Passivating Contacts Research Articles

Design, development and analysis of large-area industrial silicon solar cells featuring a full area polysilicon based passivating contact on the rear and selective passivating contacts on the front

We present SERIS’ biPoly™ technology platform on large-area (M2), n-type rear-junction silicon solar cells featuring selective poly-Si/SiOx based passivated contacts on the front side and full-area poly-Si/SiOx contacts on the rear. The selective poly-Si ‘fingers’ are formed using an industrial ink-jet masking process followed by wet-chemical etching. The metal contacts are formed by an industrial screen-printing process using high-temperature fire-though metal pastes. We obtain excellent passivation on the front and rear surfaces, resulting in iVoc values between 720 mV and 730 mV on unmetallized solar cells. After high-temperature metallization, we achieve 22% efficiency on solar cells with selective poly-Si fingers on the front. We further develop the model for biPoly™ solar cells and with the help of a detailed loss analysis and simulations, identify the various loss components to identify the device modifications required for efficiency improvements.

Application of n‐Polysilicon Rear Emitter for High‐Efficiency p‐TOPCon Solar Cells

AbstractThis study entails the examination of tunnel oxide passivated contact on p‐type silicon wafers (p‐TOPCon) passivated with n‐polysilicon for a solar cell by using Quokka‐3, a numerical simulation program. The effects of the thickness, bulk lifetime, resistivity, and selectivity of charge carriers due to the polysilicon passivated contact are investigated. Through such n‐polysilicon passivated contact, the back‐emitter solar cells engender higher internal power owing to enhanced surface passivation. This further reduces the shading loss due to front metallization; however, the reduced minority carrier lifetime of the p‐type Czochralski (Cz) wafer restricts the possibilities for high efficiency. Subsequently, the minority charge carrier lifetime of the p‐type wafer conceivably becomes an obstacle to realizing TOPCon solar cells with a high conversion efficiency. This study demonstrates that a configuration suitable for the industrial manufacturing of high‐efficiency solar cells is a crystalline silicon solar cell on a p‐type wafer through a rear‐emitter n‐polysilicon passivated contact. A roadmap toward 24.87% of the p‐TOPCon solar cells through the n‐type polysilicon passivated contact is also devised.

Potential structure of c-Si bottom sub-cell in bifacial four-terminal III-V//c-Si multijunction devices

We propose a structural innovation for a four-terminal III-V//crystalline silicon (c-Si) multijunction (MJ) device under bifacial illumination. The top cell is a double-junction gallium indium phosphide (GaInP)/gallium arsenide (GaAs) cell structure, and the bottom cell is a tunnel oxide passivating contact (TOPCon) structure with a boron-diffusion emitter and a wide-gap polysilicon oxide (poly-SiOx) passivating rear contact. To maximise the albedo-reflectance light transmitted into the c-Si absorber, the wide-gap poly-SiOx passivating contact is intended to replace the conventional narrow-gap polysilicon (poly-Si) passivating contact at the rear of the TOPCon. With an extra 0.25-sun illuminated light at the back, the bottom cell's conversion performance improves by 14.5%. When compared to mono-illumination, bifacial illumination increases the overall efficiency of the MJ device by 2.67%. The design shows great potential for enhancing further the conversion performance of the bifacial GaInP/GaAs//TOPCon MJ devices using the wide-gap poly-SiOx passivating rear contacts.

Elucidating the Role of Contact-Induced Gap States and Passivation Molecules at Perovskite/Metal Contacts

Metal halide perovskite solar cells hold great promise as an efficient and cost-effective photovoltaic technology. However, carrier recombination at their contacts impedes progress toward this goal. In this study, considering the archetypical MAPbI3 perovskite and Au as a model electrode, we employ first-principles calculations to show how the mere presence of a metal near the perovskite induces in-gap states that may impair electronically this contact because of carrier recombination and Fermi level pinning. The states are not associated with any defect. We then investigate the suppression of the contact-induced gap states by introducing various passivation molecules to displace the metal from the perovskite surface. Our results highlight from a fundamental perspective the importance of contact displacement and passivation for efficient perovskite solar cells, thereby elucidating further the role of thin molecular interlayers in experimental devices. The elimination of contact-induced gap states can greatly aid perovskite solar cells in fulfilling their promise as a future mainstream source of renewable electricity.

