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In situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) were studied in this work for the fabrication of poly-Si passivating contacts. In situ doping was targeted for enabling the full potential of the high-throughput LPCVD technique, as it could allow leaner fabrication of industrial solar cells featuring poly-Si passivating contacts than the more common ex situ doping routes. By careful optimization of the deposition temperature and the flows of the carrier gas (H-2) and the dopant precursor (PH3), high doping in the poly-Si layers was achieved with active P concentrations up to 1.3.10(20) cm(-3) . While reduction in the deposition rate (r(dep)) and thus in the throughput is a known problem when growing in situ P-doped films by LPCVD, this reduction could be limited, and the resulting r(dep) was equal to 0.078 nm/s. The developed poly-Si films were characterized both structurally and in terms of their passivation potential in poly-Si contacts. The latter yielded recombination current densities down to 1.5 fA/cm(2) in passivated (J(0, p)) and 25.6 fA/cm(2) in screen-printing metallized (J(0, m)) regions on saw-damage removed (SDR) Cz-Si surfaces, accompanied by a contact resistivity (rho(c,m)) of 4.9 m Omega.cm(2). On textured Cz-Si surfaces, the corresponding values were J(0, p) = 3.5 fA/cm(2), J(0,m )= 56.7 fA/cm(2), and rho(c,m) = 1.8 m Omega.cm(2). Optical impact of the developed poly-Si films was also assessed and a short circuit density loss of 0.41 mA/cm(2) is predicted per each 100 nm of poly-Si applied at the rear side of solar cells. The authors would like to acknowledge Rajiv Sharma from KU Leuven for his help with the interfacial oxide development, Sukhvinder Singh and Patrick Choulat from Imec for their help with the contact resistivity measurements and sample fabrication, Thomas Nuytten and Stefanie Sergeant from Imec for the Raman spectroscopy measurements, Bastien Douhard and Mustafa Ayyad from Imec for SIMS measurements, Maxim Korytov, Laura Nelissen, and Patricia van Marcke from Imec for the TEM specimen preparation and measurements, and Janusz Bogdanowicz from Imec for his help with the analysis of the Hall measurement data. This work was supported by the European Union’s Horizon2020 Programme for research, technological development, and demonstration [grant number 857793]; and by the Kuwait for the Advancement of Sciences [grant number CN18-15EE-01].

The potential of in situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) was studied for the realization of poly-Si/SiO x passivating contacts. In situ doping of poly-Si, as an alternative to ex situ methods, could enable simpler fabrication of industrial solar cells featuring these passivating contacts. With this approach, recombination current densities down to 1.7 fA/cm2 and 3.5 fA/cm2 were achieved on saw-damage removed and textured Cz-Si surfaces, respectively. It was found that the use of thermal SiO x , high active doping in the poly-Si, and hydrogenation improve the passivation quality. In addition, while post-LPCVD annealing was also beneficial, dopant loss from poly-Si at high annealing thermal budgets was observed to be detrimental to the specific contact resistivity and passivation quality, thus making it crucial to mitigate such dopant losses.

The current work is aimed at the development of the building blocks for the integration of polysilicon-based passivating contacts in co-plated n-PERT front junction solar cells. We show that both n-type poly and p-type poly layers (called n-poly and p-poly hereafter) can be obtained simultaneously by the sintering of undoped (i-poly) layers during the POCI 3 diffusion that induces the auto-doping from an existing B-doped emitter. State of the art J 0 values with very high uniformity were measured for n-poly, auto doped p-poly, and boron emitter. The total area weighted J 0 of a passivated cell before metallization is calculated to be around 32 fA/cm2• We also show the viability of a laser oxidation process to pattern the front p-poly Si layer without measurable damage. Finally, we demonstrate laser ablation and plating processes that induce only very limited damage, leading to a high iVoc of around 700 mV for asymmetric samples with p and n-poly passivated surfaces.

