Surface Passivation Of Crystalline Silicon Research Articles

POx/Al2O3 stacks: Highly effective surface passivation of crystalline silicon with a large positive fixed charge

Thin-film stacks of phosphorus oxide (POx) and aluminium oxide (Al2O3) are shown to provide highly effective passivation of crystalline silicon (c-Si) surfaces. Surface recombination velocities as low as 1.7 cm s−1 and saturation current densities J0s as low as 3.3 fA cm−2 are obtained on n-type (100) c-Si surfaces passivated by 6 nm/14 nm thick POx/Al2O3 stacks deposited in an atomic layer deposition system and annealed at 450 °C. This excellent passivation can be attributed in part to an unusually large positive fixed charge density of up to 4.7 × 1012 cm−2, which makes such stacks especially suitable for passivation of n-type Si surfaces.

Zirconium oxide surface passivation of crystalline silicon

This letter reports effective passivation of crystalline silicon (c-Si) surfaces by thermal atomic layer deposited zirconium oxide (ZrOx). The optimum layer thickness and activation annealing conditions are determined to be 20 nm and 300 °C for 20 min. Cross-sectional transmission electron microscopy imaging shows an approximately 1.6 nm thick SiOx interfacial layer underneath an 18 nm ZrOx layer, consistent with ellipsometry measurements (∼20 nm). Capacitance–voltage measurements show that the annealed ZrOx film features a low interface defect density of 1.0 × 1011 cm−2 eV−1 and a low negative film charge density of −6 × 1010 cm−2. Effective lifetimes of 673 μs and 1.1 ms are achieved on p-type and n-type 1 Ω cm undiffused c-Si wafers, respectively, corresponding to an implied open circuit voltage above 720 mV in both cases. The results demonstrate that surface passivation quality provided by ALD ZrOx is consistent with the requirements of high efficiency silicon solar cells.

Improving the Silicon Surface Passivation by Aluminum Oxide Grown Using a Non‐Pyrophoric Aluminum Precursor

In this work, an alternative nonpyrophoric Al‐precursor [dimethylaluminum isopropoxide (DMAI)] is used to deposit Al2O3 on p‐type c‐Si by plasma‐enhanced and thermal atomic layer deposition (ALD). Al2O3 films grown by thermal ALD provide a relatively modest level of crystalline silicon surface passivation compared to their plasma‐assisted ALD grown counterparts. This difference is attributed to a significant difference in the interfacial SiOx film as identified by Brewster's angle Fourier transform infrared spectroscopy. The energy barrier for DMAI to react with Si‐H groups on the surface is significantly lower compared to TMA, which impedes the formation of the interfacial SiOx layer. As thermal ALD is the strongly preferred method in the photovoltaic industry, there is a strong incentive to improve the performance of the thermal ALD process to allow for the application of this intrinsically safer Al2O3 deposition process. A chemically grown thin oxide is shown to significantly improve the level of surface passivation provided by the thermal ALD Al2O3 film resulting in an increase in implied open circuit voltage from 653 to 723 mV. This surface pretreatment thus solves a major barrier for the application of this intrinsically safer process in the photovoltaic industry.

Passivation of textured crystalline silicon surfaces by catalytic CVD silicon nitride films and catalytic phosphorus doping

Silicon nitride (SiNx) films formed by catalytic chemical vapor deposition (Cat-CVD) and phosphorus (P)-doped layers formed by catalytic impurity doping (Cat-doping) are applied for the passivation of pyramid-shaped textured crystalline Si (c-Si) surfaces formed by anisotropic etching in alkaline solution. Lower surface recombination velocities (SRVs) tend to be obtained when smaller pyramids are formed on c-Si surfaces. P Cat-doping is effective for reducing the SRV of textured c-Si surfaces as in the case of flat c-Si surfaces. We realize SRVs of textured c-Si surfaces of ∼8.0 and ∼6.7 cm/s for only SiNx passivation and for the combination of SiNx and P Cat-doping, respectively. These structures also have high optical transparency and low Auger recombination loss, and are of great worth in application for the surface passivation of interdigitated back-contact c-Si solar cells.

