Screen-printed Silicon Solar Cells Research Articles

Contribution of the photoluminescence effect of the stain etched porous silicon in improvement of screen printed silicon solar cell performance

In this work, we investigate the potential use of porous silicon (PS) nanostructures stain etched into photovoltaic technology as one possible way to increase the silicon solar cell (SC) efficiency at low cost. The process is based on a combination of texturization and porous silicon formed by metal assisted etching which used the screen printed front grid contact. The photoluminescence (PL), the reflectivity spectra of the PS layers, spectral response and electrical measurements are presented and discussed. As a result, the short circuit current density was improved by more than 21%. We observed an increase in internal quantum efficiency (iQE) in the region where the PL of the PS layers appears and the minority carriers’ diffusion length is enhanced by more than 60 . We report the correlation between the converter PL intensity and iQE and we demonstrate that the photoluminescence properties and the increased photocurrent result in an improvement of the solar cell efficiency.

Simulation-based roadmap for the integration of poly-silicon on oxide contacts into screen-printed crystalline silicon solar cells

We present a simulation-based study for identifying promising cell structures, which integrate poly-Si on oxide junctions into industrial crystalline silicon solar cells. The simulations use best-case measured input parameters to determine efficiency potentials. We also discuss the main challenges of industrially processing these structures. We find that structures based on p-type wafers in which the phosphorus diffusion is replaced by an n-type poly-Si on oxide junction (POLO) in combination with the conventional screen-printed and fired Al contacts show a high efficiency potential. The efficiency gains in comparsion to the 23.7% efficiency simulated for the PERC reference case are 1.0% for the POLO BJ (back junction) structure and 1.8% for the POLO IBC (interdigitated back contact) structure. The POLO BJ and the POLO IBC cells can be processed with lean process flows, which are built on major steps of the PERC process such as the screen-printed Al contacts and the text{Al}_text{2 }text{O}_text{3 }/text{SiN } passivation. Cell concepts with contacts using poly-Si for both polarities (text{POLO}^2-concepts) show an even higher efficiency gain potential of 1.3% for a text{POLO}^2 BJ cell and 2.2% for a text{POLO}^2 IBC cell in comparison to PERC. For these structures further research on poly-Si structuring and screen-printing on p-type poly-Si is necessary.

Photovoltaic and Charge Trapping Characteristics of Screen-Printed Monocrystalline Silicon Solar Cells with Molybdenum Oxides as Hole-Selective Layers by H2/Ar Plasma Pretreatment

Photovoltaic characteristics of screen-printed monocrystalline silicon solar cells (SPSSCs) with molybdenum oxide (MoO x ) as hole-selective layers (HSLs) were demonstrated. A H2/Ar plasma pretreatment (PPT) was incorporated into a MoO x /p-Si(100) interface, which shows the expected quality in terms of passivation. Moreover, the charge trapping characteristics of the MoO x /p-Si(100) interface were presented. The PPT parameters, including power, treated time, flow ratio of H2/Ar, and temperature, were investigated. The experimental results indicate that the Si-H bond with a relatively high intensity was demonstrated for the H2/Ar PPT. The achievement of a conversion efficiency (CE) improvement of more than 1.2% absolute from 18.3% to 19.5% for SPSSCs with H2/Ar PPT was explored. The promoted mechanism was attributed to the reduction of the interface trap density caused by the large number of Si-H bonds at the silicon substrate and MoO x interface.

Correlation between the open-circuit voltage and recombination loss at metal-silicon interfaces of crystalline silicon solar cells

For screen-printed silicon solar cells, optimization of the contact characteristics between the front metal electrode and silicon is very significant for realizing high efficiency. As technology advances, the solar cell efficiency has been steadily increased. Especially, as surface recombination becomes more important in high efficiency solar cells, understanding and controlling recombination in the metal contact area are necessary. Recombination at the metal-silicon interface is a major cause of the drop in the open-circuit voltage (Voc) of a solar cell. Thus far, the study of electrodes in silicon solar cells has been largely aimed at reducing the series resistance, and few studies on recombination due to electrodes have been performed. Quantitatively evaluating the recombination in electrodes to assess the effect on the efficiency is expected to become more important in the near future. In this paper, the contact characteristics of a screen-printed silver electrode and silicon interface were analyzed using saturation current density (Jo) measurements according to the surface doping concentration and firing temperature. The effects of the contact characteristics on Voc and recombination were also investigated. Experimental results showed that Jo.pass decreased with decreasing surface doping concentration and Jo.metal increased with increasing surface doping concentration and firing temperature. For quantitative analysis of Jo.metal, the size and distribution of Ag crystallites were observed using SEM and TEM, and the Ag concentration was analyzed by ICP-OES measurements. The larger Jo.metal was, the higher the Ag crystallite concentration, indicating that the Ag crystallites under the electrode increased Jo.metal. The effect of Jo.metal on the electrical characteristics of the solar cell was analyzed by calculating the change in the surface recombination velocity and the decreased width of Voc. Through this study, the recombination in the metallized area, which is expected to become increasingly important, and particularly the effects of the doping profile of the emitter region and silver crystallites on the surface recombination were quantitatively assessed. The amount of silver crystallites on the silicon wafer was quantitatively analyzed.

