Cell Efficiency Gain Research Articles

Determination of Metal-Induced Recombination of n-Type Bifacial Si Solar Cells Using Special Print Patterns

A systematic routine to determine the front and rear metal induced recombination losses of bifacial silicon solar cells using an automated Griddler finite-element method fit procedure is presented in this paper. The method prescribes the preparation of easy-to-fabricate test patterns printed on the front and rear side of separate cells, nonmetallized cells, and simple photoluminescence imaging. Eight different samples with similar front boron emitter and different phosphorus back surface fields (BSFs) are analyzed. Choosing an optimized BSF is shown to lead to a cell efficiency gain of ∼0.4% absolute.

Advanced fine line double printing process for manufacturing high efficiency silicon wafer solar cells

A double print technology using a front contact screen pattern of high precision ultra-fine line finger openings is demonstrated in this work. It has helped in achieving an absolute cell efficiency gain of over 0.25% compared to conventional single printing process. A suitable silver paste was applied to both single and double printing process resulting in improved results and a lower paste laydown for double printing process. This paper represents the work done after the double print process from trial phase reached a matured level with the correct selection of screen design and paste compatibility to work well with the selected screens. A significant improvement in short circuit current (Isc) of almost 90 mA was realized for double printed cells due to increased print aspect ratio and active area for light absorption.

Optimization of phosphorus dopant profile of industrial p-type mono PERC solar cells

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%.

Mass production and modeling of high efficiency n-PERT solar cells with ion implanted BSF/selective-BSF

Ion implantation is a technique which has already been successfully transferred from integrated circuits (IC) industry to photovoltaic (PV) industry, especially for p-type solar cells fabrication. Based on the configuration of Yingli n-PERT PANDA cells, phosphorus implantation is employed to fabricate n+ back surface field (BSF) as an alternative to POCl3 diffusion. A 20.3% cell efficiency on average with a maximum value of 20.5% was achieved in mass production by process optimization such as ion implantation, annealing during PECVD and passivation improvement. Resulted modeling data indicates that the selective-BSF (SBSF) cell efficiency gain was 0.2% absolute compared with rear totally homogeneous doped cell after process optimization and the SBSF cells reach the best efficiency at a wafer resistivity around 0.5–1Ωcm depending on wafer lifetime. In this work we proved that cells with efficiency higher than 21% can be realized by optimizing sheet resistance of rear surface and improving passivation.

Heavy phosphorous tube-diffusion and non-acidic deep chemical etch-back assisted efficiency enhancement of industrial multicrystalline silicon wafer solar cells

An industrial process for tube-diffused multicrystalline silicon (multi-Si) solar cells using phosphorus gettering. A cell efficiency gain of 0.5% (absolute) is achieved with heavy chemical etch-back when compared to the as-diffused cells with same final emitter.

Silicon nitride anti-reflection coating on the glass and transparent conductive oxide interface for thin film solar cells and modules

Anti-reflection coating (ARC) is well known as an important technique to enhance solar cell performance. Typical ARC has been applied on the glass surface to reduce light reflection loss at the air/glass interface. However, reflection loss occurs not only at glass surface but also at other interfaces such as glass/transparent conductive oxide (TCO) interface. The refractive index of SiNx is tunable from 1.6 to 2.7, and the range from 1.7 to 2.0 is suitable for ARC at glass/TCO interface. In this study, we examined the AR effect of silicon nitride (SiNx) deposited by plasma enhanced chemical vapor deposition at the glass/TCO interface with thin film silicon solar cell and module. Reflectivity reduction of 1.6% for glass/ZnO substrate has been obtained with optimal SiNx layer, which contribute 2.0% gain in cell efficiency. Besides, we also confirmed the relative efficiency gain of around 2% for large-sized solar module, leading to a world-record large area stabilized module conversion efficiency of 12.34%.