Effective passivation of crystalline silicon surfaces by ultrathin atomic-layer-deposited TiOx layers

We characterize the surface passivation properties of ultrathin titanium oxide (TiOx) films deposited by atomic layer deposition (ALD) on crystalline silicon by means of carrier lifetime measurements. We compare different silicon surface treatments prior to TiOx deposition, such as native silicon oxide (SiOy), chemically grown SiOy and thermally grown SiOy. The best passivation quality is achieved with a native SiOy grown over 4 months and a TiOx layer thickness of 5 nm, resulting in an effective lifetime of 1.2 ms on 1.3 Ωcm p-type float-zone silicon. The measured maximum lifetime corresponds to an implied open-circuit voltage (iVoc) of 710 mV. For thinner TiOx layers the passivation quality is reduced, however, samples passivated with only 2 nm of TiOx still show a lifetime of 612 μs and an iVoc of 694 mV. The contact resistivity of the TiOx including the SiOy interlayer between the silicon wafer and the TiOx is below 0.8 Ωcm2. The combination of excellent surface passivation and low contact resistivity has the potential for silicon solar cells with efficiencies exceeding 26%.

Detailed study of SiOxNy:H/Si interface properties for high quality surface passivation of crystalline silicon

In this work, we present a detailed study on the interface and passivation properties of the hydrogenated silicon oxynitride (SiOxNy:H) on the crystalline silicon (c-Si) and their correlations with the film composition. The SiOxNy:H films were synthesized by plasma enhanced chemical vapor deposition (PECVD) at various N2O flow rates, which results in different film composition, in particular the different H-related bonds, such as SiH and NH bonds. Fourier transform infrared spectroscopy measurements show that the concentration of NH bonds increases with the N2O flows from 0 to 30 sccm, while drops below the detection limit at N2O flows above 30 sccm. This changing trend of NH bonds correlates well with the evolution of carrier lifetime of silicon substrate passivated by SiOxNy:H film, indicating the crucial role of NH bonds in surface passivation. It is inferred that during the film deposition and forming gas anneal (FGA) a considerable amount of hydrogen atoms are liberated from the weak type of NH bonds rather than SiH bonds, and then passivate the dangling bonds at the interface, thus resulting in the significant reduction of interface state density and the improved passivation quality. In detail, the interface state density is reduced from ∼5 × 1012 to ∼2 × 1012 cm−2 eV−1 after the FGA, as derived from the high frequency capacitance-voltage (CV) measurements.

Crystalline silicon surface passivation investigated by thermal atomic-layer-deposited aluminum oxide**Project supported by the Beijing Municipal Science and Technology Commission, China (Grant No. Z151100003515003), the National Natural Science Foundation of China (Grant Nos. 110751402347, 61274134, 51402064, 61274059, and 51602340), the University of Science and Technology Beijing (USTB) Start-up Program, China (Grant No. 06105033), the Beijing Municipal Innovation and Research Base, China

Atomic-layer-deposited (ALD) aluminum oxide (Al2O3) has demonstrated an excellent surface passivation for crystalline silicon (c-Si) surfaces, as well as for highly boron-doped c-Si surfaces. In this paper, water-based thermal atomic layer deposition of Al2O3 films are fabricated for c-Si surface passivation. The influence of deposition conditions on the passivation quality is investigated. The results show that the excellent passivation on n-type c-Si can be achieved at a low thermal budget of 250 °C given a gas pressure of 0.15 Torr. The thickness-dependence of surface passivation indicates that the effective minority carrier lifetime increases drastically when the thickness of Al2O3 is larger than 10 nm. The influence of thermal post annealing treatments is also studied. Comparable carrier lifetime is achieved when Al2O3 sample is annealed for 15 min in forming gas in a temperature range from 400 °C to 450 °C. In addition, the passivation quality can be further improved when a thin PECVD-SiNx cap layer is prepared on Al2O3, and an effective minority carrier lifetime of 2.8 ms and implied Voc of 721 mV are obtained. In addition, several novel methods are proposed to restrain blistering.

Correlating the silicon surface passivation to the nanostructure of low-temperature a-Si:H after rapid thermal annealing

Using an inductively coupled plasma, hydrogenated amorphous silicon (a-Si:H) films have been prepared at very low temperatures (<50 °C) to provide crystalline silicon (c-Si) surface passivation. Despite the limited nanostructural quality of the a-Si:H bulk, a surprisingly high minority carrier lifetime of ∼4 ms is demonstrated after a rapid thermal annealing treatment. Besides the excellent level of surface passivation, the main advantage of the low-temperature approach is the facile suppression of undesired epitaxial growth. The correlation between the a-Si:H nanostructure and the activation of a-Si:H/c-Si interface passivation, upon annealing, has been studied in detail. This yields a structural model that qualitatively describes the different processes that take place in the a-Si:H films during annealing. The presented experimental findings and insights can prove to be useful in the further development of very thin a-Si:H passivation layers for use in silicon heterojunction solar cells.