© Copyright 2019 American Chemical Society. Low contact resistivity ( c ) and low recombination current density at the metallized area (J 0,metal ) are the key parameters for an electron-selective contact in solar cells, and an i-a-Si:H/TiO x /low work function metal (ATOM) structure could satisfy these criteria. In this work, to achieve strong downward band bending, an Yb ( = 2.5-2.6 eV)/Ag stack is used. Moreover, the impact of (1) substrate topography (flat or textured), (2) TiO x thickness, and (3) Ti precursor (TTIP vs TDMAT) on the ATOM contact performance is investigated. The results show that the ATOM contact is relatively insensitive to the surface topography and to the Ti precursors (TTIP or TDMAT) used for the atomic layer deposition (ALD) of TiO x . However, the TiO x thickness has a significant impact on the c and marginally on the J 0,metal of the ATOM contact. For all topography cases and Ti precursors, 1 nm thick TiO x results in the lowest c value, most likely due to E F,metal depinning. In the silicon heterojunction solar cell, this ATOM contact (i-a-Si:H/TiO x /Yb/Ag) yields a solar cell efficiency of 19.2% with a high V OC of 723 mV without the need of a doped n-a-Si:H layer. Concerning the thermal stability of these contacts, TEM images confirm that Yb does not diffuse into the i-a-Si:H layer after an annealing at 180 °C for 30 min thanks to the TiO x layer behaving as a diffusion barrier. 98% of the initial solar cell efficiency is preserved even after successive annealing treatments at 150 and 175 °C, which are values in the same temperature range used in the module lamination and the sintering of the printed Ag. These results in combination with the demonstrated efficiency underline that the ATOM contact is a promising route to realize high-efficiency solar cells. ispartof: ACS APPLIED ENERGY MATERIALS vol:2 issue:2 pages:1393-1404 status: published

Metal silicide is a well-known material for contact layers; however, it has not been tested in the context of doping-free carrier selective contacts. Thin film deposition of an appropriate metal with mild annealing treatment is an interesting alternative to the more complex depositions of other compound materials. Reaction of Yb deposited on top an i-a-Si:H passivation layer results in the formation of YbSix on top of a remnant i-a-Si:H, following a low-temperature annealing below 200 degrees C. Such a contact is an interesting candidate as a doping-free electron-selective contact. Detailed investigation of the i-a-Si/YbSix contact shows that Yb thickness, i-a-Si:H thickness and silicidation annealing conditions play a significant role in determining the recombination current density (J(0,metal)) and the contact resistivity (rho(c)). Low J(0,metal) of 5 fA/cm(2) and low rho(c) below 0.1 Omega.cm were independently demonstrated for such i-a-Si:H/YbSix contacts. We also demonstrate that lowtemperature silicidation can be combined with contact sintering (160 degrees C/25 min) or module lamination (160 degrees C/20 min), which are potential pathways for process simplification. Combining the optimized i-a-SEFI/YbSix electron contact with MoOx based hole contact in the MolYSili doping-free cell (i-a-SEH/MoOx + i-a-SEFI/YbSix), we achieved 16.7% in average efficiency and 17.0% for the champion cell. Furthermore, the YbSc contact stability was evaluated at module level and excellent thermal stability of the MolYSili laminate was demonstrated using the damp-heat test method (humidity 85%, 85 degrees C, 1000 h), where the laminated MolYSili cell did not show any degradation in the cell efficiency. This is the first proof-of-concept demonstration of a stable silicide-based contact for low-temperature processed doping-free solar cells. The authors thank Pieter Lagrain, Olivier Richard, and Hugo Bender for TEM measurement. Moreover, the authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. The work in this paper was partially funded by the Kuwait Foundation for the Advancement of Sciences under project number CN18-15EE01. imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Cho, J (corresponding author), Katholieke Univ Leuven, ESAT Dept, B-3001 Leuven, Belgium. jinyoun.cho@eu.umicore.be

