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Summary Despite the maturity of crystalline silicon photovoltaics (c-Si PV), the last 6 years have seen a string of efficiency improvements, most of which are centered around reducing the losses related to the directly metallized, heavily doped regions found in conventional c-Si solar cells. Among these advancements, polysilicon (poly-Si) passivated junctions, formed by embedding a thin silicon oxide (SiO2) layer between the c-Si wafer and a highly doped poly-Si layer, are emerging as one of the most promising alternatives, and efficiencies above 26% have already been demonstrated. The excellent performance of this junction architecture has been found to be remarkably independent of the deposition and/or doping technique used—even extending to techniques already prevalent in industry. This greatly reduces the capital and retraining expenditure needed to integrate the new technology into mainstream production lines, allowing it to be an evolutionary, rather than disruptive advancement. This has led to the rapid demonstration of large-area cells featuring poly-Si contacts by multiple PV manufacturing companies, with efficiencies above 24.5%. Although a bright future for poly-Si junctions is anticipated, as supported by the predictions of the International Technology Roadmap of Photovoltaics, several issues remain to be resolved, including those associated with the cost of and damage to the poly-Si layers due to the cell’s metallization process. This paper provides a perspective of the remaining challenges and potential of poly-Si junctions to transform the PV industry.

Passivating contacts for silicon solar cells can be fabricated by depositing a layer of intrinsic amorphous silicon (a-Si) by the plasma-enhanced chemical vapor deposition (PECVD) onto an oxidized silicon wafer, followed by a thermal POCl3 diffusion process. This article describes the influence of the main PECVD parameters, power and pressure, on the electrical performance of such phosphorus-doped polysilicon (doped-Si/SiOx) passivating contacts. We characterize their properties in terms of the passivation quality and carrier selectivity for different PECVD powers and pressures. The deposition power settings from 350 to 800 W are tried, the highest iV oc value of 721 mV is achieved at a power of 500 W. The higher deposition powers (≥650 W) lead to blistering issues and possible interface damage, while a lower deposition power (350 W) leads to incomplete decomposition of the precursor gas, resulting in a lower passivation quality. Meanwhile, the power has a marginal impact on the contact resistivity. On the other hand, the deposition pressure has only a slight impact on the passivation quality, while significant changes are observed on the contact resistivity. A lower pressure (0.1 mbar) leads to a higher contact resistivity, while the low and consistent contact resistivity values of 5.8 mΩ·cm2 are obtained at the pressures above 0.2 mbar.

Hydrogen-assisted defect engineering, via a hydrogenated silicon nitride (SiNx:H) capping layer, on doped polycrystalline silicon (poly-Si) passivating-contact structures, is explored using complem...

Summary High-performance passivating contact is a prerequisite for high-efficiency crystalline silicon (c-Si) solar cells. In this work, an electron-conductive, hole-blocking contact based on titanium nitride (TiN) deposited by reactive magnetron sputtering is presented. Quasi-metallic TiN combined with an ultrathin SiO2 passivation layer (SiO2/TiN) is demonstrated to be an effective electron-selective contact on c-Si, featuring a low-contact resistivity of 16.4 mΩ.cm2 and a tolerable recombination current parameter of ∼500 fA/cm2. By implementing the dual-function SiO2/TiN contact, which acts simultaneously as a surface passivating layer and metal electrode, an efficiency of 20% is achieved by an n-type c-Si solar cell with a simple structure. This work not only demonstrates a way to develop efficient n-type c-Si solar cells with dual-function metal nitride contacts at a low cost but also expands the pool of available carrier transport materials, from metal oxides to metal nitrides, for photovoltaic devices.

Carrier-selective contacts based on silicon films deposited onto a thin SiOx layer combine high performance with a degree of compatibility with industrial solar cell metallization steps. This paper demonstrates an approach to form electron-selective passivating contacts that maximises the overlap with common industrial equipment; it is based on depositing an intrinsic amorphous silicon (a-Si) layer by PECVD and then doping and re-crystallizing it by means of a thermal phosphorus diffusion. By optimizing the intrinsic a-Si thickness and the phosphorus diffusion temperature, a low recombination current density Joc ≈ 3 fA/cm2 and a low contact resistivity of ρc ≈ 3 mΩ-cm2 have been achieved. Additionally, these electrical parameters have been found to be sensitive to the work function of the outer metal electrode. The application of these optimized electron-selective passivating contacts to n-type silicon solar cells has permitted to achieve a conversion efficiency of 24.7%. A loss analysis has been conducted through Quokka 2 simulations, which together with quantum efficiency measurements, indicate that further optimization should focus on the front boron-doped region of the device.

