Back-contact Back-junction Research Articles

Analysis of a point-contact BC-BJ silicon solar cell by three-dimensional numerical simulations

The design of optimized and efficient solar cells is a key factor for the future development of the photovoltaic (PV) technology. This paper focuses on three-dimensional (3D) modeling of a Back Contact-Back Junction (BC-BJ) crystalline silicon (c-Si) solar cells featuring a point contact (PC) scheme in the rear emitter region in order to optimize the efficiency with respect to the use of a conventional linear contact (LC) pattern. A 3D model has been built to ensure a good accuracy in the simulation of the considered solar cell structures. The developed model includes a proper modeling of the series resistance effects related to the implemented contact pattern. The TCAD Sentaurus software provided by Synopsys has been adopted to perform comparative simulations between the considered PC BC-BJ solar cell and its LC counterpart. Our simulation results prove that the adoption of a PC scheme in the emitter region of a BC-BJ solar cell is beneficial to boost its efficiency (up to 22.43%) with respect to the conventional LC cell, while keeping lower the contacted fraction of the emitter region.

Screen‐Printed Borosilicate Glass Derived from Sol–Gel Materials for Back‐Contact Back‐Junction Solar Cells

Borosilicate glass films deposited by chemical vapor deposition are used as boron dopant sources in silicon solar‐cell manufacturing, to reduce the fabrication costs of, e.g., back‐contact back‐junction (BCBJ) solar cells. Herein, an alternative dopant source is investigated, which can replace such layers by a printing step. The necessary paste is synthesized by the sol–gel method and optimized for screen printing as demonstrated. The liquid paste can be converted to a glass by thermal annealing, as evaluated by spectroscopic investigations of the resulting thin films. The resulting layers are uniform and crack‐free with a thickness of around 150 nm on silicon surfaces. It is shown that such layers can act as boron dopant sources on silicon, sufficient for the fabrication of BCBJ solar cells, as demonstrated by prototype devices.

Understanding the impact of point-contact scheme and selective emitter in a c-Si BC-BJ solar cell by full 3D numerical simulations

Abstract This work presents a detailed analysis of the impact of a rear point-contact (PC) scheme and a selective emitter (SE) design on the performance of a crystalline silicon (c-Si) back contact-back junction (BC-BJ) solar cell featuring narrower highly-doped back surface field (BSF) areas and wider lowly-doped emitter areas. The analysis was performed both through light and dark J-V simulations by using a rigorous full three-dimensional (3D) electro-optical modeling approach to accurately include the 3D effects of several competing physical mechanisms occurring in a PC structure. Simulation results show that the adoption of a relatively thick and wide metallization contacting the silicon at the rear side only via small circular-shaped holes gives an efficiency improvement above 0.3%abs with respect to a conventional BC-BJ solar cell featuring a linear-contact (LC) metallization scheme. Moreover, the introduction of an optimized SE design, featuring a local deeper highly-doped (HDOP) profile underneath rear point contacts and a narrower lowly-doped (LDOP) profile at the non-contacted rear interfaces, can lead to a further efficiency enhancement of about 0.2%abs, which increases with the emitter width.

Ribbon interconnection of 6” BC-BJ solar cells

This work presents an interconnection approach for 6” back-contact back-junction (BC-BJ) solar cells by using conventional solder-coated copper ribbons with implemented wave structures for thermomechanical stress relief. We developed a process for production and advanced mechanical and electrical characterization for these interconnectors. In our study, mechanical stress is reduced up to 96.6 % (for ribbons) and up to 81 % (for wires) compared to non-structured interconnectors. In electrical terms, the relative effective resistance of the interconnector is increased by 3.1 % (for ribbons) and by 6.4 % (for wires). In general both, ribbons and wires, are suitable for the interconnection of 6” BC-BJ solar cells.A 4-cell module with 19.94 % efficiency in a standard module setup with 21.09 % - 21.18 % 6” BC-BJ ZEBRA cells (CTMpower = -3.8 %) is manufactured. For interconnection 8 modified ribbons (1.5 x 0.2 mm²) are used on the rear side. The cells feature a multilayer metallization with a low temperature paste. The module passes TC-200 according to IEC 61215 without degradation. Our cost analysis shows that such a stress relief structure can be realized with additional material costs of lower than 0.2 € for a 60 cell full size module.

Development of back-junction back-contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back-junction back-contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open-circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co-diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm2 by using screen printing and co-firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm2. The present work has demonstrated that the commercialization of low-cost and high-efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.

