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In this article, detailed numerical modeling is performed for front junction (FJ) and rear junction (RJ) n-type Si solar cells with screen-printed double-side poly-Si based tunnel oxide passivated contacts (TOPCon). A roadmap for efficiency projections of commercial-type RJ and FJ topologies reaching 24.8% and 23.3% efficiencies, respectively, has been developed to quantify and explain the impact of various technological innovations on the performance of each design. Understanding of mechanisms governing cell operation is crucial to explore factors that limit the efficiency potential of the two device structures. By investigating several key parameters such as front poly-Si sheet resistance and thickness, bulk material properties, and current transport in our simulation model, we determine and explain why RJ cells outperform FJ cells. Our findings reveal that FJ suffers from present technological limitations of p-type poly-Si based passivated contacts—namely, 1) large recombination observed in textured p-TOPCon layers and 2) low boron solid solubility and hole mobility in boron-doped poly-Si which results in very high sheet resistance of the front p-poly-Si emitter that contributes to fill factor degradation, especially when using thin poly-Si layer to reduce absorption losses. RJ on the contrary desensitizes the cell efficiency to front sheet resistance to allow the application of ultra-thin front n-type poly-Si layer and is therefore ideally suited for double-side TOPCon cells.

Carrier selective poly-Si/SiO x contacts have become a very strong contender for next generation high-efficiency Si solar cells as well as Si-based tandem cells. A thin unmetallized poly-Si/SiO x passivated contact on the top surface of the bottom Si cell provides an excellent opportunity for higher efficiency two-terminal Si-based tandem cells. However, this article shows that when the manufacturable low-cost screen-printed contacts are applied on the rear side of the Si cell, the exposed poly-Si/SiO x contact on the front undergoes appreciable degradation in J 0 during high temperature contact firing without a capping layer on the front poly-Si. In this article, unmetallized n -TOPCon structures with ∼10-nm poly-Si showed significant degradation after a typical simulated firing process (∼750 °C), resulting in decreased implied V oc from 728 to 708 mV and increased J 0 from 5 to 15 fA/cm2. We found that a ∼7-nm Al2O3 capping layer on poly-Si provides excellent protection with no degradation and can also be easily removed after firing for subsequent processing. For comparison, PECVD grown ∼20 nm SiN x and SiO x layers were also investigated for firing stability. The SiN x capping layer was also found to be effective in preserving stability, but the SiOx failed due to lack of hydrogen supply. Large area n -type front junction Si solar cells were fabricated with a boron diffused screen-printed emitter on the front and Ag evaporated thin n -TOPCon on the rear to demonstrate ∼0.3% higher absolute efficiency when the rear n -TOPCon was capped with Al2O3 during the firing of the front screen-printed contacts.

Herein, we present a pulsed-laser processing method for crystallization and dopant activation of a highly n-doped amorphous silicon (a-Si:H) carrier-selective layer in a tunnel oxide passivated contact (TOPCon) Si solar cell structure. The laser method provides enhanced conductivity and implied open circuit voltage while reducing emitter saturation current density and surface heating, as opposed to conventional high-temperature furnace annealing of the bulk Si wafer with a TOPCon structure. We identify an appropriate laser wavelength, fluence, and layer thickness using modeling and simulations. Raman and Hall effect measurements demonstrate increased crystallinity and dopant activation, whereas photoconductive decay shows enhanced surface and interface passivation quality. Additionally, we examine the role of subsequent SiNx deposition on further improving the passivation of laser-processed TOPCon layers to achieve a 5.9 kΩ/sq film sheet resistance, 2 ms effective carrier lifetime, 718 mV implied open-circuit voltage (iVOC), and 8.6 fA/cm2 one-side recombination current density (J0) with the potential for further passivation improvement via laser process and a-Si layer thickness optimization.