Superior silicon surface passivation in HIT solar cells by optimizing a-SiOx:H thin films: A compact intrinsic passivation layer

Abstract The microstructure of the intrinsic amorphous silicon oxide (i-a-SiO x :H) thin films is the key for crystalline silicon surface passivation in high-performance HIT solar cells. Understanding and subsequently optimizing the i-a-SiO x :H microstructure is essential in improving defect passivation and carrier transportation in a-SiO x :H films. In this work, we investigate the relationship between the bulk microstructure of i-a-SiO x :H thin films at different oxygen content and the passivation quality of the silicon surface. The results revealed that, as the oxygen content increases, the dominating growth mechanism of i-a-SiO x :H films changes. Consequently, the component contents of i-a-SiO x :H thin films were varied and the microstructure showed a first improved and then deteriorated trend with increasing oxygen content. A compact, less-defective and ordered microstructure of the a-SiO x :H film can only be obtained when there comes a tradeoff between Si O and Si H(Si 3 ) bonding formation. Correspondingly, the improved defect passivation and the enhanced carrier transportation in a-SiO x :H films lead to the increase of V oc , FF and QE of HIT solar cells, all of which are key solar cell performance parameters.

Effect of the Ti/Si ratio of spin coating solutions on surface passivation of crystalline silicon by TiOx–SiOx composite films

Passivation films or antireflection coatings are generally prepared using costly vacuum or high-temperature processes. Thus, we report the preparation of TiOx–SiOx composite films by novel spin coatable solutions for the synthesis of low-cost passivation coating materials. The desired films were formed by varying the mixing ratios of TiOx and SiOx, and the resulting films exhibited excellent surface passivation properties. For the p-type wafer, an optimal effective surface recombination velocity (Seff) of 93 cm/s was achieved at , while a surface recombination current density (J0s) of 195 fA/cm2 was obtained. In contrast, for the n-type wafer, an Seff of 27 cm/s and a J0s of 38 fA/cm2 were achieved at . This excellent surface passivation effect could be attributed to the low interface state density and high positive fixed charge density. Furthermore, the thickness of the interfacial SiOx layer was determined to be important for obtaining the desired surface passivation effect.

Crystalline silicon surface passivation by thermal ALD deposited Al doped ZnO thin films

The evidence of good quality silicon surface passivation using thermal ALD deposited Al doped zinc oxide (AZO) thin films is demonstrated. AZO films are prepared by introducing aluminium precursor in between zinc and oxygen precursors during the deposition. The formation of AZO is confirmed by ellipsometry, XRD and Hall measurements. Effective minority carrier lifetime (τeff) greater than 1.5ms at intermediate bulk injection levels is realized for symmetrically passivated p-type silicon surfaces under optimised annealing conditions of temperature and time in hydrogen ambient. The best results are realised at 450°C annealing for >15min. Such a layer may lead to implied open circuit voltage gain of 80mV.

Interface states study of intrinsic amorphous silicon for crystalline silicon surface passivation in HIT solar cell**Project supported by the National Natural Science Foundation of China (Grant Nos. 51361022 and 61574072) and the Postdoctoral Science Foundation of Jiangxi Province, China (Grant No. 2015KY12).

Intrinsic hydrogenated amorphous silicon (a-Si:H) film is deposited on n-type crystalline silicon (c-Si) wafer by hot-wire chemical vapor deposition (HWCVD) to analyze the amorphous/crystalline heterointerface passivation properties. The minority carrier lifetime of symmetric heterostructure is measured by using Sinton Consulting WCT-120 lifetime tester system, and a simple method of determining the interface state density from lifetime measurement is proposed. The interface state density measurement is also performed by using deep-level transient spectroscopy (DLTS) to prove the validity of the simple method. The microstructures and hydrogen bonding configurations of a-Si:H films with different hydrogen dilutions are investigated by using spectroscopic ellipsometry (SE) and Fourier transform infrared spectroscopy (FTIR) respectively. Lower values of interface state density are obtained by using a-Si:H film with more uniform, compact microstructures and fewer bulk defects on crystalline silicon deposited by HWCVD.

