated on multicrystalline silicon (mc-Si) samples chemically etched by Secco and Yang solutions. The samples were characterized by dislocation density (10⁵-10⁶ cm⁻²). The form of etched pits is triangular with Yang etch and like a honeycomb with Secco etch as observed with a scanning electron microscope (SEM). These textures of mc-Si wafers give a PL activity similar to that obtained with nanostructures of porous silicon (PS) as reported in the literature. The ultraviolet PL spectra observed with Yang etch shift to the blue-green spectrum range when applying Secco etch. In our experiments we have observed 3-5 μm diameter macro pores separated by a high density of nanowalls. These observations suggest that the origin of the PL activity are quantum dots resulting from the silicon nanocrystallites obtained after few minutes of chemical etching.     

Fabrication of black multicrystalline silicon surface by nanosecond laser ablation

The optimal regimes for uniform texturing of a multicrystalline silicon (mc-Si) surface by pulsed laser radiation have been determined. The morphology and reflectance spectra of the texturized mc-Si have been studied. The laser-texturized mc-Si samples with reflectance of 2?C3% over a wide spectral region have been produced. The influence of subsequent chemical etching on the reflective properties of the texturized surface has been analyzed.     

Optimized emitter contacting on multicrystalline silicon thin film solar cells

We present an optimized contacting scheme for multicrystalline silicon thin film solar cells on glass based on epitaxially crystallized emitters with a thin Al₂O₃ layer and a silver back reflector. In a first step a 6.5 µm thick amorphous silicon absorber layer is crystallized by a diode laser. In a second step a thin silicon emitter layer is epitaxially crystallized by an excimer laser. The emitter is covered by an Al₂O₃ layer with a thickness ranging from 1.0 nm to 2.5 nm, which passivates the surface and acts as a tunnel barrier. On top of the Al₂O₃ layer a 90–100 nm thick silver back reflector is deposited. The Al₂O₃ layer was found to have an optimal thickness of 1.5 nm resulting in solar cells with back reflector that achieve a maximum open‐circuit voltage of 567 mV, a short‐circuit current density of 27.9 mA/cm², and an efficiency of 10.9%. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Enhancement of optoelectronic properties in multicrystalline silicon via combination of grooving grain boundaries and porous silicon gettering

In multi-crystalline silicon (mc-Si), the detrimental effect of impurities and grain boundaries (GBs) on charge carrier transport has driven the research focus since many years. In view of curing these limitations, we present an innovative method to enhance the optoelectronic performance of mc-Si wafers via a combination between GBs grooving and porous silicon (PS) gettering. A preferential grooving of GBs was achieved using the HF/HNO₃ based solution, the PS layers were formed on both sides of the samples using stain-etching method and the gettering experiment was performed at temperatures ranging from 750 to 900 °C. As a result, it has been shown that the rapid thermal annealing process with chemical grooving gives a positive trend of improvement of the electronic quality and found to be more efficient when used in combination with PS. After removing the PS layer, the minority carrier lifetime increases by a factor of more than 27. In addition, a significant enhancement of majority carrier mobility was obtained, which led to an important decrease of the resistivity.     

Hybrid process for texturization of diamond wire sawn multicrystalline silicon solar cell

Acid texture is difficult for diamond wire sawn (DWS) multicrystalline silicon (mc‐Si) wafer owing to the inhomogeneous distribution of damage layer on the surface. In this article, metal‐assisted chemical etching (MACE) has been selected for introducing a porous seeding layer to induce acid texturing etching. SEM results show that the oval pit structures coverage get obvious improvement even on the smooth areas. Owing to the further improved light absorption ability by second MACE and nanostructure rebuilding (NSR) process, nanostructured DWS mc‐Si solar cell has exhibited a conversion efficiency of 17.96%, which is 0.45% higher than that of DWS wafer with simple acid texture process. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Femtosecond laser drilling of crystalline and multicrystalline silicon for advanced solar cell fabrication

The rear contact solar cell concept has been implemented to increase the solar cell efficiency. Practically, it necessitates rapid fabrication of a large number of via holes to form low-loss current paths. It is not a trivial task to drill a number of microscopic holes through a typical Si wafer of ??200???m thickness at reasonable processing throughput and yield. In this research, a femtosecond laser is employed to drill via holes in both crystalline silicon (c-Si) and multicrystalline silicon (mc-Si) thin wafers of ??170???m thickness with various laser parameters such as number of laser shots and pulse energy. Since a significantly high pulse energy compared to ablation threshold is mainly applied, aiming to achieve a rapid drilling process, the femtosecond laser beam is subjected to complex non-linear characteristics. Therefore, the relative placement of the sample with respect to the laser focal position is also rigorously examined. While the non-linear effect at high pulse energy regime is complex, it also facilitates the drilling process in terms of achieving high-aspect ratio, for example, by extending the effective depth of focus by non-linear effect. Cross-sectional morphological analysis in conjunction with on-line emission and shadowgraph imaging are carried out in order to elucidate the drilling mechanism.     