The rapid rise in single-junction perovskite solar cell (PSC) efficiencies, tunable bandgap, and low-cost solution processability make PSCs an attractive candidate for tandems with Si bottom cells. However, the challenge is to fabricate a high-performance semitransparent perovskite top cell in combination with an appropriate silicon bottom cell with high response to long wavelength photons that are filtered through the perovskite top cell. Currently, semitransparent perovskite cells show much lower performance compared with their opaque counterparts, while high-performance silicon bottom cells, such as heterojunction with intrinsic thin layer and interdigitated back contact, may be too expensive to meet the cost and efficiency targets for commercial viability. Here, we demonstrate a 26.7% perovskite–Si four terminal (4T) tandem cell comprising a highly efficient 17.8% CsFAMAPbIBr semitransparent, 1.63-eV bandgap perovskite top cell, and a ≥22% efficiency n-type Si bottom cell fabricated with a conventional boron diffused emitter on the front and carrier selective n+ poly-Si/SiOx passivated contact on the rear. This is among the highest efficiency perovskite/Si 4T tandems published to date and represents the first report of the use of the high temperature-resistant single side n-tunnel oxide passivated contact Si cell in a 4T configuration.

A negatively charged oxide-nitride-oxide stack for field-effect passivation of crystalline silicon solar cells is discussed. The negative charge was injected into the stack by a plasma charge injection technology. Charge stability was studied by exposing samples to AM1.5 simulation visible light and full-spectrum light at temperatures ranging from 55 to 78 °C for up to 300 h. Charge injection and loss were quantified based on shifts in the flatband voltage of capacitance–voltage curves measured with a mercury probe. The most probable mechanism of charge loss was found to be diffusion of negative charged hydrogen atoms through nitride and bottom oxide. The optimum recipe for each layer of the stack was investigated to minimize the loss of injected charge. The flatband voltage decay of the optimized stack was found to fit a power-law trend, suggesting the dispersive transport of hydrogen atoms with a dispersion parameter of ∼0.06–0.07. The optimized stack is projected to maintain a negative charge density of about 3.6 × 1012 cm−2 or more after 25 years of field operation in an environment such as Arizona, which would be sufficient for field-effect passivation under one-sun illumination. The high stability of the negative injected charge makes the plasma charging technology a safer and lower cost alternative to Al2O3-passivation technology commonly used to passivate p-type surfaces.

This paper presents fabrication and optimization of p+ passivating contact (p+ poly-Si/SiOx/c-Si) for high efficiency p-TOPCon solar cells. The p+ passivating contacts were formed by (i) in-situ doped p+ poly-Si via LPCVD and (ii) ex-situ doping of intrinsic poly-Si via APCVD boron diffusion. We studied the relationship between the passivation quality of this contact as a function of boron diffusion profile varied by altering (i) the crystallization annealing temperature, and (ii) the APCVD precursor gas flow ratio $\left( {{\emptyset _{{B_2}{H_6}}}/{\emptyset _{Si{H_4}}}} \right).$ In-situ doping resulted in higher B concentration in poly-Si, which improves field induced passivation, as well as in Si absorber near the interface, which enhances Auger recombination to degrade passivation. This trade-off resulted in lower optimum annealing temperature for in-situ process (875°C vs 950°C). However, process optimization resulted in excellent passivation with comparable iVoc and J0 for both in-situ (719mV/6.3fAcm-2) and ex-situ doped (716mV/6.6fAcm-2) un-metallized p-TOPCon. Greater than 22.0% screen-printed large area (244.32cm2) bifacial p-TOPCon solar cells were fabricated by replacing LBSF of p-PERC cells with 270 nm p+ TOPCon junction with ~10% metal coverage on the rear side. TOPCon cells gave 0.4% higher efficiency than their PERC counterpart.

Device modeling is performed to propose > 25% efficient industry-compatible rear junction double-side passivated contacts solar cell structure with full area p-TOPCon on the rear and selective area n-TOPCon under the front grid pattern (selective TOPCon). This design enables the use of thicker TOPCon (>100nm) on the front for traditional screen-printed contacts without incurring metal-induced damage, high parasitic absorption loss, and compromise in lateral transport or carrier collection on the front. Rear junction design with appropriate bulk lifetime and resistivity combination eliminate the need for heavy doping in the front field region because carriers can flow through the bulk Si without appreciable FF loss. High V OC is maintained because high-quality Si surface passivation in the field region by Al 2 O 3 /SiN gives J 0 comparable to the TOPCon. Our device modeling specifies the practically achievable properties and parameters for each region, including full area rear p-TOPCon, selective area front n-TOPCon, bulk and contacts, to achieve 25.4% efficiency screen-printed bifacial rear junction selective TOPCon cells.

