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36th European Photovoltaic Solar Energy Conference and Exhibition; 224-228, Silicon heterojunction solar cells are bifacial by nature and their bifaciality coefficient can be easily tuned, from almost 0 for mono-facial cells to typically 90% for bifacial cells, and even 100% but at the expense of the conversion efficiency. For operation in outdoor conditions offering extra light at the backside (BIFI >0), the effective performance of bifacial systems was estimated from the trade off between the cell efficiency and bifaciality. The efficiency of rear-emitter cell decreases with bifaciality and strongly drops for very high bifaciality, because of the use of limiting amorphous layers, especially too thin at backside, and the use of too resistive or too shading metal grids. Thus, an optimum can be found for a given BIFI. Varying cell bifaciality by screen printed grid dissymmetry, the system output in a large range of BIFI is maximum for cell bifaciality ranging from 85% to 90%, typically corresponding to front side metal grid pitch around 2mm and backside pitch from 0.6 to 0.9mm. For a given backside to front side dissymmetry of cell efficiency related to deposited thin layers, the use of optimal metal grid can offer 0.5% relative energy production gain. Considering specific applications where symmetrical print is preferred, the optimal grid pitch is around 1.5mm. For mono-facial systems, it was found that the use of bifacial cells gives a benefit thanks to embedded reflection in glass-back-sheet modules. The optimization of cell bifaciality pointed out in this work must also be considered with silver paste costs: saving when moving to higher bifaciality or a balance between extra cost and higher performances when dense grid for low bifaciality is preferred.

36th European Photovoltaic Solar Energy Conference and Exhibition; 235-238, We report on recent results obtained within the HETNA project, a collaboration between CEA-INES and Enel Green Power for the deployment of silicon heterojunctions solar cells and modules technologies in the production lines at 3SUN 2.0 site. SHJ cells and modules produced on the CEA-INES pilot lines show narrow efficiency distributions and good response with material and process variations: cells average efficiency ranges from 21.2% to 21.8% with the four busbars design while module peak power ranges between 366W to 374W. Increasing number of busbars from 4 to 5 enabled fill factor (FF) improvement by 0.7% abs. at both cells and modules levels without affecting the module reliability after thermal cycling. Finally, we present best cell and module (72 cells, bifacial) results obtained in 2018 reaching 23.9% and 388.4W respectively.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 437-440, We report on the first results obtained within the HETNA project: collaboration between 3SUN thinfilm modules producer and CEA-INES for the conversion of 3SUN fab into a high efficiency silicon heterojunction (SHJ) solar cells & modules factory. Improvements of SHJ samples transportation between the two sites and samples management at 3SUN site enabled absolute efficiency increase up to 4.5% for cells with hydrogenated amorphous silicon (a-Si:H) layers performed using 3SUN’s TEL Plasma Enhanced Chemical Vapor Deposition (PECVD) reactors. It was possible to reach implied open-circuit voltage (iVoc) > 744mV and implied fill factor (iFF) > 84% for 20nm thick undoped a-Si:H layers deposited on textured 156 PSQ silicon wafers. Finally, we present results obtained on 72 cells bifacial modules designed specifically for 3SUN SHJ project made using high efficiency SHJ cells. The glass-glass module architecture, exempt of any frame, demonstrated a nominal power of 380.7W under flash test measurement with dark background.

26th European Photovoltaic Solar Energy Conference and Exhibition; 853-857, In this work, we present some of the main results obtained within the project “Heterojunction Solar Cells based on a-Si/c-Si” (HETSI) [1], funded by the European Commission in the framework of the 7th Research Framework Program from 2008 to 2011. This project, based on the promising silicon heterojunction technology, represents a concerted effort of the consortium, a well balanced mix of universities, institutes of technology and industrial partners, to combine device modelling, material optimization and characterization, process development (from texturization to metallization), PV cell and module process integration, while cost and environmental issues are addressed as well. Thanks to a better understanding of the device physics from advanced characterization tools and modeling, the results obtained by this consortium have put Europe on the map, with a record cell efficiency of 20,7% on large area n-type c-Si wafers. Moreover, a dedicated module process has been developed with low losses of only 1% absolute from cell to module.

