Research And Application Of Advanced Rear-side Passivation Technology For Crystalline Silicon Solar Cells

Crystalline silicon solar cells occupy the mainstream position in the photovoltaic market,the front-side structure has been optimized extraordinarily,while the rear-side optimizing becomes a key point.Further optimizing the rear-side passivation structure is supposed to achieve a better electrical performance.Two kinds of the most attractive rear passivation schemes can be distinguished by the contact structures on the rear side:one is passivated by a dielectric film and form the local contact by opening the contact window from the passivating film;the other is passivated by a full-area passivating contact structure which has both excellent passivation quality and carrier selectivity.We carry out the following works based on these two passivation structures respectively:For local contact back passivation structure which is relatively mature,proposed to design and preparation of a low-cost bifacial mutlicrystalline PERC cell.By using a new corrosive aluminum paste to fire through the rear-side passivation films directly,forming aluminum fingers structure to replace the whole aluminum back electrode design can achieve double-sided generating capacity;For the passivating contact structures fabricated by depositing a layer of intrinsic amorphous silicon(a-Si)by plasma enhanced chemical vapor deposition(PECVD)onto an oxidized silicon wafer,followed by a thermal POCl3 diffusion process.We explore the influence of the main PECVD parameters on the material and electrical performance(passivation quility and contact resistivity)of such phosphorus doped polysilicon passivating contacts,and provide an consistent explanation.For the local contact rear-side passivating structure,it consists of two parts:non-metallic area and metallic contact area.The non-metallic area is passivated by multi-stack films of 10 nm Al2O3 film and 20 nm SiNx.It mainly depends on the field-effect passivation produced by the fixed negative charges contained in Al2O3 and Si interface.When the thickness of Al2O3 is less than 10 nm,the passivation quality characterization parameter iVoc(implied open-circuit voltage)increases with the increase of the thickness,which is due to the enhanced film uniformity and the increase of fixed charge.However,when the thickness exceeds 10 nm,iVoc is better than 660 mV,with the thickness further increasing,and the iVoc no longer significantly increasing.In addition,the superposition of 20 nm SiNx film can improve the hydrogen passivation and the iVoc further increased by more than 10 mV.On the other hand,the passivation of the metal contact area depends on the back surface field(BSF)formed by Al on the surface of silicon wafer,and the passivation quality increases with the increase of BSF thickness.The bifacial mutlicrystalline PERC cell is based on local contact rear-side passivating structure.Both the passivation quality and fire through ability of the aluminum slurry can be guaranteed with a multi-stack film of 10 nm Al2O3 film and 20 nm SiNx,and a uniform BSF layer can be formed in the metal contact area.A pre-sintering process which is added after screen printing of aluminum paste improve the formation of local BSF at the edge of aluminum fingers effectively.Finally,a champion cell with the front efficiency of 19.86%and rear efficiency of 13.65%is achieved.In addition,the light induced degradation(LID)rate can be controlled within 1.2%by both photo injection annealing and electroinjection annealing.Meanwhile,an annealing process at 385? for 3 min is explored,which is beneficial to improve the cell efficiency.In addition,it is found that the free energy loss of this kind of solar cell comes from the poor quality of mutlicystalline silicon substrate and the poor backside passivation by using the simulation analysis.The effects of silicon wafer resistivity,thickness,aluminum finger width and SiNx film thickness on cell efficiency are further analyzed by simulation in this thesis.For the full-area contact passivating contact structures,it consists of an ultra-thin interfacial silicon oxide layer and a doped polysilicon layer.The structure not only plays the role of passivation,but also undertakes the transport of majority carriers,so the relationship between passivation quality and contact resistivity must be balanced.The influence of main PECVD deposition parameters,such as temperature,power and pressure,are described in detail in this thesis.The moderate damage degree of the interfacial oxide layer can guarantee the passivation quality and obtain a lower contact resistivity.At the same time,doping concentration and crystallinity of doped polysilicon also affect them.When the deposition temperature increases,as-deposited a-Si film is obtained with more microcrystalline or polycrystalline phases,which become nucleation centers and lead to the increase of the crystallinity of the final doped poly-Si.The contact resistivity decreases with the increase of crystallinity.At the same time,higher crystallinity can also cause higher defect density of the interface layer,which will affect the passivation quality.In addition,the film density increases with the increase of the deposition temperature,leads to the diffusion coefficient of phosphorus decrease.The film deposited at a higher temperature can obtain a lower doping concentration,and the break up degree of the interfacial oxide layer is smaller.The final passivation quility increases first and then slightly decreases with the increase of the deposition temperature.Deposition power has minor effect to film crystallinity and density,which leads to a similar contact resistivity.While,lower deposition power unable to fully decompose the precursor gas,higher powers lead to increased hydrogen content,and then blistering phenomenon.Both of them can cause the reduction of the passivation quality.Low deposition pressure causes a decline in crystallinity and density,which leads to low contact resistivity.Meanwhile,the Si substrate contains a higher phosphorus concentration,resulting in a higher auger recombination and a slight reduction in the passivation quality.Finally,the optimal iVoc of 742 mV is obtained at 420?,500 W,0.2 mbar PECVD deposition conditions with a thermal diffusion temperature of 800? and a low contact resistivity of 6.4 mQ·cm2.The efficiency of the solar cell with this passivation contact structure can up to 24.7%,The main advantage of this rear-side passivation structure for solar cell by using numerical simulation is that it can effectively inhibit the recombination of the surface,which can help to obtain an excellent rear side passivation,and the horizontal transport of carriers can be avoided in the silicon substrate.