Metal oxides are interesting materials for use as carrier-selective contacts for the fabrication of doping-free silicon solar cells. In particular, MoOx and TiOx have been successfully used as hole and electron selective contacts in silicon solar cells, respectively. However, it is of paramount importance that good thermal stability is achieved in such contacts. In our work, we combined i-a-Si:H/MoOx based hole contacts with electron contacts featuring i-a-Si:H/TiOx/low work function metal (ATOM) to fabricate doping-free cells, termed MoIyATOM cells. We found that the thermal stability of the ATOM contact was improved when the i-a-Si:H was annealed (300 degrees C for 20 min in N-2) before depositing TiOx (Le. pre-TiOx annealing), which reduces the hydrogen content in i-a-Si:H by about 27 Vorei, and thereby the H-related degradation of the ATOM contact characteristics. Moreover, it was found that reducing the thickness of the low-work function metal on top of the TiOx enhanced the thermal stability of the ATOM contact. With these adaptations, the MoIyATOM cell efficiency was improved by 3.5 %(abs), with the highest efficiency of 17.6%. Moreover, the cells show improved thermal stability after the above-mentioned pre-TiOx annealing, which is confirmed by annealing tests at cell level as well as damp-heat tests at module level. The insights of this study could be used to tailor other metal-oxide based electron or hole contacts. The authors thank Praveen Dara and Johan Meersschaut for ERD measurement. Moreover, the authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. The work in this paper was partially funded by the Kuwait Foundation for the Advancement of Sciences under project number CN18-15EE-01. Imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Cho, JY (reprint author), Katholieke Univ Leuven, ESAT Dept, Kasteelpk Arenberg 10, B-3001 Leuven, Belgium; Umicore, Watertorenstr 33, B-2250 Olen, Belgium. Radhakrishnan, HS (reprint author), IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. jinyoun.cho@eu.umicore.be

One of the advantages of an epitaxially grown emitter is that its doping profile is not limited by the diffusion process and that almost any arbitrary profile shape can be realized. In this simulation study we make use of the epitaxial doping profile’s flexibility to optimize the emitter of an nPERT solar cell. Initially an optical and electrical model is set up in Sentaurus TCAD and calibrated based on measurements of an actual device with an efficiency of 22.5%. Next, simulations for a box shaped profile with varying thicknesses and doping concentrations are carried out. Additionally, a two-step emitter consisting of a combination of two box profiles – one located at the surface and another deep profile underneath the former one – is also simulated. The results clearly identify the optimal doping concentrations and thicknesses which result in the highest efficiencies. Outside of these optimal regions the most significant loss mechanisms are pointed out. To be highlighted is that the switch from a the single box profile emitter to an optimal two-step profile emitter realizes a gain of 0.4% absolute resulting in an efficiency of 22.9%. This demonstrates that the freedom in the shape of the epitaxial doping profile can be employed to further increase the solar cell efficiencies.

A possible research path to increase the photo-generated current in silicon heterojunction (SHJ) solar cells is to replace doped layers on the front-side of the cell, which result in significant parasitic light absorption losses. MoOx is one candidate to replace the p-doped a-Si:H layer in such devices, although it is claimed to be relatively unstable to thermal treatments. We found that a MoOx film with a thickness of 6 nm is sufficient to achieve a JSC of 36 mA/cm2, which is 0.5 mA/cm2 on average higher than that of our classical SHJ reference cell. We also established a contact sintering condition for printed Ag at 160 °C after MoOx deposition, without degrading the cell performance. The champion MoOx-contacted cell yielded VOC of 724 mV and FF of 74.1%, resulting in an efficiency of 19.3%. From a detailed analysis of the interfaces of the hole contact, an interfacial a-SiOx of 1.6–2 nm was observed between a-Si:H and MoOx irrespective of the MoOx thickness (6–10 nm) before and after contact sinter annealing at 160 °C. We postulate that this a-SiOx layer acts as an interfacial dipole layer and also increases the contact resistivity at this contact. The intrinsic stability of the optimised MoOx-contacted cell is studied using a one-cell mini-module under standard damp-heat testing (85 °C/85% humidity/1000 h). More than 97 %rel of the original efficiency is maintained after 1010 h of testing, which is comparable to the behavior observed in a classical SHJ reference one-cell mini-module that was similarly tested.