The plasma charge injection technology reported earlier can be a low-cost alternative to the Al 2 O 3 passivation technology. The charge stability under sunlight exposure is a key concern. We investigated the light-induced loss of injected charge for different nitrogen-to-silicon atomic ratios (x=N/Si) of SiN x , and found drastic improvements in charge loss with increasing x-value and logarithmic time dependent charge decays. The best result indicated a sufficient charge amount can sustain over the module lifetime (25 years). The charge loss is likely related to photo-generated free carriers and atomic hydrogens in SiN x .

Phosphorus (n+) and boron (p+) diffused n-type textured Cz Si wafers are passivated by two different methods: gas phase H 2 S reaction and quinhydrone-methanol (QH-MeOH) solution. The dopant diffused surface passivation quality, quantified by saturation current density (J 0 ), strongly depends on the diffusion type and the passivation method. H 2 S reaction at 550°C passivates n+ diffused surface as good as state-of-the-art Si oxide/silicon nitride stack (SiO 2 /SiN X ), while QH-MeOH passivates p+ diffused surface to the same level as state-of-the-art atomic layer deposited aluminum oxide/silicon nitride stack (Al 2 O 3 /SiN X ).

38th European Photovoltaic Solar Energy Conference and Exhibition; 7-9, Solar cells in PV modules crack during field operation, which exposes the modules to environmental stressors, including extreme weather events, such as hailstorms and hurricanes. These cracks gradually lead to power degradation. To directly address the cell-crack-induced degradation, we have reinforced the traditional screen printable silver paste, commonly used throughout the industry for front side contacts, with low-cost multiwalled carbon-nanotubes. We have shown in previous work that these metal matrix composites have little to no effect on the cell’s efficiency while enhancing the metallization’s fracture toughness and electrical gap-bridging capability. In this work, we focus on optimizing the composite metallization to achieve the best possible performance. We discover that reducing carbon nanotube agglomerations in the paste not only improves the printability for narrow gridlines, but also increases the modulus of toughness of the metallization by over 500%.

Emitted photons stemming from the radiative recombination of electron-hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the injection level. Hence, these assumptions are questionable in thermophotovoltaic devices operating in the near-field regime. In the present work, an iterative solver that combines fluctuational electrodynamics with the drift-diffusion model is developed to tackle the coupled photon and charge transport problem, enabling the determination of the spatial profile of photon chemical potential beyond the detailed balance approach. The difference between the results obtained by allowing the photon chemical potential to vary spatially and by assuming a constant value demonstrates the limitations of the conventional approaches. This study is critically important for performance evaluation of near-field thermophotovoltaic systems., Comment: 40 pages, 11 figures

Sulfur is demonstrated to be an effective and promising surface passivation element for both n- and p-type Si wafers after reacting in dilute hydrogen sulfide gas (2 - 6% in argon) at 550°C. Effective minority carrier lifetimes of > 2 ms and > 0.25 ms are achieved for n- and p-type Si wafers, respectively, comparable to the industry standard thermal oxide and atomic layer deposited aluminum oxide passivation level. Surface characterization by x-ray photoelectron and emission spectroscopy reveals that sulfur is primarily bonded in a sulfide environment.

In this work, detailed numerical modeling is performed for front junction (FJ) and rear junction (RJ) n-type Si solar cells with screen-printed double-side poly-Si based tunnel oxide passivated contacts (TOPCon). A roadmap for efficiency projections of commercial-type RJ and FJ topologies reaching ~24% and >22.5% efficiencies, respectively, has been developed to quantify and explain the impact of various technological innovations on the performance of each design. By investigating several key parameters such as front poly sheet resistance and thickness, bulk material properties, and current transport in our simulation model, we determine and explain why RJ cells outperform FJ cells. Our findings reveal that FJ suffers from present technological limitations of p-poly based passivated contacts, namely, i) large recombination observed in textured p-TOPCon layers and ii) low boron solid solubility and hole mobility in p-poly Si which results in very high sheet resistance of the front p-poly emitter that contributes to FF degradation, especially when using thin poly layer to reduce absorption loss. RJ, on the other hand, desensitizes the cell efficiency to front sheet resistance allowing application of ultra-thin front n-poly layers and is therefore ideally suited for double-side TOPCon cells.