We investigated and compared optoelectronic properties, crystallographic structures, and nanoscale surface morphologies of ex-situ phosphorus-doped polycrystalline silicon (poly-Si)/SiO x passivating contacts, formed by different deposition methods (sputtering, plasma-enhanced chemical vapour deposition (PECVD), and low-pressure chemical vapour deposition (LPCVD)). Across all these deposition technologies, a similar trend is observed: higher diffusion temperatures yield films that are more crystalline but have rougher surface morphologies due to bigger surface crystal grains. Also, the recrystallization process of the as-deposited Si films starts from the SiO x interface, rather than from the film surface and bulk. However, there are some distinct differences among these technologies. Firstly, the LPCVD method yields the roughest surface and smallest degree of crystallinity on finished poly-Si films. In contrast, the PECVD method has the smoothest surface for both as-deposited Si and annealed poly-Si films. Secondly, as-deposited sputtered and PECVD Si films contain only an amorphous phase whereas as-deposited LPCVD films has already had some crystalline phase. Thirdly, the LPCVD phosphorus in-diffusion into the substrate depends strongly on the initial film thickness, whereas for the other two methods it is weakly dependent on thickness.

We report luminescence phenomena from doped polycrystalline silicon (poly-Si) films and their applications to study carrier transport properties in passivating-contact solar cells. Low-temperature ...

Controlling the concentration of charge carriers near the surface is essential for solar cells. It permits to form regions with selective conductivity for either electrons or holes and it also helps to reduce the rate at which they recombine. Chemical passivation of the surfaces is equally important, and it can be combined with population control to implement carrier-selective, passivating contacts for solar cells. This paper discusses different approaches to suppress surface recombination and to manipulate the concentration of carriers by means of doping, work function and charge. It also describes some of the many surface-passivating contacts that are being developed for silicon solar cells, restricted to experiments performed by the authors.

This paper presents direct experimental evidence for the strong impurity gettering effects associated with the formation of both phosphorus and boron doped polysilicon/oxide passivating contacts for silicon solar cells, doped via thermal diffusion from POCl3 or BBr3 sources. Ion-implanted iron is used as a marker to quantify the gettering effectiveness via carrier lifetime measurements. The process conditions for fabricating optimum polysilicon passivating contacts are found to remove more than 99.9% of the iron from the silicon wafer bulk. The gettering effects of POCl3 and BBr3 diffused polysilicon/oxide contacts mainly arise from the dopant diffusions, as opposed to gettering by structural defects in the polysilicon films. The thin oxide interlayer hinders the gettering effectiveness at low diffusion temperatures, although its blocking effect becomes small at the moderate temperatures used to fabricate optimum polysilicon contacts. The gettering effectiveness increases with increasing diffusion temperature. The gettering of iron from the silicon wafer bulk to the surface layers is found to have a negligible impact on their ability to suppress recombination at the interface with the silicon wafer. Therefore, the formation of polysilicon/oxide passivating contacts, via thermal diffusion from POCl3 and BBr3 sources, not only achieves high quality surface and contact passivation but also has the net additional benefit of achieving very effective gettering of unwanted impurities in the silicon wafer bulk.

The formation of certain types of doped polysilicon passivating contacts for silicon solar cells is recently reported to generate very strong impurity gettering effects, revealing an important additional benefit of this passivating contact structure. This work investigates the underlying gettering mechanisms by directly monitoring the impurity redistribution during the contact formation and subsequent processes, via a combination of secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and minority carrier lifetime techniques. Microscopic features of the phosphorus and boron diffusion-doped polysilicon passivating contacts are also presented. Iron is used as a marker impurity in silicon to enable direct quantification of its concentration change in the bulk of the silicon wafers and in the surface layers that compose the contact structure. The results conclusively show that, for phosphorus-doped polysilicon passivating contacts, impurities are relocated from the silicon wafer bul...

