Low-quality Silicon Research Articles

Effective gettering of in-situ phosphorus-doped polysilicon passivating contact prepared using plasma-enhanced chemical-vapor deposition technique

In this work, we investigated the gettering of the in-situ phosphorus-doped polysilicon passivating contact that was realized through plasma-enhanced chemical-vapor deposition (PECVD) technology. Interstitial iron (Fei) induced artificially was used as the marker for estimating gettering effectiveness. The factors on gettering effectiveness are studied, including the fabrication procedures and the structures of the polysilicon passivating contact. The research suggests that the heavily phosphorus-doped polysilicon layer serves as the gettering pool for iron atoms. We found that the measures including raising the annealing temperature, extending the annealing time, increasing the thickness and doping concentration of polysilicon, and lowering the SiOx thickness are helping to improve the gettering effectiveness. Moreover, the subsequent hydrogenation promotes the lifetime and implied open-circuit voltage by reducing Shockley-Read-Hall (SRH) recombination. The advantage of the polysilicon passivating contact is the significant effect on the improvement in bulk lifetime, so that the low-quality silicon wafer has the chance to be used for solar cell fabrication with notably improved performance by using the excellent gettering and surface passivation of the polysilicon passivating contact. In summary, this work confirmed that the PECVD-prepared in-situ doped polysilicon passivating contact is an effective process to remove more than 99.99% bulk Fe contamination with an optimal formation procedure, which was essential for the industrial manufacture for silicon solar cells.

Enhancement of electrical parameters in solar grade monocrystalline silicon by external gettering through sacrificial silicon nanowire layer

In this work, we report a simple and efficient purification technique of solar grade silicon wafers suitable for high efficiency and low-cost silicon solar cells. This gettering method requires the elaboration of silicon nanowire thin layer followed by an annealing treatment and removing impurity loaded silicon nanowires. ICP-AES analysis shows a significant decrease in the metallic impurity concentration resulting considerable enhancement in the silicon purity grade. Electrical characterizations reveal a global decrease of the majority carrier concentration and a substantial improvement of the minority carrier lifetime. Furthermore, the application of this novel gettering process induces an enhancement in the silicon solar cell electrical parameters. These results give evidence of the effectiveness of the proposed purification method to produce efficient solar cells starting from low-quality silicon wafers.

A 12%-efficient upgraded metallurgical grade silicon-organic heterojunction solar cell achieved by a self-purifying process.

Low-quality silicon such as upgraded metallurgical-grade (UMG) silicon promises to reduce the material requirements for high-performance cost-effective photovoltaics. So far, however, UMG silicon currently exhibits the short diffusion length and serious charge recombination associated with high impurity levels, which hinders the performance of solar cells. Here, we used a metal-assisted chemical etching (MACE) method to partially upgrade the UMG silicon surface. The silicon was etched into a nanostructured one by the MACE process, associated with removing impurities on the surface. Meanwhile, nanostructured forms of UMG silicon can benefit improved light harvesting with thin substrates, which can relax the requirement of material purity for high photovoltaic performance. In order to suppress the large surface recombination due to increased surface area of nanostructured UMG silicon, a post chemical treatment was used to decrease the surface area. A solution-processed conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited on UMG silicon at low temperature (<150 °C) to form a heterojunction to avoid any impurity diffusion in the silicon substrate. By optimizing the thickness of silicon and suppressing the charge recombination at the interface between thin UMG silicon/PEDOT:PSS, we are able to achieve 12.0%-efficient organic-inorganic hybrid solar cells, which are higher than analogous UMG silicon devices. We show that the modified UMG silicon surface can increase the minority carrier lifetime because of reduced impurity and surface area. Our results suggest a design rule for an efficient silicon solar cell with low-quality silicon absorbers.

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.

Theoretical comparison of conventional and multilayer thin silicon solar cells

Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime (<50 ns) crystalline silicon layers. Moderate effici encies (>15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.

Theoretical comparison of conventional and multilayer thin silicon solar cells

Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime (<50 ns) crystalline silicon layers. Moderate effici encies (>15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.

Silicon Parallel Multilayer Thin Film Solar Cells

AbstractA new silicon parallel multilayer solar cell structure has recently been reported which can give high solar cell energy conversion efficiency from low quality silicon material. Advantages of this structure are described as is recent characterization work which compares the properties of grain boundaries in experimental devices to those predicted by earlier calculations.

Effect of trap location and trap-assisted Auger recombination on silicon solar cell performance

Model calculations were performed to investigate and quantify the effect of trap location and trap-assisted Auger recombination on silicon solar cell performance. Trap location has a significant influence on the lifetime behavior as a function of doping and injected carrier concentration in silicon. It Is shown in this paper that for a high quality silicon (/spl tau/=10 ms at 200 ohm-cm, no intentional doping), high resistivity (/spl ges/200 ohm-cm) is optimum for high efficiency one sun solar cells if the lifetime limiting trap is located near midgap. However, if the trap is shallow (E/sub t/-E/sub v//spl les/0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 /spl mu/s at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be /spl sim/0.2 ohm-cm, regardless of the trap location. It is shown that the presence of a shallow trap can significantly degrade the performance of a concentrator cell fabricated on high-resistivity high-lifetime silicon material because of an undesirable injection level dependence in the carrier lifetime. The effect of trap assisted Auger recombination on the cell performance has also been modelled in this paper. It is found that the trap-assisted Auger recombination does not influence the one sun cell performance appreciably, but can degrade the concentrator cell performance if the trap-assisted Auger recombination coefficient value exceeds 2/spl times/10/sup -14/ cm/sup 3//s. Therefore, it is necessary to know the starting lifetime as well as trap location in order to specify base resistivity in order to predict or achieve the best cell performance for a given one sun or concentrator cell design.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A comparatively low-quality silicon material improved for solar cell fabrication

A comparatively low-quality silicon wafer (with a purity of almost-equal-to 99.9%) was adopted to form a silicon-on-defect-layer (SODL) structure featuring improved crystalline silicon near the defect layer (DL) by means of proton implantation and subsequent annealing. Thus, the SODL technique provides an opportunity to enable low-quality silicon wafers to be used for fabrication of low-cost solar cells.

The effect of hydrogen treatment on damaged and undamaged metal-insulator-semiconductor solar cells

A metal-insulator-semiconductor structure was used to study the effect of hydrogen plasma treatment of solar cells. Cells were fabricated on single-crystal p-type silicon and treated in hydrogen plasma for different exposure times. This causes a noticeable drop in the photocurrent and density of charge carriers with exposure time. This in turn leads to a drop in the solar cell efficiency. Other cells were fabricated on damaged p-type silicon, where dislocations were created by lapping the silicon surface. Such a procedure simulates controllably the effect of dislocations and grain boundaries. The damaged silicon was then exposed to hydrogen plasma and an overall improvement in the performance of the solar cell was found. However over-exposure led to degradation as with single-crystal material. It is concluded that hydrogen plasma exposure has both beneficial and degrading effects. It improves solar cell characteristics when they have been degraded by dislocations, but it degrades cells by interacting with accepters in the p-silicon. Further studies are needed to attain the best compromise and to optimize solar cells made from low quality silicon.