© 2019 Elsevier B.V. In this work we investigated the optical and electrical performance of p-type epitaxial layers as the rear emitter of bifacial n-type PERT solar cells. In the first part of this paper, the surface morphology of epitaxial layers grown on textured surfaces is studied. Because of the epitaxial growth, a pyramids-rounding effect is observed as a result of {311} and {911} facet propagation. The growth pattern was quantified and modelled. In the second part of this paper, the optical performance of semi-device test structures is evaluated. The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25° without having to introduce any additional polishing steps to modify the morphology of the textured surface. In the last part of this paper, we present the electrical results for semi-device structures created to quantify the recombination losses in the passivated and metallized regions of those p-type epitaxial emitters. These results indicate that by introducing a rear epitaxial emitter in the bifacial n-type PERT cell structure, we can increase the implied V oc up to 17 mV compared to a diffused emitter with the same sheet resistance. ispartof: SOLAR ENERGY MATERIALS AND SOLAR CELLS vol:195 pages:43-48 status: published

We have studied the effect of different cleanings at device level for nPERT (passivated Emitter, Rear Totally diffused) cells with a rear epitaxial emitter. Cleanings based on sulfuric acid and peroxide mixtures (SPM), which are effective but also expensive, were used as reference. We found that simplified cleanings such as those based on sulfuric acid and ozone mixtures (SOM), and those based on water and ozone (IPV), are giving similar results at device level as the expensive SPM cleaning, leading to similar J0e, J0, FSF, and lifetime values. The implied VOC values of corresponding test devices fabricated using the simplified cleanings are at the same high level as the test devices using more expensive SPM cleanings. Actual nPERT devices with moderately doped epitaxially-grown emitters with a constant doping profile (box profile) showed average efficiencies above 22.0% for both SOM and IPV cleanings. Very high VOC values of up to 695 mV and implied VOC values of up to 730 mV were shown for cells with a flat rear surface. Finally, the best nPERT cells with rear epitaxial emitter and SOM cleaning showed efficiencies up to 22.5%, as externally confirmed by ISE CalLab.

This paper proposes a kerfless wafer structure with built-in p-n junctions in n-type silicon wafers grown using Crystal Solar's high throughput epitaxy technology. Compared with a conventional p-type emitter by boron diffusion, ion implantation, or epitaxy, the built-in p-type emitter has a reduced and uniform doping concentration and increased thickness. The epitaxially grown wafers and conventional Czochralski (CZ) n-type wafers were processed into solar cells. A best efficiency of 22.5% with epitaxially grown wafers was achieved, with a 6 mV gain in open-circuit voltage, suggesting a high wafer quality and superiority of the deep epitaxial emitter over a standard boron-diffused emitter. Substrate reuse associated with the kerfless epitaxy technology is studied as well, with respect to its impact on solar cell efficiency. The data suggest no degradation in cell efficiency due to substrate reuse.

Passivating contacts consisting of polycrystalline silicon (poly-Si) and thin silicon-oxide (SiOx) layers facilitate a significant reduction of recombination losses in silicon solar cells. Nevertheless, these gains come with short circuit current density (Jsc) losses due to parasitic absorption by the poly-Si. Even if the passivating contacts are employed at the rear side only, absorption, particularly due to free carriers (FCA) in the heavily doped poly-Si, may still lead to significant Jsc losses. In this work, these losses are characterized as a function of the poly-Si thickness (tpoly) by the analysis of front reflectance spectra in the infrared (IR). For this study, two sets of samples with different n-type full-area poly-Si passivating contacts at the rear are compared to references with a phosphorus(P)-diffused back surface field (BSF) instead. For the two sets, Jsc losses with respect to the references (ΔJsc) are 0.10 mA/cm2 and 0.42 mA/cm2 per 100 nm thick poly-Si, respectively. The difference between the two values is studied by Hall measurements and interpreted to be due to the over three times as high free carrier concentration (ND,act) in the poly-Si layers of the second set of samples as the first set. On the other hand, lifetime measurements showed an excellent passivation yielding an implied open circuit voltage (iVoc) up to 736 mV only for the samples with the more heavily doped poly-Si, whereas iVoc of 683 mV was measured for the first set, which indicates a trade-off between absorption losses and passivation quality.