Development of new device architectures and process technologies is of tremendous interest in crystalline silicon (c-Si) photovoltaics to drive enhanced performance and/or reduced processing cost. In this regard, an emerging concept with a high-efficiency potential is to employ low/high work function metal compounds or organic materials to form asymmetric electron and hole heterocontacts. This Letter demonstrates two important milestones in advancing this burgeoning concept. First, a high-performance, low-temperature, electron-selective heterocontact is developed, comprised of a surface passivating a-Si:H layer, a protective TiOx interlayer, and a low work function LiFx/Al outer electrode. This is combined with a MoOx hole-selective heterocontact to demonstrate a cell efficiency of 20.7%, the highest value for this cell class to date. Second, we show that this cell passes a standard stability test by maintaining >95% of its original performance after 1000 h of unencapsulated damp heat exposure, indicating...

A doping technique for p-type poly-Si/SiOx passivating contacts using a spin-on method for different mixtures of Ga and B glass solutions is presented. Effects of solution mixing ratios on the contact performance (implied open circuit voltage iVoc, contact resistivity ρc) are investigated. For all as-annealed samples at different drive-in temperatures, increasing the percentage of Ga in the solution shows a decrement in iVoc (from ∼680 to ∼610 mV) and increment in ρc (from ∼3 to ∼800 mΩ cm2). After a hydrogenation treatment by depositing a SiNx/AlOx stack followed by forming gas annealing, all samples show improved iVoc (∼700 mV with Ga-B co-doped, and ∼720 mV with all Ga). Interestingly, when co-doping Ga with B, even a small amount of B in the mixing solution shows negative effects on the surface passivation. Active and total dopant profiles obtained by electrical capacitance voltage and secondary-ion mass spectrometry measurements, respectively, reveal a relatively low percentage of electrically-active Ga and B in the poly-Si and Si layers. These results help understand the different features of the two dopants: a low ρc with B, a good passivation with Ga, their degree of activation inside the poly-Si and Si layers, and the annealing effects.

We present an electron-conductive, hole-blocking contact for silicon solar cell in this work. Quasi-metallic titanium nitride (TiN), deposited by reactive magnetron sputtering, is proven to be an effective electron-selective contact for silicon solar cell, simultaneously achieving a low contact resistivity (ρ c ) of ~ 16 mΩ∙cm2 and a tolerable contact recombination parameter (J 0c ) of ~ 500 fA/cm2. By the implementation of the dual-function SiO 2 /TiN contact, which acts simultaneously as efficient surface passivating and metal electrode, a power conversion efficiency of 20% is achieved on an n-type silicon solar cell with a simple structure. This work demonstrates a way to develop efficient n-type Si solar cells with dual-function metal nitride contacts at a low cost.

In recent years, polycrystalline silicon (poly-Si) based passivating-contact solar cells have received tremendous attention from the solar research community due to its excellent surface passivation and high carrier conductivity. However, the poly-Si films are not transparent to all wavelengths of the solar spectrum. There is often some parasitic absorption in these films. From a different standpoint, as they absorb some light, they can luminesce. This phenomenon provides us with unique opportunities to investigate optoelectronic properties of the films in a fast, contactless, and nondestructive manner. In this work, we report the luminescence phenomenon from poly-Si films used in passivating-contact solar cells. We then utilize this phenomenon to report a range of applications for solar cells including studies of carrier transport behaviors and hydrogenation inside the films.

Understanding the impact of metal contacts on the recombination within a passivated silicon wafer is crucial for the optimization of various photovoltaic devices such as passivating-contact-based solar cells. To investigate the effect of the metal work function, a selection of metals is applied to aluminum-oxide-passivated n-type crystalline silicon wafers. The saturation current density of the metalized contact (J 0m ) is determined using the quasi-steady-state photoluminescence method and used as a figure of merit to quantify the effect. We find that J 0m increases with the metal work function and that this effect is modulated with the passivation layer thickness. It is more pronounced for thinner passivation layers, which can be attributed to a significant change in the populations of electrons and holes near the silicon surface induced by the metal. Meanwhile thicker layers prevent the charge transfer between the silicon and metal more efficiently leading to insignificant changes in J 0m . Based on these findings, we suggest a suitable metal work function range to optimize contact recombination in silicon-based solar cells.

