Study Of High Efficiency Solar Cells Based On Selective Emitter And Rear Passivation

With the increasingly prominent of energy issue, solar energy, which is a renewable and clean energy, has attracted more and more attention. Also, the application prospect of solar cell is becoming more and more extensive. No matter in production or in the practical application, the crystalline silicon is the main substrate of solar cells. However, the popularity of solar cells in domestic family is still very low. Therefore, the direction of solar cell manufacturing company is cost reduction and efficiency improvement.The main content of this dissertation is passivated emitter and rear locally diffused?PERL? solar cells based on selective emitter. The investigation is focused on manufacturing method of front selective emitter, planarization method of rear surface, efficient cleaning of silicon surface before diffusion/oxidation, improvement of AlOx/SiNx passivation stacks, rear locally boron diffusion and metallization, matching of PERL structure with cast-mono substrate and light induced degradation?LID? of PERL solar cells.Firstly, laser doping of front selective emitter is studied. 1064 nm infrared Q-switched laser and 532 nm continuous laser are tested to dope phosphorus. Considering the sheet resistance and minority carrier lifetime after laser doping, 532 nm continuous laser is chosen. After balance the doping concentration and laser damage, it is found that 5 W is the proper laser power. Besides, selective emitter is able to matching high sheet resistance. In addition, finger pitch is also optimized.Then planarization method of rear surface and efficient cleaning of silicon surface before diffusion/oxidation are investigated. Two kinds of rear treatment methods?alkaline polishing and acid etching? for PERL cells are discussed. Suitable thickness of rear SiNx for alkaline polished cells is selected by analyzing LSM images, SEM images, minority carrier lifetime, cells' electrical properties, reflectivity and IQE. In order to reduce production cost and adapt to mass production, the effect of rear acid etching on PERL cells is studied. Cells' minority carrier lifetime, LSM image, reflectivity, IQE and electrical properties are discussed. The efficient cleaning includes RCA and SPM methods. Cleaning effect, stability, repeatability, material cost and time cost are compared. SPM is chosen to be applied in the following experiment.After that, AlOx/SiNx passivation stacks are optimized. First of all, the optimum thicknesses of AlOx/SiNx stacks are simulated by Matlab guided by the the optical admittance matrix method. The simulation results show the optimal thickness of AlOx and SiNx. Then the initial adjustment of the AlOx/SiNx stack, including evaluating the influence of annealing and deposition sequence of passivation layers on minority carrier lifetime, was carried out. It is concluded that 800 ? in-line annealing after ?rear AlOx—rear SiNx—front SiNx? deposition could maximize the effect of field passivation and chemical passivation. Lastly, a large number of experiments are carried out to verify the simulation results of the thickness of AlOx/SiNx stacks. If AlOx is too thick, bubbles will appear on the surface, which reduce the effect of rear passivation and rear reflection. Thus the thickness of AlOx is set to be 25 nm. After weigh the hydrogen passivation and the attenuation effect on AlOx field passivation, the thickness of SiNx is selected.Except rear passivation, a lot of research work is done on rear boron doping, such as rear boron laser doping, laser induced damage recovery, rear metallization, N2 annealing on finished PERL solar cells and PECVD prepared boron doping sources. In the section of rear boron laser doping, two kinds of lasers are investigated. Suitable laser and parameters are selected by analyzing SEM images,ECV profiles and minority carrier lifetime. In order to repair the laser damage, the doped samples are immersed in the alkaline solution. However, the damage corrosion of alkaline solution is excessive. It not only etches the laser damage, but also the boron doped area. Therefore this method is not feasible. Then FGA is attempted. FGA can effectively recover minority carrier lifetime as well as repair laser damage by hydrogenation. After that the effect of firing temperature on solar cells' performance is studied. Cells' reflectivity, IQE, electrical properties and sectional SEM images of Al/Si interface after firing are discussed. Moreover, prepared cells which undergone low temperature annealing show a 0.44% increase. The enhancement of low temperature annealing originally comes from the activation of passivated boron which is deactivated during FGA. Furthermore, to enhance the integration of fabrication steps, simplify manufacturing processes and fulfill low temperature sintering, boron source prepared by PECVD is researched. Four boron doping sources were compared: spinning-on liquid boron sources?B40 and PBF1? and PECVD-deposited solid boron sources?B₂H₆ and TMB?. PECVD-deposited solid B₂H₆ source with its outstanding doping concentration, depth and purity was selected as the most appropriate boron source. Subsequently, three kinds of boron dopant layers deposited by 13.56 MHz RF PECVD were discussed: SiO2?B? layer with different thicknesses, SiNx?B? layer with different refractive indices and a-Si?B? layer with five different firing conditions. It is concluded that a-Si?B? layer is the most suitable boron dopant layer.At last, PERL structure is applied on cast-mono substrate. The LID of cells are studied. Initially, some critical fabrication parameters, such as sheet resistance, thickness of rear passivation stacks, laser pattern and sintering temperature, were optimized. Afterwards, low temperature annealing in N2 or air was performed on prepared cells. The efficiency of cells gained 0.18% abs after 150 ? annealing in N2. Finally, light induced degradation?LID? was studied. Cast-mono silicon solar cells degenerated much less than that of mono PERL silicon solar cells due to the comparatively low interstitial oxygen concentration in cast-mono wafers.This thesis concentrates on the rear of PERL cells, such as rear polishing, rear passivation, rear doping, rear metallization. Another emphasis is to promote industrialization, for instance, light induced plating at front surface, effective cleaning, PECVD fabricated boron source. LID of heavily doped PERL solar cells was also studied. This thesis focuses on the exploration and analysis of the experimental phenomena. It is hoped that the understanding of PERL solar cells could be more intensive from this thesis, and some instructional suggestions may be given to the later investigation.