Photovoltaic and Physical Characteristics of Screen-Printed Monocrystalline Silicon Solar Cells with Laser Doping and Electroplated Copper

Photovoltaic and physical characteristics of screen-printed monocrystalline silicon solar cells (SPMSCs) were presented with electroplated copper (EPC) as the rear contact. The boron back surface field (B-BSF) formed by spin-on doping and laser doping (LD) was prepared as a seed layer for the EPC. The LD parameters, including the laser focus, laser power, laser speed, and laser line pitch, were investigated. Moreover, the effects of KOH etching on the surface properties after the LD process were explored. Furthermore, to enhance the adhesion between the B-BSF seed layer and EPC contact layer, a laser pinhole process was proposed. Finally, the EPC processes with various electroplating times were addressed. The results revealed that the mechanism of enhancements could be attributed to a continuous B-BSF seed layer and a reduction of series resistance, as well as an increase of open-circuit voltage and adhesion between the B-BSF seed layer and EPC contact layer.

Determination of Metallization-Induced Recombination Losses of Screen-Printed Silicon Solar Cell Contacts and Their Dependence on the Doping Profile

This paper presents an easy-to-implement, but rigorous method to determine the dark saturation current densities $J_{01m}$ and $J_{02m}$ under the metallized region of a silicon solar cell contact. The method is based on an autofitting procedure by the Griddler artificial intelligence finite-element simulation program to measured photoluminescence images of samples with especially designed test structures. It is found that a deeper p-n junction strongly reduces the recombination losses at the metal–silicon interface. By carefully tailoring the boron diffusion profile, the $J_{01m}$ is reduced from 839.2 to 274.2 fA/cm2.

Impact of the Presence of Busbars During the Fast Firing Process on Contact Resistances

Uniform and low contact resistances between Ag grids and Si emitters are very important for reducing series resistance losses in screen-printed silicon solar cells. In this paper, we studied systematically the contact resistance of firing-through Ag pastes on samples that were processed in a fast firing furnace with and without the presence of busbars. We fabricated p-type full-area aluminum back surface field and n-type passivated emitter rear totally diffused solar cells with various doping concentrations and surface morphologies. For all tested conditions, lower contact resistances were obtained when firing without busbars. For instance, by omitting the busbars during firing, on a 138 $\Omega$ /sq phosphorus emitter, the median contact resistance dropped from 7.5 to 0.7 m $\Omega$ $\cdot$ cm $^{2}$ , and on an 85 $\Omega$ /sq boron emitter, the median contact resistance decreased from 80 to 1 m $\Omega$ $\cdot$ cm $^{2}$ . Furthermore, we found that the pure Ag paste outperformed the AgAl paste in contacting boron emitters. An explanation of these results might be the short-circuit effect, which is a fundamental mechanism that strongly influences contact formation. In addition, in spite of the significant impact of the presence of busbars during firing on contact resistances, its impact on the $J_{0,\text{met}}$ was found to be only marginal.

Improvement on electrical properties of screen-printed silicon solar cells by light-induced electroplating of silver

The total power losses are analyzed based on two-layer electrode of light-induced electroplating after screen printing. By the introduction of scale factor in the calculation, optical losses are closer to the actual condition. Optimized two-layer contact of front side could diminish power losses distinctly and improve the current–voltage (I–V) characteristic of crystalline silicon solar cells. The relative increments of I–V parameters as a function of electroplating time are measured. The quantitative comparison of analytical results between half-ellipse shape model and half-circled shape model is performed. The numerical simulation results and experimental data show good agreement. Due to the successful verification, the simulation results could be used to optimize the two-layer electrode structure and light-induced electroplating process.

Generalized analysis of the impact of emitter sheet resistance on silicon solar cell performance

Emitter sheet resistance contributes significantly to the distributed series resistance of a solar cell. The series resistance (Rs) impacts the fill factor (FF) and in turn affects the short-circuit current (Jsc) and hence the efficiency. It is therefore, imperative to understand the properties of the emitter in order to achieve excellent ohmic contact. In this study, an analytical modeling of the dependence of open-circuit voltage (Voc), Jsc, FF, and efficiency of a silicon solar cell on emitter sheet resistance was carried out. In order to better evaluate Rs based on various metal grid designs, different grid models were used to investigate the resistive losses. And the one having the best match with the reference cell was incorporated in the simulation. It was found that for a typical three-bus-bar screen-printed silicon solar cell, the emitter sheet resistance should be in the 70–90 Ω/□ range.

