No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
High efficiency solar cell structures through the use of laser doping
... In case of laser doping, some experiments have been performed to form the full emitter by this technology [Est03]. Still, to this date laser doping is mostly applied for the selective junction formation [Tja07]. Thanks to the improvement of implanter tools, recently, this technology has demonstrated the capability to form junctions and get an excellent surface passivation [Ben12]. ...
... Depending on the technology chosen for the formation of the lowly doped emitter, there are alternatives for the selective doping around the metalized areas. Some examples include laser doping [Tja07,Wan12] or selective doping with inks [Ant10]. An interesting overview on this topic was published by Hahn in 2010 [Hah10]. ...
... Lasers are taking a wider role in other areas of the fabrication of silicon solar cells, either as a standard or as a developing technology; e.g. laser marking, laser texturing [Zol89], emitter formation by laser doping [Ame05,Tja07,Suw10], laser ablation of dielectric layers for front processing [Dub90,Kno09,Her10] or rear processing [Ago06], edge isolation [Sch04], laser fired contacts (LFC) [Sch02], or in laser-written front-side metallization [Roh85]. ...
During this thesis the seed and plate approach has been used for the front-side metallization of silicon solar cells. The potential of different metallization technologies for the seed layer formation have been evaluated. Laser writing was used as an innovative way to deposit metal contacts on silicon. A better understanding of the metal deposition by electroless plating on silicon wafers has been achieved and excellent results were obtained by combining this technique with alternative patterning methods for the front-side dielectrics. Further work is required to get these processes up to industrial standards.
... Furthermore, the presence of the SiO 2 beneath the SiN x film is known to provide relaxation of the SiN x stress [20]. Tjahjono et al. [15] compared the performance of laser doped solar cells with SiO 2 , plasma enhanced chemical vapor deposited (PECVD) SiN x and SiO 2 /SiN x . Other than the front passivation and ARC layers, the solar cell structures of these three groups are exactly the same. ...
... A low fill-factor FF in the SiN x group was assumed to be due to junction recombination/shunting, as more punctures were found in the SiN x group than in the SiO 2 group. The SiO 2 /SiN x group demonstrated the best overall performance, with good V oc , J sc , and FF [14], [15]. Further, the high-quality passivation of the SiO 2 /SiN x stack for both p + and n + surfaces [16] makes this stack suitable for laser doping applications mentioned previously. ...
... Further, the high-quality passivation of the SiO 2 /SiN x stack for both p + and n + surfaces [16] makes this stack suitable for laser doping applications mentioned previously. Sugianto et al. [13] investigated the influence of SiN x thickness (75 and 150 nm) in the SiO 2 /SiN x stacks on laser-doped solar cells by comparing illuminated current-voltage (J−V ) curves, using the same structures that Tjahjono et al. [15] used. It was found that the samples with thin SiN x (75 nm) offered a better performance in terms of FF and local ideality factor. ...
Laser doping of semiconductors has been the subject of intense research over the past decades. Previous work indicates that the use of SiO/SiN stacks instead of a single dielectric film as the anti-reflection coating and passivation layer results in laser doped lines with superior properties. In this paper, the impact of the SiN layer thickness in the SiO /SiN stacks on the properties of laser doped lines is investigated through resistance measurements of the laser doped line and the silicon-metal contact and the doping profile near the edge of the dielectric window, the latter being an important factor in determining the likelihood of high recombination or even shunting from the subsequent metallization process. Fundamentally, a problem of exposed and undoped silicon near the dielectric window is identified for most of the investigated parameter range. However, optimization of the laser parameters and dielectric film conditions is shown to be capable of preventing or at least minimizing this problem. The results indicate that for the used laser system, samples with thick dielectric stack processed using a low pulse energy and pulse distance yield the most favorable properties, such as low line resistance and low contact resistivity. Under these conditions, the laser doped regions laterally extend underneath the dielectric films, thus reducing the likelihood of high surface recombination.
... 1) Room temperature processing, eliminating the need for high-temperature furnaces and so reducing processing etching. The superiority of laser doping over conventional hightemperature tube furnace annealing has been demonstrated abundantly [1], [2], [3], [4], [5], [6], [7], [8], [9]. ...
... Performing the laser doping prior to dielectric deposition (directly on the silicon surface) can avoid this issue, as discussed in Section III-A. An alternative approach is to modify the dielectric stack to reduce the thermal expansion mismatch through the incorporation of a thin SiO 2 layer between the silicon and SiN x [1], [45], [119], [121]. ...
Laser-doped selective emitter diffusion techniques have become mainstream in solar cell manufacture covering 60% of the market share in 2022 and are expected to continue to grow to above 90% within the next five years (ITRPV). This was a very rapid uptake of technology, coming from only ∼10% penetration in 2018, and has enabled over 20 fA/cm
2
front recombination current reductions on the dominant passivated emitter and rear cell concepts in the same short period. In this article, a broad overview of key concepts in relation to laser doping methods relevant to solar cell manufacturing is given. We first discuss the basic mechanisms behind laser doping along with the benefits over conventional doping methods. The main laser doping approaches reported in the literature are then discussed, along with implications for metallization strategy, particularly in relation to selective emitter and back surface field formation in the dominant passivated emitter and rear cell technology. Different cell concepts that have benefited from the application of laser doping are also discussed. In the last section, we discuss the main defects induced by laser processing of silicon which affect the finished devices, potential and debated causes, as well as some commonly applied treatments for their mitigation.
... Silver consumption remains the highest non-silicon cost for commercial screen-printed silicon solar cells [1]. As such, the cost of producing PV devices can be reduced through the use of cheaper metallisation materials, such as nickel and copper, which can be plated to exposed silicon regions of the cell [2][3][4][5][6][7][8]. However, the transition to more widespread use of plated contacts has been slower than expected due to the continued improvement of commercially-available screenprinting technology [9][10][11] and difficulties in achieving adequate plated metal adhesion strength. ...
... The NiGD-treated samples were: (i) exposed to the NiGD solution for 5 min at 55°C; (ii) if applicable, sintered in a rapid thermal processing (RTP) furnace at 350°C for 1-10 min in a nitrogen gas (N 2 ) ambient; (iii) deglazed in 1% (w/v) HF; (iv) plated with MacDermaid 'Barrett SN' nickel sulphamate LIP solution for 2 min at 20°C using a plating current of 23 mA/cm 2 45°C and 40 mA/cm 2 . The LIP-only samples were processed with: (i) MacDermaid Barrett SN nickel sulphamate for 2 min at 23 mA/cm 2 and 20°C; (ii) sintered in an RTP furnace at 350°C for 5 min in N 2 ; (iii) exposed to aqua regia for 10 min to remove unreacted nickel and potential Kirkendall voids [41]; (iv) deglazed in 1% (w/v) HF for 30 s; ...
Nickel galvanic displacement (NiGD) plating to silicon surfaces allows for the deposition of self-limiting sub-micron nickel layers. This paper reports the use of NiGD plating as an adhesion-promoting seed layer for light-induced plated nickel and copper (LIP-NiCu) contacts on chemically-etched silicon surfaces. The improved adhesion, which is attributed to surface roughening caused by the oxidation and subsequent etching of silicon in the fluoride-containing electrolyte, is quantified by stylus-based scratch measurements and shown to increase with the duration of sintering at 350 °C. The width-normalised cut-off force for NiGD layers, sintered for 10 min, was approximated by a normal distribution with a mean of 114±32 N/mm. Unsintered NiGD and sintered LIP-only control samples, plated onto similar chemically-etched silicon surfaces, were insufficiently adherent to be measured. The poor adhesion of the unsintered NiGD contacts was attributed to the formation of voids and an oxide-rich interface layer during the galvanic displacement process. Although sintering at 350 °C appeared to reduce the thickness of the interfacial oxide and eliminate the voids, the oxide was not totally removed and contributed to the measured contact resistance of sintered NiGD-treated contacts of ~ 15 mΩ cm² being more than an order of magnitude higher than laser-ablated LIP-NiCu contacts on a similar n-type emitter surface. Adhesion priming layers formed using NiGD were used to metallise selective-emitter cells patterned using aerosol jet etching and having planarised contact regions. Although the LIP-NiCu contact grid was strongly adherent, the average area-normalised series resistance (Rs) was 0.96±0.18 Ω cm², the majority of which is attributed to high contact resistance arising from a residual interfacial oxide.