Copyright © 2018 John Wiley & Sons, Ltd. In this work, the ATOM (intrinsic a-Si:H/TiO x /low work function metal) structure is investigated to realize high-performance passivating electron-selective contacts for crystalline silicon solar cells. The absence of a highly doped Si region in this contact structure is meant to reduce the optoelectrical losses. We show that a low contact resistivity (ρ c ) can be obtained by the combined effect of a low work function metal, such as calcium (Φ 2.9 eV), and Fermi-level depinning in the metal-insulator-semiconductor contact structure (where in our case TiO x acts as the insulator on the intrinsic a-Si:H passivating layer). TiO x grown by ALD is effective to achieve not only a low ρ c but also good passivation properties. As an electron contact in silicon heterojunction solar cells, inserting interfacial TiO x at the i-a-Si:H/Ca interface significantly enhances the solar cell conversion efficiency. Consequently, the champion solar cell with the ATOM contact achieves a V OC of 711 mV, FF of 72.9%, J SC of 35.1 mA/cm 2 , and an efficiency of 18.2%. The achievement of a high V OC and reasonable FF without the need for a highly doped Si layer serves as a valuable proof of concept for future developments on passivating electron-selective contacts using this structure. ispartof: PROGRESS IN PHOTOVOLTAICS vol:26 issue:10 pages:835-845 status: published

This work focuses on the fabrication of nPERT(Passivated Emitter, Rear Totally Diffused) devices incorporating an epitaxially grown single side rear-emitter. Such epi-nPERT cells are fabricated in a simplified way using the selectivity of the epitaxial deposition, which is obtained by a PECVD-SiOxlayer, that not only mask the front but also passivates the cell. The cell performance is studied in terms of: i) various front surface fields (FSF) applied prior to emitter epitaxy and ii) usage of laser doping as an alternative to laser ablation for a front contacting scheme. The results show: i) a clear relationship between the depth of the homogeneous FSF and its impact on the open circuit voltage of the devices, with a shallow FSF having the highest V OC loss due to laser damage and ii) laser doping on devices with a relatively deep diffused FSF giving 8 mV increase in V OC as compared to devices with ablated dielectrics and a V OC increase of almost 30 mV in case a shallow selective FSF is applied. This results in a best efficiency obtained so far for the epi nPERT devices of 21.6% (226 cm 2 ).

A TiOx layer is well known as an electron-selective contact material because of its asymmetric band offsets with respect to c-Si. When applying TiOx layers as passivating electron-selective contacts, forming sub-stoichiometric TiOx is important to obtain a low contact resistivity because oxygen vacancies increase the conductivity of TiOx and provide n-type doping effects. In this work, oxygen vacancies at SiOx/TiOx interfaces are investigated by atomic depth profiling of XPS measurements. Three kinds of TiOx layers are studied grown by either e-beam evaporation, atomic layer deposition or sputtering on c-Si. In all three TiOx samples, a resulting stack of c-Si/SiOx/TiOx could be noticed XPS measurements that show SiOx peaks near the c-Si/TiOx interface. Moreover, clear TiO2 peaks, which can be measured at the surface of all three TiOx layer types, gradually change to Ti or TiSi2 peaks near the SiOx/TiOx interface. This indicates that many oxygen vacancies seem to exist at the SiOx/TiOx interface. This TiOx reduction may contribute to the formation of a dipole and increased downward band bending resulting in a lower contact resistivity in the electron-selective contacts. As a result, hetero-junction solar cells with i-a-Si:H/TiOx/Ca/Al contacts exhibit a significant series resistance reduction of about 40 % compared to solar cells with i-a-Si:H/Ca/Al contacts.

