Back-contact Back-junction Solar Cells 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.

Design of optimum rear passivated submicron Al corrugation in very thin textured silicon back-contact back-junction solar cell for absorption enhancement up to near-infrared region

The optical reflection and absorption in a very thin textured back-contact back-junction silicon (Si) solar cell are investigated. The introduction of nanotexturing on front Si surface has significantly increased the absorption in the ultraviolet (UV)–visible region with a low reflection of below 0.05. The introduction of rear surface corrugation formed by a combination of SiO2-Al has successfully enhanced the absorption up to near-infrared (NI) region. The optimum crest width, periodicity, and trough depth of corrugation are derived, which lead to high absorption up to 0.97. The internal reflection and scattering that occur near the plasmonic Al corrugation are contributing to the local maximum electric field intensity in both transverse magnetic (TM) and transverse electric (TE) modes. Since there is no perpendicular electric component in TE mode, a coupling of electric field within a corrugation trough is not observed but is only observed in TM mode. On 10-μm-thick Si, the application of Si nanocones (NCs) and optimized rear Al corrugation results in 56% improvement in photogenerated current, Jsc, compared to the reference flat Si. Thinning down the Si to only 2 μm severely limits the Jsc. Our optimized Al corrugation manages to compensate net 9% and 7% Jsc loss in 2-μm Si in respect to 10-μm-thick Si for the model with and without front Si NCs. The results seem to reveal the optimum design of rear Al corrugation for the absorption enhancement from UV up to NI wavelength region.

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.

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.

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%.

Numerical simulation of the impact of design parameters on the performance of back-contact back-junction solar cell

This work presents a study based on electro-optical numerical simulations of the impact of geometrical and doping parameters on main figures of merit of crystalline silicon back-contact back-junction solar cells. State-of-the-art physical models in combination with two-dimensional simulations performed by a TCAD tool have been adopted to carry out an extensive and detailed analysis of the influence of many fabrication parameters on performance. The studied design parameters are the doping level in front surface field (FSF), back surface field (BSF) and emitter, and the main geometrical parameters. A doping level value that allows the maximization of the efficiency for the three regions can be clearly identified. In particular, for BSF and emitter, an efficiency degradation is observed for relatively lower doping values and is ascribed to the higher contact recombination while for higher doping values the Auger recombination plays a significant role in reducing the ultimate efficiency. In FSF region the recombination due to defects at the front interface is the main limiting mechanisms for efficiency. On the basis of our analysis, a marked sensitivity of the efficiency to the gap and pitch size is caused by the series resistance increase. The efficiency exhibits a maximum value for an emitter coverage fraction (R) of 85 %. However, in the case of lower emitter coverage, Auger, Shockley---Read---Hall (SRH): in bulk and at interfaces are detrimental for the cell conversion efficiency.

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.

Electrochemical Contact Separation for PVD Aluminum Back Contact Solar Cells

This work deals with a possibility to simplify the processing of back-contact back-junction solar cells. A novel metallization process without using any additional mask is presented. The main focus is set on the contact separation resulting in the interdigitated metal pattern. In case of using evaporated aluminum as contacting material to silicon, aluminum anodizing is a convenient process to convert electrically conductive aluminum to electrically isolating aluminum oxide. In the established processes in which anodizing of aluminum is used for aluminum structuring, masks are used to achieve local anodized areas. In order to make the contact separation process by anodizing more economic, several in-situ anodizing processes either using structured processing units or printing techniques are developed and tested.

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.

Zincate processes for silicon solar cell metallization

Abstract To allow plating on aluminum, zincate processes are introduced in metallization of silicon solar cells. Different zincate solutions are investigated regarding their adhesion on typical solar cell surface morphologies. Sufficient adhesion (>1.5 N⧸mm) on PVD aluminum layers is demonstrated by zincate processes with subsequent plating of nickel, copper and silver on both random pyramid and damage etched surfaces. Small-size (4 cm²) back-contact back-junction solar cells with an aluminum or aluminum–silicon seed layer are successfully processed in this way. On screen printed aluminum, adhesion has been found to be below 1 N⧸mm. Microstructural investigation of the failure mode reveals that the aluminum peels off the silicon surface in this case.

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.

High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells

In the ALBA-II project, Q-Cells SE, Bitterfeld-Wolfen, Germany, and the Institute for Solar Energy Research Hamelin, Emmerthal, Germany, are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralski (Cz) method forms the basis of this high-efficiency solar cell development as it offers high bulk carrier lifetimes. The EWT device structure allows us to employ a simplified process sequence compared with interdigitated back-contact back-junction solar cells. High open-circuit voltages of our solar cells are achieved by different passivation layers for base and emitter surfaces and picosecond laser ablated contact openings. An optimization of the resistances along the current paths in base and emitter leads to an improvement in fill factor (FF) over former EWT solar cells. Together with the inherently high current densities of EWT solar cells, we achieve on our small-area (4-cm2, designated area without busbars) cells a short-circuit current density JSC of 40.4 mA/cm2, an open-circuit voltage VOC of 661 mV, FFs well above 80%, and, thus, cell efficiencies of up to 21.6%.

