V. Juzumasa, A. Galdikasa, A.Melninkaitisb, G. Slekysc aApplied Research Institute for Prospective Technologies
Galvydzio 5, LT-08236, Vilnius, Lithuania bVilnius University, Laser Research Center Sauletekio av. 10, LT-10223 Vilnius, Lithuania
23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain
LASER ABLATION OF PASSIVATING BARRIER LAYER COATED SILICON USING HIGH REPETITION RATE FEMTOSECOND PULSES FOR SELECTIVE EMITTER FORMATION
The advantage of femtosecond laser pulses over nanosecond and picosecond pulses is shorter penetration depth of thermal diffusion, which generates smaller or if any Heat Affected Zone (HAZ). This zone is known to be an aggregation of various defects and causes a decrease of solar cell efficiency. The use of femtosecond laser radiation is expected to increase the machining quality of silicon wafers and passivating coatings compared to nanosecond and even picosecond laser radiation. Furthermore, recently developed diode pumped solidstate (DPSS) Yb:KGW femtosecond laser systems, as one that was used in this work, allow the use of pulse repetition rates up to several hundred kilohertz. High average pulse power and high pulse repetition rates can expedite micromachining throughput considerably and have a great industrial potential.
cAltechna Co. Ltd. Konstitucijos av. 23C-640, LT-08105, Vilnius, Lithuania
ABSTRACT: Selective emitter is an interesting approach presumptively leading to the efficiency increase of silicon solar cells. In this article, femtosecond laser ablation was used to remove locally the passivating layer for the subsequent selective emitter formation. Mainly the laser ablation can be applied in several techniques: one step emitter formation by diffusion of dopants through the passivating barrier layer and technologies that require few diffusion steps or etching back the highly doped emitter. Laser pulses of the femtosecond time scale were used in order to achieve better ablation performance and to induce less damage into underlying silicon layer. Ablation thresholds for various films suitable for silicon passivation have been determined. Results for both planar and textured silicon surfaces are compared. Furthermore, the characteristics of highly doped emitter after the laser ablation of passivating coating have been studied. Dependency of the charge carrier lifetime in the ablated zone as a function of pulse eቤተ መጻሕፍቲ ባይዱergy is discussed. Keywords: laser processing, passivating coating, selective emitter
This work is intended to be applied in several selective emitter formation schemes which involve metal contacts that self-align with pattern formed in passivating films by laser ablation. For example, formation of selective openings in passivating film by femtosecond laser pulses and performing one step diffusion through the barrier mask could be considered as a new innovative
A comparison between laser ablation of various passivating coatings deposited on planar and textured silicon wafers is presented in this paper. Most of these films are formed by sol-gel technology such as SiO2, aluminum silicate glass and TiO2. These films formed in low annealing temperatures have great potential of industrial application. The ablation of this kind of variety of passivating layers with femtosecond high pulse repetition rate radiation is investigated for the first time.
2 METHODOLOGY
Boron doped crystalline <100> orientation polished silicon wafers of 7-10 cm resistivity were used during the experiments. Some of the wafers were textured with random pyramids in KOH solution. Various films were used for coating both textured and non-textured silicon wafers. Plasma enhanced chemical vapour deposition (PECVD) was used to deposit 120 nm thick SiO2 films. SiO2, aluminum silicate glass and TiO2 coatings were formed from sol-gel solution by spin coating and subsequent annealing procedure. The thicknesses of SiO2, aluminum silicate glass and TiO2 layers were respectively 170 nm, 100 nm and 70 nm.
method in solar cell technology. In this way, the high concentration deep emitter is formed in the open areas and low concentration emitter is formed under the barrier. Then the electroless nickel plating aligns the contacts with open areas. The other method include first diffusion step to create emitter of lower diffusion, passivating film coating, laser ablation and second diffusion step to create highly doped emitter areas. Also laser ablation can be performed on highly doped and coated silicon wafer. Then the contact formation and etching back the emitter to reach lower concentrations can be applied.