This work evaluates the potential of selective epitaxy to mitigate the recombination losses at the p-type contact regions of plated bifacial n-type PERT cells. Following the growth of a 500 nm, 2.5 10(19) cm(-3) boron doped epitaxial layer at the emitter contact regions defined by laser ablation of the passivating dielectrics, both an increase in V-oc (+6 mV) and FF (+ 1% absolute) are measured in the final cells compared to the reference with a homogeneous diffused emitter. These results come with an average efficiency gain of 0.3 and 0.5% absolute for front and rear side illumination, respectively. If the diffused, blanket emitter in the passivated regions is replaced by a thicker, lowly doped epitaxial profile (3 mu m, 5 10(18) cm(-3)) to further reduce recombination, an additional rise in implied V-oc after metallization of 10 mV is estimated. This increase would be the result of a reduction in J(0,pass, emitter) (down to 6 fA/cm(2)) and J(0,plated, emitter) (down to 1967 fA/cm(2)).

Eliminating a doping process could be an effective way to reduce the production cost of c-Si cells. However, in absence of highly doped Si, the formation of a high quality contact is not straightforward. The lack of field-effect passivation from a lowly doped region can lead to a high recombination current density at the contacts (J(0,metal)) and moreover, contact resistivity (rho(c)) typically increases when doping level is decreasing. In this work we focus on reducing the contact resistivity of an electron-selective contact for doping-free cells. Although the effect of low work function metals (LWMs) in combination with an i-a-Si:H layer has already been reported, the synergy effect of a LWM and a MIS (Metal-Insulator-Semiconductor) contact structure on top of the ia-Si:H has not been reported yet. Here, we demonstrate a new ATOM (i-a-Si:H / TiOX / low workfunction metal) contact structure as an electron-selective contact using an i-a-Si:H layer, a TiOX interfacial layer and Ca (Phi 2.9eV) without requiring an additional n(+) doping process. The addition of TiOX in between the i-a-Si:H layer and the Ca decreases the pc by about 2 orders of magnitude. Despite of increased J(0,metal) due to e-beam processing of TiOX, the Ca based ATOM contact increases the potential max efficiency up to 25 %. To the best of our knowledge, this is the first demonstration of an electron-selective contact comprising a low work function metal, an interfacial TiOX and an i-a-Si:H passivation layer. This type of contact could be a promising route for the optimization of doping-free cells (C) 2017 The Authors. Published by Elsevier Ltd. The authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. And part of this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270.

In the quest for lowering the manufacturing cost of silicon solar cells, imec has been working successfully on two crystalline Si ‘thin-film’ cell concepts. In a first concept, a 20 µm-thin Si solar cell is epitaxially grown on top of a porous Si-based Bragg-type reflector which is electrochemically etched in a low-cost UMG Si substrate. Large area solar cells with efficiencies of 15.2% have been made, using (semi-)industrial processing tools. This clearly demonstrates that this cell concept has almost reached the stage of industrial application. In a second, longer-term approach, a 1–5 µm-thin, stand-alone mono-crystalline film is created based on the controlled annealing of an ordered macroporous silicon layer (the ‘Epi-free’ process). With this very thin Si layer, a simple proof-of-concept solar cell has been made exhibiting an efficiency of 4%. By optimizing the cell process in terms of light trapping and passivation, efficiencies over 15% can be expected from this technology.

We report on the beneficial use of embedded segmented porous silicon broad-band optical reflectors for thin-film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well-established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm2 area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (Jsc) were well above 31 mA/cm2 for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.