... New patterning methods such as inkjet printing and laser doping [3][4][5] enable the patterning of dielectric layers for metal contacting and therefore eliminate the need to use techniques such as photolithography which enabled the earlier laboratory implementations of these cells. Metal plating, which has been successfully used for the manufacture of photovoltaic devices in industry [6], is compatible with these patterning methods and consequently presents as a commercially viable metallization method. ...
... Ideally this would be achieved by spacing the contacts closer together; however, unless the contact regions are heavily doped to minimize the minority carrier concentration at the surface, the recombination in the device will be increased resulting in a lower device voltage. Future work can also focus on how to form doped rear contact regions, either through laser-fired contacts [2], laser doping [1,3], or local Al BSFs formed by firing Al through openings in a rear passivation layer. ...
For higher-efficiency solar cell structures, such as the Passivated Emitter Rear Contact (PERC) cells, to be fabricated in a manufacturing environment, potentially low-cost techniques such as inkjet printing and metal plating are desirable. A common problem that is experienced when fabricating PERC cells is low fill factors due to high series resistance. This paper identifies and attempts to quantify sources of series resistance in inkjet-patterned PERC cells that employ electroless or light-induced nickel-plating techniques followed by copper light-induced plating. Photoluminescence imaging is used to determine locations of series resistance losses in these inkjet-patterned and plated PERC cells.
... In this work Cu penetration through LIP Ni and electroless Ni barrier layers into the underlying Si was investigated for laserdoped selective-emitter (LDSE) Si solar cells [22,23]. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were employed to identify Cu precipitates in the LDSE solar cells resulting from thermal treatments conducted at temperatures up to 400 1C. ...
... Then, the cells were immediately laser-doped using a 532 nm high-powered laser with an average power of 15 W and scan speed of 5 m/s, to form selective-emitter regions. The laser patterns the dielectric layer while melting the underlying Si allowing incorporation of P into the molten Si [23]. As a result, heavily P-doped regions were formed underneath openings in the SiN x for subsequent metallization. ...
Copper penetration through light-induced plated and electrolessly-plated nickel barrier layers into the underlying silicon of laser-doped selective-emitter (LDSE) silicon solar cells was investigated. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were employed for chemical identification of copper in silicon after thermal treatments at 200 °C and 400 °C. Copper precipitates were detected by TEM/EDX analysis in solar cells that were cooled rapidly by quenching in ethylene glycol, indicating that the nickel barrier layer may not completely prevent copper from diffusing into silicon. However, they were not detected in solar cells that were allowed to cool slowly under ambient laboratory conditions. The potential impact of copper ingression on the performance of LDSE solar cells was investigated by capturing open circuit photoluminescence images of the cells before and after heat treatments. Solar cells plated with an electroless nickel barrier layer showed more severe photoluminescence degradation after heat treatment followed by fast cooling, compared to cells plated with a nickel barrier formed by light-induced plating. The photoluminescence intensity of a solar cell with an electroless nickel barrier was partially recovered by a second heat treatment at 250 °C followed by slow cooling under ambient laboratory conditions.
... 54 Following the initial success of BP Solar with their use of Cu-plated metallisation, Suntech also adopted Cu plating for their laser-doped selective emitter (LDSE) cells. 59,155 They addressed the problem of a slow Cu plating rate by pioneering the use of LIP [50][51][52] for depositing Ni and Cu in the production of their Pluto cells. This One possible reason for this apparent durability of Cu-plated Si PV modules may be that the Si/Ni/Cu system forms a sacrificial diffusion barrier for Cu, with the Ni both reacting with the Si to form a silicide and alloying with the Cu. ...
Copper‐plated interconnects were widely adopted for volume manufacture of integrated circuits after more than a decade of intensive research to demonstrate that use of Cu would not impact device reliability. However, although Cu‐plated metallisation promises significantly reduced costs for Si photovoltaics, its adoption in manufacturing has not gained the same traction. This review identifies some key challenges facing the introduction of Cu‐plated metallisation for Si photovoltaics. These include the following: (1) increased carrier recombination due to the use of Cu for metal contact formation; (2) reduced module reliability due to adhesion or contact integrity failures; and (3) limited availability of cost‐effective processes and equipment for metal plating. For integrated circuits, Cu's low electrical resistance and high resistance to electromigration provided an impetus for the large investment in process development that was required to realise Cu‐plated interconnects. However, the technical advantages of using Cu for Si solar cell contacts are not as compelling, as solar cells can tolerate larger feature sizes thus reducing the criticality of the contact metal's conductivity and electromigration properties. Additionally, for Si photovoltaics, low cost is paramount, and new challenges arise from the need for modules to absorb light and operate in the field for 25+ years in diverse outdoor climates. However, with the scale of Si photovoltaic manufacturing expected to increase dramatically in the next decade, the use of large quantities of silver for cell metallisation will provide an incentive to address reliability concerns regarding the use of Cu for Si photovoltaic metallisation.
... Low cost silver (Ag) paste screen-printing method has emerged as the dominant technology for front-side metallization of silicon (Si) solar cells. Later, different technologies such as laser doping [14][15][16][17], self-aligning technology and screen printing, and lithography have been reported to reduce these resistances for enhancement of PCE. Recently, light-induced electroplating has been used for making front contact with silver as a low-cost promising method to improve fill factor of the solar cell [18][19][20][21]; Jensen et al 1979). ...
Fill factor of the solar cell mainly depends on series resistance and contact resistance, which are the most effective parameters to collect carriers (electrons and holes) from both electrodes of C-Si solar cells. We have used both mathematical and experimental approaches to reduce these resistances for enhancement of power conversion efficiency (PCE) by increasing fill factor. After processing by light-induced plating (LIP) for metal contact, the PCE of solar cell is obtained as 14.43%, which is 8.8% more than that before LIP processing.
... This results in the simultaneous formation of a selective emitter and self-aligned metallisation pattern. A number of high-efficiency solar cells, both on n-type and p-type substrates, have been manufactured by this methodology [90][91][92]. ...