Decoupling high surface recombination velocity and epitaxial growth for silicon passivation layers on crystalline silicon

We have critically evaluated the deposition parameter space of very high frequency plasma-enhanced chemical vapour deposition discharges near the amorphous to crystalline transition for intrinsic a-Si:H passivation layers on Si (1 1 1) wafers. Using a low silane concentration in the SiH4–H2 feedstock gas mixture that created amorphous material just before the transition, we have obtained samples with excellent surface passivation. Also, an a-Si:H matrix was grown with embedded local epitaxial growth of crystalline cones on a Si (1 1 1) substrate, as was revealed with a combined scanning electron and high-resolution transmission electron microscopy study. This local epitaxial growth was introduced by a decrease of the silane concentration in the feedstock gas or an increase in discharge power at low silane concentration. Together with the samples on Si (1 1 1) substrates, layers were co-deposited on Si (1 0 0) substrates. This resulted in void-rich, mono-crystalline epitaxial layers on Si (1 0 0). The epitaxial growth on Si (1 0 0) was compared to the local epitaxial growth on Si (1 1 1). The sparse surface coverage of cones seeded on the Si (1 1 1) substrate is most probably enabled by a combination of nucleation at steps and kinks in the {1 1 1} surface and intense ion bombardment at low silane concentration. The effective carrier lifetime of this sample is low and does not increase upon post-deposition annealing. Thus, sparse local epitaxial growth on Si (1 1 1) is enough to obstruct crystalline silicon surface passivation by amorphous silicon.

Radicals and ions controlling by adjusting the antenna-substrate distance in a-Si:H deposition using a planar ICP for c-Si surface passivation

Being a key issue in the research and fabrication of silicon heterojunction (SHJ) solar cells, crystalline silicon (c-Si) surface passivation is theoretically and technologically intricate due to its complicate dependence on plasma characteristics, material properties, and plasma-material interactions. Here amorphous silicon (a-Si:H) grown by a planar inductively coupled plasma (ICP) reactor working under different antenna-substrate distances of d was used for the surface passivation of low-resistivity p-type c-Si. It is found that the microstructures (i.e., the crystallinity, Si-H bonding configuration etc.) and passivation function on c-Si of the deposited a-Si:H were profoundly influenced by the parameter of d, which primarily determines the types of growing precursors of SiHn/H contributing to the film growth and the interaction between the plasma and growing surface. c-Si surface passivation is analyzed in terms of the d-dependent a-Si:H properties and plasma characteristics. The controlling of radical types and ion bombardment on the growing surface through adjusting parameter d is emphasized.

Suppression of the thermal quenching of photoluminescence in irradiated silicon heterojunction solar cells

We report on the decrease in photoluminescence (PL) intensity with the increase of the sample temperature (thermal quenching) in crystalline silicon and its suppression after ion irradiation. The crystalline silicon surface was passivated with intrinsic and doped hydrogenated amorphous silicon (a‐Si:H) layer stacks as for making silicon heterojunction solar cell precursors. Low energy argon ion irradiation, in the range between 5 and 17 keV, was used for controlled defect formation either in the thin a‐Si:H top layer or at the interface with the crystalline silicon beneath. The irradiation defects introduce radiative recombination centers in the a‐Si:H as can be measured from PL at low temperature. Moreover, the irradiation and annealing result in a strong modification of the thermal quenching of the PL intensity up to 500 K, showing evidence for the strong reduction of thermally activated non‐radiative recombinations at the amorphous‐crystalline interface. This result can have implications in the field of crystalline silicon surface passivation.

Ultrathin Titanium Dioxide Nanolayers by Atomic Layer Deposition for Surface Passivation of Crystalline Silicon

We demonstrate a low surface recombination velocity of 14 cm/s with only 1.5 nm thin titanium dioxide (TiO2 ) layers on undiffused 10 Ωcm p-type crystalline silicon. The TiO2 nanolayers were deposited by thermal atomic layer deposition at 150 °C and 200 °C substrate temperatures using tetrakis-dimethyl-amido titanium as the Ti precursor and water as the oxidant. The influence of a post-deposition anneal in forming gas at different temperatures was investigated. We have observed that a subsequent anneal in forming gas at 350 °C enhances the surface passivation quality of the TiO2 layers tremendously. Increasing the thickness of the TiO2 layers leads to a reduction of the surface passivation quality. Introducing a thin interfacial layer of silicon oxide (1.6 nm) grown by rapid thermal oxidation underneath the TiO2 layer improves the surface passivation of thicker TiO2 layers (5.5 and 15 nm). These results show that ultrathin TiO2 layers with a thickness of only 1.5 nm can be used to effectively passivate the c-Si surface.