Industrial Screen-Printed n-Type Rear-Junction Solar Cells With 20.6% Efficiency

Screen-printed high-efficiency industrial n-type rear-junction silicon solar cells were fabricated on 5-in commercial grade Cz wafers. A furnace-diffused boron emitter and a laser-doped phosphorous front-surface field were applied to produce n-type rear-junction cells with PECVD SiN x on the front and PECVD AlO x /SiN x on the back for surface passivation. All contacts were screen printed. An average efficiency of 20.33% was achieved, while the best efficiency was 20.65%. The initial results indicate that the potential for a higher FF can be achieved by improving the front pattern and optimizing the condition for metallization, enabling an even higher efficiency.

Characteristics on Silicon Oxynitride Stack Layer of ALD-Al2O2 Passivation Layer for c-Si Solar Cell

Silicon oxynitride that can be deposited two times faster than general SiNx:H layer was applied to fabricate the passivation protection layer of atomic layer deposition (ALD) Al2O3. The protection layer is deposited by plasma-enhanced chemical vapor deposition to protect Al2O3 passivation layer from a high temperature metallization process for contact firing in screen-printed silicon solar cell. In this study, we studied passivation performance of ALD Al2O3 film as functions of process temperature and RF plasma effect in plasma-enhanced chemical vapor deposition system. Al2O3/SiON stacks coated at 400 oC showed higher lifetime values in the as-stacked state. In contrast, a high quality Al2O3/SiON stack was obtained with a plasma power of 400 W and a capping-deposition temperature of 200 oC after the firing process. The best lifetime was achieved with stack films fired at 850 oC. These results demonstrated the potential of the Al2O3/SiON passivated layer for crystalline silicon solar cells.

Enhanced Physical and Electrical Characteristics of Screen-Printed n-Type Mono-Crystalline Silicon Solar Cells by AgAl Front Pastes with Various Aluminum Contents

Silver-aluminum (AgAl) pastes with various aluminum (Al) contents have been synthesized to enhance physical and electrical characteristics of screen-printed n-type mono-crystalline silicon solar cells (SPNMSCs). The improved series resistance and contact adhesion were demonstrated by a 4 wt% of Al contents doped AgAl front pastes (FPs). The achievement of a conversion efficiency improvement of more than 1.7% absolute from 14.6% to 16.3% in SPNMSCs with a 4 wt% of Al content in AgAl FP was explored. The promoted mechanism could be attributed to suitably front-surface-field formed at p + emitter by circulating track supplied from Al powder softened in AgAl FPs during co-firing.

Edge isolation of solar cells using laser doping

A technique of using laser doped isolation lines to separate shunted edge regions from the active area of a solar cell is presented. Photoluminescence images are used to investigate the effectiveness of the edge isolation. Screen-printed silicon solar cells are fabricated to demonstrate the ability of the technique to remove the shunt path from the front emitter to rear contact whilst minimising edge recombination. It is found that a reduction of J02 by on average a factor of more than three is possible compared to the standard method of cleaving for these laboratory cells, yielding an average improvement in FF of 1.7%abs, and 0.5%abs efficiency increase. The J02 values achieved using this process are comparable to those achieved on solar cells with edges passivated with a thermal oxide.

Ag contact properties according to the front grid width and firing temperature for silicon solar cells.

The effect of peak firing temperature and grid width on the contact properties between Ag metal and silicon (n+ emitter) was investigated for screen-printed silicon solar cells. We confirmed the factors that control the specific contact resistance as follows: (1) the Ag coverage fraction on the silicon surface, d(2) the thickness of the glass layer and (3) the etching depth on the n+ emitter region. The lowest specific contact resistance (8.27 mΩ x cm2) was obtained at the optimum firing temperature (720 degrees C). We also found that the grid width affected the contact quality of Ag paste because the contact width related to the absorbed heat of samples in RTP system. For this reason, when the grid width was further reduced, meaning more heat absorption, more Ag crystallites grew and the glass layer thickened. Light I-V results of a 6-inch silicon solar cell with minimum busbar width were similar to the PC1D simulation results. The efficiency was improved by 0.2% with the reduction of the busbar width.