This paper describes the development of a novel selective boron emitter formed a screen-printed negative resist to form vias for heavily doping regions. Ion-implantation and atmospheric pressure chemical vapor deposition (APCVD) are used to from the field (p+) and heavily doped regions (p++) of the emitters, respectively. This approach gives better control over the surface concentrations/junction depth for each region of the emitter resulting in very low full area emitter dark saturation current density (J0e) of 6fA/cm2 for the lightly doped regions and ∼93fA/cm2 for the heavily doped regions. Very low un-metallized J0e of 13fA/cm2 and 18fA/cm2 were achieved for selective emitters with $30\ \Omega/\square\ \mathrm{p}^{++}$ region in combination with 200 and $150\ \Omega/\square$ ion-implanted p+ regions respectively. A metallized J0e of 28fA/cm2 was achieved with 3% screen-printed metal grid contact coverage to p++ regions of the selective emitter. This selective emitter gave 0.4% absolute increase in efficiency over standard NPERT with a $150\ \Omega/\square$ homogeneous emitter.

This paper reports on the modeling, optimization, and implementation of p-TOPCon (tunnel oxide passivating contacts) on the rear side of a PERC to enhance its cell efficiency. Local Al-BSF of a traditional PERC was replaced by p+ poly-Si/oxide passivated contact composed of ~15A thick chemically grown tunnel oxide, capped with 120-250nm thick p+ poly-Si layer grown by LPCVD. Process optimization resulted in full-area un-metallized recombination current density (J 0b , pass ) of 0b ,) of 65fA/cm2. Detailed analysis and device simulation showed that by replacing this LBSF with 250nm TOPCon developed in the study should produce a V oc enhancement of 9.2mV, consistent with the observed cell V oc increase of 10mV.

This paper reports on a systematic and quantitative assessment of light induced degradation (LID) and regeneration in full Al-BSF and passivated emitter rear contact cells (PERC) along with the fundamental understanding of the difference between the two. After LID, PERC cells showed a much greater loss in cell efficiency than full Al-BSF cells (∼0.9% vs ∼0.6%) because the degradation in bulk lifetime also erodes the benefit of superior BSRV in PERC cells. Three main regeneration conditions involving the combination of heat and light (75 °C/1 Sun/48 h, 130 °C/2 Suns/1.5 h and 200 °C/3 Suns/30 s) were implemented to eliminate LID loss due to BO defects. Low temperature/long time (75 °C/48 h) and high temperature/short time (200 °C/30s) regeneration process was unable to reach 100% stabilization. The intermediate temperature/time (130 °C/1.5 h) generation achieved nearly full recovery and stabilization (over 99%) for both full Al-BSF and PERC cells. We discussed the effect of temperature, time and suns in regeneration mechanism for two cells.

We present the fabrication and analysis of Passivated Emitter and Rear Totally Diffused (PERT) solar cells on n-type silicon using a co-diffusion process. In a single high temperature step, a BSG/SiOx stack deposited by APCVD and a POCl3 back surface field diffuse into the wafer to form the boron doped emitter and phosphorus doped back surface field. The SiOx layer on top of BSG acts as a masking layer to prevent cross-doping of phosphorus as well as a blocking layer for boron out-diffusion. This resulted in an initial sheet resistance of 76 Ω/□ with good uniformity and a final p+ emitter sheet resistance of 97 Ω/□ after boron rich layer removal. Additionally, bulk lifetime was investigated before and after the high temperature step that resulted in an increase from 1.2 ms to 1.5 ms due to a POCl3 gettering effect. A peak cell efficiency of 20.3% was achieved and each recombination component in terms of saturation current density was calculated and analyzed to understand the cell for further efficiency enhancement.