Engineering Silicon Interfaces with Transparent AlyTiOx/ZnO/TiO2 Stack Exhibiting Exceptional Passivating Contact Performance

AbstractPassivating contact technologies are essential for fabricating high‐efficiency crystalline silicon (c‐Si) solar cells, and their application and incorporation into manufacturing lines has ranked as a hot topic of research. Generally, ideal passivating contacts should combine excellent electrical contact, outstanding surface passivation, and high optical transparency. However, addressing all these criteria concurrently is challenging since it is unlikely for any single material to exhibit both efficient carrier transport and surface‐defect passivation while demonstrating negligible parasitic absorption. In this work, several earth‐abundant, wide‐bandgap materials are combined to engineer high‐quality transparent electron‐selective passivating contact structures capable of overcoming these obstacles. A highly transparent AlyTiOx/ZnO/TiO2 stack with a total thickness of 3 nm, prepared by atomic layer deposition, is shown to provide a close‐to‐ideal passivating contact to Si surfaces by enabling dual functions of remarkable silicon surface passivation (with an effective minority carrier lifetime of 12.3 ms, an implied open‐circuit voltage of 730 mV, and a surface recombination current density prefactor of 2.6 fA cm−2), combined with efficient carrier transport with a very low contact resistivity of 3.4 mΩ cm2. These results demonstrate that low‐cost silicon interface‐engineering strategiesbased on transition metal oxides can push c‐Si solar cell performance to its theoretical limits.

Optimization of activated phosphorus concentration in recrystallized polysilicon layers for the n-TOPCon solar cell application

The pivotal structure of n-TOPCon (tunnel oxide passivating contact) solar cells is the passivating contact structure composed of a heavily doped polysilicon (poly-Si) layer and an ultrathin silicon oxide (SiOx), which can provide excellent selectively carrier transport. The activated phosphorus concentration in the passivating contact structure plays a crucial role in the passivation quality and contact resistivity performance. By adjusting PH3 flow rate, SiOx thickness and activation temperature, three different activated phosphorus concentrations in poly-Si were achieved in this work, the phosphorus concentration from SiOx to silicon bulk decreased at the same rate. It can be found that the passivation quality decreases with increasing concentration and the contact resistivity decreases significantly. With the aid of ellipsometry, we successfully extracted the refractive index n and extinction coefficient k of three poly-Si with different activated phosphorus concentrations. The extracted n and k values can well feedback the influence of concentration change on short-circuit current density in the simulation. Due to the high concentration led to the decrease of the open circuit voltage and short circuit current of the final TOPCon solar cell, but the obvious filling factor benefit can be obtained, only moderate doping concentration can obtain the best efficiency performance. Finally, the activated phosphorus concentration of about 3 × 1020 cm−1 can obtain the best efficiency, >24.8% on average.

Mitigation of shunt in poly-Si/SiO passivated interdigitated back contact monocrystalline Si solar cells by self-aligned etching between doped fingers

Mitigation of shunt in poly-Si/SiO passivated interdigitated back contact monocrystalline Si solar cells by self-aligned etching between doped fingers

Ex-situ phosphorus-doped polycrystalline silicon passivating contacts for high-efficiency solar cells by physical vapour deposition

We present ex-situ phosphorus-doped polycrystalline silicon (poly-Si) passivating contacts fabricated by the physical vapour deposition method, specifically sputtering. This technique is currently of great interest to the photovoltaics (PV) community due to its low deposition temperature, small footprint, high throughput, and use of low-hazard, solid targets and gases. We optimise the performance of the passivating contacts by adjusting various parameters from the sputtering of amorphous Si films to the formation of the poly-Si layers by a high-temperature thermal diffusion (with phosphorus oxychloride – POCl3, or phosphorus spin-on glass as the dopant sources). For the case of POCl3 diffusion, a high iVoc of ∼720 mV together with a low J0 of 2.6 fA/cm2 after a hydrogenation treatment by atomic layer deposition (ALD) AlOx and forming gas annealing (FGA), and a low contact resistivity ρc of less than 10 mΩ cm2 were achieved. Meanwhile, with spin-on doping, a low average iVoc of ∼600 mV was observed. This variation in performance could be explained by the difference in the crystallinity and the diffusion mechanism of the sputtered Si films after different subsequent thermal diffusion processes. Quokka3 simulations show that cell efficiencies as high as 25% are possible using the optimised passivating contacts. The results demonstrate the potential of these PVD poly-Si films as a promising candidate for future industrial solar cells.