23.2% laser processed back contact solar cell: fabrication, characterization and modeling

We describe the manufacturing process for interdigitated back contact back junction silicon solar cells based on laser processes, and present detailed results and analysis to our best cell efficiency of 23.24%. The manufacturing process features two laser doping steps, one for the boron doped emitter and one for the phosphorus doped back surface field. The saturation current densities of thermal oxide passivated laser doped regions are on par with furnace diffused silicon for high efficiency solar cells. Laser ablation locally defines the contact areas through the rear dielectric layer stack, and structures the rear aluminum metallization. The precision of the laser systems in conjunction with the optical setup yields line shaped doping traces with a width of 150 µm and a pitch below 500 µm. The measured optical and electrical properties of our solar cell agree well with 3D simulation results. The measured reflection, transmission, quantum efficiency and current voltage curves in dark and illuminated condition simultaneously agree well with simulation, based on the same data set, giving confidence in the result of a detailed free energy loss analysis. The bulk resistive and recombination losses are identified as the main loss contributors. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Design guidelines for a metallization scheme with multiple-emitter contact lines in BC-BJ solar cells

This work presents a study on back contact-back junction solar cells when using a metallization scheme with multiple-emitter contact lines. For this purpose, TCAD-based simulations have been carried out by taking into account realistic parameters and state-of-the-art physical models for silicon solar cells. The analysis has been performed by varying different geometrical parameters related to rear metallization, such as the number of the emitter contacts and the emitter metal coverage. In general, an efficiency improvement has been observed by uniformly placing the multiple contacts along the emitter region and by increasing the number of the emitter metal lines. An optimal emitter metallization fraction has been found for different numbers of contacts as a result of the tradeoff between recombination losses at metal/silicon interfaces and series resistance losses. Moreover, the effect of the multiple-emitter contacts has been evaluated as a function of the finger length, the emitter width, and the rear pitch.

All-Diffused Back-Contact Back-Junction Solar Cell With Aluminum-Alloyed Emitter—Experiment and Simulation

This work introduces a novel industrial feasible back-contact back-junction crystalline silicon solar cell technology with exclusively highly doped surfaces. The near-surface phosphorus doping is realized in one single tube furnace diffusion step, while the alloying of screen-printed aluminum forms the p+-doping. A first experimental verification leads to a conversion efficiency of 20.1%. Further possible improvements are proposed and verified with numerical simulation. We perform comprehensive step-by-step optimization led by insights from free energy loss analysis, resulting in a conversion efficiency potential of 22.4% under realistic assumptions for design optimization.

Front Surface Passivation Scheme for Back-Contact Back-Junction (BC-BJ) Silicon Solar Cell

Front Surface Passivation Scheme for Back-Contact Back-Junction (BC-BJ) Silicon Solar Cell

Screen-Printed Metallization Concepts for Large-Area Back-Contact Back-Junction Silicon Solar Cells

In this study, we investigate three screen-printed metallization concepts for back-contact back-junction silicon solar cells with an edge length of 156 mm: 1) a busbar-less concept with periodically interrupted contact fingers for wire-based interconnection; 2) a single-layer concept with printed busbars and periodically interrupted contact fingers; and 3) a multilayer concept consisting of continuous contact fingers, an insulation layer, and busbar metallization. A comprehensive simulation study is presented for all investigated metallization concepts, yielding their respective performance loss mechanisms. The multilayer approach is found to provide superior conversion efficiencies η, compared with the single-layer approach. For the wire-based concept, we show that contact finger interruptions up to a width of 1 mm have no significant negative impact on cell performance ( $\Delta \eta ). Furthermore, peel force measurements on test structures between soldered cell interconnectors and screen-printed metallization are discussed for the multilayer and the wire-based concept. Successful proof of principles with peel forces exceeding 1 N/mm are demonstrated for both investigated metallization concepts.

Codiffusion Sources and Barriers for the Assembly of Back-Contact Back-Junction Solar Cells

In this study, several diffusion sources are investigated, aiming at codiffusion for the fabrication of back-contact back-junction (BC-BJ) silicon solar cell. As a gaseous diffusion source, a POCl3-diffusion process is investigated, and for solid diffusion sources, phosphorus- and boron-doped silicate glass (PSG and BSG) deposited by the means of plasma-enhanced chemical vapor deposition are considered. The n+-doped areas diffused from a solid PSG layer allow for a precise adjustment of the sheet resistance $(R_{{\rm sh}})$ in the range of 40–400 Ω/sq, along with a dark saturation current density $(J_{o})$ of 55 fA/cm $^{2}$ . Subsequently, boron diffusion from solid BSG layer leads to p+-doped areas with high doping levels ( $R_{\rm sh}=$ 50 Ω/sq). However, gaseous POCl3 diffusion in combination with solid boron diffusion from the BSG layer can only be successfully performed if the BSG layer is protected with an SiO x layer. Furthermore, by adjusting the gas flows during POCl3 diffusion, n+-doped areas with $R_{{\rm sh}}$ in the range of 150– 300 Ω/sq are achieved. The corresponding surfaces feature $J_{o}$ values of 30 fA/cm $^{2}$ . The result of this study is a flexible codiffusion setup allowing for the efficient integration in advanced process chains of BC-BJ solar cells which results in the cell efficiencies well above above $\eta =$ 20%.

