Plasma-Enhanced Chemical Vapor Deposition (PECVD) is an attractive tool for TOPCon production, as it enables uniformly in situ doped amorphous silicon (a-Si) and dielectric layer depositions with high throughput. However, a lean process requires in situ interfacial oxide growth in the same tool. In this work, we use Plasma-Assisted N2O Oxidation (PANO) in an industrial kHz direct plasma reactor (centrotherm c.PLASMA) to grow the oxide and deposit in situ phosphorus doped a-Si(n) as well as SiNx on asymmetric lifetime samples. Before optimization, the oxide thickness is non uniform on the wafer, and we show that it correlates with the passivation, the contact resistivity, and the doping profile in n-type TOPCon test structures. The passivation seems to benefit more from moderate in-diffusion in the case with PANO than in the case with thermal oxidation. This is probably due to enhanced field-effect passivation compensating for lower chemical passivation, which likely results from plasma-induced damage. After studying the influence of PANO process parameters on the oxide thickness and uniformity, we optimize them to obtain a non-uniformity as low as ±2% and a recombination current density down to 2.3 fA/cm² on planar wafers.

In addition to excellent surface passivation and carrier selectivity, the structure based on the heavily doped polysilicon layer on an ultrathin silicon oxide interlayer also demonstrates strong impurity gettering effects. Herein, the gettering strength of a range of phosphorus‐ or boron‐doped polysilicon films from different fabrication techniques is assessed and compared. Iron, one of the most common metallic impurities in silicon, is used as a tracer impurity to quantify the gettering strength (segregation coefficient). A comparison of the experimental results to the literature, combined with measurements of the electrically active and inactive dopant concentrations, enables us to suggest the main gettering mechanisms in different polysilicon films. The differences in the segregation coefficients of the phosphorus‐doped polysilicon films for iron are within one order of magnitude, in spite of their different combinations of gettering mechanisms. On the other hand, boron‐doped polysilicon films show a large variation in their gettering effects, although the predominant gettering mechanisms are all attributed to electrically inactive boron, according to the current understanding of the gettering mechanisms from the literature. Finally, the impact of different polysilicon gettering effects on the efficiency of tunnel oxide‐passivated contact (TOPCon) cells is simulated and discussed.

The recombination parameter J 0 s provides an important metric to characterize surface recombination. For its calculation, numerous methods and models have to be applied. Since the models for the Auger and radiative recombination in crystalline silicon were recently revised, it is important to investigate the influence of these changes on J 0 s . The origin and possible ways of obtaining J 0 s from effective lifetime measurements as well as simulations are described in detail, including the potential to fit the full lifetime curve and a new approach that is based upon the reparameterization of the excess charge carrier density Δ n . Using the effective lifetime measurements, we find that J 0 s values determined with the older parameterization by Richter et al. will result in erroneous values up to 5 fA/cm 2 , depending on the chosen conditions. By simulating the recombination parameter J 0 s in near surface, highly doped structures, such as emitters, it is shown that these errors can even go up to 50%. If used in a simulation, we highlight the importance of having the parameterizations of surface recombination being determined with the corresponding parameterization of intrinsic recombination. Therefore, an update for the recombination at oxide-passivated and phosphorous doped surfaces is given that can be used with the new intrinsic recombination models. Finally, we give some best-practice examples on how recent improvements in effective lifetime measurements affect J 0 s values as well as possible pitfalls.

Passivating contacts based on poly‐Si/SiO x structures also known as TOPCon (tunnel oxide passivated contacts) have a great potential to improve the efficiency of crystalline silicon solar cells, resulting in more than 26% and 24% for laboratory and industrial cells, respectively. This publication gives an overview of the historical development of such contact structures which have started already in the 1980s and describes the current state‐of‐the‐art in laboratory and industry. In order to demonstrate the great variety of scientific and technological research, four different research topics are addressed in more detail: (i) the superior passivation quality of TOPCon structures made it necessary to re‐parametrize intrinsic recombination in silicon, (ii) the control of diffusion of dopants through the intermediate SiO x layer is essential to optimize passivation and transport properties, (iii) single‐sided deposition of the poly‐Si layer would reduce process complexity for industrial TOPCon cells, and (iv) silicon‐based tunnel junctions for perovskite–silicon tandem cells can be fabricated using the TOPCon technology.