Recombination in Passivating Contacts: Investigation Into the Impact of the Contact Work Function on the Obtained Passivation

Improving the passivation of contacts in silicon solar cells is crucial for reaching high‐efficiency devices. Herein, the impact of the contact work function on the obtained passivation is examined and quantified using a novel method—quasi‐steady‐state photoluminescence—which provides access to the surface saturation current density after metallization (J 0s,m). The obtained J 0s,m indicates an improvement of the surface passivation when contacts with high work function are applied onto Si wafers passivated with aluminum oxide, regardless of the wafer doping type. This improvement is mainly due to the amplification of the imbalance between the electron and hole concentrations near the Si interface. The passivation quality is reduced when using contacts with low work function in which the recombination rate increases via the charge‐assisted carrier population control. Herein, the vital importance of selecting suitable metals to minimize contact recombination in high‐efficiency solar cells is pointed.

Crystalline silicon solar cells with thin poly‐SiOx carrier‐selective passivating contacts for perovskite/c‐Si tandem applications

AbstractSingle junction crystalline silicon (c‐Si) solar cells are reaching their practical efficiency limit whereas perovskite/c‐Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c‐Si solar cells. Next to low‐thermal budget silicon heterojunction architecture, high‐thermal budget carrier‐selective passivating contacts (CSPCs) based on polycrystalline‐SiOx (poly‐SiOx) also constitute a promising architecture for high efficiency perovskite/c‐Si tandem solar cells. In this work, we present the development of c‐Si bottom cells based on high temperature poly‐SiOx CSPCs and demonstrate novel high efficiency four‐terminal (4T) and two‐terminal (2T) perovskite/c‐Si tandem solar cells. First, we tuned the ultra‐thin, thermally grown SiOx. Then we optimized the passivation properties of p‐type and n‐type doped poly‐SiOx CSPCs. Here, we have optimized the p‐type doped poly‐SiOx CSPC on textured interfaces via a two‐step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.

25.7% efficient PERC solar cell using double side silicide on oxide electrostatically doped (SILO-ED) carrier selective contacts: process and device simulation study

Passivating contacts have recently considered as a superior carrier-selective contact approach for high-efficiency silicon-based photovoltaic devices. However, the conversion efficiencies of the silicon-based passivated emitter and rear cell (PERC) are limited by contact recombination losses that reduce their performance. Therefore, we investigated a new manufacturable silicide on oxide-based electrostatically doped (SILO-ED) carrier-selective contact to suppress the contact recombination losses and reduce the saturation current density (j 0). For the first time, double side electrostatic doping is introduced to the PERC devices to form the carrier selective passivating contacts. First, a conventional PERC device was designed and the effects of surface recombination velocity (SRV) at both contacts were studied. After that, single and double SILO-ED based contacts are introduced into the device and a systematic analysis is performed to understand the tunneling phenomena and improve the conversion efficiency compared to existing PERC cells. The front SILO-ED based device with back contact SRV of 10 cm s−1 showed a power conversion efficiency of 25.4% with j 0 (14.3 fA·cm−2). In contrast, the double SILO-ED device delivered 25.7% conversion efficiency by further suppressing the j 0 to 11.8 fA·cm−2 by implementing SILO-ED approach with two different metal silicides such as erbium silicide (ErSi2) and palladium silicide (Pd2Si) on front and rear contact surface. The champion double SILO-ED PERC solar cell delivered a conversion efficiency of 25.7% with an open circuit voltage (V OC) of 742 mV. The results reported in this study would help to develop superior passivating contact-based PERC solar cells for higher efficiencies.

Bi-polysilicon passivating contact technique for crystalline silicon solar cell

Polysilicon (poly-Si) passivating contacts overcome the direct metal–semiconductor contact drawback of traditional industrial crystalline silicon (c-Si) solar cells by inserting a layer stack of poly-Si and silicon oxide layers at the rear full-area metal/c-Si interface, which is well-known as a tunnel oxide passivating contact (TOPCon). In conventional industrial TOPCon devices, the direct contact problem affects the emitter, which deteriorates the passivation quality and suppresses the open-circuit voltage (Voc). We herein introduce an innovative bi-poly-Si technique (Bi-TOPCon) featuring rear full-area passivated poly-Si and emitter locally passivated poly-Si to improve the passivation quality of the TOPCon device. The local emitter poly-Si is introduced using metal mask alignment, and its properties are optimised toward high passivation quality and low contact resistance. The Bi-TOPCon device shows a significant improvement in Voc (Voc > 700 mV). Bi-TOPCon is a promising technology for high-efficiency, next-generation industrial TOPCon devices.