In this work, we investigate the impact of hydrogenation on the performance of phosphorus doped polycrystalline silicon (poly-Si) layers for passivating contact solar cells. Integrating various characterization techniques including transmission electron microscopy, energy dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation treatment of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω.cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 μs to 2.7 ms and from 668 mV to 722 mV.

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by a thin layer of thermal atomic layer deposited (ALD) titanium oxide (TiO 2 ). Surface recombination velocities of 0.8 cm/s, 2.5 cm/s and 9.8 cm/s have been obtained on n -type 10 cm, 1 Ω cm and p -type 1 Ω cm undiffused wafers, respectively. Recombination current densities of 19 fA/cm 2 and 780 fA/cm 2 have been measured on 120 Ω/□ boron diffused p + and 100 Ω/□ phosphorus diffused n + regions. In addition to providing a superior passivation on p + over n + (c-Si) surfaces, the ALD TiO 2 layers produces a strong injection-dependent effective lifetime on the n -type substrates, , both of which are consistent with the possible presence of negative charge in the passivating layer. The recombination at the interface between TiO 2 and planar , planar and alkaline textured (c-Si) surfaces with upright pyramids is compared. We find that ( i ) a planar surface exhibits a 1.39 times higher recombination than a planar surface, and ( ii ) after area correction, the ratio of recombination on the textured and planar surfaces is 1.37. A thin film of TiO 2 deposited by ALD has been applied to the front surface of a rear locally diffused p + nn + front junction solar cell, performing the dual role of surface passivation and single-layer antireflection coating on the textured p + diffusion. The best solar cell achieved V oc =655 mV, FF =79.9% and efficiency=20.45%. The results presented in this work demonstrate that TiO 2 re-emerging as a suitable optical and passivating material to produce high performance solar cells.

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

Short-duration post-deposition thermal treatments at temperatures above those normally used for annealing activation have the potential to further improve the already excellent passivation of crystalline silicon (c-Si) achieved by Al2O3, but have so far received little attention. In this work we investigate the influence of rapid thermal annealing (RTA) on the surface passivation of c-Si by Al2O3 deposited by atmospheric pressure chemical vapour deposition (APCVD) as a function of RTA peak temperature between 500 and 900 °C, and for Al2O3 deposition temperatures between 325 and 440 °C. The saturation current density J0 of undiffused p-type surfaces is observed either to increase or decrease following RTA depending on the Al2O3 deposition temperature and the RTA peak temperature. The optimum deposition temperature depends on the post-deposition thermal processing to be applied. Films deposited at lower temperatures provide worse passivation after low temperature heat treatment, but maintain this passivation better at higher RTA temperatures. An exceptionally low J0 of 7 fA cm-2, due to the combination of a very low interface state density Dit and unusually high negative fixed charge density Qf, is achieved by the use of a short 500-550 °C RTA combined with optimised deposition conditions.

A technique to make poly-Si (p + )/SiO x contacts for crystalline silicon solar cells based on doping PECVD intrinsic amorphous silicon (a-Si) by means of a thermal BBr 3 diffusion process is demonstrated. The thickness of the a-Si layer and the temperature of the boron diffusion are optimized in terms of suppressing carrier recombination and transport losses. Different interfacial layers are studied, including ultra-thin SiO x grown either chemically or thermally, and stacks of SiO x and SiN x . While the double SiO x /SiN x interlayers do not achieve the desired performance, both kinds of single SiO x layers produce satisfactory passivating contacts, with both a low recombination current and a low contact resistivity. By adjusting the boron diffusion temperature, recombination current parameter J 0 values of ~16 fA/cm 2 to ~30 fA/cm 2 have been obtained for structures with initial a-Si thicknesses of 36–46 nm, together with a contact resistivity of ~8 mΩ cm 2 .