*****DOWNLOAD LINK: (http://handle.unsw.edu.au/1959.4/57392 )********
Recent developments in solar photovoltaic cell technology have enabled significant cost reductions so that in many markets it is now directly competitive with conventional energy generation. To maintain downward pressure on module prices and to improve efficiencies, continued developments of manufacturing processes are required. In this respect, laser processing offers advantages in achieving various fabrication steps by providing spatially precise and localised heating on short and controllable timescales, and offering continuous, high throughput, in-line processing. In designing new laser-processing approaches and in optimising existing ones, a detailed understanding of the resulting heat transfer, phase change, and other relevant phenomena that occur during the process is arguably valuable. Since experimental techniques to investigate these phenomena in detail would be very difficult and unwieldy to use as an optimisation tool, this thesis advocates an approach based in numerical modelling. The thesis first focuses on development and validation of a numerical model of heat transfer and phase change phenomena during laser-material interaction, and then implement the numerical model to reveal these phenomena in three significant laser processes used in the fabrication of solar cells: (1) laser based hydrogen passivation of defects in silicon wafers, (2) laser annealing of the absorber layer in copper zinc tin sulphide (CZTS) based solar cells, and (3) pulse laser-induced melting and solidification dynamics of the silicon wafers. The numerical model is developed in OpenFOAM, an open-source computational fluid dynamics toolbox written in C++. The developed OpenFOAM code is validated against several analytical and experimental reference cases related to the simulation of laser-semiconductor interaction, and an excellent agreement is observed between the model and the analytical and experimental results. In the first implementation of the model, the effect of continuous wave (CW) diode laserinduced heat transfer phenomena on the hydrogen passivation of silicon wafers is modelled. In the case of crystallographic defect passivation, it is demonstrated that an appropriate combinations of parameters can be chosen to enable process characteristics in the same range as those known to be optimal for conventional belt furnace or rapid thermal processing (RTP) methods, which are used to enable hydrogen release and diffusion and to passivate these defects. It is observed that the optimal temperature regime for passivation of Boron-Oxygen (B-O) defect complexes can also be obtained using different settings for the laser parameters. In addition, by coupling the thermal model with a model of the B-O defect system reaction rates, it is found that the passivated defect concentrations are significantly influenced by the processing times and the temperature distributions within the depth of the wafer. In the second implementation, the effect of CW diode laser-induced heat transfer phenomena on the processing of CZTS thin film solar cells is demonstrated. The model is applied to the situation of a CW diode laser beam annealing CZTS thin film deposited on a Molybdenum (Mo) soda lime glass substrate. It is shown that the Mo remains isothermal, whereas a temperature gradient can be observed in the CZTS thin film and the glass substrate. This temperature gradient is demonstrated to increase with the CZTS absorber layer thicknesses, which is expected to affect the absorber layer properties. Very thick absorber layers are shown to generate high thermal stress, which is associated with risk of delamination. Finally, appropriate settings of the laser-annealing parameters are determined that produce process characteristics similar to those that result in a CZTS absorber layer with optimum properties when processed via conventional methods such as the belt furnace and RTP. In the final implementation, the dynamics of laser-induced melting and subsequent resolidification of the silicon wafers are described. Silicon wafers are irradiated with a number of widely used pulse shapes, Gate, Gaussian, Weibull, Asymmetric and Q-switched, in the nanosecond regime to reveal the effect pulse shaping, i.e. the energy distribution within a single pulse, on the thermal processes and the associated melting and solidification dynamics. It is demonstrated that the transient behaviour of the heat transfer phenomena, parameterised by the surface temperature, heating and cooling rates, is significantly influenced by the variation of laser energy within the pulse. In turn, the heat transfer process controls the melting and solidification dynamics. The results suggest that in achieving a long melt duration with relatively low resolidification velocity and solid-phase thermal gradients, the pulses that ramp up quickly but deliver energy more slowly in the latter ramp-down half of the pulse would be beneficial, such as the Q-switched pulse. In summary this thesis makes a contribution to understanding heat transfer and related phenomena in key laser processing approaches used in solar cell manufacturing, providing guidance as to the selection of processing parameters and hence improved processing outcomes. It moreover demonstrates the utility of numerical models to provide this otherwise lacking information, thus potentially opening many future avenues for development and optimisation of laser processing methodologies.
... The front and rear surfaces are passivated by SiON x (75 nm) and SiO 2 (100 nm), respectively. A stack of Ni/Cu fingers deposited by light-induced plating (LIP) [12] contact the heavily doped n + openings formed by laser doping on the front of the cell. Al point contacts cover approximately 0.5% of the rear surface. ...
A detailed loss analysis is presented for a 15.9% large area ultrathin silicon (UTSi) solar cell. The loss analysis is based on a comprehensive study of the electrical and optical parameters of the champion solar cell. The results indicate that the UTSi solar cell has an efficiency potential of 19.9% using currently available technologies and is capable of achieving 22.2% efficiency in the long run.
... The laser doping method has also provided a simple, low cost way of overcoming many fundamental limitations of conventional screen-printed (SP) solar cells. Previous work has shown that by incorporating one additional laser doping step and replacing the Ag screenprinting by Ni and Cu plating to form front metal contacts (Figure 1), LD cells can achieve a considerably higher electrical performance compared to SP cells [3]. In fact, the abovementioned fabrication methods are relatively easy to retrofit on standard industrial SP lines. ...
... If no plating is used, an alignment is required to ensure reproducible front side metalization. A further sub-classification can be made with respect to the Laser doping: a wet laser doping with liquids [113] and a dry laser doping with solid films [114,115]. The concepts shown in figure 4.2 are briefly explained in the following: partial masking approach uses a semitransparent barrier like SiO 2 typically grown by thermal oxidation in a tube furnace. ...
... In this fashion, surface films containing silicon dopant elements can act as precursors for n-and p-type doping. Concurrent ablation and doping is ideally suited for aligned local contact openings and back-surface field (BSF) or emitter formation, and in recent years several studies have demonstrated locally laser doped contact openings in high efficiency cell structures [3][4][5][6]. ...
Aluminium oxide (Al2O3)functions doubly as a high-quality surface passivation material for crystalline silicon and as an aluminium (Al) p-type precursor for laser doping. Thus, p+ doping based on laser ablation of Al2O3 thin-films deposited on a silicon substrate is an attractively simplified process for concurrent local contact definition and aligned surface doping. A number of studies have demonstrated this process, but a careful examination of the influence of laser parameters on the electronic properties of the Al laser doped p+ surface itself, including the influence of post-doping annealing, has yet to be presented. Such information is valuable for establishing process windows and for providing parameters by which the contact geometry can be optimized and the performance of locally Al2O3laser doped solar cells predicted. In this work, we present accurate characterization of the electronic properties of primary importance to solar cell performance: effective surface recombination and contact resistivity. Recombination at the Al2O3laserdoped p+ surface is found to exceed that of equivalently-doped p+ silicon from furnace diffused boron, while contact resistivity to vacuum evaporated Al is up to two orders of magnitude less than screen-printed, fired Al. Based on this characterization, computersimulations demonstrate that with optimized rear contact geometries, an industrially relevant PERC cell can approach 21 % efficiency, and the high-performance UNSW PERL structure can exceed 24 %.
... The laser -assisted doping [1][2][3] provides a way to boost local regions of the silicon beneath the contacts of the solar cells to produce structures with selective emitters Metal/n + +. ...
Selective emitter solar cells were fabricated with a reduced number of technological steps. Laser doping is
often discussed in relation to silicon photovoltaic cell efficiency enhancement. In this paper, we present results of the development of a selective emitter structure for multicrystalline silicon solar cells suitable for industrial mass production. A pulsed laser is used to obtain highly doped regions that will receive the screen printed silver grid. Therefore, we designed using solidworks software a single type of pattern for square cells (multicrystalline silicon). We resolved the problem of grid mismatch between the laser pattern and metallic grid. Observation of laser treatment surface shows the perfect continuity of the lines and good flatness of the edge of finger and bus bar.
... Groups B and C performed the 20 µm 20 µm 20 µm best in terms of FF and pFF, and Group B exhibited the lowest J 02 values indicating that laser doping prior to SiN x deposition introduced the least amount of recombination in the depletion region. This is consistent with what has been previously reported in the literature, where it was hypothesised that when laser doping through a dielectric layer, defects in the silicon structure are formed along the edge of the laser doped lines due to the difference in thermal expansion coefficients of silicon and the dielectric layer [23][24][25]. In addition, the laser doped surface region itself was passivated when performing the SiN x deposition after the laser doping R sh above 1000 Ω cm 2 was achieved for Groups A, B and C, with most exceeding 10,000 Ω cm 2 . ...