The Mechanism of Hot Spots Caused by Avalanche Breakdown in Gallium-Doped PERC Solar Cells

Gallium-doped p-type passivated emitter and rear contact (PERC) solar cells, which eliminate light-induced degradation (LID) and reduce the impact of light- and elevated-temperature-induced degradation (LeTID), have completely replaced boron-doped p-type PERC cells. However, in previous experiments, we found hot spots in the center of gallium-doped PERC solar cells. In this study, it was found that gallium-doped PERC cells had uneven resistivity, which caused hot spots brought about by the avalanche breakdown of PN junctions. There were significant hot spots in the center of the tested cells, with an average resistivity of 0.4–0.5 Ωcm and nonuniformity greater than 30%, or at an average resistivity of 0.5–0.6 Ωcm with nonuniformity greater than 40%. In this paper we describe and study in detail hot spots triggered by the uneven resistivity of gallium-doped cells and analyze the causes and related influencing factors, thereby providing guidance and a reference for the improvement of the performance and reliability of gallium-doped PERC solar cells.

Revealing Surface and Interface Evolution of Molybdenum Nitride as Carrier-Selective Contacts for Crystalline Silicon Solar Cells

Molybdenum nitride (MoNx) was perceived as carrier-selective contacts (CSCs) for crystalline silicon (c-Si) solar cells due to having proper work functions and excellent conductivities. However, the poor passivation and non-Ohmic contact at the c-Si/MoNx interface endow an inferior hole selectivity. Here, the surface, interface, and bulk structures of MoNx films are systematically investigated by X-ray scattering, surface spectroscopy, and electron microscope analysis to reveal the carrier-selective features. Surface layers with the composition of MoO2.51N0.21 form upon air exposure, which induces the overestimated work function and explains the origin of inferior hole selectivities. The c-Si/MoNx interface is confirmed to adopt long-term stability, providing guidance for designing stable CSCs. A detailed evolution of the scattering length density, domain sizes, and crystallinity in the bulk phase is presented to elucidate its superior conductivity. These multiscale structural investigations offer a clear structure-function correlation of MoNx films, providing key inspiration for developing excellent CSCs for c-Si solar cells.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Improvement in the Performance of P-Type Tunnel Oxide Passivated Contact Solar Cell With Selective Tunneling Junctions Underneath the Contacts on Front Side

Industrial silicon solar cells are now moving further from passivated emitter and rear contact (PERC) toward Tunnel Oxide Passivated contact (TOPCon) technology. The efficiency of the PERC solar cell is improved by TOPCon technology, in which the rear side is fully passivated by an oxide/poly-Si tunneling junction. The performance of a TOPCon solar cell may further be improved by the incorporation of selective tunneling junctions underneath the front contacts. Some fabrications have been reported on a similar structure for an n-type Si substrate but they have not achieved the expected results. Also, a detailed study has not been done yet. Using 3-D sentaurus technology computer aided design (TCAD) simulation software, a detailed analysis has been done on the modified structure through the optimization of the tunnel oxides, the interfaces, the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> poly-Si layer, the selective (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -poly-Si) tunneling junctions underneath the front contacts, etc., which has not been done earlier. It is seen that a thick n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> poly-Si layer causes parasitic absorption of light, whereas a very thin poly-Si layer cannot provide sufficient field for tunneling of carriers. A thicker tunnel oxide produces a high series resistance, whereas a very thin tunnel oxide cannot restrict the minority carriers to reach to the contacts. Therefore, an optimization is required and the optimization is done based on a fabricated bifacial p-TOPCon solar cell reported in the literature with an efficiency of 21.2% and bulk lifetime of 200 μs. After performing all the modifications in the fabricated reference cell and optimizing different parameters, the efficiency is improved by 3.5% in absolute. The efficiency is further improved to 25.4% with the bulk lifetime of 1 ms.