Understanding the Optimization of the Emitter Coverage in BC-BJ Solar Cells

In this work, by exploiting two-dimensional (2-D) TCAD numerical simulations, we performed a study of optimum emitter coverage ratio (Ropt) to reach maximum performance on back contact-back junction (BC-BJ) solar cells. Ropt exhibits a strong dependence on pitch, emitter and back surface field (BSF) doping and bulk resistivity, ranging between 0.6 and 0.95. By fixing BSF doping, emitter doping and bulk resistivity, BSF and emitter width can be optimized independently one another. The optimum BSF width and the optimum emitter width are given by a trade-off between series resistance and electrical shading losses. From the design perspective, focusing on optimizing the BSF and emitter width is more effective than optimizing R at fixed pitch or BSF width.

Novel back-contact back-junction SiGe (BC-BJ SiGe) solar cell for improved power conversion efficiency

In this paper, a SiGe based Back-Contact Back-Junction (BC-BJ) device structure called BC-BJ SiGe solar cell has been proposed. Photo reflection is significantly reduced in UV/Visible spectrum region in case of SiC/Si3N4/SiO2 passivated BC-BJ SiGe solar cell. Result, indicates that presence of SiC play an important role in photoelectric conversion. Ray tracing and finite difference time domain (FDTD) algorithms are used to simulate optoelectronics characteristics of the device. Simulation achieves the barrier height of 0.8 eV for holes at the interface which results in a higher field. The lower interface recombination rate of the order of 1017 cmź3 sź1 has been obtained. The device shows improved photovoltaic parameters. External quantum efficiency >84 % in the spectrum range of 450---700 nm wavelength and more than 80 % in the range of 350---700 nm wavelength is obtained. Further, we obtained the fill-factor (FF) and power conversion efficiency (PCE), 79 %, 17.8 % and 79 %, 14.8 %, using FDTD and ray tracing methods, respectively. All the simulations have been done using atlas and devedit device simulator.

A simulation study of the micro-grooved electrode structure for back-contact back-junction silicon solar cell

A micro-grooved electrode structure is investigated to illustrate its advantages when applied to the back-contact back-junction (BC–BJ) silicon solar cell. The finite element analysis shows that the micro-grooved electrodes enhances the photo-carrier collection and weakens the dependence of collection ability on pitch distance. The geometries of micro-groove are found to have little impact on the cell performance. These advantages open possibilities for the implementation of low cost fabrication methods. As a demonstration, a process involving laser doping and screen printing techniques are proposed and analyzed. The simulation results show that the laser induced lattice damage causes negligible deterioration of device electrical properties and the presence of parasitic metal insulator semiconductor structure near the screen printed electrodes actually leads to a performance improvement rather than degradation. Our preliminary results indicate that the micro-grooved electrode structure is promising for fabricating low cost high efficiency BC–BJ silicon solar cells.

Back-junction back-contact n-type silicon solar cell with diffused boron emitter locally blocked by implanted phosphorus

The highest energy conversion efficiencies in the field of silicon-based photovoltaics have been achieved with back-junction back-contact (BJBC) silicon solar cells by several companies and research groups. One of the most complex parts of this cell structure is the fabrication of the locally doped p- and n-type regions, both on the back side of the solar cell. In this work, we introduce a process sequence based on a synergistic use of ion implantation and furnace diffusion. This sequence enables the formation of all doped regions for a BJBC silicon solar cell in only three processing steps. We observed that implanted phosphorus can block the diffusion of boron atoms into the silicon substrate by nearly three orders of magnitude. Thus, locally implanted phosphorus can be used as an in-situ mask for a subsequent boron diffusion which simultaneously anneals the implanted phosphorus and forms the boron emitter. BJBC silicon solar cells produced with such an easy-to-fabricate process achieved conversion efficiencies of up to 21.7%. An open-circuit voltage of 674 mV and a fill factor of 80.6% prove that there is no significant recombination at the sharp transition between the highly doped emitter and the highly doped back surface field at the device level.