We report on the understanding and optimization of ion-implanted boron emitters in combination with screen-printed contacts to produce very low recombination current density and high-efficiency cells with rear poly-Si/SiO2 passivated contact. Due to high bulk lifetime in n-base Si and very low recombination in the rear-side n+ passivated contact, recombination in the emitter limits the efficiency potential of manufacturable single-side passivated contact cells. Emitter recombination is governed by recombination under metal contacts (J0e,metal) and in the passivated regions in between (J0e,pass). Emitter profile engineering and dielectric passivation can lower the J0e,pass while paste chemistry and firing conditions can reduce J0e,metal by altering the extent of screen-printing induced emitter surface etching and the percentage of unetched dielectric islands under the metal contacts. We optimized implanted boron profiles, surface concentration and passivation, and junction depth in the sheet resistance range of 48-200 Ω/□ and achieved very low J0e,pass ( ≥ 140 Ω/□. Additionally, screen-printing paste and firing optimization reduced J0e,metal from 1172 to 707 fA/cm2 on 170 Ω/□ boron emitter by virtually eliminating emitter surface etching and creating distributed local contacts with unetched dielectric islands (~40% area coverage) underneath the grid lines. This resulted in very low total metallized J0e of 30 fA/cm2 with 3% metal coverage. Our device modeling showed the optimum boron sheet resistance is in the range of 120-170 Ω/□. The simulated results were validated by fabricating fully screen-printed 100 cm2 22.9% efficient bifacial cell with n+ poly-Si/SiO2 passivated rear contact.

This paper reports on in-depth understanding, modeling, and fabrication of 23.8% efficient 4 cm2 n-type Float Zone (FZ) silicon cells with a selective boron emitter and photolithography contact on front and tunnel oxide passivating contact on the back. Tunnel oxide passivating contact composed of a very thin chemically grown silicon oxide (∼15 A) capped with plasma-enhanced chemical vapor deposition (PECVD) grown 20 nm n+ poly Si gave excellent surface passivation and carrier selectivity with very low saturation current density (∼5 fA/cm2). A high-quality boron selective emitter was formed using ion implantation and solid source diffusion to minimize metal recombination and emitter saturation current density. Process optimization resulted in a cell $V_{{\rm{oc}}}$ of 712 mV, $J_{{\rm{sc}}}$ of 41.2 mA/cm2, and FF of 0.811. A simple methodology is used to model these cells which replaces tunnel oxide passivating contact region by electron and hole recombination velocities extracted from measured saturation current density of tunnel oxide passivating contact region and analysis. Using this approach and two-dimensional device modeling gave an excellent match between the measured and simulated cell parameters and efficiency, supporting excellent passivation and carrier selectivity of these contacts. Extended simulations showed that 26% cell efficiency can be achieved with this cell structure by further optimization of wafer quality, emitter profile, and contact design.

We report on the implementation of screen printed carrier-selective tunnel oxide passivated poly-Si rear contact for high-efficiency front junction crystalline Si solar cells. Using a 15A thick tunnel oxide grown in nitric acid at 100°C and capped with 100nm LPCVD grown n+ poly-Si, we were able to achieve excellent rear contact passivation even after the formation of screen printed contacts. The saturation current density on the rear side (J 0b’ ) without metal was 1.7 fA/cm2. After firing the screen printed Ag contacts through SiN cap on poly-Si, with 9% metal coverage, J 0b’ increased to only ~5 fA/cm2. An ion-implanted 180 Ω/□ boron doped homogeneous emitter was used on the front. This resulted in large area (239 cm2) 21.6% cells on industrial grade n-type CZ Si wafer. Smaller area 100 cm2 cells gave 22% efficiency.

In this study, we investigate the impact of the deposition of indium tin oxide (ITO) via DC magnetron sputtering on tunnel oxide passivating poly-Si contacts. Before ITO deposition to the tunnel SiO x passivating n+ poly-Si rear-contact on the cell structure with an SiN x /Al2O 3 passivating boron emitter, the implied open-circuit voltage (iV oc ) and implied fill factor (iFF) were measured to be 694±10 mV and 83±0.6%, respectively. After ITO sputtering and curing annealing, the iV oc and iFF were almost fully recovered, resulting in the iVoc of 685±11 mV and iFF of 81.9±0.8%. The characteristic of fully recovered effective lifetime is attributed to unique sputtering conditions employing a very low power density at room temperature and curing.