26th European Photovoltaic Solar Energy Conference and Exhibition; 861-864, In this paper we show the state-of-the-art on the research on standard and interdigitated back contact amorphous/crystalline heterojunction (HJ) solar cells at INES. First, we focus our study on the optimization and the comprehension of the role of the front buffer layer of our n-type HJ solar cells on the totality of the device parameters (Jsc, FF, Voc). This work allows us to reach very high Voc values over 730mV without losses on FF and efficiencies of 21%. We observe that short-circuit current is the limiting parameter on our cells, and though we go over a new structure without shadowing loses, the so-called interdigitated back contact amorphous/crystalline silicon heterojunction (IBC-HJ). Simplified cell geometry is here proposed with the emitter and BSF layers locally in contact. To avoid resistive losses (shunt effects or series resistance), it is found that the emitter stack has to be carefully designed in terms of doping profile and contact material. Using patterning steps based on screen printing technology, 25 cm2 simplified IBC Si-HJ structures have been fabricated with an efficiency of 15.7%.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2260-2263, Surface passivation tests by plasma enhanced chemical vapor deposition of hydrogenated amorphous silicon (PECVD a-Si:H) films were performed on textured n-type crystalline silicon (c-Si) wafers to monitor a heterojunction (HJ) pilot line. For our deposition recipe of intrinsic a-Si:H, an optimal thickness of about 20 to 40 nm was found to insure good sensitivity of the test. Using 40 nm films, implied open circuit voltage (Voc) up to 740 mV and effective surface recombination velocity (S) down to 8 cm.s-1, as measured at one sun by quasi-stady state photo conductivity decay have been obtained using RCA&HF or O3&HF -based cleaning sequences. This high level of surface passivation allowed a better than 19% conversion efficiency baseline of HJ cells (20.5% best cell, 148 cm2). Regarding the HJ line monitoring by passivation tests, a correlation between Voc and S of control wafers and cell performance was found, despite the other process steps than the wet ones impact the performance. This allows, especially for c-Si bulk quality, some prediction of cell performance and troubleshooting actions on the wet equipments.

26th European Photovoltaic Solar Energy Conference and Exhibition; 3822-3825, Heterojunction solar cells have demonstrated high efficiencies and are also viewed as potentially highly sustainable. A life-cycle assessment (LCA) was performed to compare the sustainability of heterojunction (HJ) solar cells, as developed in the HETSI project (FP7-ENERGY-2007-1-RTD), to conventional mono-crystalline Si solar cells. This calculation resulted in a comparison of the cumulative energy demand (MJ) and the global warming potential (kg- CO2 eq) for HJ cells versus the mono-crystalline cells. The energy payback times (EPBT) of modules of two different HJ cell designs was found to be 1.2 and 1.2 years, while that of a ‘standard’’ mono-crystalline Si module was 1.5 years. The carbon footprint for module fabrication for the mono-crystalline Si module, and the two modules with slightly different HJ cell designs were: 0.9, 0.8 and 0.8 kg CO2 equivalents/ Wp, respectively. Assuming a southern Europe illumination of 1700 kWh/m2yr, a 75% performance ratio and a 30 year lifetime and no performance degradation, these values may also be expressed as 24, 20 and 20 g-CO2 equivalent/kWh. The higher efficiency of the HJ solar cells factors into a more favourable energy payback time, and a lower carbon footprint (per Wp or per kWh). A discussion of the cumulative energy demand and environmental impact of the manufacturing processes, together with an outlook on resource depletion, points out the materials and processes which require further development for future manufacturing requirements of lower energy use and lower costs. This is the first time an LCA of c-Si/ a-Si:H HJ solar cell technology is presented.

LGEP 2010 ID = 597; International audience

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2202-2205, This work presents our progress in designing the rear emitter of a-Si:H / c-Si (n) heterojunctions solar cells (Si-HJ). We study the emitter saturation current density (J0e) of p-type a-Si:H layers and show that it greatly depends on the layer thickness and conductivity. Different p-type a-Si:H emitters are tested experimentally on Rear Emitter (RE) as well as Interdigitated Back Contact (IBC) Si-HJ devices. A low conductivity (1.4x10-6 S.cm-1) layer allows the better Voc values about 650 mV but causes resistive losses on both types of solar cells. For the IBC devices, a low emitter contact fraction induces not only fill factor (FF) losses but also a decrease of the short circuit current (Jsc) value. By optimizing the rear side geometry an efficiency of 12.7% is achieved for the Si-HJ IBC structure on 25 cm2 n-type substrate.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2242-2245, Crystalline silicon thin film processes for thicknesses in the 20 - 80 @m range are more and more interesting given the issue of silicon shortage. Layer Transfer Processes (LTP) were widely investigated to obtain thin films because of the unavailability of suitable wire sawing. LTP avoids hazardous handling with fragile layers and silicon waste during sawing. In addition, a-Si:H/c-Si heterojunction solar cells are well suited to reduce the photovoltaic power cost. High efficiency on bulk substrate has been demonstrated by Sanyo Corporation and the low temperature process, less than 250 °C, ensures economical gain. In this paper, we present the fabrication of 26 cm2 solar cells on 50 -70 @m thick CZ wafers. The emitter is obtained either by a-Si:H (n+) deposition or by phosphorus diffusion. A 10 % efficiency solar cell on 52 @m thick silicon substrate has been achieved with a-Si:H/c-Si heterojunction. Further improvement is expected with an optimized ITO deposition and a progress in fill factor. Moreover, we also evaluate the effects of rear point contacts obtained by lithography through a dielectric layer. LBIC measurement has been used and a relative 10 % enhancement of photogenerated current has been observed at wavelength 980 nm on no contacted area compared to point contacts area. Moreover defects at the silicon metallization interface can be revealed. Such characterization has been made on 45 @m thick transferred layer.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1441-1444, The aim of this work is to present our progress on heterojunction solar cells, based on a low temperature route using hydrogenated amorphous silicon (a-Si:H) ultrathin films deposited by PECVD on bulk crystalline silicon. We focus our work on both n-type and p-type monocrystalline silicon to achieve large area solar cells for industrial application. The analysis of the a-Si:H/c-Si interface quality is analyzed by the QSSPC technique that allows evaluating the passivation properties as well as qualifying the different steps. First, a new cleaning process is evaluated with respect to passivation properties, demonstrating that it is possible to obtain nearly as good passivation on textured as on polished samples with appropriate cleaning. An intrinsic layer is added to an emitter or BSF double side structure in order to show the impact of such a layer on passivation properties. Finally, full heterojunction solar cells have been characterised thanks to I-V measurements. Record Voc values up to 668 mV have been obtained on 5*5 cm² cells. Efficiencies of 16,2% and 15,8 have been obtained on P type and N type wafers, for cells of 5*5 cm² .