Two-level Metallization and Module Integration of Point-contacted Solar Cells

We present a module integration process for back junction back contact (BJBC) solar cells featuring point contacts to the back surface field (BSF). We apply two metallization layers. A first metal layer of aluminum is deposited onto the rear side of the cell and carries the current extracted from the polarity with the larger surface area fraction, e.g. from the emitter. The second metallization layer is an Al layer on a transparent substrate that we laser-weld to the small and point-shaped regions of the other polarity, e.g. the BSF region. We use a polymer for insulation between the two metal layers. The Al layer on the substrate also serves for cell interconnection, i.e., it enables module integration. Such an interconnection structure halves the fill factor losses due to the metallization. First proof-of-principle modules show a shunt free interconnection, no laser-induced damage, and an energy conversion efficiency of up to 20.7%.

Analysis of the Impact of Doping Levels on Performance of back Contact-Back Junction Solar Cells

In this work, by exploiting two-dimensional (2-D) TCAD numerical simulations, we performed a study of the impact of the doping levels on the main figures of merit in the different regions of a crystalline silicon Back-Contact Back-Junction (BC-BJ) solar cell: the emitter, the Back Surface Field (BSF) and the Front Surface Field (FSF). The study is supported by a dark loss analysis which can highlight the contribution of several recombination mechanisms to the total diode saturation current. The efficiency curve as a function of doping level exhibits a bell-shape with a clearly identifiable optimum value for the three regions. The decrease in efficiency observed at lower doping values is explained in terms of higher contact recombination for BSF and emitter, and in terms of higher surface recombination for FSF. The efficiency decrease observed at higher doping values is ascribed to the higher surface recombination for FSF and Auger recombination for all cases.

Co-Diffused Back-Contact Back-Junction Silicon Solar Cells without Gap Regions

In this paper, first generation back-contact back-junction (BC-BJ) silicon solar cells with cell efficiencies well above η = 20% were fabricated. The process sequence is industrially feasible, requires only one high-temperature step (codiffusion), and relies only on industrially available pattering technologies. The silicon-doping is performed from pre-patterned solid diffusion sources, which allow for the precise adjustment of phosphorusand boron-doping levels. Based on the investigated process technologies, BC-BJ solar cells with gap and without gap between adjacent n+ - and p+ -doped areas were processed. On the one hand, a strong reduction of the process effort is possible by omitting the gap regions. On the other hand, parasitic tunneling currents through the narrow space charge region may occur. Hence, deep doped areas were realized to avoid tunneling currents in gap-free BC-BJ cells. This paper finishes with a detailed characterization of the manufactured cells including important cell measurements like I-V, SunsVOC, quantum efficiency, and an analysis of the cell specific fill factor losses.

Simplified Front Surface Field Formation for Back Contacted Silicon Solar Cells

We investigate the formation of deep Phosphorus diffusion profiles with low surface concentrations in one single diffusion step. Such processes are suited for front surface field formation for n-type back-contact back-junction (BC- BJ) silicon solar cells or p-type silicon solar cells with alternative metallization techniques. The deposition temperature allows accurate control of the sheet resistance whereas the temperature of the in-situ oxidation strongly influences the profile-depth and surface concentration. The newly developed process leads to low dark saturation current densities well below 30 fA/cm2 on alkaline-textured surfaces and low short circuit current loss at the front side. Due to their depth and adjustable surface concentration, the obtained profiles are promising for front-surface- fields (FSF) and novel n-type emitters. A first implementation in BC-BJ solar cells with aluminum-alloyed emitter shows a good blue response of the solar cells and an improved efficiency compared to the reference process by 0.6%abs.

Large-Area Back-Contact Back-Junction Solar Cell With Efficiency Exceeding 21%

In this study, high-efficiency solar cells are presented with the use of low-cost industrially available technologies. This results in the so-called ZEBRA concept: a litho-free process in which standard 156 × 156 mm2 monocrystalline n-type Cz-Si wafers are processed into high-efficiency interdigitated back-contact solar cells. In our first attempt, we obtained energy conversion efficiencies of more than 20% on 239 cm2 area. With the help of a 3-D simulation of the device, a further improvement to more than 21.5% was determined.We indentify the open-circuit voltage and the large serial resistance as the main losses impeding from reaching the simulated efficiency. The cell results after an extensive optimization are presented in this study, focusing on the improvement of the fill factor of the cells. The optimized solar cells show an improvement in the energy conversion efficiency up to 21%. Furthermore, the simplemetallization and themodule interconnection design of the ZEBRA cells allow for a bifacial application. Indoor and outdoor I – V measurements on a bifacial one-cell-module show an enhancement in the total generated power of more than 12% as compared with a monofacial module.