Advances in the passivation of p-type and $p^{+}$ surfaces have been one of the main developments in crystalline silicon solar cell technology in recent years, enabling significant progress in p-type solar cells with partial rear contacts, and n-type solar cells with front-side boron diffusions. In this contribution, we demonstrate improvements in the passivation of p-type and boron diffused $p^{+}$ surfaces with plasma-enhanced atomic layer deposition (PEALD) gallium oxide (Ga2O3) with the addition of plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx). On 1.6 Ωcm p-type wafers, we measure an improvement in the upper limit surface recombination velocity ( $S_{{\rm eff, \text{UL}}}$ ) from 2.5 to 1.4 cm/s on optimized Ga2O3 passivated samples before and after SiNx capping. We also show an improvement in the passivation of boron diffused $p^{+}$ surfaces over previously reported data, measuring a recombination parameter ( $J_{0}$ ) of 26 fA/cm2 on a Ga2O3 passivated 85 Ω/sq boron diffusion, approaching the Auger limit of ∼21 fA/cm2 for this diffusion. In addition, we show that initial studies on the thermal stability of the Ga2O3/SiNx stack indicate that it is compatible with conventional screen-printed metallization firing procedures.

In this study, we present a novel application of thin magnesium fluoride films to form electron-selective contacts to n-type crystalline silicon (c-Si). This allows the demonstration of a 20.1%-efficient c-Si solar cell. The electron-selective contact is composed of deposited layers of amorphous silicon (∼6.5 nm), magnesium fluoride (∼1 nm), and aluminum (∼300 nm). X-ray photoelectron spectroscopy reveals a work function of 3.5 eV at the MgF2/Al interface, significantly lower than that of aluminum itself (∼4.2 eV), enabling an Ohmic contact between the aluminum electrode and n-type c-Si. The optimized contact structure exhibits a contact resistivity of ∼76 mΩ·cm(2), sufficiently low for a full-area contact to solar cells, together with a very low contact recombination current density of ∼10 fA/cm(2). We demonstrate that electrodes functionalized with thin magnesium fluoride films significantly improve the performance of silicon solar cells. The novel contacts can potentially be implemented also in organic optoelectronic devices, including photovoltaics, thin film transistors, or light emitting diodes.

With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures - the interdigitated back-contact silicon cell and the silicon heterojunction cell - both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic-inorganic perovskite materials.

Among common methods to form polycrystalline silicon (poly-Si) films for passivating-contact solar cells, physical vapor deposition, in particular sputtering, is the safest one as it does not require any toxic gaseous precursors. One of the critical parameters to control the properties of sputtered silicon films is their deposition pressure. In this work, structural and optoelectronic characteristics of ex-situ boron-doped poly-Si/SiOx passivating contacts, formed from sputtered intrinsic amorphous silicon (a-Si) deposited at different pressures on top of SiOx/c-Si substrates and subjected to a high-temperature boron diffusion step, are investigated. The deposition rate and density of the as-deposited a-Si films increase with reducing pressure. Low-temperature photoluminescence spectra captured from the as-deposited samples at different pressures do not show typical emissions from hydrogenated a-Si. Meanwhile, their Fourier-transform infrared absorption spectra all show Si–H stretching modes, indicating that hydrogen had been initially incorporated into the chemical SiOx layers and eventually hydrogenated the a-Si/SiOx interfaces during the sputtering process. After the high-temperature boron-diffusion step, all hydrogen-related peaks disappear. Lower pressure films (1.5 and 2.5 mTorr) show more consistent improved performance after hydrogen treatments, compared to higher pressure films (4 and 5 mTorr). The resultant passivating contacts at 2.5 mTorr achieve a low single-side recombination current density Jo of ~9 fA/cm2, whereas their contact resistivity is still low at 15 mΩ cm2.

This paper describes the influence of plasma enhanced chemical vapor deposition (PECVD) deposition temperature on heavily doped silicon based (doped-Si/SiOx) passivating contacts for silicon solar cells. The doped-Si films are obtained by PECVD intrinsic amorphous silicon (a-Si) and a subsequent thermal POCl3 diffusion process. By changing the deposition temperature of PECVD, a-Si films with different degrees of crystallinity and density can be obtained. These differences between the a-Si films result in different properties of the passivating contacts in terms of passivation quality and carrier selectivity. By exploring a range of PECVD deposition temperatures from 250 °C to 470 °C, the best passivation quality is obtained at a temperature of 420 °C. On the other hand, the contact resistivity decreases with increasing deposition temperature. After studying the a-Si properties and the resulting passivating contact properties, we obtain optimal passivating contacts with a high implied open-circuit voltage (iVoc) of 742 mV and a low contact resistivity ρc of 6.4 mΩ∙cm2.

