A Study Of Carrier-selective Contacts For Silicon Heterojunction Solar Cells

Crystalline silicon solar cell products are well developed and widely used.Currently the energy conversion efficiency of commercially produced monocrystalline silicon PERC solar cell has reached 22%,what is more,the energy conversion efficiency of amorphous silicon thin film silicon heterojunction（SHJ）solar cells is as high as 26.6%in laboratory.This is,however,still quite lower than the theoretical efficiency limit of crystalline silicon solar cell（29.4%）.Contact is the region between the highly recombination active metal interface and the photon absorption layer.With the improvement of silicon wafer quality,recombination loss at the contacts will become more obvious,which is also considered as the last obstacle to approach to the theoretical efficiency limit.How to overstep this obstacle,whether there has a physical model to explain the working principles of different structural silicon solar cells,whether it can compare the performance of different materials integration into silicon solar cell structure and what effect the surface states of c-Si will have,which can be answered by the carrier-selective contacts.The carrier-selective contacts can execute low minority-carrier recombination and effective majority-carrier transport,which are becoming the development direction of high efficiency silicon solar cells.Therefore,the research of carrier-selective contacts has great theoretical significance and practical application value.Carrier-selective contacts generally consist of two parts,that is,the carrier-selective transport layer such as（p）a-Si:H thin film and the passivation buffer layer such as（i）a-Si:H thin film.So,a carrier-selective contact is selective transport for one kind of charge carrier,while simultaneously passivating the c-Si surface.The physical mechanism and optimal design of silicon heterojunction（SHJ）solar cells are reviewed and the carrier-selective contacts physical model is introduced,then simulation and experiment are carried out.The main research contents and results are as follows:（1）The effect of fixed charge and work function on the（n）c-Si band bending are simulated using one-dimensional solar cell simulation software AFORS-HET.In order to acquire an apparent（n）c-Si band bending,the fixed charge density must be higher than 10¹¹ cm^-2,and the work function is lower than the work function of（n）c-Si or close to the（n）c-Si valence band edge.These studies can provide certain guidelines for choosing of carrier-selective transport layers.（2）Based on one-dimension solar cell simulation software wxAMPS,three different silicon solar cell structures are numerically simulation,which includes a diffused homojunction silicon solar cell[（p⁺）c-Si/（n）c-Si/（n⁺）c-Si],two SHJ solar cells[（p）a-Si:H/（i）a-Si:H/（n）c-Si/（i）a-Si:H/（n）a-Si:H and（n）MoO_x/（n）c-Si/（n）TiO_x].The energy band structures and the spatial distributions of carrier concentrations are discussed and the physical mechanisms of carrier-selective contacts are analyzed.The（n）c-Si band bending and asymmetric conductivities are acquired,which lead to form carrier selective contacts.We further propose the ratio of conductivity,S,to evaluate the carrier selectivity and the S values of（p⁺）c-Si,（p）a-Si:H and MoO_x contacted with（n）c-Si are 10⁹（μ_p/μ_n）,10¹³（μ_p/μ_n）and 10¹⁷（μ_p/μ_n）,respectively.（3）The effect of interface states on the carrier-selective contacts of SHJ solar cell with amorphous silicon is also simulated based on wxAMPS.For a SHJ solar cell with（n）c-Si substrate,the interface states are donor-like defects and will capture holes,then form a positive charge region.Once the interface state density reaches10¹² eV^-1cm^-2,there will has enough positive charges to shield the effect of fixed negative charge on（n）c-Si band bending,so the asymmetric carrier concentration and asymmetric conductivity decrease and the carrier selectivity degradate.（4）The（i）a-Si:H thin films are deposited on（n）c-Si surfaces with different hydrogen dilution,the passivation mechanism is analyzed and a method to calculate the values of interface state density is proposed.During the hydrogen dilution promotion,the（i）a-Si:H films with more uniform,compact microstructure and fewer bulk defects can cover more（n）c-Si surface,meanwhile H atoms can quickly reach（n）c-Si surface and enter（i）a-Si:H films to saturate dangling bonds,so the values of interface state density decline from 6.9×10¹11 eV^-1cm^-2 to 1.7×10¹11 eV^-1cm^-2.These results quantitatively demonstrate the role of hydrogen proportional control in the preparation of SHJ solar cells from the perspective of carrier selective contact passivation.（5）The hydrogen content,the hydrogen bonding mode,the mass density and the volume deficiency of（i）a-Si:H film are analyzed.The mass density decreases with the increase of hydrogen content,and volume deficiency has a transition from divacancy dominated to trivacancy dominated.The decrease of mass density involves two mechanisms,that is,the substitution of Si-atoms with H-atoms and volume deficiency transition.These results illustrate the importance of controlling the hydrogen content in（i）a-Si:H thin films.（6）The samples of（n）c-Si substrate SHJ solar cell was prepared under the condition of domestic equipment in the laboratory.The power loss and fill factor loss are quantitatively analyzed with the optimum theoretical performance parameters of the crystalline silicon solar cell.The directions in performance improving of SHJ solar cells including optimizing the chemical passivation at the heterojunction interface and the carrier-selective contacts are proposed.