Extensive Comparison of Solar Modules Manufactured with Single and Double Printed Cells

With respect to Single Printing (SP), Fine Line Double Printing (FLDP) metallization process delivers 0.21% absolute efficiency gain and 11mg paste saving in mass production. In fact, an average finger width of 47μm is achieved with FLDP, with 16μm line width decrease if compared to SP. In this work we investigate how this cell efficiency gain translates into module power by looking at extensive module production runs at Yingli. FLDP modules use 10% less Ag, have comparable Cell To Module (CTM) loss and reliability performance, and exhibit better Electroluminescence yield due to absence of interruptions.

0.4% absolute efficiency increase for inline-diffused screen-printed multicrystalline silicon wafer solar cells by non-acidic deep emitter etch-back

Emitter formation is one of the most critical and crucial process steps in the fabrication of standard silicon wafer solar cells. Typically the photovoltaic industry uses tube based phosphorus diffusion, using phosphorus oxychloride as the dopant source. Alternately, a low-cost inline diffusion using phosphoric acid as a dopant source can be used. However, proper process conditions must be used to meet solar cell energy conversion efficiencies obtained by tube diffusion. In this work, we present the application of a non-acidic homogeneous emitter etch-back process – the ‘SERIS etch’ – for inline-diffused emitters in order to raise the efficiency of multicrystalline silicon (multi-Si) wafer solar cells. We apply both light and heavy emitter etch-backs on inline-diffused emitters with sheet resistance (Rsq) values in the 40–60Ω/sq range to achieve emitters with a target Rsq of ~70Ω/sq. The emitter surface reflectance and doping uniformity are maintained even after an etch-back that results in a Rsq change of ~30Ω/sq. An average cell efficiency gain of 0.4% (absolute) is reported for cells with heavy etch-back when compared to the as-diffused non-etch-back screen-printed full-area aluminum back surface field solar cells and efficiencies up to 17.9% are achieved. Besides, best lot of the etch-back inline-diffused cells shows a 0.2% (absolute) efficiency gain over the standard tube-diffused cells. These results show that the ‘SERIS etch’ etch-back process can enable higher-efficiency industrial inline-diffused multi-Si wafer solar cells.

Fine line metallization by coextrusion technology for next generation solar cells

This paper introduces the application of a coextrusion printing as a contact and mask-free fine line metallization technology for direct printing of narrow grid lines with a strongly enhanced aspect ratio. Coextrusion printed grid lines enhance short circuit current and lower series resistance through a higher number of filigree fingers. The silver and sacrificial paste that merge inside the nozzle are simultaneously extruded onto the wafer and form a silver paste structure smaller than every design feature inside the nozzle. After contact firing the grid lines become as narrow as 35µm with an aspect ratio up to 0.7. A further shrinkage potential down to 23.7µm width with an aspect ratio up to 0.62 has been demonstrated. Coextrusion printed grid lines outperform screen printed grid lines in a whole slew of geometrical and optical properties. As for example grid line width and height variation is as low as ±0.5µm and the optical effective finger width is far smaller than the screen printed effective finger width. Due to these low effective finger widths the absolute effective shading of 96 coextrusion printed grid lines is less than half of the 80 screen printed grid lines. The calculated short circuit current gain, due to the lower effective shading was experimentally confirmed accordingly. Furthermore a benchmark test in a production like environment was performed. Hereby an absolute cell efficiency gain of +0.25% over screen printed metallization was confirmed and a cell efficiency gain of over +0.35% to +0.4% abs. can be assumed to be achieved by using a direct contacting coextrusion silver paste, which is comparable to state of the art screen printing pastes.

Novel selective emitter process using non-acidic etch-back for inline-diffused silicon wafer solar cells

In this work, a novel etch-back approach for the fabrication of a selective emitter (SE) structure is reported for inline-diffused p-type monocrystalline silicon wafer solar cells. The complete SE process, named the ‘SERIS SE’ process, involves screen printing of an etch mask, use of a HF-free etch-back solution and screen-printed metallisation. Both the emitter etch-back and etch-mask dissolution are performed simultaneously in a single processing step, thereby reducing the number of processing steps as compared to other etch-back based SE technologies. For inline-diffused emitter (ILDE) solar cells, the ‘SERIS SE’ process actually requires only one additional processing step, as an emitter etch-back is typically applied to remove the detrimental top layer. An average cell efficiency gain of 0.4% (absolute) is reported for the SE cells fabricated using the single-step SERIS SE process, as compared to etch-back homogeneous-emitter (HE) solar cells. An average batch efficiency of 18.5% is achieved for screen-printed p-type 156 mm pseudo-square Cz mono-Si full-area aluminium back surface field (Al-BSF) SE solar cells. The minimal increase in the number of processing steps, reliance on the robust screen printing process, HF-free non-acidic chemical etch-back and applicability to both tube and inline-diffused emitters make the ‘SERIS SE’ process suitable for industrial application on both mono- and multicrystalline silicon wafers.