We report direct experimental evidence for the strong impurity gettering effects associated with the formation of diffusion-doped polysilicon passivating contacts. Iron is used as a marker impurity in silicon to quantify the gettering effectiveness. By monitoring the iron redistribution from the silicon wafer bulk to the polysilicon surface layers, via a combination of carrier lifetime, secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM) techniques, the respective gettering sites in the phosphorus and boron diffusiondoped polysilicon contacts are identified. In phosphorus-doped polysilicon, iron moves to the heavily doped polysilicon layer; and in the boron-doped structure, iron is gettered to the boron-rich layer. Both gettering processes occur via an impurity segregation mechanism. Lastly, the gettering of iron to the polysilicon surface layers is found to have no impact on the passivation quality of the polysilicon contacts.

Minimizing carrier recombination losses at contact regions by using carrier-selective contact materials, instead of heavily doping the silicon, has attracted considerable attention for high-efficiency, low-cost crystalline silicon (c-Si) solar cells. Here we present a novel and stable metal nitride based hole-blocking layer for efficient silicon solar cells.The ALD-deposited tantalum nitride (TaN x ) films are demonstrated to provide excellent holeblocking property on silicon surfaces, due to their small conduction band offset and large valence band offset with silicon. Thin TaN x films are found to provide not only moderate surface passivation to silicon surfaces, but also allow a relatively low contact resistivity at the TaN x n-Si heterojunctions. An efficiency over 20% is achieved on n-type silicon solar cells featuring a simple full-area electron-selective TaN x contact, representing an absolute efficiency gain of 4.0% over the control device without TaN x contact.

Over the past three years a new family of high efficiency n-type crystalline silicon cells featuring passivated partial rear contacts (PRC) has emerged. These cells take advantage of the unique contact properties obtained by introducing thin metal compound interlayers, such as TiO $_{\mathbf {x}}$ and LiF$_{\mathbf {x}}$, between the metallic electrode and the silicon absorber. This paper explores the concept and potential advantages of the n-type passivated PRC cell. In particular, the recent fabrication of a cell at 23.1%, featuring a less-than 1% TiO$_{\mathbf {x}}$/LiF$_{\mathbf {x}}$textbf/Al passivated PRC, demonstrates the compatibility of this concept with high efficiency designs.

Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen. In this work, we demonstrate a nanoscale tantalum oxide (TaOx, ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaOx is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaOx interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications.

This paper describes the optimization of a technique to make polysilicon/SiOx contacts for silicon solar cells based on doping PECVD intrinsic polysilicon by means of a thermal POCl3 diffusion process. Test structures are used to measure the recombination current density Joc and contact resistivity ρc of the metal/n+ polysilicon/SiOx/silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiOx and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρc, with an optimum of about 1.2 nm for a thermal oxide and ∼1.4 nm for a chemical oxide. A low Joc≤5 fA/cm2 has been obtained for polysilicon thicknesses in the range of 32 nm–60 nm, while ρc increases from 0.016 Ω cm2 to 0.070 Ω-cm2 due to the bulk resistivity of polysilicon. These polysilicon/SiOx contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving Voc=674.6 mV, FF=80.4% and efficiency=20.8%, which demonstrate the effectiveness of the techniques developed here to produce high performance solar cells.

