23% efficient p-type crystalline silicon solar cells with hole-selective passivating contacts based on physical vapor deposition of doped silicon films

Of all the materials available to create carrier-selective passivating contacts for silicon solar cells, those based on thin films of doped silicon have permitted to achieve the highest levels of performance. The commonly used chemical vapour deposition methods use pyrophoric or toxic gases like silane, phosphine and diborane. In this letter, we propose a safer and simpler approach based on physical vapour deposition (PVD) of both the silicon and the dopant. An in-situ doped polycrystalline silicon film is formed, upon annealing, onto an ultrathin SiOx interlayer, thus providing selective conduction and surface passivation simultaneously. These properties are demonstrated here for the case of hole-selective passivating contacts, which present recombination current densities lower than 20 fA/cm2 and contact resistivities below 50 mΩ cm2. To further demonstrate the PVD approach, these contacts have been implemented in complete p-type silicon solar cells, together with a front phosphorus diffusion, achieving an open-circuit voltage of 701 mV and a conversion efficiency of 23.0%. These results show that PVD by sputtering is an attractive and reliable technology for fabricating high performance silicon solar cells

Charge states of the reactants in the hydrogen passivation of interstitial iron in P-type crystalline silicon

Significant reductions in interstitial iron (Fei) concentrations occur during annealing Fe-containing silicon wafers with silicon nitride films in the temperature range of 250°C-700°C. The silicon nitride films are known to release hydrogen during the annealing step. However, in co-annealed samples with silicon oxide films, which are hydrogen-lean, changes in the Fei concentrations were much less significant. The precipitation of Fei is ruled out as a possible explanation for the significant reductions. The hydrogen passivation of Fei, which is the complexing of monatomic H and isolated Fei forming a recombination-inactive hydride, is proposed as the most probable model to explain the reductions. Under the assumption that the reduction is caused by the hydrogenation of Fei, the reactants' charge states in the hydrogenation reaction are determined by two independent approaches. In the first approach, illumination is found to have a small but detectible impact on the reaction kinetics in the lower temperature range. The dominating reactants' charge states are concluded to be Fe0 + H+ as revealed by modelling the injection-dependent charge states of isolated Fei and monatomic H. In the second approach, the reaction kinetics are fitted with the Arrhenius equation over a large temperature range of 250°C-700°C. A reasonable fit is only obtained when assuming the reacting charge states are Fe0+H+. This supports the conclusion on the reacting charge states and also gives a value of the activation energy of hydrogenation in the 0.7-0.8eV range

Methods to Improve Bulk Lifetime in n-Type Czochralski-Grown Upgraded Metallurgical-Grade Silicon Wafers

This paper investigates the potential of three different methods-tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation, for improving the carrier lifetime in n-type Czochralski-grown upgraded metallurgical-grade (UMG) silicon samples. Our results show that the lifetimes in the UMG wafers used in this study were affected by both mobile metallic impurities and as-grown oxygen precipitate nuclei. Thus, the dissolution of grown-in oxygen precipitate nuclei via TR and the removal of mobile impurities via PDG step were found to significantly improve the electronic quality of the UMG wafers. Finally, we report bulk lifetimes and 1-sun implied open-circuit voltages of the UMG wafers after boron and phosphorus diffusions, as typically applied in n-type cell fabrication.This work has been supported by the Australian Renewable Energy Agency (ARENA) through research grant RND009.

In situ recombination junction between p-Si and TiO2 enables high-efficiency monolithic perovskite/Si tandem cells

Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell’s output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO2 and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells

Boron, phosphorus and aluminum gettering of iron in crystalline silicon: Experiments and modelling

A direct comparison of boron diffusion gettering, phosphorus gettering, and aluminum annealing gettering of iron is presented, using both experiment and simulation results. Float zone silicon samples were implanted with Fe and exposed to either B, P, or Al gettering at various temperatures for experimental measurements. The gettering model simulates dopant diffusion, segregation to doped layers, and diffusion of interstitial iron towards the gettering layers. The segregation model for Boron diffusion gettering agrees with the experiment results that boron diffusion gettering was completely ineffective at gettering Fe for diffusion temperatures above 850°C. Experiment results show that phosphorus diffusion gettering was effective in removing more than 90% of the interstitial iron across a range of temperatures and doses. Surprisingly, even relatively light phosphorus diffusions (145 Ω/□) were found to give very effective gettering. The phosphorus diffusion gettering model agrees reasonably with experimental results in terms of the amount gettered, but shows an opposite trend with temperature between 780°C and 850°C. Aluminum annealing gettering is very effective and the experimental detection limit prevents accurate measurements for comparison with the model

Effect of boron codoping and phosphorus concentration on phosphorus diffusion gettering

Compared with phosphorus diffusions, conventional boron diffusions for n-type solar cells are not effective at impurity gettering without the presence of a boron-rich layer. In this paper, we investigate the gettering effectiveness of light phosphorus diffusions for removing Fe impurities, applied on an underlying boron diffusion, similar to the buried emitter concept, as an option for achieving effective gettering on boron diffused substrates. Our experimental results on monocrystalline silicon samples demonstrate that the underlying boron diffusion does not affect the gettering effectiveness of the phosphorus diffusion, even though much of the phosphorus diffused region is overdoped by the boron diffusion. Furthermore, we investigate the gettering effectiveness of low surface concentration phosphorus diffusions that can result in reduced recombination in the n+ region. Our results show that the gettering effectiveness decreases when the surface phosphorus concentration is reduced, either through manipulating the deposition gas flows or through subsequent driving in. Driving in the surface phosphorus concentration from 2 × 1020 to 3.5 × 10 19 cm-3 decreased the gettering effectiveness by about one order of magnitude

Evaluating Depth Distributions of Dislocations in Silicon Wafers Using Micro-Photoluminescence Excitation Spectroscopy

AbstractCombining micro-photoluminescence spectroscopy and photoluminescence excitation spectroscopy, we are able to observe the evolution of the luminescence spectra from crystalline silicon wafers under various excitation wavelengths. By interpreting the relative change of the luminescence spectra, we can detect and examine the distributions of the dislocations, as well as of the defects and impurities trapped around them, segregated at different depths below the wafer surface. We show that in multicrystalline silicon wafers, the dislocations and the trapped defects and impurities, formed during the ingot growth and cooling, are distributed throughout the wafer thickness, whereas those generated in monocrystalline wafers by a post-diffusion thermal treatment are located near the wafer surface

External and internal gettering of interstitial iron in silicon for solar cells

The removal of dissolved iron from the wafer bulk is important for the performance of ptype multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal getteri