Simulation and optimization of polysilicon thin film deposition in a 3000 mm tubular LPCVD reactor

The high photoelectric conversion efficiency of tunnel oxide passivating contacts (TOPCon) photovoltaic (PV) cells and their process compatibility are increasingly recognized by the industry, making it of growing importance to develop deposition process technologies for high-capacity polysilicon thin films. Currently, the size of LPCVD reactors for preparing polysilicon thin films in the industry is getting enlarged, which directly leads to the degradation of axial uniformity of polysilicon thin films. In this paper, a coupled multi-physics field model is established to simulate the polysilicon thin-film Low Pressure Chemical Vapor Deposition (LPCVD) process based on COMSOL Multiphysics platform, and the validity of the model is verified by comparing experimental data with simulation results. By investigating the flow field, thermal field, chemical reaction field and surface reaction field, the effects of the process parameters such as chamber temperature and pressure are investigated, as well as characteristics of the polysilicon thin films deposition. The results show that the uniformity of thin films decreases with increasing temperature and pressure, and the deposition rate of thin films increases as temperature and pressure increase. With the help of optimized the process parameters, the deposition rate of polysilicon thin films was increased by 13.0% and the uniformity of thin films was controlled above 96%. The uniformity of the films can be further improved to 96.7% by optimizing the temperature zone parameters and adopting the wafers placement strategy.

Impedance spectroscopy characterization of c-Si solar cells with SiOx/ Poly-Si rear passivating contacts

Impedance spectroscopy characterization of c-Si solar cells with SiOx/ Poly-Si rear passivating contacts

TCAD based numerical exploration of industrially feasible tunnel oxide passivated contact on p-type silicon

Tunnel Oxide Passivated Contact (TOPCon) is considered as an immediate successor of Passivated Emitter and Rear Contact (PERC). Rear side full area hole selective passivating contact with front n+ diffused emitter on p-type silicon (Si) (p-TOPCon) would be an incremental upgrade from PERC. However, limited studies are present on p-type Si and most of them are on TOPCon with rear full area electron selective contact (n-TOPCon). In this work, we explore the potential of p-TOPCon by incorporating industry relevant material and process parameters in Sentaurus TCAD simulations. These include bulk lifetime (τb), wafer resistivity (ρ), polysilicon related rear parameters, front emitter sheet resistance (Rsheet) and its passivation (Sfront), and finger width (Wfinger). Simulation results are benchmarked against PERC and n-TOPCon. p-TOPCon outperforms PERC and has lower sensitivity to wider range of τb, ρ, and Sfront. Simulation suggests that an efficiency of 23.83% can be achieved for industrially feasible τb of 5 ms, ρ of 1 Ω-cm, emitter Rsheet of 132 Ω/sq, and Wfinger of 20μm for p-TOPCon. Replacement of diffused emitter with n-TOPCon would be ideal after accomplishing above anticipated advancements and in such a scenario, efficiency > 24.4% can be achieved on p-type Si.

Concurrently Preparing Front Emitter and Rear Passivating Contact via Continuous PECVD Deposition Plus One‐Step Annealing for High‐Efficiency Tunnel Oxide Passivating Contact Solar Cells

Although tunnel oxide passivating contact (TOPCon) solar cells (SCs) have achieved great success in the photovoltaic (PV) industry, the ultrahigh temperature to prepare boron emitters prolongs the preparation processes and increases the thermal budget. Herein, a new method is presented for simultaneously preparing the front‐side boron emitters and the rear‐side TOPCon structures through continuous plasma enhanced chemical vapor deposition of the precursor layers followed by one‐step high‐temperature annealing. The front‐side boron emitters are fabricated using a stack of nano‐SiOx/B‐doped a‐Si:H layers as the boron source, which can avoid the formation of the stacking faults and lower the annealing temperature to match the rear‐side TOPCon fabrication processes. The rear‐side TOPCon structure, consisting of a plasma‐assisted N2O oxidation (PANO) SiOx and a nitrogen‐doped a‐Si:H(n+), is used to match the B diffusion temperature with high‐quality passivation. The precursor TOPCon cell without electrodes features excellent passivation with the highest implied open‐circuit voltage (iVoc) of 726 mV. As a result, the proof‐of‐concept TOPCon SCs exhibit a remarkable efficiency of 24.07%. This work proposes a flexible and elegant process to prepare high‐efficiency TOPCon devices, which shows great potential for application in the PV industry.