Improving solar cell efficiency with optically optimised TCO layers

In this paper we analyse the capabilities and limitations of single layer index matching coatings for the glass/TCO or TCO/absorber interfaces in thin film solar cells. Optical models are used to evaluate the potential gain in cell efficiencies with hypothetical ideal matching materials to minimise losses due to reflections at internal interfaces. We suggest the range of desired optical properties of such materials, where a net gain in cell efficiency can be achieved, and discuss the commercial benefits of such intermediate matching layer. For some selected ternary and quaternary novel oxides available, the potential efficiency gain due to the inclusion of an anti-reflective matching layer in a solar cell structure is calculated and compared with the case of ideal matching materials.

Absolute Quantification of Gettering Efficiency from Luminescence Images

A multidiode simulation determines the absolute solar cell efficiency gain due to gettering. The simulation predicts current/voltage characteristics of lateral inhomogeneous industrial solar cells from luminescence images measured before and after gettering or any other processing step. Improved lifetime distributions lead to a predicted increase in solar cell efficiency Δηabs = +0.46% for a POCl3 diffusion on multicrystalline silicon wafers. The simulative approach determines the gettering efficiency of any process step on any wafer material without preselection.

Effect of Al-induced gettering and back surface field on the efficiency of Si solar cells

In silicon solar cell fabrication, impurity gettering from Si by an aluminum layer and indiffusion of Al for creating the back surface field (BSF) are inherently carried out in the same process. We have modeled these two processes and analyzed their impact on solar cell efficiency. The output of gettering and Al indiffusion modeling is used as an input for calculation of solar cell efficiency. The cell efficiency gain is obtained as a function of the processes duration. To check the relative contributions of gettering and BSF in improving the cell efficiency, their effects are evaluated together as well as separately. It is found that, for solar cells fabricated from low quality, multicrystalline Si, the efficiency gain is solely due to gettering. In solar cells made of high quality Si, the efficiency gain is primarily due to gettering, but the BSF may play a significant role if the cell thickness is less than about 200 μm. The two effects are found to be synergetic. The model provides a means for optimization of the temperature regime for both processes, as well as for maximization of solar cell efficiency.

Peripheral loss reduction of high efficiency silicon solar cells by MOS gate passivation, by poly-Si filled grooves and by cell pattern design

This paper reports a variety of methods to reduce the peripheral or edge losses in high efficiency silicon PERL (Passivated Emitter, Rear Locally-diffused) cells. A MOS (Metal Oxide Semiconductor) structure was investigated as a way to passivate the peripheral region of high efficiency PERL silicon solar cells, when this region is shaded during cell measurement. A marginal gain in the cell short-circuit current has been observed when a positive voltage is applied to the MOS gate at the cell peripheral region. When a negative bias is applied to the gate, a current loss, a significant gain in the cell fill factors and a marginal gain in cell efficiency have been observed. Two-dimensional numerical modelling has been used to analyse this performance. Although the model has predicted a similar trend, it can not fully fit to the experimental data. A weakly inverted surface channel may have resulted in the fill factor change. A higher efficiency gain is predicted if the surface channel can be eliminated. Other experimental methods to passivate scribed PERL cells are also discussed in this paper. Laser-cut grooves filled with poly-silicon at the cell edges have resulted in an improved cell performance after cell peripheral regions have been scribed off. Special design of the cells for a shingled array application has also significantly improved the cell performance, and made it possible to demonstrate 23·7% efficiency on a 21·6 cm2 large area, scribed silicon cell. Copyright © 2000 John Wiley & Sons, Ltd.