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.     

Opto‐electrical modelling and optimization study of a novel IBC c‐Si solar cell

Interdigitated back contact (IBC) crystalline silicon (c‐Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD‐based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo‐junction IBC c‐Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto‐electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front‐side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo‐junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero‐IBC cell). Simulation results show that a not‐smoothed pyramid‐textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low‐doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%_abs as compared with the reference IBC solar cell. Further, by improving both emitter and c‐Si bulk quality, a 22.84% efficient solar cell for 280‐μm thick c‐Si bulk was simulated. Copyright © 2017 John Wiley & Sons, Ltd.     

A 3‐in‐1 doping process for interdigitated back contact solar cells exploiting the understanding of co‐diffused dopant profiles by use of PECVD borosilicate glass in a phosphorus diffusion

Boron and phosphorus doping of crystalline silicon using a borosilicate glass (BSG) layer from plasma‐enhanced chemical vapor deposition (PECVD) and phosphorus oxychloride diffusion, respectively, is investigated. More specifically, the simultaneous and interacting diffusion of both elements through the BSG layer into the silicon substrate is characterized in depth. We show that an overlying BSG layer does not prevent the formation of a phosphorus emitter in silicon substrates during phosphorus diffusion. In fact, a BSG layer can even enhance the uptake of phosphorus into a silicon substrate compared with a bare substrate. From the understanding of the joint diffusion of boron and phosphorus through a BSG layer into a silicon substrate, a model is developed to illustrate the correlation of the concentration‐dependent diffusivities and the emerging diffusion profiles of boron and phosphorus. Here, the in‐diffusion of the dopants during diverse doping processes is reproduced by the use of known concentration dependences of the diffusivities in an integrated model. The simulated processes include a BSG drive‐in step in an inert and in a phosphorus‐containing atmosphere. Based on these findings, a PECVD BSG/capping layer structure is developed, which forms three different n⁺⁺−, n⁺− and p⁺−doped regions during one single high temperature process. Such engineered structure can be used to produce back contact solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Understanding of the annealing temperature impact on ion implanted bifacial n‐type solar cells to reach 20.3% efficiency

Ion implantation has the advantage of being a unidirectional doping technique. Unlike gaseous diffusion, this characteristic highlights strong possibilities to simplify solar cell process flows. The use of ion implantation doping for n‐type PERT bifacial solar cells is a promising process, but mainly if it goes with a unique co‐annealing step to activate both dopants and to grow a SiO₂ passivation layer. To develop this process and our SONIA cells, we studied the impact of the annealing temperature and that of the passivation layers on the electrical quality of the implanted B‐emitter and P‐BSF. A high annealing temperature (above 1000 °C) was necessary to fully activate the boron atoms and to anneal the implantation damages. Low J_0BSF (BSF contribution to the saturation current density) of 180 fA/cm² was reached at this high temperature with the best SiO₂ passivation layer. An average efficiency of 19.7% was reached using this simplified process flow (“co‐anneal process”) on large area (239 cm²) Cz solar cells. The efficiency was limited by a low FF, probably due to contaminations by metallization pastes. Improved performances were achieved in the case of a “separated anneals” process where the P‐BSF is activated at a lower temperature range. An average efficiency of 20.2% was obtained in this case, with a 20.3% certified cell. Copyright © 2014 John Wiley & Sons, Ltd.     

A novel strategy for preparing a selective back surface field of n-type emitter wrap through solar cells

In the present study, we have developed a novel mixed co-diffusion (MCD) process by which to prepare a selective back surface field (BSF) of n-type emitter wrap through (EWT) solar cells, which combines a plasma enhanced chemical vapor deposition (PECVD) phosphorus-doped n-type microcrystalline silicon as the dopant source, with a low temperature thermal oxidation (LTO) process. Through comparison between our MCD process and a standard co-diffusion (SCD) process, a BSF with a shallow doping depth of 0.56 µm and high doping concentration of 1.9×10²⁰ at/cm³ is easily obtained by the MCD process under the low temperature of 750 ℃. Therefore, the MCD process is shown to reduce the number of high temperature processes, which cannot produce dopant redistribution, and can accurately control the doping concentrations and depths of the BSF and emitter. In addition, the novel method also eliminates the boron-rich layer, which induces misfit dislocations and bulk lifetime degradation, without extra chemical treatment. Therefore, the MCD process' open circuit voltage, short circuit current density, conversion efficiency and fill factor of the solar cells are respectively increased by 7 mV, 6 mA/cm², 2% and 2%. These results indicate that the MCD process is a novel and potential agent for the SCD process.     