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by thermal atomic layer deposited (ALD) tantalum oxide (Ta2O5) underneath a capping silicon nitride (SiNx) layer by plasma enhanced chemical vapor deposited (PECVD). Surface recombination is investigated as a function of Ta2O5 thickness for p- and n-type Si substrates, both with and without boron (p+) or phosphorus (n+) diffusions. It is found that the recombination decreases markedly with increasing Ta2O5 thickness on p, n and p+ c-Si surfaces, but it follows an opposite trend on n+ c-Si surfaces. In all four cases, the surface recombination velocity plateaus at a Ta2O5 thickness of 12 nm. The thermal stability of surface passivation by Ta2O5/SiNx is examined by subjecting p+ and n+ diffused wafers to a typical solar cell metallization firing process, finding that it is essentially stable on p+ diffusions, but not on n+ ones, regardless of Ta2O5 thickness. We also evaluate the passivating properties of the Ta2O5/SiNx stack on planar {100}, planar {111}, and textured n-type undiffused silicon surfaces, finding that (i) planar {111} Si exhibits a 4.6-fold higher recombination than planar {100} Si, and (ii) recombination at a textured surface is approximately equivalent to that at a planar {111} after surface area correction. Furthermore, the area-corrected recombination ratio of textured to planar {100} boron diffused p+ regions is shown to be 2.2 for three different diffusions with sheet resistances at 56, 122, and 214 Ω/sq. Finally, optical simulation reveals a low reflection and negligible absorption loss for the Ta2O5/SiNx stack. The Ta2O5/SiNx stack is thus demonstrated to be an excellent surface passivation and antireflection coating for high efficiency silicon solar cells.

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a-Si:H film by means of a thermal POCl3 diffusion process and an exploration of the effect of the refractive index of the SiNx. The n+ silicon passivating contact with SiNx /SiOx double layer achieves a better result than a single SiNx or SiOx layer, giving a recombination current parameter of ∼7 fA/cm2 and a contact resistivity of ∼0.005 Ω cm2, respectively. These self-passivating electron-selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) In this Letter, the passivating contact by means of PECV (plasma-enhanced chemical vapor deposition) intrinsic a-Si:H film and thermal POCl3 diffusion process shows its great performance on the way to high efficiency silicon solar cells. In this study, n+ silicon passivating contacts with three different interfacial structures - thermal silicon oxide, silicon nitride, and double silicon nitride/silicon oxide layer - are investigated in terms of contact recombination and contact resistivity.

This paper provides an experimental proof-of-concept for simple solar cell designs on n- and p-type crystalline silicon (c-Si) substrates which utilise sub-stoichiometric MoOx (x < 3) films to collect holes. The n-type cell design (referred to as ‘moly-poly’) features a planar rear SiOx / poly-Si(n+) stack with a planar front SiOx / MoOx / ITO stack. We demonstrate an un-optimised conversion efficiency of ∼16.7±1% for a 3 x 3cm cell using a simple 10-step fabrication procedure. The p-type cell design (referred to as ‘moly-BSR’) is comprised of a simple SiNx passivated, textured, front phosphorus diffusion with a rear MoOx / Ag hole contact. A conversion efficiency of ∼16.4±1% is achieved for 2 x 2cm using an 11-step fabrication procedure. Beyond the proof-of-concept results achieved, a number of future improvements are also outlined.

This paper presents the experimental demonstration of silicon solar cells that incorporate an enhanced MIS passivated contact scheme on a phosphorus diffused surface. By depositing intrinsic a-Si:H on an ultrathin SiOx layer and alloying with an overlying aluminium layer, the interface passivation has been vastly improved over that of conventional MIS contacts, whilst maintaining a low contact resistance. This paper focuses on the optimisation of the Al/a-Si:H alloying process and the influence of the tunnelling SiOx layer thickness. A conversion efficiency of 21.0% has been achieved for n-type cells fabricated with a front boron diffusion and a full area rear MIS passivated phosphorus diffusion. The cells exhibit a moderate Voc=666 mV and FF=0.805, whereas Jsc 39.3 mA/cm2 is relatively low due to a non-optimal antireflection coating and back surface reflector, and hence will be subject to further improvement.

This work has been supported by the Australian government through the Austra-lian Renewable Energy Agency (ARENA).

Composite correlation filters are used for solving a wide variety of pattern recognition problems. These filters are given by a combination of several training templates chosen by a designer in an ad hoc manner. In this work, we present a new approach for the design of composite filters based on multi-objective combinatorial optimization. Given a vast search space of training templates, an iterative algorithm is used to synthesize a filter with an optimized performance in terms of several competing criteria. Moreover, by employing a suggested binary-search procedure a filter bank with a minimum number of filters can be constructed, for a prespecified trade-off of performance metrics. Computer simulation results obtained with the proposed method in recognizing geometrically distorted versions of a target in cluttered and noisy scenes are discussed and compared in terms of recognition performance and complexity with existing state-of-the-art filters.