... For example, the n + emitter can be formed in the whole cell surface by scanning it with a laser beam and using a spin-on dopant as a doping source [1][2]. Local point diffusions have been also proposed [3][4] and more complicated patterns can be defined by guiding the laser radiation into a chemical liquid jet [5]. Additionally, taking advantage of the easiness of laser for 2-D processing, the formation of selective emitters by means of a laser beam has been also studied taking advantage of the phosphosilicate glass grown on c-Si surface during the conventional diffusion [6][7]. ...
This paper shows an innovative and low temperature fabrication technology for crystalline silicon (c-Si) solar cells where the highly-doped regions are punctually defined through laser processed dielectric films. Phosphorus-doped silicon carbide stacks (SiCx(n)) and aluminium oxide/silicon carbide (Al2O3/SiCx) stacks are used for the creation of n+ and p+ regions respectively. These films provide excellent surface passivation on both n- and p-type substrates with effective surface recombination velocity below 20 cm/s. Moreover, a wide laser parameter window for laser processing them leads to low recombination highly-doped regions that show emitter saturation current densities of 21 and 113 fA/cm2 for n+ and p+ emitters respectively. All this is combined in the DopLa (Doped by Laser) cell structure whose fabrication process can be reduced to wafer cleaning, film depositions, laser processing and metallization. As a proof of concept, 1×1 cm2 solar cells were finished on both p- and n-type substrates with promising results. The analysis of loss mechanisms shows that optical losses and technological issues in the translation of surface passivation from the solar cell precursors to the final device are limiting efficiency. Both are not inherent to DopLa structure and suggest room for improvement in these devices. On the other hand, the crucial role played by ohmic losses in the proposed structure is identified. These ohmic losses arise from the fact that both base contacts and emitter regions are defined in point-like patterns. Firstly, base contacts are defined in a 1 mm pitch square matrix introducing additional series resistance that can be reduced by using low resistivity substrates. Secondly, the emitter consists of laser processed local diffusions with an inversion layer emitter in-between characterized by very high sheet resistances. The ohmic losses introduced by this induced emitter are closely linked to the fixed charge density located at the dielectric/c-Si interface. As a result, we conclude that low resistivity n-type substrates fit better to DopLa cell concept because emitters are based on Al2O3/SiCx stacks which have higher fixed charge density, i.e. inversion layer emitters with lower sheet resistances, than their SiCx(n) counterparts.
... Used in conjunction with LIP, FIP can enable the plating of bifacial silicon solar cells. In this work we apply LIP and FIP to the metallisation of bifacial p-type cells (see Fig. 1) where the contact regions have been formed using phosphorus (P) [8,9,[14][15][16][17] and boron (B) [18,19] laser doping. This plating method can be applied to other bifacial cells including heterojunction cells where metal is plated directly to the transparent conductive electrode. ...
In this paper we report on the fabrication of laser-doped p-type bifacial cells using self-aligned metal-plating with energy conversion efficiencies as high as 19.2%. A key fabrication step for these cells is recognising that the p-type silicon regions can be made cathodic by forward biasing the p-n junction in a process which we call here field-induced plating (FIP). Used in conjunction with light-induced plating (LIP) in the same plating apparatus, FIP can be used to form low cost nickel/copper grids on both surfaces of a cell. Furthermore, the simplicity of the FIP process means that it can potentially be performed using the same plating equipment and chemistry as used for LIP. Plating rates similar to LIP were achieved (i.e., similar to 10 mu m of copper in 10 min), however there is potential to plate at much faster rates with FIP because the junction is forward-biased. This bifacial cell plating method could be adapted to metallise a range of bifacial cells including heterojunction cells.
... In this work, we focus on a method combining direct laser-patterning of the selective emitter with a selfaligning metallization process, commonly referred to as the laser doped selective emitter (LDSE). This approach has been patented and pioneered by researchers at the ARC Photovoltaics Centre for Excellence at the University of New South Wales in Sydney, Australia [6,7]. While various traditional laser sources have been tested for this process, here were explore the potential benefit of more novel laser technologies for high-speed LDSE fabrication. ...
In recent years, laser doping of selective emitters (LDSE) has demonstrated great promise for improving c-Si solar cell performance with the potential for low cost of adaptation. Absolute cell efficiency improvement of 1 – 2 % is seen with a process pioneered at the University of New South Wales which couples narrow-line LDSE processing with a self-aligning, light-induced plating (LIP) metallization process. To help transfer this technology to the manufacturing floor, it is important to find and characterize the ideal laser source for LDSE processing. Current laser sources that may be considered include continuous wave (CW) and mode-locked (ML) quasi-CW laser sources. The output to these lasers make them well-suited for localized heating and melting of materials, and they can be built at various wavelengths, such as near infrared (NIR) at 1064 nm, green at 532 nm, and ultraviolet (UV) at 355 nm. While both CW and ML lasers might be good choices, they are vastly different in both their optical radiation output profile and their technological architectures. These differences could have important ramifications for manufacturability of LDSE solar cells. As such, we have in this work characterized both CW and modelocked laser technologies for LDSE processing. With optical emission at the 532-nm wavelength, LDSE features were created at scan speeds of 2 to 12 m/s and power levels from 12 – 15W. LIP metallization was used for final cell fabrication. Efficiencies of 17.4 – 18.4% and fill factors from 77 – 79% are typically achieved. LDSE cells fabricated with 355-nm ML laser source were also compared. Theoretical considerations with regard to the wavelength and laser type are explored; and the suitability of the various laser sources for large-scale production is discussed.
... C URRENT research in the field of crystalline silicon solar cells focuses on cost reduction of the fabrication process, as well as an increase in conversion efficiency by implementing new cell structures. The implementation of selective emitter structures has shown to increase conversion efficiency at competitive costs and has been extensively researched in recent years [1]- [6], as losses originating from the front side are reduced. Currently, a major focus of research is the development of passivated emitter and rear solar cells (PERC) [7] and their potential in large-scale production [8]- [11]. ...
We compare homogeneous and selective emitters on monocrystalline silicon solar cells with passivated surfaces and present an analysis of the saturation current densities influencing the open-circuit voltage VOC and the fill factor FF . The cells' surfaces are passivated by a thin thermal oxide. Selective emitters are fabricated by laser doping from phosphosilicate glass. On both Czochralski-grown silicon (Cz-Si) as well as float zone silicon (FZ-Si), we find higher conversion efficiencies for the cells featuring a selective emitter. An efficiency up to 20.0% is reported on FZ-Si with an area of 148.4 cm2 . For the selective emitter cells, 8 mV higher open-circuit voltages are found compared with the baseline. A saturation current analysis reveals that these cells exhibit a lower diode saturation current density of ideality 2 (J02), as well as improved shielding of the minorities in the emitter from the front contact. The selective emitter cells show a minor loss in short-circuit current density JSC of 0.5 %rel due to the presence of highly doped, illuminated areas. Front contact quality of the cells featuring a selective emitter is found to be superior compared with the cells with a homogeneously doped emitter.
... Laser doping selective emitter (LDSE) technology developed at UNSW has been proved to be a contact free and localised heating process. Such technology has a potential to be implemented in mass production for opening and doping local contact region on dielectric layers [5]. In this paper, a commercial green laser system was used to create a pseudo solar cell structure ( Fig. 1) by employing phosphorous and boron laser doping (LD) to front and rear surface of industry p-type CZ Si wafer passivated by silicon nitride (SiN x ) on front surface and silicon oxynitride (SiON x ) on rear surface. ...
