Study On The Behavior And Mechanism Of Potential Induced Degradation (PID) Of Crystalline Silicon Solar Cell And Module

In recent years,a lot of large crystalline silicon PV system solar farms have the problem of potential induced degradation?PID?,which cause a huge lost to the power station.The reason for the generation of PID is quite complex,there are few studies on the reason of PID at this moment,so it is significant to do some research on the PID problem.In this paper,crystalline silicon module of core components of PV system is studied as the object of study.From the performance of crystalline silicon cell technology and the encapsulating materials,large-scale production line is employed as the experimental platform,and the PID is studied from three aspects including micro-cracks and defects,the solar cell manufacture process of diffusion and passivation and the encapsulating materials.The mechanism of PID is deduced based on the data analysis.Main research results are summarized as follows:?1?The effect of different defect density on PID phenomenon is studied.The results showed that higher defect density has more obvious negative effect on the V_oc,J_sc and efficiency of cells.After PID test,the Rsh of cells all decreases rapidly and with the increase of defect density,the Rsh decreases faster and more serious leakeage occurs.Thererfore,a more obvious PID phenomenon is observed.Different silicon cells,including monocrystalline,polycrystalline and quasi-monocrystalline were compared in the PID test and similar law was observed,further demonstrating the close relationship between defect density and PID.The hypothetical mechanism of the correlation between high defect density and PID phenomenon is as follows:when the Na⁺moves to the defect and dislocations,it will continue to move into the silicon along with the dislocation,and be neutralized to two-dimensional quasi-metal by the electrons in the silicon wafer.Thus,the Na⁺inside the silicon wafer is consumed all the time.The Na⁺concentration inside the silicon is always lower than that at the surface,which forms a concentration gradient,promoting the diffusion of Na⁺from the surface to inside.In the end,a Na quasi-metal through the PN junction is formed inside the silicon,which results in the leakage of the PN junction,decreases the shunt resistance and finally the efficiency of solar cell.?2?The effect of different micro-crack length on the PID attenuation is studied.It is found that the longer the micro-crack of the silicon wafer,the greater the decrease in V_oc,J_sc and FF of the solar cells,and the probability of occurrence of PID is also increased with the increase of micro-crack length.The hypothetical mechanism of correlation between micro-cracks and PID phenomenon is as follows:at the surface without micro-cracks,a large number of electrons are attracted by Na⁺with positive charges,decreasing the number of electrons moved to the fingers.On the other hand,because the negative charges are accumulated at the surface of the wafer,it will repel the negative charges moving from P-type silicon to the surface,and attract the positive charges at the same time,which reduces the number of electron-hole pairs.At the surface with micro-cracks,while the electrons and holes are hindered,the movement of Na⁺to the SiN_x,P,N area is hindered.It is easy for Na⁺to accumulate at the edge of micro-cracks to capture electrons and act as recombination centers.When more and more Na⁺are accumulated at the micro-cracks,the collection of current will be decreased,leading to the current leakage.The micro-crack can also be explained as a kind of defect on the macro level,leading to the occurrence of PID phenomenon.?3?Through the experiment of different diffusion processes on the PID effect,it is found that different emitters of diffused crystalline silicon cells have a great influence on the electrical performance of the solar cell.With the increase of the emitter resistance,the PID attenuating range decreases slowly when the square resistance is between 60 and 90?.When the square resistance is greater than 90?,the PID attenuation increases rapidly.The hypothetical mechanism of the correlation between diffusion process and PID phenomenon is as follows:because the module is under negative bias,a lot of positive charges are accumulated at the SiNx/Si surface.At the SiNx/Si surface,the negative charges are induced and accumulated at the surface,which can attract holes,leading to a increase of holes;when the concentration of holes is larger than that of electrons,an anti-layer is created,part of the PN junction disappear,which cause the shunting current and eventually leading to the creation of PID,especially at high square resistance,the doping and the thickness of PN junction are not uniform,it is easier to create the anti-layer when the thickness of PN junction is thin.?4?Through the experiment of different passivation process on the PID effect,it is found that the way of SiOx and SiNx deposition can directly influence the PID phenomenon.The lifetime of minority carrier and the anti-PID performance of thermal oxidation are better than that of O₃ oxidation at normal temperature.The lifetime of minority carrier and the anti-PID performance of tubular PECVD are better than that of plate PECVD.When the thickness of SiNx is constant,with the increase of the refractive index,the anti-PID attenuation performance is gradually enhanced.When refractive index is constant,as the thickness of SiNx increases from 19nm to83nm,the anti-PID attenuation performance is gradually enhanced.The anti-PID performance of monolayer SiO₂ thin film is better than that of SiNx thin film.When the thickness is constant,with the increase of refractivity of SiNx/SiOx double layer,the anti-PID performance is gradually enhanced,and better than monolayer SiNx.The hypothetical mechanism of correlation between passivation process and PID phenomenon is as follows:When the thickness of the refractivity of passivation thin film is high.the leakage of passivation thin film is increased,resulting in a decrease of the accumulation of charges at the surface.It decrease the charges at the surface,and decreasing the PID effect.?5?The EVA film materials with different additives?Coupling agent KH570,crosslinking agent TBEC,and cross-linking agent TAIC?were prepared by screw extrusion.The results showed that the higher the volume resistivity,the better performance of anti-PID effect.The three additives in EVA material are 1.5:1:1 or1:1.5:1 or 1:1:1.5,respectively,showing a better anti PID performance.All of the three additives have an influence on the anti-PID performance,and the ration 1:1:1.5gives best performance.The hypothetical mechanism of correlation between three kinds of additive and PID phenomenon is as follows:with the additive in system increases gradually,the degree of crosslinking increase gradually,the three-dimensional network structure density in EVA is getting higher and higher,So that the volume resistivity test is rising,the consumption of the three additives achieve to a balance,the additive molecule with high mobility decreases,making the anti-PID performance become the most outstanding.