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1449-1452, The aim of the present work is to develop an alternative to ITO for silicon heterojunction solar cells. This study is motivated by cost and availability issues linked with indium material. It is proposed to replace ITO by ZnO doped with aluminum. A process for ZnO:Al films deposition is presented based on conventional RF magnetron sputtering. Experimental parameters are adapted in order to meet the specifications in terms of electrical resistivity and transmission. The microstructure of the films is also characterized by XRD. Optical simulation is carried out at the cell level in order to evaluate the optimal thickness of the Al-doped ZnO film regarding transmission. Finally, a comparison is made between a standard ITO and the newly developed ZnO:A film regarding the properties of the solar cell.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1696-1699, It has been shown recently that, on p-type c-Si, the use of an ultra-thin (3 nm) undoped (so-called intrinsic) hydrogenated polymorphous silicon (pm-Si:H) layer followed by a thin (8 nm) n-type hydrogenated amorphous silicon (a-Si:H) layer allows one to obtain a good passivation and to form a good quality emitter in (n) a-Si:H / (i) pm-Si:H / (p) c-Si heterojunction solar cells. Efficiencies of about 16.8% had been demonstrated with this type of emitter [1]. We here extend our study to the ability of (n) a-Si:H / (i) pm-Si:H stacks to form a good passivation and a high quality back surface field when deposited onto n-type c-Si wafers. We then turn to the influence of the undoped pm-Si:H buffer layer when it is coupled with p-type a-Si:H to form a p-type emitter on an n-type c-Si wafer or when such a double layer association is used as a back surface field on a p-type c-Si wafer. Passivation properties are determined by QSSPC measurements, while coplanar conductance measurements bring additional information on the interface quality. We pay a particular attention to the effect of the deposition temperature, as well as to the effects of post-annealing on the passivation quality. Finally, the influence of the thickness of the undoped polymorphous and of the doped amorphous layers are considered.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1949-1952, In this paper, interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells are investigated. Two dimensional numerical simulations for IBC-SiHJ solar cells have been performed with ATLAS 2-D and 3-D device simulation software that allows accurate bulk and interface defects modelling. We here focus on IBC-SiHJ structure on p-type crystalline silicon substrate varying the following parameters: bulk resistivity (doping concentration), BSF width, gap between the two doped layers (BSF and emitter), metal coverage of emitter contact and contact resistance. The influence of these parameters has been tested by generating the current-voltage under AM1.5 illumination. Concerning the rear side geometry of IBC-SiHJ solar cell, the short circuit current JSC is found to be limited by the width of the BSF and of the gap. Possible explanations for fill factor losses in IBC-SiHJ solar cells have been determined: high bulk resistivity, partial metal coverage of emitter contact by aluminum and high contact resistance.

International audience; In this paper we propose a novel approach to thin-film silicon solar cells technology developed at INSA-LYON, named ELIT process (Epitaxial Layer for Interdigitated back contacts solar cell Transferred using porous silicon as a weakened layer). High crystal quality Si film is grown on porous silicon (PS) by Vapor Phase Epitaxy. This single crystal silicon epitaxial layer was processed into solar cells by realizing all contacts on the back side. Then, thin film solar cell is transferred onto a low-cost substrate before texturation and deposition of antireflective coating on the front side. Preliminary experiments of transferred IBC thin-film solar cell lead to 7.7% efficiency with a 70 µm thick active layer.

National audience