Boron‐doped selective silicon epitaxy: high efficiency and process simplification in interdigitated back contact cells

The present research and development activities in crystalline silicon photovoltaics include the exploration of doping technologies alternative to the mainstream diffusion process. The goal is to identify those technologies with potential to increase the solar cell efficiency and reduce the cost per watt peak. In that respect, this work presents the selective epitaxial growth of silicon as a candidate for boron doping; showing the results of the evaluation of boron‐doped silicon epitaxial emitters on slurry and diamond‐coated wire‐sliced Czochralski material, their integration in interdigitated back contact solar cells, and the development of a novel process sequence to create the interdigitated rear junction of these devices using selective epitaxial growth. Boron‐doped silicon epitaxy is demonstrated to perform in the high efficiency range (>22%), and the use of selective epitaxial growth is proposed as a route for the simplification of the interdigitated back contact solar cell flow. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimization of ultraviolet laser doping for crystalline silicon solar cells with a novel segmented selective emitter design

This paper reports on the use of ultraviolet laser for forming segmented selective emitters on POCl ₃ n ⁺ –p–p ⁺ solar cells. Laser scan speed, pulse power, and repetition rate are optimized to minimize laser‐induced defects, which are found to enhance recombination and reduce the local open‐circuit voltage. Laser‐doped selective emitters formed by locally driving in additional phosphorous from the diffusion glass are well suited for an etchback process without the need for a mask. In this paper, we show a novel selective emitter design that is segmented instead of continuous, combined with an emitter etchback process gives an efficiency improvement of about 0.3% absolute over a standard industrial type solar cell and 0.2% absolute improvement over a non‐segmented selective emitter solar cell. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigations of parasitic shunt resistance in n‐type buried contact solar cells

It has been shown that n‐type laser‐grooved buried contact solar cells exhibit a high‐efficiency potential, both on interdigitated backside buried contact (IBBC) and double‐sided buried contact (DSBC) cell structures. As the IBBC solar cell contains heavily doped, compensated regions, the shunt mechanisms are more complicated, and are different from those of the conventional front‐collecting‐junction solar cells. In this paper, several shunting mechanisms hindering the performances of the n‐type buried contact solar cells are investigated and discussed. The main shunting routes in n‐type IBBC solar cells are concluded as follows: (1) the emitter contact metal touching the n‐type substrate, which is either due to nonuniform boron deposition or diffusion‐induced misfit dislocations; (2) the base contact metal touching the p⁺ emitter, attributed to either the phosphorus groove diffusion being unable to compensate for the boron emitter diffusion, or the junction depth located in the diffusion overlap regions not deep enough to prevent nickel from spiking through the groove diffusion. The shunt resistance of the IBBC cells increased by more than two orders of magnitude after eliminating the shunt mechanisms discussed in this study. This led to an improvement in fill factor from 0·71–0·73 to 0·74–0·76, and an increase of average absolute efficiency of more than 0·65%. Copyright © 2005 John Wiley & Sons, Ltd.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Thin Al2O3 passivated boron emitter of n‐type bifacial c‐Si solar cells with industrial process

We have presented thin Al₂O₃ (~4 nm) with SiN_x:H capped (~75 nm) films to effectively passivate the boron‐doped p⁺ emitter surfaces of the n‐type bifacial c‐Si solar cells with BBr₃ diffusion emitter and phosphorus ion‐implanted back surface field. The thin Al₂O₃ capped with SiN_x:H structure not only possesses the excellent field effect and chemical passivation, but also establishes a simple cell structure fully compatible with the existing production lines and processes for the low‐cost n‐type bifacial c‐Si solar cell industrialization. We have successfully achieved the large area (238.95 cm²) high efficiency of 20.89% (front) and 18.45% (rear) n‐type bifacial c‐Si solar cells by optimizing the peak sintering temperature and fine finger double printing technology. We have further shown that the conversion efficiency of the n‐type bifacial c‐Si solar cells can be improved to be over 21.3% by taking a reasonable high emitter sheet resistance. Copyright © 2017 John Wiley & Sons, Ltd.     

