Back-junction Silicon Solar Cells Research Articles

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Screen printed, large area bifacial N-type back junction silicon solar cells with selective phosphorus front surface field and boron doped poly-Si/SiOx passivated rear emitter

This paper reports on the effect of screen printed metallization on the passivation quality of a boron doped poly-Si/SiOx passivated contact (PC) structure composed of a very thin Si oxide (∼15 Å) capped with boron doped poly-Si. Our boron doped poly-Si/SiOx passivated contact (p-Poly Si/SiOx PC) with a SiNx capping layer gave excellent surface passivation with a very low saturation current density of ∼5 fA/cm2. After screen printed metallization on poly-Si with a metal coverage of ∼10%, this value increased to ∼17 fA/cm2. This paper also demonstrates the fabrication of screen printed, large area (239 cm2), high efficiency (∼21%) n-base bifacial back junction Si solar cells with p-Poly-Si/SiOx PC on the rear and a phosphorus implanted n++-n+ selective front surface field. Detailed analysis is performed to quantify recombination and extract the saturation current density contributions (J0) from each layer of the cell including the metallized front surface field and the tunnel oxide passivated contact. Finally, 2D device modeling of this back junction cell is performed by implementing a simple approach which replaces the p-Poly-Si/SiOx PC by an equivalent p-n junction with the same J0 and gives a good match between the measured and simulated cell parameters using the extracted J0 and recombination velocity values.

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.

Analysis of full-wafer size co-diffused BCBJ silicon solar cells with a novel screen printed boron-doping paste

This work presents an analysis of co-diffused back-contact back-junction silicon solar cells, fabricated by utilizing screen printed boron-doping paste. The feasibility of this paste has been shown in earlier work by the authors. Due to its promising performance, comparable to earlier used boron-doped silicate glass coatings, a proof-of-principle cell run was performed yielding, several small devices with various unit cell geometries and full-wafer size devices. The achieved mean conversion efficiency of nine small devices was η = 20.9 %, while the best full-wafer size device achieved η = 20.6 %. The cells are analyzed with a focus on the significant difference between small and large cells in short circuit density (ΔJSC = -4%) and the current limitations of the built cells assessed by numerical simulations. A set of improved parameters derived from earlier work at Fraunhofer ISE suggests an efficiency potential exceeding η = 23 %. The short- and long-term measures necessary to realize this potential are described in this paper.

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.

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

Loss Analysis of n-Type Passivated Emitter Rear Totally Diffused Back-Junction Silicon Solar Cells with Efficiencies up to 21.2%

In this work, we present screen-printed n-type passivated emitter rear totally diffused (n-PERT) back-junction (BJ) silicon solar cells with efficiencies up to 21.2% on total area of 239 cm2. The process sequence is based on that of p-type passivated emitter and rear cells (p-PERC), adding only a boron diffusion at the beginning. We reduce the recombination at the homogeneous phosphorus-doped front surface field by a wet-chemical etch-back of 10–20 nm and apply an advanced five-busbar layout on the front side to increase the energy conversion efficiency. We simulate the performance of the n-PERT BJ solar cell using the conductive boundary model and perform a synergistic efficiency gain analysis to identify the main limitations of our n-PERT BJ solar cells. We observe the biggest gain of 0.72% absolute after eliminating recombination at the P-doped front surface field and find that reducing recombination in general is most important for further improving our n-PERT BJ solar cells.

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%.

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.

High efficiency back-contact back-junction thin-film monocrystalline silicon solar cells from the porous silicon process

This work demonstrates the fabrication of a 45 μm thick back-contact back-junction thin-film monocrystalline silicon solar cell from the porous silicon process with an energy conversion efficiency of 18.9%. We demonstrate an efficiency improvement of 5.4% absolute compared to our prior record of 13.5% for back-contact back-junction thin-film monocrystalline silicon solar cells. This increase in efficiency is achieved by reducing the recombination at the base contact using a back surface field and by increasing the generation with a front texture. We investigate the loss mechanisms in the cell using finite element simulations. A free energy loss analysis based on experiments and simulations determines the dominating loss mechanisms. The efficiency loss by base recombination is 0.8% absolute and the loss by base contact recombination is 0.5% absolute in the 18.9% efficiency cell.

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.