Efficiencies above 19% have been achieved by many PV manufacturers by applying different selective emitter technologies on p-type CZ silicon wafers with screen printed aluminium back-surface field which limits the voltage of the cell to below 640 mV. In order to overcome this limit, in this paper, laser doping technology was applied into both surfaces of commercial grade p-type silicon wafer passivated with dielectric layer to form a pseudo solar cell structure with standard laser-doped selective emitter on the front and local back-surface field. This paper studies the passivation and thermal property of the dielectric layer. Post deposition annealing is investigated by measuring the minority carrier effective lifetime and implied Voc of the samples by photoluminescence imaging and photoconductance effective lifetime measurements. The influence of laser doping parameters on the implied Voc is also discussed in this work. As a result, implied Voc over 700 mV was achieved on commercial grade p-type CZ silicon wafer after double sided laser doping process. This implied Voc is much higher, compared to the ones obtained by single-sided selective emitter structure.
... Due to the penetration depth on the order of 1 micrometer at 532 nm wavelength, which is comparable to the emitter depth, most laser doping approaches are carried out at 532 nm [9,10,3]. IR and UV lasers are used only rarely [11,12]. ...
Lasers as production tools offer several advantages, which are especially relevant for the production of solar cells. The contactless and localized nature of the energy deposition allows new processes, such as laser selective emitter doping, laser ablation of dielectric coatings and via drilling for back contact cell concepts. A critical factor is the selection of suitable laser sources and parameters in a manner that adapts the laser process to the requirements of the material, the process nature and the solar cell properties. In this paper three laser processes are investigated with the goal to identify the most suitable laser source.
... From the annealing method used it is observed that SiON films would be suitable for solar cell structures which do not rely on high temperature processes after the SiON deposition. Such solar cell structures could be realised through the use of laser doping, sputtered aluminium and light induced plating, allowing for the optimisation of thermal treatment for surface passivation [21,22], in contrast to technologies based on screen printing which typically rely on thermal treatments optimised for the formation of a back surface field or firing of silver contacts on the front of the solar cell. This would minimise the thermal budget during the manufacturing process, reduce stresses on the wafers and allow the use of thinner and lower cost substrates. ...
This work presents a quantitative analysis on the relationship between the composition of PECVD silicon oxynitride and surface passivation on float zone silicon wafers with planar non-diffused surfaces using FTIR spectroscopy. Implied open circuit voltages of approximately 740 mV are demonstrated on both n-type and p-type substrates, with associated 1-sun effective minority carrier lifetimes of 1.8 ms and 1.1 ms respectively. Improvements in the implied open circuit voltage of up to 80 mV upon thermal annealing are presented for films with Si–H peak wavenumbers >2200cm−1 and are attributed to increasing oxygen incorporation.
... Heavy doping beneath contacts facilitates reduced contact resistance between the metal and the silicon and shields the high recombination velocity metal/silicon interface from the active regions of the cell. A potentially more effective and simpler way than previously used for achieving a selective emitter is by using laser doping 15 to produce the heavily doped regions only directly beneath the metal contacts (Fig. 4). Following top surface emitter phosphorus diffusion and silicon nitride deposition, an n-type dopant source is applied or the dopant can be incorporated into the silicon nitride layer. ...
Research and education activities at the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales (UNSW) are outlined. Research advances are reported for silicon wafer, crystalline silicon thin-film, and third generation technologies. Education activities are also expanding and the School has been fortunate to secure support for sponsorship from 2008 of undergraduate students from selected Chinese universities and postgraduate coursework and research students from China, India and South Korea.
Laser doping is a typical industrial method to introduce a local highly doped region in silicon solar cells to form a selective emitter. Such a process inherently introduces defects that can be a concern to the overall performance of the solar cell. Here, we investigate the effectiveness of laser-induced defect (LasID) passivation on lifetime test structures through different annealing processes, including high-temperature belt-furnace firing, low-temperature belt-furnace annealing, and an advanced hydrogenation process (AHP) for n+ laser-doped selective emitters. We demonstrate clear advantages of post treatment using a rapid 10 s AHP at 300 °C when the lifetime structures are prefired. For the examined laser speeds of 0.5–6 m/s (sheet resistances of 4--70 Ω/□), AHP is the most effective treatment method. For example, for a typical laser doping speed of 4 m/s, starting from the same effective carrier lifetime of 36.9±2.4 μs after laser-doping step for all the passivation treatments, the AHP not only surpasses the conventional approaches by showing the highest recovery of the effective carrier lifetime (∼79% compared with ∼63% and ∼41% for the firing and belt-furnace annealing treatments, respectively) and dark saturation current density reduction in the regions affected by LasIDs but also simultaneously suppresses light-induced degradation (maximum of 4% effective lifetime degradation with respect to the passivated state, as opposed to 14% and 16% degradation for the firing and belt-furnace annealing treatments, respectively) common in Cz grown boron-doped p-type monocrystalline silicon.
Passivated emitter and rear cells (PERC) on p-type Cz Si wafers are currently being migrated to mainstream production, applying the ongoing improvements in recent years. For PERC solar cells, the emitter recombination loss becomes the main loss of the entire cell. Currently, two-step diffusion consisting of low temperature phosphosilicate glass (PSG) deposition and high-temperature drive-in has been widely applied in production and can reduce saturation current density (J0) of the emitter in the passivated area. However, a low contact resistance under metal area cannot be guaranteed. Meanwhile, the determination of the saturation current density J0 in the passivated area is dependent on the injection density during the measurement, and exact calculation of J0,metal is not as convenient as J0,diffusion, which could lead to a wrong assessment in cell analysis. In this study, a three-step diffusion (low-temperature PSG deposition–high-temperature drive-in–low temperature PSG deposition) with low J0 and low Ag-Si contact resistance is investigated and combined with a selective emitter by an etch-back process. The results are compared with a conventional two-step POCl3 diffusion. The accuracy of J0 measurement and J0,metal test method is also discussed. With these improvements, the champion cell efficiency of our PERC solar cells fabricated on 156 × 156 mm² wafers using screen printing technology and industrial-type process has reached 22.61% with Voc of 684.4 mV and a fill factor of 81.49%, as confirmed by Fraunhofer ISE CalLab PV cells. By implementing the diffusion process to mass production, the average cell-efficiency gain is approximately 0.2% with a median efficiency of 21.7%.
Laser doping is a promising way of selective emitter formation for silicon solar cells. To quantitatively study the influence of laser parameters on the doping effect, it is necessary to develop a numerical model. This work made some improvements on an experimentally verified numerical model. The most important improvement is that the flow field and the dopant concentration profile are only computed in a subdomain instead of the whole domain. The influence of the laser power and the scanning speed on the temperature and flow field in the melt pool, the selective emitter geometry and the dopant concentration profile are investigated. Then, to accurately study how the dopant concentration profile affects the performance of the selective emitter, a semiconductor device simulation was furthermore performed based on the computed dopant concentration profiles.
Contact resistance and series resistance are the most crucial problem to collect carrier from the both electrodes. In general, contacts are made by screen printing process which is industrially established. After the screen printing process some factors arises like change of with and thickness of finger and pores are created on the top side due to solvent. For the unavoidable pores series resistance is decreased. In this paper, the mathematical modeling has done considering resistance and distribution of pores.