Diffusion‐based model of local Al back surface field formation for industrial passivated emitter and rear cell solar cells

In this work, the back surface field (BSF) formation of locally alloyed Al‐paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time‐dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full‐area Al‐BSF; therefore, a diffusion‐based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X‐ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation. Copyright © 2013 John Wiley & Sons, Ltd.     

Co‐optimization of emitter profile and metal grid of selective emitter silicon solar cells

Co‐optimization of the metallization and emitter dopant profile is fully investigated for selective emitter crystalline silicon solar cells. The simulation parameters for the laser doping selective emitter, metallization by plating, silicon nitride passivation, and aluminum back surface field are identified and reached. Internal light flux reflection is also considered in the model. In particular, the influence of the increased light trapping ability of a textured surface on the optimization results is clarified by comparing a cell with a non‐textured surface. In this paper, the optimization results, including the electrical performances of a solar cell are discussed in detail. On the basis of these simulation results, an optimized metallization and emitter dopant profile is proposed. Copyright © 2011 John Wiley & Sons, Ltd.     

背接触太阳电池的背面设计优化

利用quokka3仿真软件建立三维模型,对n型叉指背接触(IBC)单晶硅太阳电池的单元电池结构设计和栅线参数进行了仿真优化,并通过激光和丝网印刷进行了实验验证.实验结果表明,在不同IBC单元电池结构设计下,当p+发射区与n+背表面场区的宽度比值为4时,IBC太阳电池效率比宽度比值为2.3时的高0.11％.可通过减小单元...     

Bulk resistivity optimization for low‐bulk‐lifetime silicon solar cells

Guidelines are presented which are designed to achieve planar solar cell efficiencies as high as 17.5% using existing fabrication technologies and silicon substrates with lifetimes as low as 20 μs. Device simulations are performed to elucidate the need and impact of base doping optimization for different back‐surface passivation schemes, cell thicknesses, emitter profiles, and degrees of dopant–defect interaction. Results indicate that optimal resistivity is a function of back‐surface passivation, with the aluminum back‐surface field (BSF) requiring the highest resistivity, the oxide/nitride stack passivation excelling at an intermediate resistivity, and the ohmic contact needing the lowest resistivity. A comparison of simulated 300 and 100 μm cells shows that thinner cells magnify the differences in optimal resistivity for the three back‐surface passivation schemes. A lifetime model is used to account for dopant–defect interaction that can lower bulk lifetime at higher doping levels. It is demonstrated that cell efficiency decreases and optimal resistivity increases at higher levels of dopant–defect interaction. Simulated devices with an optimized base doping showed an efficiency improvement of as much as 2% (absolute) compared with identical devices with a typical base doping level (1.6 or 1.8 Ω cm) and bulk lifetime of 20 μs. Copyright © 2001 John Wiley & Sons, Ltd.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of high‐efficiency large‐area screen‐printed solar cells on direct kerfless epitaxially grown monocrystalline Si wafer and structure

This paper demonstrates the potential of epitaxially grown Si wafers with doped layers for high‐efficiency solar cells. Boron‐doped 239 cm² 180–200 µm thick 2 Ω‐cm wafers were grown with and without 15 µm thick p⁺ layer, with a doping in the range of 10¹⁷~10¹⁸ cm⁻³. A layer transfer process involving porous Si layer to lift off epi‐Si wafers from the reusable substrate was used. The pp⁺ wafers were converted into n⁺pp⁺ passivated emitter rear totally diffused (PERT) cells by forming an oxide‐passivated POCl₃‐diffused n⁺ emitter at the front, and oxide/nitride‐passivated epitaxially grown p⁺ BSF at the entire back, with local screen‐printed contacts. To demonstrate and quantify the benefit of the epi‐grown p⁺ layer, standard passivated emitter and rear cells (PERCs) with local BSF and contacts were also fabricated on p‐type epi‐Si wafers as well on commercial‐grade Cz wafers. Sentaurus 2D device model was used to assess the impact of the epi‐grown p⁺ layer, which showed an efficiency gain of ~0.5% for this PERT structure over the traditional PERC. This was validated by the cell results, which showed an efficiency of ~20.1% for the PERC, and ~20.3% for the PERT cell using epi‐Si wafers. Experimental data showed higher FF in PERT cells, largely because of the decrease in lateral resistance on the rear side. Efficiency gain, a result of higher FF, was greater than the recombination loss in the p⁺ layer because of the lightly doped thick p⁺ epi‐grown region used in this study. Copyright © 2016 John Wiley & Sons, Ltd.     

