Silica-sol-based spin-coating barrier layer against phosphorous diffusion for crystalline silicon solar cells
© Uzum et al.; licensee Springer. 2014
Received: 23 July 2014
Accepted: 17 November 2014
Published: 5 December 2014
The phosphorus barrier layers at the doping procedure of silicon wafers were fabricated using a spin-coating method with a mixture of silica-sol and tetramethylammonium hydroxide, which can be formed at the rear surface prior to the front phosphorus spin-on-demand (SOD) diffusion and directly annealed simultaneously with the front phosphorus layer. The optimization of coating thickness was obtained by changing the applied spin-coating speed; from 2,000 to 8,000 rpm. The CZ-Si p-type silicon solar cells were fabricated with/without using the rear silica-sol layer after taking the sheet resistance measurements, SIMS analysis, and SEM measurements of the silica-sol material evaluations into consideration. For the fabrication of solar cells, a spin-coating phosphorus source was used to form the n+ emitter and was then diffused at 930°C for 35 min. The out-gas diffusion of phosphorus could be completely prevented by spin-coated silica-sol film placed on the rear side of the wafers coated prior to the diffusion process. A roughly 2% improvement in the conversion efficiency was observed when silica-sol was utilized during the phosphorus diffusion step. These results can suggest that the silica-sol material can be an attractive candidate for low-cost and easily applicable spin-coating barrier for any masking purpose involving phosphorus diffusion.
Crystalline silicon solar cells currently dominate the photovoltaic market, while ongoing research is mainly focused on increasing the conversion efficiency of the solar cells and the reduction of production costs. Junction formation is one of the most crucial steps during the solar cell fabrication process. Various methods are used to form homogenous phosphorus-diffused emitters for p-type silicon solar cells. These methods differ according to the technique used to deposit the phosphorus source onto the silicon surface, including deposition of phosphorus oxychloride (POCl3) [1, 2], diluted orthophosphoric acid (H3PO4) by spray [3, 4], sol-gel sources through spin-on deposition techniques , or by using the screen-printing technique . However, the diffusion of phosphorus atoms to the rear surface cannot be avoided by either out-gas diffusion, regardless of the used phosphorus dopant source, or direct diffusion, such as in cases using POCl3. Therefore, it is necessary to mask the surfaces that one does not want to be diffused by phosphorus-doping atoms. SiO2 films are in use for many silicon device fabrications for either masking or passivation purposes. Conventionally, the deposition of SiO2 films using gas-phase deposition methods (atomic layer deposition [7, 8], chemically catalyzed chemical vapor deposition , atmospheric-pressure chemical vapor deposition (APCVD) [10, 11], low-pressure chemical vapor deposition (LPCVD) [10, 11], and plasma-enhanced chemical vapor deposition (PECVD) ) have been performed. However, particle contamination and substrate surface damage come along as disadvantages  using CVD. Moreover, in some case, the use of dangerous silane gas is introduced [10, 11]. Thermally growing oxide at high temperatures is another technique used to form oxide layers, either by dry oxidation using pure oxygen gas  or wet oxidation using oxygen/hydrogen steam . Both are widely used in the fabrication of solar cells, especially for masking [2, 14] and passivation purposes . In order to pursue simple, high-performance and cost-effective production, the development of high-performance/low-cost materials and their adaption into the silicon solar cell fabrication process is crucial. This paper introduces a spin-coating silica-sol barrier material to protect against phosphorus diffusion. The silica sol barrier layer can be also applied by spray deposition or the screen printing method, should the proper modifications and improvements be made. However, such materials have yet to be investigated and reported sufficiently. It can be simply spun on the substrate surface prior to phosphorus diffusion and directly annealed simultaneously with phosphorus after the drying step. The performance evaluation of the silica-sol barrier material was carried out mainly in terms of sheet resistance measurements, secondary ion mass spectrometry (SIMS) analysis and scanning electron microscope (SEM) measurements. P-type CZ-Si solar cells were also fabricated both with and without using the silica-sol material during the phosphorus diffusion process.
Results and discussion
Average sheet resistances of four samples measured on front and rear surfaces of the wafers
Spin speed for coating (rpm)
Front surface resistance (Ω/sq)
Rear surface resistance (Ω/sq)
On the other hand, diffusion of carbon atoms was observed in all wafers of group 2 (Figure 6). The carbon concentration in group 1 wafers was observed to be the lowest level of all, with an intensity of around 2 × 104 counts/s in a steadily decreasing attitude. However, higher carbon intensity with deeper profiles was observed for all group 2 wafers, which may come from the silica-sol paste. This diffusion of carbon has no degradation effect on the barrier film properties but needs to be investigated further. It is important to mention that although the silica-sol barrier paste in this report was investigated mainly for spin-on phosphate diffusions, some experiments were also tried out with unlimited sources like POCl3. However, only limited barrier effects could be observed for POCl3 source. Therefore, improvement of the silica-sol is in progress so that it may be applied to POCl3 diffusions as well.
Electrical characteristics of fabricated solar cells with/without using silica-sol on rear side during phosphorus diffusion
Solar cell parameters
Group 2 applied spin speed (resulted film thickness)
2,000 rpm (0.77 μm)
4,000 rpm (0.43 μm)
6,000 rpm (0.30 μm)
8,000 rpm (0.24 μm)
J sc (mA/cm 2 )
V oc (mV)
A spin-coating silica-sol material was introduced as a promising barrier material for phosphorus diffusion. The out diffusion of phosphorus could be completely prevented by using silica-sol-based film prepared using mixture of silica-sol dispersion with TMAH (9:1 in volume). After the evaluation of the material, the silicon solar cells were fabricated both with and without using the silica-sol. Conversion efficiency improvement was observed up to around 2% when utilizing silica-sol during the phosphorus diffusion step. These results can lead to the use of simple, cost-effective and high-performance silica-sol material in the silicon solar cell fabrication process. It is clear that the thermal budget of this process is lower than those of the usual techniques. The material is also cheap to produce, with the actual chemical (silica-sol) price shifting significantly according to the production volume, which will be considered in the future production stage.
Actually, it should be worth comparing the effect of the silica-sol layer with samples using currently used diffusion barrier layers (SiOx or SiNx) than with samples using no barrier at all. However, applying a non-solution-based barrier layer requires adding more steps to the process, the use of expensive equipment, etc. In any event, the development of a solution-based diffusion barrier material, as well as its application through spin coating, was the goal of this work. Indeed, the spin-coating process is used widely, mainly at laboratory scales, for fabricating thin films. We aimed to coat, and evaluate, silica sol on flat surfaces by spin coating first due to the inherent difficulties when trying to apply it to a textured surface. In the future steps, spray deposition or screen printing methods may become possible, but the necessary modifications and improvements to the paste are still under development at this time.
AU is a postdoc researcher in University of Hyogo. KF was a Master course student in University of Hyogo. YK and KT are researchers in Nissan Chemical Industry Co. Ltd. (Japan). SY and YJ are students in NAIST (Japan). YI is an Associate Professor in NAIST (Japan). YU is a Professor in NAIST (Japan). SI is an Associate Professor in University of Hyogo (Japan).
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