Laser firing processes have emerged as a technologically feasible approach for the fabrication of local point contacts or local doped regions in advanced high-efficiency crystalline-Si (c-Si) solar cells. In this work, we analyze the local impact induced by the laser pulse on the passivation layers, which are commonly present in advanced c-Si solar cell architectures to reduce surface recombination. We use microphotoluminescence (PL) measurements with a spatial resolution of 7 μm to evaluate the passivation performance at the surroundings of laser-processed regions (LPRs). In particular, we have studied LPRs performed on SiC x/Al 2O 3- and Al 2O 3-passivated c-Si wafers by an infrared (1064 nm) laser. Micro-PL results show that passivation quality of c-Si surface is affected up to about 100 μm away from the LPR border and that the extension of this damaged zone is correlated with the laser power and to the presence of capping layers. In the final part of the work, the observed decrease in passivation quality is included in an improved 3-D simulation model that gives accurate information about the recombination velocities associated with the studied LPRs.
Overall series resistance of the contact increases due to unwanted pores created during the time of contact formation in solar cell. Light induced plating is an industrial established process which is used to reduce the series resistance and to improve the contact resistance resulting in increase of fill factor of solar cell through modification of front side contact. In this paper a theoretical modeling is done to show how distributed pores affect the series resistance and solar cell performance. Here we explored several parameters like: series resistance, shunt resistance, fill factor, short circuit current, and external quantum efficiency. Through our experimental setup it is found that the short circuit current improves significantly and also it is observed that during the time of experiment due to unintentional deposition of silver nanoparticles, external quantum efficiency improved by 5.249 % and a reduction of reflectance by 2.03 %. An overall enhancement of solar cell efficiency of 1.65 % is obtained during the process.
This experimental study focoused on the analysis and optimization of some effective surface parameter like reflectance, series resistance, fill factor, surface recombination velocity to enhance the performance of Si solar cell. Near sub wave length structure is introduced to overcome the contact problem of nano texturization wher reflection loss is less than micro texturization. Screen printing technique is the low cost industrial established process but it also raises some unwanted problem which reduces the fill factor. Light Induced Plating (LIP) of c-Si solar cell is a critical process generally leading to increase in fill factor and efficiency associated with marginal reduction in short circuit current (Jsc). In this paper, the increase in Jsc is reported at the LIP treated solar cell due to unintentional deposition of silver nanoparticle on the surface of solar cell. The array of silver nanoparticle on the Si surface results into plasmonic effect as evidence by the reduction of reflectance and increase in external quantum efficiency without depositing added nano particles. Also new type of texturization, contact optimization and better LIP treatment are reported.
This study aims at investigating laser doping and laser annealing for crystalline silicon solar cells processing. Laser-processed emitters are firstly realized using three lasers and different dopants sources. The lasers are a nanosecond green laser, an excimer laser and a high-frequency ultraviolet laser. As dopants sources we used either phosphosilicate glass, phosphorus and boron-doped silicon nitrides, or phosphorus and boron ion implantation. Efficient phosphorus and boron doping are obtained using any of these laser/sources couple. In particular, low sheet resistances and low emitter saturation current densities are obtained. These laser processes are then applied to selective emitter and boron back-surface-field solar cells. Laser-doped selective emitter solar cells (using phosphosilicate glass as a dopants source) reach 18.3 % efficiency. This represents an overall gain of 0.6 %abs when compared to standard homogeneous emitter. On the other hand, laserannealed boron back-surface-field solar cells (using implanted boron as a dopants source) feature an overall gain of 0.3 %abs when compared to standard aluminium back-surface-field solar cells, thus yielding an efficiency of 16.7 %.
In the field of solar energy, in particular for solar cells, the cost and efficiency of the cell is critical. To do this, we will realize a selective emitter on multi-crystalline silicon plates. This work will focus on three parts: The first part will present the experimental approach to achieve the selective emitter laser. The second part will list the different characterizations performed on the laser-treated plates. In the last part, we will present the overall results. Finally a conclusion provides a summary of results and a perspective of work.
13.56 MHz capacitance coupled Ar plasma irradiation at 50 W for 120 s caused serious damage at SiO2/Si interfaces for n-type 500-μm-thick silicon substrates. The 635-nm-light induced minority carrier effective lifetime (τeff) was decreased from 1.7 × 10−3 (initial) to 1.0 × 10−5 s by Ar plasma irradiation. Moreover, the capacitance response at 1 MHz alternative voltage as a function of the bias voltage (C-V) was changed to hysteresis characteristic associated with the density of charge injection type interface traps at the mid gap (Dit) at 9.1 × 1011 cm−2 eV−1. Subsequent 940-nm laser annealing at 3.7 × 104 W/cm2 for 4.0 × 10−3 s markedly increased τeff to 1.7 × 10−3 s and decreased Dit to 2.1 × 1010 cm−2 eV−1. The hysteresis phenomenon was reduced in C-V characteristics. Laser annealing effectively decreased the density of plasma induced carrier recombination and trap states. However, laser annealing with a high power intensity of 4.0 × 104 W/cm2 seriously caused a thermal damage associated with a low τeff and a high Dit with no hysteresis characteristic.
Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% in-crease in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demon-strated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm 2 of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.
Laser doping of silicon is a complex process involving thermal effects and interactions between different materials far from equilibrium and over a short period. In this paper, diffused samples capped with different dielectric films (including bare surfaces) are processed using laser pulses of 20–400 ns duration and characterized by photoluminescence (PL) imaging to study the degradation of the electronic properties of the processed regions. This way, without the interference of a dopant precursor, the thermal and dielectric effects are separately investigated. It is found that the thermal effects (melting and recrystallization of the silicon) do not lead to significant damage and additional recombination, provided no severe silicon evaporation occurs. However, when a dielectric film is present, a considerable increase in recombination is observed, irrespective of laser parameters, indicating the formation of additional defects. The magnitude of the increase in recombination varies substantially, depending on the dielectric used. Repeated pulses appear to repair silicon damage introduced by the first pulse or pulses for long pulse durations but result in a slight degradation for short pulse durations. Combining the PL results and four-point probe measurement of laser-doped samples, it is demonstrated that both high dopant incorporation (sufficient silicon melting) and low recombination can, in principle, be achieved, particularly when samples are processed using long pulse durations and small pulse distances.
The advanced semiconductor finger solar cell is a silicon wafer cell designed for industrial implementation. The concept incorporates a selective emitter with a front side grid metallisation that combines the advantages of both screen printing and plating techniques. Screen-printed metal forms the busbars and a few thick but widely spaced fingers. The current is carried to these fingers via narrow, closely spaced, thinly plated, laser-doped lines formed perpendicular to the fingers. This paper explains the evolution from the original semiconductor finger cell design to this advanced version and outlines the issues encountered in the process development thus far. An encouraging result of 18.5% efficiency has been achieved on a large area cell with fully industrial equipment, and several opportunities for significant improvement are identified. Copyright © 2013 John Wiley & Sons, Ltd.
Light Induced Plating (LIP) of c-Si solar cell is a critical process generally leading to increase in fill factor and efficiency associated with marginal reduction in short circuit current (Jsc). In this paper, we report the increase in Jsc of LIP processed solar cell caused by unintentional deposition of silver nanoparticle on the surface of solar cell during LIP. The array of silver nanoparticle on the Si surface results into plasmonic effect as evidence by the reduction of reflectance and increase in external quantum efficiency.
Improvement of front surface design by a number of commercially-available techniques, together with the reduction of wafer thickness in order to reduce material cost, increases the importance of solar cell rear surface design. Although different rear surface designs have been introduced, most of them are based on high quality passivation of the rear surface and formation of a local back surface field, which traditionally includes complicated and expensive processes such as photolithography and additional diffusion. This paper investigates a simple and relatively cheap method to create a local back surface field using laser doping.