Impact of the firing temperature profile on light induced degradation of multicrystalline silicon

Light‐ and elevated temperature‐induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc‐Si wafers is analysed. Five profiles with measured high peak temperatures ≥800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N^*_t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp‐up and/or ramp‐down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Dislocation density reduction limited by inclusions in kerfless high‐performance multicrystalline silicon

Kerfless high‐performance multicrystalline silicon is an emerging material for photovoltaic applications that is characterized by having a smaller grains, and general lower average dislocation density than conventional ribbon multicrystalline silicon. Although a significant improvement over state‐of‐the‐art, dislocation reduction at the crystal growth stage is not complete. Here we employ an annealing process previously tested in conventional ingot mc‐Si to reduce dislo‐ cation clusters that remain after crystal growth. A sample is subjected to a 1390 °C annealing process for 24 h and its dislocation density reduction is evaluated. We employ infrared birefringence imaging to observe that despite achieving significant average dislocation density reduction, if inclusions are present in the sample, these may serve to nucleate new dislocations due to thermal strain. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Enhanced photon harvesting in silicon multicrystalline solar cells by new lanthanide complexes as light concentrators

Four europium complexes with enhanced luminescent properties have been synthesized. Thin transparent films of the complexes were prepared on glass panes and then were examined as efficient luminescent solar concentrators for full spectrum utilization in crystalline silicon photovoltaic cells. The complexes could behave as efficient solar concentrators since they absorb UV light, where the spectral response of the crystalline silicon solar cells is low and they emit at 615 nm where the spectral response is maximum. The glass panes covered with europium complexes act as planar waveguides of the emitted light that is collected by the solar cells, which are attached to the edges of the solar concentrators. The europium complexes as solar concentrators were examined in terms of the optimum concentrations and the number of coatings, which were determined in order to evaluate the maximum performance of the solar cells. The case of multiple glass panes with europium complexes was also examined while a 28% maximum increase in the photocurrent was finally established.     

Realization of conformal doping on multicrystalline silicon solar cells and black silicon solar cells by plasma immersion ion implantation

Emitted multi-crystalline silicon and black silicon solar cells are conformal doped by ion implantation using the plasma immersion ion implantation （PⅢ） technique. The non-uniformity of emitter doping is lower than 5 %. The secondary ion mass spectrometer profile indicates that the PⅢ technique obtained 100-rim shallow emitter and the emitter depth could be impelled by furnace annealing to 220 nm and 330 nm at 850 ℃ with one and two hours, respectively. Furnace annealing at 850 ℃ could effectively electrically activate the dopants in the silicon. The efficiency of the black silicon solar cell is 14.84% higher than that of the mc-silicon solar cell due to more incident light being absorbed.     

Formation of a honeycomb texture for multicrystalline silicon solar cells using an inkjetted mask

An effective way to reduce the reflection of a multicrystalline solar cell is the use of a honeycomb structure, which can be generated by etching a mask isotropically. In this Letter, a directly printed hexagonal inkjet mask is presented. It results in a honeycomb texture with well developed and defined etch pits at an average distance of 50.1 μm and a weighted reflection of 18.4%. The major advantage of this mask is that the masking process is simple and that it has the potential of being fast and having low costs.

     

EPR of iron centres in silicon dioxide

A review is presented of the present status of EPR-based understanding of Fe in SiO₂. As a primary goal, a guide to the literature is given. Quantitative evaluation of all relevant spin-Hamiltonian parameters is discussed, as is computer-based spectral simulation, for single crystals (alpha-quartz), powders and glasses (vitreous quartz). Identification of the numerous centres now known (e.g., for Fe³⁺:[FeO₄/M]⁰ with M=H, Li, Na, …, as well as [FeO₄]^?) is featured, discussing atomic positions, surroundings and dynamic effects. Some general chemical considerations, including comparison between Fe in SiO₂ crystals and glasses, are covered, as are future needs.     

Kinetics of high-temperature precipitation in dislocation-free silicon single crystals

The defect structure of dislocation-free silicon single crystals has been calculated using the approximate solution of the Fokker-Planck partial differential equations. It has been demonstrated that the precipitation starts to occur near the crystallization front due to the disappearance of excess intrinsic point defects on sinks whose role is played by oxygen and carbon impurities.     

A deep level transient spectroscopy study on the interface states across grain boundaries in multicrystalline silicon

Degrading the recombination activities of grain boundaries (GBs) is essential to improve the efficiency of multi‐crystalline silicon (mc‐Si) based solar cells. We apply the deep level transient spectroscopy technique to detect interface states at Σ3 and Σ9 GBs in mc‐Si. The density of interface states close to midgap is found comparable for both as‐grown GBs. Gettering or hydrogenation leads to shallower states with a smaller capture cross section and lower density. Recombination activity reduction for Σ3 GBs is stronger than for Σ9 GBs especially after hydrogenation. Both the analysis approach and experimental results could be applied for a specific GB engineering of mc‐Si based solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)