19.7% Efficient All-Screen-Printed Back-Contact Back-Junction Silicon Solar Cell With Aluminum-Alloyed Emitter

A back-contact back-junction solar cell on n-type silicon with an aluminum-alloyed emitter is introduced, where both structuring and metallization are realized by screen-printing. The process sequence for realizing the cell with a pitch of 2 mm is displayed and described. The cell parameters are shown for three different emitter coverages on the rear side (45%, 58%, and 72%). An analysis of the saturation current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ο</sub> of the different areas was carried out on lifetime samples, and the calculated V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> is compared to the measured one. The potentially critical edge of the Al finger is analyzed in a scanning electron microscopy cross section. No damaging of the aluminum paste to the passivation layer and a homogenous p+-layer over the whole finger width is observed. A conversion efficiency of 19.7% is presented for a cell with an aperture area of 16.65 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> that was exclusively processed in Fraunhofer ISE PV-TEC, which is an industrial-like fabrication environment.

Results on n-type IBC solar cells using industrial optimized techniques in the fabrication processing

Abstract Back-contact back-junction solar cell has the potential for high efficiency energy conversion due to the distinctive architecture of the device. The fabrication processing for these types of cells requires high material quality and complicated processing technology involving many masking and alignments steps which results in the increase of the manufacturing costs. Within the LIMA EU Project, we developed a novel process architecture for Interdigited-Back-Contact (IBC) front surface field (FSF) solar cells obtained by optimization of the fabrication process using only industrially feasible technology (i.e. screen-printing, laser ablation and conventional diffusion processes). With this process we have obtained cell efficiencies above 19% on n-type silicon 2x2 cm2 float zone (FZ) substrate. Moreover, we have integrated this process with innovative methodology which opens new possible solution to the already well established techniques. This approach allowed us to improve the front side with excellent proprieties of passivation and conductivity and to implement interdigitated phosphorous back surface field (BSF) and boron emitter in a single mask process. The solar cells results of this improved front side are presented in comparison to the solar cells with IBC-FSF architecture.

Back-contact back-junction silicon solar cells under UV illumination

The performance of n-type Si back-contact back-junction (BC-BJ) solar cells under illumination with high energy ultraviolet (UV) photons was investigated. The impact of the phosphorus doped front surface field (FSF) layer on the stability of the front surface passivation under UV illumination was investigated. Lifetime samples and solar cells without the front surface field showed a significant performance reduction when exposed to ultraviolet light. The surface saturation current density ( J 0e) increased from 48 to 446 fA/cm 2 after the UV exposure. At the same time the efficiency of the BC-BJ solar cells without the FSF diffusion reduced from 19.8% to 14.3%. In contrast to the lifetime samples and solar cells without the FSF diffusion, the tested n +nn + structures and the BC-BJ solar cells with applied FSF diffusion profiles were significantly more stable under UV exposure, i.e. J 0e increased only by a factor of 25% and the efficiency of these cells decreased only 0.3% abs by the UV illumination. Finally it was shown that the performance of the UV-degraded solar cells without FSF could be improved during a forming gas anneal (FGA). Due to application of FGA the efficiency almost fully recovered from 14.3% to 19.6%.

Enhanced lateral current transport via the front N+ diffused layer of n‐type high‐efficiency back‐junction back‐contact silicon solar cells

AbstractN‐type back‐contact back‐junction solar cells were processed with the use of industrially relevant structuring technologies such as screen‐printing and laser processing. Application of the low‐cost structuring technologies in the processing of the high‐efficiency back‐contact back‐junction silicon solar cells results in a drastic increase of the pitch on the rear cell side. The pitch in the range of millimetres leads to a significant increase of the lateral base resistance. The application of a phosphorus doped front surface field (FSF) significantly reduces the lateral base resistance losses. This additional function of the phosphorus doped FSF in reducing the lateral resistance losses was investigated experimentally and by two‐dimensional device simulations. Enhanced lateral majority carrier's current transport in the front n+ diffused layer is a function of the pitch and the base resistivity. Experimental data show that the application of a FSF reduces the total series resistance of the measured cells with 3.5 mm pitch by 0.1 Ω cm2 for the 1 Ω cm base resistivity and 1.3 Ω cm2 for the 8 Ω cm base resistivity. Two‐dimensional simulations of the electron current transport show that the electron current density in the front n+ diffused layer is around two orders of magnitude higher than in the base of the solar cell. The best efficiency of 21.3% was obtained for the solar cell with a 1 Ω cm specific base resistivity and a front surface field with sheet resistance of 148 Ω/sq. Copyright © 2008 John Wiley &amp; Sons, Ltd.