Effect of rapid thermal oxidation on structure and photoelectronic properties of silicon oxide in monocrystalline silicon solar cells

This paper concerns the topic of surface passivation properties of rapid thermal oxidation on p-type monocrystalline silicon wafer for use in screen-printed silicon solar cells. It shows that inline thermal oxidation is a very promising alternative to the use of conventional batch type quartz tube furnaces for the surface passivation of industrial phosphorus-diffused emitters. Five minutes was the most favorable holding time for the rapid thermal oxidation growth of the solar cell sample, in which the average carrier lifetime was increased 19.4 μs. The Fourier transform infrared spectrum of the rapid thermal oxidation sample, whose structure was Al/Al-BSF/p-type Si/n-type SiP/SiO2/SiNx/Ag solar cell with an active area of 15.6 cm2, contained an absorption peak at 1085 cm−1, which was associated with the Si–O bonds in silicon oxide. The lowest average reflectance of this sample is 0.87%. Furthermore, for this sample, its average of internal quantum efficiency and conversion efficiency are respectively increased by 8% and 0.23%, compared with the sample without rapid thermal oxidation processing.

Investigation of the mechanism of the Ag/SiNx firing-through process of screen-printed silicon solar cells

The mechanism of a Ag/SiNx firing-through process in the manufacture of crystalline silicon solar cells.

Improvement of electrical properties in screen-printed crystalline silicon solar cells by contact treatment of the grid edge

In this study, the influence of HF treatment of Ag pastes after a firing process was investigated. It was shown that the HF treatment can improve the fill factors and efficiencies of various cells including those with high initial specific contact resistances. SEM images showed that this improvement is due to the etching of the thin glass layer at the Ag-Si boundary, which exposes the Ag crystallites and colloids. These colloids electrically connect the bulk Ag to the Si through a direct contact, which reduces both the transfer length and the specific contact resistance. A model of the current path was proposed to explain the effect of HF treatment on the edge of the Ag grid.

Excellent c-Si surface passivation by thermal atomic layer deposited aluminum oxide after industrial firing activation

We demonstrate that by using a water (H2O)-based thermal atomic layer deposited (ALD) aluminum oxide (Al2O3) film, excellent surface passivation can be attained on planar low-resistivity silicon wafers. Effective carrier lifetime values of up to 12 ms and surface recombination velocities as low as 0.33 cm s−1 are achieved on float-zone wafers after a post-deposition thermal activation of the Al2O3 passivation layer. This post-deposition activation is achieved using an industrial high-temperature firing process which is commonly used for contact formation of standard screen-printed silicon solar cells. Neither a low-temperature post-deposition anneal nor a silicon nitride capping layer is required in this case. Deposition temperatures in the 100–400 °C range and peak firing temperatures of about 800 °C (set temperature) are investigated. Photoluminescence imaging shows that the surface passivation is laterally uniform. Corona charging and capacitance–voltage measurements reveal that the negative fixed charge density near the AlOx/c-Si interface increases from 1.4 × 1012 to 3.3 × 1012 cm−2 due to firing, while the midgap interface defect density reduces from 3.3 × 1011 to 0.8 × 1011 cm−2 eV−1. This work demonstrates that direct firing activation of thermal ALD Al2O3 is feasible, which could be beneficial for solar cell manufacturing.

Improved Rear-Side Passivation by Atomic Layer Deposition ${\rm Al}_{2}{\rm O}_{3}/{\rm SiN}_{x}$ Stack Layers for High $V_{\rm OC}$ Industrial $p$-Type Silicon Solar Cells

This research develops high open-circuit voltage (VOC) p-type industrial screen-printed silicon solar cells using improved rear surface passivation. It shows a significant improvement in the minority carrier lifetime by low temperature thermal atomic layer deposition of Al2O3 layers and plasma-enhanced chemical vapor deposition of SiNx passivation layers. An increase in the VOC and the short-circuit current (JSC) due to an improved long-wavelength response are also demonstrated. With the optimized stack layers, a high efficiency of 19.2% across a large area (156 cm2) is seen. Furthermore, the rear-side passivation scheme can be easily integrated into the conventional screen-printed process. This is very promising for in-line solar cell manufacturing.

Interface modeling between the printed thick-film silver paste and emitter for crystalline silicon solar cells

According to the potential barriers generated by the silver, glass layer, and N+ emitter, an analytical model is developed to explain interface contact performance between the printed thick-film silver paste and emitter for crystalline silicon solar cells. According to the model, the front contact resistance between the sintered silver and N+ emitter is simulated at different doping concentrations, temperatures, and density of state. The quantitative expression of interface contact resistance between the printed thick-film silver paste and N+ emitter is derived for the first time. The results in this study unify different viewpoints about the current transport mechanisms at the sintered silver-silicon interface. If the glass frit chemistry and silver particle size are carefully tailored, the silver consumption per watt can be reduced, and the efficiency of crystalline silicon solar cells can be further improved. The results lay the foundation for studying the screen-printed crystalline silicon solar cell front contact metallization system. Copyright © 2013 John Wiley & Sons, Ltd.