The practical realization of high efficiency laser-doped semiconductor fingers (SCF) silicon solar cell is inhibited by high contact resistance. By plating the SCF with metal, a new SCF cell concept known as the “Advanced SCF” (AdvSCF) cell that can resolve the contact resistance problem is presented. In the first AdvSCF cells demonstrated in this work, the nickel (Ni) plating coverage across the cell was found to be non-uniform with Ni voids mostly concentrated around the busbar. This was found to be avoidable by ensuring that the spin-on phosphoric acid dopant layer was uniformly thick across the whole cell area and especially at the busbar. With uniform Ni plating coverage achieved, in a batch of 6 AdvSCF cells, an average batch efficiency of 18.40 % was achieved with the highest at 18.82 %. This was achieved without any experimental optimization of the front grid design or other cell properties, implying that there is potential to achieve significantly higher efficiency levels.
Laser fired contacts (LFCs) and laser doped selective emitters can be
used to improve manufacturing throughput of photovoltaic devices without
sacrificing device conversion efficiency. However, the laser parameters
used to form these features can vary significantly. LFCs can be formed
with short pulses (hundreds of nanoseconds) while selective emitters can
be formed using either a pulsed or CW mode. Here, mathematical models
for a pulsed laser and CW laser are used to evaluate how variations in
processing parameters affects alloy formation, molten pool geometry and
dopant concentration profiles. The models solve the conservation
equations for mass, energy, and momentum to study the effects of heat
and mass transfer and fluid flow on the formation of LFCs and emitters.
Comparisons between experimental data and theoretical calculations for
molten pool geometry and concentration profiles demonstrate good
agreement. For LFCs, when assuming complete melting and mixing of the Al
contact layer, the Al concentration varies significantly with power
level, which drastically impacts the calculated pool shape. The
dimensionless Peclet number is used to understand dominant heat and mass
transfer mechanisms. Conduction is the dominant heat transfer mechanism
at power levels around 20W for both LFCs and emitters. As the power
level is increased to 50W, however, the dominant heat transfer mechanism
changes to convection. Changes in laser parameters also impact fluid
flow velocities and dopant concentration profile for emitters processed
in CW mode, which suggests that convection-based models should be used
to accurately predict concentration profiles within emitters.
This paper reports a new method for forming anodic oxides on silicon surfaces using the light-induced current of pn-junction solar cells to make p-type silicon surfaces anodic. The light-induced anodisation process enables anodic oxide layers as thick as 79 nm to be formed at room temperature in a faster, more uniform, and controllable manner compared to previously reported clip-based anodisation methods. Although the effective minority carrier lifetime decreased immediately after light-induced anodisation from initial values measured with an 17 nm thermally grown oxide on both wafer surfaces, the 1-sun implied open circuit voltage of wafers on which the thermally grown oxide on the p-type surface was replaced by an anodic oxide of the same thickness could be returned to its initial value of $635 mV (for 3–5 X-cm Cz silicon wafers) after a 400 C anneal in oxygen and then forming gas. The passivation of the formed anodic oxide layers was stable for a period of 50 days providing the oxide was protected by a 75 nm thick silicon nitride capping layer. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4829701]
Laser doping to form selective emitters offers an attractive method to increase the performance of silicon wafer based photovoltaics. However, the effect of processing conditions, such as laser power and travel speed, on molten zone geometry and the phosphorus dopant profile is not well understood. A mathematical model is developed to quantitatively investigate and understand how processing parameters impact the heat and mass transfer and fluid flow during laser doping using continuous wave lasers. Calculated molten zone dimensions and dopant concentration profiles are in good agreement with independent experimental data reported in the literature. The mechanisms for heat (conduction) and mass (convection) transport are examined, which lays the foundation for quantitatively understanding the effect of processing conditions on molten zone geometry and dopant concentration distribution. The validated model and insight into heat and mass transport mechanisms also provide the bases for developing process maps, which are presented in part II. These maps illustrate the effects of output power and travel speed on molten zone geometry, average dopant concentration, dopant profile shape, and sheet resistance.
Formation of selective emitter (SE) structures with a controlled dopant profile by laser (KrF Excimer laser at 248 nm) annealing of spin-on dopant sources is presented. Different barrier layers (BL) like spin-on glass (SoG), PECVD deposited SiN and SiOx were used as semi-transparent barrier layers for the dopant diffusion. The presence of BL at the interface between silicon substrate and the layer of dopant source controls the dopant profile of shallow emitter (ShE) during thermal diffusion, so that the etch back step could be avoided. This method allows the realization of shallow and selective emitters using a single layer of dopant source. The dopant concentration and depth with respect to the laser parameters and barrier thickness were analyzed using secondary ion mass spectrometry. It was found that the doping profile of phosphorous was precisely controlled in the shallow region upto 200 nm with a suitable emitter sheet resistance. Also, the SiN and SoG layers acted as effective phosphorous diffusion barriers for both shallow and selective emitters. On the other hand, the SiOx barrier layers, relatively lower thickness, resulted in the best electrical results at comparably lower laser fluences. In addition, laser induced damage in the silicon crystal at moderate laser fluences is nominal, and is found to be considerable at higher energies due to the enhanced energy absorption of silicon. Periodic structures were observed on the surface of laser treated silicon at the moderate laser fluences. The results were presented in detail in terms of physical behavior of the dopant diffusion with respect to laser fluence in the presence of barrier layer.
A record independently confirmed production cell efficiency of 19.3% is presented for a large area P-type CZ silicon solar cell, based on the UNSW laser doped selective emitter technology. In this work, the innovative and patented laser doping technology is simply added to a standard Centrotherm turnkey line, operating with a modified process and the addition of the laser doping and light induced plating steps. Impressively, this record efficiency is achieved by using standard commercial grade p-type CZ grown silicon wafers on standard production equipment and exceeds the previous independently confirmed record for any technology of 19.2% using a standard aluminium back surface field with full rear coverage. The avoidance of laser induced defects is discussed in this work, to overcome previous limitations of the laser doping technology using conventional Q-switched lasers or the laser chemical processing method. It is demonstrated that the use of appropriate lasers can avoid thermal cycling whilst still allowing for the sufficient mixing of dopants, and allow laser doping to be performed through a standard SiN layer with contacts formed through a self-aligning metallisation scheme.
A method of directly patterning silicon dioxide layers without the use of a mask is described. The method uses an inkjet device for the patterned deposition of a solution containing fluoride ions onto an acidic water-soluble polymer layer formed over the silicon dioxide. The deposited solution reacts with the polymer layer, at the locations where it is deposited, to form an active etchant that etches the silicon dioxide under the polymer layer to form a pattern of openings in the dielectric layer. The resulting patterned silicon dioxide layer can be used to facilitate local diffusions and metal contacts to the underlying silicon, or to enable etching of the underlying silicon. The method requires small amounts of chemicals and produces significantly less hazardous chemical waste than existing immersion etching methods.
We present an alternative method to form a blanket and selective emitter using a method that implants ion. This avoids several problems such as losing area by laser isolation and wet process for removing phosphosilicate glass formed on silicon substrate during the conventional thermal POCl3 diffusion process. It was demonstrated that laser isolation is not necessary after the ion implanted solar cells were fabricated. Furthermore, we also fabricated selective emitter solar cells. After studying their characteristics, it was clear that the solar cells with ion implanted selective emitter improved cell efficiency. This is because their blue response increased and their reverse saturation current density decreased. Using an industrially feasible process, solar cell efficiencies of >18.5% on 156mm×156mm using a shallow 100Ω/sq. emitter and an ion implanted 65Ω/sq. selective emitter were achieved.
