Screen‐printed Emitter‐Wrap‐Through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter‐Wrap‐Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen‐printing technology. Via‐holes are created by an industrially applicable high‐speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via‐holes and the rear side, allowing for a low via‐hole resistivity as well as a low resistivity contact to screen‐printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT‐cell structure can be achieved. The screen‐printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD‐anti‐reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique. Copyright © 2010 John Wiley & Sons, Ltd.     

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

N‐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 & Sons, Ltd.     

19% efficient n‐type Czochralski silicon solar cells with screen‐printed aluminium‐alloyed rear emitter

High and stable lifetimes recently reported for n‐type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n‐type Si wafers for manufacturing simple and industrially feasible high‐efficiency solar cells, we focus on back junction n⁺np⁺ solar cells featuring an easy‐to‐fabricate full‐area screen‐printed aluminium‐alloyed rear p⁺ emitter. Independently confirmed record‐high efficiencies have been achieved on n‐type phosphorus‐doped Czochralski‐grown silicon material: 18·9% for laboratory‐type n⁺np⁺ solar cells (4 cm²) with shadow‐mask evaporated front contact grid and 17·0% for front and rear screen‐printed industrial‐type cells (100 cm²). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.     

Back‐contacted back‐junction n‐type silicon solar cells featuring an insulating thin film for decoupling charge carrier collection and metallization geometry

In this study, back‐contacted back‐junction n‐type silicon solar cells featuring a large emitter coverage (point‐like base contacts), a small emitter coverage (point‐like base and emitter contacts), and interdigitated metal fingers have been fabricated and analyzed. For both solar cell designs, a significant reduction of electrical shading losses caused by an increased recombination in the non‐collecting base area on the rear side was obtained. Because the solar cell designs are characterized by an overlap of the B‐doped emitter and the P‐doped base with metal fingers of the other polarity, insulating thin films with excellent electrical insulation properties are required to prevent shunting in these overlapping regions. Thus, with insulating thin films, the geometry of the minority charge carrier collecting emitter diffusion and the geometry of the interdigitated metal fingers can be decoupled. In this regard, plasma‐enhanced chemical vapor deposited SiO₂ insulating thin films with various thicknesses and deposited at different temperatures have been investigated in more detail by metal‐insulator‐semiconductor structures. Furthermore, the influence of different metal layers on the insulation properties of the films has been analyzed. It has been found that by applying a SiO₂ insulating thin film with a thickness of more than 1000 nm and deposited at 350 °C to solar cells fabricated on 1 Ω cm and 10 Ω cm n‐type float‐zone grown silicon substrates, electrical shading losses could be reduced considerably, resulting in excellent short‐circuit current densities of more than 41 mA/cm² and conversion efficiencies of up to 23.0%. Copyright © 2012 John Wiley & Sons, Ltd.     

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

In this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN_x, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO₂/SiN_x passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO₂/SiN_x passivation layers, without inducing appreciable damage or defects on the surface of the n⁺ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 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.     

Tracking and back‐tracking

This paper presents a review of back‐tracking geometry not only for single axis but also for two‐axis tracking and analyses the corresponding energy gains. It compares the different back‐tracking strategies with the ideal tracking in terms of energy yield concluding, on the one hand, that back‐tracking is more useful for single horizontal axis than for the single vertical one, and on the other hand, that back‐tracking is more efficient when applied in the primary axis of a two‐axis tracker. Copyright © 2011 John Wiley & Sons, Ltd.     

Back‐contact solar cells: a review

Ever since the first publications by R.J. Schwartz in 1975, research into back‐contact cells as an alternative to cells with a front and rear contact has remained a research topic. In the last decade, interest in back‐contact cells has been growing and a gradual introduction to industrial applications is emerging. The goal of this review is to present a comprehensive summary of results obtained throughout the years. Back‐contact cells are divided into three main classes: back‐junction (BJ), emitter wrap‐through (EWT) and metallisation wrap‐through (MWT), each introduced as logical descendents from conventional solar cells. This deviation from the chronology of the developments is maintained during the discussion of technological results. In addition to progress on manufacturing these cells, aspects of cell modelling and module manufacturing are discussed and an outlook towards industrial implementation is presented. Copyright © 2005 John Wiley & Sons, Ltd.     

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm² Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley & Sons, Ltd.     

High efficiency screen‐printed EFG Si solar cells through rapid thermal processing‐induced bulk lifetime enhancement

This paper shows that one second (1 s) firing of Si solar cells with screen‐printed Al on the back and SiN _x anti‐reflection coating on the front can produce a high quality Al‐doped back‐surface‐field (Al‐BSF) and significantly enhance SiN _x ‐induced defect hydrogenation in the bulk Si. Open‐circuit voltage, internal quantum efficiency measurements, and cross‐sectional scanning electron microscopy pictures on float‐zone silicon cells revealed that 1 s firing in rapid thermal processing at 750°C produces just as good a BSF as 60 s firing, indicating that the quality of Al‐BSF region is not a strong function of RTP firing time at 750°C. Analysis of edge‐defined film‐fed grown (EFG) Si cells showed that short‐term firing is much more effective in improving the hydrogen passivation of bulk defects in EFG Si. Average minority‐carrier lifetime in EFG wafers improved from ∼3 to ∼33 μs by 60 s firing but reached as high as 95μs with 1 s firing, resulting in 15·6% efficient screen‐printed cells on EFG Si. Copyright © 2004 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.     