High efficiency large area n‐type front junction silicon solar cells with boron emitter formed by screen printing technology

In this paper, we report on commercially viable screen printing (SP) technology to form boron emitters. A screen‐printed boron emitter and ion‐implanted phosphorus back surface field were formed simultaneously by a co‐annealing process. Front and back surfaces were passivated by chemically grown oxide capped with plasma‐enhanced chemical vapor deposition silicon nitride stack. Front and back contacts were formed by traditional SP and firing processes with silver/aluminum grid on front and local silver back contacts on the rear. This resulted in 19.6% efficient large area (239 cm²) n‐type solar cells with an open‐circuit voltage V_oc of 645 mV, short‐circuit current density J_sc of 38.6 mA/cm², and fill factor of 78.6%. This demonstrates the potential of this novel technology for production of low‐cost high‐efficiency n‐type silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

An optimized rapid aluminum back surface field technique forsilicon solar cells

Screen-printing and rapid thermal annealing have been combined to achieve an aluminum-alloyed back surface field (Al-BSF) that lowers the effective back surface recombination velocity (S_eff) to approximately 200 cm/s for solar cells formed on 2.3 Ω-cm Si. Analysis and characterization of the BSF structures show that this formation process satisfies the two main requirements for achieving low S_eff: (1) deep p⁺ regions and (2) uniform junctions. Screen-printing is ideally suited for fast deposition of thick Al films which, upon alloying, result in deep BSF regions. Use of a rapid alloying treatment is shown to significantly improve the BSF junction uniformity and reduce S_eff. The Al-BSFs formed by screen-printing and rapid alloying have been integrated into both laboratory and industrial-type fabrication sequences to achieve solar cell efficiencies in excess of 19.0 and 17.0%, respectively, on planar 2.3 Ω-cm float zone Si. For both process sequences, these cell efficiencies are 1-2% (absolute) higher than analogous cells made with unoptimized Al-BSFs or highly recombinative rear surfaces     

Influence of boron clustering on the emitter quality of implanted silicon solar cells: an atom probe tomography study

The use of ion implantation doping instead of the standard gaseous diffusion is a promising way to simplify the fabrication process of silicon solar cells. However, difficulties to form high‐quality boron (B) implanted emitters are encountered when implantation doses suitable for the emitter formation are used. This is due to a more or less complete activation of Boron after thermal annealing. To have a better insight into the actual state of the B distributions, we analyze three different B emitters prepared on textured Si wafers: (1) a BCl₃ diffused emitter and two B implanted emitters (fixed dose) annealed at (2) 950°C and at (3) 1050°C (less than an hour). Our investigations are in particular based on atom probe tomography, a technique able to explore 3D atomic distribution inside a material at nanometer scale. Atom probe tomography is employed here to characterize B atomic distribution inside textured Si solar cell emitters and to quantify clustering of B atoms. Here, we show that implanted emitters annealed at 950 °C present maximum clusters due to poor solubility at lower temperature and also highest emitter saturation current density (J_0e = 1000 fA/cm²). Increasing the annealing temperature results in greatly improved J_0e (131 fA/cm²) due to higher solubility and a consequently lower number of clusters. BCl₃ diffused emitters do not contain any B clusters and presented the best emitter quality. From our results, we conclude that clustering of B atoms is the main reason behind higher J_0e in the implanted boron emitters and hence degraded emitter quality. Copyright © 2015 John Wiley & Sons, Ltd.