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.

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.

The preparation of phosphorus doped silicon films by DC sputtering in an Ar/PH3 atmosphere was investigated. The deposition process is determined by the dissociation of the PH3 molecule. The deposited films have active dopant concentrations >1 10(20) cm(-3) after annealing. However, the dissociation also produces hydrogen. The hydrogen trapped in the film segregates during annealing and causes blistering. The degree of blistering can be minimized by adjusting process parameters and sequence as well as film thickness. A maximum iV(oc) of 741 mV after annealing and hydrogen passivation is measured on symmetric tunnel oxide plus poly-silicon lifetime structures processed on n-type FZ silicon wafers.

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.

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%.

This work aims for the development of an industrially feasible Si bottom cell for two-terminal perovskite/Si tandem solar cells. A tunnel oxide and poly-Si based passivating front contact (TOPCon) as a replacement of the phosphorus diffused emitter is investigated to upgrade the industrial PERC technology to tandem bottom solar cells (TOPerc). It is shown that TOPCon is compatible with the temperature budget needed for alloying of the local Al rear contact and provides excellent passivation of the planar surface (iV(oc)> 720 mV). Different possible process routes and fundamental design constraints like the necessity of a hydrogenation step and a single-sided oxide removal after contact firing were investigated. In a proof-of-concept device low contact resistivity (rho(c)) and high passivation quality could be transferred from test structures to solar cells. The high open-circuit voltage (V-oc) of 691 mV and the low series resistance (R-s) of 0.7 Omega cm(2) of such a TOPerc solar cell with TCO interconnection layer highlight the potential of the proposed cell structure.

This work aims for providing a low-resistive passivating polycrystalline Si tunnel junction (poly-Si TJ) for interconnecting the sub cells in Perovskite/Si tandem solar cells. To account for the parasitic interdiffusion of dopant atoms during high-temperature processing, the incorporation of an additional diffusion blocking interlayer between the n(+) and p(+) poly-Si region of the tunnel junction is proposed. Several oxide-based interlayers have been investigated and are here discussed with respect to their deposition technique and structural properties. With the help of such interlayers, the design freedom for passivating poly-Si TJs is highly increased. The interlayers enhance the temperature stability of the layer stack and enable contact resistivities sufficiently low for tandem device application after high-temperature treatments of up to 900 or 950 degrees C. The developed front side poly-Si TJ is thereby compatible with the rear contact formation of the mainstream passivated emitter rear cell (PERC) technology.

One drawback of passivating contacts in crystalline silicon solar cells is the current loss due to parasitic absorption within the involved material layers. When employed on an illuminated side of the cell, the full spectrum of the incident light will be partly absorbed before reaching the silicon bulk. Additionally, near-infrared (NIR) absorption can substantially reduce the cell's NIR spectral response (SR) also when employed on the non-illuminated side. As those losses are hard to measure directly, optical modeling is crucial for their quantification. This paper presents an extension to an analytical light-trapping model to account for parasitic absorption in the near-surface region via a new parameter Appp (absorbed fraction per perpendicular pass). We test the model by analyzing i) reflectance measurements on samples with varying doping profiles, ii) SR measurements on TOPCon solar cells with varying back-side poly-silicon layers, and iii) SR measurements of a bifacial silicon heterojunction cell. We show that the model can well be calibrated by fitting a single value for Appp to reflectance measurements. This enables a quantification of the parasitic absorption loss without requiring knowledge of all layer's optical properties. The model also is able to predict parasitic absorption in TOPCon cells when knowing the thickness and doping density of the poly-Si layer. Having proven the usefulness of Appp to represent parasitic absorption as a single-valued quantity, we suggest Appp as a third figure of merit for the quality of a passivating contact, next to the recombination parameter J0c and the contact resistivity ρc.

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.

Polycrystalline Silicon on tunneling silicon oxide (poly-Si/SiOx) passivating contacts have shown great potential for the next-generation monocrystalline Si (c-Si) industrial photovoltaic technology. However, these cells typically suffer from strong parasitic absorption in the thick front poly-Si layer, which is designed to reduce metal-induced recombination. In previous work, we demonstrated an improved short-circuit current density, J(sc), by thinning the front poly-Si film in an SF6 plasma using the front metal grids as a self-aligned mask, but the sub - 100 nm thick poly-Si film is difficult to measure on an alkaline textured surface. Conventional optical techniques such as spectroscopic ellipsometry cannot be used due to the high scattering nature of the random pyramids. At poly-Si thicknesses below 50 nm, secondary electron microscopy (SEM) has difficulty distinguishing the poly-Si from the underlying c-Si substrate. Here, we demonstrate X-ray diffraction as an effective method to quantitatively measure the front poly-Si thickness. The thickness calculated from the diffraction peak height of the Si (111) crystallographic plane agrees well with the cross-section SEM analysis and simulations using SunSolve. We show that by thinning the front poly-Si from 200 to 60 nm, J(sc) increased by 2.4 mA/cm(2), while maintaining the same V-oc. This led to an absolute efficiency gain of 1.73%. In addition, we also discuss possible reasons for the premature loss of passivation before the removal of all poly-Si, which prevented an even higher gain in J(sc).

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.