PERC is the most common cell type in today's fabrication of crystalline Si solar cells. However, the cell design is often limited by the compromise for the front side between lateral conductivity, contact resistivity and recombination. In this work, we propose the application of the same PERC process sequence on epitaxially-grown n-type material that has a lowly-doped, deep emitter integrated at the rear side during the wafer growth. Using 3D device simulation, we investigate the influence of the doping concentration and depth of this emitter. Deep (up to 50 mu m), lowly concentrated (e.g. 1x10(17) cm(-3)) emitters are expected to be beneficial for the overall cell performance. At the best set of simulated parameters, the reference PERC is exceeded by 0.7 %(abs) in efficiency. As first steps towards a realization of the cell concept, epitaxially grown cell precursors with a variation of emitter depth of up to 20 mu m and doping concentration in the range of 8x10(16) to 1x10(19) cm(-3) are presented. An effective minority charge carrier lifetime tau(eff) approximate to 3 ms was determined for these precursors if the emitter doping concentration was below 10(18) cm(-3), even for the thickest emitter of 20 mu m indicating that the emitter at low doping concentrations does not significantly impact teff. This highlights the potential and high purity of epitaxially grown Si with stable doping concentration and sharp p/n-transitions for cell fabrication.

This paper focusses onto the first successful realization of an industry-relevant in situ B-doped direct-plasma PECVD process for p-type TOPCon as well as the fundamental understanding of the junction's working principle. A key element in this approach is controlling the diffusion of B across the poly-Si/SiOx/c-Si junction upon high-temperature annealing. More precisely, the influence of different thermally grown interfacial oxides, an intrinsic a-Si(i) layer below the highly doped a-Si(p) film as well as an additional native SiOx interlayer in between those a-Si layers are investigated. Both high passivation qualities (J(0s) < 7 fA/cm(2)) and low contact resistivities (rho(c) < 5 m Omega cm(2)) are obtained on planar surface. When implementing those layers at the rear of both-sides contacted p-type front emitter TOPCon solar cells, a very high V-oc of 719.5 mV and eta up to 22.8% were achieved. Furthermore, it is found that p-type TOPCon reveals an enhanced thermal stability upon firing up to 850 degrees C with a single SiNx layer compared to n-type TOPCon and does not require an additional AlOx layer.

In this article, we evaluate an industrially relevant alternative for the formation of selectively doped n -type tunnel oxide passivating contacts ( n -TOPCon) by means of inkjet-printing with the goal to provide a low contact resistance as well as fulfilling the requirements for screen-printing metallization. It is shown that inkjet-printing of phosphorus dopant sources for thick TOPCon layers deposited by plasma-enhanced chemical vapor deposition provides excellent surface passivation with the implied open-circuit voltage i V oc = 733 mV, implied fill factor i FF = 87%, and a high dopant concentration of N poly-Si ∼ 2 × 10 20 as required to achieve low contact resistivities when using screen-printed pastes as contacting material. The V oc values of the prepared TOPCon solar cells of 697 mV confirm that the inks and inkjet processes are suitable for integration in TOPCon solar cells. Moreover, these cells enable promising conversion efficiencies of up to η best = 22.0% and offer a valuable set-up for further investigations on the correlations between inkjet processing and solar cell performance.

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments.

n-type silicon-based tunnel-oxide passivating contact (TOPCon) solar cells are a cell concept reaching highest power conversion efficiencies. In this article, we demonstrate a substantial simplification of processing such TOPCon solar cells by reducing the number of high temperature processes. To this end, rapid vapor-phase direct doping (RVD) processes are applied for emitter formation and simultaneous annealing of the TOPCon layers within one process. RVD emitters with sheet resistances of 200 Ω sq-1 reach low emitter saturation current densities of 26 fA cm-2 on textured surfaces. Thermal interface oxides of the TOPCon layers were adapted to withstand the increased thermal budged of the RVD process. Optimized layers exhibit a saturation current density of less than 1 fA cm-2 and a contact resistance of 5 mΩ cm2. The best solar cell with the simultaneous emitter diffusion and TOPCon annealing during the RVD process reaches a confirmed efficiency of 23.3%, similar to a reference with sequential BBr3 diffusion and subsequent TOPCon deposition and annealing reaching 23.1%.

Polycrystalline-silicon/oxide (poly-Si/SiOx) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiOx interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly-Si/SiOx structures. Herein, the factors that determine the blocking effects of the SiOx interlayers are identified and investigated by examining two general types of the SiOx interlayers: 1.3-nm ultrathin tunneling SiOx with negligible pinholes and 2.5-nm SiOx with thermally created pinholes. Iron is used as a tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion-limited segregation gettering model, the blocking effects of the SiOx interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiOx interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high-temperature formation of the poly-Si/SiOx structures, a SiOx interlayer that is less stoichiometric or with a higher pinhole density is preferred.