Increased Front Surface Recombination by Rear-Side Laser Processing on Thin Silicon Solar Cells

We show the degradation of the front surface passivation by rear-side laser processing of thin silicon solar cells when using a laser with a pulse length of 8 ps. 45-μm-thick back-contact back-junction monocrystalline silicon solar cells show an energy conversion efficiency of 18.8% without rear-side laser processing, whereas they show only 7.5% with an additional rear-side laser process step for contact separation. This low efficiency is due to the degradation of the front surface passivation, which is confirmed by quantum efficiency measurements. The internal quantum efficiency at short wavelength is 0.88 without laser processing, whereas it is only 0.33 with the rear-side laser process step.

前瞻晶体硅太阳能电池未来产业化——高效N型背结前接触和背结背接触晶体硅太阳能电池

In this article, high energy-conversion-efficiency N-type back-junction front-contact and back-junction back-contact mono-crystalline silicon solar cells are explored and reviewed in terms of their research and mass-production prospects. The basic physical mechanism of such two types of high-efficiency N-type solar cells is elaborated. Moreover, the current research on N-type IBC solar cells with screen-printed Aluminum belt-firing p+-type emitter is critically addressed. It is considered to be one of the most promising candidates for the evolution of the commercialized P-type crystalline-silicon solar cells.

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.

Short-Circuit Current Losses in Back-Contacted Back–Junction Si Solar Cells: Experiment and Simulation of the Charge Collection Probability

The influence of recombination losses on the short-circuit current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> of back-contacted back-junction silicon solar cells has been investigated by experiments and simulation. The J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> losses are analyzed by 2-D simulations of the charge collection probability f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> using the reciprocity theorem. For the investigations, Gaussian-like emitter, back-surface field, and front-surface field diffusion profiles, as well as a sophisticated optical model, have been applied. Thus, experimental and simulated results of solar cells with different rear-side dimensions, i.e., pitch distance, and base resistivities could be directly compared. The diffusion profiles, the bulk charge carrier lifetime, and the surface recombination velocities of the diffused regions have been determined experimentally and used as input parameters for the simulations. It has been found that the simulated results are in good agreement with the experimental ones. The measured local and global external quantum efficiency of the solar cells could be well reproduced by the simulations. Hence, the presented simulation model provides a powerful tool to adapt the rear-side dimensions and to optimize the diffusion profiles of back-contacted back-junction silicon solar cells in order to reduce recombination losses and achieve high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> values.

Loss analysis of back-contact back-junction thin-film monocrystalline silicon solar cells

We investigate power losses in back-contact back-junction monocrystalline thin-film silicon solar cells fabricated using the porous silicon layer transfer process. Our loss analysis combines two-dimensional finite element modeling and resistance network simulations. The input parameters of the finite element modeling are determined experimentally by measuring saturation current densities and sheet resistances on test samples prepared identically to the solar cells. Characteristic solar cell parameters such as short circuit current, open circuit voltage, fill factor, and efficiency of measured and network simulated current voltage characteristics investigated in this study match within an uncertainty of 5%. Free energy loss analysis serves as comparison of all losses in units of power per area at the maximum power point. The largest loss is bulk recombination due to a carrier lifetime of 2 μs in the epitaxial Si layer. Further significant losses result from recombination at the base contacts characterized by a diode saturation current density of 50 000 fA cm−2 as well as resistive losses due to lateral majority carrier current flows within the solar cell base and contact resistance losses.

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.

Investigation of electrical shading effects in back-contacted back-junction silicon solar cells using the two-dimensional charge collection probability and the reciprocity theorem

In this study, short-circuit current losses in high-efficiency n-type back-contacted back-junction silicon solar cells caused by the electrical shading effect have been investigated by two-dimensional simulations of the charge carrier collection probability. Based on the reciprocity theorem, the homogenous partial differential equation describing the probability of charge carriers being collected by the p-n junction on the rear side of the solar cell has been solved numerically using the finite element method implemented in the partial differential equation solver COMSOL Multiphysics. The method has been applied to study the impact of geometrical parameters of the solar cell, such as the pitch distance, as well as the emitter and back surface field width, on the local and global internal quantum efficiency and on the short-circuit current density. The influence of the rear surface recombination velocity of an undiffused gap and the effective rear surface recombination velocity of the back surface field region on the short-circuit current density is also presented. It has been found that the width and the surface recombination velocity of the undiffused gap on the rear side of the solar cell have a strong impact on the charge collection probability in the base. Thus, the surface recombination velocity of the undiffused gap has to be minimized or the undiffused gap has to be reduced or omitted completely in order to increase the short-circuit current density significantly. Furthermore, it has been found that low base doping concentrations are essential for minimizing the effective rear surface recombination velocity of the back surface field. It has also been shown that in the case of low base doping concentrations, the short-circuit current density reaches its maximum value and that in this case it is nearly independent of the back surface field doping concentration.