N-type silicon wafers have been found to offer numerous advantages over p-type silicon wafers, such that they are becoming more widely used for manufacturing high-efficiency commercial solar cells. This paper focuses on work done on n-type cell structures with a screen-printed aluminum-alloyed rear junction, laser-doped selective emitter and light-induced plated front contacts to suit large-scale commercial production. However, with such a cell structure we report unusual linear shunting behavior that is only present under illumination but disappears under dark conditions. It was shown that such a phenomenon can be represented by a phototransistor model. In fact, such shunting effects are found to have detrimental impacts on the cell short-circuit current density (Jsc) and fill factor (FF), which limits the efficiency of cells in this work to 12%.
A simple but effective way to create a selective emitter is by using laser doping (LD) to selectively remove the anti-reflection coating (ARC) layer and simultaneously melt the silicon underneath it while incorporating dopants into the melted region, creating a heavily doped layer. In conjunction with LD, a plating technique is often used to selectively plate self-aligned metal contacts onto the laser doped lines. For light induced plating (LIP), the significant advantage compared to ordinary electroplating processes is that no electrical contacts are required to be connected to the front side metal grid as the illuminated solar cell itself generates the required current serving as both the cathode and anode of the reaction. This is also the reason why this plating technique has the potential to provide a very uniform layer provided a reasonably uniform light source is used to illuminate the cell surface. In this paper, we have improved the front side metallization for laser doped solar cell via a pre-treatment prior to Ni plating. The cells with homogenous Ni film have higher FF, so we have obtained over 19% conversion efficiency.
An improved technique of measuring the upper sheet resistance (Rsheet) limit of a single laser-doped (LD) line has been presented. Probing pads of a structure based on the transfer length method (TLM) principle are formed by laser doping without any further metallization step, forming a fast, simple, and inexpensive measurement technique that is applicable to both textured and planar silicon wafers with or without any silicon nitride (SiNx) dielectric. It has been demonstrated that within the scope of using a laser to form lines with Rsheet less than 15Ω/□, this TLM method seems to be an accurate and reliable method to characterize the sheet resistance of a single laser-doped line. Investigations here also suggest that the sheet resistance of a laser-doped rectangular area (LD box) formed by overlapping multiple laser-doped lines on crystalline silicon wafers and measured using the four-point probe cannot accurately or reliably represent the sheet resistance of a single LD line that is laser doped under the same laser-scribing condition. This is mainly because of (1) the tendency of the high power laser to introduce electrically detrimental cracks onto the laser-doped box, (2) the unknown appropriate spacing between adjacent LD lines that should be used when forming the LD box, and (3) the ambiguity of overdoping that makes an LD box unsuitable to represent a single LD line at all. The TLM method proposed here not only avoids these problems but is also able to self-justify its own predicted upper limit values.
Two-dimensional current flow in shallow-junction lateral transistors is analyzed. Emitter and base currents in stripe-geometry and circular devices are expressed in terms of a transform of the excess carrier concentration at the base surface. The dependence of current gain on device parameters is presented graphically. Modeling of lateral transistors is complicated by the twodimensional nature of current flow in the base. Several authors [ 11-[4] have approached the problem by using a simplified model, considering the emitter current to be the sum of one-dimensional components in the lateral and vertical directions. Other authors [ 51- [ 81 have used numerical methods for solving the two-dimensional continuity equation of minority carriers. It has been shown that for many cases the simple quasi-one-dimensional approach is plausible [ 41, and geometry-dependent factors may be used to relate two-dimensional current to that obtained from a one-dimensional model [6]. However, in the case of very shallow junctions, with depths much smaller than the lateral basewidth, the quasi-one-dimensional methods break down. For such devices (e.g., ion-implanted or mesa-structure devices) the lateral section of the transistor is very small and current flow is purely two-dimensional. The purpose of this correspondence is to present a simple approximate solution for the two-dimensional dc problem of shallow-junction lateral transistors at low-level injection. Circular devices as well as stripe-geometry devices are considered, while all the above mentioned analyses refer only to stripegeometry devices. Fig. 1 shows the device structures and de-fines the notation. Space-charge layer edges are assumed to be at x = 0. Consider first the stripe-geometry p-n-p device (Fig. l(a)) without a buried layer (xeqi-+ 00). The excess holes in the base obey the continuity equation x* (a) ( b)
This chapter explores approaches that offer higher efficiency potential of silicon solar cell. The evolution of record silicon laboratory cell efficiency reflects several stages in the evolution of cell design. After an initial period of rapid evolution in the 1950s, design stabilized for more than a decade on the conventional space cell. Key features include the use of 10-Ωcm p-type substrates to maximize high-energy radiation resistance, and the use of a nominally 40-Ω/square, 0.5-μm deep phosphorus diffusion to form the emitter region of the cell. Although it was known that lighter diffusions gave better blue response, this value was chosen because it was found to be more resistant to shunting by the top-contact metallization during processing. These cells remained the standard for space use for more than a decade and, until recently, were still specified for some space missions. Energy conversion efficiency was 10-11% under space radiation and 10-20% higher on a relative basis under terrestrial test conditions.
The intrinsic absorption spectra of high-purity single-crystal germanium and silicon have been measured at 77°K and 300°K. The spectral regions studied encompassed a range of absorption coefficient from 0.1 cm-1 to 105 cm-1 for each material. The germanium data may be interpreted as indicating a threshold for direct transitions at 0.81 ev at 300°K and at 0.88 ev at 77°K. The threshold for indirect transitions was placed at 0.62 ev and 0.72 ev for 300°K and 77°K, respectively. For silicon the data were not as readily interpreted However, there is an indication that the threshold for direct transitions should be placed at about 2.5 ev and the threshold for indirect transitions at 1.06 ev and 1.16 ev at 300°K and 77°K, respectively.
A high voltage, high efficiency silicon solar cell has been designed,
combining the better features of MIS and P-N junction cells. The MINP is
basically a shallow P-N cell with an MIS contact made to the top of the
cell. MINP performance advantages as compared to P-N junction cells are
due to the low effective recombination velocity at the silicon surface
under the MIS contact, allowing open circuit voltages of up to 678 mV.
The high voltage results in improved efficiency, and for the 2 cm x 2 cm
cell test structure, 16% total area efficiencies have been measured.
High voltage cells also offer a decreased sensitivity to increasing
temperature. In addition, the MINP device is well suited for ion
implantation as electron concentration near the surface is more
controllable, and ion implantation is a vacuum process, the preferred
technique for the top contact in MINP cells.
Self Aligning Method for Forming A Selective Emitter and Metallization in a Solar Cell
- S R Wenham
- M A Green
S. R. Wenham and M.A. Green, Self Aligning Method
for Forming A Selective Emitter and Metallization in
a Solar Cell. Patent No 6429037, August 2002:
United States.
Impact of Laser Induced Defects on the Performance of Solar Cells Using Localised Laser Doped Regions Beneath the Metal Contact, 22 nd European Photovoltaic Solar Energy Conference and Exhibition
- A Sugianto
Sugianto, A. et al, Impact of Laser Induced Defects
on the Performance of Solar Cells Using Localised
Laser Doped Regions Beneath the Metal Contact,
22 nd European Photovoltaic Solar Energy Conference
and Exhibition, 2007, Milan, Italy.
Self Aligning Method for Forming A Selective Emitter and Metallization in a
- S R Wenham
- M A Green
S. R. Wenham and M.A. Green, Self Aligning Method
for Forming A Selective Emitter and Metallization in
a Solar Cell. Patent No 6429037, August 2002:
United States.