Herein, an analysis on the impact of laser contact opening of TOPCon/SiN x stacks is presented. By etching in tetramethylammonium hydroxide (TMAH), the defect distribution in the interfacial tunnel oxide is accessed and analyzed. The defect density is significantly increased in areas where adjacent laser contact openings (LCO) overlap. Using microscopic photoluminescence (μ‐PL) spectroscopy, it is verified that correlates with an increase in the local recombination rate and thus an increase in the J 0,Met . Therefore, overlapping LCO of SiN x in TOPCon/SiN x stacks should be avoided as much as possible. Furthermore, the investigations indicate that defects in the interfacial oxide are dominantly created along exposed structures like tips and edges of (etched‐back) pyramids. A comparison of TOPCon/SiNx stacks with a variation of TOPCon thicknesses indicate that etch pits, and thus the defect density, related to LCO become more significant at lower thicknesses.

Polycrystalline‐silicon/oxide (poly‐Si/SiO x ) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiO x interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly‐Si/SiO x structures. Herein, the factors that determine the blocking effects of the SiO x interlayers are identified and investigated by examining two general types of the SiO x interlayers: 1.3 nm ultrathin tunneling SiO x with negligible pinholes and 2.5 nm SiO x with thermally created pinholes. Iron is used as tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion‐limited segregation gettering model, the blocking effects of the SiO x interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiO x interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high‐temperature formation of the poly‐Si/SiO x structures, a SiO x interlayer that is less stoichiometric or with a higher pinhole density is preferred.

The photovoltaic industry is dominated by crystalline silicon solar cells. Although interdigitated back-contact cells have yielded the highest efficiency, both-sides-contacted cells are the preferred choice in industrial production due to their lower complexity. Here we show that omitting the layers at the front side that provide lateral charge carrier transport is the key to excellent optoelectrical properties for both-sides-contacted cells. This results in a conversion efficiency of 26.0%. In contrast to standard industrial cells with a front side p–n junction, this cell exhibits the p–n junction at the back surface in the form of a full-area polycrystalline silicon-based passivating contact. A detailed power-loss analysis reveals that this cell balances electron and hole transport losses as well as transport and recombination losses in general. A systematic simulation study led to some fundamental design rules for future >26% efficiency silicon solar cells and demonstrates the potential and the superiority of these back-junction solar cells.

Industrial tunnel oxide and passivated contact (i-TOPCon) solar cells were metallized at Fraunhofer ISE using ultrashort pulse laser ablation of the passivation layers for the subsequent Ni/Cu/Ag plating process. The solar cells feature a tunnel SiOx and n-type doped polysilicon layer covered by a SiNx at the rear side, whereas the front side is made of a boron emitter passivated with a AlOx/SiNx stack. The reference i-TOPCon solar cells screen-printed at the supplier reach an efficiency of 23.46% measured by Fraunhofer ISE CalLab. The impact of the laser process on the implied open circuit voltage (iV(oc)) is characterized showing minor impact on the TOPCon side, while the emitter side reveals an increased iV(oc) loss due to laser damage. Loss analysis by simulating the plated solar cells points out the benefit of reducing the laser contact opening (LCO) area in terms of shading and contact recombination. Optimization of laser ablation and hydrofluoric acid (HF) pretreatment process result in V-oc > 700 mV and FF > 82% leading to a mean efficiency 23.6% measured in-house and a champion efficiency of 23.84% measured at Fraunhofer ISE CalLab thus outperforming the references by 0.4%(abs).

Plated Ni/Cu/Ag contacts offer the possibility to significantly reduce silver consumption for tunnel oxide passivated contact (TOPCon) solar cells. This work demonstrates industrial bifacial TOPCon solar cells with plated Ni/Cu/Ag metallization achieving champion solar cell efficiencies of up to 24.0%. The influence of reduced poly-Si thickness down to 60 nm of the TOPCon rear side on the open circuit voltage (V-oc) is analysed and the impact of laser-induced damage during laser contact opening (LCO) is characterized. Furthermore, mitigation strategies to systematic fill factor losses are developed including laser-induced nano-roughness within the LCO to increase contact adhesion and current-annealing to improve contact resistance.