Polysilicon contact structures with ultra‐thin atomic layer deposited (ALD) oxide and nitride interlayers based on SiO x , SiN x , AlO x , AlN x , and TiO x either as part of an interlayer stack when applied on top of a conventional thermally‐grown SiO x or as a single interlayer were investigated. ALD SiO x single interlayers provided a very good passivation quality with high implied open‐circuit voltage and low‐specific contact resistivity when an optimal thickness and annealing temperature was applied. Also, ALD SiN x single interlayers showed a promising passivation quality, while AlO x and AlN x interlayers only allowed for a moderate and TiO x interlayers exhibited a very poor passivation quality. ALD SiO x interlayers on planar and textured silicon solar cells with poly‐Si(p) hole‐selective contacts and poly‐Si(n) electron‐selective contacts enabled a conversion efficiency of almost 21% as a proof‐of‐concept. These results are comparable with silicon solar cells with conventional thermally‐grown SiO x , showing that ALD SiO x interlayers are an alternative to conventional thermally‐grown SiO x in polysilicon contacts structures.

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.

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.

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.

The performance of a low-resistive p+/n+ poly-Si tunneling junction (SiTJ) based on a tunnel oxide passivating contact in dependence on the thermal budget of the applied post-deposition treatment is studied. We present two approaches to reduce the performance limiting parasitic dopant interdiffusion and, thus, the contact resistivity, without impairing the passivation quality. Both, carbon-alloying of poly-Si layers and the application of diffusion blocking interlayers are effective means to maintain a low contact resistivity of ∼24 mΩcm2 at high thermal process temperatures of up to 950 °C. Those low values are obtained using either a standard furnace anneal or a rapid thermal process (RTP). We report on promising results toward a lean process sequence using only one single fast thermal treatment (RTP-only). As a main result, the flexibility for engineering and fabrication of our SiTJ was markedly improved, eventually facilitating industrially feasible perovskite/silicon tandem solar cells. One aspect being higher post-deposition temperatures needed for, e.g., bottom cell rear side contact formation and the first layers of the perovskite to cell.

In typical industrial processing of tunnel oxide passivated contact (TOPCon) solar cells, poly‐Si is deposited on the entire back of the cells. During the deposition process, a wrap‐around of poly‐Si onto the edges and the front side of the cells is virtually unavoidable if chemical vapor deposition processes are used. Plasma‐enhanced chemical vapor deposition (PECVD) is used to investigate very thin poly‐Si films and their effect on wrap‐around on bifacial TOPCon solar cells fabricated without wrap‐around etching. As a result, reduction of the poly‐Si thickness down to 30 nm significantly increases the shunt resistance, reduces the reverse bias current, and thus reduces the risk of hot spots as measured by IR imaging and microcharacterization by secondary electron microscopy. Electroplated metallization proves to be a suitable candidate for contacting such thin TOPCon layers, being less sensitive than screen‐printed metallization.

In this article, the development and optimization of carrier-selective and passivating contacts by industry-scale inline plasma-enhanced chemical vapor deposition and their successful integration into solar cells are reported. Amorphous Si thin films with varying carbon content (SiCx) were deposited on a thermally grown ultrathin tunnel oxide (TOPCon) and electrically characterized. Furthermore, the impact of a vacuum break (VB) during the deposition of a layer stack consisting of intrinsic amorphous Si [a-Si:H(i)] and boron-doped SiCx was investigated. That is, samples that were processed with VB were exposed to ambient air, and hence, a thin native oxide was formed on the a-Si:H(i) layer, which affected the boron diffusion into the absorber resulting in a distinct anneal behavior of the contacts. Upon optimization, these layers provided an excellent surface passivation quality, which was reflected in an implied open-circuit voltage of 733 mV for n-type and 716 mV for p-type TOPCon structures, respectively. In addition, very low contact resistivities of 0.3 m Omega.cm(2) for n-type and 0.5 m Omega.cm(2) for p-type TOPCon were measured, respectively. These optimized TOPCon structures were implemented into both sides contacted p-type laboratory solar cells. After a two-step furnace anneal, these cells achieved a maximum energy conversion efficiency of 22.7% with evaporated contacts.

Formation of an interfacial tunnel oxide capped by the polysilicon (poly-Si) layer is one of the most promising methods to realize carrier-selective contacts as it offers an evolutionary upgrade to the current mainstream PERC. Currently, PV industry is looking towards different technologically feasible options of transferring this cell concept from laboratory research towards industrial manufacturing. In this paper, we devise cost-driven strategies towards industrial manufacturing of TOPCon-based solar cells after assessing various process routes based on currently production-ready and upcoming future alternative process technologies. Our techno-economic assessment suggests that higher capital and operational costs required for TOPCon concept is distributed unevenly in the process value chain, with a significant fraction in diffusion/annealing and metallization steps. Nevertheless, under given assumptions, TOPCon-concepts are found to remain economically competitive against bifacial PERC in terms of levelized cost of electricity (LCOE), if a minimum absolute gain in cell efficiency Delta eta > 0.55% for most-conservative and Delta eta > 0.40% for most-progressive scenarios respectively can be maintained.

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passivate defects at the Si/SiOx interface and presumably in the doped poly-Si layer as well as their thermal stability in a low and high temperature range. To this end, hotplate annealing series were performed on textured n-type TOPCon structures and Al2O3/SiNx multi-layer stacks were exposed to fast-firing processes. Very distinct activation characteristics were detected. It was also observed that the Al2O3 capping layers enable a higher level of surface passivation and higher thermal stability compared to SiNx. When implemented in multi-layer stacks, Al2O3 acts as a hydrogen diffusion barrier und prevents effusion from the TOPCon structure that deteriorates the passivation quality irreversibly.