Numerical simulations of buried emitter back‐junction solar cells

We recently introduced the buried emitter back‐junction solar cell, featuring large area fractions of overlap between n⁺‐type and p⁺‐type regions at the rear side of the device. In this paper we analyse the performance of the buried emitter solar cell (BESC) and its generalisations by one‐dimensional device simulations and analytical model calculations. A key finding is that the generalised versions of the BESC structure allows achieving very high efficiencies by passivating virtually the entire surface of p‐type emitters by an oxidised n‐type surface layer. A disadvantage of this type of full‐area emitter passivation in the generalised back‐junction BESC is the need for an insulating layer between the metallisation of the emitter and the contact to the base, which is technologically difficult to achieve. Copyright © 2009 John Wiley & Sons, Ltd.     

Large area tunnel oxide passivated rear contact n‐type Si solar cells with 21.2% efficiency

This paper reports on the implementation of carrier‐selective tunnel oxide passivated rear contact for high‐efficiency screen‐printed large area n‐type front junction crystalline Si solar cells. It is shown that the tunnel oxide grown in nitric acid at room temperature (25°C) and capped with n⁺ polysilicon layer provides excellent rear contact passivation with implied open‐circuit voltage iV_oc of 714 mV and saturation current density J_0b′ of 10.3 fA/cm² for the back surface field region. The durability of this passivation scheme is also investigated for a back‐end high temperature process. In combination with an ion‐implanted Al₂O₃‐passivated boron emitter and screen‐printed front metal grids, this passivated rear contact enabled 21.2% efficient front junction Si solar cells on 239 cm² commercial grade n‐type Czochralski wafers. 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.     

Progress with epitaxy wrap‐through crystalline silicon thin‐film solar cells

Wafer‐Equivalents are thin‐film solar cells that use a low‐cost silicon substrate to epitaxially grow a high‐quality crystalline silicon active layer. The epitaxy wrap‐through (EpiWT) cell is a back‐contact version of the Wafer‐Equivalent that aims to increase currents and gain other benefits of back contacts. The EpiWT cell can be made in a symmetrically interdigitated configuration with 50% back emitter coverage, or using an isolation layer to lower the back emitter coverage to ∼10%, which will theoretically increase voltages. The epitaxial deposition through via holes in the substrate depends on many factors, including the sealing of the deposition chamber, and produces various thicknesses and geometrical forms of the layers in the holes. An extended process has been developed to incorporate a passivated selective emitter and the first batch has been fabricated. The best result was an efficiency of 13.2% with ∼22 µm base layer thickness. The results are limited most by the fill factors at this stage, e.g. 75% for this cell, which is due to a processing difficulty encountered with screen‐printing in via holes. A new isolation layer was tested and successfully implemented for the low back‐emitter configuration. Comparable voltages and currents were achieved but the fill factors were lower than for the 50% back emitter cells, resulting in a best efficiency of 11.2%. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

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.     

Screen‐printed epitaxial silicon thin‐film solar cells with 13·8% efficiency

Amidst the different silicon thin‐film systems, the epitaxial thin‐film solar cell represents an approach with interesting potential. Consisting of a thin active c‐Si layer grown epitaxially on top of a low‐quality c‐Si substrate, it can be implemented into solar cell production lines without major changes in the current industrial process sequences. Within this work, ∼30‐μm‐thick epitaxial layers on non‐textured and highly doped monocrystalline Czochralski (Cz) and multicrystalline (mc) Si substrates have been prepared by CVD. Confirmed efficiencies of 13·8% on Cz and 12·3% on mc‐Si substrates have been achieved by applying an industrial process scheme based on tube and in‐line phosphorus diffusion, as well as screen‐printed front and back contacts fired through a SiN_x anti‐reflection coating. An extensive solar cell characterisation, including infrared lock‐in thermography and spectral response measurements is presented. Copyright © 2003 John Wiley & Sons, Ltd.     

An efficient network‐coding based back‐pressure scheduling algorithm for wireless multi‐hop networks

Back‐pressure scheduling has been considered as a promising strategy for resource allocation in wireless multi‐hop networks. However, there still exist some problems preventing its wide deployment in practice. One of the problems is its poor end‐to‐end (E2E) delay performance. In this paper, we study how to effectively use inter‐flow network coding to improve E2E delay and also throughput performance of back‐pressure scheduling. For this purpose, we propose an efficient network coding based back‐pressure algorithm (NBP), and accordingly design detailed procedure regarding how to consider coding gain in back‐pressure based weight calculation and how to integrate it into next hop decision making in the NBP algorithm. We theoretically prove that NBP can stabilize the networks. Simulation results demonstrate that NBP can not only improve the delay performance of back‐pressure algorithm, but also achieve higher network throughput. Copyright © 2016 John Wiley & Sons, Ltd.