This work investigates in detail plating of Ni/Cu/Ag contacts as an alternative metallization approach for industrial bifacial tunneling oxide and passivating contacts (i-TOPCon) silicon solar cells. We have achieved a 23.3 % champion cell efficiency on a front and rear plated bifacial TOPCon silicon solar cell on industrial precursors on a 9 busbar design reaching the same mean efficiency level (eta = 23.0 %) as industrially processed identical screen printed references. Further, plating metallization demonstrates the potential to contact poly-Si layer thicknesses below 100 inn with reasonable J(0,met). The substitution of printed silver by plated copper leads to a significant reduction in the cost of ownership of the metallization backend for i-TOPCon solar cells of about 7.38 $ct/wafer. The integration of plated Ni/Cu/Ag contacts enables a reduction in Ag consumption of about 19.7 t/Gigawatt production capacity compared to screen printing metallization.

Combining the advantages of a high‐efficiency solar cell concept and a low carbon footprint base material is a promising approach for highly efficient, sustainable, and cost‐effective solar cells. In this work, we investigate the suitability of epitaxially grown p‐type silicon wafers for solar cells with tunnel oxide passivating contact rear emitter. As a first proof of principle, an efficiency limiting bulk recombination analysis of epitaxially grown p‐type silicon wafers deposited on high quality substrates (EpiRef) unveils promising cell efficiency potentials exceeding 25% for three different base resistivities of 3, 14, and 100 Ω cm. To understand the remaining limitations in detail, concentrations of metastable defects Fei, CrB and BO are assessed by lifetime‐calibrated photoluminescence imaging and their impact on the overall recombination is evaluated. The EpiRef wafers’ efficiency potential is tracked along the solar cell fabrication process to quantify the impact of high temperature treatments on the material quality. We observe large areas with few structural defects on the wafer featuring lifetimes exceeding 10 ms and an efficiency potential of 25.8% even after exposing the wafer to a thermal oxidation at 1050 °C.

Polysilicon/oxide (poly-Si/SiOx) passivating contacts are a promising technology for the next-generation of high-efficiency silicon solar cells. The structure can be realised by a range of fabrication techniques, which can induce very different impurity gettering effects during the formation process. Understanding the different gettering effects will enable tailored solutions to optimise the gettering efficiency in device fabrication. This paper demonstrates a method to separately quantify the impact of each component on the overall gettering effect of the poly-Si/SiOx passivating contact structures. These components consist of the heavily doped poly-Si layer, in terms of its gettering strength; the SiOx interlayer, regarding its potential blocking effect for slowing down the diffusion of impurities; and the dopant in-diffused surface regions of the silicon wafer bulk directly below the SiOx interlayer, which may have a small additional gettering effect due to heavy doping. Phosphorus in-situ doped poly-Si layers from plasma-enhanced chemical vapour deposition (PECVD), coupled with SiOx interlayers from different growth techniques, were used to demonstrate the method. The experimental and simulation results confirm that the heavily doped poly-Si layer acts as the main gettering sink and the presence of different SiOx interlayers determines the overall gettering rate. For the ultrathin SiOx interlayers studied in this work, which have a similar thickness but different stoichiometry, a standard thermally grown SiOx demonstrates the strongest blocking effect, followed by a chemically grown SiOx from hot nitric acid, and a thermal SiOx of a reduced stoichiometry (grown in a pure nitrogen ambient) demonstrates practically no blocking effect.

Recent progress in surface passivation technology and wafer pretreatment already resulted in significant improvements in the achievable minority charge carrier lifetime of crystalline silicon. Herein, this is further exemplified by studying the lifetime on lowly doped crystalline silicon wafers passivated by poly‐Si. To ensure credible lifetime measurements multiple measurement techniques are compared and good agreement between the investigated approaches is found. The resulting lifetime curves are analyzed in detail and the main limitation is very likely caused by silicon bulk recombination—most likely due to impurities. This analysis indicates that even very low impurity concentrations can be a limiting factor at the extraordinary high level of charge carrier lifetime observed in this study. Despite these limitations, lifetimes of 0.18 s on p‐type and 0.5 s on n‐type crystalline silicon wafers are measured, which to our knowledge exceed previously reported lifetimes. In both cases, these measured lifetimes correspond to an effective minority charge carrier diffusion length of ≈2.5 cm.

The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging between 2 and 10 fA/cm2 can be reported. Simultaneously, no metal induced recombination for the two lower sintering temperatures have been observed pointing towards a true passivated contact. Furthermore, contact resistivities below 10 mΩcm2 have been achieved. It seems that the industrial passivating contact matured to a fully passivated and conducting contact enabling full efficiency potential. The fact that this can be realized using either PECVD or LPCVD from various manufacturer is expected to drive costs down and contribute to the increased adoption of the TOPCon technology.