Herein we present a pulsed laser processing method for crystallization and dopant activation of a highly n-doped amorphous silicon (a-Si:H) carrier selective layer for high conductivity, high carrier lifetime, low emitter saturation current and limited surface layer heating as opposed to high temperature furnace annealing induced heating of the bulk Si wafer. We demonstrate increased crystallinity and reduced layer resistivity, without impacting surface passivation quality. Additionally, we examine the role of forming gas annealing (FGA) on further improving the passivation of laser-processed polysilicon layers to achieve a OC ), and 27 fA/cm2 recombination current density (J 0 ) with the potential for further passivation improvement via laser process optimization.

One of the ways to reduce the cost of solar electricity to 3¢/kWh, thus reaching parity with fossil-fuel-based generation, is to reduce the degradation rate of solar modules and extend their lifetime well beyond 30 years. The extended module lifetime in turn can positively influence the financial model and the bankability of utility-scale PV projects. Today, the highest-riskpriority solar module degradation mechanism is what is known as hot spots, often induced by cell cracks. In order to address this degradation mechanism, we make use of low-cost, multi-walled carbon nanotubes embedded in commercial screen-printable silver pastes. When the carbon nanotubes are properly functionalized and appropriately incorporated into commercial silver pastes, the resulting metal contacts on solar cells, after screen-printing and firing, show exceptional fracture toughness. These composite metal contacts possess increased ductility, electrical gap-bridging capability up to 50 µm, and "self-healing" to regain electrical continuity even after cycles of complete electrical failure under extreme strain.

One of the ways to reduce the cost of solar electricity is to reduce the degradation rate of solar modules and extend their lifetime well beyond 30 years. The extended module lifetime translates to increased bankability of utility-scale PV projects. In this work, we specifically address cell-crack-induced degradation by introducing crack-tolerant metallization. We make use of low-cost, multi-walled carbon nanotubes embedded in commercial screen-printable silver pastes as a highly integratable engineering solution. When the carbon nanotubes are appropriately functionalized and incorporated into commercial silver pastes, the resulting metal contacts on solar cells, after screen-printing and firing, show exceptional fracture toughness. These composite metal contacts possess increased ductility, electrical gap-bridging capability greater than 50 μm, and “self-healing” to regain electrical continuity even after cycles of complete electrical failure under large strain. While providing crack-tolerant mechanical properties, the composite paste delivers similar beginning-of-life cell performance as the conventional silver paste.

We present experimental results which show that oxygen-related precipitate nuclei (OPN) present in p-doped, n-type, Czochralski wafers can be dissolved using a flash-annealing process, yielding very high quality wafers for high-efficiency solar cells. Flash annealing consists of heating a wafer in an optical furnace to temperature between 1150 and 1250 °C for a short time. This process produces a large increase in the minority carrier lifetime (MCLT) and homogenizes each wafer. We have tested wafers from different axial locations of two ingots. All wafers reach nearly the same high value of MCLT. The OPN dissolution is confirmed by oxygen analysis using Fourier transform infrared spectra and injection-level dependence of MCLT.

Most p-type Si solar cells involves phosphorus-doped emitter by POCl 3 diffusion or phosphorus ion-implantation. Although the formation of the phosphorus emitter is known to getter impurities like Fe, the difference in the impact of these two gettering techniques on cell performance is not well quantified. Therefore, this paper compares the gettering efficiency of POCl 3 diffusion and phosphorus ion-implantation on Czochralski(Cz) and cast quasi-mono Si wafers. Cz-Si wafers were used to measure bulk lifetime and iron concentration before and after POCl 3 diffusion and phosphorus implantation with different doses. Increase in phosphorus implantation dose improved the gettering efficiency by increasing bulk lifetime and decreasing iron concentration but remained inferior to POCl 3 diffusion partly due to single side gettering as opposed to double side gettering with higher phosphorus surface concentration during POCl 3 diffusion. Moreover, large-area solar cells were fabricated on cast quasi-mono and Cz-Si wafers to quantify the impact of these emitters on cell parameters. POCl 3 diffused cast quasi-mono cells showed ~0.4% higher efficiency due to higher bulk lifetime compared to phosphorus implanted emitter. However, phosphorus implanted Cz cells gave ~0.3% higher efficiency due to lower emitter saturation current, resulting from the benefit of in-situ oxide surface passivation during the implant anneal.