The selective transport of electrons and holes to the two terminals of a solar cell is often attributed to an electric field, although well-known physics states that they are driven by gradients of quasi-Fermi energies. However, in an illuminated semiconductor, these forces are not selective, and they drive both charge carriers toward both contacts. This paper shows that the necessary selectivity is achieved by differences in the conductivities of electrons and holes in two distinct regions of the device, which, for one charge carrier, allows transport to one contact and block transport to the other contact.

The simplicity and low thermal budget of dopant-free, selective carrier contact materials is opening up new possibilities for low-cost, high performance silicon photovoltaics. This paper describes recent progress in the exploration of magnesium and related materials such as magnesium oxide and fluoride as electron-selective contacts to lightly doped n-type cSi. A direct comparison between the three materials permits to identify significant differences in transport and recombination behaviour. All three are incorporated in n-type silicon solar cells to demonstrate their actual performance at the device level. In particular, a simple aluminium electrode, functionalized with a nanoscale Mg-based layer, significantly enhances the performance of n-type c-Si solar cells, to a conversion efficiency of ~20% equivalent to the performance of the standard p-type silicon solar cells with an alloyed Al full-area hole contact.

Fuente TESIUNAM

Based on contactless photoconductance measurements of silicon wafers, we have determined the sum of the electron and hole mobilities as a function of doping, excess carrier concentration, and temperature. By separately analyzing those three functional dependences, we then develop a simple mathematical expression to describe the mobility sum as a function of carrier injection wafer doping and temperature from 150 to 450 K. This new parameterization also provides experimental validation to Klaassen's and Dorkel-Leturcq's mobility models in a range of temperatures.

We compare the passivation provided by Al2O3 deposited on planar and , and textured , boron-diffused and undiffused crystalline silicon surfaces. The passivation of surfaces is found to be somewhat worse than that of surfaces for the as-deposited films, but improves to similar values after annealing. Higher recombination at textured surfaces compared to planar surfaces can be largely, though not entirely, attributed to the difference in surface area. The passivation of both as-deposited and annealed films is found to improve over time when stored under ambient conditions. This helps give context to the myriad of results reported on planar samples – we expect J0 values measured for Al2O3 layers on such surfaces to increase by a factor of ∼2 to 3 on textured surfaces.

This paper deals with the impact of dopant compensation on the degradation of carrier lifetime and solar cells performance due to the boron–oxygen defect. The boron–oxygen defect density evaluated by lifetime measurements before and after degradation is systematically found proportional to the total boron concentration, showing that compensation cannot reduce light-induced degradation. This result is confirmed by a comparison of upgraded-metallurgical grade silicon solar cells having identical boron, oxygen and carbon but different compensation levels and in which the degradation is found more severe when the compensation is stronger.

Author(s): Bullock, J; Wan, Y; Hettick, M; Zhaoran, X; Phang, SP; Yan, D; Wang, H; Ji, W; Samundsett, C; Hameiri, Z; Macdonald, D; Cuevas, A; Javey, A | Abstract: Over the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form carrier-selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiO x /LiF x /Al electron heterocontact is presented here, which achieves mΩcm 2 scale contact resistivities ρ c on lowly doped n-type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n-type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n-type solar cells featuring undoped PRCs and confirming the unusually low ρ c of the TiO x /LiF x /Al contact. Finally, in contrast to previous versions of the n-type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.

This letter shows that intrinsic hydrogenated amorphous silicon (a-Si:H) films deposited by RF magnetron sputtering can provide outstanding passivation of crystalline silicon surfaces, similar to that achieved by plasma enhanced chemical vapour deposition (PECVD). By using a 2% hydrogen and 98% argon gas mixture as the plasma source, 1.5 Ω cm n-type FZ silicon wafers coated with sputtered a-Si:H films achieved an effective lifetime of 3.5 ms, comparable to the 3 ms achieved by PECVD (RF and microwave dual-mode). This is despite the fact that Fourier transform infrared spectroscopy measurements show that sputtering and PECVD deposited films have very different chemical bonding configurations. We have found that film thickness and deposition temperature have a significant impact on the passivation results. Self-annealing and hydrogen plasma treatment during deposition are likely driving forces for the observed changes in surface passivation. These experimental results open the way for the application of sputtered a-Si:H to silicon heterojunction solar cells. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)