Interface Processing of Amorphous–Crystalline Silicon Heterojunction Prior to the Formation of Amorphous-to-Nanocrystalline Transition Phase

Two effective interface optimizations, i.e., hydrogen-plasma treatment (HPT) and oxygen incorporation (OIn), have been investigated to optimize the interface structure and promote the formation of hydrogenated amorphous silicon (a-Si:H) with an amorphous-to-nanocrystalline transition phase based on the surface passivation of crystalline silicon. Both multistep HPT and intentional OIn are capable of producing a compact interface and a transition phase in the case of initial a-Si:H films deposited under different conditions. An appropriate multistep HPT is responsible for modifying the a-Si:H into a transition phase, whereas an effective OIn forms a disordered and more compact interface, as well as a crystallite-isolated and more transparent structure.

Surface passivation of crystalline silicon by sputtered AlOx/AlNx stacks toward low-cost high-efficiency silicon solar cells

Recently, excellent surface passivation has been achieved for both p- and n-type silicon solar cells using AlOx/SiNx:H stacks deposited by atomic layer deposition and plasma-enhanced chemical vapor deposition. However, alternative materials and deposition methods could provide practical options for large-scale manufacturing of commercial solar cells. In this study we demonstrate that AlOx/AlNx stacks fabricated by reactive radio-frequency magnetron sputtering can provide fairly good surface passivation (Smax of ∼30 cm/s) regardless of AlOx thickness, which is found to be due to the high negative fixed charge density (Qeff of −2.8 × 1012 cm−2) and moderately low interface trap density (Dit of 2.0 × 1011 eV−1·cm−2). The stacks also show fairly good antireflection performance in the visible and near-infrared spectral region. The demonstrated surface passivation and antireflection performance of in situ reactively sputtered AlOx/AlNx stacks make them a promising candidate for a surface-passivating antireflection coating on silicon solar cells.

Study of the correlation between hydrogenated amorphous silicon microstructure and crystalline silicon surface passivation in heterojunction solar cells

The properties of hydrogenated amorphous silicon (a‐Si:H) thin layers were studied by spectroscopic ellipsometry (SE) and Fourier transform infrared spectroscopy (FTIR) measurements to determine their effects on the surface passivation of crystalline silicon (c‐Si) in heterojunction solar cells. It was demonstrated that the high bulk quality of a‐Si:H layers with denser structures was not sufficient for the passivation of c‐Si surfaces in Silicon Heterojunction (SHJ) solar cells, in which the passivation performance is strongly governed by the total hydrogen content (CH) as a‐Si:H layers are ultra‐thin. High effective carrier lifetime and implied open‐circuit voltage (Voc,im), as well as low surface recombination velocity, were obtained at the appropriate CH values, and the optimal windows of CH and of the hydrogen content in the Si–H2 configuration (C2) were determined to be 5.0 ∼ 8.5 and 2.9 ∼ 6.8 at.%, respectively. For CH values higher than 8.5 at.%, the passivation effect was degraded due to the deteriorated bulk properties of the a‐Si:H layers. However, Voc,im decreased when CH was lower than 5.0 at.% because the number of H atoms was insufficient to saturate the dangling bonds at the a‐Si:H/c‐Si interface even though the a‐Si:H layers had dense microstructures.

Impact of microcrystalline silicon carbide growth using hot-wire chemical vapor deposition on crystalline silicon surface passivation

Highly crystalline microcrystalline silicon carbide (μc-SiC:H) with excellent optoelectronic material properties is a promising candidate as highly transparent doped layer in silicon heterojunction (SHJ) solar cells. These high quality materials are usually produced using hot wire chemical vapor deposition under aggressive growth conditions giving rise to the removal of the underlying passivation layer and thus the deterioration of the crystalline silicon (c-Si) surface passivation. In this work, we introduced the n-type μc-SiC:H/n-type μc-SiOx:H/intrinsic a-SiOx:H stack as a front layer configuration for p-type SHJ solar cells with the μc-SiOx:H layer acting as an etch-resistant layer against the reactive deposition conditions during the μc-SiC:H growth. We observed that the unfavorable expansion of micro-voids at the c-Si interface due to the in-diffusion of hydrogen atoms through the layer stack might be responsible for the deterioration of surface passivation. Excellent lifetime values were achieved under deposition conditions which are needed to grow high quality μc-SiC:H layers for SHJ solar cells.